Mating system evolution in sperm-heteromorphic Drosophila.

In Drosophila species of the obscura group, males exhibit sperm-heteromorphism, simultaneously producing both long sperm, capable of fertilization, and short sperm that are not. The production of multiple sperm types calls into question whether mating system correlates, such as sperm length and number trade-offs and female remating behavior, are the same as previously described in sperm-monomorphic systems. We examine three obscura group species, D. pseudoobscura, D. persimilis, and D. affinis that differ significantly in the lengths of their long fertilizing sperm, to test predictions about the relationship between sperm length and four mating system characters: male age at sexual maturity; sperm number; female remating; and male reproductive output. In D. affinis, where males produce the longest fertilizing sperm, their sexual maturity is delayed and they produce fewer long sperm compared to the other two species, as predicted if long sperm are costly to produce. Female D. affinis, although they receive fewer sperm than females of the other two species, do not remate more frequently or produce fewer progeny from a single mating. Different responses between sperm-heteromorphic and sperm-monomorphic systems underscore the complex nature of the coevolution between male and female mating system characters.