Evolution of sexual isolation in laboratory populations: Fitness differences between mating types and the associated hybrid incompatibilities

Drosophila melanogaster is known to have two races in the incipient stages of speciation that exhibit strong asymmetric premating isolation: Zimbabwe (Z) and cosmopolitan (M). In a study examining the phenotypic and genotypic evolution after secondary contact, we found that despite strong sexual selection favoring the Z-type behavior, it is the M-type behavior that comes to dominate hybrid populations. This article examines the fitness costs associated with the Z-type behavior. We have discovered that these costs are great enough to explain the failure of the Z-type behavior to prosper. Here we report that Z-type females produce approximately half the number of offspring that M-type females produce. Furthermore, crosses between populations have revealed that Z-type females mated to M-type males have approximately 20% fewer offspring than the reciprocal crosses because of an inability of M-type sperm to successfully fertilize Z-type eggs. Hybrid crosses also exhibit much-reduced numbers of viable offspring in addition to reduced hybrid male fertility. These fitness effects suggest that multiple mechanisms of postmating isolation have evolved concurrently with the divergence in behavior.