Female factors are important for the seminal Sex Peptide’s association with sperm in mated D. melanogaster

Background Male-derived seminal fluid proteins (SFPs) enter and induce a myriad of physiological and behavioral changes in mated female flies optimizing fertility. Many post-mating changes in female Drosophila melanogaster persist for ∼10-14 days, because the seminal protein that induces them, Sex Peptide (SP), is retained long-term in females by binding to sperm, with gradual release of its active domain from sperm. Several other “long-term response SFPs” (LTR-SFPs) “prime” sperm to bind SP. Whether female factors play a role in this process is unknown, though it is important to study both sexes for a comprehensive physiological understanding of this reproductive process and for consideration in models of sexual conflict. Results We report here that sperm in male ejaculates bind SP more weakly than sperm that have entered females, and that the amount of SP and other SFPs bound to sperm increases with time and transit of individual seminal proteins within the female reproductive tract. Thus, female contributions are needed for maximal and appropriate binding of SP, and other SFPs, to sperm. Towards understanding the molecular roles of the female, we found no dramatic change in pH of the female reproductive tract after mating and that initial binding of SP to sperm is normal in females with ablated or defective spermathecal secretory cells and/or parovaria, but that higher levels of SP (and sperm) are retained in the latter females. Conclusion This study reveals that the SP pathway is not entirely male-biased and that females also contribute to regulating it.