Identification of initial responsive genes to systemic dsRNA ingestion in the two-spotted spider mite, Tetranychus urticae Koch

Abstract

Ingestion of double-stranded RNA (dsRNA) is known to induce RNA interference (RNAi) in the two-spotted spider mite, Tetranychus urticae, but little is known about the underlying molecular processes. To better understand the molecular mechanisms involved in ingestion-mediated RNAi, we compared transcriptomic profiles to identify genes that respond to the T. urticae-specific (dsCOPB2) or -non-specific (dsEGFP) dsRNA. In total, 450 and 296 genes were differentially expressed in response to dsCOPB2 and dsEGFP, respectively (P<0.05). Among them, 180 genes commonly responded to both dsCOPB2 and dsEGFP, and their expression patterns were highly correlated between the two dsRNAs (r=0.892, P<0.001). Gene ontology analysis revealed that proteins associated with ‘metabolic process’ (76.2%), ‘catalytic activity’ (64.9%), and ‘membrane’ (63%) occupied the highest proportion within the categories of ‘biological process,’ ‘molecular function,’ and ‘cellular process’, respectively. The proportions of down-regulated genes were 2.2- and 2.5-fold greater than those of up-regulated genes in the categories of ‘cellular process’ and ‘molecular function’, respectively, whereas the proportion of up-regulated genes was 1.7-fold greater in the cellular component category. Interestingly, the genes that were the most highly differentially expressed following dsRNA ingestion were found to encode metabolic proteins with various catalytic functions, such as cathepsin, intradiol ring-cleavage dioxygenase, carboxylesterase, serine protease, and UDP-glycosyltransferase. Genes associated with dsRNA uptake and RNAi machinery were only rarely identified, suggesting that genes associated with dsRNA uptake and the RNAi process may not be actively engaged at 36h post-treatment, the observation time in this study. Our results suggest that metabolic enzymes possessing various types of catalytic activity appear to function before the activation of RNAi machinery in T. urticae.