Department of Biosciences, Rice University, 6100 S. Main St., Houston, TX, 77005, USA.
Microb Ecol. 2019 Feb;77(2):537-545. doi: 10.1007/s00248-018-01315-1. Epub 2019 Jan 6.
It has been proposed that holobionts (host-symbiont units) could swap endosymbionts, rapidly alter the hologenome (host plus symbiont genome), and increase their stress tolerance. However, experimental tests of individual and combined contributions of hosts and endosymbionts to holobiont stress tolerance are needed to test this hypothesis. Here, we used six green hydra (Hydra viridissima) strains to tease apart host (hydra) and symbiont (algae) contributions to thermal tolerance. Heat shock experiments with (1) hydra with their original symbionts, (2) aposymbiotic hydra (algae removed), (3) novel associations (a single hydra strain hosting different algae individually), and (4) control hydra (aposymbiotic hydra re-associated with their original algae) showed high variation in thermal tolerance in each group. Relative tolerances of strains were the same within original, aposymbiotic, and control treatments, but reversed in the novel associations group. Aposymbiotic hydra had similar or higher thermal tolerance than hydra with algal symbionts. Selection on the holobiont appears to be stronger than on either partner alone, suggesting endosymbiosis could become an evolutionary trap under climate change. Our results suggest that green hydra thermal tolerance is strongly determined by the host, with a smaller, non-positive role for the algal symbiont. Once temperatures exceed host tolerance limits, swapping symbionts is unlikely to allow these holobionts to persist. Rather, increases in host tolerance through in situ adaptation or migration of pre-adapted host strains appear more likely to increase local thermal tolerance. Overall, our results indicate green hydra is a valuable system for studying aquatic endosymbiosis under changing environmental conditions, and demonstrate how the host and the endosymbiont contribute to holobiont stress tolerance.
有人提出，后生生物（宿主-共生体单元）可以交换内共生体，快速改变后生生物组（宿主加上共生体基因组），并提高其应激耐受性。然而，需要对宿主和内共生体对后生生物应激耐受性的个体和联合贡献进行实验测试，以检验这一假设。在这里，我们使用六种绿色水螅（Hydra viridissima）菌株来区分宿主（水螅）和共生体（藻类）对热耐受性的贡献。对（1）带有原始共生体的水螅、（2）无共生体的水螅（藻类去除）、（3）新的共生体组合（单个水螅菌株分别与不同的藻类共生）和（4）对照水螅（与原始藻类重新共生的无共生体水螅）进行热休克实验，结果表明每组水螅的耐热性差异很大。在原始、无共生体和对照处理中，菌株的相对耐受性在各组内是相同的，但在新的共生体组合组中则相反。无共生体的水螅具有与带藻类共生体的水螅相似或更高的耐热性。后生生物的选择似乎比单独选择两个共生体都要强，这表明内共生体在气候变化下可能成为进化陷阱。我们的结果表明，绿色水螅的耐热性主要由宿主决定，藻类共生体的作用较小且为负向。一旦温度超过宿主的耐受极限，交换共生体不太可能使这些后生生物持续存在。相反，通过原位适应或适应能力更强的宿主菌株的迁移来提高宿主的耐受性，似乎更有可能提高局部的耐热性。总的来说，我们的研究结果表明，绿色水螅是研究水生内共生体在变化的环境条件下的一个有价值的系统，并展示了宿主和内共生体如何共同影响后生生物的应激耐受性。
