DNA methylation reprogramming (DMR) is believed to be a key process by which mammalian zygotes gain nuclear totipotency through erasing epigenetic modifications acquired during gametogenesis. Nonetheless, DMR patterns do not seem to be conserved among mammals. To identify uniform rules underlying mammalian DMRs, we explored DMRs of diverse mammalian zygotes. Of the zygotes studied, of particular interest was the bovine zygote; the paternal DNA methylation first decreased and was then rapidly restored almost to the maternal methylation level even before the two-cell stage. The 5-azadeoxycytidine treatment led to complete demethylation of the male pronucleus. The unusually dramatic changes in DNA methylation levels indicate that the bovine male pronucleus undergoes active demethylation, which is followed by de novo methylation. Our results show that, in bovine, the compound processes of active DNA demethylation and de novo DNA methylation, along with de novo H3-K9 trimethylation also, take place altogether within this very narrow window of pronucleus development.
(c) 2007 Wiley-Liss, Inc.