Abstract
The green plants (Viridiplantae) are an ancient group of eukaryotes comprising two main clades: the Chlorophyta, which includes a wide diversity of green algae, and the Streptophyta, which consists of freshwater green algae and the land plants. The early-diverging lineages of the Viridiplantae comprise unicellular algae, and multicellularity has evolved independently in the two clades. Recent molecular data have revealed an unrecognized early-diverging lineage of green plants, the Palmophyllales, with a unique form of multicellularity, and typically found in deep water. The phylogenetic position of this enigmatic group, however, remained uncertain. Here we elucidate the evolutionary affinity of the Palmophyllales using chloroplast genomic, and nuclear rDNA data. Phylogenetic analyses firmly place the palmophyllalean Verdigellas peltata along with species of Prasinococcales (prasinophyte clade VI) in the deepest-branching clade of the Chlorophyta. The small, compact and intronless chloroplast genome (cpDNA) of V. peltata shows striking similarities in gene content and organization with the cpDNAs of Prasinococcales and the streptophyte Mesostigma viride, indicating that cpDNA architecture has been extremely well conserved in these deep-branching lineages of green plants. The phylogenetic distinctness of the Palmophyllales-Prasinococcales clade, characterized by unique ultrastructural features, warrants recognition of a new class of green plants, Palmophyllophyceae class. nov.
Highlights
The green plants (Viridiplantae) are an ancient group of eukaryotes comprising two main clades: the Chlorophyta, which includes a wide diversity of green algae, and the Streptophyta, which consists of freshwater green algae and the land plants
Molecular phylogenetic and ultrastructural data indicate that the green plants split early in their evolution into two main clades: the Chlorophyta and Streptophyta[1]
The circular chloroplast genome of Verdigellas peltata (Fig. 1) is 79,444 bp long, which is smaller than most chloroplast DNAs of free-living green algae[1,29], but in the range of published prasinophyte cpDNAs15,16
Summary
The green plants (Viridiplantae) are an ancient group of eukaryotes comprising two main clades: the Chlorophyta, which includes a wide diversity of green algae, and the Streptophyta, which consists of freshwater green algae and the land plants. Recent molecular data have revealed an unrecognized early-diverging lineage of green plants, the Palmophyllales, with a unique form of multicellularity, and typically found in deep water. The Chlorophyta include a large diversity of marine, freshwater and terrestrial green algae with a wide variety of morphologies, ranging from unicellular to complex multicellular morphologies. The prasinophytes gave rise to the core Chlorophyta, which include unicellular and multicellular species that abound in marine, freshwater and terrestrial habitats. It is generally accepted that that the ancestral green plants were unicellular green algae with characters typical of most extant prasinophytes, such as the presence of flagella and organic body scales[1,19] The nature of this hypothetical ancestral green flagellate, has been a matter of debate[9,19]. A better understanding of the diversity and phylogenetic relationships among early-diverging clades of Chlorophyta and Streptophyta is central to understanding the evolution of green plants
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