VIRILITY DEFICIENCY AND THE SEX-RATIO TRAIT IN DROSOPHILA PSEUDOOBSCURA. I. SPERM DISPLACEMENT AND SEXUAL SELECTION

The Sex-Ratio (SR) phenomenon is associated with the X chromosome of many species of Drosophila. Males carrying SR transmit predominantly sperm bearing the X chromosome. SR, therefore, has a very significant advantage in segregation. This paper provides an experimental analysis of the role of virility selection in preventing the fixation of SR. SR males are found to suffer substantial virility reduction when mated with nonvirgin females but not with virgin females. The reduced virility arises because they are weaker in sperm displacement and are discriminated against by nonvirgin females. The virility deficiency of SR males is even more severe at low temperatures.