Pelle and Tube contribute to the Toll pathway-dependent antimicrobial peptide production in the red flour beetle, Tribolium castaneum (Coleoptera: Tenebrionidae)

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Abstract
Insects are solely dependent on an innate immune system. Antimicrobial peptide production is the main immune response of insects. The molecular mechanisms underlying this reaction in Drosophila melanogaster involves the induction of antimicrobial peptide genes, which is regulated by the Toll and IMD pathways. The Toll pathway is mainly activated by fungi or Gram-positive bacteria and the IMD pathway by Gram-negative bacteria. In terms of comparative immunology, we investigated the antimicrobial peptide production system in the beetle, Tribolium castaneum, which differs from that in D. melanogaster. To obtain a more detailed understanding, we examined whether Pelle and Tube, orthologues of which in D. melanogaster are the Toll pathway components, contributed to antimicrobial peptide production and immune reactions. These two genes were not induced by challenges from any type of microbe, which in this case were Gram-positive bacteria, -negative bacteria and an eukaryote. Using Pelle and Tube knockdown pupae, it was demonstrated that Pelle and Tube are involved in the induction of Cec2 as a representative Toll pathway-dependent gene in T. castaneum by Gram-positive and -negative bacteria and eukaryote challenges. Furthermore, neither Pelle nor Tube contributed to immune defences against two entomopathogenic bacteria. These results, taken together with our previous findings, led to the conclusion that the Toll pathway immune signaling reported in D. melanogaster indeed occurs in T. castaneum, and the gene sets involved in Toll signal transduction in T. castaneum did not differ significantly from those in D. melanogaster, but transduced immune signals to challenges from Gram-positive bacteria, -negative bacteria and an eukaryote, which differed from those in D. melanogaster.

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