M31,a murine homolog of Drosophila HP1, is concentrated in the XY body during spermatogenesis

The formation of the sex vesicle, or XY body, during male meiosis and pairi ng of the sex chromosomes are thought to be essential for successful sperma togenesis. Despite its cytological discovery a century ago, the mechanism o f XY body formation, particularly heterochromatinization of the sex chromos omes, has remained unclear. The HP1 class of chromobox genes are thought to encode proteins involved in the packaging of chromosomal DNA into repressi ve heterochromatin domains, as seen, for example, in position-effect varieg ation. Study of the distribution of a murine HP1-like chromodomain protein, M31, during spermatogenesis revealed spreading from the tip of the XY body in mid-stage pachytene spermatocytes to include the whole of the XY body i n late-pachytene spermatocytes. We also demonstrate that the formation of t he XY body during spermatogenic progression in neonatal mice coincides with the expression of a novel nuclear isoform of M31, M31(p21). These results support the view that a common mechanistic basis exists for heterochromatin -induced repression, homeotic gene silencing, and sex-chromosome inactivati on during mammalian spermatogenesis.