摘要：

Microevolution and origins ofBradyrhizobiumpopulations 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 toB. japonicumgroups 1 and 1a or to one of three novel lineages within the genusBradyrhizobium. 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 bracteataandDesmodium canadense). Isolates fromD. canadenseclustered withB. japonicumgroup 1, whereas those fromA. bracteatawere 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 togyrB that had a history of frequent recombination. Symbiotic gene sequences (nodC andnifH) of isolates from soybean sites and native legumes clustered in two lineages corresponding toB. japonicumandB. elkaniwith 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.

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