O-095 The persistent Y hypothesis

Abstract The Y chromosome has been touted as a wimpy relic of the X, with its survival dependent on a few critical functions in spermatogenesis and sex determination, the loss of which would signal its demise. Why then has it survived since its origin (∼165 MYA) in all but a handful of therian mammal species? This is in stark contrast to the high turnover of sex chromosomes observed in other vertebrate linages, so the mammal Y turns out to be an exception of persistence, rather than the rule. Here we propose a novel explanation for such perseverance. The Y chromosome bears so-called ‘executioner genes’ that are critical for successful meiotic progression. Their expression is required to initiate meiotic sex chromosome inactivation, but then must be subject to this very silencing they induce to ensure germ cell survival. Whenexecutioners are translocated to an autosome they escape meiotic silencing and, being pachytene-lethal, cease meiosis. Therefore, these meiotic executioners act as their own judge, jury and executioner, posing strong evolutionary constraint for the Y chromosome to persist in eutherian mammals.