Abstract
Bombardier beetles (Carabidae: Brachininae) are well known for their unique explosive defensive chemistry. These beetles are found in riparian corridors throughout the American Southwest, where they commonly form large diurnal multispecies aggregations in moist areas under rocks, in crevices, and in leaf litter. Using high throughput 16S amplicon sequencing, we provide the first microbiome survey of a bombardier beetle, Brachinus elongatulus, collected from two sites in Arizona. Two bacterial taxa were present in all individuals sampled: Enterococcus and Dysgonomonas. Enterococcus has been implicated in the production of fecal aggregation pheromone components, which have been shown to regulate aggregation in the German cockroach; it is possible that Enterococcus plays a similar role in Brachinus. Dysgonomonas was found in all the secretory cells of the defensive system and gut samples. Additional studies are needed to determine if these microbes play a role in these beetles’ unique chemical defense. Results also show that the majority of B. elongatulus individuals collected from both sites were infected with Spiroplasma. Many Spiroplasma are intracellular, vertically transmitted insect symbionts that may manipulate host reproduction (e.g., cause male-killing) or provide resistance to nematodes and/or parasitoid wasps. Defensive protection could be especially beneficial to B. elongatulus, which are frequently parasitized by horsehair worms (Nematomorpha). In sum, findings suggest several testable hypotheses on the effects bacteria may have on bombardier beetle behavior and physiology.
Highlights
Members of the genus Brachinus (Coleoptera: Brachininae) belong to a group of carabids known as bombardier beetles, which produce noxious exothermic defensive sprays to deter their predators (Arndt et al, 2015) (Figure 1A)
Specimens were briefly preserved in 100% ethanol and surfaced sterilized using an 8% bleach solution followed by a 1-min wash in phosphate-buffered saline (PBS) and a 1-min wash in 100% ethanol just prior to dissection
A total of 864,299 rDNA V3–V4 reads were obtained from 54 unique samples comprised of either whole-body individuals or a specific tissue dissected from an individual specimen (Table 1)
Summary
Members of the genus Brachinus (Coleoptera: Brachininae) belong to a group of carabids known as bombardier beetles, which produce noxious exothermic defensive sprays to deter their predators (Arndt et al, 2015) (Figure 1A). They produce defensive chemicals with a pair of pygidial glands (Figure 2). Bacterial Associates of a Bombardier Beetle benzoquinones in an exothermic reaction just before they are expelled from the body in an audible explosion (Schildknecht and Holoubek, 1961; Aneshansley et al, 1983) This intricate mechanism is broadly acknowledged as one of the most complex and sophisticated defensive systems known in the animal kingdom (Eisner, 2003)
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