Data from: Can maternally inherited endosymbionts adapt to a novel host? Direct costs of Spiroplasma infection, but not vertical transmission efficiency, evolve rapidly after horizontal transfer into D. melanogaster

Maternally inherited symbionts are common in arthropods and many have important roles in host adaptation. The observation that specific symbiont lineages infect distantly related host species implies new interactions are commonly established by lateral transfer events. However, studies have shown that symbionts often perform poorly in novel hosts. We hypothesized selection on the symbiont may be sufficiently rapid that poor performance in a novel host environment is rapidly ameliorated, permitting symbiont maintenance. Here, we test this prediction for a Spiroplasma strain transinfected into the novel host Drosophila melanogaster. In the generations immediately following transinfection, the symbiont had low transmission efficiency to offspring and imposed severe fitness costs on its host. We observed that effects on host fitness evolved rapidly, being undetectable after 17 generations in the novel host, whereas vertical transmission efficiency was poorly responsive over this period. Our results suggest that long-term symbiosis may more readily be established in cases where symbionts perform poorly in just one aspect of symbiosis.

母系遗传共生体广泛存在于节肢动物体内，且多数对宿主适应性具有关键作用。研究观察到，特定共生菌谱系可侵染远缘宿主物种，这表明新的共生关系通常可通过水平转移事件得以建立。不过已有研究证实，共生体在陌生宿主中往往生存表现不佳。我们据此提出假说：共生体所承受的选择压力足够快速，使其在新宿主环境中的不良表现可被迅速改善，从而保障共生体得以持续维持。本研究针对转染至新宿主黑腹果蝇（Drosophila melanogaster）的螺原体（Spiroplasma）菌株，对该假说展开验证。在转染后的早期世代中，该共生体的子代垂直传播效率低下，且对宿主造成了严重的适合度代价。我们观察到，宿主适合度相关的负面影响进化迅速，在新宿主中传代17代后便已无法检测到；而在此期间，其垂直传播效率却几乎未出现显著改善。本研究结果提示，若共生体仅在共生关系的某单一维度表现不佳，则长期稳定的共生关系或许更易建立。