The effect of operational sex ratio and density on the strength of sexual selection against mutant males in Drosophila melanogaster

Higher male:female operational sex ratio (OSR) is often assumed to lead to stronger sexual selection on males. Yet, this premise has been directly tested by very few studies, with mixed outcomes. We investigated how OSR affects the strength of sexual selection against two deleterious alleles, a natural ebony mutant and a transgenic GFP insertion, in Drosophila melanogaster. To this end, we estimated the relative paternity share of homozygous mutant males competing against wildtype males under different OSRs (1:2, 1:1, 2:1). We also manipulated the mating pool density (18, 36 or 54 individuals), and assessed paternity over three consecutive days, during which the nature of sexual interaction changed. The strength of sexual selection against the ebony mutant increased with OSR, became weaker after the first day and was little affected by density. In contrast, sexual selection against the GFP transgene was markedly affected by density: at the highest density it increased with OSR, but at lower densities it was strongest at 1:1 OSR, remaining strong throughout the experiment. Thus, while OSR can strongly affect the strength of sexual selection against “bad genes”, it does not necessarily increase monotonically with male:female OSR. Furthermore, the pattern of relationship between OSR and the strength of sexual selection can be locus-specific, likely reflecting the specific phenotypic effects of the mutation.

Methods
The experiments measured the relative sexual fitness of males carrying one of two mutant alleles (ebony or GFP) competing with wildtype males, depending on the operational sex ratio (OSR) and total number (density) of interacting individuals. Mating groups of virgin Drosophila individuals were set up at different operational sex ratios (1:2, 1:1, 2:1) and at different total numbers of individuals (18, 36, 54). Mutant and wildtype males were always at 2:1 ratio to each other. The mating groups interacted over 3 days. The proportional paternity of the mutant males was obtianed for offspring produced on each day.

An additional experiment was performed to compare the mating success of GFP and wildtype males. 12 GFP and 6 wildtype males were set up together with 18 wildtype females and the number of mating was observed over 8 hours.

For details, see the paper.