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 ejaculate of a single male. Rivalry within ejaculates may limit cooperation among the members of an ejaculate when they compete with sperm produced 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. Therefore, selection will favor genes expressed in males that suppress competition within ejaculates. This may explain why sperm function is largely controlled by the diploid genotype of the male progenitor, rather than by the genotypes of individual haploid sperm. Females who mate with multiple males reduce the relative advantage of a segregation distorter whenever the distorter impairs the competitive effectiveness of the ejaculates in which it occurs. If the distorter is associated with costs to organismal fitness, selection will favor female mating behavior that reduces the distorter’s equilibrium frequency. Competition within ejaculates may thus be one reason why females choose to mate with multiple males. ‘Sperm competition’ has been defined as competition within a single female between the sperm from two or more males for the fertilization of her ova (Parker, 1970; Birkhead and Marller, 1992). In effect, sperm competition is seen as a continuation of intermale conflict within the female reproductive tract. This definition ignores the possibility of competition among the haploid sperm produced by a single diploid male. If a sperm’s phenotype were determined by the diploid genotype of its male * Author for correspondence.