The epigenetic silencing phenomenon in Caenorhabditis elegans , RNA interference (RNAi), is a sequence-specific posttranscriptional gene silencing mechanism. RNAi is triggered by the introduction of dsRNA into worms and the silencing effects of RNA1 are observable in the worms exposed to dsRNA as well as in their progeny. Under most circumstances, the altered phenotype resulting from RNAi is lost at the F2 generation when the original phenotype is restored. Although a few studies have described the transmissibility of RNAi to the F2 generation and beyond, a systematic study of this phenomenon has been limited by both the assays for genetic silencing (which in some cases were either destructive or highly selective of a unique subpopulation) and by limited ability to control the nature of the RNA trigger.

We have developed an assay were we can observe the heritable RNAi (hRNAi) effect over many generations by using a gain of function allele of oma-1 (oocyte maturation defective). oma-1 is a non-essential germ line gene initially characterized and kindly provided to us by Rueyling Lin. By using this oma-1 assay, we have shown that there is a heritable but meiotically unstable RNAi effect for up to 20 generations. This has enabled us to study the heritable character of RNAi and its loss. We have investigated the character of hRNAi under numerous different experimental protocols and have established a clear dose response relationship between the triggering RNA and the resulting silencing effect. Using this assay we have also carried out a detailed analysis of, the molecular character of the trigger, determining that small RNAs as well as larger dsRNAs can induce a heritable effect. Further characterization, of the genetic linkage of hRNAi effect to the target locus, of genetic requirements for hRNAi, and of functional requirements for the hRNAi trigger lead us to a model in which at least two different mechanisms contribute to heritability. One such mechanism appears similar to classic RNAi (with many of the same genetic and molecular requirements) while the second mechanism appears to involve a novel set of characteristics.