Institute for Study of the Earth's Interior, Okayama University, Yamada, Misasa, Tottori, Japan.
PLoS One. 2013 Dec 4;8(12):e81749. doi: 10.1371/journal.pone.0081749. eCollection 2013.
Braarudosphaera bigelowii (Prymnesiophyceae) is a coastal coccolithophore with a long fossil record, extending back to the late Cretaceous (ca. 100 Ma). A recent study revealed close phylogenetic relationships between B. bigelowii, Chrysochromulina parkeae (Prymnesiophyceae), and a prymnesiophyte that forms a symbiotic association with the nitrogen-fixing cyanobacterium UCYN-A. In order to further examine these relationships, we conducted transmission electron microscopic and molecular phylogenetic studies of B. bigelowii. TEM studies showed that, in addition to organelles, such as the nucleus, chloroplasts and mitochondria, B. bigelowii contains one or two spheroid bodies with internal lamellae. In the 18S rDNA tree of the Prymnesiophyceae, C. parkeae fell within the B. bigelowii clade, and was close to B. bigelowii Genotype III (99.89% similarity). Plastid 16S rDNA sequences obtained from B. bigelowii were close to the unidentified sequences from the oligotrophic SE Pacific Ocean (e.g. HM133411) (99.86% similarity). Bacterial16S rDNA sequences obtained from B. bigelowii were identical to the UCYN-A sequence AY621693 from Arabian Sea, and fell in the UCYN-A clade. From these results, we suggest that; 1) C. parkeae is the alternate life cycle stage of B. bigelowii sensu stricto or that of a sibling species of B. bigelowii, and 2) the spheroid body of B. bigelowii originated from endosymbiosis of the nitrogen-fixing cyanobacterium UCYN-A.
大弯藻（Prymnesiophyceae）是一种具有长化石记录的沿海颗石藻，可追溯到晚白垩世（约 1 亿年前）。最近的一项研究表明，B. bigelowii 与 Chrysochromulina parkeae（Prymnesiophyceae）以及与固氮蓝藻 UCYN-A 形成共生关系的颗石藻之间存在密切的系统发育关系。为了进一步研究这些关系，我们对 B. bigelowii 进行了透射电子显微镜和分子系统发育研究。TEM 研究表明，除了细胞核、叶绿体和线粒体等细胞器外，B. bigelowii 还含有一个或两个具有内部层的球形体。在颗石藻的 18S rDNA 树中，C. parkeae 位于 B. bigelowii 分支内，与 B. bigelowii Genotype III（相似度为 99.89%）接近。从 B. bigelowii 获得的质体 16S rDNA 序列与寡营养的东南太平洋未鉴定序列（例如 HM133411）（相似度为 99.86%）接近。从 B. bigelowii 获得的细菌 16S rDNA 序列与阿拉伯海的 UCYN-A 序列 AY621693 相同，并且位于 UCYN-A 分支内。根据这些结果，我们提出：1）C. parkeae 是 B. bigelowii 或与其亲缘关系密切的物种的交替生命周期阶段，2）B. bigelowii 的球形体起源于固氮蓝藻 UCYN-A 的内共生。
