Data from: Buchnera aphidicola has changed flatmate: did the replacement of cosymbionts enable ecological expansions of aphids?

Symbiotic associations with bacteria have facilitated important evolutionary transitions in insects and resulted in long-term obligate interactions. Recent evidence suggests that these associations are not always evolutionarily stable and that symbiont replacement has occurred during the history of many insect groups. This provides us with opportunities to investigate the factors favoring one symbiont over another, but progress has been hindered by our incomplete understanding of the distribution of symbionts across phylogenetic and ecological contexts. The association of aphids with Buchnera aphidicola, established 150 Mya, is one of the most persistent obligate symbioses known to date. However, in a few species of the genus Cinara, this relationship has evolved into a “ménage à trois”, in which a former facultative endosymbiont, Serratia symbiotica, performs some of the functions usually fulfilled by Buchnera. Using deep sequencing of 16S rRNA bacterial genes from 128 Cinara species and a robust phylogeny, we show that dual symbiosis has evolved repeatedly in Cinara. Most species host both Serratia and Buchnera but, in several clades, endosymbionts related to Sodalis, Erwinia or an unnamed member of the Enterobacteriaceae have replaced Serratia. These findings confirm that the species of this genus require a co-resident symbiont to complement Buchnera in its nutritional role. We further demonstrate that cosymbiont replacement is not associated with the adaptation of aphids to new ecological conditions. We propose that symbiont succession was driven by factors intrinsic to the phenomenon of endosymbiosis, such as rapid genome deterioration or competitive interactions between bacteria with similar metabolic capabilities细菌共生关系推动了昆虫界诸多关键演化过渡事件，并形成了长期专性互作关系。近年研究证据表明，这类共生关联并非始终保持演化稳定，诸多昆虫类群的演化历史中均发生过共生体替换事件。这为我们探究为何某一共生体相较于另一共生体更具优势提供了研究契机，但目前我们对共生体在系统发育与生态背景中的分布认知尚不全面，该领域的研究进展因此受到阻滞。蚜虫与布赫纳菌（Buchnera aphidicola）的共生关系始建于1.5亿年前，是目前已知最持久的专性共生体系之一。然而在大蚜属（Cinara）的少数物种中，这一共生关系已演变为“三方共生”体系：原本作为兼性内共生体的共生沙雷氏菌（Serratia symbiotica），承担了部分通常由布赫纳菌完成的生理功能。本研究通过对128种大蚜属物种的16S rRNA细菌基因进行深度测序，并结合稳健的系统发育分析，证实双重共生在大蚜属中多次独立演化。多数大蚜物种同时携带共生沙雷氏菌与布赫纳菌，但在多个演化支中，与索氏菌属（Sodalis）、欧文氏菌属（Erwinia）或肠杆菌科（Enterobacteriaceae）未命名类群亲缘关系相近的内共生体，已取代了共生沙雷氏菌的位置。上述研究结果证实，大蚜属物种需要共存的共生体来补充布赫纳菌的营养功能。我们进一步证明，共生体的协同替换与蚜虫对新生态环境的适应性并无关联。我们提出，共生体的演替由内共生现象本身的内在因素驱动，例如快速的基因组退化，或是代谢能力相近的细菌之间的竞争互作。