Structural Diversity of Adipokinetic Hormones in the Coleopteran Suborder Polyphaga (Excluding Cucujiformia)

Beetles are the largest animal group, in general. Phylogenetically, beetles belong to the order Coleoptera, the most species-rich of the Insecta. Coleoptera is divided into four suborders: Polyphaga, Adephaga, Archostemata, and Myxophaga. Specimens from the latter two are difficult to obtain, hence, we have focused our research into the adipokinetic hormone (AKH) peptide family on the former two suborders. Data on the Adephaga were concluded in 2017. The “core Polyphaga” consists of three series: Elateriformia, Staphyliniformia, and Cucujiformia; the latter was concluded in 2019. Here, we report on the AKH sequence(s) of 23 species of beetles from 4 families of Elateriformia, namely, the Buprestidae, Cantharidae, Elateridae, and Lampyridae; and 4 families of Staphyliniformia, namely, the Hydrophilidae, Silphidae, Lucanidae, and Scarabaeidae. Sequence elucidation by mass spectrometry or Edman degradation revealed 13 octapeptides: 5 are novel, 12 are beetle-specific and Schgr-AKH-II is produced in the basal Polyphaga (Elateriformia, in Hydrophilididae and Silphidae). Since Schgr-AKH-II is also found in Adephaga, this confirms the ancestral AKH of Coleoptera. The first change in sequence is recorded in Staphylinoidea with two different residues, notably, the switch from Phe⁴ to Tyr⁴. Duplication of AKH peptides is first seen in Lucanidae, as well as the appearance of atypical AKH sequences, such as Phe², Met⁴, Leu⁴, or Phe⁷ encountered in the Scarabaeoidea. The vast majority of the pest beetles do not have beetle-specific AKHs or share the same AKH as nonharmful beetles. Ideas for finding a lead compound for green insecticides are discussed.