Plastic germline reprogramming of heritable small RNAs enables maintenance or erasure of epigenetic memories

In Caenorhabditis elegans small RNAs can regulate genes across generations. The mysterious tendency of heritable RNA interference (RNAi) responses to terminate after 3-5 generations has been referred to as "the bottleneck to RNAi inheritance." We have recently shown that the re-setting of epigenetic inheritance after 3-5 generations is not due to passive dilution of the original RNA trigger, but instead results from an active, multigenerational, and small RNA-mediated regulatory pathway. In this "Point of View" manuscript we suggest that the process that leads to the erasure of the ancestral small RNA-encoded memory is a specialized type of germline reprogramming mechanism, analogous to the processes that robustly remove parental DNA methylation and histone modifications early in development in different organisms. Traditionally, germline reprogramming mechanisms that re-set chromatin are thought to stand in the way of inheritance of memories of parental experiences. We found that reprogramming of heritable small RNAs takes multiple generations to complete, enabling long-term inheritance of small RNA responses. Moreover, the duration of this reprogramming process can be prolonged significantly if new heritable RNAi responses are provoked. A dedicated signaling pathway that is responsive to environmental cues can tune the epigenetic state of the RNAi inheritance system, so that inheritance of particular small RNA species can be extended.