SPERM (EJACULATE) COMPETITION IN DROSOPHILA-MELANOGASTER, AND REPRODUCTIVE VALUE OF FEMALES TO MALES IN RELATION TO FEMALE AGE AND MATING STATUS

In double mating experiments with D. melanogaster in which 1 male had been irradiated, sperm displacement was extensive, i.e., the 2nd male to mate displaced most of the previously stored sperm. The predominance of the 2nd ejaculate over the first increased with the interval between the 2 matings, from .apprx. P2 = .apprx. 0.83 (2nd mating on the 1st day after the 1st mating) to P2 = .apprx. 0.99 (interval between mating = 14 days) where P2 was the proportion of offspring fathered by the 2nd male. A more accurate method for calculating P2 values was developed for experiments in which sperm were labeled by irradiation treatment. Observations of the reducing egg production of the female throughout life were obtained. A model was examined which incorporated sperm competition and egg production data to predict the reproductive value to a male of a mating with a given type of female, varying in age and mating status. The relative value (in terms of probable numbers of progeny gained), of a mating with a virgin or 4 day post-mating female was about twice that of a 14 day post-mating female, mainly because of the fecundity difference. Some evolutionary aspects of sperm competition and multiple mating in insects are reviewed and discussed.