Why hybrid males are sterile in Drosophila?

Hybrid male sterility is one of the most rapidly evolving postzygotic reproductive barriers, which has received special attention in the study of speciation. In Drosophila, in majority of interspecific crosses, hybrid males are sterile while females are fertile. Why hybrid males are sterile remains a fundamental question for evolutionary biologists. A number of investigations have been carried out to understand the causes of hybrid male sterility and the results suggest that it may involve either X-Y, X-autosomes, Y-autosomes, cytoplasmic incompatibilities or a number of genes. The genetic basis of hybrid sterility remains nebulous, but it seems that it involves a large number of genes and almost all chromosomes. The only characterized speciation gene in Drosophild today is Odysseus, but no functional tests yet have been reported that support its role in hybrid sterility and its functional equivalence to the homeobox gene. To understand the mechanism of hybrid male sterility, several theories have been proposed. The three most important theories are dominance theory, faster-male theory and faster-X theory, among which the dominance theory is the most popular. Although studies on hybrid male sterility in Drosophila are well documented, no conclusive mechanism of sterility that is uniformly obeyed in all species is still known. During last two decades, the resurgence of interest in hybrid male sterility and the use of Drosophila as a model organism for such study warrant a comprehensive review on this topic to facilitate better understanding of this subject. In view of this, a brief history as well as the recent advances in the field of hybrid male sterility in Drosophila are documented in the present article.