Ecology and evolution of pyrazines in insects

Chemical communication is the oldest and most widespread form of signalling among and within organisms. Among the many compounds involved in such communication, pyrazines - nitrogen-containing heterocyclic molecules - are especially intriguing due to their widespread occurrence across the tree of life, from bacteria and fungi to insects and mammals. In this review, we focus on the role of pyrazines in insects, where they mediate both intra- and interspecific communication. Social insects, particularly within the order Hymenoptera, exhibit a greater diversity and frequency of pyrazine use compared to solitary species and other insect orders. This diversity may be associated with the complexity of communication systems required by eusociality. Pheromonal pyrazines predominantly feature alkyl and alkenyl group substituents, whereas allomonal pyrazines more often feature methoxy groups. Pyrazines have been identified in seven insect orders. Hemimetabolous insects, such as Phasmatodea, Orthoptera, and Hemiptera typically produce alkyl-substituted pyrazines, with some Hemiptera also producing methoxy variants. Methoxy-substituted pyrazines are absent in Hymenoptera but present in Coleoptera and Lepidoptera, where they serve as both pheromones and allomones. In Diptera, pyrazines are only known from a few species, and have alkyl or alkenyl substituents. Pyrazines are mainly associated with adult stages, suggesting a predominant role in later-life communication but more research is needed on early life stages. Current evidence suggests that pyrazine biosynthesis may be carried out by microbial symbionts. To understand fully the evolutionary origins and ecological functions of pyrazines in insects, comprehensive surveys across taxa and life stages alongside functional studies are essential.