Peritrophins are involved in the defense against Bacillus thuringiensis and nucleopolyhedrovirus formulations in Spodoptera littoralis (Lepidoptera: Noctuidae)

The peritrophic matrix (or peritrophic membrane, PM) is present in most insects where it acts as a barrier to mechanical insults and pathogens, as well as a facilitator of digestive processes. The PM is formed by the binding of structural PM proteins, referred to as peritrophins, to chitin fibrils and spans the entire midgut in lepidopterans. To investigate the role of peritrophins in a highly polyphagous lepidopteran pest, namely the cotton leafworm (Spodoptera littoralis), we generated Insect Intestinal Mucin (IIM⁻) and non-mucin Peritrophin (PER⁻) mutant strains via CRISPR/Cas9 mutagenesis. Both strains exhibited deformed PMs and retarded developmental rates. Bioassays conducted with Bacillus thuringiensis (Bt) and nucleopolyhedrovirus (SpliNPV) formulations showed that both the IIM⁻ and PER⁻ mutant larvae were more susceptible to these bioinsecticides compared to the wild-type (WT) larvae with intact PM. Interestingly, the provision of chitin-binding agent Calcofluor (CF) in the diet lowered the toxicity of Bt formulations in both WT and IIM⁻ larvae and the protective effect of CF was significantly lower in PER⁻ larvae. This suggested that the interaction of CF with PER is responsible for Bt resistance mediated by CF. In contrast, the provision of CF caused increased susceptibility to SpliNPV in both mutants and WT larvae. The study showed the importance of peritrophins in the defense against pathogens in S. littoralis and revealed novel insights into CF-mediated resistance to Cry toxin.