Phylogenetic relationships among nine species of the Xanthophyceae inferred from rbcL and 18S rRNA gene sequences

Gene sequence data were used to evaluate traditional classifications for the Xanthophyceae based on the level of organization of vegetative cells and to rest the hypothesis that larger morphologically complex species are highly derived members of the class. Sequences for the plastid-encoded large subunit of ribulose-1,5- bisphosphate carboxylase/oxygenase (rbcL) and nuclear-encoded small subunit ribosomal RNA (18S rRNA) genes were aligned for nine xanthophyte species. Separate and combined analyses of these data matrices yielded topologically identical trees. The molecular data do not support the monophyly of the orders Mischococcales or Tribonematales, to which coccoid and filamentous species, respectively, are traditionally assigned. Multiple, independent origins of each of these life forms are inferred from the gene tree. Thus, the level of organization of cells of species may not be a phylogenetically reliable character for ordinal classification. Although placed in the same order by some authorities, evidence is lacking for a close relationship between the siphonous, sexually reproducing genera Botrydium and Vaucheria. Vaucheria, the largest and reproductively most complex of all xanthophytes, is positioned on a long branch at the base of the ingroup and is not closely related to any other species examined. The contention that Vaucheria is the most highly advanced xanthophyte taxon is rejected by the molecular data. Results tentatively suggest that species may be divided between two major lineages characterized by the presence or absence of a bipartite cell wall.