On the initiative of a group from the Centre de Genetic Moleculaire, scientists from the CNRS (French National Research Organization) and Genoscope, the French National DNA Sequencing Centre, have just decoded the genome of Paramecium, a unicellular organism of considerable interest to evolutionary biologists. Thanks to the discovery that there were three genome duplications at different time scales, they were able to evaluate directly the consequences of this phenomenon on the evolution of species. These results will appear in the journal Nature of November 9, 2006 (online on November 1st).

Paramecium was one of the first unicellular organisms to be observed under the microscope. Since then, its large size combined with the ease of culturing it and observing its varied cellular functions have made it a favorite model for scientists. A small community of American, European and Japanese biologists have been using it for 50 years for the study of cellular organization and heredity.

Scientists from the CNRS and Genoscope who performed the decoding of the somatic¹ genome of Paramecium discovered that it contains close to 40,000 genes, versus "only" 25,000 for humans. They then demonstrated that this exceptional genetic heritage results from at least three successive duplications of the whole genome.
Genome duplications are rare events which have occurred recurrently during the evolution of eukaryotes². For a long time it was postulated that they could be the origin of major evolutionary transitions, because a doubling of the number of genes provides a large potential for innovation and therefore adaptation of species. The analysis of the genome and the observation of these three duplications at different time scales has allowed the scientists to draw three conclusions about the evolution of species :

• In agreement with previous observations on other genomes, it seems that the destiny of the majority of the duplicated genes is to lose one of the two copies progressively; this process could take several million years.
• The hypothesis stipulating that a genome duplication could lead to an explosive creation of new species has been confirmed. Indeed, Paramecium belongs to a group of 15 species which are genetically distinct but which share identical morphology and ecological niches. In fact, dating of the most recent genome duplication reveals that it took place just before the appearance of the 15 related species.
• The last conclusion, which was unexpected, is that the retention of both copies of genes after a duplication does not reflect the acquisition of new functions but rather the need for equilibrium between the different components of the cellular machinery. The majority of genes code for proteins which interact with other proteins to carry out specific cellular activities within networks. To change the quantity of one protein relative to that of the others would compromise these activities. After a duplication, the genes encoding interacting proteins are maintained as two copies and a return to the initial state due to the loss of one copy is only possible very progressively, after an accumulation of mutations which affect the level of gene expression. New functions do appear, but over longer time scales.

This research was made possible through the European Research Group composed of French, German and Polish laboratories which was created in 2002 by the Molecular Genetics Centre. In ongoing studies, this GDRE is already exploring the function of the genes which have been retained as two copies following the various genome duplications. The sequencing, which was performed at Genoscope, was funded by the Consortium National de Recherche en Genomique.

References :
Database for the biology and the genome of the Paramecium: http://paramecium.cgm.cnrs-gif.fr
To browse the Paramecium genome: http://www.genoscope.cns.fr/paramecie

Contacts :
Scientist :
CNRS
Jean Cohen
T 01 69 82 43 73
Jean.Cohen@cgm.cnrs-gif.fr

Genoscope
Patrick Wincker
T 01 60 87 25 68
pwincker@genoscope.cns.fr

Press :
CNRS
Isabelle Bauthian
T 01 44 96 46 06
isabelle.bauthian@cnrs-dir.fr

Genoscope
Bénédicte Robert
T 01 60 87 83 10
benedicte.robert@genopole.fr

¹Of all the unicellular eukaryotes, only the Ciliates, of which Paramecium is member, possess two nuclei: one serves germinal functions (sexuality) and the other serves somatic functions (function of the organism).

²Eukaryotes are uni- or multicellular organisms which possess, in addition to other organelles, a membrane-bound nucleus which contains DNA. Prokaryotes, mainly the bacteria, have circular DNA floating free in the cytoplasm.