Microevolution of symbiotic Bradyrhizobium populations associated with soybeans in east North America

• Published in: Ecology and evolution 2012-12, Vol.2 (12), p.2943-2961
• Main Authors: Tang, Jie, Bromfield, E. S. P., Rodrigue, N., Cloutier, S., Tambong, J. T.
• Format: Article
• Language: English
• Subjects: Amphicarpaea bracteata; Bacteria; Bradyrhizobium; Desmodium canadense; Domestication; evolution; Gene sequencing; Genes; Genetic diversity; Glycine max; Glycine max (soybean); Heredity; Homologous recombination; Homology; Host plants; Inoculation; Legumes; Mutation; native legumes; NifH gene; Nitrogen; Original Research; Populations; Soybeans; Studies; Symbiosis
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.404

Microevolution and origins of Bradyrhizobium populations associated with soybeans at two field sites (A and B, 280 km apart in Canada) with contrasting histories of inoculation was investigated using probabilistic analyses of six core (housekeeping) gene sequences. These analyses supported division of 220 isolates in five lineages corresponding either to B. japonicum groups 1 and 1a or to one of three novel lineages within the genus Bradyrhizobium. None of the isolates from site A and about 20% from site B (the only site with a recent inoculation history) were attributed to inoculation sources. The data suggest that most isolates were of indigenous origin based on sequence analysis of 148 isolates of soybean‐nodulating bacteria from native legumes (Amphicarpaea bracteata and Desmodium canadense). Isolates from D. canadense clustered with B. japonicum group 1, whereas those from A. bracteata were placed in two novel lineages encountered at soybean field sites. One of these novel lineages predominated at soybean sites and exhibited a significant clonal expansion likely reflecting selection by the plant host. Homologous recombination events detected in the 35 sequence types from soybean sites had an effect on genetic diversification that was approximately equal to mutation. Interlineage transfer of core genes was infrequent and mostly attributable to gyrB that had a history of frequent recombination. Symbiotic gene sequences (nodC and nifH) of isolates from soybean sites and native legumes clustered in two lineages corresponding to B. japonicum and B. elkani with the inheritance of these genes appearing predominantly by vertical transmission. The data suggest that soybean‐nodulating bacteria associated with native legumes represent a novel source of ecologically adapted bacteria for soybean inoculation. Multilocus sequence typing (MLST) and probabilistic microevolutionary analyses indicated that soybeans in east North America are predominantly colonized by diverse lineages of symbiotic bradyrhizobia originating from native legumes irrespective of the practice of recurrent bacterial inoculation. Homologous recombination events at six core gene loci of 220 isolates of soybean‐nodulating bacteria were inferred to have an effect on genetic diversification that was approximately equal to mutation. The inheritance of accessory (symbiotic) genes (nodC and nifH) appeared predominantly by vertical transmission. The data suggest that soybean‐nodulating bacteria associa