Les odeurs émises par les plantes sont souvent interprétées dans le cadre de leur interaction avec les insectes pollinisateurs, herbivores et ennemis naturels, en tant que signaux de communication dont la fonction est l'attraction ou la répulsion de ces insectes. Pourtant, la communication a une définition précise en biologie évolutive, et le terme « signal » ne doit s'appliquer qu'à des traits de l'émetteur qui ont été sélectionnés dans le cadre de l'interaction avec le récepteur par voie sensorielle. De plus, certaines études démontrent que les Composés organiques volatils (COV) émis par les feuilles participent à la défense des plantes contre divers stress abiotiques (notamment stress oxydant et thermique) et biotiques (pathogènes, induction des défenses). L'idée de cette thèse, c'est d'essayer de faire rentrer l'ensemble de ces contextes et fonctions potentielles dans un cadre commun. En utilisant comme modèle les figuiers, leurs pollinisateur et parasites spécialistes, j'ai choisi comme approche de comparer les variations des odeurs de figue à celles de odeurs de feuille, au cours du développement des figues et au cours d'une journée. Dans les deux cas, les feuilles constituent un témoin permettant de déterminer si les variations observées peuvent être adaptatives vis-à-vis de l'interaction avec les insectes, mais aussi de considérer explicitement les fonctions potentielles des odeurs émises en dehors de la réceptivité. Ainsi, les fonctions de protection contre les stress habituellement attribuées aux odeurs de feuilles sont également pertinentes pour les odeurs de figues. A partir de là, le constat que le concept de communication n'est pas pertinent pour décrire le rôle des odeurs dans les interactions figuier-parasite permet de développer une réflexion sur la manière dont les phytophages et leurs ennemis naturels sont susceptibles d'influer sur l'évolution des odeurs émises par les plantes. Enfin, dans le cas de la communication olfactive figuier-pollinisateur, l'étude du cas de Ficus septica, chez qui deux pollinisateurs écologiquement différenciés coexistent, permet d'imaginer un mécanisme potentiel de co-spéciation écologique dans lequel la divergence des modalités de communication olfactive participerait à la mise en place de l'isolement reproducteur. / The scents emitted by plants are often interpreted in the light of their interaction with pollinators, phytophagous insects and their natural ennemies, as communication signals whose function is to attract or repel those insects. However, according to the adaptive definition of communication, a trait can be called “signal” only if it has been selected for the sensory interaction with a receptor. In addition, it has been shown that the volatile organic compounds (VOC) emitted by leaves participate to the defense of the plant against abiotic (especially oxidative/heat stress) and biotic stresses (pathogens, induction of defenses). The idea underlying this thesis it to put all all the contexts of emission and functions together within a common framework. Using the fig-pollinator-parasite model, the approach I choose was to compare the variation of fig scent with that of leaf scent, along their development and daily. In both cases, leaf scent is a control trait that allows to determine if the variations observed in figs are possibly adaptive regarding the interaction with insects, and to explicitely consider the potential functions of the scents emitted out of receptivity. Stress-protective functions that are evidenced in leaf scents are thus also relevant in figs. Then, the fact that communication is not relevant to describe the role of scents in the fig-parasite interaction led me to develop a reflection on the way phytophagous insects and their natural enemies could influence the evolution of plant scents. Finally, in the case of fig-pollinator communication, studying the case of Ficus septica, that is associated to two co-occuring ecologically differenciated pollinators, allows to imagine a potential co-speciation mechanism, where the divergence of communication mode would contribute to the establishment of reproductive isolation.

Plant–animal interaction systems are complex food webs where the members—plants, pollinators, herbivores, parasites and predators of the pollinators/herbivores—interact with each other in ways which maximize their own fitness. Based on the net outcome, such interactions could be mutually beneficial to the interacting members (mutualism) or beneficial to only one of the interacting members at the cost of the other interacting members (herbivory, predation, parasitism). It is possible that such outcomes are actually a continuum and could swing in either direction from beneficial to detrimental and vice versa. Such transitions happen not only over long time scales, but could also happen within shorter time scales based on conditionalities. Conditional outcomes are those in which the outcome of an interaction between two partners is conditional on the involvement of a third partner. Thus, studying such outcomes necessitates taking into account systems beyond the classical two-partner interactions. In such complex multitrophic plant–animal interaction systems in which there are direct and indirect interactions between species, comprehending the dynamics of these multiple partners is very important for an understanding of how the system is structured. In Chapter 2 we investigate Ficus racemosa and its community of obligatory mutualistic and parasitic fig wasps that develop within the fig inflorescence or syconium, as well as their interaction with opportunistic ants. We focus on temporal resource partitioning among members of the fig wasp community over the development cycle of the fig syconia during which wasp oviposition and development occur and we study the activity rhythm of the ants associated with this community. We found that the members of the wasp community partitioned their oviposition across fig syconium development phenology and showed interspecific variation in activity across the diel cycle. The wasps presented a distinct sequence in their arrival at fig syconia for oviposition. We documented night oviposition in several fig wasp species for the first time. Ant activity on the fig syconia was correlated with wasp activity and was dependent on whether the ants were predatory or trophobiont-tending species; only numbers of predatory ants increased during peak arrivals of the wasps. In Chapter 3, we found that predatory ants (Oecophylla smaragdina) patrolling F. racemosa trees were attracted to the odour from fig syconia at different developmental phases, as well as to the odours of fig wasps, whereas other predatory ants (Technomyrmex albipes) responded only to odours of syconia from which fig wasps were dispersing and to fig wasp odour. However, trophobiont-tending ants (Myrmicaria brunnea) patrolling the same trees and exposed to the same volatiles were unresponsive to fig or fig wasp odours. The predatory ants demonstrated a concentration-dependent response towards volatiles from figs receptive to pollinators and those from which wasps were dispersing while the trophobiont-tending ants were unresponsive to such odours at all concentrations. Naıve predatory ants failed to respond to the volatiles to which the experienced predatory ants responded, indicating that the response to fig-related odours is learned. In Chapter 4 we characterise the dynamics of the volatile bouquet of the fig syconium from the initiation through pre-receptive, receptive, and late inter-floral stages which act as signals/ cues for different fig wasp species. We were also interested in diel patterns of volatile emission as some fig wasp species were strictly diurnal (the pollinator, Ceratosolen fusciceps) whereas other fig wasps such as Apocryptophagus fusca were observed ovipositing even during the nocturnal hours. We identified volatiles that were specific to syconium development phase as well as to the time of day in this bouquet. α-muurolene was identified as the sesquiterpene specific to receptive-phase as well as being present only during the day thus coinciding with the diurnal pollinator arrival pattern. Volatiles such as (E)-β-ocimene were present in increasing levels across the developmental stages of the fig and thus could act as background volatiles providing suitable information to fig wasps about host plants and their phases. Chapter 5 examines the responses of predatory and trophobiont-tending ant species to the cuticular hydrocarbon (CHC) extracts of four galler and two parasitoid fig wasp species associated with F. racemosa. Interestingly, the antennation response of both experienced and na¨ıve ants to these wasp extracts was identical indicating that prior exposure to such compounds is not necessary for eliciting such response. We also characterised these cuticular hydrocarbon extracts to find potential compounds which could as short-range cues for predatory ants. Ants were more responsive to the cuticular extracts of parasitoids rather than to those of galler wasps, implying that the CHC profile of carnivorous prey may contain more elicitors of aggressive behaviour in ants compared to herbivorous prey whose profiles may be more similar to those of their plant resources. We also find congruency between the cuticular profiles of parasitoids and their hosts suggesting that parasitoids could sequester compounds from their diet. Important findings and conclusions of the thesis are presented in Chapter 6. The first two parts of the appendices section discuss work carried out on alternative ways of analysing multivariate data sets such as plant volatiles and insect cuticular hydrocarbons. Appendix A details the use of Random Forests, an algorithm-based method of analysing complex data sets where there are more variables than samples, a situation akin to microarray data sets. This work illustrates the use of such techniques in chemical ecology, highlighting the potential pitfalls of classical multivariate tests and the advantages of newer more robust methods. Appendix B, an invited article following the publication of the earlier work, compares different data transformation procedures currently employed in such multivariate analysis. Appendix C details sex-specific differences in cuticular hydrocarbons of fig wasps, using the pollinator C. fusciceps as a case study.