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 th e Xanthophyceae based on the level of organization of vegetative cells and to rest the hypothesis that larger morphologically complex species are high ly derived members of the class. Sequences for the plastid-encoded large su bunit of ribulose-1,5- bisphosphate carboxylase/oxygenase (rbcL) and nuclea r-encoded small subunit ribosomal RNA (18S rRNA) genes were aligned for nin e xanthophyte species. Separate and combined analyses of these data matrice s yielded topologically identical trees. The molecular data do not support the monophyly of the orders Mischococcales or Tribonematales, to which cocc oid and filamentous species, respectively, are traditionally assigned. Mult iple, 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 clo se relationship between the siphonous, sexually reproducing genera Botrydiu m 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 t hat 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 b ipartite cell wall.