The mouse X chromosome is enriched for multi-copy testis genes exhibiting post-meiotic expression

According to the prevailing view, mammalian X chromosomes are enriched in spermatogenesis genes expressed before meiosis 1 – 3 and deficient in spermatogenesis genes expressed after meiosis 2 , 3 . The paucity of post-meiotic genes on the X chromosome has been interpreted as a consequence of Meiotic Sex Chromosome Inactivation (MSCI) – the complete silencing of genes on the XY bivalent at meiotic prophase 4 , 5 . Recent studies have concluded that MSCI-initiated silencing persists beyond meiosis 6 – 8 and that most X-genes remain repressed in round spermatids 7 . We report here that 33 multi-copy gene families, representing ~273 mouse X-linked genes, are expressed in the testis and that this expression is predominantly in post-meiotic cells. RNA FISH and microarray analysis show that the maintenance of X chromosome post-meiotic repression is incomplete. Furthermore, X-linked multi-copy genes exhibit expression levels similar to those of autosomal genes. Thus, not only is the mouse X chromosome enriched for spermatogenesis genes functioning before meiosis, but in addition ~18% of mouse X-linked genes exhibit post-meiotic expression.