QUANTITATIVE GENETICS OF SPERM PRECEDENCE IN DROSOPHILA-MELANOGASTER

To assess the genetic basis of sperm competition under conditions in w hich it occurs, I estimated additive, dominance, homozygous and enviro nmental variance components, the effects of inbreeding, and the weight ed average dominance of segregating alleles for two measures of sperm precedence in a large, outbred laboratory population. Both first and s econd male precedence show significant decline on inbreeding. Second m ale precedence demonstrates significant dominance variance and homozyg ous genetic variance, but the additive variance is low and not signifi cantly different from zero. For first male precedence, the variance am ong homozygous lines is again significant, and dominance variance is l arger than the additive variance, but is not statistically significant . In contrast, male mating success and other fitness components in Dro sophila generally exhibit significant additive variance and little or no dominance variance. Other recent experiments have shown significant genotypic variation for sperm precedence and have associated it with allelic variants of accessory-gland proteins. The contrast between spe rm precedence and other male fitness traits in the structure of quanti tative genetic variation suggests that different mechanisms may be res ponsible for the maintenance of variation in these traits. The pattern of genetic variation and inbreeding decline shown in this experiment suggests that one or a few genes with major effects on sperm precedenc e may be segregating in this population.