The introduction of dsRNAs into C. elegans triggers sequence-specific gene silencing (RNAi) that can spread between cells and into the progeny. RNAi can be triggered externally by soaking the worms in a solution containing bacteria expressing dsRNAs or by feeding them with such bacteria. In contrast to C. elegans, C. briggsae and C. remanei are naturally insensitive to external RNAi. In the Hunter lab, a C. elegans mutant,
sid-2, was isolated, which is defective in RNAi through externally administered dsRNAs.
sid-2 encodes a transmembrane protein, localized at the luminal membrane of intestinal cells and required for the uptake of dsRNAs. They were also able to show that the C. briggsae insensitivity to external RNAi can be rescued by transgenesis with
Cel-sid-2 (Winston et al, 2007), but it is unclear whether this transgenic approach can be applicable to other species. We and others have recently isolated new Caenorhabditis species from rotting fruits and flowers, so we tested their sensitivity upon external RNAi. We targeted actin, and found that some species are naturally sensitive to RNAi by feeding, whereas others are not. We mapped our results onto the phylogenetic tree of the Caenorhabditis genus (see abstract by Kiontke et al.) and found that loss and/or gain of this feature in the Caenorhabditis genus is complex. Interestingly, we identified one case, C. angaria, which is insensitive to RNAi targeting actin but sensitive to external RNAi targeting C. ang. RNA polymerase II gene. We also found germ line-restricted sensitivity to RNAi in the C. el. JU1580 strain, naturally infected by a nodavirus, perhaps due to the defect of somatic RNAi pathway in this wild isolate (Felix et al, 2011). Finally, we succeeded in rendering C. remanei sensitive to external RNAi after transgenesis with
Cel-sid-2, similarly to the experiment performed on C. briggsae by Winston et al, indicating that this approach can be used for sensitizing species to external RNAi. Overall, our results provide the first broad investigation of sensitivity to external RNAi within the Caenorhabditis genus, and can be valuable for designing reverse genetic experiments in these new species. We propose that the sensitivity to external RNAi is evolving fast within the Caenorhabditis genus, and this may be relevant to adaptation of nematode defense against viruses.