Abstract
In several species of Drosophila, some males carry particular X-linked alleles (referred to as sex-ratio or SR males) that cause them to sire all-female progeny. In the absence of counterbalancing selection, alleles causing such sex-ratio distortion will sweep to fixation and cause the extinction of populations or species. I report here the discovery of sex-ratio polymorphisms in two species of the Drosophila quinaria group, Drosophila recens and D. quinaria. Sex-ratio polymorphisms are now known in five of the nine species of Drosophila I have studied in the northeastern United States, which suggests that this condition may be taxonomically more prevalent than generally believed. Experiments on D. recens and D. quinaria showed that the fertilities of virgin SR and standard (ST) males were similar. However, when males were allowed to mate repeatedly, the fertility of SR males was significantly lower than that of ST males in both species. Because the ratio of females to males is expected to increase with the frequency of SR alleles, so will the potential rate of male mating. This prediction suggests a possible frequency-dependent mechanism to prevent the fixation of SR alleles after their initial appearance. A model for the subsequent evolution of sex-ratio polymorphisms is proposed.
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