Epialleles in plant evolution

Abstract

Heritable phenotypic differences caused by epigenetic modifications, rather than DNA sequence mutations, pose a challenge to our understanding of natural variation. Here, we review what is known about plant epialleles and the role of epigenetics in evolution.

Figure 1

Classes of epialleles. (a) Epigenetic modifications and associated silencing of the adjacent gene is dependent on a specific cis-element, often a repetitive element. (b) Epigenetic modification is triggered by another locus or allele. Once the modification has been established, the trigger is no longer required for its maintenance. (c) Epigenetic modification is triggered by another locus or allele, but the trigger is permanently required.

Figure 2

The potential role of inherited epigenetic changes, comparing the effects of spontaneous and induced epimutations. A population of genotypically identical individuals is shown, which contain a single locus that can exist in two epigenetic states. Like spontaneous epimutations, induced epimutations are maintained across generations, but revert randomly without the inducing environment (which almost never happens for DNA mutations). The epiallele marked in purple is disadvantageous in a normal environment (leading to increased death; red crosses). In a stress environment (indicated by a thunder bolt), the unmodified allele (shown in grey) is disadvantageous. If the environment changes randomly from generation to generation, induced epivariation is unlikely to be advantageous. If there are longer episodes of stress, induced epivariation could be advantageous, and Darwinian selection might favor alleles that can become subject to induced epivariation. However, formalization is needed to determine the boundary conditions for such a scenario.