Van Oppen M J H, Mieog J C, Sánchez C A, Fabricius K E
Australian Institute of Marine Science, PMB no. 3, Townsville MC, Qld 4810, Australia.
Mol Ecol. 2005 Jul;14(8):2403-17. doi: 10.1111/j.1365-294X.2005.02545.x.
The presence, genetic identity and diversity of algal endosymbionts (Symbiodinium) in 114 species from 69 genera (20 families) of octocorals from the Great Barrier Reef (GBR), the far eastern Pacific (EP) and the Caribbean was examined, and patterns of the octocoral-algal symbiosis were compared with patterns in the host phylogeny. Genetic analyses of the zooxanthellae were based on ribosomal DNA internal transcribed spacer 1 (ITS1) region. In the GBR samples, Symbiodinium clades A and G were encountered with A and G being rare. Clade B zooxanthellae have been previously reported from a GBR octocoral, but are also rare in octocorals from this region. Symbiodinium G has so far only been found in Foraminifera, but is rare in these organisms. In the Caribbean samples, only Symbiodinium clades B and C are present. Hence, Symbiodinium diversity at the level of phylogenetic clades is lower in octocorals from the Caribbean compared to those from the GBR. However, an unprecedented level of ITS1 diversity was observed within individual colonies of some Caribbean gorgonians, implying either that these simultaneously harbour multiple strains of clade B zooxanthellae, or that ITS1 heterogeneity exists within the genomes of some zooxanthellae. Intracladal diversity based on ITS should therefore be interpreted with caution, especially in cases where no independent evidence exists to support distinctiveness, such as ecological distribution or physiological characteristics. All samples from EP are azooxanthellate. Three unrelated GBR taxa that are described in the literature as azooxanthellate (Junceella fragilis, Euplexaura nuttingi and Stereonephthya sp. 1) contain clade G zooxanthellae, and their symbiotic association with zooxanthellae was confirmed by histology. These corals are pale in colour, whereas related azooxanthellate species are brightly coloured. The evolutionary loss or gain of zooxanthellae may have altered the light sensitivity of the host tissues, requiring the animals to adopt or reduce pigmentation. Finally, we superimposed patterns of the octocoral-algal symbiosis onto a molecular phylogeny of the host. The data show that many losses/gains of endosymbiosis have occurred during the evolution of octocorals. The ancestral state (azooxanthellate or zooxanthellate) in octocorals remains unclear, but the data suggest that on an evolutionary timescale octocorals can switch more easily between mixotrophy and heterotrophy compared to scleractinian corals, which coincides with a low reliance on photosynthetic carbon gain in the former group of organisms.
对来自大堡礁（GBR）、远东海域（EP）和加勒比地区的69个属（20个科）的114种八放珊瑚中的藻类内共生体（共生藻属）的存在情况、遗传特性及多样性进行了研究，并将八放珊瑚 - 藻类共生模式与宿主系统发育模式进行了比较。对虫黄藻的遗传分析基于核糖体DNA内部转录间隔区1（ITS1）区域。在GBR样本中，发现了共生藻属A和G类群，其中A和G较为罕见。此前曾报道过GBR的一种八放珊瑚中有B类群虫黄藻，但在该地区的八放珊瑚中也很罕见。共生藻属G迄今仅在有孔虫中发现，但在这些生物中也很罕见。在加勒比地区的样本中，仅存在共生藻属B和C类群。因此，与GBR的八放珊瑚相比，加勒比地区八放珊瑚在系统发育类群水平上的共生藻属多样性较低。然而，在一些加勒比柳珊瑚的个体群体中观察到了前所未有的ITS1多样性水平，这意味着这些群体要么同时含有多种B类群虫黄藻菌株，要么在某些虫黄藻的基因组中存在ITS1异质性。因此，基于ITS的类群内多样性应谨慎解释，尤其是在没有独立证据支持其独特性（如生态分布或生理特征）的情况下。来自EP的所有样本均无虫黄藻。文献中描述为无虫黄藻的三种不相关的GBR分类群（脆弱软柳珊瑚、纳氏真柳珊瑚和一种立体软珊瑚）含有G类群虫黄藻，并且通过组织学证实了它们与虫黄藻的共生关系。这些珊瑚颜色较浅，而相关的无虫黄藻物种颜色鲜艳。虫黄藻的进化性丧失或获得可能改变了宿主组织的光敏感性，要求动物采取或减少色素沉着。最后，我们将八放珊瑚 - 藻类共生模式叠加到宿主的分子系统发育树上。数据表明，在八放珊瑚的进化过程中发生了许多内共生的丧失/获得情况。八放珊瑚的原始状态（无虫黄藻或有虫黄藻）仍不清楚，但数据表明，在进化时间尺度上，与石珊瑚相比，八放珊瑚在混合营养和异养之间更容易转换，这与前一组生物对光合碳获取的低依赖性相吻合。
