Sex chromosome and sex determination evolution in African clawed frogs (Xenopus and Silurana)

Abstract

<p>Sex chromosomes have evolved independently multiple times in plants and animals. Sex chromosome evolution theory predicts the gradual degeneration of the sex-specific sex chromosome due to suppression of recombination, which lowers the efficacy of natural selection. Suppressed recombination also acts to resolve sexual conflict and ensure proper segregation of sex-specific alleles. However, sex chromosome degeneration is not always the case, and evolutionarily old, and young, but homomorphic (nondegenerate) sex chromosomes have been observed. African clawed frogs (<em>Xenopus</em> and <em>Silurana</em>) have homomorphic sex chromosomes due to a recent turnover event. However, occasional recombination between the sex chromosomes may contribute to the maintenance of homomorphic sex chromosomes in African clawed frogs. Mechanisms that prevent divergence of sex chromosomes may be related to polyploidization, which is frequently observed in African clawed frogs. The studies herein construct a phylogenetic framework to test alternative hypotheses for selection on sex-linked and autosomal genes involved in sex determination, map sex chromosomes and compare sex chromosomes across African clawed frogs. I have also explored the relationship between phenomena like recent turnover events, recombination and polyploidization to sex chromosome degeneration (or lack thereof). In this dissertation, I have discussed the potential for multiple mechanisms of sex determination and the unique pseudoautosomal nature of sex chromosomes within this group of frogs. This body of work provides a comprehensive study of sex chromosomes in a group lacking phylogenetic resolution and sheds light on the origin and evolution of sex chromosomes in other organisms.</p>