PHYLOGENETIC POSITION OF CHAETOSPHAERIDIUM (CHLOROPHYTA), A BASAL LINEAGE IN THE CHAROPHYCEAE INFERRED FROM 18S rDNA SEQUENCES

We sequenced the nuclear-encoded small-subunit ribosomal RNA gene (18S rDNA) of Chaetosphaeridium globosum (Nordst.) Klebahn, a microscopic freshwater epiphytic chlorophyte, to assess its phylogenetic affinities in the Chlorophyta. A phylogenetic analysis of a broad sampling of green algal taxa and Chaetosphaeridium confirmed that this alga is a member of the Charophyceae (Streptophyta) as earlier microscopical studies had suggested. However, more detailed phylogenetic analyses of the streptophyte lineage showed that contrary to expectations based on the ultrastructure of the zoospores, the presence of a unique type of setae, the oogamous mode of reproduction, and the occurrence of oscillatory rotations of the cytoplasm, Chaetosphaeridium and Coleochaete are not closely related and do not form a monophyletic clade. Instead, Chaetosphaeridium represents an early branch in the streptophyte lineage that had a near-simultaneous origin as the Charalean clade and a clade formed by all remaining streptophytes examined ( Klebsormidium, Coleochaete, Chlorokybus, Zygnematales, and bryophytes). All phylogenetic inference methods used (neighbor-joining analysis of Kimura distances, maximum likelihood, and maximum parsimony) resulted in essentially the same tree topology. No Group I introns were found in the 18S rDNA coding region of Chaetosphaeridium. Our molecular phylogenetic analysis of Chaetosphaeridium supports a recent cladistic classification of the Streptobionta by Kenrick and Crane in which Chaetosphaeridium is placed in a monotypic division and class, Chaetosphaeridiophyta and Chaetosphaeridiophyceae, respectively.