Common methylation characteristics of sex chromosomes in somatic and germ cells from mouse, lemur and human

DNA methylation of sex chromosomes was analysed using anti-5-methylcytosine antibodies on metaphase chromosomes of somatic cells from three species: h uman, lemur and mouse. Germ cells were also studied in male mouse. In femal e cells (human and mouse), the late replicating X was always the less methy lated chromosome. Compared with autosomes, the methylation of both X chromo somes was always lower in fibroblasts than in lymphocytes and the differenc e was always greater in mouse than in human. In human, mouse and lemur male cells, the labelling of the unique X chromosome was quite similar to that of the early replicating X from female cells. Except for the heterochromati c region of the human Y chromosome, strongly methylated, the overall methyl ation of the Y chromosome was low. In mouse testicular cells, a variety of DNA methylation patterns was observed according to the cell type and the st ate of differentiation. Finally, the only structures of sex chromosomes whi ch remain methylated in all conditions correspond to their pseudoautosomal regions.