The molecular evolution of sex and reproduction related genes, hybrid male sterility and spectiation in the drosophila melanogaster complex

Abstract

Haldane's rule, which states that the heterogametic sex is preferentially afflicted if one of hybrid sexes is sterile or inviable, is a general pattern in all animals that possess sex chromosomes. The hybrid sterility component of this rule is especially important because hybrid sterility is involved in the onset of postzygotic isolation. Accumulating evidence on the fast evolution of individual sex genes have stimulated us to hypothesize that the fast evolution of sex genes may be the force underlying the excess of hybrid heterogametic sterility. This study tests the evolutionary patterns of sex genes in comparison to non-sex genes, as a general group. The divergences between a group of 19 sex genes and 20 non-sex genes from X chromosome were compared between D. melanogaster, D. mauritiana, D. simulans, and D. sechellia using PCR-RFLP. Within species polymorphism data were also obtained for D. simulans and D. mauritiana. The results show a significantly higher divergence for sex genes than non-sex genes, while a comparable level of intraspecific polymorphism was revealed in both groups. Among the sex gene group, genes related to male reproduction appear to evolve faster than femalereproductive genes. The evolution of both sex and non-sex genes conforms to the neutral theory under Tajima's test and HKA test. The faster evolution of sex genes supports the fast-sex theory as an explanation for the hybrid sterility component of Haldane's rule. Localization of some examples of hybrid sterility genes is crucial to ultimately untangle the genetics of hybrid sterility. The car region of D. mauritiana, which has been shown to harbor genes that confer full effect of hybrid sterility in the D. simulans genetic background, was introgressed into the D. simulans genome by continuous backcrosses. Recombination mapping analysis, taking advantage of molecular markers, revealed that at least two regions are capable of causing hybrid sterility in this species group. The phenotypes of hybrid testes were examined during the backcross process.