Anaïs Portet

Silvain Pinaud

Guillaume Tetreau

Richard Galinier

Celine Cosseau

David Duval

Christoph Grunau

Guillaume Mitta

Benjamin Gourbal
email: benjamin.gourbal@univ-perp.fr

Integrated multi-omic analyses in Biomphalaria-Schistosoma dialogue reveal the immunobiological significance of FREP-Sm PoMuc interaction

Keywords: Compatibility polymorphism, Schistosoma mansoni, Biomphalaria glabrata, FREPs, SmPoMucs

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Introduction

Host-parasite interactions are dynamic biological systems in which the host's defense mechanisms face the parasite's infectivity mechanisms [START_REF] Combes | Selective pressure in host-parasite systems[END_REF][START_REF] Howard | Evolution of immunity and pathogens[END_REF]. The coevolution of hosts and parasites involves huge reciprocal selective pressures on both partners. Therefore, in natural environment, all living organisms have developed defense systems capable of recognizing and activating efficient effectors to eliminate most of the pathogens encountered during their life span. In invertebrate host/parasite interactions, invariable germ-lineencoded or somatically diversified pathogen recognition receptors (PRRs) that recognize a set of pathogen associated molecular patterns (PAMPs) appear to be the ultimate outcome of this molecular arms race to limit pathogen invasion [START_REF] Medzhitov | Innate immunity: the virtues of a nonclonal system of recognition[END_REF]. These polymorphic and/or diversified receptors have been identified in echinoderms (SRCR or Sp185/333 of sea urchin [START_REF] Ghosh | Sp185/333: a novel family of genes and proteins involved in the purple sea urchin immune response[END_REF][START_REF] Pancer | Dynamic expression of multiple scavenger receptor cysteine-rich genes in coelomocytes of the purple sea urchin[END_REF][START_REF] Pancer | Origins of immunity: transcription factors and homologues of effector genes of the vertebrate immune system expressed in sea urchin coelomocytes[END_REF], insects (Dscam of Drosophila melanogaster and Anopheles gambiae [START_REF] Dong | AgDscam, a hypervariable immunoglobulin domain-containing receptor of the Anopheles gambiae innate immune system[END_REF][START_REF] Watson | Extensive diversity of Ig-superfamily proteins in the immune system of insects[END_REF], crustaceans (Dscam of Daphnia [START_REF] Brites | The Dscam homologue of the crustacean Daphnia is diversified by alternative splicing like in insects[END_REF] or Litopenaeus vannamei [START_REF] Hung | Properties of Litopenaeus vannamei Dscam (LvDscam) isoforms related to specific pathogen recognition[END_REF], and molluscs (FREPs of Biomphalaria glabrata (Zhang et al., 2004)). These studies indicated that these putative immune receptor variants vary considerably between individuals, yielding an enlarged repertoire of putative recognition molecules. We can speculate that large and individual repertoires of immune receptors might confer the immune recognition specificity of invertebrate immune systems and explain in part, the issue of hostpathogen compatibility.

The present chapter explores the molecular interactions between Schistosoma mansoni and the snail Biomphalaria glabrata that provides a model of choice in evolutionary biology for investigating host-parasite co-evolutionary dynamics and the mechanisms of invertebrate innate immune response [START_REF] Lockyer | Biomphalaria glabrata transcriptome: cDNA microarray profiling identifies resistant-and susceptible-specific gene expression in haemocytes from snail strains exposed to Schistosoma mansoni[END_REF][START_REF] Mitta | Compatibility polymorphism in snail/schistosome interactions: from field to theory to molecular mechanisms[END_REF][START_REF] Mone | An example of molecular co-evolution: reactive oxygen species (ROS) and ROS scavenger levels in Schistosoma mansoni/Biomphalaria glabrata interactions[END_REF][START_REF] Pinaud | A shift from cellular to humoral responses contributes to innate immune memory in the vector snail Biomphalaria glabrata[END_REF]Zhang et al., 2004). The genus Schistosoma contains around twenty species, three of which, S. haematobium, S. japonicum, and S.

mansoni are the principal agents of human schistosomiasis that represents the second most prevalent parasitic disease in the world affecting 200 million people and killing 200,000 persons every year [START_REF] Chitsulo | Schistosomiasis[END_REF]. Adult worms mate in a vertebrate host venous system. They produce eggs that reach the aquatic environment by passing through the vertebrate intestinal or bladder wall. In water, eggs hatch and release a miracidium that specifically infects fresh water snail species.

Schistosomes, like all trematode species, needs molluscs as intermediate hosts to carry out part of their life cycle before infecting humans. Inside snail tissues, the miracidium turns into primary sporocyst (Sp1) that gives sequentially a secondary sporocyst (Sp2) and then cercaria by asexual multiplication. After leaving the snails, cercariae actively look for a definitive vertebrate host to infect [START_REF] Yoshino | Chapter 7: schistosoma sporocysts[END_REF]. In South America, B. glabrata snails have important medical and epidemiological impacts due to their role as the main vector of S. mansoni, the agent of intestinal Schistosomiasis. Consequently, a better understanding of the molecular interactions and the compatibility (susceptibility/resistance status) between B. glabrata and S. mansoni would contribute to the discovery of new ways to prevent and/or control Schistosomiasis diseases in the field (Tennessen et al., 2015b). A high level of specificity exists between the Schistosoma miracidia and the snail intermediate hosts. This specificity could be explained by the suitability/unsuitability between the physiological and biochemical states of the host and the parasite. However, the main factor supporting this specificity is the host immune system and the ability of the parasite to cope with this immunity and/or to circumvent it [START_REF] Sapp | A comparative study of mechanisms underlying digenean-snail specificity: in vitro interactions between hemocytes and digenean larvae[END_REF][START_REF] Van Der Knaap | Immune mechanisms in trematod-snail interactions[END_REF]. Thus, during specific interactions, either the host wins and the parasite is eliminated or the parasite wins and infects the host. This aspect of host/parasite interactions is described under the spelling compatibility polymorphism [START_REF] Mitta | Compatibility polymorphism in snail/schistosome interactions: from field to theory to molecular mechanisms[END_REF]) (see Fig. 5). Interestingly, investigation of the fate of miracidium parasites after penetration in the same host snail revealed that the infected snail can contain both developing primary sporocysts and encapsulated sporocysts located side by side within its tissues [START_REF] Theron | Schistosoma mansoni: distribution patterns of miracidia among Biomphalaria glabrata snail as related to host susceptibility and sporocyst regulatory processes[END_REF][START_REF] Theron | Multi-parasite host susceptibility and multi-host parasite infectivity: a new approach of the Biomphalaria glabrata/Schistosoma mansoni compatibility polymorphism[END_REF]. The phenotype (infective vs. uninfective) of a parasite is expressed as a function of the genotype of the particular host that it enters. Likewise, the phenotype (susceptible vs. unsusceptible) of a host is expressed as a function of the parasite genotype it harbours [START_REF] Theron | Multi-parasite host susceptibility and multi-host parasite infectivity: a new approach of the Biomphalaria glabrata/Schistosoma mansoni compatibility polymorphism[END_REF]. Compatibility polymorphism is thus based on a matching-phenotype model in which the compatibility is tested independently for each entering miracidium and each exposed snail [START_REF] Theron | Schistosoma mansoni: distribution patterns of miracidia among Biomphalaria glabrata snail as related to host susceptibility and sporocyst regulatory processes[END_REF][START_REF] Theron | Multi-parasite host susceptibility and multi-host parasite infectivity: a new approach of the Biomphalaria glabrata/Schistosoma mansoni compatibility polymorphism[END_REF]. Numerous studies using isolates or strains of Biomphalaria snails from diverse geographic localities that have been exposed to sympatric or allopatric strains of S. mansoni (see Fig. 5) have emphasized the high compatibility polymorphism that characterizes this host-parasite matchingphenotype [START_REF] Basch | Intermediate host specificity in Schistosoma mansoni[END_REF][START_REF] Morand | Parasite-host coevolution and geographic patterns of parasite infectivity and host susceptibility[END_REF][START_REF] Richards | Genetic factors in susceptibility of Biomphalaria glabrata for different strains of Schistosoma mansoni[END_REF][START_REF] Richards | The genetic variation of compatibility in Biomphalaria glabrata and Schistosoma mansoni[END_REF][START_REF] Theron | Multi-parasite host susceptibility and multi-host parasite infectivity: a new approach of the Biomphalaria glabrata/Schistosoma mansoni compatibility polymorphism[END_REF]. To go further in the understanding of this matching-phenotype, we must decipher the mechanisms through which snails and schistosomes interact. Integrative approaches are needed to combine a diversity of methodologies required to address the different levels of the molecular hierarchy potentially involved in host-pathogen compatibility and unravel the complexity of such biological systems (Fig. 1). The recent development of comparative genomic, transcriptomic, proteomic and epigenomic approaches, referred as multi-omic analyses, paved the way to a better understanding of the underlying molecular processes of snail/schistosome compatibility (Fig. 1). The molecular mechanisms underlying the high level of compatibility polymorphism observed in these interactions are based on the formation of immune complexes between B. glabrata fibrinogen related proteins (FREPs) and the S. mansoni polymorphic mucins (SmPoMucs), which were previously demonstrated as the most promising candidates involved in snail/schistosome compatibility [START_REF] Dheilly | A family of variable immunoglobulin and lectin domain containing molecules in the snail Biomphalaria glabrata[END_REF][START_REF] Mitta | Compatibility polymorphism in snail/schistosome interactions: from field to theory to molecular mechanisms[END_REF]Mone et al., 2010 ). On one hand, SmPoMucs are mucin-like molecules displaying a domain containing a variable number of tandem repeats (VNTR). Mucins are known to be involved in hostparasite interplay [START_REF] Buscaglia | Trypanosoma cruzi surface mucins: hostdependent coat diversity[END_REF][START_REF] Hicks | The role of mucins in host-parasite interactions. Part I-protozoan parasites[END_REF][START_REF] Rathore | An immunologically cryptic epitope of Plasmodium falciparum circumsporozoite protein facilitates liver cell recognition and induces protective antibodies that block liver cell invasion[END_REF][START_REF] Theodoropoulos | The role of mucins in host-parasite interactions: Part II -helminth parasites[END_REF]. SmPoMucs could act as pathogen associated molecular patterns (PAMPs) and would be the initiator of snail host immune recognition (Mone et al., 2010 ). On the other hand, fibrinogen-related proteins (FREPs) constitute a highly diversified family of soluble immune pathogen recognition receptors (PRRs) from B. glabrata [START_REF] Adema | A family of fibrinogen-related proteins that precipitates parasite-derived molecules is produced by an invertebrate after infection[END_REF][START_REF] Mitta | Compatibility polymorphism in snail/schistosome interactions: from field to theory to molecular mechanisms[END_REF]Mone et al., 2010 ). FREPs combine one or two N-terminal immunoglobulin superfamily (IgSF) domains with a C-terminal fibrinogen-related (FBG) domain and participate in binding trematode parasites (Mone et al., 2010;Zhang and Loker, 2004;Zhang et al., 2008a). In this review, the roles played by FREPs and SmPoMucs in compatibility polymorphism has been compiled from integrated "Multi-Omic" approaches conducted on different snail and schistosome combinations displaying different compatibility phenotypes.

What have we learned from genomic approaches on FREPs and SmPoMucs?

Genetic studies conducted in the S. mansoni/B. glabrata model demonstrated that compatibility was heritable and could be selected in the laboratory either for susceptibility/resistance of the snail or for infectivity of the parasite [START_REF] Davies | Trade-offs in the evolution of virulence in an indirectly transmitted macroparasite[END_REF][START_REF] Richards | Genetic factors in susceptibility of Biomphalaria glabrata for different strains of Schistosoma mansoni[END_REF][START_REF] Richards | Genetics of Biomphalaria glabrata and its effect on the outcome of Schistosoma mansoni infection[END_REF][START_REF] Richards | The genetic variation of compatibility in Biomphalaria glabrata and Schistosoma mansoni[END_REF][START_REF] Webster | Coevolution and compatibility in the snailschistosome system[END_REF], although the underlying genetic and molecular mechanisms were largely unknown. Compatibility may reside in a concordance of genetically determined phenotypes in snail and schistosome [START_REF] Basch | An interpretation of snail-trematode infection rates: specificity based on concordance of compatible phenotypes[END_REF][START_REF] Basch | Intermediate host specificity in Schistosoma mansoni[END_REF], a reduced set of parasite phenotypes in a population generally match with solely a fraction of the phenotypes present in the host strain [START_REF] Theron | Are Biomphalaria snails resistant to Schistosoma mansoni?[END_REF]. The first genome assembly and annotation of S. mansoni was published in 2009 [START_REF] Berriman | The genome of the blood fluke Schistosoma mansoni[END_REF]. It is composed of approximately 363 megabases in which 11,809 putative genes encoding 13,197 transcripts were identified. The functions of only a small percentage of these genes have been characterized so far. Moreover, the assembly was not perfect considering that 40% of the genome was repetitive and that the annotation mostly focused on possible therapeutic target genes against adult worms. This genomic resource has contributed to functional studies [START_REF] Protasio | Comparative study of transcriptome profiles of mechanicaland skin-transformed Schistosoma mansoni schistosomula[END_REF], and was particularly helpful for investigating the mechanism of resistance to specific drugs [START_REF] Valentim | Genetic and molecular basis of drug resistance and species-specific drug action in schistosome parasites[END_REF]. Nevertheless, it has never been used to identify which genomic regions were under selection for the interplay with its intermediate host snail. Additional efforts have been made in the characterization of gene loci in the B. glabrata genome associated with resistance or compatibility to S. mansoni infection. A first study using random amplified polymorphic DNA (RAPD) approach on several B. glabrata strains displaying a range of compatibility toward the same parasite isolate led to the identification of two heritable markers related to the resistance phenotype of adult snails [START_REF] Knight | The identification of markers segregating with resistance to Schistosoma mansoni infection in the snail Biomphalaria glabrata[END_REF]. As these RAPD markers were located in repetitive genomic regions in the B. glabrata genome, further attempts to characterize the associated genes were not successful. However, more recently a restriction site associated DNA sequencing (RADseq) study was conducted using two laboratory lines of snails that have been selected for S. mansoni resistance by experimental evolution (Tennessen et al., 2015b). RAD genotyping revealed in both lines a genomic region linked to resistance to the parasite. This region comprised 12 putative noncoding and 15 coding genes, among which 7 presented similarities for single-pass transmembrane proteins. Proteins from this family are implicated in different processes like cell migration, adhesion, growth, and typically act as receptors for extracellular signals (Tennessen et al., 2015b). The authors notably underlined their potential role in the immune system such as B-and T-cell receptors, Fc receptors, major histocompatibility complex (MHC) receptors and Toll-like receptors, which are all membrane-bound receptors recognizing foreign molecules (Tennessen et al., 2015b).

Complementary RNAseq analysis revealed that the compatibility phenotype of the snail (susceptible or resistant) was not correlated to the level of gene expression but rather to differences in allele sequences (Tennessen et al., 2015a(Tennessen et al., , 2015b). Several molecules have been identified and implicated for immunity of Biomphalaria glabrata against Schistosoma mansoni (for review, see [START_REF] Coustau | Advances in gastropod immunity from the study of the interaction between the snail Biomphalaria glabrata and its parasites: a review of research progress over the last decade[END_REF]). However, few have been characterized at the genetic or genomic level, including one particular allele of the copper-zinc superoxide dismutase (SOD1) that is associated with snail resistance to S. mansoni infection [START_REF] Blouin | Three genes involved in the oxidative burst are closely linked in the genome of the snail, Biomphalaria glabrata[END_REF][START_REF] Goodall | Biomphalaria glabrata cytosolic copper/zinc superoxide dismutase (SOD1) gene: association of SOD1 alleles with resistance/susceptibility to Schistosoma mansoni[END_REF] and of course the SmPoMuc and FREP molecules described thereafter.

The Schistosoma mansoni polymorphic mucins (SmPoMucs)

The only parasite molecules characterized at the genomic level and implicated in the interaction with the intermediate host snail are the S. mansoni Polymorphic Mucins (SmPoMucs) (for review, see (Gourbal et al., 2015)). These molecules were shown to be highly polymorphic at both population and individual level.

In a population each miracidium expresses a unique combination of SmPoMucs derived from a limited set of genes. SmPoMucs are encoded by at least 6e9 multi-gene family members (Roger et al., 2008b). These genes are composed of 15 exons: the first two exons of the 50 gene region are considered as the variable region of SmPoMuc genes, while the 13 other exons are considered as the conserved 30 region. Indeed, southern blot experiments revealed that SmPoMucs contained a variable number of repeated exon 2 that never exceeded 20 copies. The different genes were classified in four paralogous sequence groups according to their sequence coding for the conserved 30-terminal part of the gene. All members of group 4 were considered as pseudogenes. Fluorescent in situ hybridization (FISH) allowed locating SmPoMucs genes in four distinct genomic regions of chromosomes 3 and 4. This genomic clustering associated with the presence of pseudogenes strongly supports that gene duplications would be at the basis of SmPoMuc gene diversification. In SmPoMuc genes, Exon 2 (27 nucleotides) and its flanking introns are organized in 1 kb repeats, each separated by microsatellites. The high level of similarity of these genomic repeats between all members of SmPoMuc multigene family results in the frequent recombination and homogenization of the SmPoMucs genes. Diversification of SmPoMuc molecules is driven by complex genomic mechanisms. Calculation of synonymous to nonsynonymous substitutions ratios (KS/KN) and Tajima's neutrality test indicated that purifying selection acts on SmPoMuc genes (Roger et al., 2008b). In these genes, genomic repeats are subjected to concerted evolution and intron sequences are more homogenized than exons that evolve under purifying selection (Roger et al., 2008b). This results in combinatory events observed at the genomic level, probably linked to ectopic recombination and exon exchange between clusters. The SmPoMucs genes were not present in the current S. mansoni genome annotation but can be found by similarity searches. However, the current assembly (v5) around these loci contains potential errors since similarities between genes of this family as well as the genomic repeats prevented their correct assembly into the genome.

The fibrinogen-related protein (FREP) genes

Several studies have focused on B. glabrata fibrinogen-related proteins (FREPs) (for a review see [START_REF] Gordy | The role of fibrinogen-related proteins in the gastropod immune response[END_REF]) that have been shown in vitro to interact with the parasite molecules, SmPoMuc (Mone et al., 2010). FREPs are composed of a signal peptide (SP), target signal for extracellular secretion, immediately followed by an immunoglobulin superfamily domain (IgSF), which is finally connected by a length-variable interceding region (ICR) to a fibrinogen domain (FBG). Moreover two classes of FREPs are described, single-IgSF FREPs and tandem-IgSF FREPs, in which IgSF domain are arranged in tandem and separated by a short connecting region [START_REF] Hanington | The primary role of fibrinogen-related proteins in invertebrates is defense, not coagulation[END_REF][START_REF] Zhang | Parasite-responsive IgSF members in the snail Biomphalaria glabrata: characterization of novel genes with tandemly arranged IgSF domains and a fibrinogen domain[END_REF]. The two IgSF domains of the tandem-IgSF FREP are quite different in sequence and are not just a repeat of a unique domain. The two IgSF domains of tandem-IgSF FREP are more similar to each other than to the IgSF domain of a single-IgSF FREP [START_REF] Dheilly | A family of variable immunoglobulin and lectin domain containing molecules in the snail Biomphalaria glabrata[END_REF]. Fourteen FREP families have been described up to date, mainly from B. glabrata M-line strain [START_REF] Adema | A family of fibrinogen-related proteins that precipitates parasite-derived molecules is produced by an invertebrate after infection[END_REF][START_REF] Zhang | The FREP gene family in the snail Biomphalaria glabrata: additional members, and evidence consistent with alternative splicing and FREP retrosequences. Fibrinogen-related proteins[END_REF]Zhang et al., 2008a). Families 2, 4 and 14 belong to single-IgSF FREP, while families 3, 7,12 and 13 belong to tandem-IgSF FREP. The other FREP families couldn't be clearly assigned to one of these groups, because solely one IgSF or FBG domain has been described for these genes. At the genomic level complete gene sequences are available for only 4 FREP families (2, 3, 4 and 7). The FREP 2 and 4 (single-IgSF) are encoded by 4 exons and 3 introns [START_REF] Leonard | Structure of two FREP genes that combine IgSF and fibrinogen domains, with comments on diversity of the FREP gene family in the snail Biomphalaria glabrata[END_REF]Zhang et al., 2008a), and FREP 3 and 7 (tandem-IgSF) are encoded by 6 exons and 5 introns. It has been shown that introns from single-IgSF FREPs are composed of repeats that varied in length and number, and that they presented low similarity between FREP2 and FREP4 [START_REF] Leonard | Structure of two FREP genes that combine IgSF and fibrinogen domains, with comments on diversity of the FREP gene family in the snail Biomphalaria glabrata[END_REF]. The FBG region is the most conserved domain, while IgSF and ICR are the most variable domains [START_REF] Dheilly | A family of variable immunoglobulin and lectin domain containing molecules in the snail Biomphalaria glabrata[END_REF][START_REF] Leonard | Structure of two FREP genes that combine IgSF and fibrinogen domains, with comments on diversity of the FREP gene family in the snail Biomphalaria glabrata[END_REF][START_REF] Zhang | Parasite-responsive IgSF members in the snail Biomphalaria glabrata: characterization of novel genes with tandemly arranged IgSF domains and a fibrinogen domain[END_REF]Zhang and Loker, 2004). To characterize the diversity of FREPs, several approaches have been conducted. Southern blot analysis and PCR with degenerated primers on FBG domains revealed their abundance in B. glabrata genome and suggest the diversity of FREPs at this level (Zhang and Loker, 2004). Moreover, other studies have highlighted the capacity of somatic diversification by point mutation, gene conversion and ectopic recombination at the genomic level in all FREP families studied so far. Such diversification events have been reported for IgSF, ICR and FBG domains [START_REF] Dheilly | A family of variable immunoglobulin and lectin domain containing molecules in the snail Biomphalaria glabrata[END_REF]Mone et al., 2010;[START_REF] Zhang | Parasite-responsive IgSF members in the snail Biomphalaria glabrata: characterization of novel genes with tandemly arranged IgSF domains and a fibrinogen domain[END_REF]Zhang et al., , 2004)). Altogether, these results suggest a high potential of diversification of FREP molecules. The assembly and annotation of the B. glabrata genome will provide a powerful expected tool to describe and understand the mechanisms responsible for diversity of FREPs at the genomic level. Such diversity of FREP/SmPoMuc molecules provides support for the matching phenotype hypothesis and might partly explain the compatibility polymorphism observed between schistosomes and Biomphalaria strains.

What have we learned from transcriptomic approaches on FREPs and SmPoMucs?

Transcription embodies the first step of gene expression in cells. The absence or presence of specific transcripts at a precise time and/or specific localization in cells might gives insights on activated or repressed genes in specific physiological or molecular pathways. With this objective, many techniques have emerged since the 1980s to massively and easily explore the transcriptomic compartment, from suppression subtractive hybridization (SSH) and microarray to massive RNA sequencing (RNAseq). The history of the discovery and investigation of FREPs and SmPoMuc follows the improvement of next generation sequencing (NGS) methods and benefited from several methods until nowadays. In 2003, Marathon cDNA libraries brought the first information about sequence diversity of FREPs genes family. It also provided evidence for alternative splicing abilities of FREPs in Biomphalaria in response to parasite infection [START_REF] Zhang | The FREP gene family in the snail Biomphalaria glabrata: additional members, and evidence consistent with alternative splicing and FREP retrosequences. Fibrinogen-related proteins[END_REF]. Alternative splicing is described as a transcriptomic process to increase the production of host diversified recognition molecules and enhance host's ability to recognize pathogens. This provided the first insights toward the idea of a polymorphism and/or diversification in the FREPs family. In SmPoMuc, a similar pattern of alternative splicing, in addition to aberrant splicing or transsplicing, led to inclusion or exclusion of some parts of the genes that ultimately produce a high level of polymorphism from a restricted number of genes (Fig. 2), allowing to increase the diversity of proteins that are the potential targets recognized by the host immune system (Roger et al., 2008b). Concerning Biomphalaria factors, reverse transcription quantitative PCR (RT-qPCR) first revealed the differential expression of FREP family genes in Biomphalaria following infection with two species of highly infective parasites (S. mansoni and Echinostoma paraensei) [START_REF] Hertel | Differential expression of FREP genes in two strains of Biomphalaria glabrata following exposure to the digenetic trematodes Schistosoma mansoni and Echinostoma paraensei[END_REF]. This result gave support for putative roles of FREP2 and FREP4 in Biomphalaria defense. Functional invalidations were rapidly developed in Biomphalaria system to explore the involvement of specific genes in compatibility phenotypes. The first RNAi (interference RNA) gene knockdown developed in Biomphalaria strain involved the injection of double-stranded RNA (dsRNA) into the snail hemolymph or their addition to co-cultures of Biomphalaria embryonic cell line (Bge cells) and parasites [START_REF] Jiang | In vivo and in vitro knockdown of FREP2 gene expression in the snail Biomphalaria glabrata using RNA interference[END_REF]. This technique opens the way to gene inactivation of FREPs. Rapidly, the related molecules in Schistosoma have also been explored. The first transcriptomic analysis of SmPoMuc revealed that mRNAs encoding mucin-like proteins were detected at three different developmental stages (i.e., egg, miracidium and primary sporocyst Sp1), particularly at a high-level at the miracidium stage of the parasite (Roger et al., 2008a). Mucins may be involved in the penetration of miracidia into the snail and help during the transition from the free-living miracidium stage to the first intramolluscal stage Sp1 [START_REF] Theodoropoulos | The role of mucins in host-parasite interactions: Part II -helminth parasites[END_REF]. Comparison between strains of parasites revealed that less-recognized compatible strain (SmBRE) expressed 3-fold less SmPoMuc than the best-recognized incompatible one (SmGH2) [START_REF] Galinier | A multistrain approach to studying the mechanisms underlying compatibility in the interaction between Biomphalaria glabrata and Schistosoma mansoni[END_REF]Roger et al., 2008a),. These observations suggest that mucins can be the target of the recognition by the snail and that diversity of these putative antigenic proteins can increase parasite capacity to evade the host immune pathogen recognition receptors (PRRs). Proteins containing fibrinogen Ceterminal domain have been shown to be triggered in response to compatible and incompatible Schistosoma infection but also following an infection with irradiated-miracidium [START_REF] Ittiprasert | Identification of immediate response genes dominantly expressed in juvenile resistant and susceptible Biomphalaria glabrata snails upon exposure to Schistosoma mansoni[END_REF].

Based upon microarray experiments, the variation in FREP gene transcription was analyzed and revealed the over-expression of FREP2, 3, 4 and 6, the under-expression of FREP1, 5, 7, 10, 12 and divergent expression of FREP11 between 12 h and 32 days following Schistosoma infection [START_REF] Hanington | Time series analysis of the transcriptional responses of Biomphalaria glabrata throughout the course of intramolluscan development of Schistosoma mansoni and Echinostoma paraensei[END_REF]. This study provided the first evidence that FREPs constitute a complex immune-related family in which some members are over-expressed while other are underexpressed following the same infection response.

The specificity of parasite detection appears to be more efficient when the host possesses a diversified repertoire of differentially-selected PRRs that can be used in response to different pathogens. After microarrays, massive RNA sequencing (RNAseq) with Illumina/Solexa technologies have generated a lot of new information associated with the deeper sequencing resolution. By investigating B. glabrata FREPs diversity from a de novo generated transcriptome, Dheilly and collaborators [START_REF] Dheilly | A family of variable immunoglobulin and lectin domain containing molecules in the snail Biomphalaria glabrata[END_REF] revealed that within each FREP family member, a significant nucleotide variability was observed in transcripts resulting in polymorphism in the predicted amino-acid protein sequence. These authors showed that the FREP molecules did not show the highest level of transcript variants among lectin family members and also that, FREPs were not the more expressed in naïve snails. This means that putative other immune-related recognition receptor molecules within the large lectin family (including Ig-like fold, C-Type lectin, Leucinrichrepeat, Selectin, Galectin) may increase immune recognition capabilities in Biomphalaria. Several Variable Immunoglobulins (VIgLs), CREPs (C-Type lectin related protein) and GREPs (Galectin related protein) sharing IgSF domain with FREPs were also evidence. This indicates the existence of recombination or trans-slicing events between highly-variable molecules belonging to different lectin families. This demonstrates the strong evolutionary pressures playing on those closely-related molecules, which share similar putative immune-related features, which are able to generate additional polymorphism and diversity. Recent RNAseq approach confirmed the role of FREPs during a shift from a cellular to a humoral immune response in the innate immune memory of Biomphalaria [START_REF] Pinaud | A shift from cellular to humoral responses contributes to innate immune memory in the vector snail Biomphalaria glabrata[END_REF]. Despite an absence of up-regulation of FREP genes following sympatric infection, a global over-representation of FREPs was observed following immune homologous challenge. Knockdown of FREP2, 3 and 4 genes revealed that the 100% acquired resistant snails became partially susceptible (15%) after a reinfection with the same parasite and confirmed the key role of FREPs among lectin families, while highlighting that additional factors might complement FREP function in Biomphalaria innate immune memory.

4 What have we learned from epigenomic approaches onFREPs and SmPoMucs?

Heritable phenotypic diversity has traditionally been attributed to genetic diversity. We know today that this view of heritability is incomplete and must include other diversity-generating mechanisms that can be heritable such as those regulated by epigenetic elements. The epigenetic changes refer to a set of molecular processes that can affect gene expression by non-coding RNA, through methylation of nucleic acids in DNA, chemical modification of histones and re-localization of a locus inside different nucleus territories [START_REF] Keung | Chromatin regulation at the frontier of synthetic biology[END_REF]. These epigenetic factors have been shown to be under the influence of the environment and their modifications can have consequences on the chromatin compaction and therefore may affect gene expression. In this sense, epigenetic modifications can result in new phenotypes that might be adaptive. The epigenotype is mitotically and to some degree meiotically heritable, but unlike in the genotype, changes in the epigenotype are generally reversible [START_REF] Keung | Chromatin regulation at the frontier of synthetic biology[END_REF]. Therefore, the epigenetic information system (EIS, epigenetics) can provide heritable, novel phenotypes that do not rely on variation in DNA sequence [START_REF] Geoghegan | Population-epigenetic models of selection[END_REF]. If this EIS influences the capacity to generate different phenotypes, both the better adapted phenotype and the capacity to generate this phenotype will be selected and conserved in the next generation [START_REF] Gomez-Diaz | Epigenetics of hostpathogen interactions: the road ahead and the road behind[END_REF]. This hypothesis has been investigated in the context of the arm race hypothesis that occurs during the interaction between S. mansoni and its intermediate host B. glabrata. The availability of the genome for both partners [START_REF] Berriman | The genome of the blood fluke Schistosoma mansoni[END_REF] (Bg genome submitted: see Vector Base website (https://www.vectorbase.org/)) and the recent advances in genome-wide analysis have allowed the first description of different bearers of epigenetic information in different contexts. Histones are highly conserved proteins and commercial drugs and antibodies are usable to target these proteins and study their modifications in diverse species. Chromatin immunoprecipitation (ChIP) analysis targeting these modified histones has been applied to S. mansoni and allowed the visualization of the position of modified histones in vivo in different environmental contexts and at different developmental stages of this parasite [START_REF] Picard | Sex-biased transcriptome of Schistosoma mansoni: host-parasite interaction, genetic determinants and epigenetic regulators are associated with sexual differentiation[END_REF][START_REF] Roquis | The epigenome of Schistosoma mansoni provides insight about how cercariae poise transcription until infection[END_REF]. Several studies have demonstrated how histone modifications in S. mansoni are engaged to generate phenotypic plasticity during the interaction with its intermediate host.

SmPoMuc are highly conserved genes in their coding region and upstream putative promoter regions. Despite this high degree of conservation observed between compatible and incompatible strains of S. mansoni, their patterns of expression strongly differ between these two strains (Roger et al., 2008a). ChIP analysis revealed that different modified histones are present in the upstream regulatory regions of the SmPoMucs genes which could be responsible for an epigenetic-based regulation of their expression [START_REF] Perrin | Schistosoma mansoni mucin gene (SmPoMuc) expression: epigenetic control to shape adaptation to a new host[END_REF]. Other studies further showed that treatment of the parasite with drugs that target histone acetylation led to an increased compatibility with the snail [START_REF] Fneich | Epigenetic origin of adaptive phenotypic variants in the human blood fluke Schistosoma mansoni[END_REF]. The snail environment has also been demonstrated to be an epigenome modifier itself as infection of allopatric and sympatric snails with genetically identical S. mansoni resulted in the generation of different epigenotypes in each genetically identical parasite [START_REF] Roquis | Frequency and mitotic heritability of epimutations in Schistosoma mansoni[END_REF]. Furthermore, S. mansoni is a typical organism with a tRNA methyltransferase DNMT2 for which DNA methylation is really low and has been controversial [START_REF] Raddatz | Dnmt2-dependent methylomes lack defined DNA methylation patterns[END_REF]. DNA methylation can be studied by a large panel of technics such as bisulfite conversion (BS) assay that allows studying cytosine methylation at the single base resolution [START_REF] Grunau | Bisulfite genomic sequencing: systematic investigation of critical experimental parameters[END_REF], methylation-sensitive amplification polymorphism (MSAP) [START_REF] Reyna-Lopez | Differences in DNA methylation patterns are detectable during the dimorphic transition of fungi by amplification of restriction polymorphisms[END_REF] and immunological and cytochemical approaches based on the use of conserved antibodies. By combining some of these different methodologies, Geyer et al. ( 2011) studied methyl-5-cytosine (m5C) in S. mansoni and linked the presence of the cytosine methylation machinery to platyhelminth oviposition processes [START_REF] Geyer | Cytosine methylation regulates oviposition in the pathogenic blood fluke Schistosoma mansoni[END_REF]. Finally, different works reported stage and sex-specific expression of several non-coding RNAs, which indicate that they might be involved in parasite development. On the side of the intermediate host, B. glabrata epigenome has also been shown to be under the influence of the environment [START_REF] Knight | Epigenetic modulation, stress and plasticity in susceptibility of the snail host, Biomphalaria glabrata, to Schistosoma mansoni infection[END_REF]. DNA methylation and locus topography have been shown to be modified in response to S. mansoni infection [START_REF] Arican-Goktas | Differential spatial repositioning of activated genes in Biomphalaria glabrata snails infected with Schistosoma mansoni[END_REF][START_REF] Knight | Epigenetic modulation, stress and plasticity in susceptibility of the snail host, Biomphalaria glabrata, to Schistosoma mansoni infection[END_REF]. A methylome of B. glabrata has been generated by BS-Seq analysis (Cosseau et al. unpublished data and Fig. 3). This snail displays a typical invertebrate mosaic-like DNA methylation pattern, with highly methylated regions, predominantly in the CpG context, interspaced with poorly methylated regions. Unlike plants and mammals, the role of DNA methylation for regulation of gene expression is controversial in invertebrates and other bearers of epigenetic information such as small RNA, histone modifications and locus position in chromosome territories certainly deserve further attention to elucidate the involvement of epigenetic regulation events on the observed patterns of gene expression. FREP genes are typical phenotypic variants for which we anticipate an epigenetic-based gene regulation. Such a regulation is expected in the case of the generation of phenotypic diversity followed by natural selection upon environmental changes [START_REF] Cortes | A view on the role of epigenetics in the biology of malaria parasites[END_REF]. FREPs genes in Biomphalaria glabrata are mainly localized in low methylated regions (Fig. 3). The absence of DNA methylation in these regions has been hypothesized to contribute to stochastic transcriptional opportunities [START_REF] Gavery | A context dependent role for DNA methylation in bivalves[END_REF], which is typically what is expected to explain part of the transcriptional diversification observed in highly diversified FREP gene families. These FREP genes are therefore candidates for which valuable tools such as ChIP-assays against modified histones will be worth pursuing. This will certainly open new perspectives and improve the knowledge of the sophisticated molecular pathways involved in the host parasite interplay.

What have we learned from proteomic approaches on FREPs and SmPoMucs ?

Proteomic approaches allow investigating and characterizing features of immune-related actors by focusing on the last form of the gene expression process, i.e. protein that can explain the phenotype. Although genomic, transcriptomic and epigenetic approaches can inform on the sequence diversity at the gene and transcript level and on gene expression regulation, only proteomic approaches can provide information about the molecular interaction between proteins from hosts and pathogens and give insights into their function. This can come from characterization by gelbased approaches (SDS-PAGE, 2D-PAGE) or gel-free approaches (Label-Free protein sequencing) of entire proteomes coupled with tandem mass spectrometry, or by analyzing post-translational modifications of proteins, protein localization in cells, organs or tissues and/or by analyzing protein/protein interactions. All these approaches have been used in the S. mansoni/B. glabrata interaction to identify parasite antigens and the potential corresponding pathogen recognition receptors in the host's hemolymph. Following snail infection by S. mansoni, incompatible miracidia are recognized, encapsulated and killed in few hours, while compatible miracidia remain unaffected. This observation suggests constitutive antigenic differences between those strains of parasite. Thus, a global comparative proteomic approach has been conducted to characterize these antigenic differences (Roger et al., 2008a(Roger et al., , 2008c)). The main difference identified corresponded to SmPoMucs that exhibited a pronounced qualitative and quantitative polymorphism between compatible and incompatible parasite strains (Roger et al., 2008c). SmPoMuc were further investigated (Roger et al., 2008a(Roger et al., , 2008c) ) and the data obtained showed that these proteins are: (i) only expressed at larval stages interacting with the mollusc, (ii) located in the apical gland of miracidia and sporocysts, (iii) secreted and released in excretory-secretory products and finally (iv) displayed a high degree of polymorphism at the protein primary structure level and (v) are highly glycosylated. The analysis of the global glycosylation status of SmPoMucs, for both strains through chemical deglycosylation experiments, highlighted higher glycosylation levels for variants of the incompatible strain as compared to compatible strain (Roger et al., 2008a). All of these characteristics make SmPoMucs key candidates of the compatibility polymorphism and matching phenotype process, being potential pathogen antigens recognized by the host recognition receptors. Mucins have indeed already been identified as important actors of the host-parasite interaction [START_REF] Theodoropoulos | The role of mucins in host-parasite interactions: Part II -helminth parasites[END_REF], FREPs were originally discovered as circulating glycoproteins present in the plasma of B. glabrata and able to agglutinate rabbit erythrocytes [START_REF] Boswell | Isolation, characterization and functional assessment of a hemagglutinin from the plasma of Biomphalaria glabrata, intermediate host of Schistosoma mansoni[END_REF]. This agglutination was shown to be inhibited when carbohydrates were pre-incubated with plasma before the addition of rabbit red blood cells, suggesting the key role of sugar in this interaction. Further studies demonstrated that an agglutination reaction also occurred when Biomphalaria glabrata plasma was exposed to sporocyst of trematode parasites species (S. mansoni and Echinostoma paraensei) [START_REF] Couch | Humoral response of the snail Biomphalaria glabrata to trematode infection: observations on a circulating hemagglutinin[END_REF][START_REF] Loker | Schistosoma mansoni: agglutination of sporocysts, and formation of gels on miracidia transforming in plasma of Biomphalaria glabrata[END_REF][START_REF] Loker | Elevated agglutination titres in plasma of Biomphalaria glabrata exposed to Echinostoma paraensei: characterization and functional relevance of a trematode-induced response[END_REF]. Furthermore, the intensity of the reaction independent of the snail strain used regarding the infection (naïve or infected) and its resistance (susceptible or resistant) toward the parasite considered [START_REF] Bayne | Interactions between the plasma proteins of Biomphalaria glabrata (Gastropoda) and the sporocyst tegument of Schistosoma mansoni (Trematoda)[END_REF][START_REF] Couch | Humoral response of the snail Biomphalaria glabrata to trematode infection: observations on a circulating hemagglutinin[END_REF][START_REF] Loker | Alterations in Biomphalaria glabrata plasma induced by infection with the digenetic trematode Echinostoma paraensei[END_REF]. These agglutination factors were finally demonstrated to precipitate E. paraensei secretory/excretory products, which later turned out to contain FREPs [START_REF] Adema | A family of fibrinogen-related proteins that precipitates parasite-derived molecules is produced by an invertebrate after infection[END_REF][START_REF] Zhang | Parasite-responsive IgSF members in the snail Biomphalaria glabrata: characterization of novel genes with tandemly arranged IgSF domains and a fibrinogen domain[END_REF]. To shed light on the molecules from the host that specifically recognize parasite proteins and therefore trigger the appropriate immune response, an interactome approach has been performed on the Biomphalaria-Schistosoma host-pathogen system. In addition to several enzyme inhibitors, hormones and metalloenzymes, two families of lectins from B. glabrata were identified as able to interact with Schistosoma proteins: several galactose binding lectin-like proteins and FREPs (Mone et al., 2010 ). Moreover, many different proteins from the parasite interacting with host proteins were also identified including three glycoprotein families that are integral membrane protein Sm23, secretory glycoprotein k5 and SmPoMuc (Mone et al., 2010 ). To go further, co-immunoprecipitations were mandatory to demonstrate which snail protein(s) bind to SmPoMucs. CO-IP assays were thus conducted using proteins from Biomphalaria hemolymph, parasite extract containing SmPoMuc and antibody raised against C-terminal part of SmPoMucs. This revealed three proteins directly interacting with SmPoMuc include an alpha amylase, a thioester-containing protein (TEP) and a specific protein from the FREP family, FREP2 (Mone et al., 2010 ). It was hypothesized that TEP would interact with the complex primarily formed by FREP2 and SmPoMuc but the mechanisms and the binding sites involved in such an immune complex are currently unknown [START_REF] Gordy | The role of fibrinogen-related proteins in the gastropod immune response[END_REF]Mone et al., 2010 ). Finally, in B. glabrata, it has been hypothesized that these PRR molecules could serve as complementary or collaborative recognition factors that interact with each other to form complexes [START_REF] Adema | Digenean-gastropod host associations inform on aspects of specific immunity in snails[END_REF].

Among them, FREPs have been shown by western blot analysis to occur in Biomphalaria hemolymph as multimers of high molecular weight although the mechanism responsible for this multimerization and its importance in the role of FREP-based pathogen recognition has yet to be investigated [START_REF] Adema | A family of fibrinogen-related proteins that precipitates parasite-derived molecules is produced by an invertebrate after infection[END_REF][START_REF] Gordy | The role of fibrinogen-related proteins in the gastropod immune response[END_REF]Zhang et al., 2008b). Nevertheless, such multimerization could be an additional process for increasing the mollusc pathogen recognition repertoire, by generating a wide range of multimers each with a different level of parasite antigen specificity. As such, it could increase the potential for specific recognition of parasite molecules, allowing a high capacity of immune system to recognize non-self and to trigger specific anti-pathogen innate immune responses [START_REF] Adema | A family of fibrinogen-related proteins that precipitates parasite-derived molecules is produced by an invertebrate after infection[END_REF][START_REF] Schulenburg | How do invertebrates generate a highly specific innate immune response?[END_REF]. FREPs diversification would have been the evolutionary response of host to maintain its recognition capacity and could be engaged with hemocytes to mediate phagocytosis after parasite recognition [START_REF] Hanington | The primary role of fibrinogen-related proteins in invertebrates is defense, not coagulation[END_REF][START_REF] Peterson | Glycotope analysis in miracidia and primary sporocysts of Schistosoma mansoni: differential expression during the miracidium-to-sporocyst transformation[END_REF]. These results highlight how, even at the protein level, the interaction between FREPs and SmPoMuc could be at the basis of the process of compatibility between different strains of Schistosoma and Biomphalaria [START_REF] Coustau | Advances in gastropod immunity from the study of the interaction between the snail Biomphalaria glabrata and its parasites: a review of research progress over the last decade[END_REF][START_REF] Mitta | Compatibility polymorphism in snail/schistosome interactions: from field to theory to molecular mechanisms[END_REF].

A multi-omic integrative view of FREPs/SmPoMucs compatibility polymorphism

The molecular mechanisms underlying the high level of immune specificity observed in these interactions are based on the formation of complexes between a specific set of highly polymorphic or diversified immune recognition molecules named the fibrinogen-related proteins (FREPs) from B. glabrata and polymorphic parasite's glycosylated mucin molecules, named Schistosoma mansoni polymorphic mucins (SmPoMucs) [START_REF] Dheilly | A family of variable immunoglobulin and lectin domain containing molecules in the snail Biomphalaria glabrata[END_REF]Mone et al., 2010). Results obtained from "Omic" approaches on these two candidates were summarized in the Fig. 4 for different sympatric snail and Schistosome combinations displaying different levels of compatibility (see legend of Fig. 4 for details on S. mansoni and B. glabrata strain origins and phenotypes). We found that the polymorphism and expression levels of FREPs and SmPoMucs would be linked to the compatibility level between S. mansoni and B. glabrata [START_REF] Galinier | A multistrain approach to studying the mechanisms underlying compatibility in the interaction between Biomphalaria glabrata and Schistosoma mansoni[END_REF]. In BgBAR/SmLE interaction, snail hosts presented a low diversity but a high level of expression of FREPs while the parasite expressed low levels of highly diversified SmPoMucs (Fig. 4). In this context, the increased diversification combined with the decrease expression levels of the SmPoMucs glycoproteins could allow the parasite to escape from being recognized by FREP immune receptors. The same strategy could also be described for the BgVEN/SmVEN interaction, but as the host has a low expression of FREPs, the recognition selective pressures on the parasite were reduced and thus a moderate expression of SmPoMucs has been maintained. In this case, the parasite invests more in SmPoMuc diversification than in reduction of expression to circumvent recognition. A totally different strategy could be observed in BgBRE/SmBRE interaction. In this case, the snail host presents a high diversity and low expression of FREPs and the parasite a low diversity and low expression of SmPoMucs. We could hypothesis that it could be difficult for the parasite to diversify the SmPoMucs more than the host FREP molecules (BgBRE snails exhibited the highest FREP diversity) and thus parasite would reduce SmPoMuc expression to limit recognition. Finally, in BgGUA/ SmGH2 interaction, intermediate patterns can be observed, with moderate diversity/moderate expression of host FREPs and moderate diversity/high expression of parasite SmPoMucs. In this case, SmPoMucs were highly expressed and could be easily recognized by FREPs, which could explain the low compatibility observed in this host-parasite couple. On one hand, these results seemed to demonstrate that a high diversity and a low expression of SmPoMucs are required to enhance parasite infectivity. On the other hand, the diversity of FREPs seems not to be correlated to the compatibility level as it could be expected from an ongoing arms race between FREPs and SmPoMucs. Indeed, BgBRE and BgGUA (highly compatible snail strains) displayed the higher numbers of variants by comparison with BgVEN and BgBAR (low compatible snail strains). The explaining factor seems to be the high level of expression of FREPs that is required to enhance snail resistance toward S. mansoni. Allopatric interactions would also confirm these hypotheses; for example, SmVEN parasite succeed in infecting BgBAR but solely at a prevalence of 55% because SmPoMuc diversification is high but expression remains quite high and BgBAR FREPs would manage to form complexes with SmPoMucs. In SmGH2, the moderate diversity and high expression of SmPoMucs appeared to be a deleterious pattern that could perhaps explain the high incompatibility phenotype of this strain (prevalence never exceeds 60%). Based on these hypotheses, the expected compatibilities between all potential combination of the four strains of hosts and parasites were estimated and compared to the experimentally observed compatibility phenotypes 4). This discrepancy between expected and observed phenotypes would be expected. Indeed, if recognition by FREPs seemed to explain most of the compatibility, we know, based on knockdown experiments [START_REF] Hanington | A somatically diversified defense factor, FREP3, is a determinant of snail resistance to schistosome infection[END_REF][START_REF] Pinaud | A shift from cellular to humoral responses contributes to innate immune memory in the vector snail Biomphalaria glabrata[END_REF], that FREPs were not the unique determinant of compatibility and that additional factors were involved in Biomphalaria immune response. The activation of signalling pathways following recognition leading to hemocytes-dependent phagocytosis or encapsulation and to the release of humoral factors have to be considered. Finally, the results described above on B. glabrata/S. mansoni interactions, challenged our view of the role or function of FREP diversification. These results lead us to a totally new unexpected hypothesis: we argue that quantity rather than diversity of FREPs would benefit the overall recognition and elimination of a specific S. mansoni pathogen. Local adaptation or co-evolution would indeed favour the selection of a specific subset of FREPs that would be expressed at a high level to recognize a specific subset of SmPoMucs expressed by the parasite. Nevertheless, it remains unclear to what extent FREP diversity benefits immune function of B. glabrata [START_REF] Adema | Fibrinogen-related proteins (FREPs) in mollusks[END_REF]. Polymorphism and/or diversity of FREP immune receptor variants yield an enlarged repertoire of putative recognition molecules between individuals. This diversification provides a mechanism to increase host's recognition ability, and as such, the potential for specific recognition of pathogens. Indeed, Biomphalaria glabrata snails are confronted with an environment filled with complex changing communities of microorganisms and potential pathogens (bacteria, fungus, yeasts, trematodes, nematodes, etc …). Therefore, it can be expected that snails have to co-evolve with these pathogen communities and should possess sophisticated recognition systems for dealing with them. FREP diversification would be part of this enhanced immune recognition capabilities used to recognize and eliminate most of these pathogens.

(

And now, what next?

To further understand the interaction between S. mansoni and B. glabrata different lines of research can be considered. 6.1.) The interaction between FREPs and SmPoMucs is part of an immune complex involving different partners, like the thio-ester containing protein (BgTEP) and perhaps other molecules that remain to be characterized [START_REF] Gourbal | Polymorphic mucinlike proteins in Schistosoma mansoni, a variable antigen and a key component of the compatibility between the schistosome and its snail host[END_REF]Mone et al., 2010). Identification of such molecules will necessitate coimmunoprecipitation or pull-down experiments with recombinant FREP or TEP incubated with parasite extracts and snail plasma. Another question to solve is to identify the nature of the interacting domains and how molecules interact with one another. To answer this question, approaches such as those described above could be conducted using targeted mutagenesis or truncated recombinant proteins. 6.2.) To explore the function and role of highly-variable molecules like FREPs and SmPoMucs, transcriptomic approaches from RT-qPCR to NGS have suffered from technical limitations, notably the lack of genomic resources in non-model species [START_REF] Dheilly | No more non-model species: the promise of next generation sequencing for comparative immunology[END_REF]. However, emerging technologies such as single-cell RNA sequencing will provide technical breakthroughs for this field of research. We can expect that individual diversity and expression of both FREPs and SmPoMuc will be unraveled using this technology. For example, sequencing the hemocyte transcriptome for the same individual before and after pathogen infection might give insights on pathogen recognition receptor diversification in an invertebrate model. 6.3.) Analysis of transcriptomic data from different strains of B. glabrata [START_REF] Dheilly | A family of variable immunoglobulin and lectin domain containing molecules in the snail Biomphalaria glabrata[END_REF] strongly suggests the existence of other FREP subfamilies and consequently additional genes that have to be clarified with the use of the future B. glabrata genome annotation (Bg genome available at Vector Base website (https:// www.vectorbase.org/)). 6.4.) Whole-genome re-sequencing of parasite populations using PacBio and/or illumina genomic sequencing would be a powerful tool to circumvent the limits of actual genome assemblies for multi-gene families containing genomic repeats. The highlycontiguous de novo assemblies using PacBio sequencing can close gaps in current reference assemblies and longer reads would be useful to sequence through extended repetitive regions [START_REF] Rhoads | PacBio sequencing and its applications[END_REF]. Whole-genome re-sequencing might permit to explore how patterns of genetic variation change across the genome and detect, based on population genetic tests, the genes under directional and balancing selection [START_REF] Crellen | A switch in time[END_REF][START_REF] Rhoads | PacBio sequencing and its applications[END_REF]. Genetic approach by quantitative trait locus (QTL) mapping or extreme QTL (X-QTL) mapping using S. mansoni genome data would also be useful to identify genomic regions that were under selection for the interplay with its intermediate snail host. 6.5.) Methods for the functional characterization of genes of interest (i.e., invalidate and/or restore gene functions to analyze the corresponding phenotype modifications: functional genomic) are still lacking in schistosomes and snails. Except gene knockdown by RNA interference (RNAi) that is successfully used in both animal models, knockout or stable transgenic systems have to be developed now. Development of tools to introduce DNA constructs into adult and larval schistosome stages to express reporter genes have been tentatively developed using particle bombardment, electroporation or virus-based infection strategies. Unfortunately, until now non stable transformations were obtained [START_REF] Beckmann | Paving the way for transgenic schistosomes[END_REF][START_REF] Crellen | A switch in time[END_REF][START_REF] Kines | Integration of reporter transgenes into Schistosoma mansoni chromosomes mediated by pseudotyped murine leukemia virus[END_REF][START_REF] Kines | Electroporation facilitates introduction of reporter transgenes and virions into schistosome eggs[END_REF][START_REF] Mann | Transgenesis of schistosomes: approaches employing mobile genetic elements[END_REF][START_REF] Rhoads | PacBio sequencing and its applications[END_REF]. Such developments in Biomphalaria still lag behind. Finally, gene knockout using the CRISPR-Cas genomeediting technique (Komor et al., 2016) provides the ability to edit genomes within living cells or organisms representing a major advance for both host and parasite models.

The existence of polymorphic and/or diversified putative host immune receptors and parasite antigens that variy considerably in pathogen and snail populations no longer has to be demonstrated. However, understanding the function of such polymorphic or diversified molecules in invertebrates will be now a new challenge to solve and will deserve further investigations.

Figure 1

Integrative multi-Omics approaches of host-parasite interaction.

Figure 2

Transcriptional SmPoMuc polymorphism defines a specific SmPoMuc pattern for each individual in a population of S. mansoni parasites. Hypothetical exon/intron structures of SmPoMucs were used here to describe the transcriptional processes involved in the genesis of SmPoMuc diversity or polymorphism. In each table cell was indicated in the left corner the expected/observed compatibility phenotype (C: compatibility; M: moderate compatibility; IC: incompatibility) and in the right corner the prevalence (P, percentage of snail infected) and intensity values (I, number of parasites per infected snails) for all sympatric and allopatric combinations following experimental infection with 20 miracidia [START_REF] Theron | Multi-parasite host susceptibility and multi-host parasite infectivity: a new approach of the Biomphalaria glabrata/Schistosoma mansoni compatibility polymorphism[END_REF].

Expected compatibilities were estimated based on the hypothesis proposed from the observations of sympatric compatibility phenotypes (see part 5. for details). Highlighted table cells correspond to host/parasite interactions in which expected and observed phenotype data are not in concordance. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Diversity and expression levels of FREPs and SmPoMucs in four sympatric host/parasite combinations. Host and parasite were designated BgBAR/SmLE (for Belo Horizonte, Brazil), BgVEN/SmVEN (for Guaraca, Venezuela), BgBRE/SmBRE (for Recife, Brazil), and BgGUA/SmGH2 (for le Lamentin, Guadeloupe) [START_REF] Theron | Multi-parasite host susceptibility and multi-host parasite infectivity: a new approach of the Biomphalaria glabrata/Schistosoma mansoni compatibility polymorphism[END_REF]. The number of FREPs and SmPoMucs symbols represents the level of expression. The color of FREPs and SmPoMucs symbols represents the level of diversity. Mixed colors in SmGH2 represent the SmPoMucs with intermingled repeats which are a specificity of this Guadeloupian parasite strain.

Figure 5

Key definitions related to snail-schistosome interactions

• Compatibility: a pair of specific host and parasite genotypes/phenotypes in which an individual parasite infects an individual host and completes the part of its life cycle appropriate to the host. • Incompatibility: a pair of specific host and parasite genotypes/phenotypes in which an individual parasite did not infects an individual host and fails to complete the part of its life cycle appropriate to the host. • Sympatric interaction: host and parasite species coexist in the same geographic area. They experiment frequent encounters one another. • Allopatric interaction: host and parasite species originate from different geographic area. They never experiment encounters one another.

Figure 3

 3 Figure 3 Genome extract of the B. glabrata genome assembly: LGUN_random_Scaffold990:3335-119,948. Methylated cytosines are represented by blue bars. The height of the bar represents the percentage of methylation obtained for the methylated cytosines. The highest percentage of methylation observed in this window is 96%. CDS are represented according to a previously published transcriptome (Dheilly et al., 2015). Two FREPs have been annotated in this window (from position 44,955 to 47,325 and from position 99,422 to 114,087) (Richard Galinier, personal communication). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

  see table in Fig.4, experimental prevalence and intensity values are indicated). Interestingly, most of the expected and observed phenotypes were in agreement. Only two cases for which expected phenotypes did not fit with observed phenotypes were identified: (i) SmBRE in BgGUA expected to have a moderate compatibility because of the moderate expression of FREPs from BgGUA that are expected to recognize a large part of SmBRE parasites but is actually compatible (see Table Fig. 4); (ii) SmGH2 in BgBRE expected to have a moderate compatibility because of the low expression of FREPs in BgBRE and the high expression of SmPoMucs in SmGH2, and is actually incompatible (see TableFig.

Acknowledgements

We thank Ms. Anne Rognon and Ms. Nathalie Arancibia for their work and diligence in helping to produce part of the data described herein.

Funding

This work was supported by the French National Agency for Research (ANR) [grant ANR-13-JSV7-0009] Invimory to BG.