%0 Journal Article
%T The complete chloroplast DNA sequences of the charophycean green algae Staurastrum and Zygnema reveal that the chloroplast genome underwent extensive changes during the evolution of the Zygnematales
%A Monique Turmel
%A Christian Otis
%A Claude Lemieux
%J BMC Biology
%D 2005
%I BioMed Central
%R 10.1186/1741-7007-3-22
%X The 157,089 bp Staurastrum and 165,372 bp Zygnema cpDNAs encode 121 and 125 genes, respectively. Although both cpDNAs lack an rRNA-encoding inverted repeat (IR), they are substantially larger than Chaetosphaeridium and land plant cpDNAs. This increased size is explained by the expansion of intergenic spacers and introns. The Staurastrum and Zygnema genomes differ extensively from one another and from their streptophyte counterparts at the level of gene order, with the Staurastrum genome more closely resembling its land plant counterparts than does Zygnema cpDNA. Many intergenic regions in Zygnema cpDNA harbor tandem repeats. The introns in both Staurastrum (8 introns) and Zygnema (13 introns) cpDNAs represent subsets of those found in land plant cpDNAs. They represent 16 distinct insertion sites, only five of which are shared by the two zygnematalean genomes. Three of these insertions sites have not been identified in Chaetosphaeridium cpDNA.The chloroplast genome experienced substantial changes in overall structure, gene order, and intron content during the evolution of the Zygnematales. Most of the features considered earlier as typical of land plant cpDNAs probably originated before the emergence of the Zygnematales and Coleochaetales.About 450 million years ago, green algae belonging to the class Charophyceae emerged from their aquatic habitat to colonize the land [1-3]. This important event in the history of life gave rise to all the land plant species that make up the flora of our planet. The few thousand species of charophycean green algae that are alive today exhibit great variability in cellular organization and reproduction [4]. With the land plants, they form the green plant lineage Streptophyta [5], whereas all other green algae (more than 10,000 species), with perhaps the exception of Mesostigma viride, belong to the sister lineage Chlorophyta [4]. Five monophyletic groups of charophycean green algae have been recognized: the Chlorokybales, Klebsormidia
%U http://www.biomedcentral.com/1741-7007/3/22