S6-2.4 Jan. 5 Complex male X female interactions and the evolution of ejaculate characterstics PITNICK, Scott*; BJORK, Adam; PATTARINI, James; Syracuse University; Syracuse University; Syracuse University firstname.lastname@example.org
Ejaculate-female interactions are poorly understood but are expected to be widespread, complex and to have important implications for differential male fertilization success, the evolution of polyandry and diversification resulting in reproductive isolation. We discuss and integrate research on (1) genetic compatibility and female remating, (2) the role of male X female interactions in sperm competition and (3) mechanisms by which sperm achieve competitive fertilization success to better understand the significance of ejaculate-female interactions. We then describe results from two experiments. In the first, using an outbred Drosophila melanogaster population exhibiting continuous genetic variation, we quantified sperm precedence across multiple sperm competition bouts. We show that both offensive and defensive sperm competitive ability are significantly repeatable only across matings involving the ejaculates of the same pair of males competing within the same female. These repeatabilities decrease when the rival male stays the same but the female changes, and they disappear when both the rival male and the female change. In the second, experimental evolution was used to manipulate known interacting components: sperm length and female sperm-storage organ length in order to reveal the mechanisms underlying this aspect of ejaculate-female interaction. Our results are discussed with a focus on the complex nature of sperm precedence and the maintenance of genetic variation in ejaculate characteristics.