Sex-specific viability effects of mutations in Drosophila melanogaster
In populations with separate sexes, genetic load due to deleterious mutations may be expressed differently in males and females. Evidence from insect models suggests that selection against mutations is stronger in males, with a positive intersexual correlation for fitness. This pattern will reduce deleterious allele frequencies at the expense of males, such that female mean fitness is greater than expected, preserving population persistence in the face of high mutation rates. While previous studies focus on reproductive success, mutation load depends on total selection in each sex, including selection for viability. In fruit flies, we might expect minimal sex differences in viability effects, since male and female larvae behave similarly, but many genes show sex-biased expression in larvae. We measured the sex-specific viability effects of nine "marker" mutations and 33 mutagenized chromosomes. We find that both types of mutations generally reduce viability in both sexes. Among marker mutations we detect instances of sex biased selection in both directions, but mutagenized chromosomes show little sign of sex-specific mutational variance. We conclude that some mutations can indeed affect viability in a sex-specific manner, but that the pattern of male-biased mutational effects observed previously for reproductive success is not apparent at the pre-reproductive stage.