The evolution of the Green Algae
Phylogenetic analyses of complete 18S rDNAs from the green algae reveal the monophyletic origin of three independent evolutionary lineages, the classes Chlorophyceae, Trebouxiophyceae and Ulvophyceae with a basal divergence of a heterogenous assemblage of
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-Plant'-.--Systematics and Evolution
© Springer·Verlag 1997
The evolution of the Green Algae THOMAS FRIEDL Key words: Chlorophyta, Chlorophyceae, Prasinophyceae, Streptophyta, Trebouxiophyceae, Ulvophyceae, 18S rONA, coccoid green algae, lichen algae, molecular evolution.
Abstract: Phylogenetic analyses of complete 18S rDNAs from the green algae reveal the monophyletic origin of three independent evolutionary lineages, the classes Chlorophyceae, Trebouxiophyceae and Ulvophyceae with a basal divergence of a heterogenous assemblage of scaled flagellates, the prasinophytes. The Trebouxiophyceae comprise mainly coccoid green algae from terrestrial habitats or lichen symbioses. The Chlorophyceae unite green algae that have flagellated cells with basal bodies oriented in the clockwise direction or with directly opposed basal bodies. 18S rONA sequence comparisons substantiate the importance of flagellated cell morphology over vegetative cell features for tracing evolutionary relationships among the green algae. However, the counterclockwise basal body orientation which is shared among the Trebouxiophyceae and Ulvophyceae is likely a symplesiomorphy. Coccoid green algae, formerly placed in a single order (Chlorococcales), are distributed over several lineages.
The Green Algae are a eukaryotic lineage that is characterized by the presence of chloroplasts with two envelope membranes, stacked thylakoids and the chlorophylls a and h. Another important marker is the presence of starch as a reserve polysaccharide that is deposited inside the plastids. These synapomorphies clearly distinguish the Green Algae and their sister-group, the embryophytes, from other eukaryotic lineages. The monophyletic origin of all green plants is supported in phylogenetic analyses based on 18S rDNA sequence comparisons of various lineages of eukaryotes (e.g., in BHATTACHARYA & MEDLIN 1995; Fig. 1); these analyses also suggest the origin of land plants (embryophytes) from green algal ancestors (see chapter 4). Intensive ultrastructural work on the Green Algae during the past 25 years has lead to fundamental changes in green algal taxonomy compared to traditional concepts (for review: PICKETT-HEAPS 1975; MATTOX & STEWART 1984; VAN DEN HOEK & al. 1988, 1992). An exciting period in phycology started in the 1990s as molecular techniques, i.e. sequence comparisons mainly of I8S ribosomal DNAs, were introduced into the investigation of the evolution of Green Algae. The aim of the present article is to give an overview of the current status of our knowledge of green algal phylogeny as it relates to recent analyses of I8S rDNA sequences. Two datasets have been prepared from complete 18S rDNA sequences that are available from the GenbanklEBIIDDBl databases. The sequences may be directly
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accessed from the databases using the species names. The alignments used for phylogenetic analyses in this chapter are available from the author. Dataset 1 comprises 53 sequences representing all major lineages of the Chlorophyta sensu SLUIMAN (1985), Ch
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