Analysis of Gene Expression in Tenebrio molitor larvae Intoxicated by Bacillus thuringiensis Cry3Aa Toxin

The yellow mealworm, Tenebrio molitor, is sensitive to the coleopteran-specific Bacillus thuringiensis toxin Cry3Aa, but is insensitive to the lepidopteran-specific Cry1Ac.  Using paired-end sequencing, 237,000,000 reads were obtained from control, Cry1Ac-fed, or Cry3Aa-fed larvae.  SeqManNGen was used to assemble paired-end reads to a previous assembly, providing a new assembly of 14,307 contigs.  Sequences were analyzed by ArrayStar to identify statistically significant changes in gene expression in Cry3Aa-treated compared to control larvae.  Only 23 genes were differentially-expressed in Cry1Ac-challenged larvae, whereas 438 genes (428 specific to Cry3Aa) were differentially expressed in Cry3Aa-challenged larvae (>8-fold, >/=90%C.I.), reflecting the relative sensitivity of T. molitor larvae to these toxins.  Using Blast2GO, most differentially expressed genes are involved in metabolic and cellular processes, binding, and catalytic activity.  These changes are likely related to the cessation in feeding that occurs in intoxicated larvae, and induction of immune-like defenses observed in previous studies.  The most severely repressed transcripts (<0.02-fold) in Cry3Aa-intoxicated larvae included lipase, serine proteases, dipeptidyl protease, and hexamerin; the most highly increased transcripts (>50-fold) unfortunately included many unidentified or hypothetical proteins, but also serine and cysteine proteases, lysosomal proteins, lipase, and transport-related proteins.  A Fishers Exact Test (Blast2GO) indicated that GO functions related to serine proteases and serine protease inhibitors were the most significantly (FDR<0.05) enriched in the Cry3Aa dataset.  These data provide a snapshot of the dynamic intoxication process of Bt toxins in an insect, and suggest targets for RNAi or protease inhibitors to increase the sensitivity of T. molitor larvae to Cry3Aa toxin.