Sex determination, gonadal sex differentiation, and plasticity in vertebrate species

A diverse array of sex determination (<i>SD</i>) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed <i>SD</i> mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate <i>SD</i> was the discovery of the <i>SRY</i> gene in 1990. Since that time, many attempts to clone an <i>SRY</i> ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when <i>DMY/dmrt1by</i> was discovered as the <i>SD</i> gene of a small fish, medaka. Surprisingly, however, <i>DMY/dmrt1by</i> was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of <i>SD</i> genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how <i>SD</i> and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate <i>SD</i> and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various <i>SD</i> mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.