Department of Biology, Emory University, Atlanta, Georgia, 30322.
Evolution. 2022 May;76(5):1073-1081. doi: 10.1111/evo.14472. Epub 2022 Mar 31.
Reciprocal adaptation between hosts and symbionts can drive the maintenance of symbioses, resulting in coevolution and beneficial genotypic interactions. Consequently, hosts may experience decreased fitness when paired with nonsympatric partners compared to sympatric symbionts. However, coevolution does not preclude conflict-host and symbiont can act to advance their own fitness interests, which do not necessarily align with those of their partner. Despite coevolution's importance in extant symbioses, we know little about its role in shaping the origin of symbioses. Here, we tested the role of coevolution in establishing a novel association by experimentally (co)evolving a host with a protective bacterium under environmental stress. Although evolution in the presence of nonevolving bacteria facilitated host adaptation, co-passaged hosts did not exhibit greater adaptation rates than hosts paired with nonevolving bacteria. Furthermore, co-passaged hosts exhibited greater fecundity when paired with sympatric, co-passaged bacteria compared to co-passaged bacteria with which they did not share an evolutionary history. Thus, shared evolutionary history between the hosts and microbes actually reduced host fitness and has the potential to impede evolution of new beneficial associations.
宿主和共生体之间的相互适应可以促进共生关系的维持，导致共同进化和有益的基因型相互作用。因此，与非共生伙伴相比，宿主与共生体配对时可能会降低适应性。然而，共同进化并不排除冲突——宿主和共生体可以为了提高自己的适应性而行动，这并不一定与它们的伙伴一致。尽管共同进化在现存的共生关系中很重要，但我们对其在塑造共生关系起源中的作用知之甚少。在这里，我们通过在环境压力下对宿主与保护性细菌进行实验（共）进化，测试了共同进化在建立新联系中的作用。尽管在没有进化的细菌存在的情况下进化有利于宿主的适应，但与没有进化的细菌配对的宿主并没有比与没有进化的细菌共进化的宿主表现出更高的适应率。此外，与没有共同进化史的共进化细菌相比，与共进化的、同域的细菌配对的共进化宿主表现出更高的繁殖力。因此，宿主和微生物之间的共同进化历史实际上降低了宿主的适应性，并有可能阻碍新的有益共生关系的进化。
