Function of the mating plug in Drosophila hibisci Bock

The mating plug in Drosophila hibisci Beck is a firm, gelatinous structure that forms within the female's uterus during copulation. Two non-mutually e xclusive hypotheses for the function of the plug were evaluated. The plug m ay serve as a nutritional gift that females digest, using the constituents for somatic maintenance or to provision eggs as they mature within the ovar ies. Alternatively, the plug may act as a chastity enforcement device by pr eventing subsequent copulations, and thereby reducing sperm competition. Pl ug size did not decrease within females over a period of 2 days, and dietar y treatment in females did not affect plug size. The extent of ovarian prov isioning was also not related to plug size. These results weaken the nutrit ional gift hypothesis. In contrast, the probability of a second copulation increased sharply with an experimental decrease in plug size. Moreover, fem ales with plugs experimentally reduced in size were courted significantly m ore and mated significantly faster than females with larger plugs. These re sults support the chastity enforcement hypothesis. The plug retains the eja culate and concentrates sperm at the anterior end of the uterus near the ap ertures of the sperm storage organs. The presence of the plug thus probably facilitates the movement of sperm into storage by retaining sperm at the a nterior end of the uterus near the apertures of the sperm storage organs, w hich may be especially important for D. hibisci, in which sperm length is n early twofold greater than ventral receptacle length. Matings with newly ec losed virgin females were significantly shorter than with older virgins, an d copulations with the younger virgins ended more often without any sperm h aving yet entered into storage. The effectiveness of the plug in safeguardi ng a male's ejaculate may have favoured the evolution of shortened copula d urations with young virgins. One fitness advantage of shortened copula dura tion could be time liberated for the pursuit of further mating opportunitie s.