Résumé
Acacia mangium is a legume tree native of Australasia. Tt has been introduced in many tropical countries especially in the context of industrial plantations. Tt has the exceptional capacity to establish symbioses with three types of microorganisms: nitrogen-fixing bacteria, arbuscular and ecto-mycorrhizal fungi, enabling a great productivity and adaptability. In this context, many field trials were established to test the effects of controlled inoculation with selected symbiotic bacteria versus natural colonization with indigenous strains. In introduction areas, A. mangium spontaneously nodulate with local and often ineffective bacteria. When inoculated, the persistence of inoculants, their impact on local biodiversity and possible genetic recombination with local strains have to be explored. The aim ofthis study was to describe the genetic diversity ofbacteria spontaneously nodulating A. mangium in Brazil and to evaluate, by molecular tracing, the durability ofselected strains used as inoculants. Three different sites, several hundred kilometres distant, were studied, each with inoculated and non inoculated plots. About one hundred strains were isolated and sequenced on three housekeeping (g/nB, dnaK and recA) and one symbiotic (nodA) genes. We showed that, whatever the site and the treatment, A. mangium is nodulated by bacteria of the genera Bradyrhizobium and Mesorhizobium. We observed a significant genetic differentiation not only among the three sites, but also between inoculated and non-inoculated plots in each site. The two inoculated strains were never detected, even few months after inoculation, be they in nursery or in plantation, raising the question ofthe usefulness of such artificial inoculation. At last, Mesorhizobium strains, display a genetic diversity within the housekeeping genes, shared exactly the same symbiotic sequence gene, opening the door to the possible transfer of the symbiotic island from unknown symbiotic strain. (Texte integral)