Department of Entomology, The Pennsylvania State University, 501 ASI Building, University Park, PA, 16802, USA.
Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, NJ, 07030, USA.
J Chem Ecol. 2021 Mar;47(3):334-349. doi: 10.1007/s10886-021-01253-2. Epub 2021 Mar 10.
Of the approximately one million described insect species, ground beetles (Coleoptera: Carabidae) have long captivated the attention of evolutionary biologists due to the diversity of defensive compounds they synthesize. Produced using defensive glands in the abdomen, ground beetle chemicals represent over 250 compounds including predator-deterring formic acid, which has evolved as a defensive strategy at least three times across Insecta. Despite being a widespread method of defense, formic acid biosynthesis is poorly understood in insects. Previous studies have suggested that the folate cycle of one-carbon (C1) metabolism, a pathway involved in nucleotide biosynthesis, may play a key role in defensive-grade formic acid production in ants. Here, we report on the defensive gland transcriptome of the formic acid-producing ground beetle Harpalus pensylvanicus. The full suite of genes involved in the folate cycle of C1 metabolism are significantly differentially expressed in the defensive glands of H. pensylvanicus when compared to gene expression profiles in the rest of the body. We also find support for two additional pathways potentially involved in the biosynthesis of defensive-grade formic acid, the kynurenine pathway and the methionine salvage cycle. Additionally, we have found an array of differentially expressed genes in the secretory lobes involved in the biosynthesis and transport of cofactors necessary for formic acid biosynthesis, as well as genes presumably involved in the detoxification of secondary metabolites including formic acid. We also provide insight into the evolution of the predominant gene family involved in the folate cycle (MTHFD) and suggest that high expression of folate cycle genes rather than gene duplication and/or neofunctionalization may be more important for defensive-grade formic acid biosynthesis in H. pensylvanicus. This provides the first evidence in Coleoptera and one of a few examples in Insecta of a primary metabolic process being co-opted for defensive chemical biosynthesis. Our results shed light on potential mechanisms of formic acid biosynthesis in the defensive glands of a ground beetle and provide a foundation for further studies into the evolution of formic acid-based chemical defense strategies in insects.
在已描述的约一百万种昆虫物种中，步甲科甲虫（鞘翅目：步甲科）因其合成的防御化合物的多样性而长期吸引着进化生物学家的注意。这些化合物是通过腹部的防御腺产生的，代表了超过 250 种化合物，包括具有防御作用的甲酸，它在昆虫纲中至少进化了三次作为防御策略。尽管甲酸生物合成是一种广泛存在的防御方法，但在昆虫中对此知之甚少。先前的研究表明，一碳（C1）代谢的叶酸循环，即核苷酸生物合成途径，可能在蚂蚁中防御级甲酸产生中发挥关键作用。在这里，我们报告了产生甲酸的步甲 Harpalus pensylvanicus 的防御腺转录组。与身体其他部位的基因表达谱相比，C1 代谢叶酸循环中涉及的全套基因在 Harpalus pensylvanicus 的防御腺中显著差异表达。我们还发现了另外两种可能参与防御级甲酸生物合成的途径，即犬尿氨酸途径和蛋氨酸 salvage 循环。此外，我们还发现了一组在分泌叶中差异表达的基因，这些基因参与了甲酸生物合成所需的辅助因子的生物合成和运输，以及可能参与包括甲酸在内的次生代谢物解毒的基因。我们还深入了解了参与叶酸循环的主要基因家族（MTHFD）的进化，并提出在 Harpalus pensylvanicus 中，叶酸循环基因的高表达而不是基因复制和/或新功能化可能对防御级甲酸生物合成更为重要。这是鞘翅目昆虫中首次发现主要代谢过程被用于防御性化学合成的证据，也是昆虫纲中少数几个这样的例子之一。我们的研究结果阐明了步甲防御腺中甲酸生物合成的潜在机制，并为进一步研究昆虫甲酸基化学防御策略的进化提供了基础。
