The basal position of scaly green flagellates among the green algae (Chlorophyta) is revealed by analyses of nuclear-encoded SSU rRNA sequences

The prasinophytes comprise a morphologically heterogeneous assembly of most ly marine flagellates and coccoid taxa, which represent an important compon ent of the nano- and picoplankton, and have previously figured prominently in discussions about the origin and phylogeny of the green plants. To evalu ate their putative basal position in the Viridiplantae and to resolve the p hylogenetic relationships among the prasinophyte taxa, we determined comple te nuclear-encoded SSU rRNA sequences from 13 prasinophyte taxa representin g the genera Cymbomonas, Halosphaera, Mamiella, Mantoniella, Micromonas, Pt erosperma, Pycnococcus, and Pyramimonas. Phylogenetic analyses of SSU rRNA sequences using distance, parsimony and likelihood methods revealed four in dependent prasinophyte lineages (clades) which constitute the earliest dive rgences among the Chlorophyta. In order of their divergence these clades ar e represented by the genera Cymbomonas, Halosphaera, Pterosperma, Pyramimon as (clade I), Mamiella, Mantoniella, Micromonas (clade II), Pseudoscourfiel dia (strain CCMP 717), Nephroselmis (clade III), and Tetraselmis, Scherffel ia (clade IV). The coccoid Pycnococcus provasolii diverged after clade II, but before clade III. Since no other coccoid prasinophyte taxa were analyze d in this study, the phylogenetic status of this taxon is presently unresol ved. Our analyses provide further evidence for the basal phylogenetic posit ion of the scaly green flagellates among the Chlorophyta and raise importan t questions concerning the class-level classification of the Chlorophyta.