|
|
|
|
Press Info item.
11/12/2006
Ostreococcus tauri, at the root of plant ancestry
The genome of the smallest free-living eukaryote, Ostreococcus tauri, was recently sequenced by a consortium of French and Belgian laboratories, including INRA. This single-celled plankton alga is of particular interest because it is at the base of the evolutionary chain of today’s higher plants. The family of green algae to which O. tauri belongs shares many of the same physiological and molecular mechanisms as these plants. Studying the common genes between such “primitive” algae and higher plants, and the way in which they have evolved could provide insight into the function and regulation of homologous genes in plants. O. tauri could thus be as useful to plant research as yeast is to animal research.
|
| |
Ostreococcus tauri has the smallest genome of all non-parasitic eukaryotes known to date. It is about the same size as a bacterium, that is, 1 micron. Unlike bacteria, however, its chromosomes are contained in a specialised structure, the nucleus. With a nucleus, a Golgi apparatus, a single chloroplast and a single mitochondrion, O tauri has the simplest structure a photosynthetic eukaryote can have. It belongs to a green algae family that appeared some 1.5 billion years ago, making it one of the most ancient members of the green lineage. Scientists therefore expect to find in O. tauri most plant-specific genes, except those involved in multicellular differentiation.
O. tauri is commonly found in all the world’s oceans. It also makes up the majority of phytoplankton found in France’s Thau lagoon, where it was first discovered in 1994. Along with cyanobacteria, O. tauri is the first link in the marine food chain, where it harnesses solar energy and fixes carbon dioxide in order to produce organic material and oxygen for other organisms. Approximately 50% of the oxygen we breathe is generated during this photosynthetic process by single-celled phytoplankton.
 |
© CNRS / H. Moreau
As seen under the electron microscope, Ostreococcus tauri displays one starch granule (the light spot) inside a single chloroplast (around the granule, left-hand side); a nucleus (bottom); and a single mitochondrion (right).
|
A wealth of information
Sequencing the genome of an organism like O. tauri is an extremely efficient way to understand its biology. This is because information is gathered in a cumulative manner. From the genome sequence, it is possible to predict the location of genes, which in turn makes it possible to formulate hypotheses about the functions of those genes by comparing them with known genes; finally, based on the functions identified, researchers are able to suggest which metabolic chains the organism might have. Such in silico research results in hypotheses that orient future experimental research.
The information gathered from the sequencing of the O. tauri genome suggests that it is tuned to utilize nitrogen as best as possible, since it is able to efficiently assimilate not only nitrates, but also ammonium and urea. It also appears capable of C4 carbon metabolism, which is much more effective in using photosynthesis output than the C3 type.
A number of surprises
The O. tauri genome is simpler and more compact than plant genomes. It is one-tenth the size of the Arabidopsis genome, the smallest known plant genome, and has about 8,000 genes, just one-third the number found in Arabidopsis. Where plants have been observed to have complex gene families, O. tauri often has just one gene for the same function; this greatly facilitates researchers’ analysis of the function.
The O. tauri genome has 20 chromosomes, two of which are atypical. Chromosome 2 exhibits certain particularities usually associated with sex chromosomes, including an abundance of transposable elements and a particular gene structure, with many introns differing from the rest found in the genome. Although sexual reproduction mechanisms have not been observed in O. tauri, it possesses a complete set of meiosis genes. Researchers have suggested that sexual reproduction may exist as a possibility for the alga in order to establish and maintain diversity in an environment lacking strong geographical segmentation.
Of the genes found on chromosome 19, 60% have no similarity to known genes, 20% are homologs to green algae and the remaining 20% have similarities with bacterial genes, suggesting these were acquired by lateral gene transfer.
Two other Ostreococcus strains are currently being sequenced in partnership with US teams – an ocean surface strain and a “low light” strain – in order to understand the diversity and adaptation of the genus to various natural environments.
Reference: Genome analysis of the smallest free-living eukaryote Ostréococcus tauri unveils many unique features
Evelyne Derelle a, b, Conchita Ferraz b, c, Stéphane Rombauts b, d, Pierre Rouzé b, e, Alexandra Z. Worden f, Steven Robbens d, Frédéric Partensky g, Sven Degroeved h, Sophie Echeynié c, Richard Cooke i, Yvan Saeys d, Jan Wuyts d, Kamel Jabbari j Chris Bowler k, Olivier Panaud i, Benoît Piégu i, Steven G. Ball k, Jean-Philippe Ral k, François-Yves Bouget a, Gwenael Piganeau a, Bernard De Baets h, Andre Picard a, l, Michel Delseny i, Jacques Demaille c, Yves Van de Peer d, and Herve Moreau a,
Proc Natl Acad Sci U S A. 2006 Aug 1;103(31):11647-52.
a Observatoire Océanologique, Laboratoire Arago, UMR 7628, CNRS–Université Paris 6, Banyuls sur Mer, France;
b E.D., C.F., S.R. and P.R. contributed equally to this work
c Institut de Génétique Humaine, Unité Propre de Recherche 1142, CNRS, Montpellier, France;
d Department of Plant Systems Biology, Flanders Interuniversity Institute for Biotechnology and
e Laboratoire associé de l’INRA (France), Ghent University, Belgium;
f Rosenstiel School of Marine and Atmospheric Science, University of Miami;
g Station Biologique, UMR 7144, CNRS –Université Paris 6, Roscoff, France;
h Department of Applied Mathematics, Biometrics and Process Control, Ghent University, Belgium;
i Génome et Développement des Plantes, UMR 5096, CNRS–Université de Perpignan, France;
j Département de Biologie, Formation de Recherche en Evolution 2910, CNRS–Ecole Normale Supérieure, Paris
k Laboratoire de Chimie Biologique, UMR 8765, CNRS–Université Sciences et Technologies de Lille, France
|
| |
|
Written by :
INRA press service, phone: +33 (0)1 42 75 91 69
Contacts :
Pierre Rouzé
tel: 32 933 13694
pierre.rouze@psb.ugent.be
INRA Associated Laboratory, Bioinformatics Service of Ghent University (Belgium), Plant Biology Department of INRA
|
|
|
|
|
Search:
