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. Experiments involving crosses between
D. pseudoananassae and
D. bipectinata,
D. pseudoananassae and
D. parabipectinata, and
D. pseudoananassae and
D. malerkotliana have further shown that the Y chromosome has a role in hybrid male sterility. The possible interactions of Y chromosome are X-Y, Y-autosome and Y-cytoplasm. The sterile males carry a set of conspecific autosomes to the Y chromosome, which results in fertility because of the Y-dominant autosome interactions. Therefore, Y-autosome interactions are ruled out in this type of hybrid sterility. Since cytoplasmic factors can be compatible between these species, such factors are also dismissed as the cause of sterility. However, in
Drosophila paulistorum there is information suggesting that if Y chromosome and cytoplasm are from different parents, the male is usually sterile. X-Y interactions are the most likely cause of sterility in male hybrids. It has been shown that interbreeding organisms that have more genetic compatibilities have less disruption of
spermatogenesis and normal sized testes, while species who are less genetically compatible have a higher disruption in spermatogenesis and generally have atrophied testes. Another possible cause of sterility among species in which presence or absence of one or the other sex chromosome determines gender, is if one sex chromosome of one species has recessive alleles interacting with autosomal alleles of the mating species. This could cause the heterogametic sex chromosome in the hybrid to be inviable or sterile, but homogametic sex chromosome will be fertile. Consequently, in species where presence or absence of a Y chromosome determines gender, for instance, individuals carrying XY chromosomes (males) will be sterile and those carrying XX (females) will be fertile. This is closely related to
Haldane's rule. It has also been shown that microbial infections of invertebrates can cause modification of the gender and fertility of host's off-spring. For instance, infections of nematodes or of arthropods, including
Drosophila, species of the rickettsial bacteria
Wolbachia can produce a male-specific sterility, which is congenital by means of transmission through the female line. == Controversy ==