Nontransitivity of sperm precedence in Drosophila.

Sperm competition is an important component of fitness in Drosophila, but we still do not have a clear understanding of the unit of selection that is relevant to sperm competition. Here we demonstrate that sperm competitive ability is not a property of the sperm haplotype, but rather of the diploid male's genotype. Then we test whether the relative sperm competitive ability of males can be ranked on a linear array or whether competitive ability instead depends on particular pairwise contests among males. Sperm precedence of six chromosome-extracted lines was tested against three different visible marker lines (cn bw, bwD, and Cy), and the rank order of the six lines differed markedly among the mutant lines. Population genetic theory has shown that departures from transitivity of sperm precedence may be important to the maintenance of polymorphism for genes that influence sperm competitive ability. The nontransitivity seen in sperm precedence should theoretically increase the opportunity for polymorphism in genes that influence this phenotype.