Floral scent and the geographic mosaic of co-evolving plants and insects

Background/Question/Methods

The geographic mosaic theory of coevolution presents a framework for understanding how evolutionary processes mold variation in species interactions across ecosystems. As species coevolve, local populations diverge through adaptation and speciation. One important future challenge is to understand how suites of traits, rather than individual traits, vary as coevolving lineages diverge. The coevolutionary interactions between phytophagous insects and plants are potentially the most diverse forms of interaction in nature and range from antagonism (herbivory/seed predation) to mutualism (pollination/seed dispersal). We study the mutualistic interaction between plants of the genus Lithophragma (Saxifragaceae) and their main pollinators, moths of the genus Greya (Prodoxidae). The moths pollinate the plants by ovipositing into the ovaries of the host. Our studies investigate the distribution of a suite of floral and insect traits of importance for the coevolutionary interaction. Here, we present data on the distribution and variation of floral scent among and within different Lithophragma species from multiple populations across the geographic range of the interaction. We ask three specific questions: (i) what is the range of floral scent variation; (ii) how is this divergence partitioned within and among populations and species; and (iii) how does the divergence affect the responses of the coevolving moths?

Results/Conclusions

We report three major results. First, an in-depth study shows tremendous qualitative and quantitative variation in floral scent among four Lithophragma species chosen to represent a wide distribution of genetic, geographic and ecological variation. A strong correlation between field- and greenhouse samples infers that this variation is genetically determined. Second, a large-scale sampling effort from more than 70 populations across the Lithophragma genus confirms the large interspecific differences, and unravels patterns of major floral scent variation also within species and lineages. Third, Greya moths prefer to oviposit in flowers of local plants over non-locals, and floral scent is likely important for this host preference, as moth females from three focus populations navigate towards the floral scent in choice experiments, but only when the plant comes from the local population. This study shows how floral scent variation affects the potential for diversification on both sides of a mutualistic plant-insect interaction as immigrating moth or plant individuals maladapted to the local chemical communication pattern will be selected against. Future studies should be targeted towards understanding the ultimate processes responsible for this remarkable diversification in floral scent, and how this relates to geographic patterns of local coevolution in other plant and insect traits.