The molecular mechanisms of defensive-grade organic acid biosynthesis in ground beetles

Insects are known to synthesise and secrete hundreds of unique defensive chemicals, including caustic acids, pungent phenolics and citrusy terpenes. Despite efforts to characterise the defensive chemistry of ground beetles (Coleoptera: Carabidae), our knowledge of semiochemical evolution within the family and how these compounds are biosynthesised 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 characterise the defensive chemistry and generate defensive gland transcriptomes for ground beetle species representing two defensive chemical classes: the formic acid producer Platynus angustatus and the methacrylic acid producer Pterostichus moestus. Through comparative transcriptome analyses, 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 defence.