Morphological and molecular evolution of Radiolaria based on the 18S rDNA study

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The marine, holoplanktonic protists group called Phaeodarea is today represented by several hundred species. The phaeodarian skeletons consist of opaline silica as well as organic matter, and are very fragile and vulnerable to dissolution. Their tests are therefore rarely found in the fossil record, and this has caused much uncertainty around their phylogenetic evolution. In this study, three small, solitary phaeodarian species; Protocystis xiphodon (Haeckel), Challengeron diodon Haeckel, and Conchellium capsula Borgert were examined using molecular techniques in order to clarify the phylogenetic position of the Phaeodarea. The obtained phylogenetic trees for all analyses, the neighbor-joining, maximum-parsimony, and maximum-likelihood methods, combined with sequences of Polet et al. (2004) from large phaeodarian species, showed that the all phaeodarians formed a monophyletic group within Phylum Cercozoa. This result run counters to Haeckel's classical taxonomy grouping the phaeodarians together with the polycystines (i.e., nassellarians and spumellarians) and the achantharians, under the common name Radiolaria, and suggesting a monophyly for these groups. Our results strongly suggested the polyphyly of Radiolaria.