The haploid male germ cell differentiation program controls essential steps of male gametogenesis and relies partly on a significant number of sex chromosome-linked genes. These genes need to escape chromosome-wide transcriptional repression of sex chromosomes, which occurs during meiosis and is largely maintained in post-meiotic cells. A newly discovered histone lysine modification, crotonylation (Kcr), marks X/Y-linked genes that are active in post-meiotic male germ cells. Histone Kcr, by conferring resistance to transcriptional repressors, could be a dominant element in maintaining these genes active in the globally repressive environment of haploid cell sex chromosomes. Furthermore, the same mark was found associated with post-meiotically activated genes on autosomes. Histone Kcr therefore appears to be an indicator of the male haploid cell gene expression program and a notable element of genome programming in the post-meiotic phases of spermatogenesis.
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