The evolution of a placenta accelerates the evolution of postcopulatory reproductive isolation

The evolution of placentation is predicted to intensify intergenomic conflicts between mothers and offspring over optimal levels of maternal investment by providing offspring opportunities to manipulate mothers into allocating more resources. Parent-offspring conflicts can result in the evolution of reproductive isolation among populations when conflicts resolve in different ways. Postzygotic reproductive isolation is hypothesized to evolve more rapidly following the evolution of placentation due to the predicted increase in conflict. We tested this hypothesis by performing interpopulation crosses within placental and nonplacental species of Poeciliopsis to determine if the relationship between genetic distance and measures of postzygotic reproductive success differed as function of reproductive mode. We did not observe any differences in offspring viability or sterility among crosses. Offspring size declined rapidly as a function of interpopulation genetic distance within the placental species, but not among our nonplacental species. The decrease in offspring size in the placental species was beyond normal variation, likely representing a major fitness cost, consistent with the prediction that negative epistatic interactions are evolving more quickly among populations in our placental species than the nonplacental species. We discuss how our results support the role parent-offspring conflicts play in the evolution of reproductive isolation and reproductive mode.