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. 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. We might expect minimal sex differences in viability effects in fruit flies, since male and female larvae behave similarly, yet many genes show sex-biased expression in larvae. We measured the sex-specific viability effects of nine "marker" mutations and 123 mutagenized chromosomes. We find that both types of mutations generally reduce viability in both sexes. Among marker mutations we detect instances of sex-biased effects in each direction; mutagenized chromosomes show little sex-specific mutational variance, but recessive lethals show a female bias, including in FlyBase records. We conclude that mutations regularly affect viability in a sex-specific manner, but that the strong pattern of male-biased mutational effects observed previously for reproductive success is not apparent at the pre-reproductive stage.
