RNA interference (RNAi) has become a major tool for the targeted inhibition of gene function in C. elegans . New observations continue to increase the utility and ease with which interference can be achieved. For example RNAi can now be introduced by feeding or soaking. Despite these breakthroughs there are still numerous questions. For example, what is the in-vivo function of RNAi? How does interference occur? How is RNA taken up and transmitted, and is the interference effect amplified? In order to address these questions we have begun screens for mutations that diminish RNAi. We first optimized a bacterial system that efficiently drives dsRNA expression for an essential gene (
pos-1 ). From an EMS screen of 100,000 F2 animals, 80 strains were found that were able to grow on the
pos-1 bacteria. 17 of 26 strains tested are also resistant to RNAi by injection. Two of these strains define one complementation group on LGV near
unc-42 . This locus has been tentatively named
rid-1 , for RNAi deficient. The
rid-1 mutants appear strongly resistant to RNAi for all genes tested and exhibit no other phenotype. In order to facilitate cloning of RNAi genes we next searched for transposon induced alleles. Surprisingly, we found that both the
mut-2 and
mut-7 high hopper strains are themselves resistant to RNAi, although the RNAi resistance of
mut-7 is primarily specific to maternal genes. RNAi resistance and the mutator activities co-map suggesting that they have a common genetic cause. In addition, several but not all, of the new mutants exhibit sterility and mild him phenotypes correlated with
mut-2 and
mut-7 strains. These findings raise the possibility that suppression of transposon mobilization may be an in-vivo function of RNAi. Alternatively, activation of transposons and loss of RNAi may be secondary to a more general defect in gene regulation. Another interesting possibility is that the transposons themselves (once mobilized) are actively suppressing RNAi in the high hopper strains. We will present our genetic analysis of the RNAi deficient loci and describe our efforts at cloning.