Institut de Biologie de l'Ecole Normale Supérieure (IBENS), Centre National de la Recherche Scientifique (CNRS), Institut National de la Santé et de la Recherche Médicale (INSERM), Ecole Normale Supérieure, PSL Research University, 75005 Paris, France.
Philos Trans R Soc Lond B Biol Sci. 2021 Jun 7;376(1826):20200123. doi: 10.1098/rstb.2020.0123. Epub 2021 Apr 19.
DNA provides the fundamental framework for heritability, yet heritable trait variation need not be completely 'hard-wired' into the DNA sequence. In plants, the epigenetic machinery that controls transposable element (TE) activity, and which includes DNA methylation, underpins most known cases of inherited trait variants that are independent of DNA sequence changes. Here, we review our current knowledge of the extent, mechanisms and potential adaptive contribution of epiallelic variation at TE-containing alleles in this group of species. For the purpose of this review, we focus mainly on DNA methylation, as it provides an easily quantifiable readout of such variation. The picture that emerges is complex. On the one hand, pronounced differences in DNA methylation at TE sequences can either occur spontaneously or be induced experimentally across the genome through genetic means. Many of these epivariants are stably inherited over multiple sexual generations, thus leading to transgenerational epigenetic inheritance. Functional consequences can be significant, yet they are typically of limited magnitude and although the same epivariants can be found in nature, the factors involved in their generation in this setting remain to be determined. On the other hand, moderate DNA methylation variation at TE-containing alleles can be reproducibly induced by the environment, again usually with mild effects, and most of this variation tends to be lost across generations. Based on these considerations, we argue that TE-containing alleles, rather than their inherited epiallelic variants, are the main targets of natural selection. Thus, we propose that the adaptive contribution of TE-associated epivariation, whether stable or not, lies predominantly in its capacity to modulate TE mobilization in response to the environment, hence providing hard-wired opportunities for the flexible exploration of the phenotypic space. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'
DNA 为遗传性提供了基本框架，但可遗传性特征的变化不一定完全“硬连接”到 DNA 序列中。在植物中，控制转座元件 (TE) 活性的表观遗传机制，包括 DNA 甲基化，是大多数已知的与 DNA 序列变化无关的可遗传特征变体的基础。在这里，我们回顾了我们目前对 TE 含等位基因中的表观等位基因变异的程度、机制和潜在适应性贡献的认识，在这组物种中。出于本综述的目的，我们主要关注 DNA 甲基化，因为它提供了这种变异的易于量化的读数。出现的情况很复杂。一方面，通过遗传手段，TE 序列中的 DNA 甲基化可以自发或实验诱导在整个基因组中发生显著差异。这些表观变体中的许多在多个有性世代中稳定遗传，从而导致跨代表观遗传遗传。功能后果可能是重大的，但通常是有限的，尽管在自然界中可以找到相同的表观变体，但在这种情况下涉及到它们产生的因素仍有待确定。另一方面，TE 含等位基因中的适度 DNA 甲基化变异可以通过环境重复诱导，通常效果也很温和，而且这种变异大多数都会在几代中丢失。基于这些考虑，我们认为 TE 含等位基因，而不是它们的遗传表观等位基因变体，是自然选择的主要目标。因此，我们提出，TE 相关的表观变异的适应性贡献，无论是稳定的还是不稳定的，主要在于其调节 TE 动员以响应环境的能力，从而为灵活探索表型空间提供了硬连线机会。本文是主题为“表观遗传学如何影响进化过程？”的特刊的一部分。
