MULTIPLE MATING, SPERM COMPETITION AND MEIOTIC DRIVE

Most discussions of 'sperm competition' have ignored the potential for competition among the different sperm genotypes present in the ejacul ate of a single male. Rivalry within ejaculates may limit cooperation among the members of an ejaculate when they compete with sperm produce d by other males. A gene that gains an advantage in competition within an ejaculate (a segregation distorter) may increase in frequency even if it is associated with significant costs to organismal fitness. The refore, selection will favor genes expressed in males that suppress co mpetition within ejaculates. This may explain why sperm function is la rgely controlled by the diploid genotype of the male progenitor, rathe r than by the genotypes of individual haploid sperm. Females who mate with multiple males reduce the relative advantage of a segregation dis torter whenever the distorter impairs the competitive effectiveness of the ejaculates in which it occurs. If the distorter is associated wit h costs to organismal fitness, selection will favor female mating beha vior that reduces the distorter's equilibrium frequency. Competition w ithin ejaculates may thus be one reason why females choose to mate wit h multiple males.