Genome-wide identification and characterization of basic helix-loop-helix transcription factors in Spodoptera litura upon pathogen infection.

Basic helix-loop-helix (bHLH) transcription factors play an important role in a wide range of metabolic and developmental processes in eukaryotes, and bHLH proteins also participate in immune responses, especially in plants. However, their roles in insects upon entomopathogen infection are unknown. In this study, 54 bHLH genes in 41 families were identified in a polyphagous pest, Spodoptera litura, including a new bHLH gene in group B, which is specifically present in Lepidoptera and was thus named Lep. The conserved amino acids in the bHLH domain, structural architecture, and chromosomal distribution of bHLH genes in S. litura were analyzed. The bHLH genes in Plutella xylostella and Apis mellifera were also updated, and genome-wide comparison and phylogenetic analysis of bHLH members in 5 holometabolous insects were performed. The expression profiles of S. litura bHLH (SlbHLH) genes in 3 tissues at different developmental stages and their responses to S. litura nucleopolyhedrovirus (SpltNPV), Nomuraea rileyi (Nr), and Bacillus thuringiensis (Bt) infection were investigated. More SlbHLHs in group B were expressed and differentially expressed during pathogen infections, and SlbHLHs tended to be downregulated in the midgut of S. litura larvae after B. thuringiensis treatment. Our study provides an overview of bHLH family members in S. litura and their responses to different pathogens used for pest biocontrol. These findings on bHLH members may contribute to uncovering the mechanism of host-pathogen interaction.