Effect of Bacillus thuringiensis Cry3Aa intoxication on the expression of cysteine and serine peptidase transcripts in the midgut of the yellow mealworm, Tenebrio molitor

The yellow mealworm, Tenebrio molitor, is a stored-product pest found in stored grain products and is sensitive to the coleopteran-specific Cry3Aa toxin from Bacillus thuringiensis(Bt). Insect digestive peptidases catalyze the processing of Cry protoxins to toxins and can be a determining factor in toxicity and resistance. Therefore, we evaluated the effect of Cry3Aa toxin on the expression of cysteine and serine midgut peptidases genes in T. molitor larvae. High-throughput sequencing was used to obtain EST databases from the midguts of one month old T. molitor larvae fed 24 h with either a control diet or diet containing 0.1% Cry3Aa. Sequences encoding midgut cysteine peptidases from the C1 family were similar to cathepsin B, cathepsin L, and possibly cathepsin K. The expression of cysteine cathepsins was decreased overall in Bt-treated larvae, although one cathepsin L was found only in the treated animals, and the expression of cathepsin K was 1.7-fold enhanced. Serine peptidases from the S1 family were represented by presumably active peptidases and homologs (SPH) lacking functional amino acids. Predicted serine peptidases included trypsins, chymotrypsins, and elastases. Many serine peptidases and SPH in the control were not found in the Bt-treated larvae, and a major trypsin and chymotrypsin also were downregulated in treated larvae. Serine peptidases common to both groups overall were expressed 1.5-10-fold lower in Bt-treated larvae. The data are the first application of high-throughput sequencing to the study of Bt intoxication and demonstrate that Cry3Aa intoxication in T. molitor induces widespread changes in peptidase gene expression.