METHYLATION OF GENOMES AND GENES AT THE INVERTEBRATE-VERTEBRATE BOUNDARY

Patterns of DNA methylation in animal genomes are known to vary from a n apparent absence of modified bases, via methylation of a minor fract ion of the genome, to genome-wide methylation, Representative genomes from 10 invertebrate phyla comprise predominantly nonmethylated DNA an d (usually but not always) a minor fraction of methylated DNA, In cont rast, all 27 vertebrate genomes that have been examined display genome -wide methylation. Our studies of chordate genomes suggest that the tr ansition from fractional to global methylation occurred close to the o rigin of vertebrates, as amphioxus has a typically invertebrate methyl ation pattern whereas primitive vertebrates (hagfish and lamprey) have patterns that are typical of vertebrates, Surprisingly, methylation o f genes preceded this transition, as many invertebrate genes have turn ed out to be heavily methylated, Methylation does not preferentially a ffect genes whose expression is highly regulated, as several housekeep ing genes are found in the heavily methylated fraction whereas several genes expressed in a tissue-specific manner are in the nonmethylated fraction.