UC Berkeley

Ground beetles (Coleoptera: Carabidae) are fascinating in several regards. They are a hyper-diverse family with approximately 40,000 described species that produce an astounding number of chemical compounds for defense representing many chemical classes. These traits make ground beetles a fantastic model to understand the mechanisms underlying the production of defensive chemicals, and how they store compounds that could induce autotoxicity. In this dissertation, I seek to answer these questions by performing experiments on ground beetles, with a focus on bombardier beetles of the genus Brachinus. First, I review bombardier beetles and what we know about their taxonomy, anatomy, defensive chemistry, and projected biosynthetic pathways. This review highlights that carabids seem to be producing quinones using a biosynthetic pathway not known to occur in insects. Next, I performed experiments in Brachinus mexicanus and B. elongatulus to identify the defensive compounds present in different parts of their defensive gland system and test a hypothesized mechanism by which these beetles could avoid autotoxicity from cytotoxic quinone compounds they produce and store. I found that the defensive chemicals, quinones, were absent in the secretory cells of defensive glands that are thought to produce them. I further found that quinones were not actually absent, but conjugated to sugars, a process that would lead to quinones not being detected in the secretory cells and provides protection from the cytotoxic effects of quinones. Finally, I examined the microbiome in specimens across the phylogeny of Geadephaga (Carabidae + Trachypachidae + Cicindelidae) to test if microbial communities are more similar in closely related species (phylosymbiosis). I also performed experiments in B. elongatulus to determine if microbial symbionts mediate the biosynthesis of their defensive quinones. I found a weak but significant signal of phylosymbiosis in Geadephaga but find no evidence to support the mediation of quinone biosynthesis by microbes in B. elongatulus.