Mesostigmatophyceae, a new class of streptophyte green algae revealed by SSU rRNA sequence comparisons

Complete nuclear-encoded SSU rRNA sequences have been obtained from three t axa of streptophyte green algae (Klebsormidium nitens, Nitella capillaris, Chaetosphaeridium globosum) and two strains of the scaly green flagellate M esostigma viride. Phylogenetic analyses of 70 taxa of Viridiplantae (Chloro phyta and Streptophyta) and 57 taxa of streptophyte green algae and embryop hyte plants using distance, parsimony and likelihood methods revealed a nov el monophyletic lineage among the Streptophyta comprising the genera Mesost igma and Chaetosphaeridium. This lineage is described here as the Mesostigm atophyceae classis nova. Our analyses demonstrate that (1) scaly green flag ellates (prasinophytes) are polyphyletic, (2) a scaly green flagellate is a member of the Streptophyta and forms a clade with the oogamous, filamentou s Chaetosphaeridium to the exclusion of all other known streptophyte green algae, (3) a previously published SSU rRNA sequence of Chaetosphaeridium (A F113506) is chimeric and contains part of a fungal SSU rRNA, and (4) the ph ylogenetic relationships between the Mesostigmatophyceae and other streptop hyte green algae remain unresolved by SSU rRNA sequence comparisons.