Data from: Horizontal transfer of facultative endosymbionts is limited by host relatedness

Heritable microbial symbionts can have important effects on many aspects of their hosts’ biology. Acquisition of a novel symbiont strain can provide fitness benefits to the host, with significant ecological and evolutionary consequences. We measured barriers to horizontal transmission by artificially transferring facultative symbionts from the grain aphid, Sitobion avenae, and five other aphid species into two clonal genotypes of S. avenae. We found the symbiont Hamiltonella defensa establishes infections more easily following a transfer from the same host species and that such infections are more stable. Infection success was also higher when the introduced symbiont strain was more closely related to the strain that was originally present in the host (but which had previously been removed). There were no differences amongst successfully established symbiont strains in their effect on aphid fecundity. H. defensa did not confer protection against parasitoids in our S. avenae clones, although it often does in other aphid hosts. However, strains of the symbiont Regiella insecticola originating from two host species protected grain aphids against the pathogenic fungus Pandora neoaphidis. This study helps describe the extent to which facultative symbionts can act as a pool of adaptations that can be sampled by their eukaryote hosts.可遗传微生物共生体（heritable microbial symbionts）对宿主生物学的诸多维度均具有关键调控作用。获取新型共生菌株可为宿主带来适合度收益（fitness benefits），并引发显著的生态学与进化学效应。本研究通过人工转接手段，将兼性共生体（facultative symbionts）从麦长管蚜（*Sitobion avenae*）及其他5种蚜虫物种，导入2种麦长管蚜克隆基因型（clonal genotypes）中，以此测定水平传播（horizontal transmission）障碍。研究发现，共生体汉森氏菌（*Hamiltonella defensa*）从同一宿主物种转接时，更易建立侵染，且此类侵染状态更为稳定。当引入的共生菌株与宿主中原先已清除的原有菌株亲缘关系越近，侵染成功率亦越高。成功定殖的各类共生菌株对蚜虫繁殖力（fecundity）的影响无显著差异。尽管在多数其他蚜虫宿主中，汉森氏菌通常可赋予宿主抗寄生蜂（parasitoids）能力，但本研究中的麦长管蚜克隆并未观测到此类保护效应。不过，源自2种宿主物种的雷吉尔氏菌（*Regiella insecticola*）菌株，可有效保护麦长管蚜免受致病真菌（pathogenic fungus）新蚜虫疠霉（*Pandora neoaphidis*）的侵染。本研究明确了兼性共生体可作为一类适应性储备库，供真核宿主进行采样利用的潜在范围。