A contrasting picture of co-obligate nutritional endosymbiosis in aphids. undefined

Dependence on multiple nutritional symbionts forming a metabolic unit has repeatedly evolved in many insect species that feed on nutritionally unbalanced diets such as plant sap. This is the case for aphids of the subfamilies Lachninae and Chaitophorinae, which have evolved di-symbiotic systems in which the ancient obligate nutritional symbiont Buchnera aphidicola is metabolically complemented by an additional nutritional symbiont acquired more recently. Deciphering how different symbionts integrate both metabolically and anatomically in such systems is crucial to understanding how complex nutritional symbiotic systems function and evolve. In this study, we sequenced and analyzed the genomes of the symbionts B. aphidicola and Serratia symbiotica associated with the Chaitophorinae aphids Sipha maydis and Periphyllus lyropictus. Our results show that, in these two species, B. aphidicola and S. symbiotica complement each other metabolically but with distinct metabolic reactions supported by each symbiont depending on the host. Furthermore, the S. symbiotica symbiont associated with S. maydis appears to be strictly compartmentalized into specialized host cells, bacteriocytes, whereas the S. symbiotica symbiont associated with P. lyropictus exhibits a highly invasive phenotype, presumably because it is capable of expressing a larger set of virulence factors, including a complete flagellum for swimming motility. Such contrasting levels of metabolic and anatomical integration for two S. symbiotica symbionts that were recently acquired as nutritional co-obligate partners reflect distinct coevolutionary processes specific to each association.

取食植物汁液等营养不均衡膳食的诸多昆虫类群中，依赖构成代谢单元的多种营养共生体的演化模式已多次独立出现。长管蚜亚科（Lachninae）与毛管蚜亚科（Chaitophorinae）的蚜虫即为典型代表：它们演化出双共生系统，其中古老的专性营养共生体布赫纳氏菌（Buchnera aphidicola）的代谢功能，会由后期获得的额外营养共生体予以补充。阐明此类系统中不同共生体在代谢与解剖层面的整合机制，是理解复杂营养共生系统功能与演化的关键所在。本研究对毛管蚜亚科的两种蚜虫——Sipha maydis和Periphyllus lyropictus所关联的共生菌布赫纳氏菌（B. aphidicola）与共生沙雷氏菌（Serratia symbiotica）的基因组进行了测序与分析。研究结果表明，在这两种蚜虫体内，布赫纳氏菌与共生沙雷氏菌存在代谢互补关系，但二者所支撑的特异性代谢反应因宿主物种而异。此外，与S. maydis关联的共生沙雷氏菌似乎严格区室化于宿主特化的含菌细胞（bacteriocytes）中；而与P. lyropictus关联的共生沙雷氏菌则呈现出高度侵袭性表型，推测其原因在于该菌株能够表达更为丰富的毒力因子，包括一套完整的用于游动动力的鞭毛（flagellum）。这两种新近获得的营养共专性伙伴——共生沙雷氏菌，其代谢与解剖整合程度存在显著差异，反映出各自宿主-共生体关联所特有的差异化共进化进程。