Abstract
Plant odours are central for pollinator attraction. This is particularly true in obligate brood site pollination mutualisms. However, we know little about the evolution of olfactory signalling in these mutualisms. Here, we investigate geographic variation of floral odour in the obligate host-specific brood site pollination mutualism between Ficus hirta and its specialised pollinators. Floral scent samples from nine locations in China were collected using head-space adsorption and were analysed by gas chromatography-mass spectrometry. We evidence progressive geographic divergence of floral odours. The pattern of variation fits plant genetic structure for neutral genes but differs from pollinating insect structuring into species and populations. In our study system, the geographic variation of receptive floral odour presents a pattern that is not distinguishable from neutral drift. The variation is not canalised by the insects. We propose that this pattern characterises obligate brood site pollination mutualisms in which pollinators are host specific and dispersal of plant and insect is limited. Insects with their short generation times and large population sizes rapidly track any chance variation in host receptive inflorescence odours. Plants are the drivers and insects the followers. The source of the geographic variation in floral odours can be genetic or phenotypic in response to local conditions. Strict sense plant-insect co-evolution is not involved. In contrast, previous results on another Ficus-pollinating wasp association suggest that stabilizing selection could be at work in more dispersive systems.