The Evolution of Sex-related Traits and Speciation in Drosophila

Abstract

<p>The amount of genetic change associated with species differentiation, and the phenotypic characteristics of such changes between closely related species, are crucial elements to the problem of speciation. This thesis describes an analysis of morphological, protein and DNA sequence divergence between closely related species that show a common pattern of higher interspecific divergence for sex-related traits (i.e. constituents of he genitalia, traits involved in mating, fertilization, or sex differentiation) than non sex-related traits. Proteins expressed in gonodal tissues (testis and ovary) showed a higher interspecific divergence than proteins expressed in nongonodal tissues (brain and malphighian tubule) and a positive significant correlation was found between testis protein divergence and postzygotic isolation when closely related species were compared. An analysis of internal as well as external traits morphology revealed high divergence in testis length and area and genital arch area between species of the melanogaster complex. Similar levels of variation within species and non significant differences in trait asymmetry were found between sex-related and non-sex related traits. The analysis of interspecific hybrid morphology revealed between species allele complementation in non sex-related traits, while sex-related traits displayed signs of disrupted genetic interactions. A preliminary survey of DNA sequence divergence showed a high ratio of nonsynonymous to synonymous substitutions for sex-related genes. Such as elevated ratio proved to be the result of a high proportion of nonsynonymous substitutions between closely related species. Synonymous substitutions did not show any translational selective constraints that may have reduced the proportion of such changes between species.</p>