The molecular mechanisms of organic acid biosynthesis in ground beetle defensive glands

Ground beetles (Carabidae) are known to synthesize and secrete hundreds of unique defensive chemicals, including caustic acids, pungent phenolics, and citrusy terpenes. Despite efforts to characterize the defensive chemistry of ground beetles, our knowledge of semiochemical evolution within the family and how these compounds are biosynthesized remains limited. Few studies have demonstrated the likely biosynthetic precursors of select compounds in certain taxa, and only one has demonstrated which genes may be involved in the biosynthesis of formic acid. Here, we aim to expand on previous work by characterizing the defensive chemistry and reporting on the defensive gland transcriptomes of three ground beetle species spanning two defensive chemical classes: the formic acid producers Harpalus pensylvanicus and Platynus angustatus and the methacrylic acid producer Pterostichus moestus. We demonstrate that co-option of distinct primary metabolic pathways may be involved in formic acid and methacrylic acid biosynthesis in the defensive glands of these taxa. These results expand our knowledge of ground beetle defensive chemistry and provide additional evidence that co-option of existing primary metabolic pathways plays a major role in the evolution of ground beetle chemical defense.