Testing potential mechanisms of conspecific sperm precedence in Drosophila pseudoobscura.

Drosophila pseudoobscura females that co-occur with sister species D. persimilis show elevated fertilization by conspecific sperm when they mate with both a heterospecific and a conspecific male. This phenomenon, known as conspecific sperm precedence (CSP), has evolved as a mechanism to avoid maladaptive hybridization with D. persimilis. In this study, we assessed pericopulatory (during mating) and postcopulatory (after mating) traits in crosses with sympatric or allopatric D. pseudoobscura females and conspecific or heterospecific males to evaluate potential mechanisms of CSP in this system. We observed shorter copulation duration in crosses with sympatric females, but found no difference in quantity of sperm transferred or female reproductive tract toxicity between sympatry and allopatry. Our data show some support for the hypothesis that parasperm, a short, sterile sperm morph, can protect fertile eusperm from the D. pseudoobscura female reproductive tract, though it is unclear how this might affect patterns of sperm use in sympatry vs. allopatry. Overall, these results suggest that copulation duration could potentially contribute to the elevated CSP observed in sympatry.