Elucidating the principles that underlie the inheritance of non-genetic traits

Inheritance of acquired traits is a subject of fascination and controversy. Accumulating evidence suggest that information can pass from the soma to the germline, bypassing the “Weismann barrier”. Planarians are capable of reproducing asexually thereby blurring the distinction between parent and progeny. Asexual reproduction helps in overcoming the restrictions that are associated with the transfer of information from soma to germline. RNAi is routinely used to silence planarian genes systemically, but the mechanism of gene silencing is poorly understood. Adult planarian stem cells are the only dividing planarian cells, and therefore are the most likely candidate for storing and disseminating non-genetic information. Here, we studied how non-genetic gene silencing is disseminated across cells and asexually reproducing animals. Through transplantation of wildtype planarian tissues onto irradiated animals treated with RNAi, we demonstrated that non-genetic information is transmitted to stem cells from either differentiated cells or the extracellular environment. The exogenous RNAi signal decayed exponentially, which strongly suggested that dsRNA is transferred to untreated stem cells. This transfer of dsRNA could potentially be required for stable gene silencing. Our research uncovered mechanisms of non-genetic information in planarians, which has profound implications on understanding the maintenance of information in the tissue in regenerative systems.