The formation of a fertilized egg involves many interactions between males, females, and their gametes. This is even more complex in cases of multiple mating, as in Drosophila melanogaster, as the presence of ejaculates from multiple males presents the opportunity for sperm competition to occur. Male-derived seminal fluid proteins are known to influence sperm competition outcomes, in addition to other post-mating effects. While studies have shown that female genotype is also important for sperm competition outcome, the mechanisms underlying the female’s contribution to the success of a particular male’s sperm are less understood.
A previous GWAS screen based on sperm competition phenotypes across natural isolates of D. melanogaster identified candidate genes for the female’s contribution to sperm competition. Interestingly, about half of the top 33 genes have predicted neurological function, pointing to an active role for the female in sperm use and preference. We performed ubiquitous or tissue-targeted local RNAi knockdowns to assess the impact of decreased expression of these genes on sperm competition. Females were scored for sperm competition effects using progeny-phenotype assays, and for reproductive processes including fecundity, fertility, and resistance to remating. Of 29 genes tested, knockdown of 9 affected sperm competition outcomes. Genes whose knockdowns in females affect sperm competition outcomes will be further characterized for their effects on sperm storage, retention and release, and on sperm competition, by direct examination of sperm dynamics within the female, for the neurons (or other tissues) through which these genes exert their effects, and for the roles of the specific alleles found in the wild-derived genomes. Collectively, results from these experiments will provide a clearer picture as to how females are influencing sperm competition outcomes.
We thank the NIH for funding (R01-HD059060) and HHMI for a Gilliam Fellowship to SLW.