The genome of the polar eukaryotic microalga Coccomyxa subellipsoidea reveals traits of cold adaptation.
ABSTRACT Little is known about the mechanisms of adaptation of life to the extreme environmental conditions encountered in polar regions. Here we present the genome sequence of a unicellular green alga from the division chlorophyta, Coccomyxa subellipsoidea C-169, which we will hereafter refer to as C-169. This is the first eukaryotic microorganism from a polar environment to have its genome sequenced.
The 48.8 Mb genome contained in 20 chromosomes exhibits significant synteny conservation with the chromosomes of its relatives Chlorella variabilis and Chlamydomonas reinhardtii. The order of the genes is highly reshuffled within synteny blocks, suggesting that intra-chromosomal rearrangements were more prevalent than inter-chromosomal rearrangements. Remarkably, Zepp retrotransposons occur in clusters of nested elements with strictly one cluster per chromosome probably residing at the centromere. Several protein families overrepresented in C. subellipsoidae include proteins involved in lipid metabolism, transporters, cellulose synthases and short alcohol dehydrogenases. Conversely, C-169 lacks proteins that exist in all other sequenced chlorophytes, including components of the glycosyl phosphatidyl inositol anchoring system, pyruvate phosphate dikinase and the photosystem 1 reaction center subunit N (PsaN).
We suggest that some of these gene losses and gains could have contributed to adaptation to low temperatures. Comparison of these genomic features with the adaptive strategies of psychrophilic microbes suggests that prokaryotes and eukaryotes followed comparable evolutionary routes to adapt to cold environments.
Article: Slow algae, fast fungi: exceptionally high nucleotide substitution rate differences between lichenized fungi Omphalina and their symbiotic green algae Coccomyxa.[show abstract] [hide abstract]
ABSTRACT: Omphalina basidiolichens are obligate mutualistic associations of a fungus of the genus Omphalina (the exhabitant) and a unicellular green alga of the genus Coccomyxa (the inhabitant). It has been suggested that symbiotic inhabitants have a lower rate of genetic change compared to exhabitants because the latter are more exposed to abiotic environmental variation and competition from other organisms. In order to test this hypothesis we compared substitution rates in the nuclear ribosomal internal transcribed spacer region (ITS1, 5.8S, ITS2) among fungal species with rates among their respective algal symbionts. To ensure valid comparisons, only taxon pairs (12) with a common evolutionary history were used. On average, substitution rates in the ITS1 portion of Omphalina pairs were 27.5 times higher than rates in the corresponding pairs of Coccomyxa since divergence from their respective ancestor at the base of the Omphalina/Coccomyxa lineage. Substitution rates in the 5.8S and the ITS2 portions were 2.4 and 18.0 times higher, respectively. The highest rate difference (43.0) was found in the ITS1 region. These are, to our knowledge, the highest differences of substitution rates reported for symbiotic organisms. We conclude that the Omphalina model system conforms to the proposed hypothesis of lower substitution rates in the inhabitant, but that the mode of transmission of the inhabitant (vertical versus horizontal) could be a prevailing factor in the regulation of unequal rates of nucleotide substitution between co-evolving symbionts. Our phylogenetic study of Coccomyxa revealed three main lineages within this genus, corresponding to free-living Coccomyxa, individuals isolated from basidiolichens Omphalina and Coccomyxa isolated from ascolichens belonging to the Peltigerales.Molecular Phylogenetics and Evolution 01/2004; 29(3):629-40. · 3.61 Impact Factor
[show abstract] [hide abstract]
ABSTRACT: Although intracellular associations with mycorrhizal fungi are known for Ginkgo biloba, no other endosymbiotic relationships have ever been reported for this "living fossil." A protoplast culture derived from haploid explants has now revealed the existence of a green alga in vitro, whose eukaryotic status was confirmed by transmission electron microscopic studies. Phylogenetic 18S rDNA sequence analyses showed this alga to be closely related to the lichen photobiont Coccomyxa. Algae, which in host cells exist as more or less undifferentiated "precursor" forms, proliferated within necrosing G. biloba cells of a subculture derived from a zygotic embryo and were finally released into the medium. Light and electron microscopic observations showed that G. biloba cells rapidly filled up with countless green particles whose number increased up to the bursting of the hypertrophic host cells. At the beginning of reproduction no algae were visible in the nutritive medium, demonstrating that the proliferation started inside the G. biloba cells and excluding the possibility of an exogenous contamination. Occasionally, mature algae together with their precursor forms were detected by transmission electron microscopy in intact host cells of a green callus. The algae were easily identified by their similarity to the cultured algae. Eukaryotic algae have never been reported to date to reside inside higher plant cells, whereas several algal associations are well known from the animal kingdom.American Journal of Botany 05/2002; 89(5):727-33. · 2.66 Impact Factor
[show abstract] [hide abstract]
ABSTRACT: Symbiosis of green algae with protozoa and invertebrates has been studied for more than 100 years. Endosymbiotic green algae are widely distributed in ciliates (e.g. Paramecium, Stentor, Climacostomum, Coleps, Euplotes), heliozoa (e.g. Acanthocystis) and invertebrates (e.g. Hydra, Spongilla), and have traditionally been identified as named or unnamed species of Chlorella Beij. or Zoochlorella K. Brandt or referred to as Chlorella-like algae or zoochlorellae. We studied 17 strains of endosymbionts isolated from various hosts and geographical localities using an integrative approach (nuclear encoded small subunit and internal transcribed spacer regions of rRNA gene sequences including their secondary structures, morphology, physiology and virus sensitivity). Phylogenetic analyses have revealed them to be polyphyletic. The strains examined belong to five independent clades within the Trebouxiophyceae (Choricystis-, Elliptochloris-, Auxenochlorella- and Chlorella-clades) and Chlorophyceae (Scenedesmus-clade). The most studied host organism, Paramecium bursaria, harbours endosymbionts representing at least five different species. On the basis of our results, we propose a taxonomic revision of endosymbiotic 'Chlorella'-like green algae. Zoochlorella conductrix K. Brandt is transferred to Micractinium Fresen. and Zoochlorella parasitica K. Brandt to Choricystis (Skuja) Fott. It was shown that Choricystis minor (Skuja) Fott, the generitype, is a later heterotypic synonym of Choricystis parasitica (K. Brandt) comb. nov. A new species, Chlorella heliozoae, is proposed to accommodate the endosymbiont of Acanthocystis turfacea.Environmental Microbiology 02/2011; 13(2):350-64. · 5.84 Impact Factor