Li et al. recently reported how systemic RNAi occurred in the western corn rootworm (WCR), Diabrotica virgifera virgifera, one of the most important maize pests in North America and a model for understanding the mechanism of RNAi.
Efficiency of RNA interference (RNAi) varies among insect species due to the several factors: double-stranded RNA (dsRNA) degradation by endonuclease (dsRNase), dsRNA accumulation in endosomes, duplication of functional RNAi machinery genes, systemic RNAi possibility, presence of SID1 (Systemic RNA interference defective protein 1) like proteins, etc. In general, RNAi works well and is systemic in beetles.
Systemic RNAi in the worm, Caenorhabditis elegans, results from two remarkable features: RdRp (RNA-dependent RNA polymerase) and SID1 transport. RdRp is involved in an amplification step of RNAi by synthesizing siRNAs complementary to sequences upstream or downstream from the initial trigger region in the target mRNA. SID1 functions in transferring those amplified and original signals to silence gene expression from one tissue to other tissues. In insects, functional SID1-like proteins have been identified in the Colorado potato beetle, Leptinotarsa decemilnate, but no RdRp has been found in any species. Moreover, earlier studies proved that transitive and systemic aspects of RNAi found in C. elegans are not conserved in Drosophila. However, this seems to be the case coleopteran insects.
Li et al. microscopically imaged the systemic spread of RNAi response using high-sensitivity branched DNA in situ hybridization to demonstrate systemic RNAi in WCR. The mapped small RNAs isolated from WCR larvae and adults fed v-ATPase C dsRNAs were investigated whether transitive RNAi occurs in WCR. (‘transitivity’ refers to the spreading of silencing beyond the initial target sequence). The results indicated that all the siRNAs derived from the WCR did not involve extending to either the 5’ or 3’ border sequences. Li et al concluded that there is no RdRp-like activity in WCR.
Li et al also tested whether secondary siRNA production or transitive RNAi was present in WCR. The siRNA sequence profile analysis was performed to WCR fed with nucleotide mismatched dsRNA targeting v-ATPase C gene. The conclusion was that transitive RNAi was not detected in WCR after ingestion of the dsRNAs but systemic RNAi may be derived from the original dsRNA molecules taken up from the gut lumen instead of being caused by transitive pathways.
This study suggests that the initial dose of dsRNA is important factor for triggering the systemic RNAi for WCR control and protection of transgenic maize plants from WCR feeding damage. Furthermore, if we consider the other factors such as the existence of dsRNase in insects’ guts and hemolymph as a negative factor for dsRNAs delivery inside the insects, the initial dsRNA dose is crucial for a lethal systemic RNAi response to control insects.
Li, H. , Bowling, A. J., Gandra, P. , Rangasamy, M. , Pence, H. E., McEwan, R. E., Khajuria, C. , Siegfried, B. D. and Narva, K. E. (2018), Systemic RNAi in western corn rootworm, Diabrotica virgifera virgifera, does not involve transitive pathways. Insect Science, 25: 45-56. doi:10.1111/1744-7917.12382