RESUMO
We studied the genetic structure of 176 bacterial isolates from nodules of Medicago sativa, M. lupulina and M. polymorpha in fifteen sites distributed in three localities in Mexico. The strains were characterized by multilocus enzyme electrophoresis, plasmid profiles, PCR restriction fragment length polymorphism of 16S rRNA genes and of the intergenic spacer between 16S and 23S rRNA genes, and partial sequences of glnII, recA and nodB. Most of the strains were classified as Sinorhizobium meliloti, and a high genetic diversity was recorded. Six strains were classified as Sinorhizobium medicae, with no genetic variation. Phylogenetic and population genetic analyses revealed evidence of frequent recombination and migration within species.
Assuntos
Medicago sativa/microbiologia , Sinorhizobium meliloti/genética , Sinorhizobium/genética , Microbiologia do Solo , Proteínas de Bactérias/genética , DNA Bacteriano/genética , DNA Ribossômico/genética , DNA Espaçador Ribossômico/genética , Enzimas/genética , Evolução Molecular , Variação Genética , Desequilíbrio de Ligação , México , Filogenia , Plasmídeos/genética , RNA Ribossômico 16S/genética , Nódulos Radiculares de Plantas/microbiologiaRESUMO
Forty-six Mesorhizobium strains associated with the leguminous plants Leucaena leucocephala and Sesbania herbacea in an uncultivated Mexican field were characterized using a polyphasic approach. The strains were identified as Mesorhizobium plurifarium based upon the close relationships with the reference strains for this species in PCR-based restriction fragment length polymorphism analyses, sequencing of 16S rRNA genes, multilocus enzyme electrophoresis, and DNA-DNA hybridization. Although the strains isolated from both plants formed the same group in multilocus enzyme electrophoresis and cross-nodulations were observed in the laboratory, different electrophoretic types were obtained from the two plants grown in natural soils, indicating the existence of a preferable association between the plants and the rhizobia. The M. plurifarium strains from Mexico and the reference strains from Africa and Brazil formed different phenotypic clusters in a numerical taxonomy. The Mexican strains did not grow at 37 degrees C and were sensitive to salty-alkaline conditions, while the reference strains from Africa and Brazil grew at 42 degrees C and were more resistant to salty-alkaline conditions. These results demonstrate that both the plants and environmental factors affected the evolution of rhizobia and that the Mexican strains had adapted to the neutral soils and the cool climate where they were isolated.