Induced CTL-S15 gene expression by Bacillus thuringiensis declines susceptibility in Spodoptera exigua

It has been reported that C-type lectins (CTLs), which are pattern recognition receptors of the insect innate immunity response, may compete with Cry toxin for the receptor alkaline phosphatase to decrease its toxicity in insects. However, to date, which CTLs affect larval susceptibility to Bt in Spodoptera exigua is not clear. In this study, thirty-three CTL genes were identified from S. exigua. Based on the number of carbohydrate-recognition domains (CRDs) and the domain architectures, they were classified into three groups: (1) nineteen CTL-S (single-CRD), (2) eight immulectin (dual-CRD) and (3) six CTL-X (CRD with other domains). RT-qPCR analysis revealed that expression levels of SeCTL-S15, IML-4 and CTL-X6 were upregulated after challenge with Bt and Cry1Ab. Tissue and developmental stage expression analysis showed that only SeCTL-S15was mainly expressed in the midgut and larva, respectively. Knockdown of SeCTL-S15 significantly increased Bt susceptibility, as indicated by reduced survival and larval weight. These results suggest that CTL-S15 might play a vital role in the low susceptibility of larvae to Bt in S. exigua. Our results provide new insights into CTL function in insects.