THE EFFECT OF EXPERIMENTAL DESIGN ON FEMALE RECEPTIVITY TO REMATING AND ITS IMPACT ON REPRODUCTIVE SUCCESS IN DROSOPHILA MELANOGASTER.

Adaptations for sperm competition in insects fall roughly into two categories: adaptations that promote the reproductive success of the male whose sperm is in storage (e.g., Craig, 1967; Riemann et al., 1967; Fuchs et al., 1968), and adaptations that increase the reproductive success of subsequently remating males (e.g., Villavaso, 1975; Waage, 1979). The literature on Drosophila melanogaster has supported a variety of interpretations about the nature of sperm competition. In 1962, Lefevre and Jonsson presented evidence for sperm displacement in D. melanogaster females which remated rapidly. Since that time, displacement of previously-stored sperm has been invoked many times to explain (1) reduction of first-male productivity associated with remating and (2) the high proportion of second-male progeny following female remating (Lefevre and Jonsson, 1962; Minamori and Fukui, 1970; Boorman and Parker, 1976; Prout and Bungaard, 1977; Gromko and Pyle, 1978; Gilbert et al., 1981). In the same year that Lefevre and Jonsson suggested sperm displacement, Manning (1962, 1967) provided a very different view of the reproductive biology of repeated matings in this species. Instead of rapid rematings, Manning allowed female receptivity to play a larger role in the timing of remating. He suggested that first males were effective at preventing female remating, since malecontributed seminal substances caused females to become unreceptive to male courtship until sperm from the previous mating had been depleted. Under such conditions sperm displacement is not a possibility because few or no first male sperm remain in storage to be displaced. Thus, Manning's results gave evidence that males whose sperm are in storage are adapted for sperm competition, and many workers have contributed to the elucidation of mechanisms whereby male-contributed seminal substances have their effects (Garcia-Bellido, 1964; Leahy and Lowe, 1967; Merle, 1968; Bumet et al., 1973; Connolly and Cook, 1973; Richmond et al., 1980; Tompkins et al., 1980; Venard and Jallon, 1980; Gilbert et al., 198 1; Tompkins and Hall, 1981). Gromko and Pyle (1978) attempted to evaluate both the "displacement" and "effective first male" explanations within the same experimental context. They showed that the effect of displacement on reproductive success was small in comparison to the effect of first-male-contributed substances on female receptivity. Those results raised questions about the necessity of the "displacement" explanation. Gilbert (1981; Gilbert et al., 1981; Gromko et al., 1984a) attempted to discover the mechanism by which sperm displacement might occur. Extensive counts of sperm in storage gave no evidence that displacement occurred. We have, consequently, undertaken a re-examination of the evidence for sperm displacement in this species. What is the nature of the evidence for sperm displacement? Lefevre and Jonsson's (1962) argument for displacement in rapidly remating females was based on the following characteristics of the reproductive biology of D. melanogaster: (1) Female storage organs are filled to ca-