Molecular and morphological investigations of three brown algal genera with stellate plastids: evidence for Scytothamnales ord. nov. (Phaeophyceae)

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Abstract
Recent molecular phylogenies of the Phaeophyceae have demonstrated that the morphologically simple ephemeral species referred to as Ectocarpales sensu lato are not primitive in a phylogenetic sense. They form a monophyletic group and share parietal plastids with pyrenoids. The clade is distant from the orders of morphologically advanced brown algae with discoid plastids and no or reduced pyrenoids. Sequences of parts of the nuclear ribosomal cistron of species of the genera Scytothamnus, Stereocladon, and Splachnidium, which have stellate chloroplasts with a single pyrenoid in the center of the cells, provide strong evidence that none of these algae belong to the Ectocarpales sensu lato. Neither do they group with the Fucales, with which Splachnidium shares receptacles and conceptacles. Although an analysis of small subunit sequences fails to clearly separate them from the Laminariales, with which they do not share morphological, cytological, or reproductive characters, a combined analysis of partial small subunit sequences and more conserved parts of the otherwise highly variable internal transcribed spacers (ITS1 and ITS2) places them between Laminariales and Ectocarpales sensu lato. We propose Scytothamnales ord. nov. with the two closely related genera Scytothamnus and Stereocladon in the family Scytothamnaceae and the more distant Splachnidium in its own monotypic family. Adenocystis utricularis (Bory) Skottsberg, which previously has been included in Scytothamnaceae despite its different, discoid plastid type, remains a member of Ectocarpales sensu lato.

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