Divergent selection on behavioral and chemical traits contributes to isolation between populations of Drosophila melanogaster

Speciation is driven by traits that can act to prohibit mating between nascent lineages, including male courtship and female preference for male traits. Mating barriers involving these traits evolve quickly because there is strong selection on males and females to maximize reproductive success, and the tight co-evolution of mating interactions can lead to rapid diversification of sexual behavior. Using lineages of D. melanogaster that show strong asymmetrical reproductive isolation, we ask two key questions: which specific male traits are females selecting, and are these traits under divergent sexual selection? These questions have proven extremely challenging to answer, because even in closely related lineages males often differ in multiple traits related to mating behavior. We address these questions by estimating selection gradients for male courtship and cuticular hydrocarbons for two different female genotypes. We identify specific behaviors and particular cuticular hydrocarbons that are under divergent sexual selection and likely contribute to reproductive isolation. Additionally, we discovered that a subset of these traits are plastic; males adjust these traits based on the identity of the female genotype they interact with. These results suggest that even when male courtship is not fixed between lineages, ongoing selection can contribute to reproductive isolation.