A gene that modifies the sex ratio in a bisexual strain of Sciara ocellaris

The influence of an X-linked recessive mutation, sepia, on the sex determination of a bisexual strain of Sciara ocellaris was studied. It induces an alteration in the sex ratio, especially in the progeny of heterozygous females, increasing the proportion of males. These results cannot be explained by differential fecundity of the female parents of different genotypes nor by differential mortality between the sexes. The occurrence of gynandromorphs indicates that the mutant probably interferes with the processes of X-chromosome elimination. Observations that heterozygous females which received the sepia allele from their mothers produced a higher frequency of gynandromorphs than females which received the mutated allele from their male parents, suggest that the mutation is interfering with the mechanism of chromosome elimination present in the egg cytoplasm.