In the off-spring of crosses between "Drosophila simulans" and its island derivative "Drosophila mauritiana," female hybrids are fertile but male hybrids are sterile. Recent studies have shown that a critical gene for gender determination in "Drosophila" known as the sex-lethal gene is highly misregulated in "D. melanogaster" and "D. simulans" hybrids, compared with the degree of misregulation of non-sex biased genes studied. The sex-lethal gene is often abnormally expressed in male hybrids from "D. melanogaster" mothers as a result of re-localization of the male-specific complex to the X chromosome, which contributes to the male sterility. Abnormalities in sperm array were found in very few individuals during their larval stage, meaning that disruptions in spermatogenesis most likely occur during later stages in life.