Selection and the ''sex-ratio'' polymorphism in natural populations of Drosophila pseudoobscura

''Sex-ratio'' (SR) is a naturally occurring X-linked meiotic drive system, where the SR-X chromosome is transmitted to nearly all progeny of SR males. It occurs at frequencies of up to 25% in some populations of Drosophila pseudoobscura. Because of the twofold drive advantage, SR should rapidly fix in populations, causing the extinction of the species, unless opposed by strong selection. I examine several of the adult components of fitness, including the frequencies of all genotypic mating combinations, fertilities, and fecundities of flies from two populations in southeastern Arizona. Significant reduction of fecundity of SR/SR females was observed in the Tucson population. No evidence was found for either lower fertility or reduced mating success of SR males, relative to standard males. Most selection opposing SR appears to be operating at the larval stages in nature.