Assessing sex-specific selection against deleterious alleles: males don’t pay for sex

Selection acting on males can reduce mutation load of sexual relative to asexual populations, thus mitigating the two-fold cost of sex. This requires that it seeks and destroys the same mutations as selection acting on females, but with higher efficiency, which could happen due to sexual selection-a potent evolutionary force that in most systems predominantly affects males. We used replicate populations of red flour beetles (Tribolium castaneum) to study sex-specific selection against deleterious mutations introduced with ionizing radiation. Additionally, we employed a novel approach to quantify the relative contribution of sexual selection to the overall selection observed in males. The induced mutations were selected against in both sexes, with decreased sexual competitiveness contributing, on average, over 40% of the total decline in male fitness. However, we found no evidence for selection being stronger in males than in females; in fact, we observed a non-significant trend in the opposite direction. These results suggest that selection on males does not reduce mutation load below the level expected under the (hypothetical) scenario of asexual reproduction. Thus, we found no support for the hypothesis that sexual selection contributes to the evolutionary maintenance of sex.