Drosophila hybrid sterility

# Historical background
T.H. Morgan was the first to use Drosophila flies in experiments regarding heredity in 1910.  Morgan and his colleagues C.B. Bridges , A.H. Sturtevant, and H.J. Mueller developed a chromosome theory of heredity through Drosophila experiments that helped Morgan win the Nobel Prize in 1933.  Their experiments consisted of cross-breeding Drosophila mutants and documenting offspring.  Another highly regarded figure in Drosophila research was Theodosius Dobzhansky, who invented the use of genetic markers by using them to study hybrid sterility between Drosophila pseudoobscura and Drosophila persimilis (Futuyma 1997).  This method has been used heavily by scientists in similar experiments.

# Experimentation
Looking at the hybrid sterility of crosses between Drosophila simulans and its island derivative Drosophila mauritiana shows that female hybrids are fertile and hybrid males are sterile.  This seems to have happened early in their evolutionary divergence.  Recent studies have shown that a critical gene for sex determination in Drosophila known as the sex-lethal gene is highly misregulated in D. melanogaster and D. simulans hybrids, compared with misregulation of non-sex biased genes studied.  The sex-lethal gene is most abnormally expressed In the male hybrids from D. malanogaster mothers which results in a failure of localization of the male-specific complex to the X chromosome, contributing to 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.
With experiments involving crosses between D. pseudoananassae and D. bipectinata, D. pseudoananassae and D. parabipectinata, and D. pseudoananassae and D. malerkotliana it was shown that Y chromosome has a role in hybrid male sterility.  The possible interactions of Y chromosome are X-Y, Y-autosome and Y-cytoplasm (Paras 2006).  These males carry a set of conspecific autosomes to the Y chromosome, this would naturally result in fertility because of the Y-dominant autosome interactions, therefore Y-autosome interactions are ruled out in hybrid sterility.  Since cytoplasmic factors can be compatible between these species this is also dismissed as the cause of sterility, however in Drosophila paulistorum there has been 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 species who share more genetic compatibilities have less perturbance in spermatogenesis and generally result in normal sized testes with immotile sperm, while species who are less genetically compatible have a higher disruption in spermatogenesis and generally have atrophied testes. 
	Another possible cause of sterility among hybrids is if the X chromosome of one species has recessive alleles interacting with autosomal alleles of another species, this could cause the heterogametic sex in the hybrid to be inviable or sterile, but homogametic sex will not (Futuyma 1997).  Meaning that individuals carrying XY chromosomes (males) will be sterile and those carrying XX (females) will be fertile.  This is closely related to Haldane's rule.

# Controversy
Certain experiments have led scientists to believe that many observations recorded in laboratories may neglect existing polymorphism for factors in hybrid sterility due to crossing parents from non-isofemale lines, as well as possibly underestimating actual degrees of sterility caused by inaccurate measures of . Neglegence of potentially existing polymorphism could lead to misinterpretation of the scale on which hybrid sterility occurs. There is still much research to be done with Drosophila species and many controversies to be settled.  The study of Drosophila is thought to be the possible key to uncover the human genome and has already been a valuable tool for understanding evolution.