Active demethylation of the paternal genome in the mouse zygote.
DNA methylation is essential for the control of a number of biological mechanisms in mammals . Mammalian development is accompanied by two major waves of genome-wide demethylation and remethylation: one during germ-cell development and the other after fertilisation      . Most previous studies have suggested that the genome-wide demethylation observed after fertilisation occurs passively, that is, by the lack of maintenance methylation following DNA replication and cell division  , although one other study has reported that replication-independent demethylation may also occur during early embryogenesis . Here, we report that genes that are highly methylated in sperm are rapidly demethylated in the zygote only hours after fertilisation, before the first round of DNA replication commences. By contrast, the oocyte-derived maternal alleles are unaffected by this reprogramming. They either remain methylated after fertilisation or become further methylated de novo. These results provide the first direct evidence for active demethylation of single-copy genes in the mammalian zygote and, moreover, reveal a striking asymmetry in epigenetic methylation reprogramming. Whereas paternally (sperm)-derived sequences are exposed to putative active demethylases in the oocyte cytoplasm, maternally (oocyte)-derived sequences are protected from this reaction. These results, whose generality is supported by findings of Mayer et al. , have important implications for the establishment of biparental genetic totipotency after fertilisation, the establishment and maintenance of genomic imprinting, and the reprogramming of somatic cells during cloning.