Patterns of genetic variation around a locus under strong selection in wild Heliconius populations

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Wild populations of all plants and animals vary in many traits that are controlled by genetic variation. Finding the genes that control natural variation in wild populations is a major challenge, important in both applied and basic science. Evolutionary biologists are interested to know what the genes are that control diversity and adaptation in animals and plants, while applied biologists would like to identify genes involved in economically important traits such as disease resistance in crops or insecticide resistance in pests and their relatives. Here we will study a classic system in evolutionary biology, the mimetic wing patterns of tropical Heliconius butterflies. We will use modern molecular techniques to investigate a genetic locus that controls a geographic variation in mimetic pattern in the butterfly Heliconius melpomene and a within-population mimetic polymorphism in the closely-related species Heliconius numata. Mimicry, by which different chemically-defended prey share warning colours, is an ecologically well-understood adaptive trait with selection for convergence within the local prey community. Mimicry alleles vary between and within populations of the same species and are involved in speciation. The P locus in H. numata is found in the same genome region as the N-Yb-Sb locus in H. melpomene. Markers will be generated within all genes present across the region containing the mimicry locus. Variation will then be screened in wild populations of H. numata in order to search for variants that are associated with the pattern phenotype of the butterflies. For example, if a polymorphism occurs between an A and a G at a particular locus, we will investigate whether there is a tendency for the butterflies with a particular wing pattern to have either A or G at a higher frequency than expected at random. We will use high throughput, cost-effective genetic techniques that accurately estimate the association of each variable marker with colour pattern. This work will further localize the region, and possibly identify the gene, that is responsible for dramatic morphological variation and adaptation to local prey communities. In H. melpomene, where all individuals in any particular locality have the same wing pattern, we will investigate whether there is a region that shows reduced variation or a high frequency of rare variants, both of which are suggestive of the recent action of natural selection on a locus. The range margins of H. melpomene races change rapidly, so our work will provide insight into the effect of rapid adaptive range expansion on associated genetic variation.