Phylogeny of the order Scytosiphonales (Phaeophyceae) based on DNA sequences of rbcL, partial rbcS, and partial LSU nrDNA

A molecular phylogenetic study of the order Scytosiphonales (Phaeophyceae) was carried out using DNA sequences of rbcL, partial r-bcS, and partial large subunit (LSU) nrDNA (domains D1 and D2) that were determined in 14 species. Ectocarpus siliculosus (Dillwyn) Lyngbye was adopted as an outgroup taxon. Phylogenetic trees inferred in maximum-parsimony and neighbor-joining analyses were almost identical to each other and showed two large clades. A monophyletic group distributed in warm-temperate to tropical regions includes Chnoospora implexa J. Agardh, Rosenvingea intricata (J. Agardh) Borgesen, Hydroclatrus clathratus (C. Agardh) Howe, and Colpomenia sinuosa (Mertens ex Roth) Derbes et Solier. The second clade includes Colpomenia bullosa (Saunders) Yamada, Colpomenia phaeodactyla Wynne et J. Norris, four species of Scytosiphon, and three species of Petalonia, which are mostly distributed in cold-temperate regions. Colpomenia peregrina (Sauvageau) Hamel showed an ambiguous position between the two large clades. Within the second large clade, a subclade of Petalonia fascia (O. F. Muller) Kuntze, P. binghamiae (J. Agardh) Vinogradova, and Scytosiphon tenellus Kogame and another subclade of Scytosiphon gracilis Kogame and Petalonia zosterifolia (Reinke) Kuntze were supported by high bootstrap values. These results seriously question autonomy of the Chnoosporaceae and monophyly of the genera Colpomenia, Scytosiphon, and Petalonia. Morphological characters of the prostrate sporophytes, such as thallus structure and presence or absence of plurilocular zoidangia, were congruent with the molecular phylogeny. These features are likely to be more important taxonomic criteria at the generic or family level in the Scytosiphonales than is the morphology of the erect gametophytes.