Evolution of female remating behaviour following experimental removal of sexual selection

The relatively small number of ova produced by a female can be fertilized b y a single ejaculate in most species. Why females of many species mate with multiple males is therefore enigmatic, especially given that costs associa ted with remating ha iie been well documented. Recently it has been argued that females may remate at a maladaptive rate as an outcome of sexually ant agonistic coevolution: the evolutionary tug-of-war between manipulation by one sex and resistance to being manipulated by the other sex. We tested thi s hypothesis experimentally for the evolution of the female remating interv al in a naturally promiscuous species, Drosophila melanogaster. In two repl icate populations, sexual selection was removed through enforced monogamous mating with random mate assignment, or retained in polyandrous controls. M onogamy constrains the reproductive success of mates to be identical, there by converting prior conflicts between mates into opportunities for mutualis m. Under these experimental conditions, the sexually antagonistic coevoluti on hypothesis generates explicit predictions regarding the direction of evo lutionary change in female remating behaviour. These predictions are contin gent upon the mechanism of male manipulation, which may be mediated biochem ically by seminal fluids or behaviourally by courtship. Levels of divergenc e in female remating interval across lines, and in male ejaculatory and cou rtship effects on female remating, were quantified after 84 generations of selection. Data refute the hypothesis that the evolutionary change in femal e remating behaviour was due to sexually antagonistic coevolution of courts hip signal and receiver traits. The data were, however, consistent with a h ypothesis of sexual conflict mediated through ejaculate manipulation. Monog amy-line males evolved ejaculates that were less effective in inducing fema le non-receptivity and monogamy-line females evolved to remate less frequen tly, symptomatic of lowered resistance to ejaculate manipulation. The consi stency of the results with alternative hypotheses to explain female promisc uity are discussed.