RNA interference and dietary inhibitors induce a similar compensation response in Tribolium castaneum larvae

Tribolium castaneum is a major agriculture pest damaging stored grains and cereal products. The T. castaneum genome contains 26 cysteine peptidase genes, mostly cathepsins L and B, and seven have a major role in digestion. We targeted the expression of the most highly expressed cathepsin L gene on chromosome 10, TC011001, by RNA interference (RNAi), using double-stranded RNA (dsRNA) constructs of different regions of the gene (30, middle, 50 and entire coding regions). RNA sequencing and quantitation (RNA-seq) was used to evaluate knockdown and specificity amongst the treatments. Overall, target gene expression decreased in all treatment groups, but was more severe and specific in dsRNA targeting the 30 and entire coding regions, encoding the proteolytic active site in the enzyme. Additional cysteine cathepsin genes also were down-regulated (off-target effects), but some were up-regulated in response to RNAi treatment. Notably, some serine peptidase genes were increased in expression, especially in dsRNA targeting 50 and middle regions, and the response was similar to the effects of dietary cysteine protease inhibitors. We manually annotated these serine peptidase genes to gain insight into function and relevance to the RNAi study. The data indicate that T. castaneum larvae compensate for the loss of digestive peptidase activity in the larval gut, regardless of the mechanism of disruption.