Epigenetic variation in animal populations: Sources, extent, phenotypic implications, and ecological and evolutionary relevance

Laboratory experiments and fieldwork with asexually reproducing invertebrates and vertebrates clearly revealed that animal populations can produce substantial phenotypic variation despite genetic identity. This epigenetically caused phenotypic variation comes from two different sources, namely directional environmental induction and bed-hedging developmental stochasticity. Both occur together and are mediated by molecular epigenetic mechanisms like DNA methylation, histone modifications and microRNAs. These epigenetic mechanisms are also involved in insect polyphenism, phenotypic changes in early domestication, and gene expression change and chromatin rearrangement during speciation. Epigenetic variation is particularly important for asexual populations helping them to stay in the game of life when the environmental conditions change. However, it is also relevant for sexually reproducing populations, as shown for genetically impoverished invasive groups, cave animals and sessile taxa that cannot evade unfavourable environmental conditions. Experiments revealed that epigenetic marks can be transgenerationally inherited and persist for several generations. First evidence suggests that inherited epimutations with phenotypic effects may end-up in phenotype-fixing genetic mutations by accelerated mutation of methylated nucleotides. Refined concepts, suitable animal models, fast and affordable new omics techniques that require only small tissue samples, and appropriate data interpretation tools are now available enabling future investigations in ecological and evolutionary epigenetics with high accuracy.