Localized efficacy of environmental RNAi in Tetranychus urticae

Environmental RNAi has been developed as a tool for reverse genetics studies and is an emerging pest control strategy. The ability of environmental RNAi to efficiently down- regulate the expression of endogenous gene targets assumes efficient uptake of dsRNA and its processing. In addition, its efficiency can be augmented by the systemic spread of RNAi signals. Environmental RNAi is now a well-established tool for the manipulation of gene expression in the chelicerate acari, the two-spotted spider mite, Tetranychus urticae. Even though environmental RNAi is efficient in inducing phenotypic changes in treated mites and reducing their fitness, it has a limited ability to decrease the expression of constitutively- expressed target genes. To understand the discrepancy between these RNAi effects, we investigated the spatial range of the effectiveness of environmental RNAi. Using dsRNAs that target eight constitutively-expressed genes encoding proteins with essential cellular functions, we examined the spatial distribution of cells that displayed histological changes associated with the environmental RNAi. We show that regardless of the target gene tested, the observed phenotypes were restricted to cells that are in direct contact with the dsRNA- containing gut lumen. In contrast, cells that are not the primary recipients of dsRNA, but are adjacent to the gut, appear phenotypically normal, indicating that environmental RNAi has localized efficiency in T. urticae.