Nucleus-Independent Transgenerational Small RNA Inheritance in C. elegans

Studies using C. elegans nematodes demonstrated that, against the dogma, animals can transmit epigenetic information transgenerationally. While it is now clear that in these worms ancestral RNA interference (RNAi) responses continue to regulate gene expression for many generations, it is still debated whether the primary agent that perpetuates heritable silencing is RNA or chromatin, and whether the information is communicated to the next generation inside or outside of the nucleus. Here we take advantage of the tractability of gene-specific double stranded RNA-induced heritable silencing to answer these questions. We demonstrate that RNAi can be inherited independently of any changes to the chromatin or any other nuclear factors via mothers that are genetically engineered to transmit only their ooplasm but not the oocytes nuclei to the next generation. Nucleus-independent RNA inheritance depends on ZNFX-1, an RNA-binding germ granule resident protein. We find that upon manipulation of normal germ granules functions (in pptr-1 mutants) nucleus-independent RNA inheritance becomes stronger, and can occur even in znfx-1 mutants. Utilizing RNA sequencing, chimeric worms, and sequence polymorphism between different C. elegans isolates, we identify endogenous small RNAs which, similarly to exogenous siRNAs, are inherited in a nucleus-independent manner. From an historical perspective, nucleus-independent inheritance of small RNAs might be regarded as partial vindication of discredited cytoplasmic inheritance theories from the 19th century, such as Darwins "pangenesis" theory.