Molecular phylogeny of crustose brown algae (Ralfsiales, Phaeophyceae) inferred from rbcL sequences resulting in the proposal for Neoralfsiaceae fam. nov.

The order Ralfsiales was established to accommodate the brown algal taxa having a crustose thallus, an isomorphic life history, discoid early development of the thallus and containing a single, plate-shaped chloroplast without pyrenoids in each cell. However, the validity of the order has been questioned by many researchers because several exceptions to these criteria have been found within the order. Molecular phylogenetic analysis of the taxa assigned to the order, using rbcL DNA sequences, reveals that Ralfisiales is not a monophyletic group but is separated into two major groups, excluding Lithodermataceae, which were not included in the present analysis: clade 1, comprising the members of Ralfsiaceae, Mesosporaceae, Analipus japonicus and Heteroralfsia saxicola; and clade 11, consisting of Diplura species, sister to the Ishigeales clade. On the basis of these results, we propose emendment of the Ralfsiales to contain only species having (1) discoidal early development of the thallus; (2) intercalary plurilocular gametangia with terminal cells and terminal unilocular zoidangia; and (3) a crustose phase in the life history. Furthermore, we propose the establishment of the new family Neoralfsiaceae to accommodate the new genus Neoralfsia, on the basis of Ralfsia expansa.