Résumé
The alpha-proteobacteria displayed diverse and often unconventional life- styles. In particular, they keep close relationships with the eucaryotic cell. Their genomic organization is often atypical. Indeed, complex genomes, with two or more chromosomes that could be linear and sometimes associated with plasmids larger than one megabase, have been described. Moreover, polymorphism in genome size and topology as well as in replicon number was observed among very related bacteria, even in a same species. Alpha-proteobacteria provide a good model to study the reductive evolution, the role and origin of multiple chromosomes, and the genomic fluidity. The amount of new data harvested in the last decade should lead us to better understand emergence of bacterial life-styles and to build the conceptual basis to improve the definition of the bacterial species.Original Abstract: Les alpha-proteobacteries ont des modes de vie varies et originaux. Elles sont, en particulier, capables d'etablir des relations etroites avec la cellule eucaryote. Leurs structures genomiques sont souvent atypiques avec des chromosomes multiples, parfois lineaires pouvant etre associes a de grands plasmides de plus d'une megabase. de plus, un fort polymorphisme de taille, de topologie et de nombre de replicons est souvent observe au sein d'une meme espece ou entre bacteries phylogenetiquement tres proches. Ainsi, les alpha- proteobacteries representent un modele particulierement pertinent pour etudier le phenomene de reduction genomique, l'origine et le role biologique des chromosomes multiples, ainsi que la dynamique et la fluidite genomique. La somme des donnees acquises ces dernieres annees sur le genome des alpha- proteobacteries ameliore la comprehension de l'emergence des modes de vie bacteriens et enrichit les concepts a la base de la definition de l'espece bacterienne.