Alterations in the expression of certain midgut genes of Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) larvae and midgut histopathology in response to Bacillus thuringiensis Cry1C toxin

Bacillus thuringiensis (Bt) utilization as a biological control agent is highly recommended due to its safety, specificity, and efficiency. Importance of the entomocidal Cry proteins secreted by Bt is dramatically increased subsequent Cry genes transformation into a number of economic crops, rendering them protection against insect attack. In the last decade, insect resistance against transgenic Bt crops is gradually raised in several lepidopteran pests. A better understanding of the processing of Bt Cry1C toxin in the larval midgut of the lepidopteran pest species, the cotton leaf worm, Spodoptera littoralis (Boisd.), is very important to characterize the main regulatory elements of Bt tolerance. The present study aimed to define factors that are involved in insect tolerance toward Bt Cry1C through evaluating the mRNA level of trypsin (Try), aminopeptidase N (APN), alkaline phosphatase (ALP), cadherin (Cad), and cytochrome P450 (CYP) in both susceptible and cry1C tolerant strains of S. littoralis. Total RNAs were extracted from susceptible and tolerant strains to construct cDNAs. Quantitative real-time polymerase chain reaction (qPCR) showed a significant upregulation of CYP gene in tolerant strain. In contrast, the levels of expression of Try, ALP, and Cad were significantly downregulated in tolerant strain. APN relative mRNA expression did not show significant differences between susceptible and tolerant strains. Histologically, the midgut of late third-instar larvae of tolerant population S. littoralis showed vacuolization of the epithelium and disruption of both the peritrophic membrane and the striated boarder compared to the susceptible strain. Obtained data indicated a relationship between exposing to Bt Cry1C toxin and alteration of CYP, Try, ALP, and Cad expression in midgut of S. littoralis. These results may be an evidence for the important roles of CYP, Try, ALP, and Cad in the resistance development and toxicity to Bt Cry1C. The results are useful for further illustrating of Bt Cry1C processing and S. littoralis tolerance.