In C. elegans, RNA-dependent RNA polymerase (RdRP) amplifies the primary siRNA forming secondary dsRNAs that feed back into the front end of the RNAi pathway ( Sijen et al., IDH tumor 2001). However, core RNAi machineries for siRNA production are not involved in systemic spreading of RNAi, and siRNA amplification is not necessary for the systemic RNAi effect ( Tomoyasu et al., 2008). In general, the core RNAi machineries are conserved among all insects species examined, while RdRP homologs have never been identified, even in those showing robust systemic

RNAi ( Tomoyasu et al., 2008). Nonetheless, RdRP-like activity via alternative enzymes has been reported in Drosophila cells ( Lipardi et al., 2005). SID-1 is a dsRNA-selective dsRNA-gated channel (Shih and Hunter, 2011) and its role in dsRNA uptake is the key to systemic spreading of RNAi in C. elegans. In insects, the presence of SID-1-like

(SIL) proteins appears to vary phylogenetically, e.g., being notably absent in Dipterans ( Tomoyasu et al., 2008). However, several studies cast doubts on their roles in dsRNA uptake. First, sensitivity to RNAi is not always associated with the presence of sil. For instance, the silkmoth Bombyx mori Linnaeus possesses three sid-1 orthologs but is not susceptible to experimental RNAi. However, when ectopically expressed in Bombyx cells, C. elegans SID-1 could aid dsRNA uptake thereby greatly enhance the cells’ sensitivity Epigenetic inhibitor to RNAi ( Kobayashi et al., 2012). Second, for those species with robust systemic RNAi that also possess sils, the sils are actually dispensable with Decitabine cell line regard to the RNAi effect ( Luo et al., 2012; Tomoyasu et al., 2008). Further, insect sils appear to share more similarity in sequence with C. elegans tag-130, which is not involved in RNAi ( Tomoyasu et al., 2008). Therefore, insects amenable to systemic RNAi must possess alternative mechanism(s) for the systemic spreading of RNAi signal. In insects other than D. melanogaster, research on RNAi has largely focused on the non-cell autonomous (environmental and

systemic) RNAi response. Until recently, most investigations of RNAi in insects have involved delivery of in vitro synthesized dsRNAs into embryos or the hemocoel by microinjection. This method of dsRNA delivery has provided a powerful reverse genetic tool for investigating gene function in species lacking well developed genetics as well as a means to evaluate the relative sensitivity of a given species to systemic RNAi. However, microinjection is obviously not a useful means to deliver dsRNA for pest control. The potential utility of RNAi for insect pest control was suggested by two studies published in 2006 demonstrating that RNAi can be elicited in insects by oral administration of dsRNA (Araujo et al., 2006; Turner et al., 2006).