Phylogenetic analyses of the polycystine Radiolaria based on the 18s rDNA sequences of the Spumellarida and the Nassellarida

Acantharea, Polycystinea, and Phaeodarea have members that are widely distributed in the marine plankton. Many biologists use the conventional term “Radiolaria” to include these three classes. However, on the basis of an 18S rDNA study, Polet et al. (2004, Protist 155, 53-63) recently suggested that the Phaeodarea should be moved into the Phylum Cercozoa. In the present paper, the phylogenetic relationships of the Acantharea and the Polycystinea, especially the phylogenetic positions of Nassellarida and Spumellarida, were inferred from 18S rDNA sequences including those we have determined from the Family Spongodiscidae (Class Polycystinea, Order Spumellarida) and the Family Pterocorythidae (Class Polycystinea, Order Nassellarida). Among major eukaryotic lineages, the Polycystinea were shown to constitute a paraphyletic group: in the phylogenetic trees for the relationships among polycystines, the collosphaerid, sphaerozoid, and thalassicollid spumellarians and the pterocorycid nassellarians constantly formed a monophyletic group, from which the spongodiscid spumellarians were excluded. This conclusion is not consistent with the current taxonomy of the “Radiolaria,” and leads us to consider that the collosphaerid, sphaerozoid, and thalassicollid spumellarians and the pterocorycid nassellarians may have evolved from an ancestor with triradiate branched spicules.