Radiolaria Divided into Polycystina and Spasmaria in Combined 18S and 28S rDNA Phylogeny

Radiolarians are marine planktonic protists that belong to the eukaryote supergroup Rhizaria together with Foraminifera and Cercozoa. Radiolaria has traditionally been divided into four main groups based on morphological characters; i.e. Polycystina, Acantharia, Nassellaria and Phaeodaria. But recent 18S rDNA phylogenies have shown that Phaeodaria belongs within Cerocozoa, and that the previously heliozoan group Taxopodida should be included in Radiolaria. 18S rDNA phylogenies have not yet resolved the sister relationship between the main Radiolaria groups, but nevertheless suggests that Spumellaria, and thereby also Polycystina, are polyphyletic. Very few sequences other than 18S rDNA have so far been generated from radiolarian cells, mostly due to the fact that Radiolaria has been impossible to cultivate and single cell PCR has been hampered by low success rate. Here we have therefore investigated the mutual evolutionary relationship of the main radiolarian groups by using the novel approach of combining single cell whole genome amplification with targeted PCR amplification of the 18S and 28S rDNA genes. Combined 18S and 28S phylogeny of sequences obtained from single cells shows that Radiolaria is divided into two main lineages: Polycystina (Spumellaria+Nassellaria) and Spasmaria (Acantharia+Taxopodida). Further we show with high support that Foraminifera groups within Radiolaria supporting the Retaria hypothesis.

放射虫（Radiolarians）是海洋浮游原生生物，与有孔虫（Foraminifera）、丝足虫（Cercozoa）同属于真核生物超类群根足虫总界（Rhizaria）。传统分类学中，放射虫依据形态特征被划分为四大主要类群：多孔虫纲（Polycystina）、等辐骨虫目（Acantharia）、罩笼虫目（Nassellaria）及泡沫虫纲（Phaeodaria）。但近期基于18S核糖体DNA（18S rDNA）的系统发育研究表明，泡沫虫纲（Phaeodaria）隶属于丝足虫门（Cercozoa），而原归类为太阳虫类（heliozoan）的足针虫目（Taxopodida）应当被纳入放射虫类群。
尽管18S rDNA系统发育分析尚未明确放射虫主要类群间的姐妹群关系，但其结果显示，泡沫虫目（Spumellaria）乃至多孔虫纲（Polycystina）均为多系演化群。迄今为止，除18S rDNA序列外，从放射虫细胞中获取的其他序列极为有限，这主要源于放射虫无法进行人工培养，且单细胞聚合酶链式反应（single cell PCR）的成功率极低。
为此，本研究采用将单细胞全基因组扩增（whole genome amplification）与18S、28S核糖体DNA（28S rDNA）基因的靶向聚合酶链式反应（targeted PCR）相结合的全新方法，探究了放射虫主要类群间的相互演化关系。基于单细胞获取的序列构建的18S与28S联合系统发育树显示，放射虫可划分为两大主要演化支：多孔虫纲（Polycystina，包含泡沫虫目Spumellaria与罩笼虫目Nassellaria）与痉挛虫演化支（Spasmaria，包含等辐骨虫目Acantharia与足针虫目Taxopodida）。此外，本研究以高支持度证实，有孔虫（Foraminifera）隶属于放射虫类群，这一结果支持了网状虫假说（Retaria hypothesis）。