Sex Chromosomes Evolution in Mammals

The hypothesis that the sex chromosomes evolved from a pair of autosomes has been accepted by many evolutionary geneticists. The latest findings revealed that، mammalian sex chromosomes evolved from a pair of autosome، 166 million years ago. While the X is highly conserved in size، gene content and gene order among different species of placental mammals، Y chromosome has been undergone many degeneration and lost more than 1000 genes during the evolution. It is predicted that، at the observed rate of attrition، the human Y chromosome will be gone within 5-10 million years. Stepwise cessation of recombination initiated with an inversion in the Y، is the main cause of Y chromosome attrition. Higher mutation rate due to special structure of Y، accumulation of transposable elements and inefficiency of selection in non-recombinant parts of Y for elimination of deleterious mutations، are the major causes of Y chromosome degeneration. It is predicted that after complete extinction of Y، a new sex determination should be evolved. In two groups of rodents، the mole vole (Ellobius) and the Japanese spiny rat (Tokudaya)، Y chromosome has been removed completely and a new sex determination system evolved. However، it is yet to be detected.