Genetic mechanisms of sex determination in mammals lacking a Y chromosome
aKoltzov Institute of Developmental Biology, RAS
119334 Moscow, Vavilova, 26, Russia
bVavilov Institute of General Genetics, RAS
119991 Moscow, Gubkina, 3, Russia
*e-mail: i.bakloushinskaya@idbras.ru
Genetic sex determination is realized by a complex cascade of genes. In mammals, sex is determined by genes whose precise temporal and spatial regulation leads to the development of testes or ovaries from bipotential embryonic gonads. Genetic development programs along the female or male pathway are not completely different, a significant part of the genes involved in both pathways. In placental mammals, activation of the Y-linked gene Sry (sex-determining region Y) initiates testicular development. When Sry is not present, as in XX individuals, the bipotential gonads develop into ovaries. Y chromosome and Sry gene were lost in two rodents genera, mole voles genus Ellobius and spiny rats Tokudaia. In four mole vole species without Y chromosomes, a few Y-linked genes escaped to the X chromosome and exposed in male and female genomes. Two X chromosomes in males and females of E. talpinus and E. tancrei are morphologically identical, but functional differences are evident in meiosis. So far, no explanation has been received for existence of males and females in mole voles and spiny rats along with the Sry loss and possible upregulation of the Sox9 gene. Several genes, usually localized in a Y-chromosome, exist in female and male genomes of mole voles, such as a factor spermatogonial proliferation (Eif2s3y, Eukaryotic translation initiation factor 2 subunit 3, Y-linked). In species with two X-chromosomes in individuals of both sexes, the morphological identity of sex chromosomes masks functional heteromorphism, which can be detected in the meiosis only. The existence of males and females for the mole voles Ellobius and the spiny rats Tokudaia in the absence of the Sry gene and a possible violation of expression of the Sox9 gene has not yet been explained. To date, it remains unclear how variable the cascade of genes involved in the sex determination can be. Study of different deviations in the structure of the genome, such as XX or X0 females and males, full or partial loss of the Y chromosome, as well as in the structure of the gonads, may be crucial for understanding the sex determination genetics.
