The Complete Mitochondrial DNA Sequences of Nephroselmis olivacea and Pedinomonas minor: Two Radically Different Evolutionary Patterns within Green Algae

Abstract

Green plants appear to comprise two sister lineages, Chlorophyta (classes Chlorophyceae, Ulvophyceae, Trebouxiophyceae, and Prasinophyceae) and Streptophyta (Charophyceae and Embryophyta, or land plants). To gain insight into the nature of the ancestral green plant mitochondrial genome, we have sequenced the mitochondrial DNAs (mtDNAs) of Nephroselmis olivacea and Pedinomonas minor . These two green algae are presumptive members of the Prasinophyceae. This class is thought to include descendants of the earliest diverging green algae. We find that Nephroselmis and Pedinomonas mtDNAs differ markedly in size, gene content, and gene organization. Of the green algal mtDNAs sequenced so far, that of Nephroselmis (45,223 bp) is the most ancestral (minimally diverged) and occupies the phylogenetically most basal position within the Chlorophyta. Its repertoire of 69 genes closely resembles that in the mtDNA of Prototheca wickerhamii , a later diverging trebouxiophycean green alga. Three of the Nephroselmis genes ( nad10 , rpl14 , and rnpB ) have not been identified in previously sequenced mtDNAs of green algae and land plants. In contrast, the 25,137-bp Pedinomonas mtDNA contains only 22 genes and retains few recognizably ancestral features. In several respects, including gene content and rate of sequence divergence, Pedinomonas mtDNA resembles the reduced mtDNAs of chlamydomonad algae, with which it is robustly affiliated in phylogenetic analyses. Our results confirm the existence of two radically different patterns of mitochondrial genome evolution within the green algae. INTRODUCTIONOf the three main kingdoms of evolutionarily advanced eukaryotes (animals, fungi, and land plants), plants have the most clearly defined antecedents. Combined molecular, biochemical, and ultrastructural data demonstrate that the unicellular progenitors of land plants lie within the green algae, which together with land plants form a monophyletic lineage that is characterized by the presence of chloroplasts surrounded by two membranes and containing stacked thylakoids and chlorophylls a and b (Bhattacharya and Medlin, 1998;Chapman et al., 1998). The evolutionary coherence of land plants and green algae is apparent at the levels of both the nuclear and chloroplast genomes; however, at the level of the mitochondrial genome, this affiliation breaks down (see below; Gray et al., 1989).Phycologists generally recognize five classes of green algae: the Charophyceae, which include the closest relatives of land plants; the Chlorophyceae; the Trebouxiophyceae; the Ulvophyceae; and a nonmonophyletic group known as the Prasinophyceae (reviewed in Friedl, 1997;Bhattacharya and Medlin, 1998;Chapman et al., 1998). The latter class is extremely diverse and is thought to include descendants of the earliest diverging green algae (Melkonian, 1990a). Comparison of nuclear small subunit (SSU) rRNA gene sequences suggests that extant green eukaryotes ("green plants") divide into two major evolutionary lineages: the Streptophyta, containing the charophytes, land ...

Mosses share mitochondrial group II introns with flowering plants, not with liverworts

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Chlamydial symbionts in the enigmatic Xenoturbella (Deuterostomia)

“…In such case, the intron must have been lost in F. eriococci. The intron is similar to a group of chloroplast and mitochondrial rRNA gene introns from chlorophyte algae (Everett et al, 1999b;Turmel et al, 1999Turmel et al, ,2002Lucas et al, 2001;Pombert et al, 2006). Members of Chlamydia have not yet been found in algae, so the intron might have originated from the food of its earlier host, as already suggested by Everett et al (2005).…”

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“…Isolation and sequencing of cpDNA A + T-rich organelle DNA was separated from nuclear DNA by CsCl-bisbenzimide isopycnic centrifugation (Turmel et al 1999a). Both the chloroplast and mitochondrial genomes were completely sequenced as described previously (Pombert et al 2004), using as templates plasmid clones originating from the organelle DNA fraction as well as PCR fragments spanning uncloned regions.…”