Increasing ocean temperatures are causing dysbiosis between coral hosts and their symbionts. Previous work suggests that coral host gene expression responds more strongly to environmental stress compared to their intracellular symbionts; however, the causes and consequences of this phenomenon remain untested. We hypothesized that symbionts are less responsive because hosts modulate symbiont environments to buffer stress. To test this hypothesis, we leveraged the facultative symbiosis between the scleractinian coral Oculina arbuscula and its symbiont Breviolum psygmophilum to characterize gene expression responses of both symbiotic partners in and ex hospite under thermal challenges. To characterize host and in hospite symbiont responses, symbiotic and aposymbiotic O. arbuscula were exposed to three treatments: (1) control (18°C), (2) heat (32°C), and (3) cold (6°C). This experiment was replicated with B. psygmophilum cultured from O. arbuscula to characterize ex hospite symbiont responses. Both thermal challenges elicited classic environmental stress responses (ESRs) in O. arbuscula regardless of symbiotic state, with hosts responding more strongly to cold challenge. Hosts also exhibited stronger responses than in hospite symbionts. In and ex hospite B. psygmophilum both down-regulated gene ontology pathways associated with photosynthesis under thermal challenge; however, ex hospite symbionts exhibited greater gene expression plasticity and differential expression of genes associated with ESRs. Taken together, these findings suggest that O. arbuscula hosts may buffer environments of B. psygmophilum symbionts; however, we outline the future work needed to confirm this hypothesis.

中文翻译：

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在热挑战下，共生调节共生体的基因表达，但不调节珊瑚宿主的基因表达

海洋温度升高导致珊瑚宿主及其共生体之间的生态失调。先前的研究表明，与细胞内共生体相比，珊瑚宿主基因表达对环境压力的反应更强烈。然而，这一现象的原因和后果仍未得到检验。我们假设共生体的反应较弱，因为宿主会调节共生体环境来缓冲压力。为了检验这一假设，我们利用石珊瑚珊瑚Oculina arbuscula与其共生体Breviolum psygmophilum之间的兼性共生来表征共生伙伴在住院和出院后在热挑战下的基因表达反应。为了表征宿主和寄宿共生反应，共生和非共生O。 丛枝暴露于三种处理：（1）对照（18°C），（2）热（32°C）和（3）冷（6°C）。用B重复了该实验。 从O培养的psygmophilum。  arbuscula来表征前院共生体反应。这两种热挑战都引发了O中的经典环境应激反应 (ESR) 。 无论共生状态如何，丛枝花的宿主对寒冷挑战的反应更强烈。宿主也表现出比寄宿共生体更强的反应。住院和出院B .  psygmophilum在热挑战下均下调与光合作用相关的基因本体通路；然而，寄宿共生体表现出更大的基因表达可塑性以及与 ESR 相关的基因的差异表达。综上所述，这些发现表明O . 丛枝宿主可以缓冲B的环境。 嗜热共生体；然而，我们概述了证实这一假设所需的未来工作。