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Abstract
A combined data set, which consisted of molecular and nonmolecular traits, was used to address sister- group relationships of diatoms. The result of the cladistic inference indicated that the closest living relative of Bacillariophyceae (Skeletonema costatum (Grun.) Cleve) is a clade composed of representatives of Phaeophyceae (Costaria costata (C. A. Agardh) Saunders), Xanthophyceae (Tribonema aequale Pascher), Chrysophyceae (Ochromonas danica Pringd) and Synurophyceae (Mallomonas papillosa Harris & Bradley) rather than any single one of these taxa. Analyses of suboptimal diatom sister-group topologies demonstrated that the most parsimonious phylogeny is statistically superior to all the alternative hypotheses. The robustness of the preferred sister-group relationship is further supported by bootstrap and weighted analyses of the combined data. Homoplasies in the data are also discussed and it is concluded that the biochemical, cytological and ultrastructural characters show the highest number of extra changes per character.