Reinforcement and selection against migrants maintain sperm competitive and genetic differences among populations of <i>Drosophila pseudoobscura</i>

Abstract Sexual interactions among closely related species can select for divergence in reproductive traits to avoid the costs of hybridization. This selection, termed ‘reinforcement’, produces a canonical pattern of stronger isolation in sympatric populations versus in allopatric populations. However, unless there is selection against migrants between geographic contexts, this pattern of stronger sympatric isolation will be transient. We tested for evidence of this selection against migrants in the context of sperm competition in Drosophila pseudoobscura. Sympatric genotypes have an advantage in sympatric contexts, via elevated conspecific sperm precedence—a form of reproductive isolation—against congeneric D. persimilis. Here, using experimental crosses that simulated migration between populations, we show that sympatric genotypes are at a sperm-competitive disadvantage against allopatric male genotypes. The combination of selection against sympatric males in allopatry and an advantage of sympatric genotypes in sympatry produces a scenario where selection can maintain or even strengthen sympatric/allopatric differences. We also document sequence and expression differentiation in reproductive genes between allopatry and sympatry, including genes with known roles in sperm competition. Most of these changes were unique to individual sympatric populations and might reflect the unique ways that selection has acted on sperm competition genes in sympatry.