CONICET | Buscador de Institutos y Recursos Humanos

How females store and utilize sperm after remating can generatepostcopulatory sexual selection on male ejaculate traits. Variationin ejaculate performance traits is generally thought to beintrinsic to males, yet is likely to interact with the environmentin which sperm compete (e.g., the female reproductive tract). Ourunderstanding of female contributions to competitive fertilizationsuccess is limited, however, in part due to challenges of observingevents within the reproductive tract of internally fertilizing specieswhile discriminating among sperm from competing males. Here,we used females from crosses among isogenic lines of Drosophilamelanogaster, each mated to two genetically standardized males(first with green- and second with red-tagged sperm heads)to demonstrate heritable variation in female remating interval,progeny production rate, sperm-storage organ morphology anda number of sperm performance, storage and handling traits.We then used multivariate analyses to examine relationships betweenthis female-mediated variation and competitive paternity.In particular, the timing of female ejection of excess second-maleand displaced first-male sperm was genetically variable and, byterminating the process of sperm displacement, significantly influencedthe relative numbers of sperm from each male competingfor fertilization and, consequently, biased paternity. Our resultsdemonstrate that females do not simply provide a static ´arena´for sperm competition but rather play an active and pivotal role inpostcopulatory processes. Resolving the adaptive significance ofgenetic variation in female-mediated mechanisms of sperm handlingis critical for understanding sexual selection, sexual conflict,and the coevolution of male and female reproductive traits.