Gene expression in murine germ cells

In mammals, the X and Y chromosomes are subject to meiotic sex chromosome inactivation (MSCI) during prophase I in the male germline, but their status thereafter is currently unclear. An abundance of X-linked spermatogenesis genes has spawned the view that the X must be active [1-8]. On the other hand, the idea that the imprinted paternal X of the early embryo may be pre-inactivated by MSCI suggests that silencing may persist longer [9-12]. To clarify this issue, we establish a comprehensive X-expression profile during mouse spermatogenesis. Here, we discover that the X and Y occupy a novel compartment in the post-meiotic spermatid and adopt a non-Rabl configuration. We demonstrate that this post-meiotic sex chromatin (PMSC) persists throughout spermiogenesis into mature sperm and exhibits epigenetic similarity to the XY body. In the spermatid, 87% of X-linked genes remain suppressed post-meiotically, while autosomes are largely active. We conclude that chromosome-wide X-silencing continues from meiosis to the end of spermiogenesis and discuss implications for proposed mechanisms of imprinted X-inactivation.

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Namekawa SH, Park PJ, Zhang LF, Shima JE, McCarrey JR, Griswold MD, Lee JT