Acquisition and proliferation of algal symbionts in bleached polyps of the upside-down jellyfish, Cassiopea xamachana
Acquisition and proliferation of algal symbionts in bleached polyps of the upside-down jellyfish, Cassiopea xamachana
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DOI：
10.1016/j.jembe.2018.08.010
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发表时间：
2018-11-01
影响因子：
2
通讯作者：
Martindale, Mark Q.
中科院分区：
文献类型：
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作者：
Newkirk, Casandra R.;Frazer, Thomas K.;Martindale, Mark Q.
共生微藻属的鞭毛藻与刺胞门的动物，特别是珊瑚之间的共生关系，构成了海洋环境中最著名的共生关系之一。这种关系的本质是，这些共生藻类为它们的刺胞宿主提供光合作用的产物，而藻类则在宿主的内胚层细胞中得到庇护。Cassiopea xamachana（又名“倒立水母”）是一种可以在实验室环境中进行整个生命周期饲养的孢子动物，使用这种模型系统可以研究刺胞动物和藻类的共生关系，而不受珊瑚和其他刺胞动物的许多限制。本研究利用C. xamachana的非共生水螅阶段来可视化宿主动物的内胚层细胞获取藻类细胞的过程，这是向自由生活的水母（胞胞）过渡的必要现象。在实验之前，从环境源中携带藻类的珊瑚虫受到高温的影响，以启动藻类的排出，并获得完全没有共生体的珊瑚虫（非共生体）；用高分辨共聚焦显微镜证实了白化。随后将非共生水螅体引入少量共生菌中，并根据每隔7天产生的计数数据计算藻类的内在生长率。在实验试验中用于重新引入的共生菌来自三个来源：(1)商业培养的藻类细胞（A194型），(2)在实验室热诱导漂白后从宿主组织中排出的藻类，以及(3)从成年水母中分离出来的藻类。这三种藻类细胞来源被用来代表可能的水库，在漂白事件发生后，被漂白的动物可能从中补充其共生体种群。在所有三个处理组中，C. xamachana息肉都获得了共生菌细胞，尽管培养的共生菌A194在宿主的内胚层细胞中表现出最慢的平均内在生长速率（0.054 d(-1)）。因此，对于那些携带A194型共生体的息肉来说，平均到卵化的时间较慢（大约120天）。事实上，含有其他来源共生菌的珊瑚虫平均在70天内就会发生交配。虽然我们知道，在水螅阶段，如果没有共生体的获取，就不会发生互生，但本研究提供的结果清楚地表明，在互生时，共生体的数量可能会有很大的变化。在共生发生之前，这种共生数量的变化证实了之前的文献证据，即共生不仅是由共生的获得所刺激的，也是由其他触发因素引起的。我们的方法允许使用定量方法来发现这些物种之间成功建立共生关系所涉及的其他成分。
The mutualistic relationship between dinoflagellates in the genus Symbiodinium microadriaticum and animals in the phylum Cnidaria, specifically corals, make up one of the most well-known symbioses in the marine environment. The nature of the relationship is such that these symbiotic algae provide products of photosynthesis to their cnidarian hosts, while the algae are afforded refuge within the endodermal cells of the host. Use of the model system Cassiopea xamachana (a.k.a. the 'upside down jellyfish'), a scyphozoan that can be reared throughout its entire life cycle in a laboratory setting, allowed for the study of the cnidarian-algal symbiosis without many of the constraints involved when working with corals and other cnidarians. The aposymbiotic polyp stage of C. xamachana was used in this study to visualize the acquisition of algal cells by endodermal cells of host animals, a requisite phenomenon for the transition to a free living medusae (strobilation). Polyps harboring algae from environmental sources prior to experimentation were subjected to high temperatures to initiate the expulsion of algae and to obtain polyps that were completely free of symbionts (aposymbiotic); bleaching was confirmed with high resolution confocal microscopy. Aposymbiotic polyps were subsequently introduced to low numbers of Symbiodinium, and the intrinsic growth rate of algae calculated based on count data generated at 7-day intervals. Symbiodinium used for reintroduction in experimental trials were obtained from three sources: (1) commercially cultured algal cells (type A194), (2) algae that had been expelled from their host tissues following heat-induced bleaching in the laboratory, and (3) algae freshly isolated from an adult medusa. These three sources of algal cells were used to represent the possible reservoirs from which bleached animals might replenish their symbiont population following a bleaching event. Symbiodinium cells were acquired by C. xamachana polyps in all three treatment groups, although cultured Symbiodinium A194 exhibited the slowest mean intrinsic growth rate (0.054 d(-1)) once they were housed within endodermal cells of their host. Mean time to strobilation was accordingly slow (> 120 days) for those polyps harboring type A194 symbionts. In fact, polyps harboring other sourced Symbiodinium strobilated in < 70 days on average. Though it is known that strobilation does not occur without the acquisition of symbionts during the polyp stage, the results provided in this study demonstrate clearly that the number of symbionts at the time of strobilation can vary greatly. This variation in symbiont number just prior to strobilation, substantiates previous documented evidence that strobilation is not only stimulated by acquisition of Symbiodinium, but by other triggers as well. Our methods allow quantitative methods to be used to discover additional components involved in the establishment of successful symbiosis between these species.