Two different methylation levels in vertebrate genomes

Investigations aimed at understanding the biological significance of DNA methylation in vertebrates have been carried out along two lines. 

A summary of the results of Table 5.3 indicates (Table 5.4) that the average methylation values in fishes (1.70%) and amphibians (1.98%) are roughly twice as large as those found in mammals (0.88%), birds (1.02%) and reptiles (1.00%). Since average GC levels are close to 42.5% for all five vertebrate classes, this means that approximately 9% of all cytosines are methylated in fishes and amphibians compared with approximately 4.5% in mammals, birds and reptiles. 

The correlation between 5mC and genomic GC levels is understandable if one recalls that CpG frequency is linearly correlated with the GC content of the genome (Bernardi, 1985 Aissani and Bernardi, 1991a,b) and that CpG is the major site of methylation, methylation on other dinucleotides (Woodcock et al., 1987) and on CpNpG trinucleotides (Clark et al., 1995) being extremely limited. A similar correlation was also found in plant 

Reptilia Crocodylia Crocodylidae Crocodilus niloticus blood 44.4 1.06 6  

Aves Rheiformes Rheidae Rhea americana blood 44.0 0.98 6  

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