How to be a dioecious fig: Chemical mimicry between sexes matters only when both sexes flower synchronously.

M Hossaert-Mckey,C C L Soler,J-M Bessière,R M Borges,C Chen,B Schatz,M Proffit

doi:10.1038/srep21236

Abstract

In nursery pollination mutualisms, which are usually obligate interactions, olfactory attraction of pollinators by floral volatile organic compounds (VOCs) is the main step in guaranteeing partner encounter. However, mechanisms ensuring the evolutionary stability of dioecious fig–pollinator mutualisms, in which female fig trees engage in pollination by deceit resulting in zero reproductive success of pollinators that visit them, are poorly understood. In dioecious figs, individuals of each sex should be selected to produce odours that their pollinating wasps cannot distinguish, especially since pollinators have usually only one choice of a nursery during their lifetime. To test the hypothesis of intersexual chemical mimicry, VOCs emitted by pollen-receptive figs of seven dioecious species were compared using headspace collection and gas chromatography-mass spectrometry analysis. First, fig-flower scents varied significantly among species, allowing host-species recognition. Second, in species in which male and female figs are synchronous, intersexual VOC variation was not significant. However, in species where figs of both sexes flower asynchronously, intersexual variation of VOCs was detectable. Finally, with one exception, there was no sexual dimorphism in scent quantity. We show that there are two ways to use scent to be a dioecious fig based on differences in flowering synchrony between the sexes.

Highlights

In nursery pollination mutualisms, which are usually obligate interactions, olfactory attraction of pollinators by floral volatile organic compounds (VOCs) is the main step in guaranteeing partner encounter
Among the sexual strategies to avoid inbreeding in flowering plants, sexual dimorphism promotes the evolution of dioecy[1]
Comparing fig species in which males and females flower synchronously and asynchronously, we examined the potential of volatile chemical signals in mediating intersexual mimicry in dioecious nursery pollination mutualisms by estimating the degree of intersexual chemical similarity in species with varied flowering phenologies

Summary

Introduction

In nursery pollination mutualisms, which are usually obligate interactions, olfactory attraction of pollinators by floral volatile organic compounds (VOCs) is the main step in guaranteeing partner encounter. For the deceptive sex it is crucial to maintain signals that mimic the signals of the rewarding sex in order to attract pollinators and ensure mating success, thereby maximizing the fitness of both sexes Among these signals, while floral scents (i.e. bouquets of volatile organic compounds (VOCs) emitted by flowers) are often crucial signals for pollinator attraction[8,9], they may be among the most variable traits of the plant phenotype[10]. Wasps that exit male figs during the time of synchronous female receptivity have no choice, and due to strong chemically mediated species-recognition mechanisms based on chemical signals[13], they will be attracted to the deceptive female figs. The wasp population in these species is maintained by the emergence of a few wasps, at the end of the period of female flowering, which enter some of the few male figs that are receptive at that time and in which they can reproduce[20,24]

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Conclusion