Background: In many female insects, peptides transferred in the seminal fluid induce postmating responses (PMR), such as a drastic increase of egg laying and reduction of receptivity (readiness to mate). In Drosophila melanogaster, sex-peptide (SP) elicits short- and long-term PMR, but only the latter in the presence of stored sperm (sperm effect).
Results: Here, we elucidate the interaction between SP and sperm by immunofluorescence microscopy. Transgenic males were used to study the effects of SP modification on the PMR of females in vivo. We report that SP binds to sperm with its N-terminal end. In females, the C-terminal part of SP known to be essential to induce the PMR is gradually released from stored sperm by cleavage at a trypsin cleavage site, thus prolonging the PMR. These findings are confirmed by analyzing the PMR elicited by males containing transgenes encoding modified SPs. SP lacking the N-terminal end cannot bind, and SP without the trypsin cleavage site binds permanently to sperm.
Conclusion: By binding to sperm tails, SP prolongs the PMR. Thus, besides a carrier for genetic information, sperm is also the carrier for SP. Binding to sperm may protect the peptide from degradation by proteases in the hemolymph and, thus, prolong its half-life. Longer sperm tails may transfer more SP and thus increase the reproductive fitness of the male. We suggest that this could explain the excessive length of sperm tails in some Drosophila species.