Epigenetics and the Extreme Stress Response

Abstract

Environmental conditions can be highly unfavorable for many organisms, imposing a variety of extreme stresses onto animal inhabitants. Winters are typically synonymous with shorter photoperiods, lack of food resources, and subzero temperatures. While some species migrate to avoid these conditions, many others have evolved defensive responses to combat these otherwise lethal situations. Such strategies can be classified into major categories including: freeze tolerance, freeze avoidance, anoxia tolerance, diapause, and hibernation. These types of strategies have been documented in a range of organisms including soil microfauna, intertidal marine invertebrates, insects, mammals, and various ectothermic vertebrates including some turtles, snakes, salamanders, and frogs. Extreme survival responses are possible thanks in part to metabolic rate depression (MRD), in which animals dramatically suppress energy expenditure and production to varying degrees. MRD necessitates holistic changes to the transcriptome of these specialized species. Unsurprisingly, recent research is showing that epigenetic mechanisms are invaluable contributors to stress adaptation, as is also true of gene silencing by noncoding RNAs. Epigenetic controls are a collection of regulatory mechanisms that alter gene expression without changing the DNA sequence itself, thereby making them ideal for implementing rapid, transient changes in phenotype as is characteristic of seasonal MRD. The current review will summarize the recent literature regarding epigenetic regulatory mechanisms, MRD, and adaptation to extreme environmental conditions. We also document where current research is directed, and what the most consequential and pressing inquiries are in the field.