Identifying the presence and strength of reproductive isolating barriers is necessary to understand how species form and then remain distinct in the face of ongoing gene flow. Here we study reproductive isolation at two stages of the speciation process in the closely related mushroom-feeding species Drosophila recens and D. subquinaria. We assess three isolating barriers that occur after mating, including the number of eggs laid, the proportion of eggs laid that hatched, and the number of adult offspring from a single mating. First, all three reproductive barriers are present between D. recensfemales and D. subquinaria males, which are at the late stages of speciation but still produce fertile daughters through which gene flow can occur. There is no evidence for geographic variation in any of these traits, concurrent with patterns of behavioral isolation. Second, all three of these reproductive barriers are strong between geographically distant conspecific populations of D. subquinaria, which are in the early stages of speciation and show genetic differentiation and asymmetric behavioral discrimination. The reduction in the number of eggs laid is asymmetric, consistent with patterns in behavioral isolation, and suggests the evolution of postmating prezygotic isolation due to cascade reinforcement against mating with D. recens. In summary, not only may postmating prezygotic reproductive barriers help maintain isolation between D. recens and D. subquinaria but they may also drive the earliest stages of isolation within D. subquinaria.

Understanding the pleiotropic consequences of gene drive systems on host fitness is essential to predict their spread through a host population. Here we study Sex-ratio (SR) X-chromosome drive in the fly Drosophila recens, where SR causes the death of Y-bearing sperm in male carriers. SR males only sire daughters, which all carry SR, thus giving the chromosome a transmission advantage. The prevalence of the SR chromosome appears stable, suggesting pleiotropic costs. It was previously shown that females homozygous for SR are sterile, and here we test for additional fitness costs of SR. We find that females heterozygous for SR have reduced fecundity and that male SR carriers have reduced fertility in conditions of sperm competition. We then use our fitness estimates to parameterize theoretical models of SR drive and show that the decrease in fecundity and sperm competition performance can account for the observed prevalence of SR in natural populations. In addition, we find that the expected equilibrium frequency of the SR chromosome is particularly sensitive to the degree of multiple mating and performance in sperm competition. Together our data suggest that the mating system of the organism should be carefully considered during the development of gene drive systems.