title: Genome analysis of the smallest free-living eukaryote Ostreococcus tauri unveils many unique features.
abstract: 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.
authors.affiliation: 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.
authors.forename: Evelyne
authors.initials: E
authors.lastname: Derelle
authors.forename: Conchita
authors.initials: C
authors.lastname: Ferraz
authors.forename: Stephane
authors.initials: S
authors.lastname: Rombauts
authors.forename: Pierre
authors.initials: P
authors.lastname: Rouzé
authors.forename: Alexandra Z
authors.initials: AZ
authors.lastname: Worden
authors.forename: Steven
authors.initials: S
authors.lastname: Robbens
authors.forename: Frédéric
authors.initials: F
authors.lastname: Partensky
authors.forename: Sven
authors.initials: S
authors.lastname: Degroeve
authors.forename: Sophie
authors.initials: S
authors.lastname: Echeynié
authors.forename: Richard
authors.initials: R
authors.lastname: Cooke
authors.forename: Yvan
authors.initials: Y
authors.lastname: Saeys
authors.forename: Jan
authors.initials: J
authors.lastname: Wuyts
authors.forename: Kamel
authors.initials: K
authors.lastname: Jabbari
authors.forename: Chris
authors.initials: C
authors.lastname: Bowler
authors.forename: Olivier
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authors.lastname: Panaud
authors.forename: Benoît
authors.initials: B
authors.lastname: Piégu
authors.forename: Steven G
authors.initials: SG
authors.lastname: Ball
authors.forename: Jean-Philippe
authors.initials: JP
authors.lastname: Ral
authors.forename: François-Yves
authors.initials: FY
authors.lastname: Bouget
authors.forename: Gwenael
authors.initials: G
authors.lastname: Piganeau
authors.forename: Bernard
authors.initials: B
authors.lastname: De Baets
authors.forename: André
authors.initials: A
authors.lastname: Picard
authors.forename: Michel
authors.initials: M
authors.lastname: Delseny
authors.forename: Jacques
authors.initials: J
authors.lastname: Demaille
authors.forename: Yves
authors.initials: Y
authors.lastname: Van de Peer
authors.forename: Hervé
authors.initials: H
authors.lastname: Moreau
mesh_terms: D000818:Animals; D000460:Chlorophyta / Q000235:genetics*; D002875:Chromosomes; D005057:Eukaryotic Cells; D019143:Evolution, Molecular; D016678:Genome; D008969:Molecular Sequence Data; D017422:Sequence Analysis, DNA