Genetic architecture of a body color cline inDrosophila americana

Abstract

AbstractPhenotypic variation within a species is often structured geographically in clines. InDrosophila americana, a longitudinal cline for body color exists within North America that appears to be due to local adaptation. Thetanandebonygenes have been hypothesized to contribute to this cline, with alleles of both genes that lighten body color found inD. americana. These alleles are similar in sequence and function to the allele fixed inD. americana’smore lightly pigmented sister species,Drosophila novamexicana. To test this hypothesis, we examined the frequency and geographic distribution ofD. novamexicana-like alleles oftanandebonyinD. americana. Among alleles from over 100 strains ofD. americanaisolated from 21 geographic locations, we failed to identify additional alleles oftanorebonywith as much sequence similarity toD. novamexicanaas the alleles previously described. However, using genetic analysis of 51D. americanastrains derived from 20 geographic locations, we identified one new allele ofebonyand one new allele oftansegregating inD. americanathat are functionally equivalent to theD. novamexicanaallele. An additional 5 alleles oftanalso showed marginal evidence of functional similarity. Given the rarity of these alleles, however, we conclude that they are unlikely to be driving the pigmentation cline. Indeed, phenotypic distributions of the 51 backcross populations analyzed indicate a more complex genetic architecture, with diversity in the number and effects of loci altering pigmentation observed both within and among populations ofD. americana. This genetic heterogeneity poses a challenge to association studies and genomic scans for clinal variation, but might be common in natural populations.