Sex ratio, relative frequency, and mating success in two genotypes of Drosophila melanogaster

When two types of Drosophila are in competition, the frequency dependence of mating success is measured routinely in our laboratory and many others by direct observation of mating pairs in observation chambers. In these experiments, the Sex Ratio (number of males divided by the number of females) is usually 1:1. Various authors have shown that the male sexual activity depends on the Sex Ratio. We wished to investigate a possible influence of Sex Ratio on the frequency dependence of mating success and the "rare-type advantage." "General" Sex Ratio changes do affect mating success. The influence of the so-called "relative" Sex Ratio (number of males of the first type, A, divided by number of females of the second type, B) seems very low. Male Ratio and Female Ratio changes (i.e., changes of Genotype Ratio for one sex only, the number and type of the other sex flies remaining constant) have approximately the same influence: in these experiments the relative success of both types is also frequency dependent.