Comparison of RNA Interference Genes Among Hessian Fly, Flour Beetle, Fruit Fly, Mosquito and Nematode

Many gall midges are significant pests in agro-ecosystems. Among these is the Hessian fly (Mayetiola destructor), which is a pest of wheat and frequently causes significant yield losses in wheat production areas worldwide.  One approach to discover the function of genes involved in pathogenicity is to perturb function by reverse genetic methods such as RNA interference (RNAi).  Caenorhabditis elegans and Tribolium castaneum exhibit robust systemic RNAi responses; however, the diptera Drosophila melanogaster, Anopheles gambiae and Hessian fly do not.  This difference is proposed to be due to a lack of core component genes for systemic RNAi in the dipteran genomes. To test our hypothesis we utilized the recently available Hessian fly genome sequence to identify homologs of genes involved in RNAi-mediated gene silencing and systemic RNAi response.  We compared these results with the core component genes of T. castaneum, D. melanogaster, A. gambiae and C. elegans. Comparisons were made at the amino acid level, with particular attention given to the conservation of protein motifs. In all five species, the environmental RNAi pathway was found to possess all genes needed for the endocytotic uptake of dsRNA. However, the PAZ domain of dicer-2 was highly variable in the diptera, which also lacked an integral membrane protein gene, sid-I, needed to propagate a response via the systemic RNAi pathway. Other core components displayed significant motif differences that may determine capacity to conduct systemic RNAi.