Defensive Chemical Evolution in the Carabidae with an Emphasis on Formic Acid, Methacrylic Acid, and Benzoquinone-producing Taxa
Open Access
- Author:
- Rork, Adam
- Graduate Program:
- Entomology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 07, 2021
- Committee Members:
- Shaun Mahony, Outside Unit & Field Member
Julie Urban, Major Field Member
Etya Amsalem, Major Field Member
Tanya Renner, Chair & Dissertation Advisor
Gary Felton, Program Head/Chair - Keywords:
- Carabidae
Semiochemistry
Chemical defense
Morphology
Evolution
Exocrine glands - Abstract:
- Of the approximately one million described insect species, ground beetles (Coleoptera: Carabidae) have long captivated the attention of evolutionary biologists due to their taxonomic diversity and array of defensive compounds secreted. Indeed, over 250 compounds have been documented from the defensive abdominal glands of ground beetles. This array of defensive strategies across the family make them ideal for understanding the evolution of insect chemical defense systems. Despite their fascinating array of defensive compounds, there have been no investigations into the evolution of ground beetle pygidial glands. My overarching goal is to understand the underlying genetics, biochemistry, and evolution of ground beetle chemical defense as well as morphological adaptations of the glands to storing and secreting cytotoxic compounds. To do this, I am studying six ground beetles from three subfamilies: Harpalus pensylvanicus and Platynus angustatus are formic acid producers, Pterostichus moestus is a methacrylic acid producer, and the bombardier beetles Brachinus elongatulus and Pheropsophus jessoensis are benzoquinone producers from the subfamily Brachininae while Goniotropis kuntzeni is a benzoquinone producer from the subfamily Paussinae. My research is driven by the following hypotheses: 1) Formic acid-producing ground beetles have co-opted the folate cycle of C1 metabolism for defensive formic acid biosynthesis. 2) Methacrylic acid-producing ground beetles have co-opted the valine catabolic pathway for defensive methacrylic acid biosynthesis. 3) Benzoquinone-producing ground beetles have co-opted a suite of oxidoreductase genes for the oxidation of phenolics, ultimately leading to benzoquinone biosynthesis. My research is also driven by an exploratory interest in identifying potential morphological adaptations in the pygidial glands to defensive chemical resistance. To address these hypotheses, RNA-Seq data were generated for each species’ defensive glands and glandless bodies. Differential gene expression analyses were conducted between tissues, and significantly upregulated genes flagged as candidates for defensive chemical biosynthesis based on functional annotations. Isotopic precursors were also fed to select taxa and defensive chemicals harvested for gas chromatography-mass spectroscopy for confirmation of precursor incorporation. Our results show that genes involved in the folate cycle of C1 metabolism are significantly upregulated in the defensive glands of formic acid producers compared to regular body tissue with isotope tracing results currently pending. Our results also support the hypothesis that L-valine catabolism plays a role in the biosynthesis of methacrylic acid and other related compounds in P. moestus. While we do find support for the presence of oxidoreductases in the secretory lobes of the three benzoquinone producers, it remains uncertain which if any are involved in benzoquinone biosynthesis. However, we have provided evidence for the role that peroxidases likely play in converting hydroquinones to their benzoquinone end-products, as well as genes involved in hydrogen peroxide biosynthesis in the Brachininae. The glands of all species examined thus far, including H. pensylvanicus, B. elongatulus, and Metrius contractus (Paussinae) have been found to contain a high proportion of the elastomeric protein resilin in the collecting ducts, which is resistant to chemical and physical stress. By comprising the ductwork of the gland system, resilin presumably allows for chemicals to be transported to the reservoir chambers without corroding the system. Generally found in arthropod body segments, this novel use of resilin in the gland system suggests it may be an adaptation to defensive chemical storage and transport. My dissertation research thus provides novel insight into the evolution of insect defensive glands using the Carabidae as a case study. It is likely that the co-option of primary metabolic pathways through gene upregulation in the defensive glands is a key factor in the defensive chemical capabilities of ground beetles, and that morphological innovations such as resilin incorporation into gland tissue are important adaptations to defensive chemical storage and secretion.