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...母系遗传的共生体（symbionts）在节肢动物中极为常见，其中多数对宿主的适应性演化具有关键作用。特定共生体谱系能够侵染亲缘关系较远的宿主物种，这一观测结果表明，新的共生关系通常可通过侧向转移事件（lateral transfer events）得以建立。然而已有研究显示，共生体在新型宿主中往往适配性不佳。我们提出假说：共生体所承受的选择压力足够快速，使其在新型宿主环境中的不良适配性可被快速改善，从而得以在新宿主中维持存续。本研究针对一株被转染至新型宿主黑腹果蝇（Drosophila melanogaster）的螺原体（Spiroplasma）菌株，对该假说进行验证。在转染后的最初几代中，该共生体向子代的垂直传播效率较低，且对宿主造成了严重的适合度（fitness）代价。我们观察到，共生体对宿主适合度的负面影响快速演化：在新型宿主中传至17代后，该负面影响已无法被检测到；而同期内，共生体的垂直传播效率几乎未出现显著改善。我们的……