As sea surface temperatures increase, many coral species that used to harbour symbionts of the genus Cladocopium have become colonized with the thermally tolerant genus, Durusdinium. Here, we asked how gene expression in the symbionts of one genus changes depending on the abundance of another symbiont genus within the same coral host, and what effect this interaction has on the host. Symbiont gene expression was overwhelmingly driven by whether the genus was the minority or the majority within the host, which affected 79% (Durusdinium) and 96% (Cladocopium) of all genes. Particularly strong effects in both genera were observed for photosynthesis components (upregulated in the minority state) and proteins putatively associated with cell motility (upregulated in the majority state). Importantly, there was no distinct gene expression signature associated with the mixed symbiosis state when both genera were represented in comparable proportions within the host, which could lead to more intense competition. The mixed symbiosis was also not associated with elevated host stress: in fact, after heat treatment, stress signatures were the lowest in mixed-symbiosis corals compared to both Cladocopium- and Durusdinium-dominated corals. In conclusion, during shuffling between Cladocopium and Durusdinium both symbiont genera go through extensive and largely reciprocal physiological transitions, but there is no evidence of intensifying antagonistic interactions that are detrimental to the host. Unless the mixed-symbiosis corals in this study are not representative of the typical transition between Cladocopium and Durusdinium, the process of shuffling from one symbiont genus to another appears to be cost-free for the coral host, and even appears to be associated with lower stress susceptibility. This raises optimism for the future corals, which will probably have to rely on symbiont shuffling more and more to withstand environmental challenges.

中文翻译：

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Cladocopium 和 Durusdinium 之间的洗牌广泛地改变了每个共生体的生理机能，而不会给珊瑚宿主造成压力

随着海面温度的升高，许多曾经栖息着枝角藻属共生体的珊瑚物种已经被耐热的杜鲁氏珊瑚属定殖。在这里，我们询问了一个属的共生体中的基因表达如何根据同一珊瑚宿主内另一个共生体属的丰度而变化，以及这种相互作用对宿主有什么影响。共生基因的表达主要由该属是宿主内的少数还是多数驱动，这影响了 79% ( Durusdinium ) 和 96% ( Cladocopium )) 的所有基因。对于光合作用成分（在少数状态下上调）和推定与细胞运动相关的蛋白质（在多数状态下上调），观察到两个属中特别强烈的影响。重要的是，当两个属在宿主中的比例相当时，没有与混合共生状态相关的明显基因表达特征，这可能导致更激烈的竞争。混合共生也与宿主压力升高无关：事实上，在热处理后，混合共生珊瑚中的压力特征与以枝叶属和杜鲁斯属为主的珊瑚相比是最低的。总之，在Cladocopium和Durusdinium之间的洗牌过程中两个共生菌属都经历了广泛且很大程度上互惠的生理转变，但没有证据表明对宿主有害的拮抗相互作用加剧。除非本研究中的混合共生珊瑚不能代表Cladocopium和Durusdinium之间的典型过渡，否则从一个共生属到另一个共生属的改组过程似乎对珊瑚宿主来说是免费的，甚至似乎与较低的压力敏感性。这使人们对未来的珊瑚感到乐观，它们可能不得不越来越依赖共生体洗牌来抵御环境挑战。