PRASINOPHYTES FORM INDEPENDENT LINEAGES WITHIN THE CHLOROPHYTA - EVIDENCE FROM RIBOSOMAL-RNA SEQUENCE COMPARISON

Complete nuclear-encoded small-subunit ribosomal RNA (rRNA) sequences were determined from Nephroselmis olivacea Stein, Pseudoscourfieldia marina (Throndsen) Manton, Scherffelia dubia (Perty) Pascher, and Tetraselmis striata Butcher (Chlorophyta) to investigate the evolutionary position of these scaly green flagellates. Results of neighbor-joining and maximum parsimony phylogenetic analyses demonstrate at least two independent prasinophyte lineages defined by N. olivacea/P. marina and S. dubia/T. striata, which together with the Chlorophyceae, Pleurastrophyceae, and Ulvophyceae form a monophyletic group. Within this assemblage, N. olivacea and P. marina represent an early-diverging lineage that is evolutionarily distinct from the later-diverging S. dubia/T. striata clade. The branch point of the S. dubia/T. striata clade precedes the near-simultaneous radiation of the Chlorophyceae, Ulvophyceae, and Microthamniales. Though interrelationships between these three latter groups of algae are not resolved, the phylogenetic analyses demonstrate that the Prasinophyceae (sensu Moestrup and Throndsen) and the Pleurastrophyceae (sensu Mattox and Stewart) are not monophyletic classes.