The mouse X chromosome is enriched for multicopy testis genes showing postmeiotic expression

According to the prevailing view, mammalian X chromosomes are enriched in spermatogenesis genes expressed before meiosis 1 , 2 , 3 and deficient in spermatogenesis genes expressed after meiosis 2 , 3 . The paucity of postmeiotic 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 , 7 , 8 and that most genes on the X chromosome remain repressed in round spermatids 7 . Here, we report that 33 multicopy gene families, representing ∼273 mouse X-linked genes, are expressed in the testis and that this expression is predominantly in postmeiotic cells. RNA FISH and microarray analysis show that the maintenance of X chromosome postmeiotic repression is incomplete. Furthermore, X-linked multicopy genes exhibit a similar degree of expression as 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 are expressed in postmeiotic cells.