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Journal of Bacteriology, July 2007, p. 5223-5236, Vol. 189, No. 14
0021-9193/07/$08.00+0     doi:10.1128/JB.00105-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Horizontal Gene Transfer and Homologous Recombination Drive the Evolution of the Nitrogen-Fixing Symbionts of Medicago Species

Laboratoire des Symbioses Tropicales et Méditerranéennes, UMR 113 IRD-Cirad-SupAgro-UM2/USC INRA, Campus de Baillarguet, 34398 Montpellier Cedex 5, France,¹ Institut des Sciences de l'Evolution de Montpellier, UMR 5554 CNRS-UM2, Montpellier, France²

Received 18 January 2007/ Accepted 1 May 2007

Using nitrogen-fixing Sinorhizobium species that interact with Medicago plants as a model system, we aimed at clarifying how sex has shaped the diversity of bacteria associated with the genus Medicago on the interspecific and intraspecific scales. To gain insights into the diversification of these symbionts, we inferred a topology that includes the different specificity groups which interact with Medicago species, based on sequences of the nodulation gene cluster. Furthermore, 126 bacterial isolates were obtained from two soil samples, using Medicago truncatula and Medicago laciniata as host plants, to study the differentiation between populations of Sinorhizobium medicae, Sinorhizobium meliloti bv. meliloti, and S. meliloti bv. medicaginis. The former two can be associated with M. truncatula (among other species of Medicago), whereas the last organism is the specific symbiont of M. laciniata. These bacteria were characterized using a multilocus sequence analysis of four loci, located on the chromosome and on the two megaplasmids of S. meliloti. The phylogenetic results reveal that several interspecific horizontal gene transfers occurred during the diversification of Medicago symbionts. Within S. meliloti, the analyses show that nod genes specific to different host plants have spread to different genetic backgrounds through homologous recombination, preventing further divergence of the different ecotypes. Thus, specialization to different host plant species does not prevent the occurrence of gene flow among host-specific biovars of S. meliloti, whereas reproductive isolation between S. meliloti bv. meliloti and S. medicae is maintained even though these bacteria can cooccur in sympatry on the same individual host plants.

Published ahead of print on 11 May 2007.

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• Rangin, C., Brunel, B., Cleyet-Marel, J.-C., Perrineau, M.-M., Bena, G. (2008). Effects of Medicago truncatula Genetic Diversity, Rhizobial Competition, and Strain Effectiveness on the Diversity of a Natural Sinorhizobium Species Community. Appl. Environ. Microbiol. 74: 5653-5661 [Abstract] [Full Text]  
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