Table_1_Transmission of Bacterial Symbionts With and Without Genome Erosion Between a Beetle Host and the Plant Environment.XLSX

Many phytophagous insects harbor symbiotic bacteria that can be transmitted vertically from parents to offspring, or acquired horizontally from unrelated hosts or the environment. In the latter case, plants are a potential route for symbiont transfer and can thus foster a tripartite interaction between microbe, insect, and plant. Here, we focus on two bacterial symbionts of the darkling beetle Lagria villosa that belong to the genus Burkholderia; the culturable strain B. gladioli Lv-StA and the reduced-genome strain Burkholderia Lv-StB. The strains can be transmitted vertically and confer protection to the beetle’s eggs, but Lv-StA can also proliferate in plants, and both symbiont strains have presumably evolved from plant pathogens. Notably, little is known about the role of the environment for the transmission dynamics and the maintenance of the symbionts. Through manipulative assays, we demonstrate the transfer of the symbionts from the beetle to wheat, rice and soybean plants, as well as leaf litter. In addition, we confirm that aposymbiotic larvae can pick up Lv-StA from dry leaves and the symbiont can successfully establish in the beetle’s symbiotic organs. Also, we show that the presence of plants and soil in the environment improves symbiont maintenance. These results indicate that the symbionts of L. villosa beetles are still capable of interacting with plants despite signatures of genome erosion and suggest that a mixed-mode of bacterial transmission is likely key for the persistence of the symbiosis.

多数植食性昆虫（phytophagous insects）体内携带共生细菌（symbiotic bacteria），这类细菌可通过垂直传播从亲本传递至子代，也可通过水平传播从非亲缘宿主或环境中获得。在后一种传播途径中，植物是共生体传播的潜在载体，因此可促成微生物、昆虫与植物之间的三方互作。本研究聚焦于暗拟步甲（darkling beetle）Lagria villosa的两种伯克霍尔德菌属（Burkholderia）共生细菌：可培养菌株唐菖蒲伯克霍尔德菌（B. gladioli）Lv-StA，以及基因组简化菌株伯克霍尔德菌Lv-StB。这两种菌株均可通过垂直传播传递，并为甲虫的卵提供保护；其中Lv-StA还可在植物体内增殖，且两种共生菌株据推测均起源于植物病原菌。值得注意的是，目前学界对环境在共生体传播动态（transmission dynamics）与共生体维持过程中的作用知之甚少。通过操控性实验（manipulative assays），本研究证实共生体可从甲虫传播至小麦、水稻、大豆植株以及枯落叶（leaf litter）中。此外，本研究证实无共生体幼虫（aposymbiotic larvae）可从干燥叶片中获取Lv-StA，且该共生体可成功定殖于甲虫的共生器官（symbiotic organs）中。同时，本研究表明环境中存在植物与土壤可促进共生体的维持。上述结果表明，尽管存在基因组侵蚀（genome erosion）的特征，但Lagria villosa的共生体仍具备与植物互作的能力；同时提示，细菌的混合传播模式可能是共生关系得以存续的关键。