Reef-building corals depend upon a nutritional endosymbiosis with photosynthetic dinoflagellates of the family Symbiodiniaceae for the majority of their energetic needs. While this mutualistic relationship is impacted by numerous stressors, warming oceans are a predominant threat to coral reefs, placing the future of the world's reefs in peril. Some Symbiodiniaceae species exhibit tolerance to thermal stress, but the in hospite symbiont response to thermal stress is underexplored. To describe the underpinnings of symbiosis and heat stress response, we compared in hospite and free-living transcriptomes of Durusdinium trenchii, a pan-tropical heat-tolerant Symbiodiniaceae species, under stable temperature conditions and acute hyperthermal stress. We discovered that symbiotic state was a larger driver of the transcriptional landscape than heat stress. The majority of differentially expressed transcripts between in hospite and free-living cells were downregulated, suggesting the in hospite condition is associated with the shutdown of numerous processes uniquely required for a free-living lifestyle. In the free-living state, we identified enrichment for numerous cell signalling pathways and other functions related to detecting and responding to a changing environment, as well as transcripts relating to mitosis, meiosis, and motility. In contrast, in hospite cells exhibited enhanced transcriptional activity for photosynthesis and carbohydrate transport as well as chromatin modifications and a disrupted circadian clock. Hyperthermal stress induced drastic alteration of transcriptional activity in hospite, suggesting symbiotic engagement with the host elicited an exacerbated stress response when compared to free-living D. trenchii. Altogether, the dramatic differences in gene expression between in hospite and free-living D. trenchii indicate the importance of considering symbiotic state in investigations of symbiosis and hyperthermal stress in Symbiodiniaceae.

中文翻译：

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在稳定和热应激条件下，耐热珊瑚内共生体的自由生活和共生生活方式表现出截然不同的转录组。

造礁珊瑚主要依靠营养共生和共生菌科的光合鞭毛藻来满足其大部分能量需求。尽管这种相互关系受到许多压力因素的影响，但海洋变暖是珊瑚礁的主要威胁，使世界珊瑚礁的未来陷入危险。一些共生植物对热应激具有耐受性，但尚未充分研究其对热应激的共生共生体反应。为了描述共生和热应激反应的基础，我们在稳定的温度条件下和急性高温胁迫下，对全热带耐热共生菌科杜氏硬藻Dustusdinium poundii的住院和自由生活转录组进行了比较。我们发现，共生状态比热应激是转录环境的更大驱动因素。住院和自由生活细胞之间大多数差异表达的转录本均被下调，表明住院条件与自由生活独特的众多过程的关闭有关。在自由生活状态下，我们确定了丰富的细胞信号通路和与检测和响应不断变化的环境有关的其他功能的富集，以及与有丝分裂，减数分裂和运动相关的转录本。相反，在住院医院中，细胞的光合作用和碳水化合物运输以及染色质修饰和昼夜节律被破坏时，转录活性增强。高温胁迫诱导了临终者转录活性的急剧变化，这表明与自由共生的海。相比，与宿主的共生参与引起了加剧的应激反应。总而言之，在院内和自由生活的海D之间基因表达的巨大差异表明，在研究共生菌和共生体高温共生的研究中，考虑共生状态的重要性。