Table_2_Bacterial Associates of a Gregarious Riparian Beetle With Explosive Defensive Chemistry.docx

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.

射炮步甲（Bombardier beetle）隶属于步甲科（Carabidae）气甲亚科（Brachininae），以其独特的爆炸性防御化学机制而闻名。该类群广泛分布于北美西南部的河岸廊道生境中，常于日间在岩石下、缝隙及落叶层等潮湿区域形成大规模的多物种聚集群。本研究采用高通量16S扩增子测序技术，对采自美国亚利桑那州两个采样点的长射炮步甲（Brachinus elongatulus）开展了首次微生物组调查。所有采样个体均携带两种核心细菌类群：肠球菌属（Enterococcus）与狄氏菌属（Dysgonomonas）。其中，肠球菌属已被证实可合成粪便聚集信息素组分，该组分已被证明能调控德国小蠊的聚集行为，推测其在射炮步甲属（Brachinus）中可能发挥类似的聚集调控功能。狄氏菌属则存在于该甲虫防御系统的所有分泌细胞及肠道样本中。仍需开展进一步研究以明确这些微生物是否参与射炮步甲独特的化学防御过程。研究结果同时显示，两个采样点采集的大部分长射炮步甲个体均感染了螺原体属（Spiroplasma）。多数螺原体属物种为胞内共生菌，可经垂直传播感染昆虫宿主，其既可能调控宿主繁殖（如引发雄性致死），也可为宿主提供抗线虫、抗寄生蜂的防御能力。鉴于长射炮步甲常被铁线虫（Nematomorpha）寄生，这类防御性保护对其而言尤为关键。综上，本研究结果提出了若干可验证的科学假说，用以阐释细菌对射炮步甲行为与生理机能的潜在影响。