Sex Peptide Is Required for the Efficient Release of Stored Sperm in Mated Drosophila Females

The Drosophila seminal fluid protein (SFP) sex peptide (SP) elicits numerous post-mating responses, including increased egg laying and decreased sexual receptivity, in the mated female. Unlike other SFPs, which are detectable in mated females for only a few hours post mating, SP is maintained-and its effects are sustained-for several days. The persistence of SP in the mated female's reproductive tract is thought to be a consequence of its binding to, and gradual release from, sperm in storage, which maintains SP's ability to act within the female reproductive tract. Recent studies have shown that several other SFPs, acting in a network, are needed for SP's localization to sperm and are necessary for the efficient release of sperm from storage. This result suggested an additional new role for SP modulating the release of sperm from storage. We tested for this possibility by examining sperm storage parameters in mated females that did not receive SP. We found that while sperm accumulation into storage was unaffected, sperm depletion from storage sites was significantly decreased (or impaired) in the absence of SP. Mates of males expressing a modified SP that is unable to be released from sperm showed a similar phenotype, indicating that release of sperm-bound SP is a necessary component of normal sperm depletion. Additionally, SP null males were more successful in a sperm competitive environment when they were first to mate, which is likely a consequence of higher retention of their sperm due to defective sperm release. Our findings illustrate a direct role for SP in the release of sperm from storage.