Analysis of the Toll and Spaetzle Genes Involved in Toll Pathway-Dependent Antimicrobial Gene Induction in the Red Flour Beetle, Tribolium castaneum (Coleoptera; Tenebrionidae).

Insects rely only on their innate immune system to protect themselves from pathogens. Antimicrobial peptide (AMP) production is the main immune reaction in insects. In , the reaction is regulated mainly by the Toll and immune deficiency (IMD) pathways. Spaetzle proteins, activated by immune signals from upstream components, bind to Toll proteins, thus, activating the Toll pathway, which in turn, induces AMP genes. Previous studies have shown the difference in immune systems related to Toll and IMD pathways between and . In , nine Toll and seven spaetzle (spz) genes were identified. To extend our understanding of AMP production by , we conducted functional assays of Toll and spaetzle genes related to Toll-pathway-dependent AMP gene expression in under challenge with bacteria or budding yeast. The results revealed that and double-knockdown and knockdown strongly and moderately reduced the Toll-pathway-dependent expression of AMP genes, respectively. Moreover, and double-knockdown pupae more rapidly succumbed to entomopathogenic bacteria than the control pupae, but knockdown pupae did not. The results suggest that and play a large role in Toll-pathway-dependent immune reactions, whereas plays a small part.