DNA methylation is a key epigenetic modification in the vertebrate genomes known to be involved in biological processes such as regulation of gene expression, DNA structure and control of transposable elements. Despite increasing knowledge about DNA methylation, we still lack a complete understanding of its specific functions and correlation with environment and gene expression in diverse organisms. To understand how global DNA methylation levels changed under environmental influence during vertebrate evolution, we analyzed its distribution pattern along the whole genome in mammals, reptiles and fishes showing that it is correlated with temperature, independently on phylogenetic inheritance. Other studies in mammals and plants have evidenced that environmental stimuli can promote epigenetic changes that, in turn, might generate localized changes in DNA sequence resulting in phenotypic effects. All these observations suggest that environment can affect the epigenome of vertebrates by generating hugely different methylation patterns that could, possibly, reflect in phenotypic differences. We are at the first steps towards the understanding of mechanisms that underlie the role of environment in molding the entire genome over evolutionary times. The next challenge will be to map similarities and differences of DNA methylation in vertebrates and to associate them with environmental adaptation and evolution. 1. Environmental Epigenetics and DNA Methylation In vertebrates, cytosine DNA methylation is a heritable epigenetic modification that occurs mostly at the CpG dinucleotides except for the CpGs in CpG islands [1]. Recently, it has become extremely attractive given its involvement in a diverse range of cellular functions including tissue-specific gene expression, cell differentiation [2], development [3, 4] and reprogramming ([5] see references therein), genomic imprinting, X chromosome inactivation, and regulation of chromatin structure and disease states [6–9]. Notably, the epigenome contains hypervariable regions that could be a source of cellular diversity [10] or could underlie disease states or provide an engine for neutral selection at cell or tissue level [11]. Such hypervariability might be influenced by metabolite fluctuations, temperature variation, and other environmental agents that exert their action on chromatin-modifying enzymes and gene regulation [12–15]. A clear example of how environment plays an important role in shaping the epigenome is represented by monozygotic twins, who are epigenetically indistinguishable early in life but with
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