Evolution of Genes Involved in Mammalian Reproduction and Sex Determination.

Abstract

Traits that influence reproductive success and reproductive isolation in plants and animals have been the central focus of evolutionary biology since Darwin. Many genes were recently identified as important players during various phases of fertilization and sex determination. Although our understanding of the mechanism underlying the evolution of these genes is not complete, the emerging trend indicates that genes involved in reproduction are often rapidly evolving. Elucidating the mechanisms and factors that influence this fast pace of evolution has broad implications to human health and fertility, as well as the process of speciation, which is of fundamental relevance to the theory of evolution. Herein, I present case studies on three genes CATSPER1, SED1 and Sry, which are essential in sperm motility, sperm adhesion, and sex determination, respectively, and propose mechanisms by which natural selection has shaped their evolution. CATSPER1 is a calcium voltage-gated ion channel expressed on the plasma membrane of the principal tailpiece of spermatozoa and is essential for sperm motility. My findings document the first known case of positive Darwinian selection acting on the length of a protein (Podlaha and Zhang 2003). Specifically, natural selection promotes fixation of insertions and deletions in the CATSPER1 N-terminus, both in primates and rodents (Podlaha et al. 2005), affecting the ion channel’s rate of inactivation. Through the length variation of the N-terminus, channel’s rate of inactivation may have direct impact on sperm motility, which is an important determinant of sperm competition. SED1 is an important sperm-egg binding protein. My study provides evidence for a functional shift in SED1 due to a lineage specific loss of a protein-protein binding domain (Podlaha, Webb, and Zhang 2006). This domain loss was accompanied by positive selection in other parts of the protein in the ancestor of New World and Old World monkeys. The multifunctional nature of SED1 protein, however, obscures identification of the selective agent underlying this functional shift. Lastly, my analysis of the sex determination gene Sry addresses questions about the molecular mechanism that gives rise to fertile sex reversed females in akodont rodents.