Phylogenetic relationships of the brown algal orders Ectocarpales, Chordariales, Dictyosiphonales, and Tilopteridales (Phaeophyceae) based on RUBISCO large subunit and spacer sequences

Phylogenetic relationships among 23 species of morphologically simple brown algae belonging to the Ectocarpales sensu stricto, Chordariales, Dictyosiphonales, and Tilopteridales sensu stricto, Phaeophyceae (Fucophyceae), were analyzed using chloroplast-encoded RUBISCO large subunit gene sequences (rbcL) and the associated RUBISCO spacer sequences. Comparison of the observed and expected sequence divergence at the three codon positions of rbcL showed that the level of mutational saturation within the brown algae is minor. Thus, rbcL is well suited for phylogenetic studies in this group. Unweighted parsimony analyses and a neighbor-joining distance analysis were performed using unambiguously aligned rbcL sequences from the above four orders, one marine raphidophyte and two Tribophyceae (Xantophyceae). Polyphyly of Tilopteridales sensu lato (i.e. including Dictyosiphonales) is verified; we therefore recommend the use of Tilopteridales in the strict sense. The Ectocarpales, Chordariales, and Dictyosiphonales are paraphyletic with respect to each other, forming a highly interwoven clade. A separate parsimony analysis of the RUBISCO spacer as well as a combined rbcL and spacer analysis supported the close relationship among the latter three orders, adding to the evidence that they should be subsumed into the Ectocarpales sensu lato.