Plants Are the Drivers of Geographic Variation of Floral Scents in a Highly Specialized Pollination Mutualism: a Study of Ficus Hirta in China

Abstract BackgroundFloral volatiles play an important role in pollinator attraction. This is particularly true in obligate brood site pollination mutualisms. The plants generally produce inconspicuous flowers and depend on odours to attract to their inflorescences specialised pollinators that breed in their floral structures. Little is known about the processes shaping the micro-evolution of these floral odours. Here, we investigate geographic variation of floral odour in an obligate host-specific brood site pollination mutualism where plant and pollinator genetic structures are different, Ficus hirta and its specialised pollinators.ResultsWe evidence progressive geographic divergence of floral odours. The pattern of variation fits plant genetic structure but differs from pollinating insect structuring into species and populations. In our study system, the evolution of receptive floral odour presents a pattern that is not distinguishable from neutral drift that is not canalised by the insects.ConclusionWe propose that this pattern characterises obligate brood site pollination mutualisms in which pollinators are host specific and dispersal is limited. Insects with their short generation times and large population sizes track variation in host receptive inflorescence odours. Plants are the drivers and insects the followers. Strict sense plant-insect co-evolution is not involved. In contrast, stabilizing selection may be at work in more dispersive brood site pollination mutualisms, while pollinators may mediate local interspecific plant floral odour convergence when plant species share local pollinators.