ACTIN PHYLOGENY IDENTIFIES MESOSTIGMA VIRIDE AS A FLAGELLATE ANCESTOROF THE LAND PLANTS

Green algae and land plants trace their evolutionary history to a uniq ue common ancestor. This ''green lineage'' is phylogenetically subdivi ded into two distinct assemblages, the Chlorophyta and the Streptophyt a. The Chlorophyta includes the Chlorophyceae, Trebouxiophyceae, Ulvop hyceae, and Prasinopohyceae, whereas the Streptophyta includes the Cha rophyceae plus the bryophytes, ferns, and all other multicellular land plants (Embryophyta). The Prasinophyceae is believed to contain the e arliest divergeneces within the green lineage. Phylogenetic analyses u sing rDNA sequences identify the prasinophytes as a paraphyletic taxon that diverges at the base of the Chlorophyta. rDNA analyses, however, provide ambiguous results regarding the identity of the flagellate an cestor of the Streptophyta. We have sequenced the actin-encoding cDNAs from Scherffelia dubia (Prasinophyceae), Coleochaete scutata, Spirogy ra sp. (Charophyceae), and the single-copy actin gene from Mesostigma viride (Prasinophyceae). Phylogenetic analyses show Mesostigma to be t he earliest divergence within the Streptophyta and provide direct evid ence for a scaly, biflagellate, unicellular ancestor for this lineage. This result is supported by the existence of two conserved actin-codi ng region introns (positions 20-3, 152-1), and one intron in the 5'-un translated region of the actin gene shared by Mesostigma and the embry ophytes.