Bendif El Mahdi, Probert Ian, Young Jeremy R, von Dassow Peter
Marine Biological Association of the UK, Plymouth, UK.
Université Pierre et Marie Curie (Paris VI), Station Biologique de Roscoff, Roscoff, France; Centre National de la Recherche Scientifique, FR2424, Roscoff Culture Collection, Station Biologique de Roscoff, Roscoff, France.
Protist. 2015 Jul;166(3):323-36. doi: 10.1016/j.protis.2015.05.003. Epub 2015 May 14.
The coccolithophore genus Gephyrocapsa contains a cosmopolitan assemblage of pelagic species, including the bloom-forming Gephyrocapsa oceanica, and is closely related to the emblematic coccolithophore Emiliania huxleyi within the Noëlaerhabdaceae. These two species have been extensively studied and are well represented in culture collections, whereas cultures of other species of this family are lacking. We report on three new strains of Gephyrocapsa isolated into culture from samples from the Chilean coastal upwelling zone using a novel flow cytometric single-cell sorting technique. The strains were characterized by morphological analysis using scanning electron microscopy and phylogenetic analysis of 6 genes (nuclear 18S and 28S rDNA, plastidial 16S and tufA, and mitochondrial cox1 and cox3 genes). Morphometric features of the coccoliths indicate that these isolates are distinct from G. oceanica and best correspond to G. muellerae. Surprisingly, both plastidial and mitochondrial gene phylogenies placed these strains within the E. huxleyi clade and well separated from G. oceanica isolates, making Emiliania appear polyphyletic. The only nuclear sequence difference, 1bp in the 28S rDNA region, also grouped E. huxleyi with the new Gephyrocapsa isolates and apart from G. oceanica. Specifically, the G. muellerae morphotype strains clustered with the mitochondrial β clade of E. huxleyi, which, like G. muellerae, has been associated with cold (temperate and sub-polar) waters. Among putative evolutionary scenarios that could explain these results we discuss the possibility that E. huxleyi is not a valid taxonomic unit, or, alternatively the possibility of past hybridization and introgression between each E. huxleyi clade and older Gephyrocapsa clades. In either case, the results support the transfer of Emiliania to Gephyrocapsa. These results have important implications for relating morphological species concepts to ecological and evolutionary units of diversity.
颗石藻属的球石藻包含一个遍布全球的远洋物种组合，其中包括能形成水华的大洋球石藻，并且在奈氏球石藻科中与标志性的颗石藻赫氏艾氏藻密切相关。这两个物种已经得到了广泛研究，在培养物保藏中心也有很好的代表性，而该科其他物种的培养物则很缺乏。我们报告了使用一种新颖的流式细胞术单细胞分选技术，从智利沿海上升流区的样本中分离培养出的三株新的球石藻菌株。通过扫描电子显微镜进行形态分析以及对6个基因（核18S和28S rDNA、质体16S和tufA，以及线粒体cox1和cox3基因）进行系统发育分析，对这些菌株进行了表征。颗石的形态测量特征表明，这些分离株与大洋球石藻不同，最符合米勒球石藻。令人惊讶的是，质体和线粒体基因系统发育都将这些菌株置于赫氏艾氏藻分支内，并且与大洋球石藻分离株明显分开，这使得艾氏藻属显得是多系的。唯一的核序列差异，即在28S rDNA区域中的1个碱基对，也将赫氏艾氏藻与新的球石藻分离株归为一类，而与大洋球石藻分开。具体来说，米勒球石藻形态型菌株与赫氏艾氏藻的线粒体β分支聚集在一起，该分支与米勒球石藻一样，与寒冷（温带和亚极地）水域有关。在可以解释这些结果的假定进化情景中，我们讨论了赫氏艾氏藻不是一个有效的分类单元的可能性，或者，另一种可能性是每个赫氏艾氏藻分支与较古老的球石藻分支之间过去存在杂交和基因渗入。无论哪种情况，结果都支持将艾氏藻属归入球石藻属。这些结果对于将形态物种概念与生态和进化多样性单元联系起来具有重要意义。
