The role of DNA methylation in facilitating life history trait diversity in fishes

The incorporation of epigenetics (i.e., change in gene activity without change in DNA sequence) into studies of gene regulation and phenotypic expression has contributed to a significant improvement in our understanding of the evolution of life history traits. One important epigenetic mechanism is DNA methylation, which in vertebrates generally means the addition of a methyl group to a cytosine thus altering gene expression. Here, we discuss progress and gaps in our knowledge of the role of DNA methylation in facilitating diversity across four life history trait classes in fishes: developmental processes, size and growth rates, aging and sexual maturity, and sex regulation. We discuss insights into the regulatory aspect of gene expression in fish which can ultimately influence phenotypic diversity and speciation. We discuss how temperature influences methylation patterns affecting multiple traits. DNA methylation influence on gene expression varies depending on tissue types and the location within the genome of the methylated site (i.e., DNA methylation can increase or decrease gene expression). The role of DNA methyltransferases is also a common denominator across all tissue types in influencing the global methylome status regardless of species or environmental stressor. Organismal development stage is equally important, a decrease in global methylation throughout early development generally corresponds to elevated gene expression associated with growth and development. Finally, we discuss general limitations of DNA methylation studies with a focus on fish. We then provide recommendations for future research.