Discovery and Description of the Bacterial Associates of a Gregarious Riparian Beetle with Explosive Defensive Chemistry

Abstract

Bombardier beetles (Carabidae: Brachininae), well known for their unique explosive defensive chemistry, are found in riparian corridors throughout the American Southwest where they commonly form large diurnal aggregations in moist areas under rocks, in crevices, and in leaf litter. Using high throughput 16S amplicon sequencing, we provided the first microbiome survey of a bombardier beetle, Brachinus elongatulus. Results include the discovery that this species has a core set of six bacterial phylotypes found in all 13 specimens collected from two sites in Arizona separated from one another by over 80 kilometers. The core microbiome includes three species of Lactobacillales, including representatives of Enterococcus and Weissella. Members of these genera have been implicated in the production of volatile carboxylic acids that act as fecal aggregation pheromone components, which regulate aggregation in the German cockroach, and they may also play a role in Brachinus aggregation behavior. Our results also show that some B. elongatulus individuals collected from both sites are heavily infected with another bacterium, Spiroplasma. Various species of this facultative endosymbiont have been shown to have either a commensal, mutualistic or pathogenic relationship with their host. Most interestingly, from the perspective of B. elongatulus natural history, some Spiroplasma species have been shown to provide their dipteran hosts defensive protection against nematode infection, which is also a constant threat for Brachinus species. Our findings reveal intriguing possibilities that bacteria may influence bombardier beetle behavior and physiology, and this research lays the groundwork for future experiments to directly assess these beetle-bacterial relationships.