Molecular phylogeny of Laminariales (Phaeophyceae) inferred from small subunit ribosomal DNA sequences

In order to elucidate the molecular phylogeny of the order Laminariales, complete small subunit (SSU) rDNA sequences were determined for 11 species, including representatives from all the families of the order: Pseudochorda gracilis Kawai et Nabata and P. nagaii (Tokida) Inagaki of the Pseudochordaceae, Chorda filum (L.) Stackhouse of the Chordaceae, Alaria fistulosa Postels et Ruprecht, Ecklonia cava Kjellman and Egregia menziesii (Turner) Areschoug of the Alariaceae, Agarum clathratum f. yakishiriense, I. Yamada, Kjellmaniella crassifolia Miyabe and Laminaria japonica Areschoug of the Laminariaceae and Lessonia nigrescens Bory and Postelsia palmaeformis Ruprecht of the Lessoniaceae. The published data of Halosiphon tomentosus (Lyngbye) Jaasund and Saccorhiza polyschides (Lightfoot) Batters were included for tree construction. Our SSU rDNA sequences show that the Pseudochordaceae and Chordaceae are clearly separated from the strongly monophyletic group consisting of the Alariaceae, Laminariaceae and Lessoniaceae. The sequence data also show that the Pseudochordaceae is monophyletic and is distant from H. tomentosus and S. polyschides. Considering our molecular data and the reported morphology, life history and sex pheromones of the family, it appears likely that the Pseudochordaceae might have branched off first from the laminarialean lineage that leads, through the Chordaceae, to the advanced Laminariales (i.e. the Alariaceae, Laminariaceae and Lessoniaceae). The limited resolution that results from the close similarities among the SSU rDNA sequences gives a need for more informative molecular markers in order to resolve the circumscription and phylogeny of the Laminariales.