DNA methylation at CpG is a common epigenetic mechanism which controls gene expression. In many species, paternal genome is subjected to genome-wide active demethylation before the DNA replication commences. This study was designed to examine the dynamics of paternal genome demethylation in pronuclear bovine zygotes produced by IVF and ICSI with non-dried or freeze-dried/+4 °C-stored spermatozoa. Zygotes were fixed and immunostained for 5-methyl-cytosine at 8, 10, 14 and 18 h post IVF (hpi). Relative methylation level of male to female pronucleus decreased significantly from 0.92 at 8 hpi to 0.69 at 10 hpi with no additional decrease at 14 and 18 hpi (0.67 and 0.64, respectively). These decreases were mainly due to the higher proportions of zygotes showing relative methylation <0.60 when compared to 8 hpi. Since it takes about 2 h until sperm cells penetrate into oocytes in bovine IVF, zygotes at 6 and 12 h after ICSI may be developmentally comparable with those at 8 and 14 hpi, respectively. However, higher proportion of zygotes 6 h after ICSI showed relative methylation <0.60 when compared to that of IVF-derived zygotes. The relative methylation levels at 12 h were 0.66 and 0.59 in zygotes produced by ICSI with non-dried and freeze-dried sperm, respectively. Surprisingly, the proportion of ICSI zygotes produced with freeze-dried sperm and showed relative methylation <0.60 at 12 h was higher than that in non-dried sperm derived zygotes. These results indicate that demethylation of paternal genome occurs in IVF- and ICSI-derived pronuclear bovine zygotes, and that freeze-drying may increase the susceptibility of paternal genome to demethylation activity of oocyte cytoplasm.
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