Projected increases in sea surface temperatures will exceed corals' ability to withstand heat stress within this century. Experimental evolution of cultured symbionts (Symbiodiniaceae) at high temperatures followed by reintroduction into corals can enhance coral heat tolerance. Several studies have selected for enhanced tolerance in Cladocopium goreaui (C1) over multiple time scales and then compared the performance of coral juveniles infected with the heat‐tolerant C1 selected strain (SS) to the performance of juveniles infected with the C1 wild type (WT). To derive lessons about host benefits when symbionts are experimentally selected, here we compare the performance of SS‐ and WT‐juveniles after 21 cell generations of heat selection versus longer periods (73–131) in recently published experiments. After 21 generations, we found rapid improvement in heat tolerance of SS through an overall shift in the mean tolerance to temperature. This did not translate to improved growth and survivorship of the coral. Specifically, survival did not differ significantly between juveniles of Acropora tenuis hosting WT versus SS at any temperature. Juveniles infected with WT exhibited greater skeletal growth than those infected with SS at 27 and 31°C but not at 32.5°C. SS‐juvenile symbiont cell densities increased significantly at 27°C relative to SS‐juveniles in the 31 and 32.5°C. Photosynthetic efficiencies in SS‐juveniles were higher compared to WT‐juveniles at 31°C, equal at 27°C, and lower at 32.5°C. These results suggest that selection over longer generation (>130) times will be needed to confer host benefits and will be dependent on the stability of this association being maintained in nature.

中文翻译：

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珊瑚藻内共生藻（Cladocopium goreaui）的实验演变：十年压力实验的经验教训，以提高珊瑚的耐热性

在本世纪内，预计海洋表面温度的升高将超过珊瑚承受热应激的能力。在高温下对共生共生体（Symbiodiniaceae）进行实验性进化，然后再引入珊瑚中可以增强珊瑚的耐热性。选择了几项研究来增强刺槐（Cladocopium goreaui）的耐受性（C1）在多个时间尺度上，然后将耐热C1选择菌株（SS）感染的珊瑚幼鱼的性能与C1野生型（WT）感染的幼鱼的性能进行了比较。为了从实验中选择共生体时获得有关宿主益处的经验教训，在这里，我们比较了最近发表的实验中，经过21个细胞代的热选择与较长时期（73-131）后，SS和WT少年的表现。在21代之后，我们发现SS的耐热性通过平均温度耐受性的整体变化而迅速提高。这并没有转化为改善珊瑚的生长和生存能力。具体来说，Acropora tenuis的幼体之间的存活率没有显着差异在任何温度下托管WT与SS。在27和31°C时，受WT感染的少年比受SS感染的少年表现出更大的骨骼生长，但在32.5°C时则没有。相对于31和32.5°C的SS青少年，SS青少年的共生细胞密度在27°C时显着增加。在31°C时，SS幼虫的光合效率高于WT幼虫，在27°C时相等，而在32.5°C时则较低。这些结果表明，需要较长时间的选择（> 130次）来赋予宿主收益，并且取决于自然维持这种联系的稳定性。