Small RNAs in the Transgenerational Inheritance of Epigenetic Information

In recent years it has become evident that RNA interference-related mechanisms can mediate the deposition and transgenerational inheritance of specific chromatin modifications in a truly epigenetic fashion. Rapid progress has been made in identifying the RNAi effector proteins and how they work together to confer long-lasting epigenetic responses, and initial studies hint at potential physiological relevance of such regulation. In this review, we highlight mechanistic studies in model organisms that advance our understanding of how small RNAs trigger long-lasting epigenetic changes in gene expression and we discuss observations that lend support for the idea that small RNAs might participate in mechanisms that trigger epigenetic gene expression changes in response to environmental cues and the effects these could have on population adaptation. Small RNAs have long been known to be involved in specifying and stabilizing distinct chromatin states. Only recently has it become evident that small RNAs have the capacity to trigger epigenetic gene silencing.Once initiated by primary small RNAs, the repressed states can be propagated across multiple generations in different model organisms, commonly through the amplification of secondary small RNAs in stable feedback loops, while the primary small RNAs become dispensable.Small RNAs have also been demonstrated to direct the deposition of phenotypically plastic epigenetic marks, which mediate repression under specific conditions only.These observations lend support to the hypothesis that small RNAs might be involved in sensing environmental conditions and triggering epigenetic gene expression changes that may lead to increased population fitness of an organism that lives in a dynamic habitat.