Antonella Ruggiero

Pascal Lapébie

Carine Barreau

Evelyn Houliston
email: houliston@obs-vlfr.fr

Wnt signaling in multipotent stem cell formation and differentiation during larval development in the hydrozoan Clytia

Keywords: stem cell, hydrozoan, Wnt, neurogenesis, germ plasm

Acknowledgement

First of all I would like to thank Evelyn Houliston for giving me the possibility to do my PhD in her Laboratory and to Carine Barreau for being present in everyday lab life.

ank you for giving me the freedom to develop my style and my ideas and for taking care of me.

In particular I would like to say merci to Remi who o ered me a helping hand.

I would like to say thanks to all the present and the past Lab members: Patrick, Pascal, Luca, Chiara, Sandra, Gonzalo, Tsuyo and Sophie. It has been a pleasure to work with you, thank you for the support (technical and moral!).

A special thanks to Pascal that was my big-brother (I would prefer to say fratm) and to Lucas and Chiara those were always there for me.

I am also very grateful to all the people in the unit that I have met over these four years, in particular to Janet for the hospitality and to Stefano who was the rst to welcome me here.

Merci Sophie, Justine et Carole pour être toujours là dans cette longue marche dans le désert.

Vorrei ringraziare Stefania per essere stata allo stesso tempo: ricercatrice, capo, psicologa, mamma, amica... grazie per essere stata tutto questo! Grazie di cuore alla mia famiglia e ai miei amici piu cari che mi hanno guardato da lontano!

General Summary

The aim of this work was to extend our understanding of the mechanisms regulating stem cell formation, specification and differentiation by studies in the non-bilaterian metazoan model Clytia hemisphaerica. Clytia, like other hydrozoan cnidarians, possess a particular population of multipotent stem cells called interstitial cells (i-cells), present during larval development and in the adult medusa, which are able to give rise both to somatic cell types and to gametes.

In bilaterian animals Wnt/β-catenin signalling regulates fundamental developmental processes such as germ layer segregation and primary body axis specification, but also regulates stem cell proliferation, lineage specification and differentiation.

I investigated the role of Wnt/β-catenin signalling in i-cell specification and differentiation.

I established a set of markers for known i-cell derivatives: neural cells, nematocytes, gland cells and germ cells, which I used to monitor the presence of these cell types in Wnt3 morphant embryos. The results obtained suggest that Wnt/β-catenin signalling is involved in the last step of differentiation for certain neuronal cell types including nematocytes, but not for somatic cell fate choice from the multicellular i-cell pool. I found no evidence for germ cell fate specification from the multipotent i-cell pool prior to adult stages.

In the second part of my study I investigated the role of Wnt/β-catenin signalling in i-cell formation during embryogenesis. The results indicated that during normal development icell formation is Wnt/β-catenin independent and probably driven by inheritance of germ plasm containing localised mRNAs from the egg animal pole. In contrast in embryo repatterning following embryo bisection, Wnt/β-catenin signalling appears to be necessary for de novo i-cell formation in the absence of germ plasm. Thus two distinct mechanisms can lead to i-cell formation during embryogenesis.

Overall the results obtained provided a better picture of how i-cells and their derivatives arise during embryogenesis and larval development.

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List of the Figures

Figure 1 : Stem cell potential. Figure 2 : Asymmetrical cell divisions. Figure 3 : Wnt signalling pathways. Figure 4 : Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure 5 : Embryonic axis and Wnt β-catenin signalling. Figure 6 : One hypothesis for the phylogenetic relationships between major metazoan clades.

Figure 7 : Cnidarian model in developmental biology. Figure 8 : Clytia life cycle. Figure 9 : Clytia embryonic development. Figure 10 : Clytia medusa anatomy. Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult. Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa. Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures. Figure 15 : Structure of Clytia female gonad. Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification. Figure 17 : Molecular markers for Clytia Nematogenesis. Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs. Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions. Hydrozoan cnidarians were among the first non-bilaterian metazoan animals in which stem cells were described. A particular population of multipotent stem cells termed interstitial cells (i-cells) can differentiate into somatic cells such as nerve cells, gland cells and nematocytes, as well as into gametes.

My thesis work addressed the regulation of the i-cell population in the hydrozoan Clytia hemisphaerica. Various aspects of stem cell regulation were addressed, namely their formation during embryogenesis, proliferation, specification and differentiation of derivative fates. The main aspects treated in this manuscript will be specification (how the i-cells chose their fate), and differentiation (how once specified they reach their terminal state), as well as their embryological origin.

In the first part of this Introduction (1), I will describe general stem cell features and the mechanisms that regulate their formation and differentiation in various taxa and tissues.

The second part [START_REF] Hansen | Control of the proliferation versus meiotic development decision in the C. elegans germline through regulation of GLD-1 protein accumulation[END_REF] is focussed on the cnidarian biology, in particular on i-cell biology.

I will describe in detail the known i-cell derivative cell types: their morphology and gene expression characteristics, and what is known about the molecular mechanisms controlling their specification and differentiation.

1) A general overview of stem cell features and regulation

Stem cell properties

A general definition of a stem cell, as commonly found in cell biology textbooks (e.g. Molecular Biology of the Cell, [START_REF] Alberts | Somatic stem cells express Piwi and Vasa genes in an adult ctenophore: ancient association of "germline genes" with stemness[END_REF], is as an undifferentiated cell capable of both self-renewal and multi-lineage differentiation, displaying an unlimited proliferative activity that lasts (in vivo) the lifetime of the animal. Based on this definition a stem cell should have the following characteristics:

• No particular morphological distinctive features.

• Be able to generate at least one daughter cell with the same potential as the mother cell.

• Unlimited mitotic proliferation.

• Potential to generate large numbers of differentiated functional progeny.

Stem cell potential

Based on their potential to differentiate into any particular type of cell, stem cells can be classified into 4 different categories: totipotent stem cells, pluripotent stem cells, multipotent stem cells and unipotent stem cells (Figure 1). 1) Totipotent stem cells: a totipotent cell can give rise to various differentiated cell types including those of extra-embryonic tissues. The zygote is considered by some to be totipotent stem cells, although it doesn't have the property of self-renewal, at least not directly.

2) Pluripotent stem cells: animal pluripotent stem cells are able to differentiate into all cell types of the three embryonic germ layers, i.e., ectoderm, mesoderm, and endoderm. The best-know example is the embryonic stem cell (ES cell) [START_REF] Martin | Isolation of a pluripotent cell line from early mouse embryos cultured in medium conditioned by teratocarcinoma stem cells[END_REF]. In mammals, ES cells derive from the inner cell mass of the blastocyst. The inner cell mass comprises a group of cells arising very early during pre-implantation development, which give rise to the embryo proper. The outer cell layer of the blastocyst instead gives rise only to extraembryonic tissues.

In all the animals that lack extra embryonic tissues, the terms totipotent and pluripotent are equivalent.

3) Multipotent cells: multipotent stem cells show a restricted pattern of differentiation into a limited number of cell types. A good example of a multipotent stem cell system is provided by the hematopoietic system, responsible for producing mature blood cells.

Hematopoietic stem cells called hemocytoblasts give rise to "transit amplifying" progenitors of the myeloid and lymphoid lineages. Transit-amplifying cells are fated cells that proliferate and generate fully differentiated cells. The myeloid progenitor cells will then generate erythrocytes, megakaryocytes and myeloid cells, which in turn proliferate and differentiate into eosinophil, basophil, neutrophil and monocytes. The lymphoid cells produce T lymphocytes, B-lymphocytes and NK lymphocytes.

Multipotent stem cells are defined as adult stem cells if they are found in adult stage and can generate all the cell types of the tissue from which they originate, as is the case for the hematocytoblasts.

4) Unipotent stem cells: unipotent stem cells have a very restricted cell differentiation capacity, and can generate only one single type of cell. Melanocyte stem cells provide one example of unipotent stem cells, producing the pigmented cells (melanocytes) present in hair follicles and in epidermis. Melanocyte stem cell systems were characterised for the first time in mouse hair follicles [START_REF] Nishimura | Dominant role of the niche in melanocyte stem-cell fate determination[END_REF]. During each cycle of hair regeneration and regression, melanoblasts proliferate and differentiate into fully mature melanocytes, which enter into the developing hair.

As mentioned in the hematopoiesis example above, stem cells divide to generate intermediate cells, termed transit-amplifying cells. This type of temporal and developmental hierarchical organisation reduces the number of cell divisions that stem cells must undergo [START_REF] Morrison | Stem cells and niches: mechanisms that promote stem cell maintenance throughout life[END_REF]. However this ordered progression, from stem cells to differentiated cells is not rigid. Indeed in particular conditions committed, differentiating or differentiated cells can revert to the self-renewing stem cells state. Thus tissue stem cells have the potential to revert (to dedifferentiate in the most extreme examples) to stem cells under certain circumstances. An example of this cellular plasticity is provided by studies of mouse spermatogonia [START_REF] Nakagawa | Functional Hierarchy and Reversibility Within the Murine Spermatogenic Stem Cell Compartment[END_REF]. Spermatogenic cells are organised within a compartment populated by germ cells and by somatic cells with support functions. The germ cell precursors, the spermatogonia give rise to the spermatocytes. Several subpopulations of spermatogonia, can be distinguished by morphology and by gene expression patterns reflecting different degrees of differentiation.

During regeneration after injury, spermatogonia at the most advanced differentiating stages contribute to the re-population of the spermatogenetic compartment by reconverting into stem cells.

Generating cell type progenitors

The progenitors of particular cell types are generated when stem cell progeny loses their stem cell properties (proliferation and self renewal) and moves on the pathway towards differentiation. In order to provide progenitors of particular differentiated cell types while The cartoon shows how stem cells gradually commit towards a differentiation pathway. The cells acquire a restricted fate, decreasing in potential. Pluripotent cells (in red) are able to give rise to multipotent stem cell here represented in three different colours (green, pink and blue). Multipotent stem cells have a reduced potential. Curved arrows illustrate the self-renewal ability of these stem cells. On the right, an illustration of the zygote that in mammals is a totipotent cell. also renewing the stem cell population, the stem cells divide asymmetrically giving rise to two sister cells that have different fates, a feature that can be recognised by differences in size, morphology, gene expression pattern, or the number of subsequent cell divisions undergone by the two daughter cells [START_REF] Horvitz | Mechanisms of asymmetric cell division: Two Bs or not two Bs, that is the question[END_REF]. The asymmetric choice between self-renewal and differentiation can be regulated by extracellular cues coming from a "niche" acting preferentially on one of the daughter cell (a process termed environmental asymmetric cell division), or by intrinsic programs within the cell, which is known as divisional asymmetric cell division [START_REF] Wilson | Bone-marrow haematopoietic-stem-cell niches[END_REF] or unequal segregation of localised cytoplasmic determinants (Figure 2).

In particular conditions such as during the formation of the stem cells pool, stem cell can divide symmetrically. A) Localised determinants direct the fate of the terminal differentiated cell, activating an intrinsic genetic program. B) Extrinsic cues determine the fate of one of the two daughter cells inducing the specification of cell precursor (CP) that will move into the differentiation pathway. Modified from [START_REF] Wilson | Bone-marrow haematopoietic-stem-cell niches[END_REF].

Generating progenitors via unequal determinant segregation

Two main events are necessary for setting up divisional asymmetric cell division: cell fate determinants have to be asymmetrically localised and the mitotic spindle has to be oriented with respect to the determinant axis [START_REF] Betschinger | Dare to be different: asymmetric cell division in Drosophila, C. elegans and vertebrates[END_REF].

One example of asymmetric cell division of a stem cell involving unequal determinant distribution is seen in Drosophila neuroblasts [START_REF] Knoblich | Asymmetric cell division during animal development[END_REF]), which are central nervous system progenitor cells. Neuroblasts derive from the neuroectoderm tissue. Committed neuroblasts migrate into the sub neuroectodermal layer in a process termed delamination, and then generate by asymmetric cell division a small basal daughter cell called the ganglion mother (GMC) and a large apical daughter cell that retain a stem cell nature. The GMC is the neural progenitor as it will divide one more time and gives rise to two post mitotic neurons. In contrast the large apical cell retain a neuroblasts identity and continue to divide in a stem cell manner. The apical-basal polarity is set up by the PAR protein complex, which is localised apically. The PAR complex comprises a core of the three proteins: Par3, Par6 and aPKC (atypical Protein Kinase C), which have a role in asymmetric cell division across species and in different cell types [START_REF] Guo | par-1, a gene required for establishing polarity in C. elegans embryos, encodes a putative Ser/Thr kinase that is asymmetrically distributed[END_REF].

The PAR complex regulates the localisation of the neural fate determinants in the GMC cells such as the membrane-associated Notch pathway suppressor Numb, the transcription factor Prospero, and specific adaptor proteins responsible for their basal localisation Miranda and Pon (Partner of Numb). Miranda is a coiled-coiled protein that binds Prospero and translocates it to the cell cortex, from where it is later released in the daughter cell to activate neural cell fate gene transcription. 1.3.2 Generating progenitors via environmental cues: the "Niche" Stem cell "niches" are spatially distinct microenvironments that can include neighbouring cells, signalling molecules and other extracellular materials [START_REF] Scadden | The stem-cell niche as an entity of action[END_REF]. These have been studied extensively in Drosophila and Caenorhabditis elegans gonads, which can be conveniently manipulated through genetic and molecular approaches. The C. elegans adult gonad shows an ordered progression in germ line formation from germ stem cells (GSCs) occupying the most distal extremity, to mature gametes positioned in the proximal part. A key population of undifferentiated somatic mitotic cells called "distal tip cells" (DTCs) covers the distal extremity of the gonad. DTCs are the major regulator of germ line proliferation and are the main component of the gonad stem cell niche [START_REF] Kimble | On the control of germ cell development in Caenorhabditis elegans[END_REF]. The stem cell properties of the GSCs depend upon their contact with the DTCs. As GSCs move further away from the distal tip, they terminate their mitotic activity and enter into meiosis. DTCs cells control GSC differentiation through a Notch-like signal. DTCs express on the surface the delta-like ligand LAG-2, which activates the Notch-like receptor GLP-1, localised on the mitotic germ cells. In the cell contacting the DTCs, GLP-1 is activated and translocates to the nucleus, where it downregulates meiosis promoting genes like gld-1, gld-2 and nos3, required to maintain the stem cell state [START_REF] Hansen | Control of the proliferation versus meiotic development decision in the C. elegans germline through regulation of GLD-1 protein accumulation[END_REF]).

Maintaining the undifferentiated stem cell state

Maintenance of the undifferentiated stem cell state relies on a variety of regulatory mechanisms acting at different levels including transcriptional regulation, translational regulation and epigenetic modification of chromatin. During my thesis I used extensively as stem cell markers two genes involved in maintaining pluripotency through regulation of mRNA metabolism and translational control, Piwi and Nanos. Interestingly these two genes along with other mRNA binding proteins such as Vasa and PL10 which have traditionally been considered as "germ line genes" because of their highly conserved expression in germ cell precursors (PGC), are now emerging as more widely functioning stem cell regulators with functions outside the germ line.

Before introducing Piwi, Nanos in more detail, I will mention briefly in this section the main actors in transcriptional regulation and epigenetic regulation of stem cell identity.

Transcriptional control

A set of transcriptional factors (TFs) have been well characterised in vertebrate embryonic stem cells, including Nanog, Oct4, Klf4 and Sox2, which act to repress somatic differentiation programs [START_REF] Chambers | Functional Expression Cloning of Nanog, a Pluripotency Sustaining Factor in Embryonic Stem Cells[END_REF][START_REF] Niwa | Quantitative expression of Oct-3/4 defines differentiation, dedifferentiation or self-renewal of ES cells[END_REF][START_REF] Boyer | Core transcriptional regulatory circuitry in human embryonic stem cells[END_REF].

Discovered in 1998 Oct4 was characterised as an essential TF in maintaining the pluripotent state [START_REF] Nichols | Formation of Pluripotent Stem Cells in the Mammalian Embryo Depends on the POU Transcription Factor Oct4[END_REF]. Oct4 belongs to the POU transcription factor family and can either activate or repress target gene expression by binding to the octameric sequence consensus motif AGTCAAAT. This consensus motif can be found in regulatory sequences including those in introns. Although POU domains that bind this sequence are conserved across metazoans [START_REF] Gold | The Early Expansion and Evolutionary Dynamics of POU Class Genes[END_REF]), Oct4 orthologs have not been identified outside vertebrates.

Nanog is required for the maintenance of a pluripotent state and to prevent the differentiation. It is a tripartite protein with a central homeodomain. Transcriptional regulation is mediated by the C-terminal domain, which is characterised by two subdomains, WR and CRD2 [START_REF] Pan | The stem cell pluripotency factor NANOG activates transcription with two unusually potent subdomains at its C terminus[END_REF]. Recent phylogenetic analysis shows that Nanog like Oct4 is not present in C. elegans or in Drosophila or in non vertebratechordates like Ciona and Amphiouxus [START_REF] Satou | Regulatory genes in the ancestral chordate genomes[END_REF]) but is present only in vertebrates [START_REF] Ma | Bioinformatic analysis of the four transcription factors used to induce pluripotent stem cells[END_REF]. Sox2 belongs to the SR-HMG protein family. It is needed for the maintenance of a pluripotent state. Indeed loss of Sox2 causes the activation of differentiation pathways [START_REF] Niwa | How is pluripotency determined and maintained? Development[END_REF]. Sox2 can cooperate with Oct3/4 in the control of target gene transcription. Kfl4 is a member of the KFL (Krüppel-like factor) transcription factor family. It is a zinc finger protein, which can activate or repress target genes involved in cell proliferation and or differentiation.

Epigenetic mechanisms

Stem cell regulation mechanisms involve epigenetic mechanisms and non-coding RNAmediated gene silencing. Epigenetic in this context refers to changes in DNA structure without modifications of the DNA sequence. Epigenetic mechanisms include histone modifications and DNA methylation (chemical modifications to the cytosine residues of DNA). The tri-methylation of particular residues on the DNA sequence can identify the state of the chromatin. The methylation of the Lysine 4 on the Histone 3 (metK4H3), is correlated with transcriptional activity, while compact chromatin, not accessible for transcription, is marked by methylation of the Lysine 27 on Histone 3 (metK27H3). A protein complex involved in chromatin modification is the Polycomb group complex (PcG). PcG proteins function as a repressive complex, is required for the long-term silencing of chromatin and has an important role in the differentiation of stem cells as well as in early embryonic development [START_REF] Simon | Occupying chromatin: Polycomb mechanisms for getting to genomic targets, stopping transcriptional traffic, and staying put[END_REF]. Three proteins form the core complex: enhancer of zeste (Ezh2), embryonic ectoderm development (Eed) and suppressor of zeste 12 (Suz12). Eed binds Suz12, which is a histone methyltransferase while EZh2 maintains the silent tri-methylated state of the Lysine 27 residue of the target DNA.

PcG proteins are highly conserved across animal species and have been studied also in two hydrozoans: Podocoryne [START_REF] Lichtneckert | Evolutionary conservation of the chromatin modulator Polycomb in the jellyfish Podocoryne carnea[END_REF] and Hydra [START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF][START_REF] Genikhovich | Discovery of genes expressed in Hydra embryogenesis[END_REF]. The Hydra Eed ortholog HyEED is expressed in hydrozoan multipotent stem cells termed interstitial cells (i-cells) and in nematoblasts (stinging cells peculiar to cnidarians) [START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF][START_REF] Genikhovich | Discovery of genes expressed in Hydra embryogenesis[END_REF]), which will be introduced in detail respectively in section 2.2.1 and 2.2.3 of this Introduction chapter. As HyEDD is expressed in the early-stage nematoblasts located in Hydra gastric region, but never detected in differentiated nematocytes located in the head and in the foot.

Overexpression of a construct in which HyEED was fused to eGFP under a ubiquitous actin promoter, revealed degradation of the fusion protein, and thus probably of the endogenous protein, during nematocyte differentiation. Furthermore, following treatment with an inhibitor of the proteasome machinery (MG132), HyEED expressing cells were detected in mature nematocytes located at the extremity of the Hydra polyps. This study points to a role for EED in Hydra in preventing nematocyte differentiation from nematoblasts by polycomb type chromatin remodelling.

Regulatory RNAs

Small non-coding regulatory RNAs, along with epigenetic mechanisms, facilitate the continued expression of stem cell TFs. Stem cell TFs contribute to maintaining pluripotency by preventing stochastic and aberrant induction of differentiation. Thus regulatory RNAs, such as microRNAs (miRNA), endogenous small interfering RNAs (siRNA), long non-coding RNAs (LncRNA) and Piwi-interacting RNA (piRNA) play an important role in the control of pluripotency.

In the last few years the role of the LncRNAs has been widely investigated [START_REF] Flynn | Long noncoding RNAs in cell-fate programming and reprogramming[END_REF]. LncRNAs are non-coding transcripts greater then 200 base pairs in length. A specific LncRNA has been identified that participates in the regulation of pluripotent ES cells [START_REF] Guttman | lincRNAs act in the circuitry controlling pluripotency and differentiation[END_REF]. Knockdown of these LncRNAs in mouse ES cells results in downregulation of the expression of pluripotency markers including Oct4, Nanog, Sox2 and Klf4. siRNAs and miRNAs bind to Argonaute/Piwi family proteins of the Ago sub-type which are widely expressed in different tissue types [START_REF] Wutz | Transcriptional and Translational Regulation of Stem Cells[END_REF]. They are characterised by three highly conserved domains: the PAZ, MID and PIWI domains. The N-terminal PAZ domain and the MID domain together bind small RNAs [START_REF] Song | Identification, expression, and purification of a unique stable domain from human HSPC144 protein[END_REF], while the Cterminal PIWI domain, whose structure is similar to that of RNase H enzymes, cleaves the RNA/DNA hybrids formed when the regulatory RNAs hybridise to complementary sequences in the genomic DNA [START_REF] Okamura | Distinct roles for Argonaute proteins in small RNA-directed RNA cleavage pathways[END_REF]. Piwi proteins bind to a specific type of regulatory RNAs called piRNAs, which are mostly restricted to germ line and stem cells [START_REF] Thomson | The biogenesis and function of PIWI proteins and piRNAs: progress and prospect[END_REF], as described in more detail in the following sections. piRNAs have sequence complementary to those of transposable elements : mobile DNA sequences that can insert themselves into new positions in the genome where they can cause genome instability and loss of long-term fitness. Piwi-piRNA interaction leads to the degradation of piRNA/transposable elements hybrids, thus preventing specific transposons from integrating into the genome [START_REF] Brennecke | Discrete Small RNA-Generating Loci as Master Regulators of Transposon Activity in Drosophila[END_REF]. A major function of Piwi and of pi-RNAs is thus thought to be to protect the genome from transposable mobile elements.

More generally, RNA-binding proteins regulate every aspect of RNA metabolism, including pre-mRNA splicing, mRNA trafficking, stability, and translation. Among these, Nanos is an RNA binding protein with translational repressor function. In the next section I will give a more detailed introduction to the classical germ line genes, Nanos and Piwi that I used to track the multipotent stem cell population in Clytia.

A shared gene toolkit for stem cells and germ line

In the last few year molecular studies conducted in non-bilaterian organisms such as sponges and cnidarians, and in some invertebrates like planarians, have indicated the presence of a common molecular signature shared between some stem cell populations and the germ cells. These genes have been referred to as Germline Multipotency Program (GMP) genes (Juliano et al. 2010). The GMP gene set includes Nanos, Piwi, Vasa and PL10, all coding for RNA binding proteins. The GMP gene products have frequently been described as associated in granule-like structures in germ line cells variously termed nuage, chromatid bodies, germ plasm, and pole granules. These structures extensively described in primordial germ cells and developing oocytes, are composed of ribonucleoprotein complexes located in the perinuclear region in PGCs which often translocate to subcortical regions of the oocyte during oogenesis [START_REF] Kloc | The Balbiani body and germ cell determinants: 150 years later[END_REF]). Germ plasm presence in oocytes of different species is rather patchy across the animal phylogenetic tree leading to speculation that it may have arisen several times during evolution [START_REF] Extavour | Mechanisms of germ cell specification across the metazoans: epigenesis and preformation[END_REF]. Traditionally its role has been considered to be as a germ line determinant, which causes the cells of the embryo that inherit it to adopt a PGC fate. This determinant role for germ plasm has been shown by ablation or transplantation experiments in various insect species and in frogs [START_REF] Kloc | The Balbiani body and germ cell determinants: 150 years later[END_REF]).

Piwi proteins in the germ line and stem cells

Piwi, Nanos and Vasa proteins and mRNAs, have repeatedly been found to be enriched in germ plasm in species where the germ plasm is present. More generally, across all species examined whether they have germ plasm in their oocytes or not, expression of the GMP genes is characteristic of the germ cell developmental lineage ('germ line'). In nonbilaterian animals, including sponges and cnidarians, as well as in some bilaterian species, expression of Nanos, Piwi and other "germ line" genes has been documented also in various populations of non-germ line multipotent stem cells. This has led to the idea that the presence of these proteins may be a molecular signature of pluripotent/multipotent stem cells rather than exclusively a germ cell marker. The findings relating to this conclusion are described in more detail in the following sections.

• Piwi, a universal marker of germ line

Historically Piwi (p-element induced wimpy testis) was identified in Drosophila from genetic screening for mutants defective in stem cell division. Characterisation of the Piwi phenotype revealed a role in germline stem cell (GSC) maintenance via self-renewal [START_REF] Lin | A novel group of pumilio mutations affects the asymmetric division of germline stem cells in the Drosophila ovary[END_REF]. In Drosophila, Piwi is expressed in the germ line and in a somatic population of cells located apically with respect to the GSCs [START_REF] Cox | A novel class of evolutionarily conserved genes defined by[END_REF]. In C. elegans prg-1 and prg-2 (Piwi related genes) are required for the maintenance of mitotic germ cells.

The zebrafish Piwi homolog Ziwi is also required for maintenance and development of germ cells [START_REF] Houwing | A Role for Piwi and piRNAs in Germ Cell Maintenance and Transposon Silencing in Zebrafish[END_REF], while Miwi expression in mouse is restricted to the male germ line, and affects spermatogenetic progression [START_REF] Deng | miwi , a Murine Homolog of piwi , Encodes a Cytoplasmic Protein Essential for Spermatogenesis[END_REF]. In the polychaete Capitella teleta, two Piwi orthologs were found, Ct-Piwi1 and Ct-Piwi2 [START_REF] Giani | Somatic and germline expression of piwi during development and regeneration in the marine polychaete annelid Capitella teleta[END_REF].

Expression pattern analysis detected these two transcripts in overlapping domains of proliferating somatic stem cell in the posterior growth zone and in the male and female PGCs, localised in the mature gonads.

• Piwi a marker of stem cell in non-bilaterian metazoans

After the discovery of Piwi, in 1998 Cox and colleagues were the first to hypothesise an extended role of the Piwi family protein. They proposed that Piwi represents the first of a group of genes that acts both in germ line and in somatic stem cells to regulate stem cell maintenance, proliferation and differentiation. This hypothesis has now been confirmed thanks to the increasing number of studies showing the presence of Piwi protein in somatic stem cells in a variety of animal species.

One example of multipotent stem cells expressing Piwi is sponge archeocytes, which display morphological and biological stem cells characteristics [START_REF] Simpson | Cell Biology Sponges[END_REF]. Recent studies on the fresh water sponge (demonsponge) Ehirava fluviatilis demonstrate that archeocytes self-renewal, proliferate and possess the capacity to differentiate into several cell types including gametes [START_REF] Funayama | The stem cell system in demosponges: Insights into the origin of somatic stem cells[END_REF]. Archeocytes expressed two Piwi orthologs EflPiwiA and EflPiwiB. EflPiwiA is also expressed in another cell type specific to the sponge called choanocytes, flagellated cells whose main function is nutrient uptake. Upon spontaneous disorganisation and reorganisation of the cells in Ehirava, choanocytes can trans-differentiate into archeocytes and then re-differentiate into spermatocytes [START_REF] Gaino | Origin of male gametes from choanocytes in Spongia officinalis (Porifera, Demospongiae)[END_REF].

In demosponges, it was recently proposed that a stem cell system based on two stem cells populations exist: the archeocytes and the choanocytes [START_REF] Funayama | The stem cell system in demosponges: Suggested involvement of two types of cells: Archeocytes (active stem cells) and choanocytes (food-entrapping flagellated cells)[END_REF]. The archeocytes give rise to all cell types [START_REF] Funayama | The stem cell system in demosponges: Insights into the origin of somatic stem cells[END_REF] and the choanocytes manifest their stem cell potential in special conditions, during sexual reproduction or during regeneration. In the latter case choanocytes could accelerate the processes of sponge body recovery [START_REF] Funayama | The stem cell system in demosponges: Suggested involvement of two types of cells: Archeocytes (active stem cells) and choanocytes (food-entrapping flagellated cells)[END_REF]).

In the ctenophore Pleurobrachia pileus, two Piwi orthologs are found to be expressed in somatic stem cells and in germ cells (Alié et al. 2011). In particular Piwi proteins are expressed in undifferentiated cells in tentacle roots, in the stem cells that give rise to neuronal types at the aboral pole, and in the ciliated polster cells in the combs.

Piwi gene expression is also characteristic of the i-cell population in Hydra, Hydractinia and Clytia, (Nishimiya-Fujisawa and Kobayashi 2012, Kanska and[START_REF] Kanska | New roles for Nanos in neural cell fate determination revealed by studies in a cnidarian[END_REF][START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF][START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF].

A further example of Piwi expression in somatic stem cells is seen in planarian neoblasts.

Neoblasts are totipotent stem cells that can generate all somatic cell types as well as the germ line [START_REF] Baguna | Planarian neoblast[END_REF]). In the planarian Schmidtea mediterranea two Piwi orthologs have been identified: Smedwi1 and Smedwi2. Functional studies by RNAi treatments show that Smedwi2 is necessary for the production of neoblast progeny [START_REF] Reddien | SMEDWI-2 is a PIWI-like protein that regulates planarian stem cells[END_REF].

Despite the reported expression of Piwi genes in a wide range of somatic stem cell types, their function in these cells remains unclear. They may be involved both in maintenance of the stem cell pool, as seen in the Smedwi2 case, and in protection of the genome from transposons, which may compromise regeneration capacity or the ongoing tissue renewal seen in hydrozoans, ctenophore tentacles and sponges.

Nanos regulates transcription and translation repression

Nanos is an RNA binding protein that contains a CCHC zinc finger domain with wellconserved functions involved both transcriptional regulation and translational repression.

Nanos was first identified in Drosophila as an essential factor in early embryonic patterning [START_REF] Weismann | RNA regulatory elements mediate control of Drosophila body pattern by the posterior morphogen nanos[END_REF]; however it has since become clear that this role is particular to Drosophila, while the conserved function of Nanos is associated with germ line.

• Nanos is associated with germ line

Nanos is expressed in primordial germ cells (PGCs) in widely divergent metazoans and its function is required for PGCs maintenance and migration in different organisms including zebrafish [START_REF] Koprunner | A zebrafish nanos -related gene is essential for the development of primordial germ cells[END_REF], C. elegans [START_REF] Kraemer | NANOS-3 and FBF proteins physically interact to control the sperm-oocyte switch in Caenorhabditis elegans[END_REF] and Xenopus [START_REF] Lai | Xenopus Nanos1 is required to prevent endoderm gene expression and apoptosis in primordial germ cells[END_REF]. To execute its role as a translational repressor, Nanos acts together with a protein partner called Pumilio. mRNAs identified as Nanos/Pumilio targets have been characterised in several organisms and include the transcription factor Hunchback, involved in the establishment of anterior-posterior polarity in Drosophila [START_REF] Murata | Binding of Pumilio to maternal hunchback mRNA is required for posterior patterning in Drosophila embryos[END_REF], and VegT an endoderm fate TF in Xenopus [START_REF] Lai | Xenopus Nanos1 is required to prevent endoderm gene expression and apoptosis in primordial germ cells[END_REF].

A role for Nanos in transcriptional repression in the germ line has also been shown in Drosophila where somatic gene transcription is prematurely activated in mutants lacking Nanos [START_REF] Kobayashi | Essential role of the posterior morphogen nanos for germline development in Drosophila[END_REF].

• Nanos somatic expression

In Drosophila, Nanos also acts outside the germ line directing dendrite morphogenesis [START_REF] Brechbiel | Spatial regulation of nanos is required for its function in dendrite morphogenesis[END_REF] and some data also available suggest a possible function for Nanos in somatic stem cell maintenance. In the sponge Sycon ciliatum, Nanos is expressed in oocytes and in macromeres that give rise to the pynacoderm, the external layer of the adult sponge [START_REF] Leininger | Developmental gene expression provides clues to relationships between sponge and eumetazoan body plans[END_REF]. Thus sponge Nanos is expressed in both germ line and somatic cells. In several hydrozoan species examined, the Nanos1 genes are expressed in icells and germ cells. In contrast the paralog Nanos2 genes are additionally expressed at other sites, including the epithelial endodermal hypostome cells in Hydra. Hydra Nanos2 expression increases during head regeneration, suggesting a role in head morphogenesis [START_REF] Mochizuki | Expression and evolutionary conservation of nanos-related genes in Hydra[END_REF]. Clytia and Hydractinia Nanos2 paralogs, beside i-cells and germ line, are expressed in differentiating nematoblasts [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF][START_REF] Kanska | New roles for Nanos in neural cell fate determination revealed by studies in a cnidarian[END_REF], indicating presumably a role for Nanos2 in nematocyte formation. In the anthozoan cnidarian Nematostella Nanos orthologs have been characterised in two different studies.

However the 2 reports show some discrepancies concerning the expression pattern of Nanos1 and Nanos2. Torras and colleagues show a localised maternal distribution of Nanos1 mRNA [START_REF] Torras | Posterior expression of nanos orthologs during embryonic and larval development of the anthozoan Nematostella vectensis[END_REF], while the results of Extavour and colleagues suggest that Nanos1 is only expressed zygotically from the early gastrula stage, in a population of ectodermal scattered cells (Extavour et al. 2005). Torras and colleagues also claim that Nanos2 is first detected at gastrula stage, then it localised in the posterior region of the planula larva and in developing tentacles, suggesting that Nanos2 could regulate the formation of posterior structures [START_REF] Torras | Posterior expression of nanos orthologs during embryonic and larval development of the anthozoan Nematostella vectensis[END_REF]. In contrast Extavour and colleagues provide a detailed Nanos2 expression pattern showing for the first time the transcript at late blastula in a population of cells that in later stages will participate in the formation of the presumptive endoderm. In primary polyps Nanos2 expression is restricted to distinguishable patches of cells localised along the length of the mesenteries. These cells have the typical PGCs morphology (with large nucleus and Nanos cytoplasmic localisation). Moreover these cells are Vasa immunoreactive, suggesting that they might correspond to Nematostella PGCs. In Nematostella it remains to be resolved whether Nanos proteins have a somatic stem cell and/or germ line function.

In planarians expression of Nanos, unlike Piwi, is restricted to germ line precursors and its function is linked to gonad development and in gonad regeneration condition [START_REF] Wang | Nanos Function Is Essential for Development and Regeneration of Planarian Germ Cells[END_REF]) while in ctenophores the Nanos expression pattern has not yet been described.

Wnt signalling: a candidate for stem cell regulation

Extrinsic factors, notably acting via intercellular signalling pathways, play a major role in regulating the balance between stem cell differentiation and maintenance (section 1.3.2).

Many different signalling pathways including Wnt, ERK-MAPK, BMP-TFGβ-Smad and Notch [START_REF] Nusse | Wnt Signaling and Stem Cell Control[END_REF][START_REF] Zhang | BMP signaling and stem cell regulation[END_REF][START_REF] Campos | Beta1 integrins activate a MAPK signalling pathway in neural stem cells that contributes to their maintenance[END_REF][START_REF] Androutsellis-Theotokis | Notch signalling regulates stem cell numbers in vitro and in vivo[END_REF]

• Overview of Wnt signalling

Wnt signalling regulates crucial aspects of cell fate determination, cell migration, cell polarity, organogenesis and primary body axis polarity during embryonic development.

Wnt ligands and their Frizzled (Fz) receptors are evolutionary conserved during metazoan evolution. Indeed they are present throughout the animal kingdom [START_REF] Croce | Evolution of the Wnt pathways[END_REF], including in organisms that diverged early from the bilaterians such as sponges and cnidarians, implying their presence in the metazoan common ancestor.

Wnt signalling can occur via β-catenin mediated regulation of gene transcription, described as the "canonical" or Wnt/β-catenin pathway (Figure 3A). Wnt signalling can also occur through β-catenin-independent pathways, including the Wnt/PCP (Figure 3B) and Wnt/Ca2+ (Figure 3C) pathways. The core proteins of the Wnt/PCP pathway regulate two different phenomena: coordination of individual cell polarity in the plane of the tissue (Planar cell polarity), which can occur without Wnt ligands, and morphogenetic movements [START_REF] Lapébie | Dissecting the PCP pathway: One or more pathways?: Does a separate Wnt-Fz-Rho pathway drive morphogenesis[END_REF].

Wnt/Ca2+ pathway activation causes the release of intracellular Ca2+ with consequent activation of Cam Kinase II and Protein Kinase C which in turn drive the activation of nuclear factors like CREB and NFAT that promote the transcription of specific genes, including ones involved in organogenesis and establishment of the dorso-ventral axis [START_REF] De | Wnt / Ca 21 signaling pathway : a brief overview The Non-canonical Wnt Signaling Cascade[END_REF].

1.6.1 The Wnt/ β-catenin pathway Wnt/β-catenin pathway core components include: § Wnts Ligand. § Receptors: Frizzled (Fz) family seven-pass-transmembrane proteins, and LRP5/6 co-receptors. § The multi-domain protein Dishevelled (Dsh), which binds to the cytoplasmic domain of Fz. § The transcriptional co-regulator β-catenin. § A cytoplasmic destruction complex including the multi-domain protein adenomatous polyposis coli tumor suppressor (APC), the multifunction enzyme glycogen synthase kinase 3β (GSK3β), and the multidomain protein Axin. § The transcription factor T-cell factor/lymphoid enhancing factor (TCF/Lef).

The Wnt/β-catenin pathway is activated by the binding of Wnt ligands to the Fz-LRP5/6 complex (Figure 3A). In the absence of Wnt ligands newly synthetized β-catenin accumulates in the cytosol and is phosphorylated by GSK3β. Phosphorylated β-catenin binds the APC-Axin destruction complex for degradation by the proteasome machinery.

Upon Wnt/Fz interaction Dsh is phosphorylated and translocates to the plasma membrane [START_REF] Rothbächer | Dishevelled phosphorylation, subcellular localization and multimerization regulate its role in early embryogenesis[END_REF].

Phosphorylated Dsh recruits the APC-Axin destruction complex to the plasma membrane, thereby preventing destruction of β-catenin and resulting in its stabilisation and accumulation in the nucleus. Once in the nucleus, β-catenin interacts with the TCF/Lef transcription factor.

In the Wnt-off state the TCF/Lef complex binds the transcription repressor Groucho, forming a repressive complex which blocks transcription of Wnt target genes. In the Wnt on state, Groucho is replaced by the β-catenin converting the TCF complex from a transcriptional repressor to a transcriptional activator. A) The "canonical", β-catenin dependent pathway starts with the binding of the wnt ligands to the Frizzled (Fz) and LRP5/6 receptor complex. This complex induces the stabilisation of β-catenin via the bound protein Dishevelled (Dsh). Increased β-catenin stabilisation is accompanied by its translocation to the nucleus. In the nucleus, β-catenin bound to the transcriptional activator TCF/LEF induces the transcription of the genes normally suppressed by TCF/LEF in the absence of Wnt. In the absence of Wnt signalling, β-catenin associates with APC and AXIN in destruction complex. B) Interactions between the Fz/PCP core components (Fz, Dsh, Strabismus and Prickle) to assure planar cell polarity do not require the Wnt ligand (see Figure 4). Wnt binding can, however, regulate downstream cell behaviours via the so-called Wnt-PCP pathway through Fz and Dsh, which activates the small GTPase RhoA and its effector ROK (Rho-associated kinase) in order to regulate the actin cytoskeleton. Another downstream effector of Dsh is JNK which can be activated by RhoA. C) The Wnt-Ca 2+ pathway is activated through wnt ligand binding to Fz receptors. Fz induces the activation of heterotrimeric G-proteins, which regulates calcium-calmodulin kinase 2 (CamK2) and protein kinase C (PKC). Modified from [START_REF] Habas | Dishevelled and Wnt signaling: is the nucleus the final frontier[END_REF].

The Wnt/β-catenin pathway can be negatively or positively stimulated by pharmacological treatments with molecules that target different components of the pathway, resulting in either its inhibition or activation. Commonly GSK3β has been targeted by treatments with LiCl or by paullone derivatives such as Alsterpaullone and BIO, resulting in artificial stabilisation of β-catenin and ectopic activation of the pathway.

The Wnt/ Planar cell polarity pathway

The use of the term "Wnt/PCP pathway" is rather confusing since Fz-mediated Planar Cell Polarity (PCP) does not necessarily involve Wnt. It was recently proposed to separate the effects mediated by the core components Fz, Dsh, Strabismus and Prickle into two pathways [START_REF] Lapébie | Dissecting the PCP pathway: One or more pathways?: Does a separate Wnt-Fz-Rho pathway drive morphogenesis[END_REF]. One set of interactions termed Fz/PCP would coordinate planar cell polarity, while a second Wnt/Fz/Rho pathway involved in activating morphogenetic movements. These two pathways would act through different downstream modules.

• Fz/PCP and tissue polarity

In multicellular organisms, epithelial cells and other tissue sheets are not only polarized along the apicobasal axis, but also within the epithelial plane, a phenomenon known as planar cell polarity (PCP). In Drosophila PCP core components coordinate polarity between adjacent cells along the anterior-posterior axis of the embryo or the proximaldistal axis of the wings (Figure 4A). The PCP core components are Frizzled (Fz), Dishevelled (Dsh), the Lim domain protein Prickle (Pk), the four-pass-transmembrane protein Vang Gogh/Strabismus (Vang/Stbm), the ankyrin repeat protein Diego (Dgo) and the seven-pass-transmembrane atypical cadherin Flamingo/Starry night (Fmi/Stan) [START_REF] Cho | Action of fat, four-jointed, dachsous and dachs in distal-toproximal wing signaling[END_REF]. One of the key features of this pathway is the asymmetric distribution of certain components between the two sides of the cell with respect to the direction of planar polarity. This asymmetrical distribution allows the propagation and coordination of the signalling from cell-to-cell (Figure 4B). The Stbm/Pk complex is localized on one side, while, Fz/Dsh/Dg complex is localised on the opposite side, such that Stbm and Fz interact extracellularly in neighbouring cells [START_REF] Devenport | The cell biology of planar cell polarity[END_REF].

Fz/PCP was first described in the Drosophila adult wing to account for the polarity of cuticular hairs and bristles that point towards the posterior pole [START_REF] Gubb | A genetic analysis of the determination of cuticular polarity during development in Drosophila melanogaster[END_REF] (Figure 4A). In vertebrates, PCP underlies the organisation and orientation of stereocilia in the sensory epithelium of the inner ear and the organisation of hair follicles (Figure 4A). PCP core components are extremely conserved throughout the animal kingdom.

Their function in cilia orientation is conserved in hydrozoans, as is the case of Clytia. The Clytia Strabismus (Stbm) ortholog is required for the coordination of ectodermal cilia along the oral-aboral axis in the larva [START_REF] Momose | A conserved function for Strabismus in establishing planar cell polarity in the ciliated ectoderm during cnidarian larval development[END_REF]) (Figure 4A). Disrupting PCP in Clytia embryo by Stbm morpholino injection impairs cilia orientation.

• Wnt/Fz/Rho: the Morphogenetic module

In chordates the Wnt/Fz/Rho module has been shown to regulate different morphogenetic movements in the embryo, notably convergent extension in the so-called Keller explants.

The Keller explant is a piece of tissue including dorsal mesoderm and neural ectoderm from Xenopus early gastrula embryo. Two explants derived from two different embryos are cultured together. In wild type condition the neural ectoderm and the dorsal mesoderm undergo convergent extension (narrowing and elongating) forming a stereotyped morphology with two different domains (an elongated domain and a collar region interface). Keller explants are used to characterise the function of conserved heterologous proteins during gastrulation. Using this technique, it was shown that Nematostella Wnt5 and Wnt11 proteins impair convergent extension in Xenopus Keller explants [START_REF] Rigo-Watermeier | Functional conservation of Nematostella Wnts in canonical and noncanonical Wnt-signaling[END_REF]. Ectopic expression of Nematostella Wnt5 and Wnt11 induce opposite phenotypes. Wnt5 overexpression induces the formation of explants with a more elongating domain compared to the stereotyped morphology, whereas Wnt11 overexpression caused shorter explants, suggesting that these ligands affect cell convergent extension movements differently. It is not clear if Wnt5 and Wnt11 affect the establishment of the mesodermal cell polarity directly. Interestingly the overexpression of these two genes in Xenopus ventral blastomeres at the four-cell stage does not produce a secondary axis, unlike Nematostella Wnt1 overexpression. This suggests that Wnt5 and Wnt11 do not trigger the Wnt/β-catenin dependent pathway and that a β-catenin independent pathway was already present in the last common metazoan ancestor.

Dishevelled

The Wnt/Fz/Rho module not only regulates morphognetic movements in vertebrates but also in cnidarians. In Hydra inhibition of JNK, one of the of the downstream elements of the Wnt/Fz/Rho module, by a chemical compound that specifically acts on JNK (not on p38 or ERK), impairs Hydra bud evagination [START_REF] Philipp | Wnt/beta-catenin and noncanonical Wnt signaling interact in tissue evagination in the simple eumetazoan Hydra[END_REF].In Clytia, it remains to be seen whether Wnt/Fz/Rho dependent movements, oriented by Fz/PCP, are responsible for elongation of the Clytia embryo during gastrulation, which is disrupted when Stbm is inhibited [START_REF] Momose | A conserved function for Strabismus in establishing planar cell polarity in the ciliated ectoderm during cnidarian larval development[END_REF].

1.6.3 Developmental roles of Wnt/β-catenin pathway: Germ layers and body axis Wnt/β-catenin signalling is widely deployed during early embryonic development to regulate germ layer formation and body axis [START_REF] Clevers | Wnt/beta-catenin signaling in development and disease[END_REF][START_REF] Van Amerongen | Towards an integrated view of Wnt signaling in development[END_REF], suggesting ancestral and highly conserved roles of this pathway. Wnt pathway via βcatenin stabilisation leads to endoderm/endomesoderm specification. This process has been demonstrated in a wide range of phyla including cnidarians, echinoderm, hemichordates and ascidians [START_REF] Logan | Nuclear beta-catenin is required to specify vegetal cell fates in the sea urchin embryo[END_REF][START_REF] Imai | Beta)-Catenin Mediates the Specification of Endoderm Cells in Ascidian Embryos[END_REF][START_REF] Wikramanayake | An ancient role for nuclear beta-catenin in the evolution of axial polarity and germ layer segregation[END_REF][START_REF] Darras | Catenin specifies the endomesoderm and defines the posterior organizer of the hemichordate Saccoglossus kowalevskii[END_REF][START_REF] Hudson | β-Catenin-Driven Binary Fate Specification Segregates Germ Layers in Ascidian Embryos[END_REF]. Studies in the solitary ascidians Phallusia mammillata and Ciona intestinalis revealed that the fate choice between endoderm and mesoderm is mediated through a β-catenin binary ON-OFF switch [START_REF] Hudson | β-Catenin-Driven Binary Fate Specification Segregates Germ Layers in Ascidian Embryos[END_REF].

Hyperactivation or downregulation in isolated blastomere can convert the fate choice. For example it was shown that if an isolated blastomere retains an ON β-catenin state from 8 to 32 cell stage it will have an endoderm fate while in a OFF state the cell will give rise to the ectoderm [START_REF] Hudson | β-Catenin-Driven Binary Fate Specification Segregates Germ Layers in Ascidian Embryos[END_REF].

More widely discussed is the role of β-catenin nuclearisation as the key step in the establishment of the antero-posterior axis during bilaterian embryogenesis and the oralaboral axis in non-bilaterian animals (Petersen & Reddien 2009;[START_REF] Marlow | Ectopic activation of the canonical wnt signaling pathway affects ectodermal patterning along the primary axis during larval development in the anthozoan Nematostella vectensis[END_REF].

During gastrulation β-catenin nuclearisation occurs and specifies oral-aboral identity, at the posterior pole in bilateria and the oral pole in cnidarians (Figure 5A). This correlation suggests that regionalised nuclear localisation of the β-catenin to establish embryo polarity was already a feature of embryogenesis in the common ancestor of metazoans.

• Wnt signalling in Cnidarians

Genome and transcriptome analyses in cnidarian species have identified all the core components of Wnt and Fz/PCP signalling pathways, including Wnt ligands and Fz receptors [START_REF] Kusserow | Unexpected complexity of the Wnt gene family in a sea anemone[END_REF][START_REF] Guder | The Wnt code: cnidarians signal the way[END_REF][START_REF] Lee | A WNT of things to come: evolution of Wnt signaling and polarity in cnidarians[END_REF]. In cnidarians Wnt/βcatenin signalling has a key role in regulating axis formation, as well as other features of embryonic patterning, notably germ layer formation. This regulation of the cnidarian oralaboral axis by Wnt signalling continues throughout the life cycle. In the anthozoan Nematostella vectensis, pharmacological activation of the Wnt/β-catenin pathway during development induces ectopic endoderm formation [START_REF] Wikramanayake | An ancient role for nuclear beta-catenin in the evolution of axial polarity and germ layer segregation[END_REF][START_REF] Röttinger | A Framework for the Establishment of a Cnidarian Gene Regulatory Network for "Endomesoderm" Specification: The Inputs of ß-Catenin/TCF Signaling[END_REF]) while β-catenin knockdown, by morpholino injection into 1 cell stage embryos prevents endoderm formation and gastrulation (Leclère et. al under revision). Similarly in Clytia, downregulation of Wnt3 (to abolish β-catenin stabilisation) severely delays gastrulation, although endoderm formation recovers by the larva stage.

In Hydra and in Hydractinia polyps, hyperactivation of Wnt signalling, via pharmacological stabilisation of GSK3b, induces ectopic "oral identity" in the polyp body column [START_REF] Hobmayer | WNT signalling molecules act in axis formation in the diploblastic metazoan Hydra[END_REF]Broun et al. 2005;[START_REF] Muller | Wnt signaling in hydroid development: ectopic heads and giant buds induced by GSK-3beta inhibitors[END_REF]. Furthermore the transplantation of pieces of the "oralised" body column into a wild type polyps induce the formation of a second axis (Broun et al. 2005).

In Clytia developing embryos Wnt3 ligand is expressed on the future oral side, derived from the egg animal pole along with the Frizzled receptor, Fz1, while a second Frizzled, Fz3 is expressed at the opposite pole, (Figure 5B). Wnt β-catenin signalling through Fz1 specifies oral identity, while Fz3 acts as a Wnt/β-catenin pathway inhibitor and determines the aboral identity (Figure 5B). Wnt3 is expressed earlier than any other Wnt ligand during Clytia embryogenesis, although 4 other ligands are later expressed in nested domains in the oral ectoderm (Momose et al. 2008). Moreover Wnt3 disruption via morpholino injections abolishes the expression of the other Wnt ligands consistent with its predominant role in axis formation. In Clytia that Wnt/β-catenin (via Wnt3) thus controls aboral-axis determination but is not essential for endodermal formation (Momose et al. 2008).

1.6.4 Wnt/β-catenin signalling and stem cell regulation Wnt/β-catenin signalling not only specifies the anterior-posterior axis in many metazoan species (Petersen & Reddien 2009) but also has other developmental roles including the regulation of stem cell dynamics. Within different stem cell systems (e.g. hematopoietic, intestinal stem cell system), Wnt signalling has been found to act to maintain tissue cell homeostasis by the regulation of cell proliferation and differentiation, promoting lineage specification and maintaining a pluripotency state [START_REF] Sokol | Maintaining embryonic stem cell pluripotency with Wnt signaling[END_REF][START_REF] Nusse | Wnt Signaling and Stem Cell Control[END_REF].

One of the well-studied systems in which Wnt/β-catenin signalling controls cell homeostasis by regulating cell proliferation and differentiation is the mouse crypt-villus axis in the intestinal stem cell system. At the base of the villi are small invaginations termed crypts, which represent the functional unit of the intestinal epithelium. Stem cells of the crypt-villus system are located at the bottom of each crypt. These cells divide and produce transit-amplifying progenitors that will finally differentiate in the villus. In contrast Paneth cells, which are differentiated cells that control the intestinal microbial environment, are located below the stem cells. Wnt signalling can promote both the proliferation of the stem cell and the differentiation of the Paneth cells. In vivo studies using mouse transgenic lines showed that abolition of Wnt/β-catenin signalling has dramatic effects on the crypt compartments. Mice mutants lacking the transcription factor TCF4 show complete depletion of the crypt stem cells [START_REF] Pinto | Canonical Wnt signals are essential for homeostasis of the intestinal epithelium service Canonical Wnt signals are essential for homeostasis of the intestinal epithelium[END_REF], while the hyperactivation of Wnt/βcatenin promotes de novo formation of the Paneth cells [START_REF] Andreu | A genetic study of the role of the Wnt/beta-catenin signalling in Paneth cell differentiation[END_REF].

Wnt/β-catenin also controls neuronal lineage specification from neural crest cells. During the development of the vertebrate nervous system, neural crest cells generate peripheral nervous system neurons and several non-neural derivatives such as melanocytes [START_REF] Dorsky | Environmental signals and cell fate specification in premigratory neural crest[END_REF]. The activation of the β-catenin pathway in neural crest cells promotes neural fate specification, such that ablation of the β-catenin gene in mouse neural crest cells causes the loss of sensory cells and melanocytes [START_REF] Hari | Lineage-specific requirements of β-catenin in neural crest development[END_REF]. Conversely when β-catenin is constitutively activated the number of neural crest derivatives increases.

Going back to a stem cell system, Wnt/β-catenin signalling has been proposed to maintain self-renewal and pluripotency in mouse and human ES cells [START_REF] Sato | Maintenance of pluripotency in human and mouse embryonic stem cells through activation of Wnt signaling by a pharmacological GSK-3-specific inhibitor[END_REF]). The pharmacological hyperactivation of the Wnt/β-catenin pathway in ES cells promotes a state of pluripotency as suggested by morphological and molecular markers (i.e. increased levels of the pluripotent stem cell marker Oct3/4, as described in Introduction section 1.4).

The role of Wnt/β-catenin in stem cell regulation is not restricted to vertebrates. Studies on i-cells, in hydrozoans indicate that the Wnt/β-catenin pathway regulates the i-cells differentiation [START_REF] Teo | An evolutionary conserved role of Wnt signaling in stem cell fate decision[END_REF][START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF]. In Hydra and in Hydractinia adult polyps, increasing Wnt/β-catenin signalling, via GSK3 inhibition induces ectopic differentiation of the nematocytes and nerve cells (only in Hydractinia). In Hydra the activation of Wnt/β-catenin, promotes the ectopic differentiation of the nematoblasts (see Introdution section 2.3.3). In Hydra activation of Wnt/β-catenin promotes the supernumerary differentiation of nematoblasts (see Introduction section 2.3.3). In Hydractinia nematogenesis is also stimulated, and it appears that pharmacological stabilisation of Wnt/β-catenin first causes a transient increase in proliferating cells as shown by BrdU labelling assays, followed by an increase in the number of differentiated nerve cells and nematocytes.

How are the functions of Wnt signalling in regulating body axis formation and maintenance, germ layer segregation, stem cell multipotency and fate commitment related and coordinated? This remains an important question to address.

2) Cnidarian models in developmental biology

Most studies in developmental biology involve a relatively small number of "model" organisms such as mouse, C. elegans, Drosophila and Xenopus. These are not sufficient to represent the wide diversity of life. It is becoming increasingly apparent that organisms outside these classical laboratory systems can make important contributions to understanding biological processes both evolutionary conserved and unique to particular animal groups.

In the last decade or so, the analysis of the increasing amount of genome and transcriptome data collected from non-conventional model species has revealed that most of the components of key signalling pathways involved in the regulation of fundamental developmental and cellular mechanisms are highly conserved across metazoans. For example genome analysis of the sponge Amphimedon queenslandica revealed the conservation of several gene families involved in fundamental mechanisms such as cell adhesion, body plan specification, and cell type differentiation [START_REF] Srivastava | The Amphimedon queenslandica genome and the evolution of animal complexity[END_REF].

More specifically in cnidarians, a large and diverse phylum positioned as sister group to the main animal clade of Bilateria (Figure 6), genome sequences analysis has revealed that their genomes share from many gene families with bilaterians [START_REF] Putnam | Sea anemone genome reveals ancestral eumetazoan gene repertoire and genomic organization[END_REF][START_REF] Chapman | The dynamic genome of Hydra[END_REF][START_REF] Hwang | The evolutionary emergence of cell typespecific genes inferred from the gene expression analysis of Hydra[END_REF][START_REF] Soza-Ried | The transcriptome of the colonial marine hydroid Hydractinia echinata[END_REF], even though cnidarians differ from bilaterian animals by the lack of key features such as mesoderm and a central nervous system. Cnidarians possess a "complete" Wnt gene repertoire [START_REF] Lee | A WNT of things to come: evolution of Wnt signaling and polarity in cnidarians[END_REF][START_REF] Kusserow | Unexpected complexity of the Wnt gene family in a sea anemone[END_REF] whose function in the establishment of the embryonic axis seems to be conserved as discussed above (see Introduction section 1.6.3).

Interest in using cnidarians as experimental models in developmental biology has grown, partly because of their morphological simplicity, high regenerative capabilities, and phylogenetic position but also because of their ontogenic plasticity. For example some cnidarians can rest in low metabolic states to survive a critical period, while in particular circumstances such as after an injury they can reactivate genetic cell proliferation and morphogenesis [START_REF] Boero | On the origins and evolution of hydromedusan life cycles (Cnidaria , Hydrozoa). Sex Origian and Evolution[END_REF][START_REF] Piraino | Reverse development in Cnidaria[END_REF]). The addition of genomic data in recent years provides an opportunity to study these processes at molecular level in cnidarians.

Cnidarians are subdivided in to two classes Anthozoa and Medusozoa (Figure 7A). The Medusozoa traditionally are split in four classes: Hydrozoa, Cubozoa, Scyphozoa, and Staurozoa. The Anthozoa (including sea anemones and corals), usually have larval and benthic polyp forms, the Medusozoa ancestrally have larvae, benthic polyps and freeswimming jellyfishes in their life cycles.

Cnidarians generally exhibit only one single body axis, the oral-aboral axis, with two germ layers (diploblastic): endoderm and ectoderm and are radially symmetric. However among Metazoan phylogenetic tree showing in light grey the Bilateria clade and in light blue the "basal branching" metazoans. Some of the topology is controversial but most recent studies agree that Cnidaria is the sister group to all bilaterinas. Cnidaria possess most of the key family genes involved in developmental mechanism, including a family of about 12 Wnt family genes.Modified from [START_REF] Ryan | Hox, Wnt, and the evolution of the primary body axis: insights from the early-divergent phyla[END_REF].

cnidarians some anthozoans possess a second body axis orthogonal to the oral aboral axis (directive axis) imparting bilateral symmetry (Figure 7B). In Nematostella the directive axis is defined by a plane that cuts symmetrically the mouth, the pharynx, the siphonoglyph and the retractor muscles.

Several cnidarian species have now been established as experimental models allowing gene function studies in developmental biology, notably Nematostella [START_REF] Darling | Rising starlet: The starlet sea anemone, Nematostella vectensis[END_REF], Hydra [START_REF] Galliot | Hydra, a fruitful model system for 270 years[END_REF] and Hydractinia (Plickerk et al. 2012) (Figure 7C) as well as less experimentally amenable species such as Podocoryne [START_REF] Galliot | Cnidarians as a model system for understanding evolution and regeneration[END_REF] and Acropora [START_REF] Ball | Coral development: From classical embryology to molecular control[END_REF]. They have provided important insights into evolutionary questions such as the correspondence between the oral-aboral axis of Cnidaria and the anterior-posterior axis of Bilateria [START_REF] Sinigaglia | The Bilaterian Head Patterning Gene six3/6 Controls Aboral Domain Development in a Cnidarian[END_REF][START_REF] Hobmayer | WNT signalling molecules act in axis formation in the diploblastic metazoan Hydra[END_REF], and the homology between the vertebrates striated muscle and the muscle of the medusa [START_REF] Steinmetz | Independent evolution of striated muscles in cnidarians and bilaterians[END_REF]. They have also been employed to address particular biology processes such as cell proliferation, differentiation (David 2012), and trans-differentiation [START_REF] Schmid | Transdifferentiation and regeneration in vitro[END_REF].

The hydrozoan Clytia hemisphaerica is a useful addition to these existing cnidarian models [START_REF] Houliston | Clytia hemisphaerica: a jellyfish cousin joins the laboratory[END_REF], particularly to address developmental questions given the easy access to all embryonic stages, gonads and oocytes ,and regeneration questions, since the Clytia medusa displays whole body regeneration. Moreover Clytia embryos, larvae, polyps and jellyfishes are very robust, easy to handle and transparent. The time required to complete the full life cycle is relatively short (around three months) and Clytia is easily kept in laboratory conditions, where spawning can be experimentally controlled by light after a period of darkness [START_REF] Houliston | Clytia hemisphaerica: a jellyfish cousin joins the laboratory[END_REF].

Clytia life cycle and development

Clytia is also known by other names including Phialidium and Campanularia. This confusion is due in part to the existence of multiple life stages, such that Clytia polyps and jellyfishes were not initially recognised as alternative forms of the same species. (2011[START_REF] Houliston | Clytia hemisphaerica: a jellyfish cousin joins the laboratory[END_REF], [START_REF] Bradshaw | Distinct mechanisms underlie oral vs aboral regeneration in the cnidarian Hydractinia echinata[END_REF], [START_REF] Leclère | RGM Regulates BMP-Mediated Secondary Axis Formation in the Sea Anemone Nematostella vectensis[END_REF].

Historically the polyp was named Clytia (Linnaeus 1767) and the medusa was named Phialidium (Leuckart 1856). Clytia has a tri-phasic life cycle: the free-swimming jellyfish, the larva and the polyp colony (Figure 8). Male and female adult Clytia have separate sexes (Figure 8b-c). Medusae spawn daily following a light-dark cycle, with a short light stimulus being sufficient to trigger oocyte maturation after 2 hours of darkness [START_REF] Amiel | Clytia hemisphaerica: A Cnidarian Model for Studying Oogenesis[END_REF].

Following fertilisation, the egg (Figure 8d) develops into a planula larva (Figure 8e), which after 3 days is able to settle down onto a substrate and metamorphose to give rise to a colony of polyps.

In natural conditions, metamorphosis is induced by a bacterial film, so in sterile seawater planulae will not metamorphose. In laboratory conditions metamorphosis can be induced efficiently by GLWamide neuropeptides. GLWamides were first identified as metamorphosis inducing factors in the hydrozoan Hydractinia with the name metamorphosine A (Leitz et al.1994). Clytia undergoes very dramatic metamorphosis. The aboral pole of the planula attaches to the substrate and there is subsequently complete loss of larval structures before the polyp forms. The first structure to form is the primary polyp, which is the founder of the polyp colony (Figure 8f). It has been shown that cells from the oral pole of the larva contribute to the hypostome (mouth) of the primary feeding polyp (Freeman 1981b). Once it starts feeding, the primary polyp propagates vegetatively by extending stolons out across the seabed or other substrate. The mature colony is polymorphic with a connected system of feeding gastrozoid polyps (Figure 8h) and gonozoid polyps (Figure 8i) specialised for the budding of new baby medusae (Figure 8j).

Gastrozoids produce new medusae clonally, and so laboratory medusae produced from a single colony are genetically identical. This is particularly advantageous for genetic studies, since polymorphism is highly reduced. In our laboratory most experiments have now been performed for over 10 years using 2 or 3 individual genotypes obtained from the same 'immortal' colonies. In collaboration with R. Copley (LBDV), M. Manuel's laboratory (UPMC Jussieu) and the Genoscope (Evry), the Clytia genome has been sequenced, assembled and is currently undergoing annotation (publication in preparation). An extensive Clytia transcriptome data set published last year (Lapébie et al. 2014) (see Result -II) is available on the Compagen web site, which is comparative genomic platform for early branching metazoans (www.compagen.org). A library containing about 8000 unique fully sequences cDNA sequences corresponding to about 90,000 ESTs is also available. Current work in the laboratory is generating, using Illumina Seq technologies, further transcriptomic data to cover particular life stages or tissues (not published). These bioinformatics resources will be available in the near future to analyse gene expression profiles throughout the life cycle. Clytia adult medusa (a) spawn daily after a short light stimulus. Female and male medusa release unfertilised eggs and sperm in the water (female and male gonads b-c). Following the external fecondation, the fertilised eggs cleave after 1 hour. Embryonic development generates the larval stage, about 48 hours post fertilisation at 18°C (d). The3-4 day old planula larva metamorphoses into a primary polyp (f) that is the founder of the polyp colony (g). The colony first develops the gastrozoids (h) that are the feeding zooids. Later, gonozoids, form that bud and release baby immature jellyfishes (j). Released baby jellyfishes are 1-3 mm in diameter and grow by feeding to become sexually mature after about 2-3 weeks in laboratory conditions, reaching a maximum size of 1-2 cm (a). Scale bar 1mm for adult jellyfishes and polyps, 100μm for the other stages. Modified from [START_REF] Houliston | Clytia hemisphaerica: a jellyfish cousin joins the laboratory[END_REF].

Clytia embryogenesis and larval development

Clytia eggs are transparent and measure between 150 and 180 µm in diameter (Figure 8d).

First cleavage is unipolar and occurs 50-60 minutes after fertilisation (Figure 9A). In hydrozoans the initiation site of the first cleavage coincides with the region of the egg that extruded the polar bodies (the animal pole), and later the site of cell ingression during gastrulation and the future oral pole of the larva (Freeman 1981a) (Figure 9A).

For experimental reproducibility we control the timing of developmental events in Clytia in the laboratory by maintaining a constant temperature (18-20 0 C). During the first six hours of development at 18°C, a series of divisions, produces a single cell-layered blastula, covered by cilia for swimming, prior to the onset of gastrulation (Figure 9A). At 18°C Gastrulation starts, 10 hours post fertilization (hpf) by unipolar cell ingression at oral pole [START_REF] Byrum | An analysis of hydrozoan gastrulation by unipolar ingression[END_REF]) (Figure 9A). Endodermal cells and Piwi/Nanos1 positive-i-cells ingress to enter the cavity of the gastrula. Therefore a mixed population of cells that can be found inside the gastrula. Some of these cells will contribute to the endoderm (i.e. FoxB expressing cells), while others are founders of the i-cell lineage. After 24hpf, gastrulation is complete and planula larva elongates along the oral-aboral axis (Figure 9A), but endoderm is still not differentiated. Except at the mid-gastrula stage, embryos and planulae show a strong ability to re-pattern along the oral-aboral axis; when cut into presumptive oral and aboral halves they are able to reform well-patterned mini-larvae (Freeman 1981b). At 48hpf, ectoderm and endoderm are fully formed and are separated by a thin acellular mesoglea (Figure 9A-B). The ectodermal cells are ciliated and ectodermal cilia are oriented along the oral-aboral axis to drive the directional swimming towards the aboral pole, [START_REF] Momose | A conserved function for Strabismus in establishing planar cell polarity in the ciliated ectoderm during cnidarian larval development[END_REF]. At 48hpf planula stage the ectodermal layer shows the presence of several differentiated cell types: nematocytes, nerve cells and gland cells. In contrast, the endoderm hosts a population of endodermal gland cells and the multipotent i-cells stem cells (Figure 9B).

Clytia medusa anatomy

The Clytia medusa is a small transparent jellyfish of about 1cm in diameter, with a laboratory life span of 2 to 3 months. The main anatomical feature of the jellyfish is the After 6 hours, the embryo reaches the blastula stage. In some blastulae at this stage it possible to visualise the animal pole thanks to a deep cleft between the two lobes. At the early gastrula stage it is possible to recognise the first sign of polarity. Embryos acquire an ellipsoid shape and the cells are more elongated. At 24hpf gastrulation is complete, however presumptive endoderm is not yet differentiated (green cells). At 48hpf ectoderm (red cells) and endoderm (green cells) are well formed and the larva shows the typical torpedo shape. The line represent the developmental time in hours at 18°C. Modified from Freeman (1981b). B) Longitudinal section of a "late differentiated planula" (>48hpf). At this stage all the differentiated somatic cells are present as indicated by labels. Endoderm in green, ectoderm in red, mesoglea in black. Modified from Bodo & Bouillon (1968).

bell-shaped umbrella (Figure 10A). The umbrella is convex above (exumbrella) and concave below (subumbrella) with the medusa manubrium and the mouth centrally positioned. The mouth has a single, cross-shaped opening connected to a large internal cavity that expands to form a simple stomach (Figure 10A). From the stomach four radial canals extend to the margin of the bell where tentacles are attached. The regions where the tentacles grow are defined as tentacle bulbs (Figure 10B). The tentacle bulbs are highly proliferative regions. Clytia gonads, the sexual organs of the medusa, are attached to the subumbrellar side of the umbrella on each of the four radial canals. Sensory organs are present in the form of simple statocysts (balance organs) located between tentacle insertion points. Two condensed nerve rings run around the periphery of the umbrella: one (outer ring) organised for integrating sensory inputs and the other (inner ring) for coordinating motor responses [START_REF] Satterlie | Neuronal control of swimming in jellyfish: a comparative story[END_REF]) (Figure 10A). A diffuse nerve net runs through the subumbrella layer, which includes fast-contracting circular muscles for swimming, and the manubrium.

Hydrozoan cell types

Hydrozoans possess a relatively limited number of cell types compared to bilaterian animals. Medusae and larvae share to a large extent the same cellular components that will be described in the next section. The hydrozoan planula possesses two germ layers, the endoderm comprising endodermal gland cells, and endodermal epithelial cells, and the ectoderm include ectodermal epithelial cells, nerve cells, nematocytes and gland cells. I-cells are distinct from any germ layers and migrate between the two depending on stage and structure, but in larval stages are located among the endodermal cells.

I-cells: a hydrozoan multipotent stem cell system

I-cells are migratory cells without any particular morphological distinctive feature such as neural processes or polarised granules. They are capable of generating daughter cells with the same potential as the mother cells, have unlimited proliferation potential and are able to produce differentiated functional progenies. Therefore i-cells can clearly be considered to be a hydrozoan stem cell population. They have the particularity of being able to give rise to both somatic cell types and gametes (Figure 11A) [START_REF] Bosch | Stem cells of Hydra magnipapillata can differentiate into somatic cells and germ line cells[END_REF][START_REF] Bode | The interstitial cell lineage of hydra: a stem cell system that arose early in evolution[END_REF]. A) Hydra i-cell. Multipotent i-cell (green) give rise to all somatic progenitor (pink) and germ cell precursor (pink). All the progenitors differentiate producing mature cell types (sky blue). Prec: precursor, NeuroB: neuroblast, NematoB: nematoblast, PGC: germ cell precursor. In Hydra multipotent cell (green) are localised mostly in the gastric region in the interstitial spaces between the epithelial cells, differentiating cell are disperse throughout the Hydra body in the endodermal region (pink) while differentiated cells (sky blue) are mostly localised in the ectoderm layer at body extremities (tentacles, foot, hypostome). ec: ectoderm, en: endodermModified from Hobmayer et al. (2012). B) Clytia i-cell. In Clytia i-cell are distinguishable, from a morphological point of view at early gastrula stage (green line). In adult stage the known site of i-cell localisation are the proximal part of the tentacle bulbs, the base of the manubrium and the gonads. Fated cell (pink line) have been characterised in the planula endoderm and in the medusa. Comparable to Hydra differentiated cells are located in the ectodermal layer, however mature gland cells are localised in both layer ectoderm and endoderm. ec: ectoderm; en: endoderm; TB: tentacle bulbs. I-cells have been found exclusively in hydrozoans. Currently there is no evidence for the presence of this cell type, either in other medusozoan classes or in anthozoans . However the presence of multipotent stem cells in other cnidarians cannot be excluded. Several studies suggest that the amebocyte cells might be putative stem cells in anthozoans [START_REF] Gold | Stem cell dynamics in Cnidaria: are there unifying principles?[END_REF].

August Weismann in 1883 was the first to detect the i-cells during his morphological studies on the Hydractinia adult polyps [START_REF] Weismann | RNA regulatory elements mediate control of Drosophila body pattern by the posterior morphogen nanos[END_REF]. Weismann termed these cells Stammzellen, to illustrate their ability to generate committed progenitors (germ cells).

Later i-cells were described in Hydra adult polyps, as small cell (5-6 µm diameter) with a round or spindle shape, large nucleus, a dense cytoplasm filled with ribosomes, few mitochondria, a small Golgi apparatus, and little endoplasmic reticulum [START_REF] Lentz | THE FINE STRUCTURE OF DIFFERENTIATING INTERSTITIAL CELLS IN HYDRA[END_REF].

These morphological characteristics are common to all hydrozoans i-cells described so far.

A large part of the current extensive knowledge of i-cell biology and more generally of hydrozoan cell types derives from studies performed in Hydra and Hydractinia. Therefore I will first give a short introduction on i-cell distribution and molecular signatures in Hydra and in Hydractinia, followed by a more detailed description of what is known in Clytia.

I-cells localisation and molecular signature • Hydra and Hydractinia

In Hydra adult polyps, i-cells are found in the gastric region between endodermal and ectodermal interstices (Figure11A). Within the i-cell population it is possible to distinguish two sub-populations: large and small i-cells. The large i-cells, found alone or in pairs, are referred to as true multipotent stem cells, while the small i-cells represent fate-committed icell populations, as shown for germ cells precursors [START_REF] David | Distribution of interstitial stem cells in Hydra[END_REF][START_REF] Nishimiya-Fujisawa | Germline stem cells and sex determination in Hydra[END_REF]. Multipotent i-cells reside constantly in the body column, which can be considered to represent a stem cell niche. Even though several studies have addressed the structural details of the Hydra stem cell niche [START_REF] Bosch | The Hydra polyp: nothing but an active stem cell community[END_REF], Bosch 2009b), at present little functional data is available to support the hypothesis that an "environmental" signal controls extrinsic or intrinsic cell program (see Introduction 1.3.1 and 1.3.2 sections of the Introduction). In contrast to the i-cell population, i-cell derivatives mainly reside at the body extremities (head and foot) (Figure 11A).

In Hydractinia, i-cells appear at early gastrula stage during development [START_REF] Plickert | Cell proliferation and early differentiation during embryonic development and metamorphosis of Hydractinia echinata[END_REF][START_REF] Frank | Stem Cells in Marine Organisms[END_REF]). In the planula, they are located in the endoderm and some of them translocate through the mesoglea and migrate to the ectoderm. In the polyps, i-cells are mainly located in the stolonal compartment where they generate all the derivatives [START_REF] Frank | Stem Cells in Marine Organisms[END_REF].

From a molecular point of view, i-cells, in both Hydra and Hydractinia express the germ cell/stem cell markers Nanos and Piwi (see section 1.5). In Hydra two Nanos gene paralog and the two Piwi paralog Hywi and Hyli are expressed both in multipotent i-cells and in committed i-cells, [START_REF] Mochizuki | Expression and evolutionary conservation of nanos-related genes in Hydra[END_REF][START_REF] Nishimiya-Fujisawa | Germline stem cells and sex determination in Hydra[END_REF]. In Hydractinia Nanos1 is female specific and is not detectable during development while Nanos2 is maternally expressed in a germ plasm like structure, in the future oral pole [START_REF] Kanska | New roles for Nanos in neural cell fate determination revealed by studies in a cnidarian[END_REF]Plickert et al 2012).

In the less studied system Podocoryne carnea, Piwi transcripts have been detected throughout the life cycle in somatic and in germ line cells (Seipel et al. 2004).

Recently the expression pattern of Nanos and Piwi genes has been reported in Siphonophores, which are colonial hydrozoans. Nanos and Piwi positive cells were identified in the growth zone of the colony and in the female and male gonads [START_REF] Siebert | Stem cells in a colonial animal with localized growth zones[END_REF], showing again expression in germ cells and in somatic cells as for the other hydrozoans.

• Clytia

In the course of Clytia development, i-cells can be identified morphologically during gastrulation in ingressing cells in the oral region and then dispersed in the central core of the planula endoderm (Bodo et Bouillon 1968) (Figure 8B,11B). Presumptive i-cell derived somatic cell types (ganglion cells, nematocytes, gland cells) are distributed in the late planula ectoderm and endoderm layers.

As in Hydra and Hydractinia, Clytia i-cells express Nanos1, Piwi, Vasa, Nanos2 and PL10 genes [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]. In this study I will focus on Nanos1 and Piwi. In Clytia these genes are expressed throughout the life cycle (Figure 12A-B). Both genes are expressed maternally and the maternal mRNAs are localised within the egg in a "germ plasm-like" region surrounding the egg pronucleus. Transcripts of both genes are detected at cells located at the animal pole at the blastula stage and in ingressing cells at gastrula stage (Figure 12A). Presumably during this period of development zygotic transcription of these genes starts and the spatial continuity suggests that new transcription could well begin in the same cells that inherit the maternal mRNA. In planula larvae Piwi and Nanos1 are expressed in a population of cells with i-cell morphology (Figure 12A). In the adult medusa, Piwi and Nanos1 expression is detected in male and female gonads, around the mouth and in the proximal part of the tentacle bulbs (Figure 12B). Piwi and Nanos1 are not exclusively expressed in multipotent i-cells but also in some somatic fated cells and in germ cells (Figure 12B). Piwi and Nanos1 expression in hydrozoans including Clytia is not restricted to multipotent, non-committed i-cells since it has also been reported in some early somatic derivative cells and in germ cells. These two genes may thus not be perfect markers to study i-cells ontogeny, but so far in Clytia I have been unable to uncover a marker specific to uncommitted i-cells (i.e. not expressed in germ cells or in somatic derivatives).

It is evident that there is a lack of information concerning i-cell ontogeny, since the majority of i-cell studies were carried out in the adult Hydra polyp. Even though a pioneer study in the hydrozoan Pennaria tiarella investigated the i-cell origin during embryonic development [START_REF] Summers | The Ontogeny o f Interstitial Cells in Pennaria tiarella[END_REF]. In this studies i-cell presence were scored for the first time at planula stage in central endoderm, by morphological criteria. It is possible however that based on this this criteria delayed the timing of i-cell formation was incorrectly estimated. Interestingly in this study i-cells are thought to derive from endodermal cells rather than to arise as a germ layer-independent population. My study aims at compensating the lack of information with respect to i-cell origins during larval development. I investigated the appearance of i-cells during embryogenesis, and monitored i-cell derivatives appearance during Clytia larval development with the use of molecular markers. At the early gastrula stage they are expressed at the oral pole, in cells that will ingress with the presumptive endoderm cells. In the planula the genes are expressed in a population of cells with morphological characteristics of i-cell interspersed in the endodermal region. Scale bar 50μm. On the left schematic representations of i-cell localisation (orange dots). Light grey: endoderm, dark grey: ectoderm. B) Piwi, Nanos1 and Vasa expression in the medusa. Piwi, Nanos1, Vasa mRNAs are detected in three main regions: the proximal area of the tentacle bulbs, at the base of the manubrium, and in the gonads. On the left schematic representation of i-cell distribution in the medusa (orange dots). In the pink square, schematic representation of the area boxed in A3, illustrating the medusa manubrium. Scale bar 100μm for the whole medusa, 25μm for the other images. From [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF].

Nerve cells

The hydrozoan nervous system comprises morphologically distinct cell types, categorised as sensory cells and ganglion cells. Sensory cells convert specific chemical and physical stimuli such as light and touch into nerve impulses, while ganglion cells are often organised in intermediate stations termed plexuses that transmit impulses to other nerve elements. Some species, like the hydromedusa Aglantha digitale, possess specialised nerve cells with giant axons [START_REF] Roberts | The giant axon escape system of a hydrozoan medusa, Aglantha digitale[END_REF]. In Aglantha these nerve cells mediate the so-called escape swimming behaviour. Giant axon cells receive a sensory excitation from the bell margin, this signal is then conducted around the margin, provoking tentacles contraction [START_REF] Roberts | The giant axon escape system of a hydrozoan medusa, Aglantha digitale[END_REF].

Even though the cnidarians (though hydrozoans) nervous system is considered to be one of the simplest metazoans nervous systems, it shows a wide diversity in its composition. In Hydra and in Hydractinia polyp nerve cells are localised in the ectodermal layer. In Hydra all the nerve cells are derived from the i-cells [START_REF] Bosch | Stem cells of Hydra magnipapillata can differentiate into somatic cells and germ line cells[END_REF]. The nerve cell precursors, called neuroblasts are localised in the gastric region and acquire their final fate only after reaching the ectodermal layer.

Morphological studies in Clytia planula described the presence of sensory and ganglionic cells in the planula ectoderm (Bodo & Bouillon 1968). Sensory cells are oriented perpendicular to the mesoglea placed at the base of the two epithelial layers, with the cell bodies located within the ectoderm [START_REF] Martin | Development of Nerve Cells in Hydrozoan Planulae: I. Differentiation of Ganglionic Cells[END_REF]. Ganglion cells, (unipolar, bipolar or multipolar), are arranged in a nerve plexus oriented parallel to the mesoglea between the two epithelial sheets.

Unlike in Hydra polyps, nerve cells in Clytia larvae are not all i-cell derived. I-cell-depleted embryos are able to develop a subpopulation of neurosensory cells, but not ganglion cells, suggesting an ectodermal origin for some sensory cell types cells (Thomas & Martin 1987).

To describe the neuroanatomical features in planulae, jellyfishes and polyps of cnidarians, immunoreactivity toward some conserved neuropeptides is used as a common tool [START_REF] Conzelmann | Antibodies against conserved amidated neuropeptide epitopes enrich the comparative neurobiology toolbox[END_REF]. Neuropeptides are dominant transmitters molecules in cnidarians [START_REF] Grimmelikhuijzen | Mini-review: the evolution of neuropeptide signaling[END_REF]. The two types of antibodies used by the cnidarian community are those raised against RFamide and GLWamide neuropeptides.

Ectodermal sensory cells expressing GLWamides have been described at the aboral pole in

Hydractinia echinata, Clava multicornis and Pennaria tiarella planula [START_REF] Piraino | Complex neural architecture in the diploblastic larva of Clava multicornis (Hydrozoa, Cnidaria)[END_REF][START_REF] Gajewski | LWamides from Cnidaria constitute a novel family of neuropeptides with morphogenetic activity[END_REF][START_REF] Plickert | The role of alpha-amidated neuropeptides in hydroid development--LWamides and metamorphosis in Hydractinia echinata[END_REF]. RFamide positive ganglion cells are positioned at the base of the ectoderm and form a nerve net all along the mesoglea [START_REF] Piraino | Complex neural architecture in the diploblastic larva of Clava multicornis (Hydrozoa, Cnidaria)[END_REF][START_REF] Plickert | The role of alpha-amidated neuropeptides in hydroid development--LWamides and metamorphosis in Hydractinia echinata[END_REF][START_REF] Martin | Characterization of a RFamide-positive subset of ganglionic cells in the hydrozoan planular nerve net[END_REF]. In Hydra polyps, RFamide neuropeptides are detected in the epithelial nerve net around the mouth (Figure 13G) while in the hydrozoans medusa Eirene RFamide neuropeptides are detected in the nerve ring and in the tentacle bulbs ectoderm (Figure 13F). However depending on the species, RFamides and GLWamides are not restricted to only one cell type as showed in Clava multicornis [START_REF] Piraino | Complex neural architecture in the diploblastic larva of Clava multicornis (Hydrozoa, Cnidaria)[END_REF].

Furthermore there may be several distinct precursors producing neuropeptides with slightly different sequences expressed in different cell populations and at different life cycle stages: for example there are 3 RFamide precursor genes in Hydra and two different GLWamide precursors in Clytia, only one of which is expressed in the larva.

It is clear that RFamide and GLWamide antibodies label different neural cell subpopulations [START_REF] Piraino | Complex neural architecture in the diploblastic larva of Clava multicornis (Hydrozoa, Cnidaria)[END_REF][START_REF] Koizumi | The nerve ring in cnidarians: its presence and structure in hydrozoan medusae[END_REF], and correspondingly that the two neuropeptides have different roles. Several reports indicate that GLWamides participate in the induction of metamorphosis [START_REF] Plickert | The role of alpha-amidated neuropeptides in hydroid development--LWamides and metamorphosis in Hydractinia echinata[END_REF]. In Clytia as in Hydractinia addition of some synthetic GLWamide peptides to the seawater can induce metamorphosis. In the light of wide utilisation of these two neuropeptides in cnidarians, I decided to use these two markers to monitor the appearance of differentiated neural cell types during Clytia larval development.

Nematocytes

In cnidarians, nematocytes are also known as stinging cells or cnidocytes. The terms "cnidocyte" and indeed "cnidarian" derive from the greek "cnida", meaning nettle.

Nematocytes can be considered to be a neural cell type. They are specialised mechanosensitive neurosecretory cells, whose function is essentially in food capture and defence [START_REF] Watanabe | Immortality and the base of multicellular life: Lessons from cnidarian stem cells[END_REF], Galliot & Quiquand 2011).

Each nematocyte contains a special organelle called the nematocyst (the capsule) and a sensory apparatus termed the cnidocil. The nematocyst is a Golgi-derived capsule that contains a highly folded tubule that is expelled from the capsule where it is held under pressure upon stimulation to deliver a toxin into the prey or predator.

The nematocyst discharge mechanism is one of the fastest in nature and take less then 3 ms [START_REF]An Ultrahigh-Speed Analysis of Exocytosis: Nematocyst Discarge[END_REF]. The energy for the high intracapsular osmotic pressure is generated by a high concentration of the Poly-γ-glutamate (PGA) [START_REF] Weber | Poly(-glutamic Acid)s Are the Major Constituents Hydra of Nematocysts Hydroxoa , Cnidaria )*[END_REF]. PGA is a polyanionic molecule that can bind to the cationic molecules [START_REF] Szczepanek | Poly-γ -glutamate synthesis during formation of nematocyst capsules in Hydra[END_REF]. Nematocyte capsules are birefringent, facilitating detection with polarized light and the intrinsic "chemical "nature of the nematocyte capsule allows a particular staining method to detect mature nematocytes (described in Material and Methods). During this study this technique was used to define when and where the first mature nematocyte appear in embryonic stages. In Hydra and Hydractinia polyps nematocytes are mostly located in the ectoderm at the extremities of the body, while their precursors termed nematoblasts are mostly localised in the gastric body column. As it is the case for the nerve cells, nematoblasts acquire their final fate only in the ectoderm.

In Clytia planula, nematocytes are localised in the ectoderm when they are completely differentiated, while their intermediates, the nematoblasts are localised in the endodermal region. Thus nematoblasts migrate from the endoderm to the ectoderm to reach their final fate. In the adult jellyfish nematocytes are present thought the body but highly concentrated in the tentacles and in the mouth ectoderm [START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF]).

Gland cells

From a morphological point of view gland cells are polarised cells, with granules concentrated in the cytoplasm toward the apical pole, and the nucleus located near the basal membrane. In cnidarians gland cells have secretory and digestive functions. In Hydra gland cells derive from the i-cells (Bode 1987, Siebert et al 2008) and are localised both in the endoderm and in the ectoderm. They are mostly scattered along the Hydra endoderm with a major density at the sub hypostomal region (Lentz, 1966[START_REF] Chera | Silencing of the hydra serine protease inhibitor Kazal1 gene mimics the human SPINK1 pancreatic phenotype[END_REF].

In Clytia planulae, three different kinds of gland cells have been described: i) glandulaire spumeuse (hypostomal mucus cells) that arise at planula stage and persist in the aboral larval ectoderm; ii) glandulaire spheruleus (zymogen cells) that appear at the planula stage, located in the ectoderm but less numerous than the hypostomal mucus cells, and iii) granuleuse cells, present in the endoderm and in the ectoderm (Bodo & Bouillon 1968) (Figure 8B). In the adult jellyfish, gland cells are distributed in the gastrodermis in the manubrium and in the radial canal (Harrison and Westfall 1991). In particular the mouth opening is characterised by the presence of spumeuse and spheruleus gland cells (Bouillon 1966).

Gametes

Epithelial cells

Among the few cell types that compose the hydrozoans body, epithelial cells represent the majority. Hydrozoan epithelial cells are the main component of the outer ectoderm and the inner endoderm. Both layers contain basal muscle processes that run parallel to the mesoglea. Muscle fibres are attached to the mesoglea and interconnect to form longitudinal and circular sheets that coordinate movement under neural control [START_REF] Brusca | Les cellules glandulaires des hydroïdes et hydroméduses. Leur structure et la nature de leurs sécrétions[END_REF]. In Hydra, epithelial cells are considered as an additional stem cell lineage to the icells, because in the absence of i-cells (explained above) they are sufficient to drive morphogenesis and budding (Marcum and Campbell 1978). Further evidence of the role of the epithelial cells in morphogenesis was gained by in vivo tracking of epithelial cells (Wittlieb et al. 2006a). This technique allowed to determine the contribution of the GFP labelled endodermal epithelial cells in the bud formation. In Hydractinia, i-cells can give . . rise to the epithelial cells, so the epithelial cells may not have to behave as stem cells (Plickert et al. 2012).

I-cell dynamics

The interstitial cell system provides a good model to investigate the specification and the differentiation of stem cells, since in planulae, polyps and medusae, the number of cells is limited and the final differentiated cells are common to all late stages. I-cells can give rise to nerve cells, nematocytes, gland cell and germ cells. Since the discovery of their multipotential capacity, i-cells have been the subject of numerous studies to unravel the basic mechanisms that regulate cell specification, proliferation and commitment.

Analysis of i-cell multipotency

In Hydra, analysis of i-cell potential was possible by using cytotoxic compounds that specifically eliminate i-cells, without killing the animal. Among these chemicals the most commonly used are Hydroxiurea (HU) and colchicine. HU blocks early S-phase cells by inactivating the nucleotide reductase enzyme, which is necessary for DNA synthesis, while colchicine is a microtubule inhibitor which causes M phase arrest. Following treatment by HU or colchicine, cell-cycle arrested and i-cells become susceptible to phagocytosis by epithelial cells [START_REF] Campbell | Elimination by Hydra interstitial and nerve cells by means of colchicine[END_REF], causing their loss. Treatments with colchicine and HU generate the so termed "epithelial polyps" or "i-cell free polyps". The cellular content of the epithelial polyps was analysed by Hydra maceration technique [START_REF] David | A Quantitative Method for Maceration of Hydra Tissue[END_REF]. With this technique, Hydra tissues can be easily dissociated into individual cells, which retain their in vivo morphology and hence are recognizable as distinct cell types. Epithelial polyps lack nematocytes, all the nerve cell types and the gametes [START_REF] Campbell | Elimination by Hydra interstitial and nerve cells by means of colchicine[END_REF][START_REF] Bode | The interstitial cell lineage of hydra: a stem cell system that arose early in evolution[END_REF].

Differently from polyps, in planula i-cells can be microsurgically deleted. Using this approach it was possible to investigate the origin of the i-cell derived nervous system in Clytia planula. During Clytia development, at middle gastrula stage, it is possible to separate an oral half containing oral ectoderm, presumptive endoderm and i-cells, and an aboral half composed only by the ectoderm. The planulae derived from the oral-half, form proper i-cells, and i-cell derivatives. In contrast the planula derived from the aboral-half lacked nematocytes and ganglion cells. Surprisingly the latter displayed the presence of neurosensory cells. These observations suggest that in Clytia planula differently from Hydra, some sensory neurons derive from the ectodermal layer rather than from the i-cells (Martin and Thomas 1987), although the possibility of residual i-cells in the aboral part cannot be ruled out.

During the last 50 years, chemical and microsurgical techniques allowed, to unravel the icells potentialities. More recently the development of transgenic lines, in Hydra and in Hydractinia allowed in vivo i-cell tracking. With the help of this technique it was possible to validate the i-cell potentiality [START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF]) and to investigate the migratory icell behaviour. When Hydra i-cells are transplanted in a wild type host polyp, they do not move, maintaining their position. Conversely, if the transplantation is done in epithelial polyps, i-cell began to move and to repopulate the i-cell free polyp (Boehm & Bosch 2012), suggesting that i-cell homeostasis is highly regulated.

Generating i-cell progeny: i-cell derivatives specification and differentiation

In Hydra polyps, and to some extent in Clytia planulae nematocytes and nerve cells originate from i-cells following distinct pathways of differentiation. The differentiation pathways of these two cell types are very different and are described below.

• Nematocyte differentiation

Nematocytes derive from precursor cells called nematoblasts, which are an i-cell derived cell population. The nematoblasts divide synchronously forming a cell cluster of 4 to 32 cells connected by intra-cytoplasmatic bridges [START_REF] David | Cell cycle kinetics and development of Hydra attenuata. III. Nerve and nematocyte differentiation[END_REF]. Once they stop proliferating the differentiation of the nematocytes capsule starts. During nematogenesis the Golgi complex enlarges and increases in size. In later stage of differentiation, the nucleus is displaced to one side of the cell and there is a strong increase in the amount and organisation of the ER. Vesicles fuse together to give rise to a tubule, which will generate the harpoon thread, and the cnidocil apparatus appears (Figure 14 A). In the last stage of differentiation, the tubule invaginates within the nematocyst, which reaches its maximum size, while the other organelles shrink and the ER system breaks down [START_REF] Slautterback | The development of the cnidoblasts of Hydra; an electron microscope study of cell differentiation[END_REF]. As a consequence of this final process of capsule maturation, the nematoblast cluster disperses and individual cells migrate into the ectodermal layer where the mature nematocytes become embedded into the epithelium. In Hydra nematoblasts traverse long distances; nematogenesis occurs mainly in the central region of the polyp but differentiated nematocytes are particularly abundant in the head region, especially in the tentacles.

• Neuronal differentiation

Neurogenesis follows a more direct differentiation pathway than nematogenesis. In Hydra polyps the neural precursors, the neuroblasts, are located along the body column.

Responding to local cues, differentiating neurons migrate toward the extremities of the body, (head and foot) to acquire their final fate. The analysis of the Hydra nervous system (including nematocytes and nerve cells) using molecular markers and functional assays has shown that, despite their distinct differentiation pathways, nematocytes and nerve cells derive from common precursors.

• Genetic program modulating nematocytes and neural cell specification and differentiation

Cnidarians possess transcription factors belonging to most of the groups known to be involved in neurogenesis from studies in bilaterian models. In bilaterians, the TF families that participate in neurogenesis include bHLH transcription factors, homeodomain proteins, Zinc finger proteins and Sox subgroup HMG box factors.

In cnidarians the bHLH repertoire is largely expanded [START_REF] Gyoja | A genomewide survey of bHLH transcription factors in the coral Acropora digitifera identifies three novel orthologous families, pearl, amber, and peridot[END_REF]. Bilaterian proneural bHLH factors belong to two main families: Atonal (ato) and Achete-scute (asc).

In bilaterians, factors belonging to these two families are expressed in ectodermal cells where they are able to initiate neural development under the influence of Notch signalling [START_REF] Bertrand | Proneural genes and the specification of neural cell types[END_REF].

In Hydra, the Achete-scute factor CnAsh is expressed in two distinct populations of cells, nerve cells developing in the apical/oral region and nematoblasts in polyp body column (Figure 16). In Podocoryne, two Achaete-scute genes have been characterised: Ash1 and Ash2 [START_REF] Müller | Evolutionary aspects of developmentally regulated helix-loop-helix transcription factors in striated muscle of jellyfish[END_REF]Seipel et al. 2004a). Ash1 expression is restricted to nematocytes, while Ash2 is expressed in endodermal cells both, in the planula and in the medusa. In the planula these cells are positioned in the aboral endoderm, and in the medusa in the manubrium endoderm.

The cellular identity of this cell population is not clear but they have been proposed to be neurosecretory cells on the basis of position and morphology. Phylogenetic analysis shows that Ash2 belongs to the Asc-B family, and that it is an ortholog of the mouse gene Ash3. In mouse, Ash3 is expressed in the gland cell lineage.

Among the class of homeodomain proteins present in cnidarians [START_REF] Ryan | The cnidarianbilaterian ancestor possessed at least 56 homeoboxes. Evidence from the starlet sea anemone, Nematostella vectensis[END_REF], two genes belonging to the Paired-like class, Prdl-a and Prdl-b have been characterised in Hydra [START_REF] Gauchat | prdl-a, a gene marker for Hydra apical differentiation related to triploblastic paired-like head-specific genes[END_REF]. Prdl-a is expressed in a subpopulation of apical neurons surrounding the hypostome ectoderm (Figure 16), however during head regeneration it is transiently expressed in the endoderm of the regenerative tip [START_REF] Gauchat | The orphan COUP-TF nuclear receptors are markers for neurogenesis from cnidarians to vertebrates[END_REF]). Hydra Prdl-b is expressed in dividing nematoblasts net where nematocytes start to differentiate and migrate. Co-expression experiments using Prdl-a and Prdl-b probes reveal three different cell populations: a nerve cell population expressing Prdl-a, a dividing nematoblast population expressing Prdl-b and a third population expressing both genes, suggesting that nematocytes and nerve cell derive from the same neuronal precursor [START_REF] Gauchat | The orphan COUP-TF nuclear receptors are markers for neurogenesis from cnidarians to vertebrates[END_REF])(Figure 16). Molecular studies such as these suggest that TF gene families involved in neurogenesis in vertebrates are involved in both neurogenesis and nematogenesis in hydrozoans, reinforcing the hypothesis that nematogenesis is a modified neural pathway [START_REF] Watanabe | Immortality and the base of multicellular life: Lessons from cnidarian stem cells[END_REF]). The Hydra Zic gene is another example of a neural gene involved in nematogenesis. Zic genes code for a family of C2H2 zing finger transcription factors.

Vertebrate Zic genes are involved in germ layer differentiation, organogenesis, and have been shown to have a common role in neurogenesis by studies in cnidarians and ctenophores [START_REF] Aruga | A wide-range phylogenetic analysis of Zic proteins: Implications for correlations between protein structure conservation and body plan complexity[END_REF][START_REF] Layden | Expression and phylogenetic analysis of the zic gene family in the evolution and development of metazoans[END_REF]. In Hydra polyps, HyZic is expressed in proliferative nematoblasts in the body column, [START_REF] Lindgens | Hyzic, the Hydra homolog of the zic/odd-paired gene, is involved in the early specification of the sensory nematocytes[END_REF]), prior to the expression of nematocyte structural genes such us Nowa (Figure 16). In Nematostella five Zic genes have been characterised. During development these genes are first expressed in the early gastrula at the aboral pole, while in the late planula and in the budding stage they are expressed in the presumptive and developing tentacles, in the ectoderm and in the endoderm.

Various studies in Hydra have indicated that neuroblasts and nematoblasts can derive from a common population of fate-committed i-cells. Another homeodomain TF, the Gsx orthologue Cnox2, appears to play a key regulatory function in these cells. Cnox-2 is expressed in bipotent interstitial cells that give rise both to apical neurons and gastric nematoblasts. RNAi experiments indicate that Cnox-2 functions upstream of HyZic, Prdl-a, and CnAsh in both nerve cell and nematocytes [START_REF] Miljkovic-Licina | Head regeneration in wild-type hydra requires de novo neurogenesis[END_REF]), but upregulated the expression level of the TF COUP-TF. COUP-TF belongs to the steroid/thyroid hormone receptor superfamily of nuclear receptors, present across metazoans, whose members are involved in neurogenesis in vertebrates and in invertebrates [START_REF] Gauchat | The orphan COUP-TF nuclear receptors are markers for neurogenesis from cnidarians to vertebrates[END_REF][START_REF] Galliot | Developmental Biology[END_REF][START_REF] Tang | COUP-TFs regulate eye development by controlling factors essential for optic vesicle morphogenesis[END_REF]. In Hydra COUP-TF presents an expression pattern similar to Prdl-b and might promote the entry into nematocytes pathway [START_REF] Miljkovic-Licina | Head regeneration in wild-type hydra requires de novo neurogenesis[END_REF]).

The SOX gene family is also commonly associated with neurogenesis. Sox proteins posses a highly conserved DNA binding domain termed HMG. These transcription factors maintain many cell homeostasis and self-renewal/differentiation pathways. Sox genes are often deployed as neurogenesis regulators in bilaterians, they can define neural identity, maintain neural stem cell populations and govern neural differentiation pathways [START_REF] Pevny | SOX genes and neural progenitor identity[END_REF]Placzek 2005, Sandberg et al. 2005). At present among hydrozoans Sox genes have been characterised only in Clytia [START_REF] Jager | Expansion of the SOX gene family predated the emergence of the Bilateria[END_REF]Jager et al. 2011). In contrast Sox genes have been widely studied in anthozoans [START_REF] Magie | Genomic inventory and expression of Sox and Fox genes in the cnidarian Nematostella vectensis[END_REF][START_REF] Shinzato | Sox genes in the coral Acropora millepora: divergent expression patterns reflect differences in developmental mechanisms within the Anthozoa[END_REF]Richards & Rentzsch 2014). A recent study in Nematostella showed for the first time the existence of a neural restricted multipotent population expressing the ortholog of the conserved neurogenic gene SoxB( 2). This multipotent cell population divides asymmetrically, giving rise to sensory neurons, ganglion neurons and nematocytes (Richards & Rentzsch 2014) which represent the totality of neural cell types.

• Clytia SOX complement

In Clytia 10 SOX genes have been described (Jager et al. 2011). Based on their expression profiles they were grouped into three categories: endodermal genes (SoxF), genes expressed in stem cell/undifferentiated cells (Sox3, Sox1, Sox12) and nematocytes/neural genes (Sox2, Sox13, Sox14, Sox15, Sox10). For my study I was particularly interested in Sox15, which represented a good candidate for a role in neurogenesis/nematogenesis. Sox15 in the adult jellyfish is expressed in the nematogenic tentacle bulbs ectoderm, while in the planula it is detected in an endodermal populations (putative nematoblasts), and in an ectodermal cell population identified as maturing nematocytes (Jager et al. 2011) (Figure 17).

• Nematogenesis in Clytia tentacle bulbs

Studies conducted in different cnidarian species identified minicollagen proteins as the major component of the nematocyst capsule [START_REF] Kurz | Mini-collagens in hydra nematocytes[END_REF]. Minicollagens are small collagen-like proteins, with long polyproline stretches flanked at the N and C-terminal by cysteine rich domains (CRD). These CRDs are responsible for stabilization of the capsule wall by disulphide bonds. Minicollagen mRNA is not detected in fully differentiated nematocytes either in Hydra or in Nematostella. Therefore minicollagen in situ hybridisation specifically identifies nematoblasts with an immature capsule in Clytia tentacle bulbs. In Clytia medusa the gene Mcol3-4a is expressed in the manubrium ectoderm and in the tentacle bulb ectoderm. Clytia medusa nematogenesis is particularly active in the tentacles bulbs [START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF], more precisely in the tentacle bulb ectoderm (TBE). An ordered progression of nematogenesis occurs along the proximaldistal axis of the tentacle, such that the TBE can be subdivided in four regions depending on their degree of differentiation and proliferation state (Figure 14 C). Interestingly,

Mcol3-4a positive cells partially overlap with cell expressing the stem cell gene Piwi, suggesting that nematocyte-fated i-cells transiently retain Piwi expression. Piwi expressing cells in the tentacle bulb are highly proliferative [START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF]. Clytia tentacle bulbs,

Mcol3-4a and Sox15 expression domains overlap extensively (Figure 17C). I used these three genes to detect nematogenetic cells during Clytia embryonic development.

• Gland cell formation I-cells also give rise to gland cells, however this i-cell differentiation pathway has been less studied. In Hydra the formation of gland cells was investigated by classical i-cell labelling approaches and recently by in vivo zymogen cell tracking [START_REF] Schmidt | Gland cells in Hydra: cell cycle kinetics and development[END_REF]Siebert et al. 2008). These in vivo tracking experiments show that mucous gland cell and zymogen cells are i-cell derived. Hydra gland cells express the serine protease inhibitor Antistastin [START_REF] Holstein | The primitive metazoan Hydra expresses antistasin, a serine protease inhibitor of vertebrate blood coagulation: cDNA cloning, cellular localisation and developmental regulation[END_REF][START_REF] Hwang | The evolutionary emergence of cell typespecific genes inferred from the gene expression analysis of Hydra[END_REF]).

Serine protease inhibitor (SPI) was characterised for the first time in salivary glands extract of the Mexican leech Haementeria officinalis. It has a potent anticoagulant action, (which inhibits blood coagulation factor Xa activity and disrupts the coagulation cascade) and an anti-metastatic effect [START_REF] Tuszynskisb | Isolation and Characterization of Antistasin[END_REF]. In this study I used the Clytia Antistasin gene to describe the distribution of gland cells during embryonic development.

• Germ cells origin

In hydrozoans the germ line is not separated from somatic cell lineages. Indeed interstitial cells give rise to both lineages [START_REF] Bosch | Stem cells of Hydra magnipapillata can differentiate into somatic cells and germ line cells[END_REF]. The first to infer an i-cell origin for the germ cells was August Weissmann. He described, in Hydractinia histological preparation, the presence of a population of cells that he defined as "distinct population of cell, determined in advance, that undergo transformation in germ cells" [START_REF] Weismann | RNA regulatory elements mediate control of Drosophila body pattern by the posterior morphogen nanos[END_REF]. However the first experimental evidence for an i-cell origin for germ cells dates back to 1957. Hauenschild performed transplantation experiments in the hydrozoan medusa Eleutheria dichotoma. He transferred i-cells from a sexual strain to an asexual medusa.

Following the transplantation the medusa acquired the capacity to form gametes [START_REF] Haueschild | Experimentelle Untersuchungen über die Entstehung asexueller Klone bei der Hydromeduse Eleutheria dichotoma[END_REF]. In Clytia the germ cell origin has been little investigated, but it appears that i-cells give rise to germ cells only within the gonads. In this study I sought to identify molecular markers expressed early during germ cell development from i-cells in order to assess whether i-cells committed to be germ cell precursors already arise during larval development. Among conserved factors regulating gametogenesis, the Boule/Daz gene family is conserved across animal kingdom (Eirín-López & Ausió 2011; [START_REF] Kuales | Boule-like genes regulate male and female gametogenesis in the flatworm Macrostomum lignano[END_REF][START_REF] Shah | Widespread presence of human BOULE homologs among animals and conservation of their ancient reproductive function[END_REF]. Boule/DAZ genes play a variety of roles in germ cell development. For example, in the flatworm Macrostomum lignano three boule genes have been identified, of which macbol1 and macbol3 regulate respectively spermatogenesis and oogenesis [START_REF] Kuales | Boule-like genes regulate male and female gametogenesis in the flatworm Macrostomum lignano[END_REF]. In C.elegans DAZ-1 homolog was shown to be a crucial factor for female meiosis, while in Drosophila boule absence is associated with a defects in spermatogenesis [START_REF] Karashima | Caenorhabditis elegans homologue of the human azoospermia factor DAZ is required for oogenesis but not for spermatogenesis[END_REF][START_REF] Eberhart | Meiotic cell cycle requirement for a fly homologue of human Deleted in Azoospermia[END_REF]. Given the widespread involvement of Boule/DAZ genes in gametogenesis, I decide to identify and characterise for the first time Boule genes as potential early/specific expressed genes for germ cell in Clytia.

Wnt signalling regulates i-cell differentiation

As discussed above (section 1.6.4), in bilateria Wnt signalling regulates embryonic patterning and stem cell behaviour. The role of Wnt signalling in hydrozoan stem cell regulation was investigated in Hydra and Hydractinia polyps [START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF][START_REF] Teo | An evolutionary conserved role of Wnt signaling in stem cell fate decision[END_REF]). The approach used in these two studies was similar. The authors hyperactive Wnt signalling by global treatment using inhibitors of the kinase GSK3b (a negative regulator of Wnt signalling). In Hydra Wnt signalling overexpression resulted in a change of the positional value along the oral-aboral axis, which affected the nematoblast population.

Early nematoblasts expressing nbo35, located in the gastric region of the polyp were abolished following Wnt hyperactivation, while later nb031 expressing nematocytes, normally located in the tentacles became dispersed all along Hydra body. These results suggest that nematoblasts are forced to differentiate into nematocytes by the increase of the Wnt signalling, which promotes oral positional values. This hypothesis was confirmed by grafting experiments in which wild type i-cells were transferred in treated polyps. In these conditions i-cells behave as they have reached the final destination to become nematocytes.

These observations suggest that Wnt signalling modulate i-cell differentiation in nematocytes by providing oral positional information.

Coherently with these results, hyperactivation of Wnt signalling in Hydractinia induces an increase in the number of nematocytes. In addition in these altered condition the numbers of RFamide positive neurons increased around the hypostome. Even though these results clearly show an implication of Wnt in i-cell differentiation, it is still unclear whether Wnt signalling acts only via oral-aboral positional identity or also has a direct role in differentiation and/or earlier steps of neurogenesis or nematogenesis.

The embryological origin of i-cells

Clytia hemisphaerica i-cells express a set of conserved "germ cell/stem cell" markers including Piwi and Nanos1. In vertebrates, these genes are known to be part of the "germ plasm", a ribonucleopreotein complex inherited exclusively by the cells that will become germ cell precursors (See section 1.5). In Clytia these genes are expressed maternally and the mRNAs localise in a "germ plasm-like" region surrounding the egg pronucleus, as shown by electron microscopy (Figure 18). These mRNA are detected at the animal/oral pole during Clytia embryonic and larval development, (see Figure 12, Leclère et al. 2012).

In Hydractinia eggs Vasa protein is concentrated in a similar position and is probably another germ plasm component [START_REF] Rebscher | The germ plasm component vasa allows tracing of the interstitial stem cells in the cnidarian Hydractinia echinata[END_REF]. The presence of morphologically recognisable germ plasm structures, suggests that i-cell formation could be determined by inheritance of the germ plasm. Previous studies investigated the participation of the germ plasm into i-cell formation through the production of "germ plasm free" embryos. To separate animal (germ plasm present) and vegetal halves (germ plasm absent) embryos bisection experiment at 8-cell stage (single blastomeres isolation) and at blastula stage were performed (Figure 19). Fragments lacking germ plasm are able to re-generate Piwiexpressing i-cells (Figure 19), indicating the existence of cell-signalling mechanisms acting in de novo i-cell formation [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]. Among the signalling pathways regulating stem cell formation the Wnt/β-catenin pathway is a prime candidate for regulating i-cells. Wnt/β-catenin signalling regulates proliferation and differentiation in Hydractinia and i-cell terminal differentiation in Hydra [START_REF] Teo | An evolutionary conserved role of Wnt signaling in stem cell fate decision[END_REF][START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF]). In Clytia Wnt3 ligand expression at the " animal" or "oral" pole of the embryo during the larval development corresponds partially to the expression domain of Nanos1 and Piwi as shown in figure 20.

Therefore in the present study I investigated the impact of Wnt/β-catenin signalling on:

-i-cell formation during undisturbed embryogenesis.

de novo i-cell formation in isolated vegetal embryo fragments.

-fate specification and differentiation of i-cell derivatives.

Animals

• Culture system

The complete Clytia life cycle is reproduced under laboratory conditions in a system of small aquaria each with a capacity of 5-6 litres, linked in series, and supplied by a circulating flow of artificial seawater. Some aquaria contain colonies of polyps, fixed on glass plates and other contains mature medusae, separated into males and females. Mature medusae are cultured in closed compartments in which the light is controlled by a day/night cycle, in order to synchronise spawning. Polyp colonies and jellyfishes are fed twice a day with Artemia salina larvae. More details of the Clytia culture system can be found at http://clytia.wikispaces.com/.

For all the experiments performed, Clytia hemisphaerica medusa cultured in Villefranchesur-Mer from established laboratory colonies were used [START_REF] Chevalier | Polarised expression of FoxB and FoxQ2 genes during development of the hydrozoan Clytia hemisphaerica[END_REF].

Clytia eggs were collected by placing the female medusae in glass beakers filled with artificial seawater (ASW-Red Sea Salt brand), and were allowed to settle by gravity. The sperm released by adult males was collected with a pipette and added to the collected eggs.

In Clytia there is no obvious sign of successfully fertilisation. It is possible to estimate under the stereomicroscope if the amount of sperm surrounding the eggs is sufficient for fertilisation and successful development. At the four cells stage, embryos are transferred to clean glass beakers containing Millipore filtered Artificial seawater (MFSW). The embryos were cultured at 18 ºC in MFSW.

• Egg microinjection and embryo manipulation

Morpholino oligonucleotides were suspended in water to a concentration of 4mM. As laboratory routine, a working solution of 1 mM morpholino oligos was prepared. The sequence of the morpholino oligonucleotide against Wnt3, injected at 200 μM have been already published (Momose et al. 2008). Microinjection in unfertilised egg was performed using an Eppendorf ''Femtojet' microinjection system.

Embryos bisection experiments were performed at the 8-cell stage or blastula stage.

Embryos were cut using fine tungsten needles on 2% agarose-coated Petri dishes. Embryo fragments were cultured in artificial filtered seawater. Although Clytia embryos exhibit variable morphologies during early development, for the cutting experiment at the blastula stage we used only the "peanut" shape embryos in which the animal-vegetal axis can be deduced [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF].

• Phylogenetic analyses

Alignments for the phylogenetic analyses were constructed using published sequences from previous work (Lapébie et al. 2014;[START_REF] Shah | Widespread presence of human BOULE homologs among animals and conservation of their ancient reproductive function[END_REF]) and sequences retrieved by TblastN searches on public databases (Genbank) or at www.compagen.org [START_REF] Hemmrich | Molecular signatures of the three stem cell lineages in hydra and the emergence of stem cell function at the base of multicellularity[END_REF]) for a set of representatives phyla . Sequences were aligned using CLUSTALW in the BioEdit package (A. Hall 1999) and the alignment corrected manually or using MUSCLE (Edgar 2004). Maximum-likelihood (ML) analyses were performed using the PhyML program. Trees were then visualised and annotated with FigTree v. 1.4.2 (http://tree.bio.ed.ac.uk/software/figtree/). Branch support was tested with bootstrapping (550replicates).

• Single and double in situ hybridisation

Single-probe in situ hybridisations was performed using the InsituPro robot (Intavis) as described previously (Lapébie et al. 2014). For double fluorescent in situ (FISH) using DIG-UTP and Fluorescein-UTP labelled probes, the protocol was modified as follows.

Embryos were fixed overnight at 20°C in HEM buffer (0.1M Hepes pH 6.9, 50mM EGTA pH7.2, 10mM MgSO4), containing 3.7% formaldehyde, then washed five times in PBST (Phosphate Buffered Saline containing 0.1% Tween 20) for 10 minutes, dehydrated through a PBST/50% methanol step and stored in methanol at -20°C. After rehydration in PBST/50% methanol and three 5-minute washes in PBST, samples were transferred to the InsituPro robot plate. The robot program was as described previously with increased (12h) antibody incubation steps using peroxidase anti-DIG or anti-Fluorescein antibodies diluted 1:2000. Samples were then washed twice in fresh colour reaction buffer (0.0015 % H202 in PBS) for 30 minutes. Signal was developed with fluorophore-conjugated tyramide (Perkin Elmer; 1:400 in colour reaction buffer) for 1hour. Residual enzyme activity was inhibited by10-minute incubation in 0.01N HCl followed by 2 PBST washes prior to addition and development of the second peroxidase-conjugated antibody. Following completion of the second colour reaction, samples were mounted in Citifluor mounting medium (Citifluor Ltd) and images acquired using a Leica SP5 microscope. To verify the specificity of the images obtained for each probe combination, two controls were performed in parallel with single probe, imaging in both channels.

• Cells counting procedure

For co-expression studies the number of labelled cells was counted using the "cell Counter" ImageJ Plugin (cell_counter.jar). Cells were identified by HOECHST counterstaining and counted in each Z-plane, imaged using a Leica SP5 confocal microscope.

• Nematocyst staining and Immunofluorescence

Mature nematocytes were counted by staining the poly-γ-glutamate contents of their nematocyst capsules with DAPI, adapting a protocol from previous studies [START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF] and viewing them under the FITC channel of an epifluorescence microscope. Under these conditions only nematocyst capsules, and not nuclei, were visible.

Samples were fixed for 1 h in 3.7% formaldehyde/PBS at room temperature, washed 3 times in PBS for 10 minutes. They were incubated for 30 min in 50 μg/ml DAPI (from a 1mg/ml stock) in PBS. Samples were rinsed 3 times and mounted in Citifluor. Staining was observed by epifluorescence on a Zeiss Axiovert 200 microscope (excitation 450-490 nm, emission >515 nm).

Immunofluorescence was performed as previously described (Amiel and Houliston, 2009).

RFamide positive neurons were visualised using a specific Clytia RFamide antibody (kindly gift of Dr. Gáspár Jékely, Max Plank Institute for developmental Biology, Tübingen, Germany), final dilution of 1:250, overnight at 4 °C, followed by a rhodamine coupled antirabbit IgG (Sigma, final dilution 1:100) for 3 h at room temperature, with Hoechst dye 33258 (Sigma, 0.3 μg/ml final) included to stain DNA. Specimens were mounted in Citifluor mounting medium (Citifluor Ltd) and imaged using a Leica SP5 confocal microscope.

1.Background and Questions

In this chapter I will present the analyses that allowed me to achieve a first molecular cartography of i-cells and their derivatives during Clytia embryonic and larval development.

Whit the exception of an elegant histological description, which illustrates the different cell types of the Clytia planula larva (Bodo & Bouillon 1968; Figure 9B), and electron microscope descriptions of cell types in normal and i-cell-free larvae (Thomas and Martin 1987), little was previously known concerning the distribution of i-cells derivatives in the Clytia larva.

In order to track the appearance of different types of i-cell derivatives: nematocytes, nerve cells, gland cells and germ cells, I selected a set of potential molecular markers based on published work in Clytia, Hydra and Podocoryne [START_REF] Hwang | The evolutionary emergence of cell typespecific genes inferred from the gene expression analysis of Hydra[END_REF][START_REF] Gauchat | prdl-a, a gene marker for Hydra apical differentiation related to triploblastic paired-like head-specific genes[END_REF]Seipel et al. 2004;Jager et al. 2011;[START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF][START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF]). In addition I took advantage of the transcriptomic data derived from sorted endodermal, ectodermal and Nanos expressing (i-cell lineage) cells in Hydra [START_REF] Hemmrich | Molecular signatures of the three stem cell lineages in Hydra and the emergence of stem cell function at the base of multicellularity[END_REF] at http://compagen.zoologie.uni-kiel.de/blast.html) to verify that the genes with identifiable Hydra orthologs were expressed in Hydra multipotent i-cell lineage (Table 1 Annexes).

In this study I used Piwi and Nanos1 interchangeably as i-cell markers. By co-localisation experiments (double in situ hybridisation) at different developmental stages I was able to show the two genes are largely expressed in the same cell population (Figure 21).

It should be noted that defining i-cells on the basis of Piwi and Nanos1 expression means that both multipotent i-cells and some "fated i-cells" are included in the same population.

Nematogenesis progression during Clytia larval development.

In order to investigate the nematogenetic lineage during Clytia larval development I aimed to track the appearance of nematoblast progenitors (fated i-cells), nematoblasts and differentiated nematocytes. To do this I used a combination of approaches. I detected mature nematocytes using a DAPI staining technique (see Materials and Methods for details). At high concentrations, DAPI binds the anionic molecule poly-γ-glutamate that is one of the major components of the nematocyte capsule [START_REF] Weber | Poly(-glutamic Acid)s Are the Major Constituents Hydra of Nematocysts Hydroxoa , Cnidaria )*[END_REF]) allowing the visualisation of the fully mature differentiated nematocytes under a FITC filter. These mature nematocyte cells also have a characteristic splinter-shaped morphology (Figure 22A) and are highly bi-refringent under polarised light. No nematocytes could be detected by DAPI staining in 24hpf planulae (Figure 22B), indicating that the nematocyte differentiation has not yet occurred. At 36hpf a few nematocytes (1-2 for each planula analysed) were detected in the ectodermal layer (Figure 22B). At 48hpf the average number of nematocytes in each planula increased to 25-30. By 72hpf the number had further increased, to reach 30 to 50 nematocytes per planula (Figure 22B). This observation shows that nematocytes first appear in the planula at around 36hpf and that their numbers then increase progressively, suggesting ongoing nematogenesis in the planula.

1.1.2 Mcol3-4a-expressing nematoblasts are largely restricted to the endodermal region Minicollagens are the main component of the nematocytes capsule [START_REF] Holstein | Fibrous mini-collagens in hydra nematocysts[END_REF], their transcripts can be detected prior the capsule formation, in the nematoblast cell population. Among several minicollagens genes characterised in cnidarians [START_REF] David | Evolution of complex structures: minicollagens shape the cnidarian nematocyst[END_REF], to describe the distribution of nematoblasts in Clytia embryos and larvae I performed in situ hybridisation with the Minicollagen 3-4a gene (Mcol3-4) since it was previously shown to be expressed in the Clytia medusa nematoblast [START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF].

Expression of Mcol3-4a mRNA was detected for the first time at 24 hpf in cells concentrated in the oral 2/3 of the endodermal region (Figure 23A). As seen also in Clytia Co-localisation studies using the Mcol3-4a probe in combination with the Sox15 probe, showed that all Mcol3-4a positive cells also expressed Sox15 (Figure 24B), and that Sox15 is also expressed in differentiating nematocytes in the planula ectoderm, in which Mcol3-4a expression is no longer detected. These findings suggest that Sox15 expression is maintained longer than the structural capsule gene Mcol3-4a and also that Sox15 may be expressed in two different cell populations, rather than being restricted to the nematocytes. In order to investigate whether Sox15 is expressed in early nematoblasts, I performed colocalisation study using Sox15 and Piwi at the early gastrula stage, the earliest stage in which Sox15 is detected and before the onset of Mcol3-4a expression. It was possible to distinguish co-expression of Sox15 and Piwi in a few cells, presumably corresponding to early stage nematoblasts expressing Sox15 (Figure 24C). However it was unclear if there was a population of Sox15-expressing cells lacking Piwi expression. The Hydra Sox15 ortholog is found to be weakly expressed in Nanos sorted i-cells, supporting the hypothesis of an i-cell origin for some of the Sox15 positive cells (Table1 Annexes).

Based on these observations, it appears that Sox15 represents a "pan nematogenetic", gene which has an early function in nematogenesis commitment and remains expressed right through the process of nematogenesis. Its function may not, however be restricted to nematogenesis, at least at early stages, accounting for the apparently wider distribution of Sox15 expression across a broader oral domain in the early gastrula. In this context it cannot be excluded that Sox15 is involved in both nematogenesis and neurogenesis. The timing and the localisation of Sox15 expression are compatible with additional expression in a putative neuronal lineage, although we have yet to identify a neurogenesis gene expressed as early as the early gastrula stage (see following section). This scenario could thus be similar to the one described in Nematostella, in which SoxB( 2) is expressed in common neuronal progenitors that give rise to nematocytes, neurosensory cells and ganglionic cells (Richards & Rentzsch 2014). Functional studies will be required to test the role of Sox15 in Clytia nematogenesis and potentially in neurogenesis.

Neurogenesis during larval development

To follow neuroblasts and differentiated nerve cells I sought to use gene markers associated with neural fate choice and early neurogenesis (eg bHLH family genes) as well as late nerve cell markers.

In hydrozoans, transcription factors of the bHLH (Achaete-Scute, atonal-like), PRD (aristaless-like, prdl-a, prdl-b, gsc, rx, repo, PaxA ⁄ C, PaxB) and C2H2 Znf (Glis, Zic) families have been identified as regulators of neurogenesis (See introduction section 2.2.3 Reviewed in Galliot and Quiquand 2011). I characterised some of these genes as putative candidates to follow Clytia neurogenesis. As potential markers of neural precursors I selected Prdl-a, Asc-b and Zic-C. As markers of differentiating and mature nerve cells, I used two neuropeptide precursors identified in the available Clytia transcriptome data. One was a precursor of the RFamide neuropeptide family, widely used as neurotransmitters in many animal species. The other coded for a GLWamide precursor known to be specifically involved in regulating larval settlement in hydrozoan planulae [START_REF] Plickert | The role of alpha-amidated neuropeptides in hydroid development--LWamides and metamorphosis in Hydractinia echinata[END_REF].

I performed in situ hybridisation at successive developmental stages: early gastrula, 24hpf, 36hpf (intermediate stage between the end of gastrulation and full differentiation of the endodermal layer), 48hpf and 72hpf planula. None of the genes selected showed detectable expression at early gastrula stage (not shown) or at 24hpf, indicating that neural fate specification occurs only once the polarity of the developing larva is well established and gastrulation completed.

Clytia Prdl-a as putative neuronal marker

By reciprocal blast using the Hydra Prdl-a sequence I identified the Clytia Prdl-a ortholog from the available Clytia transcriptome (validated through phylogenetic analysis; Figure 25). Prdl-a codes for a homeodomain proteins belonging to the wider Paired family. In Hydra polyps, Prdl-a acts as regulator of neurogenesis during head regeneration. It is expressed in a subpopulation of neurons localised around the Hydra hypostome and also transiently in the endoderm during regeneration [START_REF] Gauchat | prdl-a, a gene marker for Hydra apical differentiation related to triploblastic paired-like head-specific genes[END_REF]. During embryonic development Clytia Prdl-a expression was detected beginning at 36hpf in cells scattered through the aboral half of the planula endodermal region (Figure 26A), suggesting that these cells are ready to migrate into the ectoderm prior to differentiation. Others Prdl-a positive cells were positioned more centrally and showed an i-cell-like morphology (round shape with a large nucleus, Figure 26B) consistent with expression in nematoblasts. At later stages (48hpf and 72hpf) the expression of Prdl-a was only weakly detectable, suggesting that wave of production of a wave of Prdl-a expressing neurons is followed by reduced expression in later stages of development (Figure 26A).

To validate Prdl-a as a neural marker I analysed its expression pattern in the adult jellyfish (Figure 26C). Prdl-a expression was detected in the tentacle bulb on the proximal side of the differentiation zone (Figure 26C), defined as the β region by Denker et colleagues, (see introduction section 2.2.3 Figure 17) and in a subset of ectodermal nerve cells in the manubrium ectoderm (Figure 26C). To confirm the neural identity of these Prdl-a expressing cells, it would be useful to examine their morphology in more detail, or by comparing their distribution with that of neurons expressing different neuro-transmitters.

The Hydra Prdl-a gene is similarly expressed in neurons surrounding the mouth [START_REF] Gauchat | prdl-a, a gene marker for Hydra apical differentiation related to triploblastic paired-like head-specific genes[END_REF]. No Prdl-a expression was detected in the gonads, in the nerve ring or in the circular canal (Figure 26C). These observations indicate that Prdl-a is expressed in a subpopulation of nerve cells of the medusa, but also in some putative neuroblasts. This hypothesis is supported by the analysis of expression of the Hydra Prdl-a ortholog in sorted i-cells (Table1 Annexes). To address this hypothesis I performed double in situ hybridisation using Prdl-a probes in combination with Piwi probes at larval stages. I was unable to demonstrate any co-expression, however this result is not conclusive because of the relatively weak expression of Prdl-a.

To resolve this question it would be useful to perform co-localisation experiments at the medusa stage, to see if Piwi and Prdl-a have overlapping expression domains, as is the case for Piwi and Mcol3-4a (see Figure 17 introduction). 1.2.3 Clytia Asc-b is a expressed in the endoderm in both planulae and medusae Among the many described proneural gene families I decided to investigate a member of the Achete scute family, Clytia Asc-b. Asc proteins form a subfamily of the wider bHLH transcription factor family, involved in promoting neural progenitor commitment [START_REF] Bertrand | Proneural genes and the specification of neural cell types[END_REF]. Clytia Asc-b was isolated as a full-length clone by BLAST searches using the Hydra CnASH sequence from available Clytia transcriptome data. In Hydra CnASH is expressed both in differentiating sensory neurons (Hayakawa et al. 2004) and in differentiating nematoblast [START_REF] Lindgens | Hyzic, the Hydra homolog of the zic/odd-paired gene, is involved in the early specification of the sensory nematocytes[END_REF]. A detailed phylogenetic analysis, including bHLH proteins from Nematostella, Hydra, Acropora, Drosophila, and Human (sequences extracted from [START_REF] Gyoja | A genomewide survey of bHLH transcription factors in the coral Acropora digitifera identifies three novel orthologous families, pearl, amber, and peridot[END_REF], determine the relationships between 29 Clytia bHLH proteins (Figure 27). These included Clytia orthologs of numerous proneural genes including human Neurogenin a key regulator of neuronal fate [START_REF] Kageyama | Roles of bHLH genes in neural stem cell differentiation[END_REF]. The Clytia gene that I had selected, for further analysis, Asc-b, was found to be the ortholog of Podocoryne Ash2 gene (Seipel et al. 2004) and not of CnASH (Asc-a). In Podocoryne Ash2 expression is restricted to the endoderm, both in medusa and in planula. It is unclear which type of cells expresses the gene, but they were tentatively proposed to be secretory cells in Clytia developing larvae Asc-b expression was observed in few endodermal cells at 36hpf, restricted to the aboral half of the embryo. At 48hpf the expression was weaker and at 72hpf the expression was barely detectable (Figure 28A).

Expression of this gene at medusa stage was detected in the endodermal cells of the tentacle bulbs, in the circular canal endoderm and in the manubrium endoderm. No expression was detected in gonad endoderm (Figure 28B) or in the radial canals. I could not identify with certainty the cell types in which Asc-b is expressed. Asc-b, as seems the case of Ash2 in Podocoryne (see Introduction 2.3.2) may not be expressed in neurons. Detailed morphological analysis, combined perhaps with histological characterisation (e.g. PAS staining) would be useful to determine the cell type that expresses Asc-b. Whatever the identity of these cells, they are probably derived from i-cells since the Hydra Asc-b ortholog is expressed in i-cell sorted population [START_REF] Hemmrich | Molecular signatures of the three stem cell lineages in Hydra and the emergence of stem cell function at the base of multicellularity[END_REF]) (see Table1 Annexes). 1.2.4 Zic-C is expressed in a sub-type of neural cells specific to medusa tentacles.

Zic family genes encode C2H2-type zinc finger transcription factors. In vertebrates, Zic genes are present in multiple copies (5 copies in mice and in Xenopus) and are involved in a plethora of biological events at different times during development. For example, during gastrulation Zic genes are involved in germ layer specification while during organogenesis they are required for neural tube formation. Moreover, Zic genes show a redundant role in central nervous system development [START_REF] Houtmeyers | The ZIC gene family encodes multi-functional proteins essential for patterning and morphogenesis[END_REF]. Several studies in cnidarians and ctenophores suggest an ancestral role for Zic genes in neuronal development [START_REF] Layden | Expression and phylogenetic analysis of the zic gene family in the evolution and development of metazoans[END_REF][START_REF] Lindgens | Hyzic, the Hydra homolog of the zic/odd-paired gene, is involved in the early specification of the sensory nematocytes[END_REF]). In Nematostella five Zic genes have been identified. These genes show overlapping expression domains in presumptive tentacle ectoderm in the planula and in tentacle ectoderm in the polyps, which are considered highly neurogenic structures. In Hydra Hyzic is expressed in early nematoblast precursors, thus it implicated in the nematogenesis process [START_REF] Lindgens | Hyzic, the Hydra homolog of the zic/odd-paired gene, is involved in the early specification of the sensory nematocytes[END_REF].

At start of this study I could only identify one Zic gene in the available Clytia transcriptome dataset. Phylogenetic analysis revealed that this gene is the ortholog of Nematostella Zic-C (Figure 29), and so the gene was named Zic-C and that it groups in a clade with other Phylogenetic analysis of Zic/ Gli family zinc finger motifs. The sequence were extracted from [START_REF] Layden | Expression and phylogenetic analysis of the zic gene family in the evolution and development of metazoans[END_REF]

Neuropeptide expression defines mature neural subpopulations

Neuropeptides are major neurotransmitter molecules in cnidarians [START_REF] Grimmelikhuijzen | Peptides in the Nervous Systems of Cnidarians: Structure, Function, and Biosynthesis[END_REF] and have been used extensively to visualise the cnidarians nervous system (see Introduction 2.2.2). To monitor nerve cells differentiation I used GLWamide and RFamide neuropeptide precursors as markers. The mRNA of one of the RFamide precursor transcript (PP5) was detected in few ectodermal cells at 36hpf planula stage (Figure 31A). At 48hpf PP5 expression was detected in cells positioned in the lateral ectoderm along the oral-aboral axis (Figure 31A), but mainly excluded from the polar region, as reported for the other hydrozoan planulae [START_REF] Plickert | The role of alpha-amidated neuropeptides in hydroid development--LWamides and metamorphosis in Hydractinia echinata[END_REF].

Immunostaining with a Clytia specific RFamide antibody (kindly gift of Dr. Gáspár Jékely Max Plank Institute for developmental Biology, Tübingen, Germany) showed an extensive nerve net between lateral ectoderm and endoderm of the planula (Figure 31B). Cells-bodies were positioned at the base of the ectoderm. These RFamide neuropeptide expressing cells have been described as ganglionic cells [START_REF] Martin | Development of Nerve Cells in Hydrozoan Planulae: I. Differentiation of Ganglionic Cells[END_REF]).

In the medusa PP5 mRNA was detected throughout the manubrium ectoderm, in the medusa tentacle, in the tentacle bulb ectoderm and in the nerve ring (Figure 31C).

In situ hybridisation revealed that expression of the GLWamide precursor PP2 is restricted to larval stages (Figure32A). At 36hpf, only a few cells express the PP2 transcript, while the number of cells increases at 48 and 72hpf. The mRNA was detected in a band of cells localised around the aboral pole from which neural projections connect the outer and the inner cell layer, typical of sensory cells (Figure 32A).

These observations indicate that differentiated nerve cells first start to appear at 36hpf, and that Clytia RFamide and GLWamide neuropeptides are expressed in two different populations of neurons, likely corresponding to ganglionic and sensory neuron respectively. The absence of the PP2 GLWamide precursor in medusae (Figure 32B) is consistent with its specific role only in metamorphosis. A second GLWamide precursor named PP11 is expressed in the medusa (G. Quiroga and P. Lapébie, unpublished results).

Gland cell formation in Clytia planulae and in the medusae

I followed gland cell formation during Clytia larval development and in the medusa by in situ hybridisation using the Antistasin gene Antistasin is a serine protease inhibitor detected in Hydra gland cells with a digestive function (zymogen cells). Sequence analysis revealed that Clytia Antistasin contains seven antistasin domains, confirming that it belongs to the antistasin multigenic family. It was not possible to perform a phylogenetic analysis since the number of domains and the type of domain is highly variable among species. Antistasin expression analysis revealed that at 24hpf, the transcript is expressed in the oral endodermal region. At later stages, Antistasin expressing cells increased in number and were found through the endoderm (Figure 33A), with some cells also detected weakly in the ectoderm at 72hpf (Figure 33A), where other types of gland cell have also been described [START_REF] Bodo | Etude histologique du developpement embryonnaire de quelques Hydrome ´duses de Roscoff[END_REF].

To confirm the identity of the Antistasin expressing cells I performed expression analysis in the medusae. At this stage Antistasin was detected strongly and exclusively in gland cells of the manubrium (Figure 33B). These cells show a club shape, typical of zymogen cells (Figure 33C) and are located in the manubrium endoderm and more specifically in the stomach region.

These observations suggest that Antistasin positive cells are gland cells and that in the planula they could correspond to the "granuleuse" gland cells described by [START_REF] Bodo | Etude histologique du developpement embryonnaire de quelques Hydrome ´duses de Roscoff[END_REF] since they are the only type of gland cells described in both layers at this stage. In addition an i-cell origin for Antistasin positive gland cells is supported by the level of Hydra Antistasin ortholog in separated i-cells and in the endodermal cells (Table 1, Annexes).

Germ cell genes in Clytia

It is not known when the germ cells fate is specified during the Clytia life cycle, i.e. whether this fate choice is already made for some cells from i-cell in the planula, in the circulating icells in the stolon system of the polyp colony or not until i-cells reach the gonad in the medusa stage. To try to identify germ cell precursors during larval stages in Clytia, I tested multiple members of several conserved gene families associated with germ cell formation in bilaterian species, specifically the Boule/DAZ family and Tudor domain proteins.

Boule/DAZ family members are RNA binding proteins and include Boule, DAZ and DAZL. Ten Boule-related sequences were identified from the Clytia transcriptome collection and three full-length cDNA clones were retrieved from a cDNA library for which ESTs were available [START_REF] Chevalier | Polarised expression of FoxB and FoxQ2 genes during development of the hydrozoan Clytia hemisphaerica[END_REF]. Seven sequences from the Hydra vulgaris genome and four sequences from the Nematostella vectensis genome were also identified as belonging to the Boule/DAZ family. Phylogenetic analysis suggested that a duplication event occurred specifically in the cnidarian lineage (Figure 34A). I selected three boule genes for in situ hybridisation: Boule1-a, one of the two orthologs of the Bilaterian Boule/Daz gene, as well as genes from a cnidarian specific group, Boule-like-1a, and Boulelike-1-b. The Boule 1-a amino acid sequence includes the conserved motifs RNP1 and RNP2, as well as conserved amino acids present at diagnostic positions considered as the molecular signature of Boule proteins (Figure 34B). The Boule1-a transcript was detected exclusively in the germ cells in both male and female Clytia jellyfish gonads (Figure 35A), but not at other i-cell sites such as tentacle bulbs. The profile of expression of Boule-1a in both male and female gonads was very similar to that reported for Nanos1 and Piwi, suggesting the Boule1-a is expressed in i-cells but only in the gonads. Conversely during embryonic development no Boule-1a expression was detected. In contrast, Boule-like1-a and Boule-like-1b transcripts were detected both during larval development and in the medusa.

Boule-like-1a transcripts were detected for the first time at 24hpf in scattered cells located between the endodermal and the ectodermal layers (Figure 35A), as described for Nanos1

and Piwi [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]). In the adult medusa its expression appeared restricted to tentacle bulbs, in a population of cell located between ectoderm and endoderm. Boule-like1-b mRNA was detected in both ectoderm and endoderm at 24 and 48hpf, while it was barely detectable at 72hpf (Figure 35A). In the adult Boule-like1b expression was restricted to female gonads and it was ubiquitously expressed in oocytes and eggs (Figure 35-B).

These results suggest that Boule1-a is exclusively expressed in germ cells and can therefore be used as a specific germ cells marker and that germ cell fate is not specified until the medusa stage. Tudor domain proteins are characterised by the presence of the tudor domain [START_REF] Ponting | Tudor domains in proteins that interact with RNA[END_REF], but can also contain other domains in a variety of combinations. This is reflected in the diversity of biological processes in which tudor domain proteins are involved. The role of Tudor itself in germ cell formation has been demonstrated in different taxa. Drosophila tudor mutants fail in to form pole cells (Thomson & Lasko 2005), while Tdrd1/Mtr1 gene deletion in mouse leads to complete male sterility [START_REF] Chuma | Tdrd1/Mtr-1, a tudor-related gene, is essential for male germ-cell differentiation and nuage/germinal granule formation in mice[END_REF]. mRNA was detected for the first time at planula 48hpf, and was broadly distributed in the endoderm also at later stages. TDP-2 transcripts were detected for the first time in the early gastrula at future oral pole (gastrulation site). During development it is expressed in a population of cells located within the endoderm region which could correspond to an i-cell subpopulation. Scale bar 50μm. B) Medusa stage. TDP-1 expression detected in the medusa was female specific, with ubiquitous mRNA distribution detected in the developing oocytes. The transcript was detected also in the tentacle bulbs at the boundary between endoderm and ectoderm. TDP-2 mRNA was detected in both male and female gonads and in tentacle bulbs, at the interface between endoderm and ectoderm. Scale bar 50μm.

In the Clytia transcriptome I identified two tudor domain containing proteins, baptised TDP-1 and TDP-2. Expression pattern analysis showed that TDP-1 can be first detected at 48hpf, in the endodermal region and in a few cells in the ectoderm towards the oral pole, while at later stages TDP-1 expression appears restricted to the endodermal region (Figure 36-A). In the medusa TDP-1 mRNA was barely detectable in male gonads, while in female gonad it was detected in oocytes of all stages of oogenesis, and in tentacle bulbs at the boundary between endoderm and ectoderm (Figure 36B). In contrast TDP-2 was detected already at early gastrula stage, widely across the gastrulation site at the future oral pole. At larval stage (24,48,72hpf) TDP-2 transcripts were detected in a few cells located in the central endodermal region which might correspond to an i-cell subpopulation (Figure 36A).This hypothesis is supported also by high level of Hydra TFDP-2 ortholog in separated i-cells population (Table1 Annexes). In the medusa TDP-2 was detected in both male and female gonads, (predominantly in small oocytes) and in the tentacle bulbs, at the boundary between endoderm and ectoderm (Figure 36B). Interestingly TDP-2 sequence analysis reveals the presence of two dimethylarginine/lysine binding sites. Recently it has been showed that PIWI proteins contain dimethyl arginine residues. Certain Tudor domain proteins can through the dimethyl/arginine binding sites, associate specifically with PIWI proteins [START_REF] Siomi | How does the royal family of tudor rule the PIWI-interacting RNA pathway?[END_REF]. This could suggest a specific function of this Clytia tudor protein in i-cell regulation through Piwi (e.g. genome protection). Neither of the Tudor domain proteins chosen here for analysis have the profile characteristics expected for germ cell fate specification genes, both being expressed in the tentacle bulbs, but they may have a wider roles in i-cell biology. I identified the gene Thyn1 as an additional putative germ-cell specific marker. Originally I selected this gene on the basis of reported icell lineage specific expression in a micro-array based screen in Hydra [START_REF] Hwang | The evolutionary emergence of cell typespecific genes inferred from the gene expression analysis of Hydra[END_REF]).

Thyn1 stays for thymocyte nuclear protein 1. This gene was originally identified is a screen in avian lymphocyte cultures, as a cellular protein that mediates apoptotic events. Thyn1 gene is present in a single copy for each species (see phylogenetic analysis Figure 37A) and the proteins appear to be highly conserved among bacteria, plants, and throughout the animal kingdom [START_REF] Miyaji | Molecular cloning and characterization of the mouse thymocyte protein gene[END_REF]. These proteins are characterised by the presence of the so-called EVE domain, whose function is still unknown (Figure 37B). The EVE domain belongs to the PUA domain superfamily involved in RNA binding. The role of this protein has been little studied in vertebrates. Functional analysis in thymocyte cell cultures show that it prevents apoptosis, and that increased protein levels correlate with a decrease of apoptosis [START_REF] Toyota | Thy28 partially prevents apoptosis induction following engagement of membrane immunoglobulin in WEHI-231 B lymphoma cells[END_REF]. I found that Clytia Thyn1 expression was restricted to the male and female gonads, and that the mRNA is distributed uniformly in the eggs (Figure 38). The expression of Thyn was very strong in the male gonad. It is known that the apoptosis mechanism used to regulate the number of proliferating germ cells and it has been shown in Hydra spermatogenesis is regulated through apoptotic event [START_REF] Kuznetsov | Role of epithelial cells and programmed cell death in Hydra spermatogenesis[END_REF].

Background and Questions

Depending on the cellular context, Wnt ligands activating the Wnt/Fz/β-catenin pathway regulate embryonic axis polarity, maintain stem cell populations, control cell proliferation and govern cell fate decisions throughout the animal kingdom. For PAPER 2, I participated in the identification of additional and novel genes expressed differentially along the developing oral-aboral axis and implicated in embryonic development. The genes were identified by comparing the transcriptomes of unmanipulated early gastrulae and of early gastrula in which the Wnt/β-catenin pathway was inactivated.

PAPER 1: Summary of the results

Investigating Wnt/β-catenin signalling in i-cell formation and differentiation during

Clytia embryonic development.

In order to investigate the influence of the Wnt/β-catenin signalling activity on i-cell dynamics during Clytia embryonic development I monitored the presence of i-cell and icell derivatives: nerve cells, gland cells and nematocytes in embryos in which Wnt signalling was abolished by Wnt3 morpholino injection (Wnt3 morphants).

To verify if i-cell differentiation was affected in Wnt3 morphants at different developmental stages (early gastrula, 24 and 48hpf), I used the set of i-cell derivatives markers described in the previous ResultI chapter.

To assess the involvement of the Wnt/β-catenin pathway in i-cell maintenance and i-cell proliferation I used both Nanos1 and Piwi i-cell markers. The main finding were as follows:

• I-cell formation is Wnt3 independent Wnt3 morphants have a slightly delayed gastrulation, have a spherical shape and lack polarity as indicated by the loss of orientation in their swimming behaviour and by loss of expression of oral markers such as the Clytia Brachyury ortholog Bra1 (Momose et al. 2008). Wnt3 morphant embryos fail to elongate even at 24 and 48hpf, making it easy to verify in each experiment that Wnt/β-catenin signalling has been largely inhibited. Despite the very different morphology I could not distinguish any difference between wild type embryos and Wnt3 morphants with respect to i) the timing of onset of Nanos1 and Piwi expression, ii) the number of cells expressing Nanos1 and Piwi (Figure 39). Corresponding analysis of expression levels of these genes in Wnt3-MO embryos showed no significant difference (Figure 43).

These observations suggest that i-cell formation is not dependent on the Wnt/β-catenin pathway and therefore that formation and maintenance of the i-cell population is regulated independently from axial polarity. was not affected or slightly delayed (Mcol3-4a) in Wnt3 morphants, however RFamide but not GLWamide expression was completely abolished at 48hpf Wnt3-MO planulae (Figure 40). 48hpf Wnt3-MO planulae also lacked nematocytes as shown using DAPI staining, and by 72hpf only a few nematocytes (between 5-7) could be detected in Wnt3 morphants in comparison with controls that had between 70-80 nematocytes per planula. [START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF][START_REF] Teo | An evolutionary conserved role of Wnt signaling in stem cell fate decision[END_REF]. More specifically, "oralised" Hydra polyps treated with alsterpaullone, (ALP, which inhibits the negative Wnt regulator GSK3b kinase) show ectopic expression of a specific tentacle nematoblast marker nbo31 in the gastric region. Indeed the gastric nematoblasts acquire the fate of the tentacle nematobasts (that normally expressed the nbo31 gene) and stop expressing the nbo35 gene that is a marker of this population. This is consistent with the role of Wnt/β-catenin signalling in defining positional information along the O-A axis. Consistently, grafting experiment in which untreated i-cells were transplanted to ALP treated polyps showed that i-cells behaved as if they had reached their final destination in the tentacles in terms of neural marker expression [START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF]. Hydractinia polyps treated with ALP showed an increase in the number of nematocytes (by morphological analysis) and RFamide cells, suggesting that also in Hydractinia polyps Wnt signalling influences the final differentiation of neural cell types. In Clytia the absence of Wnt3 in the oral ectoderm, with consequent down regulation of the Wnt/β-catenin signalling prevented the formation of the nematocytes and the RFamide nerve cells. This is again consistent with a role for Wnt/β-catenin signalling in promoting differentiation of oral type nerve cells. Interestingly the GLWamide neuronal subpopulation was not affected by the absence of Wnt/β-catenin signalling. This population is also positioned in the ectoderm, but at the aboral pole of the planula. This is consistent with a model in which neuroblasts can differentiate into GLWamide cells at a lower level of Wnt/β-catenin signalling, Wnt ligands in Clytia planulae are probably distributed in a gradient from oral to aboral pole because of a nested/overlapping expression of several ligands in the planula (Momose et al. 2008). In accordance with this model genes expressed in the neural and nematocyte precursors (e.g. are not inhibited by Wnt3 downregulation (Figure 41).

Mcol3

• Wnt3 does not affect gland cells formation

I monitored the expression of the gland cell marker Antistasin in Wnt3 morphants, to see whether gland cell formation is Wnt/β-catenin dependent. The Antistasin transcript could be detected in Wnt3 morphants in 24hpf and 48hpf planulae. As in the controls, Antistasin expression was strongly detected in the endoderm and also in the ectoderm .

These data indicate that Wnt3 is not involved in gland cell development or differentiation.

Taken together these data indicate that: ü i-cell formation during embryonic development is Wnt/β-catenin pathway (via Wnt3) independent ü Wnt/β-catenin signalling regulates, directly or indirectly, the final step of differentiation for the oral/lateral neural cell types including nematocytes and RFamide neurons.

ü Somatic commitment for i-cells is not regulated by Wnt/β-catenin signalling ü Gland cell formation is not Wnt/β-catenin signalling dependent • Wnt/β-catenin signalling contributes to i-cell formation in embryos lacking germ plasm Previous work [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]) left open the question of whether i-cells formation during Clytia development occurs by inheritance of maternal germ plasm (containing Piwi and Nanos1 mRNAs) (see Introduction 2.4) from the egg animal pole. I-cells can also form "de novo" in isolated vegetal embryos fragments, implying that signalling mechanisms also contribute to their formation. These fragments can restore an oral -aboral axis, despite the loss of much of the maternal Wnt3 and Fz1, concentrated in animal half of the embryos.

The results presented in the previous section show that Wnt/β-catenin signalling is not required for i-cell formation during normal embryonic development, but I wanted to test its role also in i-cell formation during embryo re-patterning. The presence of i-cells was monitored by in situ hybridisation with probes for Piwi or Nanos1. These mRNAs are detectable in the i-cells and in the germ plasm-bearing blastomeres at early stages. To generate i-cell depleted embryos, animal/oral halves containing germ plasm, and vegetal/aboral halves lacking germ plasm were separated by bisection. To investigate the role of Wnt/β-catenin pathway on i-cell formation in the context of axis re-patterning, Wnt3-MO was injected into unfertilised eggs, and embryo bisection performed at the blastula stage. At this stage the morphology of most embryos allows the A-V axis to be distinguished. Alternatively, blastomeres were separated individually at 8-cell stage. The presence of Piwi or Nanos1 expressing cells was assessed in the planulae after one or two days of development. There was a clear difference in the ability of embryo vegetal halves or vegetal 1/8 blastomeres to support i-cell formation in the presence or absence of Wnt signalling. While almost all vegetal fragments from uninjected embryos formed i-cells, a significant numbers failed to reform i-cells following Wnt3-MO injection.

From these results the following conclusion can be made: ü Germ plasm contributes to i-cell formation ü Two distinct mechanisms can ensure the formation of i-cells during embryogenesis ü Wnt3 drives (directly or indirectly) the formation of i-cells in germ plasm-depleted embryos.

In this scenario Wnt/β-catenin signalling is potentially the inductive signal responsible for i-cell formation in the absence of germ plasm. It is likely that Wnt signalling acts indirectly, allowing the re-establishment of an oral pole territory, which is a pre-requisite for i-cell restoration. Other signalling pathways such as peptide-based signalling, TGFb or Notch signalling could be more directly involved in the homeostasis of the i-cell population downstream of Wnt signalling. In situ hybridisation using Nanos1 (A-D) and Piwi (E-H) probes on non-injected and Wnt 3 morphants following animal/vegetal separation. A-D embryo bisection experiment performed at blastula stage. All the mini planulae (non injected and Wnt3 morphants) animalderived (A-B) present Nanos1expression, while only the 50% of the Wnt3 injected vegetal -halves present Nanos1 expression. In 24hpf larvae derived from separated 8-cell stage blastomeres (E-H), in situ hybridisation using Piwi probe revealed that 30% of the noninjected embryos are spherical and lack Piwi-expressing cells (G), as compared to 56% in the case of Wnt3-MO injection (H). Scale bars: 50μm for A-H.

Introduction

Hydrozoan cnidarians are animals which have certain biological features more frequently associated with plants. They typically propagate vegetatively as well as by sexual reproduction, and also show exceptional regeneration properties including restoration of missing parts of the three possible life cycle forms: planula larva, polyp and medusa.

Central to these two related features is a remarkable population of multipotent stem cells called interstitial cells (i-cells), present through the entire life cycle [START_REF] Watanabe | Immortality and the base of multicellular life: Lessons from cnidarian stem cells[END_REF]) [START_REF] Bosch | The Hydra polyp: nothing but an active stem cell community[END_REF] (Plickert et al., 2012;[START_REF] Watanabe | Immortality and the base of multicellular life: Lessons from cnidarian stem cells[END_REF]. The i-cells are small cells with rounded morphology and relatively large nuclei that occupy spaces at the bases of the two layers of self-maintaining epitheliomuscular cells. The epitheliomuscular cells are the basic cell type of the two cnidarian germ layers (ectoderm and endoderm) and can also generate certain specialized epithelial cell types as well as some sensory neurons [START_REF] Bosch | The Hydra polyp: nothing but an active stem cell community[END_REF]Martin and Thomas, 1981a;[START_REF] Martin | The origin of the nervous system in Pennaria tiarella, as revealed by treatment with colchicine[END_REF]. I-cells have been extensively characterised in Hydra polyps, where they have been shown to generate multiple somatic cell types including secretory gland cells, nerve cells and nematocytes (the stinging cells characteristic of cnidarians, which can be considered to be specialised mechanoreceptor cells), as well as germ cells [START_REF] Bosch | Decision making in interstitial stem cells of Hydra[END_REF]David, 2012;Galliot and Quiquand, 2011;[START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF][START_REF] Watanabe | Immortality and the base of multicellular life: Lessons from cnidarian stem cells[END_REF]. In the colonial polyps of Hydractinia, icells have an even wider potential, being able in experimental tests to generate all adult cell types including epitheliomuscular cells [START_REF] Müller | Totipotent migratory stem cells in a hydroid[END_REF]Plickert et al., 2012). Once committed to different somatic cell fates, neuronal and nematocyte precursors migrate into the ectodermal layer to take up their positions along the body axis, termed oral-aboral (OA) in cnidarians, and complete differentiation. These processes have been described not only to hydrozoan polyps, but also at the "planula" larva stage [START_REF] Martin | III. Some interstitial cells traverse the ganglionic pathway in the endoderm[END_REF][START_REF] Martin | Migration of interstitial cells and their derivatives in a hydrozoan planula[END_REF]. In hydrozoan planula larvae i-cells are largely confined to the inner layer of endoderm cells [START_REF] Bodo | Etude histologique du développement embryonnaire de quelques Hydroméduses de Roscoff[END_REF][START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]Plickert et al., 2012), despite early histological suggestions of an ectodermal origin in some species (see discussion in [START_REF] Genikhovich | Discovery of genes expressed in Hydra embryogenesis[END_REF].

With the notable difference that they generate somatic cell types as well as germ cells, hydrozoan i-cells in many ways resemble a classical animal "germ line", a developmental cell lineage devoted to producing gametes which in many animals is segregated from the "soma" early during embryogenesis. Thus i-cells are the source of germ cells, and express orthologues of many genes associated with totipotency and commonly considered to be germ line markers, such as Nanos, Piwi, PL10 and Vasa [START_REF] Genikhovich | Discovery of genes expressed in Hydra embryogenesis[END_REF][START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF][START_REF] Rebscher | The germ plasm component vasa allows tracing of the interstitial stem cells in the cnidarian Hydractinia echinata[END_REF]. Furthermore, during embryonic development Clytia i-cells arise near the site of endoderm formation derived from the egg animal pole, mirroring the endoderm-primordial germ cell association seen in many chordate species in which both derive from the egg vegetal hemisphere.

Correspondingly the animal region of hydrozoan eggs and vegetal region of bilaterian eggs harbor two types of developmental determinant: maternally localized mRNAs that contribute to endoderm fate, and "germ plasm". Germ plasm is a morphologically distinguishable egg cytoplasmic material containing localized germ line mRNAs and/or proteins inherited by primordial germ cells, which in species of anuran amphibians, nematodes and insects has been demonstrated experimentally to determine germ line fate [START_REF] Extavour | Mechanisms of germ cell specification across the metazoans: epigenesis and preformation[END_REF][START_REF] Ikenishi | Germ plasm in Caenorhabditis elegans, Drosophila and Xenopus[END_REF]. Germ plasm like material has been detected in hydrozoan eggs as animal pole aggregates of Vasa protein in Hydractinia [START_REF] Rebscher | The germ plasm component vasa allows tracing of the interstitial stem cells in the cnidarian Hydractinia echinata[END_REF] and of Vasa, Nanos1, Nanos2, Piwi, PL10 mRNAs in Clytia [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]. Although i-cells are first detected in the corresponding future oral pole region (cell ingression site) at the early gastrula stage in Clytia, there is no experimental evidence that this germ plasm contributes to determining their fate. Indeed, germ plasm inheritance is certainly not the exclusive route to i-cell formation, since both animal and vegetal blastomeres from 8 cell stage embryos can develop into "mini-larvae" containing i-cells [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]. This observation suggests that the intercellular signaling mechanisms responsible for the strong re-patterning ability of vegetal embryo fragments (Freeman, 1981) can promote i-cell formation ectopically in the absence of germ plasm.

We have addressed the intercellular interactions regulating different aspects of i-cell regulation during larval development in Clytia: their initial formation, the commitment to different derivative fates through the specification of progenitor cell populations, and the differentiation of final specialized cell types. We focussed on Wnt signaling, known for its role in regulating stem cell dynamics in mammalian stem cells [START_REF] Reya | Wnt signalling in stem cells and cancer[END_REF].

Two previous studies in hydrozoan polyps found that over-stimulation of the Wnt/bcatenin pathway by pharmacological inhibition of the negative regulator GSK3b caused excess formation of nematocytes and of some nerve cell types at the expense of progenitors [START_REF] Hensel | Lineage-specific evolution of cnidarian Wnt ligands[END_REF][START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF][START_REF] Teo | An evolutionary conserved role of Wnt signaling in stem cell fate decision[END_REF]. Interpretation of the exact role of Wnt signaling in stem cell regulation from these studies is complicated by its simultaneous function in defining positional values along the OA axis, because Wnt activation confers oral identity in cnidarians [START_REF] Brouns | A Swiss Army Knife of Immunity[END_REF][START_REF] Duffy | A heat shock protein and Wnt signaling crosstalk during axial patterning and stem cell proliferation[END_REF][START_REF] Hobmayer | WNT signalling molecules act in axis formation in the diploblastic metazoan Hydra[END_REF][START_REF] Kusserow | Unexpected complexity of the Wnt gene family in a sea anemone[END_REF][START_REF] Wikramanayake | An ancient role for nuclear beta-catenin in the evolution of axial polarity and germ layer segregation[END_REF] probably reflecting an ancient role in animal body axis specification (Petersen and Reddien, 2009). The critical role of Wnt signaling in axial patterning is well illustrated during Clytia embryogenesis. In the future oral half of the early embryo, the ligand Wnt3 and receptor Fz1, both synthesized from animally localized maternal mRNAs, activate the Wnt/b-catenin ('canonical') pathway to cause stabilization and nuclear localization of the transcriptional co-regulator b-catenin, while the negatively acting receptor Fz3 blocks this pathway in the future aboral territory (Momose et al., 2008;Momose and Houliston, 2007). Wnt-dependent gene transciption in the oral territory shows two main expression patterns, one involved in axial patterning through maintained expression at the oral tip throughout larval development, and the other distinguishing various ingressing cell populations including presumptive endoderm cells but also the i-cell progenitors (Lapébie et al., 2014).

Using molecular markers for i-cell derivatives we mapped their appearance and differentiation patterns in relation to the developing OA axis of embryos and larvae. We then assessed the influence of Wnt/b-catenin signaling on different aspects of i-cell development, achieving inhibition of this pathway by injection into the egg of Morpholino oligonucleotides targeting Wnt3 (Wnt3-MO), previously demonstrated to prevent bcatenin cytoplasmic accumulation and nuclear localisation (Momose et al., 2008). We also used in some experiments the pharmacological inhibitor PKF118-310, which interferes with the interaction between b-catenin and the conserved transcription factor Tcf/LEF, required to regulate gene expression via this pathway [START_REF] Lepourcelet | Small-molecule antagonists of the oncogenic Tcf/beta-catenin protein complex[END_REF].

Materials and Methods

Manipulation of Eggs and Embryos

Eggs obtained by light-induced spawning of laboratory-raised medusae were microinjected with Wnt3-MO prior to fertilization as described previously (Momose and Houliston, 2007). At the four or eight cell stage, any unfertilized or abnormally dividing embryos were removed. Planulae were fixed for in situ hybridization after 24, 48 or 72 hours of culture (hpf for hours post ferilization) at 18°C in Filtered Artificial Sea Water (FASW; Red Sea Salt brand). Blastula bisection and blastomere separation were performed using fine Tungsten wire loops in agar-lined petri dishes as described previously [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]. PKF118-310 was used at 0.9 µM in FASW, diluted from a 15 mM stock immediately before use.

Gene cloning and in situ hybridization

In situ hybridization probes were synthesized from cDNA clones retreived from our EST collection when available. For the remaining sequences, cDNAs were cloned by PCR using the pGEM-T Easy system (Promega). All sequences were verified before probe synthesis.

DIG-labeled and Fluorescein-labeled antisense RNA probes for in situ hybridization were synthesized using Promega T3/T7/Sp6RNA polymerases, purified using ProbeQuant G-50 MicroColumns (GE Healthcare) and taken up and stored at -20°C in 100 ml of 50% formamide.

In situ hybridization was performed using the InsituPro robot (Intavis) as described previously (Lapébie et al., 2014).

Immunofluorescence and nematocyte staining

Immunofluorescence was performed as previously described (Amiel and Houliston, 2009).

RFamide positive neurons were visualized using a commercial FMRF-amide antibody (Immunostar, final dilution of 1:800) overnight at 4 °C, followed by a rhodamine coupled anti-rabbit IgG (Sigma, final dilution 1:100) for 3 h at room temperature, with Hoechst dye 33258 (Sigma, 0.3 μg/ml final) included to stain DNA. Specimens were mounted in Citifluor mounting medium (Citifluor Ltd) and imaged using a Leica SP5 confocal microscope. The nematocyte staining protocol was adapted from previous studies [START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF][START_REF] Wolenski | Characterizing the spatiotemporal expression of RNAs and proteins in the starlet sea anemone, Nematostella vectensis[END_REF]. Samples were fixed for 1 h in 3.7% formaldehyde/PBS at room temperature, then washed 3 times in PBS for 10 minutes. They were incubated for 30 min in 50 μg/ml DAPI (from a 1mg/ml stock) in PBS. Samples were rinsed 3 times and mounted in Citifluor. Staining was observed by epifluorescence on a Zeiss Axiovert 200 microscope (excitation 450-490 nm, emission >515 nm).

Sequence analysis

In order to assign orthology to newly-identified Clytia genes, we performed phylogenetic analysis for the multigenic families, retrieving homologous protein by reciprocal blast from our transcriptome collection and from publicly available genomic database (NCBI). We checked the domain composition characteristic to the protein family using EXPASY PROSITE domain prediction tools on-line, and added sequences to previously published alignments when available. Species were selected to represent the different taxa (Deuterostomes, Lophotrochozoa, Ecdysozoa) and the cnidarians Hydra vulgaris (Hydrozoa) and Nematostella vectensis (Anthozoa). Sequences were aligned using MUSCLE (Edgar, 2004)at http://www.ebi.ac.uk/Tools/msa/muscle/. Maximum Likelihood analysis using the MPI version of PhyML 2.4 was run on the Observatoire de la Cote d'Azur calculation cluster using the LG amino-acid substitution model {Le:2008fp}.

Gamma shape parameters and the proportion of invariable sites were estimated, and 4 substitution rate categories were used. Phylogenetic trees are available in File S2. For Antistasin and Tudor-containing proteins, whose domain sequence alignment was unsatisfactory, BLAST and domain analysis identified putative orthologues in the closely related species Hydra vulgaris.

Accession numbers for the newly identified sequences are given in Table S3. This table also lists Hydra magnipapillata and Hydra vulgaris orthologues of these genes, and the number of matching reads from the transcript dataset (HAEP) available at http://compagen.zoologie.uni-kiel.de/blast.html [START_REF] Hemmrich | Molecular signatures of the three stem cell lineages in Hydra and the emergence of stem cell function at the base of multicellularity[END_REF] for separated cell populations (endoderm, ectoderm and nanos-positive cells), normalized with respect to total read number.

(Figure 1N), however those in the "vegetal" group showed no detectable i-cells in 9/18 cases (Figure 1P). These data indicate that i-cell formation in the vegetal region is Wnt3 dependent. Development of i-cells in 50% of half-planulae in the "vegetal group" could be accounted for by contamination with animal material including germ plasm at about this frequency (see above).

Similar results were obtained following separation of individual blastomeres from non-injected and Wnt3-MO embryos at the 8-cell stage. For each embryo, 4 blastomeres (derived from the egg animal hemisphere) will contain germ plasm and the other 4 will be germ plasm free in most cases [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]. This blastomere-separation approach has the advantage of more reliable separation of germ plasm from non-germ plasm regions, but the disadvantage that it is not possible to distinguish animal from vegetal blastomeres due to a lack of polarity landmarks at the 8-cell stage. In the experimental conditions used for this experiment, 1 or 2 of the 8 mini-embryos derived from each 8-cell stage embryo failed to develop an OA axis, as assessed by elongation and directional swimming, even at 48 hpf (data not shown). This suggests that some vegetal-blastomere derived mini-embryos lacked sufficient "organizer" activity to re-establish an oral peak of Wnt/b-catenin signaling necessary for axis recovery. We observed a correlation between axis formation and i-cell development; Piwi-expressing cells could clealy be detected in 36/51 (70%) of mini-embryos derived from uninjected 8-cell stage embryos, which had all attained an elongated morphology 24 hpf , while the remaining mini-embryos showed spherical morphology and no i-cells could be clearly distinguished (Table 2, experiment 1) .

For mini-embryos derived from Wnt3-MO embryos, which were all spherical, the number that developed Piwi-positive i-cells was further reduced to 56% (31/55 in Table 2,expt1; Figure 1Q-T). The most likely explanation for this result is that the mini-embryos generated from germ plasm-containing animal blastomeres can develop i-cells despite the lack of OA polarity, while those generated from vegetal blastomeres cannot.

Taken together these results indicate that two distinct mechanisms can generate icells during Clytia larval development. During normal development Wnt is not required, likely because of fate determination by inherited germ plasm. In fragments lacking germ plasm, however Wnt signaling is required for a distinct i-cell specification process, either directly or indirectly through its role in re-establishing the OA axis.

Sequential specification of i-cell derivatives during larval development

To address the involvement of Wnt signaling in cell fate choice from i-cell pool, we developed a panel of gene markers for likely i-cell derivative cell types, focusing on the known i-cell derivatives in Hydra: gland cells, nerve cells and nematocytes (Figure 2; see Supplementary file S1 for expression patterns of these genes in adult medusae) and germ cells (Figure 3).

As a gland cell marker we adopted the Clytia orthologue of the Hydra gene Antistasin, a serine protease inhibitor suggested to protect endodermal cells from their own secreted digestive enzymes [START_REF] Holstein | The primitive metazoan Hydra expresses antistasin, a serine protease inhibitor of vertebrate blood coagulation: cDNA cloning, cellular localisation and developmental regulation[END_REF], whose expression in the Clytia medusa was strongly detected in the digestive part of the manubrium (Supplementary file S1). In Hydra all endodermal gland cells (mucous gland cells and zymogen cells) are i-cell derived (Siebert et al., 2008), however some mucous cells in the Clytia larva may not be (Martin and Thomas, 1981a). During larval development we could detect weak Antistasin expression as early as 24 hpf, ie in the very early planula just completing gastrulation (Figure 2A-C). Expression was largely confined to the central presumptive endoderm region consistent with expression in cells that participate in the formation of the central endodermal cavity by autolysis.

To follow nematocyte formation we used as a marker the Minicollagen 3/4a gene, which codes for a structural component of the nematocyst capsule [START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF].

Minicollagen 3/4a mRNA could be detected from 24 hpf through larval development in the nematoblasts and differentiating nematocytes in the central part of the endodermal region, but not during later stages of differentiation and migration into the ectoderm (Figure 2D-F).

Finally, to characterise neurogenesis we identified the Clytia orthologue of the Hydra paired-class homeobox gene Prdl-a (phylogenetic analysis in Supplementary file S2), expressed in subset of i-cell derived neural cells occupying the head region of Hydra and Podocoryne polyps [START_REF] Gauchat | prdl-a, a gene marker for Hydra apical differentiation related to triploblastic paired-like head-specific genes[END_REF][START_REF] Reya | Wnt signalling in stem cells and cancer[END_REF]. In Clytia medusae, Prdla was similarly expressed in a population of cells with neural-like morphology in the manubrium ectoderm and in the proximal part of the the tentacle bulb ectoderm (Supplementary file S1). During larval development we could first detect Prdl-a expression in the endodermal region at 36 hpf (not shown), its expression at 48 hpf and 72 hpf being slightly less concentrated towards the oral pole (Figure 2G-I). We found that a basic Helix Loop Helix (bHLH) gene related to the Drosophila neurogenic gene Achaete Scute, Asc-b,

No evidence for germ cell precursor formation during larval development

In hydrozoans, differentiating germ cells detected by morphological criteria have only been described in adult stages (medusae or polyps, depending on the species life cycle), either in the gonads or in some cases migrating towards them [START_REF] Künzel | Migration and differentiation potential of stem cells in the cnidarian Hydractinia analysed in eGFP-transgenic animals and chimeras[END_REF]Momose and Houliston, 2007). To address whether committed germ line precursors might already be generated during larval stages, we sought to identify early-expressed molecular markers of the germ line in Clytia by testing in adult medusae the expression of six candidate genes identified in our C. hemisphaerica transcriptome (Lapébie et al., 2014;[START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF] Drosophila (by the original Tudor gene) and mammalian spermatogenesis [START_REF] Pek | Tudor domain proteins in development[END_REF]Thomson and Lasko, 2005). The two Clytia Tudor Domain Proteins TDP-1 and TDP-2 that we tested contain four or six Tudor domains respectively, as well as one RING domain and one BBOX protein domain for TDP-1, while TDP-2 contains two binding sites for dimethylated arginin providing a putative binding site for Piwi proteins. The Clytia Thyn1

gene was selected for characterization due to the i-cell restricted expression of the Hydra ortholog Hmp_06895 [START_REF] Hemmrich | Molecular signatures of the three stem cell lineages in Hydra and the emergence of stem cell function at the base of multicellularity[END_REF][START_REF] Hwang | The evolutionary emergence of cell typespecific genes inferred from the gene expression analysis of Hydra[END_REF]. Thyn1 (For Thymocyte nuclear protein 1) proteins show wide sequence conservation of the "EVE" putative RNA binding motif among animals and plants and even bacteria (Phylogeny in Supplementary File S2), but little is known about their function, except for the regulation of apoptosis and cell cycle progression in thymocytes by mammalian and avian Thyn1 proteins [START_REF] Miyaji | Molecular cloning and characterization of the mouse thymocyte protein gene[END_REF]Momose et al., 2008;[START_REF] Toyota | Thy28 partially prevents apoptosis induction following engagement of membrane immunoglobulin in WEHI-231 B lymphoma cells[END_REF].

In situ hybridization patterns in adult medusae (Figure 3) identified two of the six candidate markers tested, Clytia Boule1-a (Fig. 3A) and Thyn1 (Fig. 3F) as early germ line makers. Expression of these two genes was detected exclusively in developing male and female germ cells and in the region of the adult gonad close to the bell where i-cells and the early stages of germ cell development are most concentrated (Amiel and Houliston, 2009;[START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF]. Germ-line restricted expression of Clytia Boule1-a is consistent with a highly conserved germ line role across animal species, whereas the Clytia Thyn1 expression was unexpected given its wider i-cell expression in Hydra [START_REF] Hwang | The evolutionary emergence of cell typespecific genes inferred from the gene expression analysis of Hydra[END_REF]. Expression of , TDP-1 and TDP-2 was also detected in male and/or female germ cells but also in other sites, notably in the i-cell rich region at the base of the tentacle bulbs (Figure 3C, D, E), suggesting wider or multiple developmental roles. Boule like1-a was not expressed in the gonad, while Boule like1-b was expressed in developing female but not male germ cells (Figure 3B).

No expression of either of the putative germ line specific genes that we identified, Boule1-a or Thyn1, was detected in planula larvae, even at 72 hpf when the larvae are fully developed and competent to metamorphose. These analyses thus support a model in which commitment of i-cells to a germ cell fate is delayed until adult stages in the Clytia life cycle.

Wnt signaling is not required for i-cell somatic fate choices

We tested the requirement for wnt signaling in somatic cell type generation from icells using the molecular markers characterised above (Figure 4) as well as other staining techniques to detect differentiated neurons and nematocytes (Figure 5). In Wnt3-MO larvae, Antistasin (Fig. 4A-B), Prdl-a (Fig. 4E-F stages is also prevented (Momose et al., 2008), but we cannot exclude the possibility that other Wnt ligands function during later periods of larval development. To address this possibility we treated larvae from 24 hpf to 48 hpf with the TCF/ß-catenin interaction inhibitor PKF118-310 (Figure 6). In PKF118-310 treated larvae, Piwi (Fig. 6B) and

Minicollagen 3-4a (Fig. 6D) expression was detectable by in situ hybridization at 48 hpf as it was in non treated controls (Fig. 6A,C) and in Wnt3-MO larvae (Fig. 1F and Fig. 4D),

showing that Wnt signaling is dispensable for maintenance of i-cells and nematoblast formation. On the other hand RFamide precursor (PP5) and GLWamide precursor (PP2) expression was abolished, suggesting that Wnt signaling acts at later stages of larval development. Assessment of the numbers of final nematocytes by DAPI staining and RFamide expressing cells by immunostaining confirmed this conclusion (data not shown).

These PKF118-310 experiments are consistent with the Wnt3-MO findings, but have to be treated with caution since we cannot rule out non-specific effects of the drug.

Discussion

The hydrozoan planula larva offers an excellent opportunity to undertand how alternative somatic cell types can be generated from a common stem cell pool and also to address the embryological origin of i-cells. In this study we followed successive steps of i-cell development during embryogenesis and larva formation in Clytia, and defined successive distinct roles for Wnt signaling. We found that Wnt signaling is not essential either to establish or to maintain the i-cell pool during normal development, nor for them to engage in somatic fate pathways via the formation of nematoblasts, neuroblasts or gland cells.

Wnt/ß-catenin inhibition did, however, prevent terminal differentiation of nematocytes and some ganglionic cell types. We also uncovered another significant feature of i-cell regulation which requires, directly or indirectly, Wnt signaling: their formation by a 'recovery' pathway activated in the absence of maternal germ plasm from the egg animal pole.

The two principal areas addressed by our study are discussed in more detail below: Wnt signaling is not required to specify i-cell somatic fates but for their differentiation.

Our conclusion that the appearance during Clytia larva development of precursor cells of various somatic i-cell derivatives does not require Wnt signaling, but that final differention of nematocytes and some neural cell types does so, is compatible with most of the observations from previous studies in Hydractinia and Hydra. We found a strong reduction in the number of differentiated nematocytes in Clytia Wnt3-MO and PKF118-310 treated embryos, while in polyps of both the other species, treatment with paullones to stimulate b-catenin accumulation by blocking GSK-3b resulted in a large excess of developing and mature nematocytes [START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF][START_REF] Teo | An evolutionary conserved role of Wnt signaling in stem cell fate decision[END_REF]. In Hydra polyps the increase in late stages of nematocyte differentiation induced by alsterpaullone was found to occur at the expense of cells expressing the "nematoblast nest" marker nb035.

We can thus propose that in both these species of hydrozoan polyp as well as the Clytia planula larva, Wnt signaling drives the terminal steps of nematocyte differentiation. Wnt expression in the planula ectoderm is organised as overlapping/nested domain of several ligands (Momose et al., 2008), which could produce an oral-aboral descending gradient of Wnt/b-catenin activity. The oral pole peak coincides with a concentration of nematocytes at this site [START_REF] Bodo | Etude histologique du développement embryonnaire de quelques Hydroméduses de Roscoff[END_REF]Plickert et al., 2012) (Figure 5). Likewise in Hydra it has been proposed that Wnt activity promotes nematocyte differentiation relating to hyper-development of mouth and tentacle structures at the oral tip of the polyp where this pathway is locally favored [START_REF] Bosch | Decision making in interstitial stem cells of Hydra[END_REF][START_REF] Hobmayer | WNT signalling molecules act in axis formation in the diploblastic metazoan Hydra[END_REF][START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF][START_REF] Lengfeld | Multiple Wnts are involved in Hydra organizer formation and regeneration[END_REF].

While all three studies of Wnt function during i-cell development support a stimulatory role for Wnt signaling in nematogenesis, interpretations vary concerning which steps are being regulated. The Hydra study [START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF] and our Clytia data indicate that neither maintenance of the multipotent i-cell pool nor initial entry into the nematogenesis pathway are regulated by Wnt signaling. In apparent contradiction, transient stimulation of cell proliferation and b-catenin nuclear localization in cells of the endodermal region was observed in alsterpaullone-treated Hydractinia larvae and polyps [START_REF] Teo | An evolutionary conserved role of Wnt signaling in stem cell fate decision[END_REF], but it is possible that the cell population affected by Wnt pathway stimulation in that study was not restricted to the i-cell lineage. The delay we observed in Minicollagen 3-4a expression in Wnt3-MO Clytia embryos also appears at first sight to contradict the hypothesis that early nematogenesis steps are Wnt-independent, but may reflect the general delay in zygotic gene expression in i-cells and other cells that ingress during gastrulation when the translation of Wnt3, Fz1 or the PCP core component gene Strabismus are disrupted (Lapébie et al., 2014).

As for nematogenesis, Wnt signaling was found to affect final differentiation but not early regulatory steps during i-cell derived neurogenesis. In Clytia, Prdl-a expression occurred on schedule in Wnt3-MO embryos, but we observed cell-type specific effects on neuronal differentiation. The sensitivity of these cell types correlates with their distribution along the OA axis. Thus RFamide ganglion cells formation along the sides of the Clytia planula, may be stimulated by "higher" values of a Wnt gradient in the ectoderm along the OA axis, as may the RFamide neurons preferentially associated with tentacles and other oral structures in Hydractinia polyps [START_REF] Teo | An evolutionary conserved role of Wnt signaling in stem cell fate decision[END_REF]. Conversely development of aboral localized neurons such as GLWamide expressing sensory neurons in the Clytia planula does not seem to be sensitive to Wnt3 down-regulation, although it was disrupted by the TCF pharmacological inhibitor, indicating that GLWamide neuron differntiaton could require a lo Wnt/β-catenin w level of Wnt/β-catenin signaling. In Hydra, paullone treatments had no obvious effects on neuronal populations expressing the peptide Hym355, which is expressed extensively along the body column [START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF], and thus not obviously under the influence of the oral Wnt source [START_REF] Hobmayer | WNT signalling molecules act in axis formation in the diploblastic metazoan Hydra[END_REF][START_REF] Lengfeld | Multiple Wnts are involved in Hydra organizer formation and regeneration[END_REF]. Oral specific effects of Wnt signaling on neural differentiation could well be mediated by peptides secreted by the epithelial cells that could affect interstitial cell differentiation by inhibiting commitment or migration of nerve precursor cells [START_REF] Bosch | Ancient signals: peptides and the interpretation of positional information in ancestral metazoans[END_REF][START_REF] Watanabe | Immortality and the base of multicellular life: Lessons from cnidarian stem cells[END_REF]. In Hydra, Hym-355 (a PRGamide) produced by neurons stimulates neural differentiation while -PW family peptides secreted by epithelial cells inhibit it [START_REF] Takahashi | Important roles for epithelial cell peptides in Hydra development[END_REF]. Wnt3/β-catenin Finally, endodermal digestive cell differentiation, as assayed by Antistasin expression, was not affected in Clytia Wnt-3 MO embryos. Since Wnt expression in the planula is restricted to the ectoderm, the endodermal region may be largely out of the influence of Wnt signaling both for early and late stages of i-cell development.

To conclude, studies of Wnt signaling in hydrozoan polyps and larvae can be reconciled according to a model in which Wnt signaling stimulates the final differentiation of nematocytes (normally concentrated at the planula oral pole and in polyp oral tentacles), and also neuronal differentiation in ectodermal ganglion cells. In future experiments it would be interesting to address the signaling pathways responsible for the i-cell regulation processes occuring in the endodermal region of the planula; drug experiments in Hydra suggest that the Notch pathway is involved in nematoblast specification [START_REF] Käsbauer | The Notch signaling pathway in the cnidarian Hydra[END_REF][START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF] while secreted peptides may be involved in the feedback regulation that maintains the i-cell pool [START_REF] Steele | Developmental signaling in Hydra: what does it take to build a "simple" animal?[END_REF]. Cnidarian-specific secreted peptides and 7tm receptors expressed preferentially in the cells that ingress during gastrulation (including i-cells) (Lapébie et al., 2014) provide candidates for these signals.

Two mechanisms for i-cell fate determination during Clytia embryogenesis

Previously we showed that germ plasm including Vasa, PL10, Nanos1 and Nanos2 mRNAs is positioned around the female pronucleus at the egg animal pole. In this position it is inherited by the i-cell precusors, although inheritence by this lineage has not been demonstrated. This would require tracking of the germ plasm in live animals, perhaps in the future by gene editing to tag UTRs of the mRNA allowing their detection with fluorescent binding proteins, for instance using the MS2 system [START_REF] Bertrand | Localization of ASH1 mRNA particles in living yeast[END_REF]. In this study we provide the first experimental evidence that germ plasm has i-cell fate determining activity in Clytia. We found that i-cells could develop in whole embryos and in (germ plasm containing) animal embryo fragments whether or not Wnt signaling was inhibited, and thus irrespective of OA axis development. In contrast, i-cells did not form from (germ plasm free) vegetal regions if Wnt signaling was inhibited. The most simple explanation for this result is that the Wnt independent i-cell formation is driven by germ plasm inherited from the egg animal pole.

Although we can thus be reasonably confident that germ plasm participates in determining i-cell fate during normal embryogenesis, clearly an alternative mechanism is available to account for i-cell formation from vegetal fragments. Germ line specification in many animals is delayed until late stages of embryogenesis, despite deriving in undisturbed embryos from an identifiable cell lineage (Juliano et al., 2010). The sea urchin embryo provides an example in which plasticity of the germ line lineage has been demonstrated experimentally; germ line cells normally derive from the small micromere lineage (which actually generates the entire adult primordia ahead of larval metamorphosis) but when the small micromeres are removed, regulatory mechanisms allow fertile adults to develop. The Clytia example is, however, the first case to our knowledge of germ line restoration following precursor ablation in an animal where "germ plasm" in undisturbed conditions contributes to determining the fate of the germ cell lineage. The germ plasm in this case contains aggregates of mRNAs from several germ lines genes (Vasa, PL10, Piwi, Nanos1, Nanos2) and also probably the Vasa protein as observed in Hydractinia [START_REF] Rebscher | The germ plasm component vasa allows tracing of the interstitial stem cells in the cnidarian Hydractinia echinata[END_REF], but it is important to note that these mRNAs are also present throughout the embryo in a less organised state, which may facilitate formation of i-cells following animal pole removal [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]Momose et al., 2008). A variety of post transcriptional mechanisms may contribute to mRNA and protein accumulation during this process, as described for sea urchin Vasa and Nanos [START_REF] Gustafson | Developmental Biology[END_REF][START_REF] Oulhen | The 3'UTR of nanos2 directs enrichment in the germ cell lineage of the sea urchin[END_REF][START_REF] Yajima | Thyn1 Phylogenetic analysis of metazoan Eve domain proteins. Thyn1 proteins are highly conserved, identifiable in both bacteria and mammals. Amino acid sequences from Aplysina californica (Ac , Capitella teleta (Cte); Saccoglossus kowalevskii (Sk); Lottia gigantean (Lgi); Xenopus tropicalis (Xt); Trichoplax adherens (Tca); Gallus gallus (Gg); Mus musculus (Mm); Clytia hemisphaerica (Che); Hydra vulgaris (Hv); Homo sapiens (Hsa); Nematostella vectensis (Nv), were identified by reciprocal blast. The following sequences containing Eve domain were used for phylogenetic analysis[END_REF].

In Clytia, i-cell regeneration was found to be Wnt3 dependent, both from vegetal blastula halves and from isolated 8-cell stage blastomeres. This could mean that Wnt/b-catenin signaling directly stimulates i-cell formation, and/or that it does so indirectly by allowing restoration of the OA axis via formation of an oral organizer region, as characterised in Hydra polyps (Broun, 2005;[START_REF] Hobmayer | WNT signalling molecules act in axis formation in the diploblastic metazoan Hydra[END_REF]. New transcription of the germ plasm genes would not be required, at least initially, since their maternal mRNAs are present in a diffuse form in the vegetal region as well as in animal pole aggregates [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF].

Our findings in Wnt3-MO embryos were reinforced by the observation that some vegetal blastomeres from uninjected 8-cell stage embryos failed to recover OA axes and also to "regenerate" i-cells. This failure of some vegetal blastomeres to recover axial polarity was somewhat surprising given the high recovery rates reported previously from similar experiments in C. gregaria (Freeman, 1981) and C. hemisphaerica [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]. It partially resembled the results obtained in Podocoryne carnea [START_REF] Momose | Animal pole determinants define oral-aboral axis polarity and endodermal cell-fate in hydrozoan jellyfish Podocoryne carnea[END_REF] or Nematostella vectensis [START_REF] Fritzenwanker | Early development and axis specification in the sea anemone Nematostella vectensis[END_REF][START_REF] Lee | Asymmetric developmental potential along the animal-vegetal axis in the anthozoan cnidarian, Nematostella vectensis, is mediated by Dishevelled[END_REF], in which axis restoration capacity is animally localized. Axis re-patterning ability in C. hemisphaerica may be sensitive to variation in experimental/culture conditions. This ability depends initially on the balance between the pathway activators Wnt3 and Fz1 provided from animal localized maternal mRNAs, and the inhibitory Fz3 from vegetally localized mRNAs (Momose et al., 2008;Momose and Houliston, 2007). Subsequently secreted factors produced orally and aborally are proposed to act to maintain polar identities [START_REF] Lapébie | Differential Responses to Wnt and PCP Disruption Predict Expression and Developmental Function of Conserved and Novel Genes in a Cnidarian[END_REF]. The failure of vegetal fragment to form an oral pole could thus reflect a failure of residual Wnt3/Fz1 signaling to overcome Fz3-mediated inhibition of oral identity during these processes. Whatever the reasons, this failure provided us with a useful confirmation of the correlation between axis re-patterning and i-cell regeneration capacities in the Clytia larva. Tables (positively regulated by Wnt3) from which the top 20 of the list were chosen for further analysis (Figure 43, full list of transcripts identified PAPER 2, file S1).

Polarised expression pattern of Wnt3 target genes

In situ hybridisation experiments were performed for the selected transcripts at different developmental stage:

-Early gastrula stage, the same stage that was used to harvest mRNA originally for the differential gene expression analysis.

-24 hpf planula, when gastrulation is completed, polarity is completely established and presumptive endoderm cells ingressed but endoderm is not yet differentiated.

-48hpf planula in which ectoderm and endoderm are both well formed and all the differentiated somatic cell types are present.

In situ hybridisation analysis revealed 4 types of spatio-temporal expression profiles, allowing the genes to be grouped in the following 4 categories: The DGE approach was entirely validated by our systematic in situ hybridisation analysis.

The positive "targets" (direct or indirect) of Wnt3 exclusively had the O and IE expression patterns (oral), and the negative "targets" of Wnt3 exclusively had the A/D patterns (aboral or delayed). C) Summary table describing the different categories identified by comparing expression profile of the differentially expressed genes. Amongst the total number of genes differentially expressed, 114 were significantly downregulated in Wnt3 morphants, and 40 significantly upregulated in Wnt3 morphants. The biological validation by in situ hybridisation and selected qPCR (not shown) of the more sensitive "top"20 and "top"18 respectively downregulated and upregulated in Wnt3 morphants, allowed us to define four different expression profile groups: O (oral), IE (ingressing endoderm), A(aboral), D( delayed) based on the expression profile. Among the IE genes, 5 genes show an expression profile reminiscent of the germ cell /stem cell genes Nanos1 and Piwi. However Nanos1,Piwi expression was not significantly affected by Wnt3 disruption.

IE genes show an i-cell like expression pattern

Some of the genes identified as positively regulated by Wnt3 showed an expression pattern reminiscent of the classical i-cell markers Nanos1 and Piwi [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]) and so were of particular interest in my study. Since Fox-A is considered a conserved endodermal marker [START_REF] Fritzenwanker | Analysis of forkhead and snail expression reveals epithelial-mesenchymal transitions during embryonic and larval development of Nematostella vectensis[END_REF][START_REF] Oliveri | Repression of mesodermal fate by foxa, a key endoderm regulator of the sea urchin embryo[END_REF]) I considered it unlikely that this gene would be associated with i-cells.

Different responses of Wnt3-MO responsive genes to Fz1-MO

We further analysed the response of the transcripts set defined above in Fz1 morphants.

The responses were not strictly correlated, implying that Fz1 down-regulation has effects both relating to the Wnt/β-catenin pathway and independent of it. Specifically, we found that the genes with "axial expression" profiles, A and O, were weakly affected in early gastrula Fz1 morphants, while the genes in IE and D expression profile group were more strongly sensitive to Fz-1 MO. In the IE category, genes tend to be strongly down-regulated in Fz1-MO early gastrula and D category genes to be up-regulated in Fz1 morphants, while "axial genes" (O and A) are less affected (Paper 2 Figure 7). We were further able to show that this differential response was due to the sensitivity of certain genes to PCP disruption, since Fz1 is a PCP core component. To do that we compared the transcriptome profiles from Fz1 morphants and Stbm morphants as Stbm is exclusively involved in Fz/PCP but not in Wnt/β-catenin signalling, and found that the responses of all 4 categories of genes were very similar under these two conditions.

Additional Results: Characterisation of putative novel i-cell genes underexpressed in Wnt3 morphant early gastrulae.

• Znf845 and Mos3 are not expressed in Nanos1 and Piwi expressing i-cells

The Znf845 and Mos3 genes both showed expression territories very similar to Piwi and Nanos1 expression at different stages of planula development (Figure 44A). To examine whether Znf845 and Mos3 are expressed in multipotent i-cells I assessed the expression of these two transcripts in the medusa in the gonads, in the base of the manubrium [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF] and in tentacle bulbs (the proximal part close to the umbrella) [START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF] where i-cells are concentrated.

Unlike Piwi and Nanos1 neither Znf845 nor Mos3 mRNAs could be detected in the gonads or in spawned eggs (Figure 44B). The four genes were expressed in the tentacle bulbs and in the manubrium, although their patterns were slightly different (Figure 44B).

Znf845 expression was also detected in tentacle bulbs and in the base of the manubrium, as is the case of Mos3. However Znf845 expression in the tentacles bulbs was detected in two patches of cells located between endoderm and ectoderm in a similar position of Nanos1 and Piwi (Figure 44B). In contrast, in the manubrium Znf845 mRNA was detected in fewer cells than Nanos1 and Piwi mRNA, localised in the ectoderm at the base of the manubrium, Mos3 was strongly detected in the tentacle bulb ectoderm in wider domain in comparison in Nanos1 and Piwi, and in ectodermal cells in the manubrium (Figure 44B). These expression profiles suggest that in medusa this gene is expressed in a more extensive cell population than the i-cells.

The absence of Znf845 and Mos3 expression in the gonads means that neither gene is expressed in all i-cells. Thus I investigated the possibility that Znf845 and Mos3 are expressed in i-cells, committed to a somatic fate, by co-localisation studies using either Mos3 or Znf845 probes in combination with Piwi and with the somatic early precursors (e.g. Antistasin).

• Znf845 is expressed in i-cells subpopulation

Znf845 is a conserved zinc finger domain protein. Znf845 genes have been little studied and their role has not previously been investigated. The Hydra Znf845 ortholog is highly expressed in Nanos positive cells (i-cells) and it s transcript is thus strongly represented in the i-cell population [START_REF] Hemmrich | Molecular signatures of the three stem cell lineages in Hydra and the emergence of stem cell function at the base of multicellularity[END_REF]) (Table1 Annexe).

My co-localisation studies in Clytia using Znf845 probe in combination with Piwi probe showed that a subpopulation of Piwi expressing cells also expressed Znf845, but no cells could be identified expressing Znf845 alone (Figure 48C-D). By quantification of a total of 5 planulae I found that around 50% of the Piwi positive cells also expresses Znf845, although the total number of cells was variable between different planula (Table 1). These observations suggest that the Znf845 gene may be expressed in a subpopulation of i-cells committed to a somatic cell fate corresponding to half of the total Piwi positive population.

To narrow down the possible identity of the cells expressing Znf845, I compared its expression pattern in larval development and in the medusa to that of known i-cell derivative markers characterised in Results-I.

• Znf845 does not co-localise with Antistasin in medusae

Co-localisation studies were performed at the medusa stage, when Antistasin expression is much stronger then in the planula. As showed in Result I, Antistasin expression is restricted to gland cells, positioned in the manubrium endoderm, distributed along the four ridges of the mouth. Znf845 expression in medusae was detected in the ectoderm in a few cells showing i-cell like morphology (Figure 46A). No cells expressing both probes could be identified (Figure 46C). Furthermore these cells were located in two different germ layers: Antistasin-expressing cells were located in the endoderm, while Znf845expressing cells were located in the ectoderm at the base of the manubrium. However since i-cells are highly migratory cells it is possible that the few ectodermal cells, could correspond to gland cell early precursors that then migrate into the endodermal layer before differentiation. The absence of detectable co-localization between Znf845 and Antistasin expression does not allow to be ruled out this possibility.

• No Znf845 expression in nematoblasts

Co-localisation studies using the Mcol3-4a probe in combination with Znf845 at 24hpf revealed only cells expressing either Znf845 or Mcol3-4a (Figure 41D). The cells expressing Znf845 and Mcol3-4a are clearly distinct although they are localised in a similar region of the planula endoderm. Again, it remains possible that Znf845 is involved in the specification of early nematoblasts, despite the lack of co-localisation, given its early expression at early gastrula. To test this idea I performed co-localisation studies using Znf845 in combination with Sox15 (Result 1) at the early gastrula stage and at 48hpf. The early gastrula experiment unfortunately failed to give any reliable results (probably due to the timing of the fixation of the embryos) while at 48hpf no co-expression was observed (Figure 47). From these preliminary observations I can conclude only that if Znf845 is involved in nematogenesis, it is expressed only transiently in early precursors. To address this hypothesis it would be necessary to repeat the double in situ hybridisation experiment at the early gastrula stage. Another possibility is that Znf845 is expressed early during development of the neuronal lineage. To distinguished between these hypothesises it would be useful to perform functional studies by morpholino injection, followed by assaying using gene marker expression to determine whether nematoblasts, nematocytes and/or other somatic cells types are affected by Znf845 down-regulation. The study presented in this thesis should be placed in a wider context of stem cell biology.

An important aim of stem cell research is to understand the intrinsic and extrinsic mechanisms regulating proliferation and differentiation of multipotent undifferentiated stem cells. Knowledge in this field has come from a broad range of model organisms as well as from in vitro stem cells systems like mammalian ES cells. Among these models are hydrozoan i-cells, which have long been appreciated to provide a great opportunity to study the mechanisms responsible for fate choices and differentiation signals from a multipotent stem cell population [START_REF] Bode | The interstitial cell lineage of hydra: a stem cell system that arose early in evolution[END_REF][START_REF] Frank | Stem Cells in Marine Organisms[END_REF]Plickert et al. 2012). Previous studies in the Clytia medusa have further demonstrated that this "full life-cycle" species could be an attractive model for the study of ordered nematogenesis from i-cells in the tentacle bulb [START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF]Jager, et al. 2011). My work has focussed on developing the Clytia larva as a model system to experimentally address the embryonic origin of i-cells, as it is more accessible than the medusa for analysing gene function (via microinjection of Morpholinos /synthetic RNAs etc. into the egg). I hope that the work reported here has illustrated the potential of this system to provide new insights into metazoan stem cell research.

The particular focus of my project was to address the impact of Wnt/β-catenin signalling on i-cell dynamics during Clytia larval development; I use the term "dynamics" here to include i-cell formation during embryogenesis, as well as fate specification and differentiation of i-cell derivatives.

To unravel the effects of Wnt/β-catenin signalling on i-cells dynamics I first had to determine the distribution of i-cells and their derivatives. To do this I characterised molecular markers for each i-cell derived cell type lineage. I then exploited these molecular markers to outline a spatial and a temporal map of i-cell derivatives during larval development.

The data collected through this study provided a first framework for investigating several different aspects relating to the origin of i-cells and of their derivatives. In the first section of this general discussion chapter, I will focus on the origin and the regulation (in particular by Wnt/β-catenin signalling) of the cells that make up the hydrozoan nervous system: nematocytes and nerve cells. The second section is dedicated to the embryological 12). The colours correspond to the genes listed in E. Bi-coloured dots indicate co-localisation. At 36hpf only the expression of the putative neural genes was verified. E). Table summarising the cell types and the assigned markers.

One possible candidate regulatory transcription factor that could be expressed in bi-potent progenitors or be involved exclusively in the nematogenesis pathway in Clytia larvae is Sox15. SOX genes have been shown in Clytia to be expressed in developing neurosensory cells and in nematoblasts (Jager et al. 2011). In vertebrates, SOX transcription factors have been widely investigated for their role in pluripotency (see Introduction section 2.3.2). Some have dual functions, for instance mouse Sox2 can regulate both neural stem cell maintenance and nerve cell differentiation in the eye and the brain [START_REF] Wegner | SOX after SOX: SOXession regulates neurogenesis[END_REF].

Bilaterian SOX genes can be classified in five major groups (named B, C, D, E and F), with SoxB genes being commonly expressed only transiently during neurogenesis. Phylogenetic analysis was not able to assign Clytia Sox15 to any of the previously identified SOX gene groups, so the exact identity of Sox15 is still obscure, however it fulfils several criteria to be considered as gene expressed in early neural /nematocyte precursors. As shown in Results-I, section 1.2.2, Sox15 mRNA can be detected at the early gastrula stage in Piwi positive cells, as well as in differentiating nematoblasts in the endoderm, where it is co-expressed with Mcol3-4a (Figure 24). Its expression territory in the larva ectodermal includes differentiating nematocytes but could also include nerve cells. Sox15 is the only Clytia SOX gene to have detectable expression in both germ layers, while the other putative nematogenic SOX markers, Sox14, Sox5 and Sox2 were detected exclusively in populations of endodermal cells (Jager et al. 2011). Furthermore Sox15 expression is Wnt3 dependent at early gastrula stage (Lapébie et al. 2014) SOX gene function has not been studied so far in hydrozoans, but in anthozoans the neural functions of certain SOX genes have been suggested or demonstrated. In the coral Acropora analysis by in situ hybridisation showed the expression of the gene SoxC (which belongs to the SoxC family) in a sub population of ectodermal neurosensory cells [START_REF] Shinzato | Sox genes in the coral Acropora millepora: divergent expression patterns reflect differences in developmental mechanisms within the Anthozoa[END_REF]. A recent study in Nematostella demonstrated that SoxB(2) regulates neurogenesis (Richards & Rentzsch 2014). By using a stable transgenic line in which the SoxB(2) promoter drives the expression of the fluorescent reporter mOrange it was shown that SoxB [START_REF] Hansen | Control of the proliferation versus meiotic development decision in the C. elegans germline through regulation of GLD-1 protein accumulation[END_REF] is expressed in sensory neurons, ganglionic cells and nematocytes (by colocalisation studies using Nematostella minicollagen specific antibody). Furthermore morpholino injection experiments show that SoxB [START_REF] Hansen | Control of the proliferation versus meiotic development decision in the C. elegans germline through regulation of GLD-1 protein accumulation[END_REF] functions upstream in the development of all these three neural cell populations.

To complete our study of Clytia Sox15 in i-cells and to test whether it regulates neurogenesis and/or nematogenesis it would be interesting to perform functional studies using morpholino injection following by in situ hybridisation of neural and nematocyte markers. In addition, double staining with the anti-RFamide antibody could help determine whether Sox15 is also expressed in neural cell types in the planula ectoderm.

Other transcription factor families involved in neurogenesis /nematogenesis should also be investigated. It would be particularly informative to perform co-expression studies, combining for example the neuronal marker Prdl-a (this study) and orthologs of genes mRNA is detected at the early gastrula stage in a population of 'i-cell like' ingressing cells at the oral pole and at larval stages in a few cell located in the endodermal core region, that could correspond to an i-cell sub-population on the way to a particular neuronal fate.

Future research could involve detailed characterisation of genes such as HlhIE1 by As discussed in the Introduction (section 1.4), stem cell maintenance is regulated at several levels, including transcriptional control by the means of transcription factor. In vertebrates a set of transcriptional factors have been well characterised as regulators of embryonic stem cells, including Nanog, Oct4, Klf4 and Sox2, which act to repress somatic differentiation programs. These genes do not have clear ortholog in invertebrates. Event though recent study in Hydractinia revealed the presence of a POU domain-containing gene, the Polynem gene (Pln), which phylogenetic analysis could not assign to a specific POU gene class. Pln ectopic expression in polyp epithelial cells induces the formation of neoplasm, mostly composed by morphologically recognisable Nanos and Piwi expressing i-cells [START_REF] Millane | Induced stem cell neoplasia in a cnidarian by ectopic expression of a POU domain transcription factor[END_REF]. Downregulation by RNAi treatment in polyp increases the number of mature nematocytes. These findings suggest that POU class gene could have a role in stem cell maintenance and in differentiation also in early branch metazoan. Studies in Hydra show that FoxO plays a crucial role in the regulation of stemness [START_REF] Boehm | Migration of multipotent interstitial stem cells in Hydra[END_REF]. The FoxO gene is expressed in all three the stem cell lineage (i.e. ectodermal cells, endodermal cell and i-cells), all along the Hydra body column [START_REF] Boehm | Migration of multipotent interstitial stem cells in Hydra[END_REF]). Overexpression of FoxO in i-cell lineage increases the proliferation ratio and forces stem cell to differentiate into nematocytes as shown by the atypical Piwi expression in fully differentiated nematocytes.

Clytia FoxO [START_REF] Chevalier | Polarised expression of FoxB and FoxQ2 genes during development of the hydrozoan Clytia hemisphaerica[END_REF] is ubiquitously expressed at early stages of embryogenesis and at both poles in the planula. Thus in Clytia too FoxO may control stem cell behaviour, not only in i-cells but more widely in epithelial stem cells. In other biological systems FoxO proteins often play a role in regulating cell proliferation and growth in response to a variety of extracellular signals, notably via the TOR pathway but also Wnt signalling [START_REF] Eijkelenboom | FOXOs: signalling integrators for homeostasis maintenance[END_REF]. A functional characterisation of Our findings suggest that in Clytia Wnt/β-catenin signalling does not prevent specification of nematoblasts or neuroblasts, but impairs the differentiation of nematocytes and of some neural sub-types. Particularly sensitive was the differentiation of ganglion cells expressing a RFamide precursor distributed laterally in the planula between the endoderm and ectoderm epithelial layers, while aboral sensory neurons expressing GLWamide, which are involved in mediating the settlement response, were prevented by β-catenin/TCF pharmacological inhibition but not by Wnt-3 MO (Figure 6 PAPER 1). One possible explanation for this discrepancy is that the drug treatment had non-specific effects, causing loss of all late cell type markers. Following TCF inhibitor (PKF-118) treatment, embryos morphology was not well maintained. To test whether it has a general toxic effect on development, it would be useful to monitor the expression of other lateexpressed genes in drug treated embryos.

On the other hand, it is possible that the inhibition of GLWamide sensory neuron development in PKF-118 treated embryos but not in Wnt3 morphants reflects the requirement for a low level of Wnt/b-catenin signalling for the differentiation of aboral type neurons and a higher dose for oral ones. Under this hypothesis the aboral GLWamide subpopulation would need a lower level of active nuclear b-catenin/TCF to acquire its fate, still available in Wnt3-MO embryos but not following PKF-118 treatment. In this context the Clytia planula O-A Wnt gradient can be compared to the A-P Wnt gradient along the vertebrate neural plate. In vertebrates, an anterior-posterior Wnt gradient is crucial for the specification of positional information along the neural plate. For instance in the fish Medaka this gradient has been shown to be achieved via modulation of Wnt signalling by the inhibitor SRFP1 which defines the anterior neural plate domain. SFRP1 overexpression thus induces expansion of the forebrain territory (anterior neural plate) accompanied by axis duplication and posterior truncation [START_REF] Lopez-Rios | The Netrin-related domain of Sfrp1 interacts with Wnt ligands and antagonizes their activity in the anterior neural plate[END_REF].

Similarly in cnidarians it the oral-aboral Wnt activity gradient specifies positional values [START_REF] Marlow | Ectopic activation of the canonical wnt signaling pathway affects ectodermal patterning along the primary axis during larval development in the anthozoan Nematostella vectensis[END_REF]) that could influence neural differentiation via the production of local cues. In Hydra, neuroblasts after entering the neuronal fate lineage migrate to their site of differentiation, at the oral and oral extremities of the polyp, and their final identity is then specified in relation to their position along the O-A axis by neuropeptides [START_REF] Takahashi | A novel neuropeptide, Hym-355, positively regulates neuron differentiation in Hydra[END_REF][START_REF] Bosch | Symmetry Breaking in Stem Cell of the Basal Metazoan Hydra[END_REF]). One example is the neuropeptide Hym355. Hym355 precursor is expressed in neuronal cell population present both in the foot and in the tentacle. In adult polyps treated with this neuropeptide, the number of nerve cells increases initiating a positive regulation of neural cell differentiation. Other secreted peptides are specifically expressed either in the foot or the head region, and so can influence the regional identity of neuron differentiation.

Our findings in Clytia planulae, along with the stimulation of nematocyte and RFamide nerve cell differentiation by Wnt signalling hyperactivation in Hydra and Hydractinia polyps (discussed also in the section 2.1.1, Result-II), indicate that Wnt signalling may have a role in neural differentiation directly and/or via its role in defining positional values. We can speculate that Wnt signalling could also have an active role in nematogenesis and in neurogenesis in the Clytia medusa, since Wnt3 is expressed, among other sites, at the opening of the mouth and in the nematogenic ectoderm in the tentacle bulbs, (T.

Condamine and M. Jager, personal communication). Involvement of Wnt/β-catenin signalling in the neural lineage has been proposed also from studies in the ctenophore P.Pileus. Expression studies show that Wnt signalling genes are expressed in various neural structures throughout the adult ctenophore body [START_REF] Jager | Evidence for Involvement of Wnt Signalling in Body Polarities, Cell Proliferation, and the Neuro-Sensory System in an Adult Ctenophore[END_REF]. The majority of the Wnt gene expression has been reported in the Polar field, which is an aboral neuro-sensory complex in which a neural structures have been identified together with sub population of RFamide immunoreactive neural subpopulation (Jager et al. 2011).

Equating i-cell formation to germ line segregation mechanisms

The germ line is the cell lineage that will give rise to gametes carrying the genetic information to the next generation. The segregation of germ line from somatic cell lineages, with the subsequent formation of primordial germ cells (PGC), can occur through two different mechanisms termed preformation and epigenesis. In preformation (Drosophila, zebrafish, Xenopus), germ cells can be identified very early in embryogenesis.

Their identity is determined by inheriting localised germ plasm (see Introduction section 1.5). For example the specification of germ line in the crustacean Parhyale occurs by early germ plasm segregation, starting at first cleavage. The blastomere bearing germ plasm, named g, it is able to generate the germ line (Extavour 2005). Single blastomere isolation experiments demonstrated that all isolated blastomeres are able to develop independently of each other, but only the germ plasm-inheriting blastomere (tracked using a Vasa Given the scattered distribution of epigenesis and preformation as modes of germ cell development across the metazoan phylogeny (Figure 49), there is still much debate concerning which of the two mechanisms is ancestral. The observation that late germ line segregation (after embryogenesis) occurs in several animals and does not appear to rely on maternally localised factors, supports the idea that epigenesis is the ancestral mechanism and that probably preformation derived from epigenesis multiple times during evolution (Extavour, 2007;[START_REF] Extavour | Mechanisms of germ cell specification across the metazoans: epigenesis and preformation[END_REF]. This view is supported also in a recent study in which molecular evolution rates were correlated with PGC specification [START_REF] Evans | Acquisition of germ plasm accelerates vertebrate evolution[END_REF]. Evolution rates of protein coding sequences were compared between two sister taxa that segregate their germ lines by epigenesis vs preformation (urodele vs. anuran amphibians), using mammals as out-group. It was shown that the taxa that use the preformation mechanism show a faster rate of sequence evolution, suggesting that the acquisition of germ plasm is an evolutionary advantage.

On the other hand, the recent identification of germ plasm in an increasing number of taxa and of its presence in basal branch metazoans such as Clytia, indicates that the preformation mechanism in the common metazoan ancestor should be not ruled out [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF], perhaps originally existing in parallel with epigenetic mechanisms.

A previous study showed the presence of localised "germ plasm" in Clytia oocytes and eggs [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]) by morphological criteria in electron microscopy oocyte sections and by detection of localised mRNAs for Vasa, PL10, Nanos2, Nanos1 and Piwi in spawned eggs (Figure 18). These 'germ plasm' mRNAs are aggregated in granules at the animal pole

• Does germ plasm determine i-cell fate?

Hydrozoan i-cells can be considered as an atypical germ line since they give rise to both germ cells and somatic cells. The i-cell formation mechanism in Clytia can thus be equated with the mechanism of germ line formation. To determine whether germ plasm has an instructive role in i-cell formation (and as a consequence in germ line development), embryo fragments in which germ plasm was absent (vegetal halves) were produced by embryo bisection or blastomeres isolation [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF]; see also Results II section 2.1.1, Figure 42). Vegetal fragments lacking germ plasm were able to develop Piwiexpressing cells by the planula stage. The planulae derived from the vegetal halves (germ plasm absent) and animal halves (germ plasm present) were similar with respect to the time and number of Piwi-expressing cells. This result indicates that in vegetal fragments at least germ plasm is not instructive in the formation of i-cells, and that an inductive signal is able to promote i-cell de novo formation during embryo re-patterning. In this study we addressed whether Wnt/β-catenin signalling was involved in this process. Critically, we found that vegetal fragments from Wnt3-MO injected embryos had a reduced capacity to recover Piwi/Nanos1 expressing cells at the planula stage compared to vegetal fragments from uninjected embryos (Figure 1 and Table 1 in PAPER1). These results indicate that in the absence of germ plasm, Wnt3 function is necessary (directly or indirectly) for de novo icell formation. Conversely Wnt3 is not required for i-cell formation during normal development (see Figure 39, ResultsII), suggesting that Wnt3 is involved in i-cell ontogeny exclusively during axis re-establishment. A correlation between de novo i-cell formation and axis regulation was observed also in some embryo fragments derived from uninjected embryos; when the embryos halves were not able to regulate their axis, as indicated by the loss of elongated embryo shape and directed swimming, Piwi expressing cells tended to be absent. Wnt-dependent de novo i-cell formation during embryo re-patterning thus quite possibly relates to the critical role of Wnt signalling in establishing axial properties during embryogenesis. Oral-aboral axis development is initiated in Clytia from the egg, building on the asymmetrical distribution of the Wnt3 and Fz mRNAs (Momose & Houliston 2007;Momose et al. 2008). During re-patterning of vegetal fragments a sufficient quantity of Wnt3 and Fz1 from the animal region is presumably required to allow re-establish an oralaboral axis through feedback mechanisms involving unknown secreted molecules. In vegetal fragments, the lack of germ plasm combined with the Wnt/β-catenin downregulation prevents i-cell de novo formation, suggesting that axis formation is required. In animal fragments germ plasm can determine i-cell fate even in the absence of oral-aboral polarity.

Taken together these results indicate that since embryos lacking germ plasm and Wnt3 signalling are not able to form i-cells, germ plasm normally has an instructive function, not directly linked to early germ lineage segregation but to i-cell lineage segregation. How Wnt3 drives the de novo i-cell formation is not yet understood. One hypothesis could be that following blastomere isolation or blastula bisection, reactivation of Wnt signalling via Wnt3 causes re-aggregation of pre-existing Nanos1 and Piwi mRNAs, allowing reformation of the germ plasm and thus i-cell formation. In this context is important to note that the concentrations of Nanos and Piwi mRNAs between the vegetal and the animal halves at the blastula stage are surprisingly similar [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF], despite the presence of germ plasm in the animal fragments. However we should not rule out another possibility, involving traces of Wnt3 protein that could persist in the animal hemisphere of Wnt3 morphant embryos. In this scenario, in both Wnt3-MO whole embryos and animal halves, residual maternal localised Wnt3 protein could be sufficient to promote i-cell formation independently of its role in OA axis formation.

Signalling pathways other than Wnt may also be involved in de novo i-cell formation in Clytia reformed oral territories. During mouse embryogenesis BMP signalling and Wnt/βcatenin signalling collaborate to induce PGC formation [START_REF] Ohinata | A Signaling Principle for the Specification of the Germ Cell Lineage in Mice[END_REF]). In the mouse epiblast, PGCs are specified by BMP signalling coming from the extra embryonic ectoderm. BMP signalling induces the expression of the gene Blimp1. Blimp1 protein is a repressor of transcription of somatic genes and it is a key factor in PGCs specification.

Wnt3 function is also essential to induce the expression of Blimp1, ensuring that these cells can respond correctly to BMP4 to be specified as germ cells [START_REF] Ohinata | A Signaling Principle for the Specification of the Germ Cell Lineage in Mice[END_REF].

Interestingly, transcripts of the potential BMP and Wnt inhibitor Cerberus are underexpressed in Clytia Wnt3 early gastrula morphants (Figure 3 PAPER 2), suggesting that also in Clytia BMP and Wnt signalling might collaborate.

The Clytia transcriptome contains several probable BMP ligands and Smad effectors (Lapébie unpublished data). Preliminary expression pattern analysis has shown that the Clytia eggs and/or embryos express mRNAs for a BMP5/8 ortholog and a possible BMP2/4 ortholog as well as Smad1/5 and Smad 2/3 orthologs.

The expression pattern of BMP signalling molecules had already been associated with germ cells development in other cnidarians. For example in Hydra Hysmad1 levels are upregulated during oogenesis in Hydra sexual strains [START_REF] Hobmayer | Identification and expression of HySmad1, a member of the R-Smad family of TGFβ signal transducers, in the diploblastic metazoan Hydra[END_REF]

while in

Nematostella the expression domain of BMP 2/4 ortholog coincides with clusters of cells localised in the mesenteries that correspond to presumptive Nematostella PGCs (Extavour et al. 2005;[START_REF] Finnerty | Origins of bilateral symmetry: Hox and dpp expression in a sea anemone[END_REF]). To test the involvement of BMP signalling in i-cell formation during embryogenesis in Clytia, one approach would be to hyper-activate the pathway by injection into egg of 4-hydroxychalcone, a recently identified chemical activator of the BMP pathway [START_REF] Vrijens | Identification of Small Molecule Activators of BMP Signaling[END_REF]. In crickets, hyperactivation of the BMP pathway using 4-hydroxychalcone increased the number of PGCs clusters and the level of phosphorylated Smad4 indicative of the pathway activation. It will be interesting to investigate the role of these molecules both in the formation of i-cells during normal development and during embryo re-patterning.

To conclude, this study has provided experimental support for the hypothesis that two different mechanisms for the formation of i-cells coexist in Clytia. During undisturbed development, germ plasm appears to have an instructive role, while in re-patterning conditions signalling molecules participate in an inductive process.

From an evolutionary point of view, these findings can contribute to the debate concerning which mechanism, epigenesis or preformation, was present in the common metazoan ancestor [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF][START_REF] Extavour | Mechanisms of germ cell specification across the metazoans: epigenesis and preformation[END_REF]. Specifically, they support the possibility that both mechanisms were available in the common ancestor of metazoans, and that in certain animal lineages either epigenesis or preformation was reinforced as the primary mechanism [START_REF] Kumano | Evolution of germline segregation processes in animal development[END_REF].

Figure 20 :

 20 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 21 :

 21 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 22 :

 22 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 23 :

 23 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 24 :

 24 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 26 :

 26 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 27 :

 27 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 28 :

 28 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 29 :

 29 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 30 :

 30 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 31 :

 31 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 32 :

 32 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 33 :

 33 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 34 :

 34 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 35 :

 35 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 36 :

 36 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure 37 :

 37 Figure1: Stem cell potential. Figure2: Asymmetrical cell divisions. Figure3: Wnt signalling pathways. Figure4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. Figure5: Embryonic axis and Wnt β-catenin signalling. Figure6: One hypothesis for the phylogenetic relationships between major metazoan clades.Figure7: Cnidarian model in developmental biology. Figure8: Clytia life cycle. Figure9: Clytia embryonic development. Figure 10 : Clytia medusa anatomy.Figure 11 : I-cell and i-cell derivatives localisation i the Hydra polyp and in Clytia developmental stages and adult.Figure 12 : Distribution of i-cell expressing Piwi, Nanos1, Vasa during larval development and in the medusa.Figure 13 : RFamide immunoreactivity in hydrozoans nervous system. Figure 14 : Nematocytes structures.Figure 15 : Structure of Clytia female gonad.Figure 16 : Genetic circuitry involved in nematocytes and neuronal cell specification.Figure 17 : Molecular markers for Clytia Nematogenesis.Figure 18 : Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia eggs.Figure 19 : Piwi expressing i-cells can form from both vegetal and animal regions.Figure 20 : Nanos1, Piwi and Wnt3 expression pattern comparison in eggs and at the early gastrula stage.Figure 21 : Co-localisation of Piwi and Nanos1 transcripts.Figure 22 : Appearance of nematocytes during larval development.Figure 23 : Mcol3-4a expression allows tracking of the nematoblast population during larval development. Figure 24 : Sox15 expression pattern in nematoblasts and in differentiating nematocytes. Figure 25 : Gene orthology of the metazoan paired-class homeobox transcription factors.Figure 26 : Prdl-a expression during larval development and in the adult medusa.Figure 27 : Gene orthology of the metazoan bHLH family.Figure 28 : Asc-b expression during larval development and in the adult medusa.Figure 29 : Gene orthology of the metazoan Gli/Zic.Figure 30 : Zic-C expression during larval development and in the adult medusa.Figure 31 : RFamide expression during larval development and in the adult medusa.Figure 32 : GLWamide expression during larval development and in the adult medusa.Figure 33 : Antistasin expression during larval development and in the adult medusa.Figure 34 : Gene orthology of the metazoan Boule/DAZ family members.Figure 35 : Boule/DAZ family gene expression during larval development and in the adult medusa.Figure 36 : Tudor domain gene expression during larval development and in the adult medusa.Figure 37 : Phylogenetic analysis of the metazoan EVE domain protein.

Figure :

 : Figure : Thyn1 expression during larval development and in the adult medusa. Figure : Nanos1 and Piwi expression is not inhibited in Wnt3 morphants larvae. Figure : Wnt3 is required for differentiation of RFamide neurons. Figure : A Wnt gradient is required for neural fate acquisition. Figure : Wnt signalling participates in de novo i-cell formation.Figure : Comparative transcriptomic approach to unravel new genes involved in Clytia embryonic patterning . Figure : Nanos1,Piwi,Mos3 and Zfn845 expression pattern during larval development in young medusa and in adult jellyfish. Figure : Co-localisation of Piwi with Mos3 and Znf845. Figure : Znf845 is not expressed in gland cells or in nematocytes. Figure : Znf845 is not expressed in Sox15 positive cells. Figure : I-cell cartography. Figure : Germ plasm distribution in metazoans.

2

 2 Figure : Thyn1 expression during larval development and in the adult medusa. Figure : Nanos1 and Piwi expression is not inhibited in Wnt3 morphants larvae. Figure : Wnt3 is required for differentiation of RFamide neurons. Figure : A Wnt gradient is required for neural fate acquisition. Figure : Wnt signalling participates in de novo i-cell formation.Figure : Comparative transcriptomic approach to unravel new genes involved in Clytia embryonic patterning . Figure : Nanos1,Piwi,Mos3 and Zfn845 expression pattern during larval development in young medusa and in adult jellyfish. Figure : Co-localisation of Piwi with Mos3 and Znf845. Figure : Znf845 is not expressed in gland cells or in nematocytes. Figure : Znf845 is not expressed in Sox15 positive cells. Figure : I-cell cartography. Figure : Germ plasm distribution in metazoans.

Figure 1 :

 1 Figure 1: Stem cell potential. The cartoon shows how stem cells gradually commit towards a differentiation pathway. The cells acquire a restricted fate, decreasing in potential. Pluripotent cells (in red) are able to give rise to multipotent stem cell here represented in three different colours (green, pink and blue). Multipotent stem cells have a reduced potential. Curved arrows illustrate the self-renewal ability of these stem cells. On the right, an illustration of the zygote that in mammals is a totipotent cell.

Figure 2 :

 2 Figure 2: Asymmetric cell divisions. A) Localised determinants direct the fate of the terminal differentiated cell, activating an intrinsic genetic program. B) Extrinsic cues determine the fate of one of the two daughter cells inducing the specification of cell precursor (CP) that will move into the differentiation pathway. Modified from[START_REF] Wilson | Bone-marrow haematopoietic-stem-cell niches[END_REF].

  have been shown to regulate stem cell proliferation and/or differentiation in vivo and in vitro, in different species and in different contexts of development and tissue homeostasis. Ongoing studies in our group are focussed on the key participation of Wnt signalling in embryonic and larval development in Clytia, and so I addressed, during my thesis work the role of this pathway in i-cell development. In this part of the Introduction I will give an overview of the various Wnt signalling pathways.

Figure 3 :

 3 Figure 3: Wnt signalling pathways. Diagram illustrating Wnt β-catenin dependent and independent signalling pathways.A) The "canonical", β-catenin dependent pathway starts with the binding of the wnt ligands to the Frizzled (Fz) and LRP5/6 receptor complex. This complex induces the stabilisation of β-catenin via the bound protein Dishevelled (Dsh). Increased β-catenin stabilisation is accompanied by its translocation to the nucleus. In the nucleus, β-catenin bound to the transcriptional activator TCF/LEF induces the transcription of the genes normally suppressed by TCF/LEF in the absence of Wnt. In the absence of Wnt signalling, β-catenin associates with APC and AXIN in destruction complex. B) Interactions between the Fz/PCP core components (Fz, Dsh, Strabismus and Prickle) to assure planar cell polarity do not require the Wnt ligand (see Figure4). Wnt binding can, however, regulate downstream cell behaviours via the so-called Wnt-PCP pathway through Fz and Dsh, which activates the small GTPase RhoA and its effector ROK (Rho-associated kinase) in order to regulate the actin cytoskeleton. Another downstream effector of Dsh is JNK which can be activated by RhoA. C) The Wnt-Ca 2+ pathway is activated through wnt ligand binding to Fz receptors. Fz induces the activation of heterotrimeric G-proteins, which regulates calcium-calmodulin kinase 2 (CamK2) and protein kinase C (PKC). Modified from[START_REF] Habas | Dishevelled and Wnt signaling: is the nucleus the final frontier[END_REF].

Figure 4 :

 4 Figure 4: Planar cell polarity coordinates the orientation of cilia and hairs in metazoan tissues. A) Examples of PCP coordination in the Drosophila wing blade, mammalian epidermis and Clytia planula ectodermal cilia. Cilia and hairs point in a single direction along the tissue axis, as indicated in the scheme. Ectodermal cilia in Clytia planula are oriented along the oral/aboral axis. Blue lines refer to a schematic illustrating that PCP components are localised at plasma membrane and are segregated along the epithelial plane. Red and green lines indicate the distribution of PCP core component at proximal/anterior to distal/posterior side. B) Asymmetrical distribution of PCP pathway components within neighbouring cells in the Drosophila wing blade. In blue Pk and Vang (Stbm) at anterior pole, in red Dg, Dsh and Fz localised at posterior side. Modi ed from Devenport (2014), Momose et al. (2012).

Figure 6 :

 6 Figure 6: One hypothesis for the phylogenetic relationships between major metazoan clades.Metazoan phylogenetic tree showing in light grey the Bilateria clade and in light blue the "basal branching" metazoans. Some of the topology is controversial but most recent studies agree that Cnidaria is the sister group to all bilaterinas. Cnidaria possess most of the key family genes involved in developmental mechanism, including a family of about 12 Wnt family genes.Modified from[START_REF] Ryan | Hox, Wnt, and the evolution of the primary body axis: insights from the early-divergent phyla[END_REF].

Figure

  Figure 7: Cnidarian models in developmental biology. A) A current view of phylogenetic relations among cnidarians. Nematostella belongs to the anthozoans, while Hydractinia, Hydra and Clytia are hydrozoans. B) Schematic representation of the Nematostella directive axis determined by the shape of the pharynx and the asymmetric position of the retractor muscles within the mesenteries. C) Representative cnidarian model systems used in developmental biology. From top to bottom: Nematostella vectensis, Hydractinia echinata, Hydra vulgaris and Clytia hemisphaerica.For each organism is indicated the adult stage (polyps and/or medusae) and "developmental" stages (cleavage and larval stages). Modified from[START_REF] Technau | Evolutionary crossroads in developmental biology: Cnidaria[END_REF][START_REF] Houliston | Clytia hemisphaerica: a jellyfish cousin joins the laboratory[END_REF],[START_REF] Bradshaw | Distinct mechanisms underlie oral vs aboral regeneration in the cnidarian Hydractinia echinata[END_REF],[START_REF] Leclère | RGM Regulates BMP-Mediated Secondary Axis Formation in the Sea Anemone Nematostella vectensis[END_REF].

Figure 8 :

 8 Figure 8: Clytia life cycle.Clytia adult medusa (a) spawn daily after a short light stimulus. Female and male medusa release unfertilised eggs and sperm in the water (female and male gonads b-c). Following the external fecondation, the fertilised eggs cleave after 1 hour. Embryonic development generates the larval stage, about 48 hours post fertilisation at 18°C (d). The3-4 day old planula larva metamorphoses into a primary polyp (f) that is the founder of the polyp colony (g). The colony first develops the gastrozoids (h) that are the feeding zooids. Later, gonozoids, form that bud and release baby immature jellyfishes (j). Released baby jellyfishes are 1-3 mm in diameter and grow by feeding to become sexually mature after about 2-3 weeks in laboratory conditions, reaching a maximum size of 1-2 cm (a). Scale bar 1mm for adult jellyfishes and polyps, 100μm for the other stages. Modified from[START_REF] Houliston | Clytia hemisphaerica: a jellyfish cousin joins the laboratory[END_REF].

Figure 9 :

 9 Figure 9: Clytia embryonic development.A) Schematic illustration showing Clytia embryonic development. The site of the first cleavage, which is unipolar, corresponds to the oral pole of the planula (blue asterisks). After 6 hours, the embryo reaches the blastula stage. In some blastulae at this stage it possible to visualise the animal pole thanks to a deep cleft between the two lobes. At the early gastrula stage it is possible to recognise the first sign of polarity. Embryos acquire an ellipsoid shape and the cells are more elongated. At 24hpf gastrulation is complete, however presumptive endoderm is not yet differentiated (green cells). At 48hpf ectoderm (red cells) and endoderm (green cells) are well formed and the larva shows the typical torpedo shape. The line represent the developmental time in hours at 18°C. Modified fromFreeman (1981b). B) Longitudinal section of a "late differentiated planula" (>48hpf). At this stage all the differentiated somatic cells are present as indicated by labels. Endoderm in green, ectoderm in red, mesoglea in black. Modified fromBodo & Bouillon (1968).

Figure 10 :

 10 Figure 10: Clytia anatomy. A) Clytia anatomical features The exumbrella and subumbrella are separated by a thick mesoglea. The nervous system includes a diffuse nerve net in the subumbrella region and in the manubrium, as well as inner and out nerve rings that run around the bell margin. From the stomach 4 radial canals extend to the bell margin. Each radial canal hosts one of the 4 gonads. B) Morphology of Clytia tentacles and tentacle bulbs (TBs). From left to right electronic microscopy picture of TB showing the site of the attachment to the bell margin; DIC image showing the internal cavity of the TB; Confocal optical section of an isolated TB stained with DAPI to visualise nuclei (red) and Phalloidin for cell contours (cyan). Scale bar 25μm. From Houliston et al (2010), Denker et al. (2008).

Figure 11 :

 11 Figure 11: I-cell and i-cell derivatives in the Hydra polyps and in Clytia developmental stages and adult.A) Hydra i-cell. Multipotent i-cell (green) give rise to all somatic progenitor (pink) and germ cell precursor (pink). All the progenitors differentiate producing mature cell types (sky blue). Prec: precursor, NeuroB: neuroblast, NematoB: nematoblast, PGC: germ cell precursor. In Hydra multipotent cell (green) are localised mostly in the gastric region in the interstitial spaces between the epithelial cells, differentiating cell are disperse throughout the Hydra body in the endodermal region (pink) while differentiated cells (sky blue) are mostly localised in the ectoderm layer at body extremities (tentacles, foot, hypostome). ec: ectoderm, en: endodermModified fromHobmayer et al. (2012). B) Clytia i-cell. In Clytia i-cell are distinguishable, from a morphological point of view at early gastrula stage (green line). In adult stage the known site of i-cell localisation are the proximal part of the tentacle bulbs, the base of the manubrium and the gonads. Fated cell (pink line) have been characterised in the planula endoderm and in the medusa. Comparable to Hydra differentiated cells are located in the ectodermal layer, however mature gland cells are localised in both layer ectoderm and endoderm. ec: ectoderm; en: endoderm; TB: tentacle bulbs.

Figure12:

  Figure12: Distribution of i-cells expressing Piwi, Nanos1,Vasa during larval development and in the medusa. A) Piwi, Nanos1 and Vasa expressing i-cells in embryos and larval stages. By In situ hybridisation Piwi, Nanos1 and Vasa mRNAs are detected at the animal/oral pole during development. At the blastula stage the genes are expressed at the animal pole (on the two main lobes of blastulae showing a "peanut" morphology).At the early gastrula stage they are expressed at the oral pole, in cells that will ingress with the presumptive endoderm cells. In the planula the genes are expressed in a population of cells with morphological characteristics of i-cell interspersed in the endodermal region. Scale bar 50μm. On the left schematic representations of i-cell localisation (orange dots). Light grey: endoderm, dark grey: ectoderm. B) Piwi, Nanos1 and Vasa expression in the medusa. Piwi, Nanos1, Vasa mRNAs are detected in three main regions: the proximal area of the tentacle bulbs, at the base of the manubrium, and in the gonads. On the left schematic representation of i-cell distribution in the medusa (orange dots). In the pink square, schematic representation of the area boxed in A3, illustrating the medusa manubrium. Scale bar 100μm for the whole medusa, 25μm for the other images. From[START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF].

Figure 13 :

 13 Figure 13: RFamide immunoreactivity in hydrozoan nervous systems. Confocal images of different type of neurons immunodetected with a RFamide antibody in planulae, medusae, and polyps. (A-D) Clava multicornis planula (48hpf) showing an aboral neuronal plexus. In A punctuate staining characteristic of neuropeptide release is visible. B high magnification of the neural plexus, NN: nerve net. C high magnification showing a neurosensory cell (NS), D high magnification showing a ganglionic cell (Gang). E shows RFamide mature neurons at 36hpf. In the medusa Eirene (F), RFamide positive neurons are distributed in the nerve ring (NR) and in the manubrium. In Hydra polyps (G) an RFamide condensed nerve net (NN) is observed around the hypostome. Scale bar: A 40μm, B 25μm, C 10μm, D 3μm, E 30μm, F 200μm, G 100μm. Modified from Piraino et al. (2011), Koizumi et al. (2014); Takahashi & Hatta (2011).

Figure 14 :

 14 Figure 14: Nematocytes structuresThe nematocyst is contained in the cnidocytes. The nematocyst consists of capsule to which is link a tube with a harpoon thread (a). Upon cnidocil stimulation the folded tube is expulsed from the capsule and injected in the prey (b). (From Boundless. "Phylum Cnidaria." Boundless Biology. Boundless, https://www.boundless.com/biology/te xtbooks/boundless-biologytextbook/invertebrates-28/phylumcnidaria-167/phylum-cnidaria-644-12947/).

  Gametes and germ cell precursors are localised in the sexual organs in Hydra and in Clytia adult medusa gonads. Clytia gonads are attached to the subumbrellar side of the bell on each of the four radial canals. The oocytes are arranged on each side of the endodermal cavity (Figure15). Proliferating germ cell precursors and oocytes undergoing the first stages of meiosis lie close to the medusa bell. Growing oocytes are positioned more distally. Isolated Clytia gonads, can survive for 2-3 days. During this time they are still able to spawn in response to dark-light cycles, although they have lost physical contact with the rest of the medusa body. Thus isolated Clytia gonads can be used as a model to study mechanisms of oocyte maturation[START_REF] Houliston | Clytia hemisphaerica: a jellyfish cousin joins the laboratory[END_REF]).

Figure 15 :

 15 Figure 15: Structure of Clytia female gonad. Schematic representation, of the female gonad. Oocytes are positioned between endoderm (red) and ectoderm (green) layers. Early oocytes are localised in the proximal region of the gonad (close to the bell margin), while the bigger late stage oocytes, are more common in the distal part of the gonad. From Amiel & Houliston (2009).

Figure

  Figure 16: Genetic circuitry involved in nematocytes and neuronal cell specification. Diagram illustrating the genes that mediate nerve cells and nematocytes specification and differentiation in Hydra. Shaded in pink nematogenesis, shaded in yellow neurogenesis. Genes highlighted by double circles are expressed in the common neural progenitor (NPG). ISC :interstitial stem cell. Black encircled genes represent the family genes studied in this work. Modified from Galliot & Quiquand (2011).

Figure 17 :

 17 Figure 17: Molecular markers for Clytia Nematogenesis. A) Expression pattern analysis of the nematogenic marker Mcol3-4a in the medusa. Mcol3-4a is expressed in the mouth and in the tentacle bulb ectoderm. Light blue and green arrowheads indicate Mcol3-4a expression at the base of manubrium and the base of the tentacle. On the right lower panel, high magnification of Mcol3-4a-expressing-cells in the tentacle bulb (external side). Yellow dotted lines highlight nematoblast nests as typical nematocytes conformation. In situ hybridisation showing overlapping expression domains of Mcol3-4a and Piwi. Scale bar First row: 250μm, second row: 25μm, 2.5μm, third row 25μm, 25μm, 10 μm B) Sox15 expression in adult medusa and planula. In late planula (>48hpf), Sox15 mRNA is detected in a population of endodermal cells and in differentiating nematoblasts in the ectoderm, as shown by the high magnification in the lower right panel. In the medusa Sox15 expression coincides with Mcol3-4a expression in tentacle bulbs. Scale bars 50μm, 10μm (planula), 20μm in tentacle bulbs. C) Schematic representation of a tentacle bulb, including the distribution of the different cell types and molecular markers. From Jager et al. (2011), Denker et al. (2008).

Figure 19 :

 19 Figure 19: Piwi expressing i-cells can form from in both vegetal and animal regions.On the right, a schematic representation of the approach used to separate animal and vegetal halves at blastula stage. Note that at this stage the animal pole is distinguishable by the deeper cleft of the typical peanut shape. Bisection successfully eliminated germ plasm from about 60% of vegetal fragments, as assessed by immediate fixation prior to Piwi in situ hybridisation (C1, C4, C5). At the early gastrula stage (C2, C6) and at 24hpf (C3, C7) Piwi-expressing cells were detected in almost all samples, indicating that fragments lacking germ plasm are able to re-form i-cells. FromLeclére et al. (2012).

Figure 18 :

 18 Figure 18: Morphological and molecular evidence for perinuclear germ plasm localisation in Clytia oocytes. A, C: Characteristic perinuclear distribution of Nanos1and Piwi mRNAs in very early oocyte stages G: TEM sections of growing oocytes. Perinuclear gem plasm is indicated by red arrowheads er: endoplasmic reticulum, nm: nuclear membrane, pm: cell membrane, n: nucleus, nu: nucleolus, mit: mitochondria, cyt: cytoplasm, nu: nucleus, Scale bars 5μm. From Leclére et al. (2012).

Figure 20 :

 20 Figure 20: Nanos1, Piwi and Wnt3 expression pattern in eggs and at early the gastrula stage. In situ hybridisation analysis shows that mRNAs for Nanos1, Piwi and Wnt3 are all localised at the animal cortex in the unfertilised eggs (left panel for each line). At the early gastrula stage Nanos1 and Piwi (right panel) are expressed in a population of ingressing cells at oral pole, while Wnt3 expression is detected mainly in the ectodermal cells. Scale bar 50μm.Left panel from Momose et al. (2008). Scale bar 40μm.

  Fated i-cells have already left the stem cell pool and chosen a particular differentiated cell type fate. I considered cells that co-express Piwi/Nanos1 with early somatic markers to be fated i-cells; depending on the lineage these could be called for instance early nematoblasts or early neuroblasts.To identify early progenitors and differentiated i-cells, I tried to identify markers for each step of differentiation. Because hydrozoan medusae and planulae share to a large extent the same cellular components, therefore I included in this study the adult jellyfish, in which structures are easier to recognise then in the planula. Comparison between the expression patterns in the planulae and in the medusae helped me to propose identities for the cells expressing the genes. I also used other staining techniques to identify differentiated nematocytes and some neural sub-types. This work was the first attempt to provide a molecular description of the appearance of the i-cell derivatives during Clytia larval development. It will provide a basic foundation for further molecular characterisation and for functional analyses on the regulation of i-cell dynamics (commitment and differentiation), and provided the background for the studies of Wnt signalling regulation of i-cells development presented in Results chapter II.

  Figure 21: Co-localisation of Piwi and Nanos1 transcripts. A) Representative confocal image showing co-localisation of Nanos1 (green) and Piwi (pink) mRNA. Merged images show that Piwi and Nanos1 co-localise at the early gastrula and at 72hpf (not shown). The identified cells, co-express both genes even though the superposition is not highlighted by a third different colour (e.g yellow). Scale bar 50μm. B) Negative control. Confocal image showing a single FISH, using Piwi probe (or Nanos1, not shown) at 24hpf. To verify the reliability and the specificity of the signal detected, the samples were incubated only with the Piwi probe and treated for a double FISH and imaged in both channels. Scale bar 50μm.

  From a molecular point of view, it has previously been shown that in the adult medusa and in the late planula (>48hpf) Mcol3-4a and Sox15, are expressed in nematoblasts and in nematocytes[START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF] Jager et al. 2011) (Figure 17 Introduction). Therefore I used these markers to follow the nematogenetic pathway during Clytia development.1.1.1 Distribution of nematocytes during embryonic developmentTo determine exactly when and where mature nematocytes first appear during embryogenesis I scored the presence of nematocyte capsules by DAPI staining (Figure22A) during development at24, 36, 48, 72-hour post fertilisation (hpf). 24hpf corresponds to the end of gastrulation, with completion of the ingression of presumptive endoderm cells. By 48hpf the endodermal cells are organised into an endodermal epithelium and an internal cavity has formed through destruction and digestion of a central core of cells. At 72hpf larval development and cell type differentiation is complete and the planula is competent to metamorphose.

Figure 22 :

 22 Figure 22: Appearance of nematocytes during larval development. A) Nematocytes labelling. Nematocytes were labelled by incubation with high DAPI concentration; the image shows a 48hpf planula. Scale bar 50μm. In the right panel detailed view, of the area boxed in A. Nc: nematocytes capsule. Scale bar 10μm. Lower panel schematic illustration of the nematocytes and the nematocyst capsule. B) Nematocytes counts. Nematocytes were counted using epifluorescence microscopy and were grouped on the basis of nematocyte number. Light blue represents planulae with no detectable nematocytes, orange 1-10, green 11-20, yellow 21-30, dark blue more then 30 nematocytes detected. C) Schematic representation of Clytia larval development, indicating the nematocytes position during development. Nematocytes are represented using the same drawing in A. en: endoderm. ec: ectoderm. Oral pole at the top.

  tentacle bulbs, Mcol3-4a expressing cells showed a typical nematoblast morphology and organisation (nests of 2,4 or 8 cells, known from Hydra studies to be interconnected by cytoplasmic bridges) (Figure17Introduction). In later stages, (48hpf and 72hpf) some cells expressing Mcol3-4a could be detected at the interface between endoderm and ectoderm layers suggesting that these cells are preparing to migrate into the ectoderm where they mature to form definitive nematocytes (see above). In fully differentiated nematocytesMcol3-4a mRNA can no longer be detected.In the adult medusa Mcol3-4a is expressed throughout the nematoblast population, including nematoblast progenitors in which Piwi is still expressed[START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF]. To map the distribution of early stage nematoblasts at planula stages I performed coexpression studies using Mcol3-4a and Piwi probes. Using this approach I could distinguish three different cell populations (Figure23B): Piwi expressing i-cells, (in red, including some multipotent i-cells and probably some early precursors of other cell lineages), early nematoblast progenitors (expressing both Piwi and Mcol3-4a, in yellow) and dividing nematoblasts organised in clusters (expressing only Mcol3-4a, in green).These three cell populations were all located in the central region of the planula endoderm but without any consistent relative distribution or position along the oral-aboral axis.In the medusa, the expression of Mcol3-4a widely overlaps in nematoblasts with other genes including the Clytia Sox family transcription factors Sox5, Sox13, Sox14, Sox12 and Sox15(Jager et al. 2011). I was particularly interested in Clytia Sox15, because of its very early Wnt3-dependent expression at the early gastrula stage (see Results II-PAPER 2) and its expression in both nematoblasts and differentiated nematocytes in the late planula (>48hpf)(Jager et al. 2011). Thus I combined analysis of the expression of Sox15 andMcol3-4a to map the appearance of nematoblasts and nematocytes during larval development.1.1.3 Sox15 is expressed during an extended period of nematogenesis I performed Sox15 in situ hybridisation at the early gastrula, 24hpf and 48hpf planula stages. At the early gastrula stage, I detected Sox15 expression in a subpopulation of ingressing cells at the oral pole, distributed in a similar manner to the i-cell markers, Piwi and Nanos1 (Figure24A, for Piwi and Nanos1 seeFigure 12, 44). Interestingly the expression of Sox15 at this stage covers a broader domain then Nanos1 and Piwi. At 24hpf Sox15 mRNA was more weakly detected in the endoderm, with a few cells detected also in the ectoderm (Figure24A). At this stage detection of the transcript was difficult, due to the weak intensity and diffuse distribution of the in situ hybridisation signal. This expression could reflect the onset of the expression of the gene in different cell populations dispersed in the planula ectoderm and endoderm. In contrast at 48hpf stage, Sox15 expression was clearly detected in two different cell populations, one population scattered in the central endoderm and one in the ectoderm (Figure24A-B). Sox15 positive cells in the ectodermal layer had a characteristic morphology and are likely differentiating nematocytes(Jager et al. 2011).

Figure 23 :

 23 Figure 23: Mcol3-4a expression allows tracking of the nematoblast population during larval development. A) Mcol3-4a mRNA is detected in cells in the planula endodermal region. In situ hybridisation with Mcol3-4a probe from blastula to planula 72hpf stages. From left to right: blastula, early gastrula, 24, 48, 72hpf planula stages. Mcol3-4a mRNA is not detected at blastula stage. Gene expression is detected at 24hpf in the endodermal region in a group of cells positioned mainly in the oral half. White asterisks indicate putative nematoblasts migrating into the ectoderm at 48hpf and at 72hpf.ec: ectoderm, en: endoderm. Scale bar 50μm. B) Mcol3-4a expression coincides partially with Piwi expression. Confocal images showing the expression pattern of Mcol3-4a (red) and Piwi (green) at 24hpf planula. From right to the left: Mcol3-4a, Piwi, Merge (only one Z plane). Yellow colour indicates cells that show the co-expression of both genes. Oral pole at top. Scale bar 50μm. Lower image represent high magnification, showing a double-labelled cells (yellow). Scale bar 10μm.

Figure 24 :

 24 Figure 24: Sox15 expression in nematoblasts and in differentiating nematocytes. A) Expression pattern of Sox 15 during Clytia development. In situ hybridisation of Sox15 from early gastrula to planula 48hpf. Sox15 expression was detected at the oral pole in the early gastrula, in a wide population of cells. At 24hpf, it was weakly detected in few cells located both in the ectoderm and in the endoderm (black asterisks). At 48hpf Sox15 mRNA was detected in two distinct cell populations, in the endoderm and ectoderm regions. Scale bar 50μm. B) Some Sox15 expressing cells in the endoderm are nematoblasts. Confocal images showing the expression of Sox15 (green) and Mcol3-4a (red), and the merged images (only one Z-plane) at 48hpf. Mcol3-4a expressing cells are restricted to the endoderm, while Sox15 mRNA is found in the nematoblasts and in differentiating nematocytes (blue arrows). Yellow arrows indicate some cells expressing both genes, with a red core representing Mcol3-4a mRNA and green dots representing Sox15 mRNA. C) Sox15 and Piwi double in situ at the gastrula stage. Confocal image showing double FISH using Piwi (pink) and Sox15 (green) probes. From left to right: Piwi, Sox15, Merged image. Yellow arrows indicate cells expressing both genes. Dashed lines represent the gastrula contour. Cyan: HOECHST stained nuclei. Oral pole at top of all the images. Scale bar 50μm.

  Figure 25: Gene orthology of metazoan paired-class homeobox transcription factors.Phylogenetic analysis of Prdl-a transcription factors. The amino-acid alignment of paired subfamily sequences was extracted from (Lapébie et al. 2014; Ryan et al. 2006). Clytia and Hydra Prdl-a orthologs pairs can be clearly identified (shaded in blue). Clytia genes are indicated in red. The following sequences from Clytia hemisphaerica have been added (Sequence name: Accession number). Prdl-b:KT318135, Prdl-d:KT318136, HOP: KT318137, Pitx: KT318138, Rx: KT318139, Otx-related-2:KT318140, Ot: KT318141, Otx: KT318142, Otx-related-1: KT318143, Prdl-e: KT318144, Prdl-c:KT318145, Unc:KT318146, PaxB:KT318147, PaxC :T318148

Figure 26 :

 26 Figure 26: Prdl-a expression during larval development and in the adult medusa. A) In situ hybridisation of Prdl-a during larval development at 24,36, 48 and 72 hpf. Prdl-a mRNAwas first detected at 36hpf, concentrated in the aboral endoderm. A later stages the expression is more widespread but weaker. Scale bar 50μm. Oral pole at the top of all the images. B) High magnification showing cell morphology of Prdl-a expressing cells at 36hpf. Scale bar 10μm. ic: icells. C) Medusa stage. Prdl-a expression was detected in a subpopulation of ectodermal cells in the upper part of the tentacle bulbs, and in subpopulation of ectodermal cells in the manubrium. Scale bar 100μm. Tentacle bulbs are oriented with their proximal pole on the top, manubrium oriented with mouth on the bottom, in all images. The Medusa cartoon shows the site of Prdl-a expression (light blue) in the adult, in particular in tentacle and tentacle bulbs (TB) (corresponding to the differentiation region β, modified from Denker et al. 2008) and in the manubrium (M).

Figure

  Figure 27: Gene orthology of metazoan bHLH family. Phylogenetic analysis of bHLH transcription factor domaincontaining sequences.The sequence were extracted from(Lapébie et al. 2014). The analysis confirms the placement of the Clytia proteins within the Achaetescute family. Asc-b is the ortholog of Podocoryne Ash2, Hydra vulgaris HvAsh-like and Human Ascb3 (Grey box). Note that the Clytia gene HlhIE1 belongs to the same clade but it was named previously according to its expression characteristics. Clytia genes are indicated in red. The following sequences from Clytia hemisphaerica have been added (Sequence name: Accession number) PTFb: KT318149, Neurogenin: KT318150, Paraxis: KT318151, AP4: KT318152, Asca-1: KT318153, Hlh-4: KT318154, HEY/HES-2: KT318155, HAND: KT318156, Hlh-6: KT318157, Twist: KT318158, Net: KT318159, Hlh-5:KT318160, Hlh-1: KT318161, Hlh-2: KT318162, HEY/HES-1:KT318163, USF:KT318164, KIAAA2018: KT318165, Bmal: KT318166, Hlh-3: KT318167, Ahr-b: KT318168, ARNT: KT318169, MITF: KT318170, Asca-2: KT318171, MIST-b: KT318172, Ahr-a: KT318173, MIST-a: KT318174.

Figure 28 :

 28 Figure 28: Asc-b expression during larval development and in the adult medusa. A) In situ hybridisation of Asc-b at 24,36, 48, 72 hpf. Asc-b was first detected at 36hpf, weakly in the aboral region of the endoderm. Scale bar 50μm. B) Medusa stage. Asc-b mRNA was detected in a subpopulation of cells in the endodermal region of tentacle bulbs. Scale bar 10μm. Boundary between ectoderm and endoderm indicated by white dotted lines. On the left, a higher magnification view of the area boxed in red showing Asc-b expression in the endoderm of the circular canal (cc). Scale bar 10μm. Asc-b expression in the manubrium endoderm. No expression was detected in mature female gonads. Ec: endoderm, ec: ectoderm, cc: canal circular. The Medusa cartoon shows the site of Asc-b gene expression (Cyan) in the adult, in particular in tentacles and tentacle bulbs (modified from Denker et al 2008).

Figure

  Figure 29: Gene orthology of metazoan Gli/Zic superfamily.Phylogenetic analysis of Zic/ Gli family zinc finger motifs. The sequence were extracted from[START_REF] Layden | Expression and phylogenetic analysis of the zic gene family in the evolution and development of metazoans[END_REF]. Genes identified in this study are in red. Clytia Zic-C groups with the uncharacterised gene Hydra gene HmZnfe with Nematostella Zic-C, and with Scalionema suvaense Zic gene (shaded in red). The additional Clytia sequence that was found is orthologous of the Hydra gene HyZic, involved in early nematogenesis[START_REF] Lindgens | Hyzic, the Hydra homolog of the zic/odd-paired gene, is involved in the early specification of the sensory nematocytes[END_REF]).

Figure 30 :

 30 Figure 30: Zic-C expression during larval development and in the medusa. A) In situ hybridisation of Zic-C during larval development at 24, 36,48, 72hpf. Zic-C mRNA was first detected at low levels at 48hpf stage in the aboral endoderm. Scale bar 50μm. B) Medusa stage. Zic-C was detected strongly in cells in the tentacle and tentacles bulbs ectoderm; Zic-C was not detected either in the manubrium ectoderm or in the gonad ectoderm, or in the radial/circular canal or in the nerve ring. Scale bar 100μm. Medusa cartoon showing the site of Zic-C gene expression (dark blue) in the adult, in particular in tentacle and tentacle bulbs (corresponding to the β, γ, δ differentiating/ differentiated tentacle areas, modified from Denker et al. (2008).

Figure 31 :

 31 Figure 31: RFamide expression during larval development and in the adult medusa. A) In situ hybridisation of the RFamide precursor gene PP5 during larval development at 24, 36,48, 72 hpf. PP5 mRNA is detected in ectodermal cells, and starting from 36hpf, in the lateral ectoderm. Scale bar 50μm. B) Clytia specific RFamide antibody staining at 48hpf. On the left a higher magnification view of the area boxed in red, showing the morphology of neuronal RFamide expressing cells. Scale bar 10μm. C) Medusa stage. RFamide expressing neurons are strongly detected in tentacles, in tentacle bulb ectoderm and in the nerve ring (Nr), as well as in scattered cells in the bell epithelia. Scale bar 100μm. Nr: nerve ring. The Medusa cartoon shows the site of PP5 expression (purple) in the adult, in particular in tentacle and tentacle bulbs (corresponding to the β, γ, δ differentiating/ differentiated tentacle areas, modified from Denker et al 2008).

Figure 32 :

 32 Figure 32: GLWamide expression during larval development and in the adult medusa. A) In situ hybridisation of the GLWamide precursor PP2 during larval development at 24, 36,48, 72 hpf. PP2 mRNA in the planula is detected in ectodermal cells starting for 36hpf, positioned at the aboral pole. Scale bar 50μm. B) Medusa stage. PP2 mRNA could not be detected in the medusa. Black asterisks indicate non specific signal. Scale bar 50μm.

Figure 33 :

 33 Figure 33: Antistasin expression during larval development and in the adult medusa. In situ hybridisation of Antistasin during larval development at 24,48,72 hpf. Antistasin is first detected at24hpf stage in the oral endodermal region. At later stages the gene expression territory extends throughout planula endoderm. At 72hpf some Antistasin-expressing gland cells are found in the ectoderm. Scale bar 50μm. B) Medusa stage. In medusae, Antistasin expression is restricted to gland cells located in manubrium endoderm. Scale bar 100μm. C) High magnification showing Antistasin cell morphology following fluorescence in situ hybridisation: Antistasin (green), Hoechst stained nuclei (blue). Scale bar 10μm. The Medusa cartoon shows the site of Antistasin gene expression (green).

Figure 34 :

 34 Figure 34: Gene orthology of metazoan Boule/DAZ family member. A) Phylogenetic analysis using the RRM domain. Numbers below the branches are maximum likelihood bootstrap values. Note the lineage specific duplication event in cnidarians. Clytia genes are indicated in red. Members of the Musashi orthology group from Drosophila, Xenopus and sea urchin were chosen as outgroup. Species abbreviations: Api (Apis millifera), Bmo (Bombix mori) Che(Clytia hemisphaerica), Cte (Capitella teleta), Ce(Caenorhabditis elegans), Hv (Hydra vulgaris), Hs (Homo sapiens) Mli (Macrostoma lignanum), Mmu (Mus musculus), Xt (Xenopus tropicalis), Nv (Nemtostella vectensis), Tca (Trichoplax adherens), Sp(Strongylocentrotus purpuratus). Sequence name: Accession number HvBoule-like3: XP_012556641.1, NvBoule-like1-a: XP_001620626.1, HvBoule-like1-e: XP_004211430.1, NvBoule: XP_001632599.1, HvBoule-like2b: XP_004207589.1, NveBoule-like1-e: XP_001635220.1, NveBoule-like1-b: XP_001637248.1, HvBoule-like2-a: XP_002163170.2, NvBoule-like2-a: XP_001623902.1, DmGJ12977: XP_002046609.1, XtMusashi :XP_012816891.1, SpMusashi: XP_001199490.2, CheBoulelike1-c: KT318175, CheBoulelike3: KT318176, CheBoulelike1-d: KT318177, CheBoule1-b: KT318178, CheBoulelike1-e: KT318179, CheBoulelike2-a: KT318180, B) RRM sequence alignment. Alignment of RRM sequences; the highly conserved motifs RNP-1 and RNP-2 are indicated in grey. The two amino acids present in all Boule ortholog are highlighted in blue.

Figure 35 :

 35 Figure 35: Boule/Daz family gene expression during larval development and in the adult medusa. A) In situ hybridisation of Boule1-a, Boule-like1-a, Boule-like1-b during larval development at early gastrula 24,48,72hpf. Boule-like1-a and Boule-like1-b were first detected at 24hpf. During development, Boule like1-a expressing cells were found in the oral endodermal region, in a similar distribution to the one reported for Nanos1 and Piwi. Boule like1-b was detected in both ectodermal and endodermal layers at 24, 48 and weaker at 72hpf. Scale bar 50μm. Boule1-a was not detected in the larva. Scale bar 50μm. B) Medusa stage. Boule1-a expression in the adult was detected only in male and female gonads. Boule1-a mRNA was distributed uniformly in the unfertilised egg. Boule-like1-a mRNA was detected exclusively in the tentacle bulbs, in two lateral patches of cell localised at boundary between endoderm and ectoderm. Boule-like1-b gene expression was only detected in the female gonad and the mRNA was ubiquitously expressed in the unfertilised egg. Scale bar 50μm. Medusa cartoon showing the site Boule1-a gene expression.

Figure 36 :

 36 Figure 36: Tudor-domain gene expression during larval development and in the adult medusa. A) In situ hybridization of TDP-1 and TDP-2 during larval development at early gastrula 24,48,72hpf. TDP-1mRNA was detected for the first time at planula 48hpf, and was broadly distributed in the endoderm also at later stages. TDP-2 transcripts were detected for the first time in the early gastrula at future oral pole (gastrulation site). During development it is expressed in a population of cells located within the endoderm region which could correspond to an i-cell subpopulation. Scale bar 50μm. B) Medusa stage. TDP-1 expression detected in the medusa was female specific, with ubiquitous mRNA distribution detected in the developing oocytes. The transcript was detected also in the tentacle bulbs at the boundary between endoderm and ectoderm. TDP-2 mRNA was detected in both male and female gonads and in tentacle bulbs, at the interface between endoderm and ectoderm. Scale bar 50μm.

Figure 37 :

 37 Figure 37:Phylogenetic analysis of metazoan EVE domains proteins Thyn1 proteins are highly conserved from bacteria to mammals. Sequences from Aplysina californica, Capitella teleta,Saccoglossus kowalevskii, Lottia gigantean,Xenopus tropicalis, Nematostella vectensis, Trichoplax adherens, Gallus gallus, Mus musculus, Clytia hemisphaerica, Hydra vulgaris, Homo sapiens, were identified as belonging to Eve family by reciprocal blast. Species abbreviations:Ac (Aplysina californica), Cte (Capitella teleta), Sk (Saccoglossus kowalevskii), Lgi (Lottia gigantea), Xt (Xenopus tropicalis), Nv (Nematostella vectensis), Tca (Trichoplax adherens), Gg (Gallus gallus), Mm (Mus musculus), che (Clytia hemisphaerica), Hv (Hydra vulgaris), Hsa (Homo sapiens).The following sequences containing Eve domain were used for phylogenetic analysis. Sequence name;: Accession numberLgiThyn1: XP_009059928.1, TcaThyn1: XP_009059928.1,HvThyn1: XP_012558048.1, AcThyn1: XP_005096716.1, XtThyn1 :NP_001120782.1, CteThyn1: ELU16431.1GgThyn1 :NP_989618.1, MmThyn1: XP_006510751.1, HsaThyn1: NP_054893.1, SkThyn1: XP_006821718.1, NvThyn1: XP_001635839.1 B) EVE domain analysis. Amino acid sequence analysis shows the presence of the EVE domain in the Thyn1 protein sequence (in green).

  As described in the Introduction, in hydrozoans, including Clytia, Wnt/β-catenin dependent signalling (Figure3A) drives oral/aboral axis formation. During Clytia embryonic development the Wnt/β-catenin pathway specifically establishes the oral pole through the activation of the Wnt3 ligand and the Fz1 receptor produced from maternal mRNAs localised in the egg animal half(Momose et al. 2008)(seeIntroduction 2.1.3). In Hydra and Hydractinia polyps Wnt/β-catenin signalling controls body axis polarity, but it is also involved in the regulation of i-cell differentiation (Introduction, sec 1.6.4). In Hydra the hyperactivation of the Wnt/β-catenin signalling via GSK3 inhibition, induces ectopic nematocyte differentiation; in Hydractinia it also causes an increase in the number of nematocytes and of RFamide neural subpopulations. These observations suggest that in both these hydrozoan polyps Wnt signalling controls i-cell differentiation. The experiments described in this chapter address the functions of the Wnt/β-catenin signalling in Clytia i-cell regulation in relation to axis establishment in the embryo and larva. The chapter includes two manuscripts, one in preparation for publication in Developmental Biology (Ruggiero et al in prep) (PAPER 1) and one published in 2014 (Lapebie, Ruggiero at al., PLoS Genetics 2014) (PAPER 2). PAPER 1 addresses the function of the Wnt/β-catenin pathway in the formation and differentiation of i-cell during embryonic development by a loss of function approach.

Figure 39 :

 39 Figure 39: Nanos1 and Piwi expression is not inhibited in Wnt3 morphants.In situ hybridisation of Nanos1 and Piwi in untreated (left panels) and Wnt3 morphants (right panels) at early gastrula and planula 24,48hpf. At the early gastrula stage Piwi mRNA expression in Wnt3 morphants embryos is slightly weaker than in untreated embryos. In later stages the expression of Piwi and Nanos1in Wnt3 morphants is comparable to the untreated embryos. All control embryos are oriented with the oral pole at the top. Scale bar 50µm.

Figure 40 :

 40 Figure 40: Wnt3 is required for the differentiation of the RFamide neurons. In situ hybridisation of Mcol3-4a, Prdl-a, PP5 (RFamide precursor), PP2 (GLWamide precursor), Antistasin in untreated (left panels) and Wnt3 morphants (right panels) at 24hpf and 48hpf. Prdl-a, RFamide and GLWamide mRNAa were not detected at 24hpf in the control or in Wnt3 morphants. At 24hpf Mcol3-4a expression is absent in Wnt3 morphants, however at 48hpf Mcol3-4a transcript became detectable. At 48hpf, the expression of Prdl-a and PP2 was detected in Wnt3 morphants, while PP5 expression was completely absent. Antistasin was detected in control and in Wnt3 morphants at 24,48hpf planula. Scale bar 50µm.

Figure 41 :

 41 Figure 41:A Wnt gradient is require for neural fate acquisition. Schematic illustration of untreated planula 48hpf and Wnt3 MO at the same stage of development. The distribution of the different somatic cell types is represented according to the expression profile of the gene markers and to morphological features: Nem = nematocytes, i-cells (Piwi and Nanos1), Gland cell (Antistasin), putative neuroblast precursors (Prdl-a). RFamide and LWamide indicate two different neuronal cell subpopulations. Wnt3 morphants lack oral identity, but gastrulate forming ectoderm (grey) and endoderm (light grey). Wnt3 expression domain indicated in yellow, correspond to the ectodermal oral pole absent in Wnt3 morphants. Wnt3 morphants lack nematocytes and RFamide neuronal cell type.

Figure 42 :

 42 Figure 42: Wnt3 signalling is required for de novo i-cell formation. In situ hybridisation using Nanos1 (A-D) and Piwi (E-H) probes on non-injected and Wnt 3 morphants following animal/vegetal separation. A-D embryo bisection experiment performed at blastula stage. All the mini planulae (non injected and Wnt3 morphants) animalderived (A-B) present Nanos1expression, while only the 50% of the Wnt3 injected vegetal -halves present Nanos1 expression. In 24hpf larvae derived from separated 8-cell stage blastomeres (E-H), in situ hybridisation using Piwi probe revealed that 30% of the noninjected embryos are spherical and lack Piwi-expressing cells (G), as compared to 56% in the case of Wnt3-MO injection (H). Scale bars: 50μm for A-H.

  coding for various putative RNA binding proteins, 3 Boule/DAZ related proteins, 2 Tudor domain proteins and the EVE domain nuclear protein Thyn1. Boule/DAZ family members have a characteristic combination of RRM domains and Daz repeats and are widely associated with germ cell development across the animal kingdom {Fu:2015cv} but have not been previously characterised in cnidarians. Clytia Boule1-a is an orthologue to all bilaterian Boule genes (including the Daz or DAZL subfamily specific to vertebrates), while Boule like1-b and Boule like 1-a result from independent expansion of the Boule/DAZ family in the cnidarian linage (Phylogenetic analysis in Supplementary File S2). Tudor domain proteins also have widespread roles in RNA metabolism including the piRNA processing and DNA damage responses, but also in pole cell formation in

  ) and Asc-b (Fig.4G-H) expression was detectable at 48 hpf as it was in uninjected controls, despite the spherical phenotype, showing that neither Wnt3 function nor oral territory development are required for gland cell or nerve cell development. Minicollagen 3-4a expression (Fig.4C-D) also was clearly detected at 48 hpf but not at 24 hpf, indicating that nematoblast formation occurs with a delay. Despite the Wnt3 independence of neural and nematocyte precursors formation, we found that the numbers of differentiated nematocytes and of some neural cells types was severely reduced in Wnt3-MO embryos. Intact 48 hpf and 72 hpf planulae showed on average 23 and 69 nematocytes respectively as visualised with DAPI staining (Fig.5A, C; n=22 and n=21 embryos respectively). In contrast, no nematocytes were detected in Wnt3-MO planulae at 48 hpf and 72 hpf (Fig.5B, D; average=0 and 0,7 for n=26 and 33 respectively). RFamide expressing neurons were not detectable in Wnt3-MO planulae at 48 hpf by either in situ hybridization (Figure4J) or immunofluorescence (Fig.5E-F). On the other hand GLWamide expressing sensory neurons were clearly detectable in the Wnt3-MO larvae at 48 hpf (Figure4K-L). These observations suggest that differention of some icell derivatives may depend on Wnt3 signaling, either directly or through its role in patterning the OA axis. b-catenin nuclear localisation is abolished in Wnt3-MO embryos, at least until the early gastrula stage, and the expression of two Wnt ligands expressed orally during later

  defining the Wnt dependent steps of somatic cell development from i-cells, and uncovering alternative mechanisms to specify formation of the initial i-cell pool. Another point of interest in this study was our identification of two genes expressed early during germ line development, Boule1-a and Thyn1. No expression of either of these could be detected in the planula, supporting a model in which germ cell fate specification in Clytia is delayed until adult stages, for instance being induced by signals from the somatic gonad tissues.

Figure 1 .

 1 Figure 1. Wnt3 requirements for i-cell development in embryos and embryo fragments.Images showing in situ hybridization using Nanos1 (A-F) and Piwi (G-L) probes on noninjected and Wnt 3-MO embryos fixed at the early gastrula stage (A, B, G, H) and in planula larvae fixed at 24 hpf (C, D, I, J) and 48 hpf (E, F, K, L). Cells expressing both probes were detected at all three developmental stages, in intact embryos as well as in embryos lacking Wnt3. In 26 hpf embryos derived from animal (M-N) or vegetal (O-P) halves of blastula stage embryos, clear Nanos1 expressing cells were detectable in all the non-injected or Wnt3-MO injected mini-planulae except for 50% of the "vegetal" group Wnt3-MO injected (P), in which Nanos1 signal is absent. In 24 hpf larvae derived from separated 8-cell stage blastomeres (Q-T), in situ hybridization using Piwi probe revealed that 30% of the non-injected embryos are spherical and lack Piwi-expressing cells (S), as compared to 56% in the case of Wnt3-MO injection (T). Scale bars: 50 μm for M-T.

Figure 2 .

 2 Figure 2. Expression of putative i-cell somatic derivative markers during larval development.Images showing in situ hybridization on planula larvae fixed at 24 hpf and 48 hpf using Antistasin (A-C) probes for gland cells, Minicollagen 3-4a (D-F) for nematocytes, Prdl-a (G-I) for differentiating neurons, Asc-b (J-L) for an uncharacterised endoderm-associated cell population, the RFamide precursor gene PP5 (M-O) for ganglionic neurons and the GLWamide precursor PP2 (P-R), for aboral sensory neurons. Scale bars: 50 μm.

Figure 3 .

 3 Figure 3. In situ hybridization analysis of potential germ line markers.In situ hybridization images of Clytia medusae and planula larvae to characterize expression of a set of potential germ cell expressed genes, Boule-1a (A1-A7) , Boule like1-b (B1-B7), Boule like1-a (C1-C7), TDP-1 (D1-D7), TDP-2 (E1-E7) and Thyn1 (F1-F7). TDP-1 expression in adult medusa was detected only in female gonads. The strong signal in small oocytes (red arrows in A1') is typical of oocyte expressed genes. The only genes exclusively expressed in the germ line, Boule-1a and Thyn1, were not detectably expressed in larval stages. AI', BI,'CI',DI',EI',FI' high magnification of the areas boxed in AI,BI,CI,DI,EI,FI,.

Figure 4 .

 4 Figure 4. I-cell derivative expression in Wnt3-MO embryos. In situ hybridization images of planula larvae fixed at 24 hpf and 48 hpf developed in parallel from non-injected (left panels) and Wnt3-MO injected (right panels) eggs. Probes used: Antistasin for gland cells (A-B), Minicollagen 3-4a for nematocytes (C-D), Prdl-a for differentiating neurons (E-F), Asc-b for undefined endoderm-associated cells (G-H), PP5 for RFamide ganglionic neurons (I-J) and PP2 for GLWamide aboral sensory neurons (K-L). Scale bars: 50 μm.

Figure 5 .Figure 6 .

 56 Figure 5. Wnt3 is required for nematocyte and ganglion cell differentiation. A-D) Poly-gamma-glutamate staining of nematocytes capsules using DAPI at high concentration. Needle shape capsules are clearly visible in 48 hpf planulae (A). Their numbers increase at 72 hpf (C). In Wnt3-MO embryos, no nematocytes were detected (B, D). E-F) RFamide antibody staining of 48 hpf non-injected (E) or Wnt3-MO injected (F) planula larvae. Ganglion cells stained in non-injected condition are totally absent from Wnt3-MO larvae. Neuronal cell bodies are indicated by red arrows. Scale bars: X for A-D; 50 μm for E-F

  -like1-e (XP_001635220.1); NveBoule-like1-b (XP_001637248.1); HvBoule-like2a (XP_002163170.2); NvBoule-like2-a (XP_001623902.1); DmGJ12977 (XP_002046609.1); XtMusashi (XP_012816891.1); SpMusashi (XP_001199490.2);, CheBoulelike1-c (KT318175); CheBoulelike3 (KT318176); CheBoulelike1-d (KT318177); CheBoule1-b (KT318178); CheBoulelike1-e (KT318179); CheBoulelike2-a (KT318180) CheBoule1-a (XXXX); CheBoule1-like-1a (xxxx); CheBoule1-like1-b (xxx).

O)

  Oral genes expressed at oral pole in the ectoderm at all three stages (Paper 2 Figure 2). A) Aboral genes expressed in the aboral ectoderm at early gastrula stage and maintained during planula stage in different aboral pole cell population (Paper 2 Figure 3). IE) Ingressing/Endoderm genes, detected in different population of ingressing/ingressed cells at oral pole during gastrulation, and restricted to the endodermal region during planula stage (Paper 2 Figure 2) D) Delayed Expressed, these transcripts could be detected only very weakly and uniformly at early gastrula stage, while later expression among the genes of this category was variable, covering both endoderm and ectoderm along the OA axis. (Paper 2 Figure 3) These may reflect genes whose expression is generally repressed by Wnt signalling during gastrulation.

2 J

 2 All these genes belonged to the IE category and include a Zn-finger domain protein Znf845, Mos3, two bHLH genes (HlhIE1 and HlhIE2) and the Forkhead transcription factor Fox-A. (Figure34) (Paper 2 Figure2 J,K,L,M,Q,).These 5 genes have similar expression pattern at early gastrula, the transcripts are localised at oral pole in ingressing/ ingressed cells during gastrulation, while at planula stage, HlhIE1 (24hpf and 48 hpf stage) is expressed in few cells in the endoderm (Paper 2 Figure 2 M). HlhIE2 is expressed in few cells in the planula 24hpf and all along the endoderm layer in planula 48hpf (Paper 1 Figure 2 Q). Znf845, Mos3 and Fox-A, showed in larval stages expression patterns similar to those reported for Nanos1 and Piwi (Paper 2 Figure

Figure 44 :

 44 Figure 44: Nanos1, Piwi, Mos3 and Znf845 expression during larval development in young and in adult jellyfishes. A) Expression pattern of the four genes during embryonic development. For each gene expression pattern is shown (from left to the right): early gastrula, 24,48,72hpf. At the early gastrula stage, Nanos1,Piwi, Znf845 and Mos3 transcripts are detected at the oral pole in a group of ingressing cells at site of gastrulation and in the larval stages in the central endodermal region . B) Expression in the young medusa and in the unfertilised eggs. Gonads, eggs, tentacle bulbs, manubrium. Mos3 and Znf845 transcripts were not detected in the gonads or in the eggs. In the young and in the adult medusa (C) their expression is restricted to the tentacle bulbs and to the manubrium ectoderm. Tentacle bulbs are oriented with the proximal side on the top, base of the manubrium at the top. Scale bar 50µm.

  Among these genes, Mcol3-4a (nematoblast marker) and Antistasin (gland cell marker) showed an expression profile that resembled the Znf845 profile in the larval and in medusa stages (see Result-I Figure 33). I thus performed co-localization experiments using the Znf845 probe in combination with Mcol3-4a and Antistasin probes at different developmental stages.

Table 1 :

 1 Znf845 is expressed in about 50% of the total Piwi expressing i-cell population. Quantification performed for 5 planulae. Columns indicate the sample number, the number cells positive only for Znf845, only for Piwi and for both probes.

  Figure 45: Co-localisation of Piwi with Mos3 and Znf845. A) Representative confocal images of double in situ experiments using Piwi and Mos3 probes at early gastrula stage. From left to right: Piwi (in red) Mos3 (in green), last panel merged. Cell expressing both Piwi and Mos3 are concentrated in ingressing cells positioned at oral pole. Scale bar 50µm. B) Detailed view of the area boxed in last panel in A. High magnification image showing that Piwi and Mos3 genes are not expressed in the same cell population. Dashed line represent the contour of the early gastrula. Scale bar 10µm. C) Representative confocal image demonstrating the co-localisation of Piwi (green) Znf845 (red) in planula 24hpf. Merged images show that Piwi and Znf845 co-localise in same cells. Yellow colour in the merged image indicates co-expression. Scale bar 50µm D) Detailed view of the area boxed in last panel in C. High magnification image showing that Piwi and Znf845 co-localise in some cells. Scale bar 10µm.

  Figure 46: Znf845 is not expressed in gland cells or in nematoblasts. A) Representative confocal image of Znf845 positive cells, highlighting the i-cell like morphology of the Znf845 expressing cells in the manubrium ectoderm. Highlighted cell with an i-cell like morphology (outlined by dashed lines in the left panel). B) Schematic drawing showing the distribution of Znf845 and Antistasin positive cells in the medusa manubrium. Znf845 (in red) and Antistasin (in green). Piwi positive cells are also shown (cyan). C) Representative confocal images of Znf845 and Antistasin expression patterns in young medusa. The orientation of the images follows the section traced in the scheme in B. Znf845 (red), Antistasin (green), merged image. Dashed lines mark endoderm (en), ectoderm (ec). Yellow asterisks indicated unspecific Znf845red signal bleeding through from Antistasin strong staining. D) Representative confocal image of double in situ hybridisation using Mcol3/4a and Znf845 at planula 24hpf stage. From right to the left: Znf845 (green), Mcol3-4a (red), last panel merge. Scale bar 50µm.

  Figure 47: Znf845 is not expressed in Sox15 positive cells. A) Representative confocal images of double in situ experiments using Znf845 and Sox15 probes at p48hpf stage. From left to right: Znf845 (in magenta) Sox15 (in green), last panel merged. Scale bar 50µm. B) Detailed view of the area boxed in last panel in A. High magnification image showing that Znf845 and Sox15 genes are not expressed in the same cell population. Scale bar 10µm.

Figure 48 :

 48 Figure 48: I-cell cartography. Schematic representation of the distribution of i-cell derivatives during larval development and in the adult medusa. Each putative i-cell derived lineage is illustrated on separate lines: A) Nematogenesis B) Neurogenesis C) Gland cell formation D) Gametogenesis. Nanos-and Piwi-expressing i-cells (in orange) are represented only in the nematogenesis line (A), indicate the co-expression of Piwi with Mcol3-4a. Nanos1 and Piwi i-cell population is detected throughout larval development and in the medusa ( see Figure12). The colours correspond to the genes listed in E. Bi-coloured dots indicate co-localisation. At 36hpf only the expression of the putative neural genes was verified. E). Table summarising the cell types and the assigned markers.

  , which could explain the delay in Mcol3-4a expression in Wnt3-MO embryos (Figure 40 ResultsII). The expression pattern and the Wnt3-MO response are compatible with a role for Sox15 in a bi-potent progenitor population, or as a nematogenesis specific gene.

  expressed early during nematogenesis in Hydra like Prdl-b and COUP-TF. I identified a Clytia Prdl-b ortholog sequence (Figure 25, Results-I), but no clear ortholog of Hydra COUP-TF in the extensive transcriptome data available for Clytia (data not shown). COUP-TF in Hydra and in Hydractinia is thought to promote neural differentiation and to down-regulate nematogenesis (see above; Miljkovic-Licina et al. 2007; Duffy & Frank 2011). In Hydractinia COUP-TF is positively regulated by the Wnt/β-catenin pathway. Thus Hydractinia polyps treated with LiCl (which inhibits GSK3β) show increased COUP-TF expression. Although Wnt pathway disruption during Clytia embryogenesis did not prevent Prdl-a expression, it caused a transient delay in the appearance of early nematogenesis markers (Mcol3-4a). Whether this represents a direct of indirect consequence of the Wnt3-MO phenotype, the transcriptomic data derived from the Wnt3 early gastrula morphants (Lapébie et al. 2014) could provide some good candidate transcription factors with possible roles in early fate specification events. Among the genes under-expressed in Wnt3 morphants, the gene HlhIE1 shows an expression profile that could fulfil the criteria required to be considered as gene expressed in the neural precursor cells (Figure 2M, PAPER 2). HlhIE1 can be placed by phylogenetic analysis (Figure 27, Results-I) into the Achaete Scute family, whose members are often involved in neural fate commitment (see Introduction 2.3.2 and ResultsI). During Clytia development HlhIE1

FoxO

  in Clytia larvae and in adult medusa in relation to homeostasis of different cell populations would be very interesting. • Differentiation of some neural subtypes requires Wnt signalling Wnt signalling has been variously implicated in each step of neural development: neural stem cell population maintenance, neural lineage specification, neural precursor proliferation and differentiation (Reviewed in Inestrosa & Arenas 2010; Mulligan & Cheyette 2012).

  antibody) is able to develop germ cell features, suggesting that germ plasm is instructive for germ line segregation.The second mechanism of germ line specification involves extrinsic signals from surrounding tissues to a group of embryonic or other undifferentiated cells that will acquire germ cell identity. A recent study in the cricket(Donoughe et al. 2014) showed that BMP signalling, as is the case for mouse (Saitou & Yamaji 2012), is the signalling pathway responsible for PGC induction. Alteration of BMP signalling by RNAi tretments targetting BMP pathway components (e.g. the BMP ligands BMP2/4, BMP5/7/8 and the effector Smad1/5/8) impairs PGC formation independently of the effect on mesoderm induction.

Figure 5: Embryonic axis and Wnt/β-catenin signalling.

	A) Comparison of the axial
	localisation of Wnt transcripts
	and β-catenin nuclearisation in
	metazoans.	Wnt	expression
	domains are represented with
	green lines, while β-catenin
	localisation is illustrated with sky
	blue dots.
	In cnidarians Hydra, Clytia and
	Nematostella (shaded in red), β-
	catenin nuclearisation occurs at
	the oral pole. (Modified from
	Petersen & Reddien 2009).
	B) Model of axis determination
	in Clytia. Wnt/β-catenin pathway
	core components are distributed
	asymmetrically in the egg. On the
	animal side Wnt3 mRNA (in
	red), Fz1 mRNA (in green); on
	the vegetal side Fz3 mRNA (in
	blue). In Clytia embryos, β-
	catenin	nuclearisation	is
	restricted to the animal side and
	coincides with Wnt3 and Fz1
	activity. Wnt3 and Fz1 via β-
	catenin drive the establishment of
	the oral identity. In contrast Fz3
	on the aboral side inhibits β-
	catenin	localisation.	From
	Momose et al. (2008).

7: Cnidarian models in developmental biology.

	A) A current view of phylogenetic
	relations	among	cnidarians.
	Nematostella belongs to the
	anthozoans, while Hydractinia,
	Hydra and Clytia are hydrozoans.
	B) Schematic representation of
	the Nematostella directive axis
	determined by the shape of the
	pharynx and the asymmetric
	position of the retractor muscles
	within the mesenteries.
	C) Representative cnidarian model
	systems used in developmental
	biology.
	From top to bottom: Nematostella
	vectensis, Hydractinia echinata,
	Hydra	vulgaris	and	Clytia
	hemisphaerica.
	For each organism is indicated the
	adult	stage	(polyps	and/or
	medusae) and "developmental"
	stages (cleavage and larval stages).
	Modified from Technau & Steele

TABLE 1 : I-cell development following Blastula bisection.

 1 Phylogenetic analysis of metazoan paired-class homeobox transcription factors. The predicted amino acid sequences of Paired subfamily proteins was extracted from[START_REF] Ryan | The cnidarian-bilaterian ancestor possessed at least 56 homeoboxes. Evidence from the starlet sea anemone, Nematostella vectensis[END_REF] Lapébie et al. 2014). Maximum likelyhood tree with bootstrap values next to each node. Clytia genes are indicated in red. Clytia hemisphaerica Prdl-a and Prdl-b were named according to their orthology with Hydra Prdl-a and Prdl-b. Scale bar represents 0.6 substitions per site. The following Clytia hemisphaerica sequences were added to the alignment provided in(Lapébie et al. 2014). Accession numbers in parentheses.Phylogenetic analysis of metazoan DAZ/Boule family RRM proteins. Sequences from Hydra vulgaris and Nematostella vectensis genomes and Clytia hemisphaerica transcriptomes were identified as belonging to Boule/Daz family by reciprocal blast and added to the sequence list below. Species abbreviations: Api (Apis millifera), Bmo (Bombix mori) Che(Clytia hemisphaerica), Cte (Capitella teleta), Ce(Caenorhabditis elegans), Hv (Hydra vulgaris), Hs (Homo sapiens) Mli (Macrostoma lignanum), Mmu (Mus musculus), Xt (Xenopus tropicalis), Nv (Nemtostella vectensis), Tca (Trichoplax adherens), Sp(Strongylocentrotus purpuratus) Members of the Musashi orthology group from Drosophila, Xenopus and sea urchin were been chosen as an outgroup. Divergent parts of the alignment have been removed, and the RRM domain and Daz repeats retained. The sequences published in(Tsai et al. 2014) have been used , and the following sequences from Clytia, Nematostella and Hydra have been added, with Accession numbers in parentheses.

	6 hpf blastula		Non-injected			Wnt3-MO injected
	30min after blastula bisection	Uncut	Animal half	Vegetal half	Uncut	Animal half	Vegetal half
	Embryos with Nanos1 positive cells (%)	100	100	38	100	91	50
	Embryos without Nanos1 positive cells (%)	0	0	62	0	9	50
	Total	26	11	8	24	22	12
	26 hpf planula		Non-injected			Wnt3-MO injected
	20h after blastula bisection	Uncut	Animal half	Vegetal half	Uncut	Animal half	Vegetal half
	Embryos with Nanos1 positive cells (%)	100	100	92	100	95	50
	Embryos without Nanos1 positive cells (%)	0	0	8	0	5	50
	Total	27	14	12	30	21	18

MATERIAL & METHODS

Acknowledgements :

We thank Sandra Chevalier for invaluable technical assistance and Doug Houston (Iowa) for his participation in cutting experiments. We also thank Doug and all our group members, especially Lucas Leclère and Chiara Sinigaglia for stimulating discussions.

RESULTS I:

Monitoring the spatial and temporal distribution of i-cell derivatives during Clytia larval development.

cnidarian genes. A second Clytia Zic gene (named CheZic), found later is the ortholog of the Hydra Hyzic.

I investigated the expression pattern of Clytia Zic-C gene during embryonic development and in the adult medusa. In planulae Zic-C mRNA was weakly detected at the aboral pole, at 48 and 72hpf (Figure 30A). In contrast, in the medusa, Zic-C expression was strongly detected in the tentacle and in the tentacle bulb ectoderm. Zic-C expressing cells are most likely neurosensory cells as indicated by their neuronal process (Figure 30-B). Zic-C neuronal cells appear to be specifically associated with the tentacles, since no expression was detected in the manubrium ectoderm or in gonad ectoderm or in nerve ring.

Consistently with previous data in non-bilaterian species, these observations indicate that Zic-C is expressed late during development and in a tentacle specific neuronal subtype.

In support of this hypothesis the Hydra Zic-C ortholog is not enriched in sorted i-cell population (Table1 Annexes).

Potentially Thyn1 could thus be involved in regulating germ cell degeneration in Clytia gonads. The restriction of its expression to the gonad, as in the case of Boule-1a, is consistent with a scenario in which germ cell fate specification occurs only in sexually mature medusae.

Overall these data suggest that Boule-1a and Thyn1 are germ cell markers, and indicate that germ cell specification occurs only after larval metamorphosis. It will be interesting to test the functional role of these putative germ cell specific genes during germ cell development.

The level of these two transcripts could be downregulated using RNAi (currently being tested in the group, S. Chevalier unpublished) on the whole medusa (at different stages of sexual maturation, or on the isolated gonads techniques), or by gene editing using the efficient CRISPR-Cas9 approach (T. Momose, unpublished).

RESULTS II:

Wnt/β-catenin signalling in embryonic patterning, i-cell formation and i-cell differentiation during Clytia embryionic and larval development We conclude that during Clytia embryogenesis Wnt signaling is required for i-cell formation in the absence of germ plasm (directly or through axis establishment) and for nematocyte and neuronal differentiation, but not for somatic cell fate specification from icells. Our findings also support a role for germ plasm in i-cell fate determination during normal embryogenesis.

Highlights

• Germ plasm inheritance determines i-cell fate during normal embryogenesis • I-cell formation in the absence of germ plasm is possible via Wnt signaling • Somatic fate specification from i-cells in the larva is Wnt independent.

• Final differentiation of nematocytes and some neurons requires Wnt signaling • No evidence for germ cell lineage specification during larval development.

Results

I-cells can form by Wnt3 dependent and independent mechanisms

To test the role of wnt signaling in i-cell formation we blocked Wnt/b-catenin signaling during embryogenesis by injection prior to fertilization of Wnt3-MO (Momose et al., 2008) and monitored i-cell formation by in situ hybridization using Piwi and Nanos1

probes. The i-cells detected by these two probes detect both the multipotent stem cell population and some cells already embarked on the developmental pathway to form derivative cell types such as nematocytes, as shown by double fluorescent in situ with the nematoblast-expressed gene Minicollagen 3/4a [START_REF] Denker | Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)[END_REF] (data not shown). In Wnt3-MO embryos fixed at the early gastrula stage, 24 hpf (early planula) and 48 hpf (mid planula), i-cells were clearly detected in the vast majority of embryos in parallel with their appearance in non-injected controls, despite the spherical morphology indicating loss of the oral territory (Fig. 1A-L). These observations indicate that during normal development neither Wnt signaling nor oral fate determination is required for i-cell formation or maintenance of the population, and are compatible with a direct mechanism of i-cell fate determination by inherited germ plasm. We addressed this possibility using the two different embryo manipulation approaches employed previously [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF], blastula bisection or separation of blastomeres at the 8-cell stage.

Non-injected and Wnt3-MO pre-injected blastula-stage embryos (about 5-6 hpf)

were bisected into putative animal and vegetal halves, using the distinctive lobed morphology observed in some embryos to orient the OA axis. Nearly all "animal" blastula halves from Wnt3-MO also developed clear i-cells by 26 hpf showed a very similar expression profile to Prdl-a through larval development (Fig. 2J-L).

Clytia Asc-b is the orthologue of Podocoryne Ash2 (phylogenetic analysis in Supplementary file S2), expressed in putative secretory cells in the endoderm [START_REF] Hensel | Lineage-specific evolution of cnidarian Wnt ligands[END_REF][START_REF] Khalturin | Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation[END_REF]Seipel et al., 2004;[START_REF] Teo | An evolutionary conserved role of Wnt signaling in stem cell fate decision[END_REF]. Expression of Clytia Asc-b in medusa was confined to cells in endodermal structures (circular but not radial canals and manubrium), whose precise identity remains to be established (Figure S1). Trancriptomic data from separated cell populations from Hydra polyps [START_REF] Hemmrich | Molecular signatures of the three stem cell lineages in Hydra and the emergence of stem cell function at the base of multicellularity[END_REF] supported an i-cell origin for Prdl-a, Asc-b and Antistasin-expressing cells, since Hydra ortholog transcripts for all these genes were all detected in Nanos-positive cells, and more strongly in endodermal cells for Asc-b and Antistasin (Table 3). The timing of Clytia Prdl-a and Ascb expression compared to Minicollagen 3/4a expression indicates that neurogenesis occurs later than nematocyte differentiation during Clytia larval development, although this should be confirmed by identifying genes expressed earlier during neurogenesis. We cannot rule out the possibility of a common neural-nematogenesis precursor as described in Nematostella (Richards and Rentzsch, 2014) and in Hydra [START_REF] Galliot | Developmental Biology[END_REF].

Finally to map the formation of differentiated neurons we identified the Clytia cDNA sequences coding for precursors of RFamide neuropeptides, widely expressed in cnidarian i-cell derived ganglionic cells (Hayakawa et al., 2004;[START_REF] Martin | III. Some interstitial cells traverse the ganglionic pathway in the endoderm[END_REF]Momose et al., 2008;Momose and Houliston, 2007), and one of two GLWamide precursor genes identified in the Clytia transcriptome, PP2. GLWamides are expressed in sensory neurons concentrated at the aboral pole of hydrozoan larvae and are involved in triggering metamorphosis [START_REF] Piraino | Complex neural architecture in the diploblastic larva of Clava multicornis (Hydrozoa, Cnidaria)[END_REF]Takahashi and Hatta, 2011). All the neurons expressing GLWamide precursor genes in Hydra are absent in i-cell free polyps [START_REF] Lapébie | Differential Responses to Wnt and PCP Disruption Predict Expression and Developmental Function of Conserved and Novel Genes in a Cnidarian[END_REF][START_REF] Mitgutsch | Expression and Developmental Regulation of the Hydra-RFamide and Hydra-LWamide Preprohormone Genes in 1-15[END_REF], implying an i-cell origin, although we cannot rule out the possibility that the sensory neurons of ectodermal origin described in hydrozoan larvae (Martin and Thomas, 1981a;Momose et al., 2008;Thomas et al., 1987)

SUPPLEMENTARY Files

Supplementary Figure S1

S1) Putative somatic i-cell derivative gene expression in adult medusa

Images showing in situ hybridization on adult medusa using Antistasin

PAPER 2: Summary of the Results

The study of the Wnt pathway is one of the main interests of our laboratory. During the course of my PhD I participated in a group project aimed at identifying via a comparative transcriptomic approach novel genes involved in embryo patterning directly or indirectly regulated by Wnt signalling.

To gain a more exhaustive understanding of the gene toolkit involved in generating and maintaining Clytia axial polarity, the transcriptome of un-manipulated wild type early gastrula stage embryos was compared with that of Wnt3 early gastrula morphants (Figure 43). Previous observations from our group indicated that the first zygotic expression of many genes starts at the beginning of gastrulation.

My participation in this project was mainly in the systematically characterisation of the expression pattern by in situ hybridisation of genes significantly over or under expressed in Wnt3 morphant early gastrula embryos. I took advantage of this new transcriptome dataset to look for novel i-cell associated genes. Indeed by comparing the expression patterns of Wnt3 target genes I found that five genes in particular showed expression patterns similar to those of the i-cell markers Nanos1 and Piwi. I went on to investigate in more detail two of these, Mos3 and Znf845 as described in the additional results presented after PAPER 2 in this chapter.

Identification of novel Clytia embryos patterning genes

To identify novel regulators implicated in Clytia axial polarity and embryogenesis we compared the transcriptome of un-manipulated gastrula embryos with the transcriptome of the Wnt3 early gastrula morphants. To distinguish the influence of Wnt3/Fz/β-catenin signalling and Fz1/PCP we included in this study the transcriptomes of the Fz1 and Stbm early gastrula morphants.

Differential gene expression analysis based on stringent selection of the most significantly over and under expressed genes identified: 1) 40 genes significantly upregulated in Wnt3

early gastrula morphants (negatively regulated by Wnt3) of which 18 were selected for further validation. 2) 114 transcripts downregulated in Wnt3 early gastrula morphants

The fact that IE (ingressing/endoderm) profile genes and D profile genes (repressed until early gastrula) are particularly sensitive to PCP disruption is yet to be fully understood. In Clytia PCP disruption by these morpholinos leads to loss of ectodermal planar cell polarity and in to defects in planula elongation as well as altered gastrulation, which could well result in a delay in the expression of genes associated specifically with ingressing cells. Published in PloS Genetics September 2014

• Mos3 is not expressed in the same cells as Piwi

The kinase Mos has been widely investigated for its role in oocyte development in numerous taxa. Mos is a serine-threonine protein kinase, which activates the MAPK (mitogen-activated protein kinase) pathway and regulates meiotic divisions and oocyte specific processes such as polar body formation and the "cytostatic" (CSF) cell cycle arrest of the egg awaiting fertilisation [START_REF] Dupré | Mos in the oocyte: how to use MAPK independently of growth factors and transcription to control meiotic divisions[END_REF].

MAPK/ERK signalling is activated in both eggs and in somatic cells where it plays a pivotal role in regulating cellular proliferation, cellular differentiation and in some circumstances apoptosis [START_REF] Yoon | The extracellular signal-regulated kinase: multiple substrates regulate diverse cellular functions[END_REF]. In somatic cells the activation of this pathway depends phenotype, although at higher doses the organisation of the planula endoderm appeared to be disrupted. Further analysis will be required to determine if the phenotype is specific to Mos3-MO or is due to the high dose used.

To further investigate this intriguing gene, with its unexpected expression pattern in the embryo, several basic aspects need to be clarified. First of all it would be useful to determine if Mos3 kinase has "CSF" activity, assayed as the ability to cause arrest of blastomere division via the activation of the MAPK cascade by the classical assay of mRNA injection into Xenopus embryos, as performed previously for Mos1 and Mos2 (Amiel et al. 2009). It would also be interesting to verify if in Mos3 expressing cells MAPK/ERK is active by double labelling (IF-FISH) using Mos3 probe and an antibody recognising the phosphorylated (active) form of ERK, in order to see when during development the pathway becomes active.

These studies of i-cell fate pathways would obviously be greatly aided by the development of transgenic lines expressing a fluorescent reporter in i-cells as has been developed in Hydra (Wittlieb et al. 2006b) or under the control of linage specific promoters as achieved for SoxB [START_REF] Hansen | Control of the proliferation versus meiotic development decision in the C. elegans germline through regulation of GLD-1 protein accumulation[END_REF] in Nematostella (Richards & Rentzsch 2014). Such transgenic reporter lines would enable more easily to relate the expression of different genes to the development of each particular derivative cell type.

Discussion & Perspectives

origins of the Nanos1 and Piwi expressing i-cells and the relationship between i-cell formation and germ line segregation mechanisms.

Neurogenesis and Nematogenesis

The expression patterns of gene markers coupled with other methods to detect differentiated nematocytes and RFamide-expressing ganglion/intermediate neurons was used to characterise the appearance and distribution of i-cell derivative intermediates space during larval development, and in the adult, as summarised in Figure 48. This i-cell cartography, based on the expression of selected genes at successive stages, revealed that nematoblasts expressing the capsule protein Mcol3-4a appear earlier (at 24hpf, i.e. just at the end of the gastrulation) than expression of the neurogenesis gene Prdl-a (first detected at 36hpf). In Hydra Prdl-a is expressed in a particular sub-population of developing apical neurons [START_REF] Gauchat | prdl-a, a gene marker for Hydra apical differentiation related to triploblastic paired-like head-specific genes[END_REF][START_REF] Miljkovic-Licina | Neuronal evolution: analysis of regulatory genes in a first-evolved nervous system, the hydra nervous system[END_REF]). The relative timing of Mcol3-4a

and Prdl-a expression suggests that specification of the nematocyte lineage may occur before that of the nerve cell lineage in the Clytia larva. In Hydra it has been proposed that neurons and nematocytes derive from a common precursor expressing the parahox gene [START_REF] Miljkovic-Licina | Head regeneration in wild-type hydra requires de novo neurogenesis[END_REF]. Conversely the expression of Coup-TF, which is thought to repress nematocyte proliferation, was upregulated [START_REF] Gauchat | The orphan COUP-TF nuclear receptors are markers for neurogenesis from cnidarians to vertebrates[END_REF].

In Clytia Cnox-2/Gsx is expressed only in the medusa, in the distal part of the tentacle bulbs, described as the nematocyte differentiation zone [START_REF] Chiori | Are hox genes ancestrally involved in axial patterning? Evidence from the hydrozoan Clytia hemisphaerica (cnidaria)[END_REF]) but where neural differentiation may also be occurring (see Prdl-a expression in Figure 26, Results-I).

If Gsx function is conserved in Clytia, this lack of expression in the larva could suggest that nematogenesis and neurogenesis at this stage do not involve a common precursor, although it remains possible that another regulatory transcription factor substitutes for Gsx at the larval stage.

expression pattern analysis including double in situ to verify an i-cell origin, followed by functional studies.

Impact of Wnt/β-catenin signalling on i-cell dynamics

• I-cell population maintenance is not dependent on Wnt signalling

Wnt signalling can promote cell proliferation and stem cell renewal (see Introduction 1.6.4). In Clytia larvae I found no evidence for a role for Wnt signalling in i-cell renewal.

The development and maintenance of Nanos1 and Piwi i-cell expressing populations were not affected in Wnt3 morphants (see Figure 39). This finding is supported by transcriptome analysis, which reveals that the Nanos1 and Piwi expression levels are not significantly affected by Wnt3 disruption (Figure 43C).

The Nanos1 and Piwi-expressing i-cell population includes not only multipotent cells, but also some "fated" cells, probably committed towards different somatic fates, as identified To conclude, the results reported here suggest that Wnt signalling is not involved either in i-cell maintenance or early precursor specification, even though more early makers need to be tested in Wnt3 morphants. These findings are in contrast with the results obtained in Hydractinia (discussed also in the section 2.1.1, ResultII) in which the hyperactivation of the Wnt signalling boost the number of S-phase cells in stolons. It is possible that is effect is due to the global polyp treatment, and that not only i-cell are responsive to Wnt signalling.

from the egg, inherited through cleavage stages until the blastula stage. They continue to be detected, probably as a result of new transcription from the zygotic genome, in cells that form at a corresponding position at the early gastrula stage, which then migrate into the endodermal region of the planula during late gastrulation (Figure 12). Despite this spatial "continuity" in the expression of germ plasm genes during Clytia development [START_REF] Leclère | Maternally localized germ plasm mRNAs and germ cell/stem cell formation in the cnidarian Clytia[END_REF] we could not conclude prior to this study that in Clytia germ cell lineage fate is determined by germ plasm inheritance. On one hand, it was not known whether at the larval stage the i-cell population was uniform in terms of potential, or whether a subpopulation was already committed to an exclusive germ line fate and thus represents a true germ line. On the other hand, there was no functional evidence that the maternal germ plasm had a determinant function. Although not conclusive, my study provides new evidence relating to both of these points.

• Is hydrozoan Germ plasm inherited by the i-cell lineage or a dedicated germ line?

Given that the "germ line" markers including Nanos1 and Piwi in Clytia show wider expression in i-cells and their derivatives, to monitor the appearance of germ cell precursors during development I searched for genes involved specifically in gametogenesis.

Among the conserved family genes implicated in gametogenesis I focussed on the Boule and Tudor families.

In the available Clytia transcriptome dataset I found 10 boule-like genes, including one, Boule1-a, that fulfilled the criteria to be considered a germ cell markers. Phylogenetic analysis revealed that Boule1-a is an ortholog of the vertebrate Boule/Daz family gene group (Figure 34 ResultI), and possesses two conserved amino acids at positions considered as the Boule signature [START_REF] Shah | Widespread presence of human BOULE homologs among animals and conservation of their ancient reproductive function[END_REF][START_REF] Kuales | Boule-like genes regulate male and female gametogenesis in the flatworm Macrostomum lignano[END_REF]. In situ hybridisation analysis showed that Boule1-a expression is restricted to medusa gonads (both female and male) and is not detected in other stem cell areas, like tentacle bulbs. Importantly no Boule1-a expression was detected in larval stages, suggesting that germ cell specification occurs only in the sexual medusa. A second gene with an identical expression profile was also identified (Thyn). Neither of the tudor domain containing genes selected, TDCP1 and TDCP2, showed an expression pattern compatible with a germ cell marker (Figure 36).

These observations indicate that despite the presence of localised germ plasm in the eggs, This table lists Hydra magnipapillata and Hydra vulgaris orthologs of selected Clytia genes analysed in this study, and the number of matching reads from the transcript dataset (HAEP) available at http://compagen.zoologie.uni-kiel.de/blast.html [START_REF] Hemmrich | Molecular signatures of the three stem cell lineages in Hydra and the emergence of stem cell function at the base of multicellularity[END_REF] for separated cell populations (endoderm, ectoderm and nanos-positive cells), normalized with respect to total read number.