Abstract
Restriction endonucleases were used to determine the degree of methylation at the sequences CCGG and GCGC in a wide range of animal DNAs. Both total DNA methylation and ribosomal DNA methylation were studied. Whole DNA methylation was indetectable in arthropods, fractional in other invertebrate phyla, and high in the vertebrates. Ribosomal DNA was predominantly unmethylated in all animals except fish and amphibia, where it was heavily methylated. We discuss the evolutionary and functional implications of these results, and suggest that the large differences between genome types are the result of evolutionary changes in the relative size of heavily methylated and unmethylated compartments.