Divergent patterns of correlated evolution in primary and secondary sexual traits of cactophilic Drosophila

Abstract

The rapid diversification of sexual traits is a common phenomenon accompanying the evolution of reproductive isolation, yet the evolutionary mechanisms driving such diversification are often unknown. Based on experimentally evolved strains of two sister species of cactophilic Drosophila, we investigated the correlated evolution of primary and secondary sexual traits to semi-natural environments enriched in secondary metabolites. We compared patterns of morphological evolution in the size and shape of male wing and genitalia of Drosophila buzzatii and Drosophila koepferae selected for different levels of alkaloid intensities for 20 generations. We found similar modes of selection operating among organs but not among species. The evolution of these traits in D. koepferae were compatible with patterns of stabilizing selection, while in D. buzzatii were characterized by directional changes. We also found that allometric variation was an important component of genital shape evolution, whereas changes in the wing morphology were less pronounced and mostly non-allometric. Overall, our data suggest that the diversification of sexual traits in this species pair is related to the evolution of dissimilar genetic architectures and reinforced by divergent ecological responses.