Abstract
This study reports on extensive experimental material covering more than 30 years of studying the genetics of mole voles. Sex chromosomes of Ellobius demonstrate an extraordinary case of mammalian sex chromosomes evolution. Five species of mole voles own three types of sex chromosomes; typical for placentals: XY♂/XX♀; and atypical X0♂/X0♀; or XX♂/XX♀. Mechanisms of sex determination in all Ellobius species remain enigmatic. It was supposed that the Y chromosome was lost twice and independently in subgenera Bramus and Ellobius. Previous to the Y being lost, the X chromosome in distinct species obtained some parts of the Y chromosome, with or without Sry, and accumulated one or several copies of the Eif2s3y gene. Along with enormous variations of sex chromosomes, genes of sex determination pathway and autosomes, and five mole vole species demonstrate ability to establish different meiotic mechanisms, which stabilize their genetic systems and make it possible to overcome the evolutionary deadlocks.