Department of Biology, University of Richmond, VA 23173, USA.
Biol Rev Camb Philos Soc. 2012 Nov;87(4):804-21. doi: 10.1111/j.1469-185X.2012.00223.x. Epub 2012 Mar 8.
Zooxanthella symbioses are arguably the most important ecological interaction on coral reefs because they energetically subsidize the entire community, and enhance the calcification process that provides structure for all other organisms. While we have developed a detailed understanding of the diversity among and within the Symbiodinium clades, we currently lack a mechanistic explanation for which factors favoured zooxanthella invasion of the intracellular habitat in heterotrophic hosts, and for what molecular mechanisms permit residence within the cell. We propose two hypotheses that explain important evolutionary and ecological features of zooxanthella symbioses. The magnesium inhibition hypothesis (MIH) states that increases in the Mg/Ca ratio in sea water that occurred over the last 100 million years created a situation where Mg(2+) inhibited Ca(2+) transport to zooxanthellae. The MIH predicts, among other things, that the intracellular niche was invaded as a response to this abiotic stressor. The arrested phagosome hypothesis (APH) states that Symbiodinium spp. mimic host cell endosomal digestive machinery via the symbiosome to appear like digesting prey through perpetual release of zooxanthella-derived compounds. The APH represents a subtle but important distinction from previous hypotheses regarding interactions between symbiont and host at the cellular level. The APH predicts that symbionts tune rates of material release to match expectations of host cellular machinery. An outcome of the APH is that intra-host residence time becomes a vital parameter to consider. Both hypotheses shift control of the symbiosis away from the host, and instead focus attention on the niche requirements of Symbiodinium spp.
虫黄藻共生关系可以说是珊瑚礁上最重要的生态相互作用，因为它们为整个群落提供能量，并增强了钙化过程，为所有其他生物提供了结构。虽然我们已经对共生藻属内和属间的多样性有了详细的了解，但我们目前还没有一个机械解释来说明哪些因素有利于虫黄藻入侵异养宿主的细胞内栖息地，以及哪些分子机制允许其在细胞内生存。我们提出了两个假设来解释虫黄藻共生关系的重要进化和生态特征。镁抑制假说（MIH）认为，过去 1 亿年来海水中镁/钙比值的增加导致了镁（Mg2+）抑制了向虫黄藻的钙（Ca2+）运输。MIH 预测，除其他外，细胞内小生境是作为对这种非生物胁迫的反应而被入侵的。吞噬体被捕假说（APH）认为，共生藻属通过共生体模拟宿主细胞内体消化机制，通过不断释放虫黄藻衍生化合物，使其看起来像是在消化猎物。与以前关于共生体和宿主在细胞水平上相互作用的假设相比，APH 代表了一个微妙但重要的区别。APH 预测，共生体将物质释放的速度调整到与宿主细胞机制的预期相匹配。APH 的一个结果是，宿主内的居留时间成为一个需要考虑的重要参数。这两个假说都将共生关系的控制权从宿主转移开，而是将注意力集中在共生藻属的小生境需求上。
