Genome analysis of the smallest free-living eukaryote Ostreococcus tauri unveils many unique features.
The green lineage is reportedly 1,500 million years old, evolving shortly after the endosymbiosis event that gave rise to early photosynthetic eukaryotes. In this study, we unveil the complete genome sequence of an ancient member of this lineage, the unicellular green alga Ostreococcus tauri (Prasinophyceae). This cosmopolitan marine primary producer is the world's smallest free-living eukaryote known to date. Features likely reflecting optimization of environmentally relevant pathways, including resource acquisition, unusual photosynthesis apparatus, and genes potentially involved in C(4) photosynthesis, were observed, as was downsizing of many gene families. Overall, the 12.56-Mb nuclear genome has an extremely high gene density, in part because of extensive reduction of intergenic regions and other forms of compaction such as gene fusion. However, the genome is structurally complex. It exhibits previously unobserved levels of heterogeneity for a eukaryote. Two chromosomes differ structurally from the other eighteen. Both have a significantly biased G+C content, and, remarkably, they contain the majority of transposable elements. Many chromosome 2 genes also have unique codon usage and splicing, but phylogenetic analysis and composition do not support alien gene origin. In contrast, most chromosome 19 genes show no similarity to green lineage genes and a large number of them are specialized in cell surface processes. Taken together, the complete genome sequence, unusual features, and downsized gene families, make O. tauri an ideal model system for research on eukaryotic genome evolution, including chromosome specialization and green lineage ancestry.
Derelle
Evelyne
E
Observatoire Océanologique, Laboratoire Arago, Unité Mixte de Recherche 7628, Centre National de la Recherche Scientifique-Université Pierre et Marie Curie-Paris 6, BP44, 66651 Banyuls sur Mer Cedex, France.
Ferraz
Conchita
C
Rombauts
Stephane
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Rouzé
Pierre
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Worden
Alexandra Z
AZ
Robbens
Steven
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Partensky
Frédéric
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Degroeve
Sven
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Echeynié
Sophie
S
Cooke
Richard
R
Saeys
Yvan
Y
Wuyts
Jan
J
Jabbari
Kamel
K
Bowler
Chris
C
Panaud
Olivier
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Piégu
Benoît
B
Ball
Steven G
SG
Ral
Jean-Philippe
JP
Bouget
François-Yves
FY
Piganeau
Gwenael
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De Baets
Bernard
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Picard
André
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Delseny
Michel
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Demaille
Jacques
J
Van de Peer
Yves
Y
Moreau
Hervé
H
D000818:Animals; D000460:Chlorophyta / Q000235:genetics*; D002875:Chromosomes; D005057:Eukaryotic Cells; D019143:Evolution, Molecular; D016678:Genome; D008969:Molecular Sequence Data; D017422:Sequence Analysis, DNA