Mosaic heterochromatin of the inactive X chromosome in vole Microtus rossiaemeridionalis.

During early development in female mammals, one of the two X chromosomes recruits a variety of protein complexes that establish repressive chromatin modifications and thus becomes transcriptionally silenced. This process is termed X chromosome inactivation (XCI). Imprinted XCI of the paternal X chromosome occurs in the extraembryonic lineages of some eutherian species (e.g., rodents). In the cells of the embryo proper, the choice of the X chromosome for XCI is random. In this study we compared the distribution of some histone modifications on metaphase spreads from extraembryonic endoderm and fibroblast cell lines in vole Microtus rossiaemeridionalis, which are examples of imprinted and random XCI, respectively. The X chromosome of M. rossiaemeridionalis bears a large constitutive heterochromatic block enriched with repeated DNA, making this species a useful model for studying chromatin structure. In vole fibroblasts and the majority of extraembryonic endoderm cells, the silencing of the inactive X chromosome appears to involve two types of facultative heterochromatin. The first is defined by H3K27 trimethylation and H2A ubiquitylation and colocalizes with previously described Xist RNA banding, whereas the second is associated with H3K9 trimethylation and the heterochromatic protein HP1. The block of constitutive heterochromatin on the M. rossiaemeridionalis X chromosome has the same pattern of chromatin modifications as the second type of facultative heterochromatin. The distribution of histone modifications, HP1 protein, and Xist RNA on vole inactive X chromosome is the same during both the imprinted and the random XCI.