Methylation levels of maternal and paternal genomes during preimplantation development

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Abstract
The methylation status of three highly repeated sequences was studied in sperm, eggs and preimplantation embryos with different combinations of parental chromosomes. High levels of methylation of the IAP and MUP sequence families were found in sperm and in eggs, whereas the LI repeat was found to be highly methylated in sperm but only about 42 % methylated in eggs. To assess how the two parental genomes behaved during preimplantation development, normal, fertilised embryos were compared with parthenogenetic embryos where the chromosomes are exclusively of maternal origin. It was observed that the high levels of methylation at the IAP and MUP sequences were retained through early development, with the first signs of demethylation at the IAP sequences apparent on both parental chromosomes in the blastocyst. Methylation at the sperm-derived LI sequences dropped to about the same level as that of the egg-derived sequences by the late 2-cell stage, both then remain at this intermediate level until around the time of cavitation when levels fell to about 10 % in the blastocyst. High levels of DNA methylase were detected in germinal vesicle and metaphase II oocytes; these high levels were maintained in fertilised and parthenogenetic embryos through into the morula and then declined to be undetectable in the blastocyst. Our comparison of maternal and paternal genomes suggests that methylation levels at repeat sequences are remarkably similar at the time of fertilization or, as in the case of the LI sequences, they become so during the first few cell cycles. Hence, there do not appear to be global methylation differences between the genomes that are retained through preimplantation development. The marked loss of methylation from sperm-derived LI sequences during the first two cell cycles may be indicative of the re-modelling of paternal chromosomes in the egg.