Sexually antagonistic coevolution of a postmating-prezygotic reproductive character in desert Drosophila

Rapid divergence in postmating-prezygotic characters suggests that selectio n may be responsible for generating reproductive barriers between closely r elated species. Theoretical models indicate that this rapid divergence coul d he generated by a series of male adaptations and female counteradaptation s by means of sexual selection or conflict, but empirical tests of particul ar mechanisms are generally lacking. Moreover, although a male-female genot ypic interaction in mediating sperm competition attests to an active role o f females, molecular or morphological evidence of the female's participatio n in the coevolutionary process is critically needed. Here we show that pos tmating-prezygotic variation among populations of cactophilic desert Drosop hila reflects divergent coevolutionary trajectories between the sexes. We e xplicitly test the female's role in intersexual interactions by quantifying differences in a specific postmating-prezygotic reproductive character, th e insemination reaction mass, in two species, Drosophila mojavensis and Dro sophila arizonae. A series of interpopulation crosses confirmed that popula tion divergence was propelled by male-female interactions, a prerequisite i f the selective forces derive from sexual conflicts. An association between the reaction mass and remating and oviposition behavior argues that diverg ence has been propelled by sexually antagonistic coevolution, and potential ly has important implications for speciation.