A comprehensive phylogeny of the Phaeophyceae based on nrDNA sequences resolves the earliest divergences

The present study is the first comprehensive phylogeny of the class of the Phaeophyceae. For 67 species representing all orders of the class, the sequ ences of the 3 ' -end of the small and the 5 ' -end of the large subunit nr RNA genes were aligned and analysed. A further analysis based on sequences of the 3 ' -end of the small and of the complete sequences of the 28S gene of the large subunits was also performed, but for only eight taxa. In both analyses, Tribonema marinum (Xanthophyceae) was used as outgroup. The analy ses showed the Dictyotales as diverging first, followed by the Sphacelarial es, then the Syringodermatales. Most of the orders currently accepted were confirmed as monophyletic groups but the Laminariales and Tilopteridales re mained polyphyletic. The relationships of the remaining orders to each othe r were not resolved with the present data set. Ascoseira, included for the first time in a molecular study, appeared as a separate lineage without any clear relationship with other algae possessing conceptacles (Splachnidium and Fucales). Algae with stellate plastids, never studied in a global conte xt, were polyphyletic; this result is consistent with their plastid ultrast ructure and is discussed in detail. As further result of the present study, the South African genus Bifurcariopsis appeared as the sister taxon of the North Atlantic genus Himanthalia, and Xiphophora appeared as the sister ta xon of Hormosira rather than as a member of the Fucaceae; the taxonomic pos ition of these genera is discussed. (C) 2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.