Evolutionary changes in CpG and methylation levels in the genome of vertebrates

We have analysed the levels of 5-methylcytosine (5mC) in DNAs from 42 vertebrates, and compiled, including data from literature, a table of genomic 5mC and GC levels (as well as the available c-values, i.e., the haploid genome sizes) of 87 species from all vertebrate classes. An analysis of the data indicates that (i) two positive correlations hold between the 5mC and GC levels of the genomes of fishes/amphibians and mammals/birds, respectively; (ii) the genomes of fishes and amphibians are, on average, about twice as methylated as those of mammals, birds and reptiles, this difference being unrelated to the amounts of repetitive DNA sequences; (iii) the 5mC and CpG observed/expected values show no overlap between the two groups of vertebrates and suggest the existence of two equilibria. The transition separating the two equilibria appears to have taken place at the time of appearance of reptiles. Its possible cause(s) and its implications are discussed.