Phenotypic Plasticity: What Has DNA Methylation Got to Do with It?

Simple Summary Phenotypic plasticity, the genome producing multiple phenotypes, is central to an animal's ability to respond to environmental change, expected or otherwise. A prominent example of this is the behavioural maturation of a honey bee worker over its lifetime. These multiple phenotype outcomes are based on changes in gene expression precipitated by internal and external signals. How these signals are translated from the environment into changes in gene expression is an active area of research. One avenue of investigation has been the responsiveness of DNA methylation, chemical modifications of cytosines, to environmental changes and its influence on gene expression. We review this active field of research and find that cytosine methylation is often altered by environmental changes. However, we find no strong, broad links between changes in cytosine methylation and changes in gene expression. Although there is some evidence that species-specific links between the two occurs. While this is currently the case, we also do not believe the field has arrived at a conclusive answer. Better experimental designs, appropriate biological replication, newer computational tools, and, most importantly, the use of genetic manipulations will provide definitive answers as to the link between phenotypic plasticity and DNA. How does one genome give rise to multiple, often markedly different, phenotypes in response to an environmental cue? This phenomenon, known as phenotypic plasticity, is common amongst plants and animals, but arguably the most striking examples are seen in insects. Well-known insect examples include seasonal morphs of butterfly wing patterns, sexual and asexual reproduction in aphids, and queen and worker castes of eusocial insects. Ultimately, we need to understand how phenotypic plasticity works at a mechanistic level; how do environmental signals alter gene expression, and how are changes in gene expression translated into novel morphology, physiology and behaviour? Understanding how plasticity works is of major interest in evolutionary-developmental biology and may have implications for understanding how insects respond to global change. It has been proposed that epigenetic mechanisms, specifically DNA methylation, are the key link between environmental cues and changes in gene expression. Here, we review the available evidence on the function of DNA methylation of insects, the possible role(s) for DNA methylation in phenotypic plasticity and also highlight key outstanding questions in this field as well as new experimental approaches to address these questions.