Table_3_Episyrphus balteatus symbiont variation across developmental stages, living states, two sexes, and potential horizontal transmission from prey or environment.XLSX

IntroductionEpisyrphus balteatus is one representative Syrphidae insect which can provide extensive pollination and pest control services. To date, the symbiont composition and potential acquisition approaches in Syrphidae remain unclear.
MethodsHerein, we investigated microbiota dynamics across developmental stages, different living states, and two sexes in E. balteatus via full-length 16S rRNA genes sequencing, followed by an attempt to explore the possibility of symbiont transmission from prey Megoura crassicauda to the hoverfly.
ResultsOverall, Proteobacteria and Firmicutes were the dominant bacteria phyla with fluctuating relative abundances across the life stage. Cosenzaea myxofaciens is dominant in adulthood, while Enterococcus silesiacus and Morganella morganii dominate in larvae and pupae of E. balteatus, respectively. Unexpectedly, Serratia symbiotica, one facultative endosymbiont commonly harbored in aphids, was one of the predominant bacteria in larvae of E. balteatus, just behind Enterococcus silesiacus. In addition, S. symbiotica was also surprisingly most dominated in M. crassicauda aphids (92.1% relative abundance), which are significantly higher than Buchnera aphidicola (4.7% relative abundance), the primary obligate symbiont of most aphid species. Approximately 25% mortality was observed among newly emerged adults, of which microbiota was also disordered, similar to normally dying individuals. Sexually biased symbionts and 41 bacteria species with pairwise co-occurrence in E. balteatus and 23 biomarker species for each group were identified eventually. Functional prediction showed symbionts of hoverflies and aphids, both mainly focusing on metabolic pathways. In brief, we comprehensively explored the microbiome in one Syrphidae hoverfly using E. balteatus reared indoors on M. morganii as the model, revealed its dominated symbiont species, identified sexually biased symbionts, and found an aphid facultative endosymbiont inhabited in the hoverfly. We also found that the dominated symbiotic bacteria in M. crassicauda are S. symbiotica other than Buchnera aphidicola.
DiscussionTaken together, this study provides new valuable resources about symbionts in hoverflies and prey aphids jointly, which will benefit further exploring the potential roles of microbiota in E. balteatus.

引言
黑带食蚜蝇（Episyrphus balteatus）是一类具有代表性的食蚜蝇科（Syrphidae）昆虫，可提供广泛的传粉与害虫防治服务。截至目前，食蚜蝇科昆虫的共生菌群组成及其潜在的获取途径仍不明确。

方法
本研究通过全长16S核糖体RNA（16S rRNA）基因测序技术，解析了黑带食蚜蝇不同发育阶段、不同生活状态以及雌雄个体的菌群动态变化；同时尝试探究共生菌从猎物厚喙长管蚜（Megoura crassicauda）向食蚜蝇传播的可能性。

结果
总体而言，变形菌门（Proteobacteria）与厚壁菌门（Firmicutes）为优势细菌门，其相对丰度在整个生活史中呈动态波动。粘质科森扎菌（Cosenzaea myxofaciens）为成虫期优势菌，而西里西亚肠球菌（Enterococcus silesiacus）与摩根氏摩根菌（Morganella morganii）分别为黑带食蚜蝇幼虫与蛹期的优势菌。出乎意料的是，通常定殖于蚜虫体内的兼性内共生菌（facultative endosymbiont）——虫生沙雷氏菌（Serratia symbiotica），亦是黑带食蚜蝇幼虫体内的优势细菌之一，其丰度仅次于西里西亚肠球菌。此外，厚喙长管蚜体内的优势菌同样为虫生沙雷氏菌（相对丰度达92.1%），其丰度显著高于多数蚜虫物种的专性初级共生菌——蚜虫布赫纳菌（Buchnera aphidicola，相对丰度仅4.7%）。本研究观察到约25%的新羽化成虫死亡，这些个体的菌群结构同样失调，与自然死亡个体的菌群特征一致。最终，本研究鉴定出具有性别偏好的共生菌、41种在黑带食蚜蝇体内呈成对共现模式的细菌，以及每个类群对应的23种生物标记物种。功能预测结果显示，食蚜蝇与蚜虫的共生菌群均主要富集于代谢通路。简言之，本研究以室内饲养、取食摩根氏摩根菌的黑带食蚜蝇为模型，全面解析了该食蚜蝇科昆虫的菌群组成，明确了其优势共生菌物种，鉴定出具有性别偏好的共生菌，并发现一种蚜虫兼性内共生菌可定殖于食蚜蝇体内。同时我们证实，厚喙长管蚜体内的优势共生菌为虫生沙雷氏菌，而非蚜虫布赫纳菌。

讨论
综上，本研究同时为食蚜蝇与猎物蚜虫的共生菌群研究提供了新的宝贵数据资源，将有助于后续深入探究菌群在黑带食蚜蝇体内的潜在功能。