Opportunities offered by piRNA mediated gene silencing in pest control

RNAi is best known as a genetic tool that silences gene expression through introduction of a double-stranded RNA (dsRNA). It is used to control insects pests where ingestion dsRNAs targeted to essential genes leads to mortality. The effect of dsRNA is mediated by its conversion into small-interfering RNAs (siRNAs) which guide gene silencing factors to target mRNAs. In insects, siRNAs represent one of three RNAi mechanisms, and among them is the most evolutionarily labile. Therefore, siRNAs might not be an appropriate choice in some instances for pest control technology. As an alternative to siRNA-based technology we have explored piRNA-type RNAi. piRNAs were initially thought to be germline specific; however, studies have found piRNAs in the somatic tissues of most insects. piRNAs are Dicer independent, being derived from fragments of transcripts targeted by preexisting piRNAs. There are multiple benefits associated with piRNA-based gene silencing compared to siRNAs. One benefit is the homogenous structure of dsRNAs, which makes them vulnerable to gut dsRNases, can be avoided. With piRNA-technology, RNA molecules that could be designed around any structural motif. Another benefit is the trigger would be more species specific compared to siRNAs as they would exploit the identities of endogenous piRNAs. Also, piRNAs are longer than siRNAs, indicating the probability of a 26-30nt piRNA sequence occurring in a transcriptome is several orders of magnitude lower compared to a 21nt siRNA. Here we provide results from several problematic pest species examining the viability of this potentially superior approach to RNAi-based biopesticides.