Methylation of DNA in early development: 5-methyl cytosine content of DNA in sea urchin sperm and embryos.

By separating formic acid hydrolysates with high pressure chromatography on an Aminex-10 column, we determined the ratio of 5-methyl cytosine to cytosine and other bases of DNA from sea urchin sperm and nuclei of embryos from early cleavage through pluteus stages. Contrary to several previous reports, we could not find any measurable changes in the methylation levels of embryonic nuclear DNAs at different stages of development. We also found no consistent differences between the methylation levels of sea urchin sperm and embryonic nuclei or the 5-methyl cytosine content of fish (Mugil cephalus) sperm and liver nuclei. While these measurements would not have detected subtle variations associated with differentiation, they would have indicated the gross changes previously reported for embryos or between sperm and somatic nuclei had those changes been present.