Rodrigo Augusto

Santinelo Pereira¹
email: raspereira@yahoo.com.br

Finn Kjellberg²
email: finn.kjellberg@cefe.cnrs.fr

Mutualism as a source of evolutionary innovation: insights from insect-plant interactions

Keywords: co-diversification, co-evolution, ecological interactions, mutualism conflicts, plant defences, pollination

Mutualism is ubiquitous in nature and probably all species in the world are involved in some form of such ecological interaction. The evolution of mutualisms is often envisioned in a framework where individual selection leads to reciprocal exploitation that results in net benefits to all the interacting parties, i.e. in a framework in which individual selection intrinsically destabilises the mutualistic association. However, mutualisms are stable, and rarely turn into parasitism. At a macroevolutionary scale, mutualisms have been a source of major evolutionary innovations and have benefited all the species involved. Indeed, complementary traits of the partners enable them to colonise new ecological niches. In this chapter we discuss mutualism as a source of evolutionary innovation, using some insect-plant associations as examples. Firstly, we dedicate a topic to theoretical aspects and hypotheses on evolutionary forces stabilizing mutualisms (and their pitfalls). We present some examples of mutualisms that allowed insects and/or plants to expand their ecological niches. Finally, we focus on fig tree -animal interactions that, putatively, allowed this plant lineage to diversify in subtropical and tropical ecosystems and to form a group of keystone species for the functioning of many forest ecosystems. Learning objectives: After completing this chapter, you should understand the following:  To understand the evolution of mutualisms we need to look at natural selection operating at different levels  The general architecture of mutualistic associations  Mutualisms are ecologically and evolutionarily stable  Mutualistic associations may boost species diversification by creating new ecological opportunities (innovations) for the interacting species

Introduction

Mutualisms, defined as interspecific interactions that are beneficial to all the involved partners, are ubiquitous in nature. Probably all species in the world are involved in some form of mutualistic interaction [START_REF] Bronstein | The evolution of plant-insect mutualisms[END_REF]. In this chapter we will address direct mutualisms in which the partners are in physical contact. Direct mutualisms may be further divided into symbiotic and non-symbiotic mutualisms [START_REF] Boucher | The ecology of mutualism[END_REF]. In symbiotic mutualisms, individuals are physiologically integrated, whereas in non-symbiotic mutualism, despite some physiological co-adaptation, individuals of the interacting species are at least at some stage of their life cycle physiologically independent [START_REF] Boucher | The ecology of mutualism[END_REF]. Many symbiotic mutualisms involve the exchange of nutritional and energetic services (e.g. photosynthesis). Mutualisms, and particularly non-symbiotic ones, can also involve protection and dispersal of gametes or propagules.

Often, individuals benefit directly from their mutualistic behaviour. Many definitions restrict mutualism to these cases. However, in a number of interactions that constitute undisputed examples of mutualisms, the individuals performing the act that is beneficial to the other species do not benefit directly from this act. This is typically the case in gynodioecious Ficus species (Box 1). Pollinator individuals that ensure pollination by entering functionally female inflorescences (figs) die without reproducing: their reproductive value is equal to zero [START_REF] Kjellberg | Biology, ecology and evolution of fig-pollinating wasps (Chalcidoidea, Agaonidae)[END_REF][START_REF] Pereira | Polinização por vespas[END_REF]. A definition of mutualism that would exclude half of Ficus species and many other undisputed cases of mutualisms would be useless. In the case of the Hence, in many situations, mutualism benefits cannot be defined at the level of individual interactions. Defining mutualism at the population level could in a number of cases be operational as the an interaction may vary from mutualistic to parasitic across populations [START_REF] Addicott | Variation in the costs and benefits of mutualism: the interaction between Yuccas and Yuccas months (Tegeticula yuccasella)[END_REF][START_REF] Thompson | The Evolution of Species Interactions[END_REF][START_REF] Friberg | Extreme diversification of floral volatiles within and among species of Lithophragma (Saxifragaceae)[END_REF]. However, the structuring of mutualists into populations and species may be so different that a population level approach is not operational. For instance, Ficus hirta present gradual genetic variation among populations while its pollinating wasp in South-East China constitutes a single population [START_REF] Yu | Multiple parapatric pollinators have radiated across a continental fig tree displaying clinal genetic variation[END_REF]). In such cases, selective processes do not occur at the same geographic scales for the partners.

Different geographic population structure between mutualists is probably the rule rather than the exception in non-symbiotic mutualisms [START_REF] Alvarez | Phylogeography and historical biogeography of obligate specific mutualisms[END_REF]. Therefore, in this chapter, we define mutualisms as interspecific interactions that are beneficial for the species involved and that result in adaptive innovation. Obligate interactions resulting from infection of a host and subsequent adaptation of the host to its pathogen do not enter within the limits of this definition if they do not result in biological innovation. Such interactions can become obligate when curing the host from its pathogen results in self-poisoning [START_REF] Dedeine | Removing symbiotic Wolbachia bacteria specifically inhibits oogenesis in a parasitic wasp[END_REF].

Generally, organisms that associate in a mutualism differ radically in biological traits and life habits. The combination of these traits confers them new biological capacities, allowing them to colonise new ecological niches. At a macro-evolutionary scale, mutualisms have been at the source of major biological innovation [START_REF] Margulis | Symbiosis as the source of evolutionary innovation[END_REF][START_REF] Wheat | The genetic basis of a plant insect coevolutionary key innovation[END_REF][START_REF] Jr | The evolution of mutualism[END_REF]. A classic example is the endosymbiontic origin of mitochondria and plastids through the inclusion of prokaryotic organisms into the pre-eukaryotic cell [START_REF] Gray | Lynn Margulis and the endosymbiont hypothesis: 50 years later[END_REF]. This association improved the energetic machinery of eukaryotes and allowed some of them to become primary producers [START_REF] Margulis | Archaeal-eubacterial mergers in the origin of Eukarya: Phylogenetic classification of life[END_REF]. Several other symbiotic mutualisms have been the source of key innovations at the origin of major new modes of life. For instance, representatives of at least seven phyla (Annelida, Arthropoda, Ciliophora, Mollusca, Nematoda, Platyhelminthes and Porifera) are known to associate with chemosynthetic bacteria [START_REF] Dubilier | Symbiotic diversity in marine animals: the art of harnessing chemosynthesis[END_REF]. This type of mutualism allows representatives from these diverse lineages of animals to colonize habitats presenting high concentrations of reduced energy sources, such as sulphide and methane. These habitats are generally transient but at different time scales ranging from whale carcasses and sunken wood on the deep-sea floor to somewhat more stable habitats, such as hydrothermal vents, cold seeps, shallow-water coastal sediments and continental margins (e.g. mangrove areas) [START_REF] Dubilier | Symbiotic diversity in marine animals: the art of harnessing chemosynthesis[END_REF]. Herbivores, including insects of different orders and vertebrates derive their digestive capacities from bacteria, fungi, protozoa and other microorganisms. The fine mechanisms involved have been investigated for cockroaches, termites, attine ants, sap-feeding insects and ruminant vertebrates [START_REF] Mueller | The origin of the Attine ant-fungus mutualism[END_REF][START_REF] Nalepa | Detritivory, coprophagy, and the evolution of digestive mutualisms in Dictyoptera[END_REF][START_REF] Aanen | The evolution of fungus-growing termites and their mutualistic fungal symbionts[END_REF][START_REF] Kamra | Rumen microbial ecosystem[END_REF][START_REF] Baumann | Biology of bacteriocyte-associated endosymbionts of plant sap-sucking insects[END_REF]; [START_REF] Koike | Fibrolytic rumen bacteria: their ecology and functions[END_REF][START_REF] Douglas | The microbial dimension in insect nutritional ecology[END_REF][START_REF] Caldera | Insect symbioses: a case study of past, present, and future fungus-growing ant research[END_REF]. While taxonomically and biologically highly heterogeneous, these digestive mutualisms are all based on the capacity of the animal to collect carbon rich resources that they cannot digest by themselves. The associated microorganisms provide the metabolic capacities to degrade theses carbon rich resources, such as cellulose, hemicelluloses and lignin, to detoxify secondary plant compounds and to convert nitrogen into available amino acids. The association of plants with mycorrhizal fungi (e.g. fungi belonging to the Phylum Glomeromycota and forming arbuscular mycorrhiza) is most often mutualistic, improving water and nutrient uptake by the plant, especially the uptake of phosphate and nitrogen [START_REF] Brundrett | Diversity and classification of mycorrhizal associations[END_REF]. Plant fine roots are constrained in how thin they can become because of the size of their genome which results in large nuclei. Fungi have much smaller genomes allowing smaller nuclei and hence allowing fungal filaments to be much thinner than plant fine roots. Therefore, mycelia explore the soil at a much finer scale and at lower constitutive costs than plant roots. Mycorrhizal fungi are associated with roots of 70 -90% of land plant species, representing one of the most extensive terrestrial symbioses [START_REF] Parniske | Arbuscular mycorrhiza: the mother of plant root endosymbioses[END_REF]. It may have played a central role in the colonisation of terrestrial habitats by enabling plants to exploit mineral soils. The association between plants and nitrogen-fixing bacteria is another example of mutualism allowing ecological innovation. The bacteria metabolise inert N2 present in the atmosphere into the utilizable form ammonia (NH3) through the action of nitrogenases. This mutualism is present in a diversity of plant lineages including ferns, gymnosperms, mono-and eudicots. The plants host the endophytic bacteria in their roots. Among them, the root nodule symbiosis is particularly sophisticated and complex. The root nodules house high bacteria densities in a structure that provides an anaerobic microenvironment favourable for nitrogenase activity [START_REF] Markmann | Evolution of root endosymbiosis with bacteria: how novel are nodules?[END_REF][START_REF] Soto | Mutualism versus pathogenesis: the give-and-take in plant-bacteria interactions[END_REF]). The mutualisms exemplified above are just a few examples of known mutualisms that represent biological innovations central to the colonisation of earth and the functioning of extant ecosystems. There are many other examples, as for instance lichens associating fungi and algae/cyanobacteria [START_REF] Nascimbene | Freshwater lichens of the Italian Alps : a review[END_REF], or sessile/slow-moving marine animals and algae/cyanobacteria [START_REF] Venn | Photosynthetic symbioses in animals[END_REF]).

In the following, we first present some theoretical considerations on mutualisms. Indeed, developing a comprehensive theory of mutualisms is still an open endeavour. We propose a framework within which a theory may emerge. Then we develop some examples of insect-plant and insect-fungi mutualisms that are at the origin of major evolutionary innovation. Finally, we focus on the fig tree -associated animal interactions that allowed this plant lineage to diversify in subtropical and tropical ecosystems and to become keystone species for the functioning of some forest ecosystems. An intriguing example is pollination by deceit in gynodioecious Ficus species [START_REF] Kjellberg | The stability of the symbiosis between dioecious figs and their pollinators: A study of Ficus carica L. and Blastophaga psenes L[END_REF]; Figure 1). Functionally "male" trees produce pollinating wasps (pollen vectors) and pollen, and female trees produce seeds but neither pollen nor wasps. Female trees bear figs in which all the styles of pistillate flowers are too long to allow wasp oviposition. Wasps entering figs of female trees pollinate but die without reproducing. Functionally male trees bear figs in which all pistillate flowers have short styles. These flowers may receive an egg and produce a wasp, but they rarely produce seeds. As a result, although male figs present both male and female flowers, they are functionally male. This is a case of pollination by deceit and figs on For the compounds that are perceived by the pollinators, the inflorescences of female and "male" trees emit exactly the same relative proportions [START_REF] Hossaert-Mckey | How to be a dioecious fig: Chemical mimicry between sexes matters only when both sexes flower synchronously[END_REF][START_REF] Proffit | Chemical signal is in the blend: bases of plantpollinator encounter in a highly specialized interaction[END_REF]. Hence, a first answer is male-female mimicry.

However, a better formulation is probably, male-female mimicry makes distinguishing between male and female trees complicated. Then the question becomes, could it be worthwhile to take the time and efforts to distinguish male and female trees, can being choosy be selected? In some Ficus species, there is hardly any selection for avoiding female trees because figs on female and "male" trees are not receptive at the same time of the year, so that the wasps never get a chance to choose between sexes. Most pollinators emerge from figs on "male" trees at a time when there are no receptive male figs. They will fail to reproduce irrespective of whether they avoid female trees or not [START_REF] Kjellberg | The stability of the symbiosis between dioecious figs and their pollinators: A study of Ficus carica L. and Blastophaga psenes L[END_REF]). In Ficus species in which "male" and female trees are receptive at the same time, the race to enter receptive figs [START_REF] Conchou | Daily Rhythm of Mutualistic Pollinator Activity and Scent Emission in Ficus septica: Ecological Differentiation between Co-Occurring Pollinators and Potential Consequences for Chemical Communication and Facilitation of Host Speciation[END_REF] selects against taking the time required to distinguish highly similar phenotypes.

Theoretical remarks

The main pitfall of a vision in which adaptive innovation drives the evolution of mutualistic associations is formulations suggesting that a species acts for the sole benefit of another one and reciprocally. Such formulations cannot be defended. There is no way escaping that individuals are selected to maximize their fitness. In a mutualism this may, or may not, be at the expense of the partner species. In this context, the evolution of mutualisms is often envisioned in a framework where individual selection leads to reciprocal exploitation that results in net benefits to all the interacting parties, i.e. in a framework in which individual selection intrinsically destabilises the mutualistic association [START_REF] Axelrold | The evolution of cooperation[END_REF][START_REF] Herre | Conflict of interest in a mutualism: documenting the elusive fig wasp-seed trade-off[END_REF]. Therefore, it has been proposed that host sanction against noncooperative partners may be important for mutualism stability. Host sanctions would be any traits evolved specifically to reduce the fitness of a partner that becomes non-cooperative.

Lists of mechanisms limiting non-cooperative behaviour or partner overexploitation and derived from intraspecific cooperation theory have been proposed [START_REF] Herre | The evolution of mutualisms: exploring the paths between conflict and cooperation[END_REF][START_REF] Yu | Parasites of mutualisms[END_REF][START_REF] Sachs | The evolution of cooperation[END_REF][START_REF] Doebeli | Models of cooperation based on the Prisoner's Dilemma and the Snowdrift game: Prisoner's Dilemma and the Snowdrift game[END_REF][START_REF] West | Social semantics: altruism, cooperation, mutualism, strong reciprocity and group selection[END_REF][START_REF] Bergmüller | On the further integration of cooperative breeding and cooperation theory[END_REF]. While cooperation models have stimulated theoretical studies on mutualism, they do not find consistent empirical support to explain the stability of a range of mutualistic associations. For example, models based on retaliation (sanctions or refusing future interactions) are probably only of importance for animals with developed cognition capacity, as they require the ability to remember past interactions and behave accordingly. Sanction has been invoked to explain cooperation reinforcement in some mutualistic association, such as legume plants -N2-fixing bacteria, fig trees-pollinating fig wasps and yucca -pollinating yucca moths [START_REF] Pellmyr | Evolutionary stability of mutualism between yuccas and yucca moths[END_REF][START_REF] Kiers | Measured sanctions: legume hosts detect quantitative variation in rhizobium cooperation and punish accordingly[END_REF][START_REF] Kiers | Sanctions, Cooperation, and the Stability of Plant-Rhizosphere Mutualisms[END_REF][START_REF] Jandér | Host sanctions and pollinator cheating in the fig tree-fig wasp mutualism[END_REF][START_REF] Jr | The evolution of mutualism[END_REF][START_REF] Jandér | Precision of host sanctions in the fig tree-fig wasp mutualism: consequences for uncooperative symbionts[END_REF]. In these examples, when the Rhizobium bacteria do not provide nitrogen, or when the fig wasps/yucca moths do not pollinate, the non-cooperating partner pays a cost, as the O2 flux to the radicular modules where bacteria are housed stops or the flower ovaries or inflorescences where fig/yucca pollinators laid their eggs provide less nutrients or abort.

However, there is no demonstration that these host traits that have been selected to respond to non-cooperative partners, i.e. that they qualify as sanctions [START_REF] Frederickson | Rethinking Mutualism Stability: Cheaters and the Evolution of Sanctions[END_REF]. Further no theoretical framework has been proposed within which sanction could evolve. Indeed, there is an intrinsic problem with the strong differences in generation time and populations sizes between the interacting species (see Box 1). Individuals of the slow-reproduction small population size species interact with many individuals and many generations of their associated species. Such asymmetry is constitutive of most mutualistic associations. As a consequence, in visions of mutualism stability based on an evolutionary race between species, the partner with larger population sizes and the shorter generation time should win the race and reap the highest profits possible from the association. In the process, it would overcome any adaptation of the other species to limit overexploitation.

Contrary to the predictions based on ideas stemming from within species cooperation theory, mutualistic associations are robust to the presence of non-cooperative partners.

Indeed, phylogenetic evidences from a range of biological systems evidences that mutualisms rarely evolve into parasitic interactions in nature [START_REF] Frederickson | Mutualisms Are Not on the Verge of Breakdown[END_REF]. A striking example of this robustness is provided by the diffuse mutualism involving seed dispersion by vertebrates.

Seed dispersion networks are structured into complex mosaics of frugivorous guilds associated with plant guilds and involve plant traits such as seed size, fruit colour, fruit flesh nutrient content, etc, with few specialist species [START_REF] Shanahan | Fig-eating by vertebrate frugivores: a global review[END_REF][START_REF] Albert | The role of macaca spp. (Primates: Cercopithecidae) in seed dispersal networks[END_REF][START_REF] Sarmento | Partitioning of seed dispersal services between birds and bats in a fragment of the Brazilian Atlantic Forest[END_REF]. Poor dispersers and seed predators are frequent among members of the frugivorous guilds [START_REF] Howe | Seed dispersal by fruit-eating birds and mammals[END_REF][START_REF] Compton | Seed dispersal in an African fig tree: Birds as high quantity, low quality dispersers?[END_REF][START_REF] Shanahan | Fig-eating by vertebrate frugivores: a global review[END_REF]. Nevertheless, even with this widespread occurrence of non-cooperating partners, about 70-90% of tropical trees present adaptions favouring animal seed dispersion [START_REF] Howe | Ecology of Seed Dispersal[END_REF]. Thinking about the stability of mutualisms and their role in biological innovation obliges to envision their evolution from two different perspectives. The individual selection perspective is a truism: only genes that increase in frequency in a population become fixed. Genes that decrease in frequency will disappear. Therefore, individual selection may lead to population size decrease and to species extinction by increasing the relative frequency of a gene that reduces population size. On the other hand, one of the defining characters of life is that living organisms have a history. Indeed, life on earth has survived for over 3 billion years. Species that have survived and given rise to new ones are those in which short-term individual selection was compatible with, or even favored, long-term species survival. Consequently, extant species derive from species in which some intrinsic biological traits resulted in lack of short-term selection favoring genes that would lead to species extinction. Only lineages in which there was no such destructive short-term selection have survived. If a lineage loses these biological traits, it will go extinct. This intrinsic property of life is illustrated by sexual reproduction. Most species on earth engage in some form of sex. Nevertheless, asexual lineages regularly appear. In the phylogenies, there are no asexual lineages but only isolated asexual species at the end of phylogenetic branches. This means that asexual species are evolutionary dead ends [START_REF]The Evolution of Sex[END_REF]. Why is loss of sex so rare that it does not drive life on earth to extinction? Simply because extant lineages derive from species in which some intrinsic trait resulted in short term selection for sex and most often species inherit this trait from their ancestors [START_REF] Gouyon | Sex: a pluralist approach includes species selection. (One step beyond and it's good.)[END_REF]).

As a result, loss of sex remains exceptional. We can apply the same line of thinking to mutualisms. Only mutualisms in which there is no short-term selection against the mutualism, or mutualisms in which no mutation that destabilizes the mutualism can arise, survive over evolutionary times. This explains why mutualisms are intrinsically stable [START_REF] Frederickson | Mutualisms Are Not on the Verge of Breakdown[END_REF] and, hence, why we do not expect to find adaptations specifically evolved to punish cheating mutualists (i.e. that would take the benefits from the mutualism without reciprocating). Therefore, sanctions do not explain mutualism stability [START_REF] Frederickson | Rethinking Mutualism Stability: Cheaters and the Evolution of Sanctions[END_REF].

The mutualism between Yucca and Yucca-moths provides an example of the pitfalls associated with thinking in terms of sanctions. Yuccas are pollinated by Tegeticula moths that lay eggs in the flowers. The moth collects pollen with its modified labial palps and deposits some of it on the flower stigma after oviposition. The moth larva feeds on the developing young seeds. If the larva eats too many developing seeds in the ovary, the flower aborts leading to the death of the larva. From these observations, it is tempting to suggest that Yucca plants have developed a mechanism to limit oviposition by its mutualistic pollinator. If the pollinator turns into a parasite eating too many seeds, it is punished. However, Tegeticula belong to a group of moths that do limited damages in the ovaries. Lack of host overexploitation predates the mutualism (Yoder et al. 2010). Similarly, Yucca belong to a lineage of plants in which there is abundant early abortion of developing fruits. Abortion rates of fruits containing few developing seeds and of damaged flowers are high. Hence, the mechanisms limiting over-exploitation predated the evolution of the mutualism and do not qualify as sanctions. We propose that the evolution of the mutualism was made possible by pre-existing traits of the associates [START_REF] Frederickson | Rethinking Mutualism Stability: Cheaters and the Evolution of Sanctions[END_REF]. This is further demonstrated that one Tegeticula pollinated species, Hesperoyucca whipplei, is not a Yucca. The moth has jumped host, and has become the active pollinator of a new lineage [START_REF] Pellmyr | Yuccas, Yucca Moths, and Coevolution: A Review[END_REF]. This lineage did not have a history of co-adaptation with the moth and therefore could not have developed adaptations against overexploitation by mutualist Tegeticula moths. Despite this, the association has thrived demonstrating that the adaptations limiting exploitation pre-existed to the mutualism. Successful host switching has occurred in several active pollination mutualisms. The complementarity of species traits in mutualisms are generally stricking. In both symbiotic and non-symbiotic mutualism, in general, there is a combination of a structural component, representing goods provided by a long-lived partner (e.g., 3D structure, sheltering or food), and a service supplied by a short-lived partner (e.g., gamete transport, resource transport from the surroundings, protection against natural enemies or biosynthetic capacities) (Leigh Jr 2010). In addition, mutualisms involve members of distant taxonomic groups, as exemplified by marine invertebrates -bacteria, mammals/insects -gut microorganisms, plant -root bacteria/fungi, fungi -algae/cyanobacteria, and plant -insect associations. Large phylogenetic distance between the partners is probably important for mutualism stability, as it enhances the combination of contrasting abilities, and decreases niche overlap. As a consequence, the potential for selective conflicts between partners is reduced, and the potential for overcoming them is enhanced. Indeed, the larger organism generally controls the arena in which the life of the smaller, shorter-lived organism is plaid out.

Controlling the arena may canalise selective forces acting on the smaller partner. In this topic we argue that mechanisms to reinforce cooperation, acting exclusively at individual (or gene) level are not sufficient to stabilize mutualistic associations, although these mechanisms play a role in reducing conflicts between partners. One example of such mechanism is the vertical transmission of the symbiont that occurs in sap-sucking insects -gut bacteria [START_REF] Baumann | Biology of bacteriocyte-associated endosymbionts of plant sap-sucking insects[END_REF] and ant/beetle -cultivated fungi [START_REF] Aanen | The evolution of fungus-growing termites and their mutualistic fungal symbionts[END_REF][START_REF] Mueller | The Evolution of Agriculture in Insects[END_REF], which increases the symbiont endogamy, limiting the selection of non-cooperative traits via kin selection. In other types of mutualism, specially the non-symbiotic ones, it is necessary to consider spatiotemporal dynamics at community level and the multilevel selection to accommodate an evolutionary framework of mutualisms [START_REF] Gomulkiewicz | Coevolution in Variable Mutualisms[END_REF][START_REF] Wilson | Evolution "for the Good of the Group[END_REF][START_REF] O'gorman | For the good of the group? Exploring group-level evolutionary adaptations using multilevel selection theory[END_REF][START_REF] Nowak | The evolution of eusociality[END_REF]. Mutualism as source of evolutionary innovation that expands the partners capacities appears as a general aspect of + / + interactions. Such emergent attribute can sometimes transcend pure genetic determinism if one considers cultural evolution in animals with developed cognitive capacities, as for example corvids and primates, that have allowed them to better explore the ecosystem, and eventually use new niches [START_REF] Wimsatt | Genes, Memes, and Cultural Heredity[END_REF][START_REF] Castro | The evolution of culture: From primate social learning to human culture[END_REF][START_REF] Marzluff | Cultural Coevolution: How the Human Bond with Crows and Ravens Extends Theory and Raises New Questions[END_REF][START_REF] Vale | The Cultural Capacity of Human and Nonhuman Primates: Social Learning, Innovation, and Cumulative Cultural Evolution[END_REF].

Some examples of mutualisms involving insects and plants

Most seed plants are sessile during their sporophytic phase, i.e. during most of their life cycle.

Hence, many of their responses to ecological challenges, such as defence against herbivores, gamete transfer and seed dispersal, may rely on mutualistic associations with mobile animals (see previous topics). On the other hand, animals may expand their ability to use plant resources thanks to mutualistic interactions with microorganisms [START_REF] Douglas | The microbial dimension in insect nutritional ecology[END_REF] or by tending phytophagous insects [START_REF] Heil | Protective ant-plant interactions as model systems in ecological and evolutionary research[END_REF]. Those mutualisms can be classified according to the services provided by one of the partners, usually the one smaller in size and with shorter generation time (Table 1). In the following, we present some examples of insect-plant mutualisms involving defence against herbivores, digestion of plant products, and pollination by insects (Figure 2).

Protection mutualisms

Animals and plants can obtain protection against natural enemies from mutualistic associations. Particularly, two interrelated systems have been extensively studied: ant-plantherbivore and ant-hemipteran interactions. Insect herbivory is a key factor in plant communities that effects plant productivity, survival and reproduction [START_REF] Showalter | Insect ecology: an ecosystem approach[END_REF].

Plants can derive protection against herbivores by associating with predatory ants. As ants are among the most important predators of arthropods, they can constitute an effective plant defence. Indeed, while many herbivores have overcome plant chemical defences, few have evolved efficient adaptations against predatory ants [START_REF] Heil | Protective ant-plant interactions as model systems in ecological and evolutionary research[END_REF].

Plants can attract facultative mutualist ants by providing direct rewards (energy-rich extrafloral nectar and food bodies), as well as indirect rewards such as carbohydrate-rich excretions (honeydew) of phloem-feeding hemipterans (trophobionts). In this later tripartite mutualism, ants defend their trophobionts from predatory and parasitoid insects, resulting in another nested defensive interaction [START_REF] Delabie | Trophobiosis between formicidae and hemiptera (Sternorrhyncha and Auchenorrhyncha): an overview[END_REF][START_REF] Heil | Protective ant-plant interactions as model systems in ecological and evolutionary research[END_REF][START_REF] Mckey | The trophic structure of tropical ant-plant-herbivore interactions: community consequences and coevolutionary dynamics[END_REF].

The direct and indirect rewards provided by the plants for the ants are poor in nitrogen and present highly unbalanced amino acid compositions. Ants may complement this unbalanced diet by hunting herbivorous insects or by harvesting some of their hemipteran trophobionts [START_REF] Del-Claro | Loss and gains in ant-plant interactions mediated by extrafloral nectar: fidelity, cheats, and lies[END_REF][START_REF] Calixto | Protection mutualism: an overview of ant-plant interactions mediated by extrafloral nectaries[END_REF]. Some ants, especially species adapted for living in the canopy, are associated with endosymbiotic microbes that help them to cope with such nutritional imbalance. The need of supplementation of nitrogen from external sources, seems to have driven the selection of prey-foraging strategies and physiological adaptations [START_REF] Heil | Protective ant-plant interactions as model systems in ecological and evolutionary research[END_REF][START_REF] Mckey | The trophic structure of tropical ant-plant-herbivore interactions: community consequences and coevolutionary dynamics[END_REF].

In over 100 genera of tropical angiosperms, plants are involved in more permanent, in some cases obligatory, mutualisms with protective ants. These plants have specialised structures, called domatia, and used as nest by the ants. Domatia can occur in hollow stems (e.g., Cecropia, Leonardoxa and Macaranga), thorns (Acacia), petioles (Piper), or leaf pouches (Hirtella, Maieta, Scaphopetalum and Tococa). In addition to domatia, plants may offer food rewards, such as extrafloral nectar, food bodies or both. Plants benefit from protection services and, additionally, from nutrients mobilised by the ants. It is estimated that 80% of the carbon in Azteca ant's bodies can be derived from their Cecropia host tree, whereas about 90% of the plant's nitrogen comes from ants debris (i.e., exuviae, dead larvae, workers, and remains of arthropod prey) [START_REF] Heil | Protective ant-plant interactions as model systems in ecological and evolutionary research[END_REF].

Defensive mutualisms have evolved many times between insect species. Ants show trophobiotic relationships with lepidopteran larvae of the familes Lycaenidae, Riodinidae, and Tortricidae, and heteropteran species of the families Coreidae, Pentatomidae and Plataspidae.

In addition to ants, hemipteran species have trophobiotic relationships with a range of other insect groups, such as Anthribidae, Coccinellidae, Apoidea (and other aculeate Hymenoptera), Tachinidae, Syrphidae and Neuroptera [START_REF] Delabie | Trophobiosis between formicidae and hemiptera (Sternorrhyncha and Auchenorrhyncha): an overview[END_REF]). However, protection mutualism is more frequent and better studied in ant-hemipteran interactions. Sap-feeding hemipterans are particularly vulnerable to predation. Some species are sessile at some life-stages. Even in more mobile species, to access the phloem, hemipteran insects introduce their stylets deep into plant tissues; a process that takes minutes to hours. While they are attached to the host plant, they are not able to remove the mouthparts quickly to escape predators. Moreover, because of their feeding mode, hemipteran insects excrete large quantities of honeydew. These insects have one additional problem; they have to get rid of such excretion that can accumulate on their bodies and serve as substrate for fungus growing. Thus, trophobiosis has this "cleaning" benefit for sap feeding insects [START_REF] Delabie | Trophobiosis between formicidae and hemiptera (Sternorrhyncha and Auchenorrhyncha): an overview[END_REF].

Digestive mutualism

Mutualism with microorganisms has allowed several groups of animals to feed on plants.

Indeed, plant tissues constitute a source of energy and nutrients rich in cellulose and allelochemicals, which are primarily inaccessible to animal digestion. Plant feeding insects and vertebrates derive their digestive capabilities from bacteria, fungi, protozoa and other microorganisms. The most studied groups include cockroaches, termites, attine ants, plant sap-feeding insects and ruminants [START_REF] Mueller | The origin of the Attine ant-fungus mutualism[END_REF][START_REF] Nalepa | Detritivory, coprophagy, and the evolution of digestive mutualisms in Dictyoptera[END_REF][START_REF] Aanen | The evolution of fungus-growing termites and their mutualistic fungal symbionts[END_REF][START_REF] Kamra | Rumen microbial ecosystem[END_REF][START_REF] Baumann | Biology of bacteriocyte-associated endosymbionts of plant sap-sucking insects[END_REF]; [START_REF] Koike | Fibrolytic rumen bacteria: their ecology and functions[END_REF][START_REF] Douglas | The microbial dimension in insect nutritional ecology[END_REF][START_REF] Caldera | Insect symbioses: a case study of past, present, and future fungus-growing ant research[END_REF]).

Despite their remarkable taxonomic and ecological diversity, digestive mutualisms all share the attribute of making use of the large metabolic capacities of microorganisms to degrade carbon rich sources (i.e. cellulose, hemicelluloses and lignin), detoxify secondary plant compounds and convert nitrogen in available amino acids.

Detritivorous termites and cockroaches are associated with bacteria, fungi and protozoa that degrade cellulosic material and recycle nitrogen from insect waste [START_REF] Nalepa | Detritivory, coprophagy, and the evolution of digestive mutualisms in Dictyoptera[END_REF][START_REF] Douglas | The microbial dimension in insect nutritional ecology[END_REF]). Analogous interactions occur in ruminant vertebrates, allowing them to assimilate carbon present in cellulosic compounds and metabolise anti-nutritional and toxic substances of plants. Cattle support an impressively diverse and complex microorganism community. One millilitre of rumen liquor can enclose as many as 10 10 -10 11 cells of 50 bacterium genera, 10 4 -10 6 ciliate protozoa from 25 genera, 10 3 -10 5 zoospores of five anaerobic fungus genera and 10 8 -10 9 bacteriophages [START_REF] Kamra | Rumen microbial ecosystem[END_REF]. Nevertheless, the bacterial diversity is largely underestimated, as many rumen bacteria cannot be cultivated in the laboratory. Species survey based on DNA sequences suggest that 300-400 species of bacteria are present in the rumen [START_REF] Koike | Fibrolytic rumen bacteria: their ecology and functions[END_REF].

Despite of the large metabolic capacity of microorganisms, the amount of vegetal material that insects can process is ultimately constrained by their gut volume. This limitation is overcome by insects that cultivate fungi in their nests [START_REF] Douglas | The microbial dimension in insect nutritional ecology[END_REF]. Fungus cultivation is well known in the Neotropical Attini ant tribe and in the Old-World termite subfamily Macrotermitinae [START_REF] Mueller | The origin of the Attine ant-fungus mutualism[END_REF][START_REF] Aanen | The evolution of fungus-growing termites and their mutualistic fungal symbionts[END_REF][START_REF] Caldera | Insect symbioses: a case study of past, present, and future fungus-growing ant research[END_REF]). However, fungus cultivation is much more widespread and is carried out by siricid woodwasps, cerambycid beetles and plant-ants [START_REF] Douglas | The microbial dimension in insect nutritional ecology[END_REF][START_REF] Defossez | Ant-plants and fungi: a new threeway symbiosis[END_REF]. All the 210 plus attine ant species rely on cultivation of fungi of the tribe Leucocoprineae as their main food. The majority of cultivated fungi belong to two genera, Leucoagaricus and Leucocoprinus. Basal attine species cultivate fungi on dead-plant matter and caterpillar frass, while derived lineages collect fresh leaves and flowers as substrate for fungi, suggesting that the development of a new mode of fungi cultivation is an innovation that has allowed ant species diversification. The fungi cultivated by derived attine ants are also highly specialized on the mutualism. They have evolved a specialised structure rich in nutrients, the gongylidia, that serves as ant food. This structure is formed by densely packed clusters of hyphae that are easily harvested by the ants [START_REF] Mueller | The origin of the Attine ant-fungus mutualism[END_REF]. The mutualism involving fungus-growing termites is analogous with the attine-fungus association, as it allows termites to make use of a diversity of vegetal food sources such as wood, dry grass, and leaf litter. This mutualism is restricted to a single termite subfamily, Macrotermitinae, which is associated with fungi of the genus Termitomyces. In contrast with fungal symbionts of the attine ants that are usually propagated clonally and vertically by dispersing queens, Termitomyces reproduce sexually and are most often horizontally transmitted [START_REF] Aanen | The evolution of fungus-growing termites and their mutualistic fungal symbionts[END_REF].

A particular mutualism involves sap-feeding insects and intracellular bacteria. This system is one of the most specialized digestive mutualisms in terms of partner integration.

Pollination mutualism

Gamete transfer in seed plants is strongly constrained by the sessile nature of their predominant life stage, the sporophyte. Therefore, seed plants must rely on animal and abiotic pollen vectors (wind and sometimes water) to achieve cross fertilisation. Several groups of insects including bees, wasps, beetles, dipterans, lepidopterans and thrips, are involved in nonsymbiotic and symbiotic pollination mutualisms. Non-symbiotic mutualisms include cases of diffuse reciprocal adaptation, where independent plant lineages share convergent floral attributes called pollination syndromes associated with pollinator guilds that present particular sensory biases and particular pollen transfer attributes [START_REF] Rech | Síndromes de polinização: especialização e generalização[END_REF][START_REF] Dellinger | Pollination syndromes in the 21 st century: where do we stand and where may we go?[END_REF]. In those mutualisms, insects benefit from food (nectar, pollen, oil), shelter (mainly for mate encounter) as well as other resources used for nest construction or their reproductive behaviour (oil, resin and floral scent) [START_REF] Agostini | Recursos florais[END_REF].

Symbiotic pollination mutualisms are associations in which the plant provide breeding sites for their pollinators. Most of them qualify as nursery pollination mutualism. Larvae of pollinators feed on ovules/seeds or other floral parts [START_REF] Sakai | A review of brood-site pollination mutualism: plants providing breeding sites for their pollinators[END_REF][START_REF] Dufaÿ | Conflicts between plants and pollinators that reproduce within inflorescences: evolutionary variations on a theme[END_REF][START_REF] Hembry | Diversification and coevolution in brood pollination mutualisms: Windows into the role of biotic interactions in generating biological diversity[END_REF]. Among these, brood-site pollination mutualism (sensu Hembry and

Althoff 2016, i.e. association where pollinators feed on developing plant ovules) has evolved independently in several groups of plants and insects [START_REF] Dufaÿ | Conflicts between plants and pollinators that reproduce within inflorescences: evolutionary variations on a theme[END_REF]. It is often obligate (e.g., fig trees -fig wasps, yucca -yucca moths and leafflowers -leafflower moths, senita cactus -senita moths), but can also be facultative when the plant is pollinated both by insects whose offspring develop feeding on seeds and by generalist insect pollinators that visit flowers to feed on pollen and/or nectar; e.g. Silene -Hadena/Perizoma, Lithophragma -Greya and globeflower -globeflower flies [START_REF] Kephart | Pollination and seed predation by moths on Silene and allied Caryophyllaceae: evaluating a model system to study the evolution of mutualisms[END_REF][START_REF] Hembry | Diversification and coevolution in brood pollination mutualisms: Windows into the role of biotic interactions in generating biological diversity[END_REF]. In these cases, the relationship with the seed eating taxa varies locally from mutualism to antagonism depending on the local pollination efficiency of generalist pollinators.

In brood-site pollination mutualisms the plants are selected to attract pollinators that will feed on developing seeds. We propose that the pollinating insects are selected to be host specialists so that they collect pollen from plants of and oviposit in plants of the same species, thus ensuring successful seed to feed their offspring. In most cases, the plants are selected to try to kill the developing pollinator larvae as the pollinator offspring will not carry pollen from their natal plant, and the system remains stable because this selection has not succeeded [START_REF] Addicott | Ev•oolution of mutualistic life-cycles: yucca moths and fig wasps[END_REF]). For instance, in the fig tree -fig wasp system, about 50% of the plant ovules are consumed by the pollinator larvae [START_REF] Kjellberg | Biology, ecology and evolution of fig-pollinating wasps (Chalcidoidea, Agaonidae)[END_REF]. However, breeding pollinators does not come at a cost in Ficus as most consumed ovules produce female pollinating wasps, that will carry pollen from their natal fig (Jousselin and Kjellberg 2001;[START_REF] Kjellberg | Pollination mode in fig wasps: the predictive power of correlated traits[END_REF]Jousselin et al. 2003a). [START_REF] Sakai | Artocarpus (Moraceae)-gall midge pollination mutualism mediated by a male-flower parasitic fungus[END_REF][START_REF] Sakai | Thrips pollination of androdioecious Castilla elastica (Moraceae) in a seasonal tropical forest[END_REF][START_REF] Zerega | Pollination in the New Guinea Endemic Antiaropsis decipiens (Moraceae) Is Mediated by a New Species of Thrips, Thrips antiaropsidis sp. nov. (Thysanoptera: Thripidae)[END_REF][START_REF] Olotu | Effects of forest fragmentation on pollination of Mesogyne insignis (Moraceae) in Amani Nature Reserve forests, Tanzania: Effects of forest fragmentation on pollination of Mesogyne insignis[END_REF][START_REF] Araújo | From anthesis to diaspore dispersal: reproductive mechanisms of rare herbaceous Moraceae species endemic to Brazil[END_REF]).

Brood

Among these, only in Ficus do the wasp larvae feed on developing plant ovules. This is probably the sole Moraceae genus in which fertilisation female flowers generally directly benefits the wasp's offspring. Fig trees stand out for their generalised obligate-specialized pollination mutualism and their taxonomic diversity. Ficus is about seven times more specious than the second moracean genus in number of species (i.e., Dortenia with 113 spp, which is followed by Artocarpus with ca. 70 spp) (Figure 3). Ficus is, in addition, functionally diversified.

The genus presents diverse life-forms (freestanding and hemi-epiphytic trees, shrubs and climbers), breeding systems (monoecious and gynodioecious), pollination mode (active and passive), seed dispersion syndromes (birds and volant and non-volant mammals, as well as reptiles and fishes)] that allow them to occupy a range of (micro)habitats [START_REF] Shanahan | Fig-eating by vertebrate frugivores: a global review[END_REF]Jousselin et al. 2003b;[START_REF] Harrison | Figs and the diversity of tropical rainforests[END_REF][START_REF] Coelho | Water availability determines the richness and density of fig trees within Brazilian semideciduous forest landscapes[END_REF]. Each Ficus species is pollinated by one or some species of host species-specific agaonid wasp [START_REF] Ramírez | Host specificity of fig wasps (Agaonidae)[END_REF][START_REF] Kjellberg | Biology, ecology and evolution of fig-pollinating wasps (Chalcidoidea, Agaonidae)[END_REF] [START_REF] Berg | Classification and distribution of Ficus[END_REF]. Fertilised pollen-loaded pollinating female wasps are attracted by a blend of volatile substances released by the receptive figs [START_REF] Grison-Pigé | Specific attraction of fig-pollinating wasps: Role of volatile compounds released by tropical figs[END_REF][START_REF] Souza | Diversity of fig glands is associated with nursery mutualism in fig trees[END_REF]. The pollinating wasp enters the fig through the ostiole, a pore closed by floral involucral bracts and pollinates (actively or passively) the pistillate flowers. In some of them, the wasp inserts its ovipositor through the style to lay one egg precisely between the nucellus and the inner integument [START_REF] Jansen-Gonzalez | Mutualism from the inside: coordinated development of plant and insect in an active pollinating fig wasp[END_REF].

Ovaries that receive a wasp egg turn into galls where the pollinator larvae will develop feeding on endosperm, and those that have been pollinated and have escaped wasp oviposition will produce seeds. Approximately four to eight weeks later (up to 9 months later for the pollinator of the common fig!), depending on the temperature [START_REF] Pereira | Phenological patterns of Ficus citrifolia (Moraceae) in a seasonal humid-subtropical region in Southern Brazil[END_REF], the offspring complete their development. The males emerge first, locate and copulate with the females still enclosed in their natal galls. Then, the fertilised females emerge from their galls, actively collect pollen from staminate flowers at anthesis (in passively pollinated species the pollen from dehiscing anthers adheres spontaneously to the wasp's body) and leave the natal fig to

search for another tree bearing receptive figs [START_REF] Frank | The behavior and morphology of the fig wasps Pegoscapus assuetus and P. jimenezi: descriptions and suggested behavioral characters for phylogenetic studies[END_REF]. Subsequently the figs ripen and become attractive to a diversity of vertebrate frugivores that can act as seed dispersers [START_REF] Shanahan | Fig-eating by vertebrate frugivores: a global review[END_REF]. The figs that are not consumed by frugivores fall to the ground and turn into a resource for a diverse range of more or less specialised animals, mainly insects [START_REF] Palmieri | The role of non-fig-wasp insects on fig tree biology, with a proposal of the F phase (Fallen figs)[END_REF].

Gynodioecius Ficus species present two types of plants -one that produces figs containing only pistillate flower (i.e., 'female' tree). The pollinator attracted to a fig in a female tree pollinates but cannot lay eggs, as the styles are too long for the wasp ovipositor to reach the ovary of these flowers (Figure 1B). Thus, the pistillate flowers of female trees are pollinated by deceit, as the pollinator is attracted by a deceptive resource signalling. Pollen production, on the other hand, occurs in the functionally male trees, which have figs that

enclose both pistillate and staminate flowers (Figure 1B). Wasp can oviposit in 'male' trees as the pistilate flowers have short styles. 'Male' trees do not produce seeds because either the wasps deposit pollen precisely on the stigmas of the flowers into which they oviposit (active pollination) or because of poor germination of pollen in 'male' trees (Jousselin and Kjellberg 2001). However, pollen is dispersed by the pollinator offspring in the same way of in monoecious Ficus species (Figure 1A; [START_REF] Weiblen | How to be a fig wasp[END_REF].

The ancestral mode of pollination of Moraceae and their closest relatives Cannabaceae and Urticaceae is wind pollination involving an explosive mechanism of pollen dispersal with stamens inflexed in the flower bud that, when they distend cause a rapid anther movement, releasing large amounts of pollen [START_REF] Pedersoli | Anatomy solves the puzzle of explosive pollen release in wind-pollinated urticalean rosids[END_REF]. This pollen release mechanism can achieve an exceptional initial velocity of Mach 0.7 (232 m.s -1 ) in mulberry plants [START_REF] Taylor | High-speed pollen release in the white mulberry tree[END_REF]. This mechanism may be an adaptation to circumvent the wind limitation in the tropical forest understorey [START_REF] Bawa | Wind-Pollination in the Understorey of a Rain Forest in Costa Rica[END_REF]. In this context, the fig -fig wasp mutualism brings out a singular innovation in pollen dispersion, by combining initial wind dispersal of the wasps followed by wasp chemotaxis allowing cross-pollination at amazingly low population densities [START_REF] Ware | Dispersal of adult female fig wasps: 2. Movements between trees[END_REF]. This is achieved despite the dispersing wasps only surviving 24-48 hours outside figs [START_REF] Kjellberg | Longevity of a fig wasp (Blastophaga psenes)[END_REF][START_REF] Jevanandam | Climate warming and the potential extinction of fig wasps, the obligate pollinators of figs[END_REF]. Despite this temporal constraint, agaonid wasps can regularly disperse pollen over remarkably long distances comparatively to usual insect pollinated plants. It is demonstrated that average pollination distances by agaonid wasps within a fig population may reach several tens of kilometres [START_REF] Compton | The colonization of the Krakatau Islands (Indonesia) by fig wasps and other chalcids (Hymenoptera, Chalcidoidea)[END_REF][START_REF] Nason | Paternity analysis of the breeding structure of strangler fig populations: Evidence for substantial long-distance wasp dispersal[END_REF][START_REF] Nazareno | What the reasons for no inbreeding and high genetic diversity of the neotropical fig tree Ficus arpazusa?[END_REF][START_REF] Ahmed | Wind-borne insects mediate directional pollen transfer between desert fig trees 160 kilometers apart[END_REF]).

Long-distance pollen dispersal, at least in monoecious Ficus species, is mediated by wind.

Collection of aerial plankton shows that wasps pollinating monoecious Ficus species are dispersed by the wind above the forest canopy over long distances [START_REF] Compton | The flight heights of chalcid wasps (Hymenoptera, Chalcidoidea) in a lowland bornean rain forest: Fig wasps are the high fliers[END_REF][START_REF] Harrison | Dispersal of fig pollinators in Asian tropical rain forests[END_REF]. When a wasp detects the plume of receptive fig scent released from a receptive tree, it moves down, out of the main wind current and then flies upwind to reach the tree [START_REF] Ware | Dispersal of adult female fig wasps: 2. Movements between trees[END_REF].

The pollination mutualism seems to have opened other evolutionary opportunities in Ficus, such as the active pollination that increases the efficiency of pollen transfer from the plant to the insect. As a consequence, the plants can invest in other aspects of male function such as producing more male inflorescences or in the case of Ficus, breeding more pollinator offspring [START_REF] Sakai | A review of brood-site pollination mutualism: plants providing breeding sites for their pollinators[END_REF][START_REF] Pellmyr | Active pollination drives selection for reduced pollen-ovule ratios[END_REF]). Active pollination is present in two thirds of the Ficus species. Agaonid wasps that actively pollinate fig trees transport the pollen clumped into body containers (i.e., pollen pockets), which apparently shares functional analogies with the cohesive pollen dispersion in pollinia observed in Asclepiadaceae and Orchidaceae [START_REF] Ramírez | Pollination analogies between Orchidaceae, Ficus (Moraceae) and Asclepiadaceae[END_REF]. It is postulated that pollen clumping can improve the pollination success as it decreases the pollen waste during transport, and increases its probability of being deposited on a conspecific stigma [START_REF] Johnson | The structure and function of orchid pollinaria[END_REF]. However, this is not ever true in Ficus as actively pollinating wasps may carry limiting quantities of pollen [START_REF] Kjellberg | Some pollinators are more equal than others: Factors influencing pollen loads and seed set capacity of two actively and passively pollinating fig wasps[END_REF]. Indeed, in the fig system as in other actively pollinated systems, the wasps, not the tree decide how much pollen they load into their pockets. In general, canopy fig tree species produce synchronous crops within trees but asynchronous among trees, making fig crops available at population level year-round [START_REF] Mckey | The trophic structure of tropical ant-plant-herbivore interactions: community consequences and coevolutionary dynamics[END_REF][START_REF] Windsor | Phenology of fruit and leaf production by "strangler" figs on Barro Colorado Island, Panama[END_REF][START_REF] Figueiredo | Phenology and pollination ecology of three Brazilian fig species (Moraceae)[END_REF][START_REF] Pereira | Phenological patterns of Ficus citrifolia (Moraceae) in a seasonal humid-subtropical region in Southern Brazil[END_REF] [START_REF] Sakai | Thrips pollination of androdioecious Castilla elastica (Moraceae) in a seasonal tropical forest[END_REF][START_REF] Zerega | Pollination in the New Guinea Endemic Antiaropsis decipiens (Moraceae) Is Mediated by a New Species of Thrips, Thrips antiaropsidis sp. nov. (Thysanoptera: Thripidae)[END_REF][START_REF] Olotu | Effects of forest fragmentation on pollination of Mesogyne insignis (Moraceae) in Amani Nature Reserve forests, Tanzania: Effects of forest fragmentation on pollination of Mesogyne insignis[END_REF]. These three genera have 1-3 species each, and there is no evidence that those insect morphology, display the same set of associated anatomical characters as modern species [START_REF] Peñalver | Fig wasps in dominican amber (Hymenoptera: Agaonidae)[END_REF][START_REF] Compton | Ancient fig wasps indicate at least 34 Myr of stasis in their mutualism with fig trees[END_REF][START_REF] Farache | First record of a non-pollinating fig wasp (Hymenoptera: Sycophaginae) from Dominican amber, with estimation of the size of its host figs[END_REF].

A major challenge in the study of mutualisms is how to go beyond particular examples and biological models and draw generalisations. We may list some challenges: generally brood-site pollination mutualisms, and case studies of mutualisms have something to tell us, beyond fascinating stories. We have tried to approach generalisation. This is a first sketch, and as such, we have made a number of provocative statements. The aim was to suggest alternative perspectives, to stir reflexion. A very important point is that as long as we do not know enough about a biological model it is very easy to make false inferences. To understand mutualisms we need strong biological data on the different systems.

Conclusion

The view that mutualism represents reciprocal exploitation leads to (1) a false assumption of an inherent conflict of interest between interacting parties, and (2) that the evolution of strategies that limit non-cooperative behaviours, such as sanctions, is condition for mutualism stability. However, phylogenetic evidences from a range of mutualistic systems point out that traits claimed as sanctioning acts were preexisting adaptations. Indeed, mutualisms are stable at macroevolutionary timescales, and rarely evolve to parasitic associations. In general, mutualisms involve members of distant taxonomic groups that combines a structural (goods provided by a long-lived partner) and a service component supplied by a short-lived partner.

Thus, mutualism is a source evolutionary innovations that expand the partners ecological  What should be the insights resulted from expanding mutualism theory to non-pure biological interactions (i.e. culture vs. animals)?

  fig pollinating wasp mutualism, the wasps maximise their fitness by trying to reach receptive

Box 1 .

 1 Pollination by deceit in fig trees.

  female trees constitute a lethal trap for the wasps. Considering the difference in generation time between fig trees (counted in years) and fig wasps (counted in months) and the huge difference in population sizes (ranging from 10 th of thousands to millions of times larger insect population sizes), a simple prediction is that the capacity to recognise female trees and avoid them will evolve rapidly. Nevertheless, approximately half of the 800 Ficus species are functionally dioecious and they correspond to a limited set of monophyletic lineages. Why is it so? The pollinating fig wasps are attracted to the fig tree by volatile compounds released by the receptive inflorescences.

Figure 1 .

 1 Figure 1. Reproductive systems in fig trees. A -in monoecious species seeds and pollen-loaded wasps are produced in the same fig. B -in gynodioecious (functionally dioecious) species, functionally male trees produce pollen-loaded wasps and female trees produce seeds.

  It has also been shown that Epicephala moths have colonised and pollinate actively four different lineages of plants within Phyllanthaceae converting pollinated lineages pollinated by generalist insects into lineages engaged in active pollination mutualisms (Kawakita 2010). Further, some chalcid fig-wasp lineages that do not belong to the Agaonidae have become figpollinators despite 70 Ma year co-diversification of the mutualistic association between Ficus and Agaonidae (Jousselin et al. 2001). Hence, we have numerous examples of new associations that became mutualisms because of pre-exiting traits of one or both partners involved in the new mutualism.

Figure 2 .

 2 Figure 2. Examples of mutualisms involving insects and plants. Protection mutualism: A -Ectatomma tuberculatum ant probing an extrafloral nectary in Leguminosae. Digestive mutualisms: B -Aconophora sp. (Auchenorrhyncha: Membracidae), associated with Baumannia (γ-proteobacteria) and Sulcia (Bacteroidetes); C -Atta laevigatta carring a floral bud. Pollination mutualisms: D -Editha magnifica wasp visiting Vernonia flowers; nonspecialized insects may eventually act as pollinators. E -immature seeds and fig wasps in the monoecious F. maxima; note that seeds (s) are produced in flowers closer to the fig wall, while wasps (w) are produced closer to the fig cavity. F -fig wasps collecting pollen (arrows) in the monoecious F. albert-smithii. Photo credits: A -D (Kleber Del-Claro), E -F (Finn Kjellberg).

  Intracellular bacteria reside in specialized host cells (bacteriocytes) that constitute an organ called the bacteriome and are vertically transmitted -the symbionts migrate to the ovaries and enter the germ cells. This specialised mutualism is restricted to the monophyletic clade constituted by members of the two hemipteran suborders, Sternorrhyncha (psyllids, whiteflies, aphids, and mealybugs) and Auchenorrhyncha (sharpshooters)]. This clade comprises over 10,000 representatives. Within this clade different insect lineages are associated with different bacterium genera: Buchnera (aphids), Carsonella (psyllids), Portiera (whiteflies), Tremblaya (Mealybugs) and Baumannia (sharpshooters). Plant sap is an unbalanced diet for insects as it is rich in carbohydrates relative to free amino acids and it is deficient in essential amino acids. The symbiotic bacteria convert non-essential amino acids into essential ones, allowing sap-feeding insects to circumvent their intrinsic nutritional limitation. In contrast to other sap-feeders that feed on sap circulating in the phloem, sharpshooters feed on xylem. The bacteriome in Homalodisca sharpshooters is bilobed, with one portion hosting Baumannia bacteria that can synthesize vitamins and cofactors, and the other portion hosting Sulcia bacteria that can synthesize essential amino acids[START_REF] Baumann | Biology of bacteriocyte-associated endosymbionts of plant sap-sucking insects[END_REF][START_REF] Moya | Learning how to live together: genomic insights into prokaryote-animal symbioses[END_REF][START_REF] Douglas | The microbial dimension in insect nutritional ecology[END_REF]).

  -site pollination mutualism in fig trees: did it boost species and lifeform diversifications? Fifty years ago, William Ramirez and Jacob Galil/Dan Eisikowitch independently discovered active pollination by some fig wasps (Hymenoptera: Agaonidae). Female actively pollinating wasps collects pollen into specialised pockets before leaving their natal figs, and later remove pollen from the pockets and deposit it on the stigmas in another fig containing receptive pistillate flowers[START_REF] Galil | Note on pollen transport, pollination and protection of ovaries in Ficus sycomorus[END_REF][START_REF] Ramírez | Fig wasps: mechanism of pollen transfer[END_REF]). These observations marked the modern evolutionary studies of fig trees (Ficus spp) and their associated animals, making it a model system to investigate the evolution of mutualisms[START_REF] Borges | Fifty years later, figs and their associated communities[END_REF]. The fig trees belong to Moraceae family, which includes ca. 39 genera. With a pantropical/subtropical distribution, Ficus encompasses approximately 70% of all 1,100 described Moracea species[START_REF] Zerega | Biogeography and divergence times in the mulberry family (Moraceae)[END_REF][START_REF] Gardner | Phylogeny and biogeography of Maclura (Moraceae) and the origin of an anachronistic fruit[END_REF]). Wind pollination is ancestral in Moraceae(Gardner et al. Taxon, accepted). Nevertheless, representatives of a series of genera are involved in nursery pollination mutualisms. They include representatives of genera Artocarpus (gall midges), Antiaropsis/Castilla (thrips), Dorstenia (flies) and Mesogyne (bees), and Ficus (chalcid wasps)

Figure 3 .

 3 Figure 3. Phylogeny of Moraceae tribes according to Gardner et al. (Taxon, accepted). Terminal widths are proportional to the squared root (number of species).

  pollinations parallel the fig -fig wasp mutualism in terms of pollination efficiency. Patterns of diversification/extinctions in Ficus lineages support a diversification hypothesis based on new ecological opportunities. Bruun-Lund et al. (2018), based on a dated and comprehensive phylogenetic hypothesis, demonstrated that fig trees follow the evolutionary model of 'museum of diversity', with gradual accumulation of species over time coupled with very low extinction rates (Figure 4A). They showed that key innovations directly or indirectly associated with the reproductive biology of fig trees correlated with higher diversification rates in clades where those features were present. For instance, actively pollinated Ficus species diversify faster and present lower extinction rates than passively pollinated ones (Figure 4B). Similarly, Those features make the fig -fig wasp association very robust and successful. Indeed, fossil evidences support that the fig -fig wasp mutualism is stable along its evolutionary history. Fossil specimens of a pollinating fig wasp from a limestone bed in England (∼34 Ma), and pollinating and non-pollinating fig wasps from Dominican amber (10-20 Ma), as well as pollen

"

  Most fig papers start with the statement that the fig-fig wasp mutualism is a model system. Is it a model system of anything else than figs?" Richard T. Corlett "What will matter a lot to whether the paper is accepted (…) is how you frame the results. (…) They absolutely have to convincingly inform broader concepts in ecology, evolution and/or behavior that aren't system specific. That's always a little hard with figs." Judith L. Bronstein "Fig wasps are wonderful" E. Allen Herre What we hope to have shown in this chapter is that figs and fig wasps and more



  capacities. The brood-site pollination in the fig tree -fig wasp mutualism resulted in a set of innovations, such as efficient long-distance pollination and year-round massive seed rain, that opened new ecological niches and allowed a remarkable net diversification at a macroevolutionary timescale. Mechanisms to limit non-cooperative behaviour  Mutualism as source of evolutionary innovations Questions  Do the architecture "goods vs. services" occur in other examples of mutualisms? In addition to Ficus, are lineages involved in mutualistic associations in general more diverse than close-related lineages not engaged in mutualisms?

Table 1 .

 1 Examples of mutualisms involving insects and plants 332

	Mutualism Classes	Provided services	Systems	Partner integrations	Sub-divisions	Some examples	References
					Extra-floral nectar, domatia and food bodies	Myrmecophilic plants (several of flowering plants and ferns)	Heil & Mckey (2003)
	Protection mutualism	Protection against herbivores	Predatory insects -plants	Non-symbiotic	Honey dew from phloem-feeding (trophobionts) hemipterans	Hemiptera -Formicidae mainly, but also Anthribidae, Neuroptera Tachinidae, Syrphidae and Coccinellidae, Apoidea,	Delabie (2001)
	Digestive mutualism	Degrade carbon rich sources (i.e. cellulose, hemicelluloses and lignin), detoxify secondary plant compounds and convert nitrogen in available amino acids	Insects -microorganisms	Symbiotic	General feeders Plant sap feeders Fungus -growing insects	Blattodea, Coleoptera and Psocoptera Hemiptera -Baumannia, Buchnera, Carsonella, Portiera, Sulcia and Tremblaya bactéria, and clavicipitacean fungi Attine ants, Macrotermitinae termites and Scolytinae (ambrosia beetles) -several fungi	Nalepa et al. (2001), Douglas (2009) Douglas (2009) Mueller et al. (2005)
				Non-	Nectar, pollen and	bees, beetles, dipterans,	Rech et al.
				symbiotic	sheltering as reward	lepidopterans, thrips and wasps	(2014)
	Pollination mutualism	Gamete transport	Several insect orders -seed plants	Symbiotic	Brood-site pollination	Obligate: fig trees -fig wasps, leafflowers -leafflower moths and yucca -yucca moths senita cactus -senita moths, Silene -Hadena/Perizoma, Lithophragma -Greya and globeflower -globeflower flies	Sakai (2002), Weiblen (2002) Kephart et al. (2006), Hembry & Althoff (2016)

  . Approximately half of Ficus species are monoecious, bearing urn shaped inflorescences (also called syconium or fig) that enclose both pistillate and staminate flowers. The other species are structurally gynodioecious (see next paragraph), but functionally dioecious

  . Trees are selected to produce synchronous crops for wasp attraction as in some Ficus species producing small figs consumed by local animals (e.g. F. guianensis, F. These pollinating figs were receptive several weeks earlier, leading to the selection of figs producing crops that would pollinate them. Hence, there is frequency dependent selection favouring year-round production of crops due to the strong protogyny of figs and long wasp development time. Compton et al. 2000; Harrison 2003), usually produce huge fig crops that consequently result in a massive production of small seeds. Thus, capacity to be pollinated at very low densities coupled with the massive seed dispersion by canopy frugivores allow fig trees to colonise highly transient habitats and unlikely (in terms of frequency) habitats.

	Year-round fig production makes figs keystone resources for the year round survival of highly
	diversified frugivorous vertebrates, allowing different seed-dispersion mutualism to arise
	(Shanahan et al. 2001). Moreover, monoecious fig trees, which are adapted to long-distance
	pollen dispersion (
	caulocarpa, F. subpisocarpa), fig receptivity is synchronised within crop, wasp emergence is
	synchronised within crop, but fig ripening is scattered over a longer period of time (Chiang et
	al. 2018). Synchronous fig crops lead to year-round fruiting because of strong within fig
	protogyny associated with the production of synchronised crops. If receptive figs are

particularly abundant at one period of the year, then there is selection on trees to produce crops that will pollinate these figs, i.e. that will release wasps at that time.

We postulate that brood-site pollination mutualism has boosted species diversification in Ficus, by opening new adaptive opportunities. As a matter of comparison the sister group of Ficus (i.e., tribe Castilleae, represented by 10 genera and approximately 60 species) is nearly 13 times less speciose than Ficus

[START_REF] Gardner | Phylogeny and biogeography of Maclura (Moraceae) and the origin of an anachronistic fruit[END_REF]

. Pollination biology in Castilleae is not well known, but pollination by thrips is reported for Antiaropsis decipiens and Castilla elastica, and potential bee/vespid pollination for Mesogyne insignis