Characterization of RNA interference in the model cnidarian Nematostella vectensis reveals partial target silencing but lack of small RNA amplification

RNA interference (RNAi) is an antiviral mechanism based on processing of viral double-stranded RNA (dsRNA) into short interfering RNAs (siRNAs) leading to dsRNA destruction. It is considered as an ancestral mechanism which invertebrates rely on for defense against viruses, while vertebrates have evolved instead the interferon pathway. Recent studies showed that sea anemones, members of the basally-branching phylum Cnidaria, might possess an antiviral response with more vertebrate characteristics than previously thought; however, it is unknown whether cnidarians also employ RNAi as an immune response similarly to nematodes and insects. In this study we characterize the antiviral RNAi response in the model cnidarian Nematostella vectensis . We injected dsRNA with GFP sequence to GFP-expressing transgenic zygotes and show that siRNAs mapping to the GFP sequence are generated and induce a moderate but significant knockdown of GFP expression. In addition, we find no evidence of secondary siRNA generation, despite their crucial role in the amplification of antiviral response in other organisms. Notably, RNAi pathway components are specifically upregulated upon dsRNA injection while microRNA pathway components are downregulated. Overall, our findings reveal that N. vectensis possesses an antiviral RNAi system with no secondary amplification, suggesting that it serves only as a short-term antiviral mechanism.