| The diversity and relationships of Bradyrhizobium legume nodule symbionts are not well understood, studies have focused on sampling a small geographic region or a limited number of host legume taxa. To address this lack of comparative sampling, I analyzed 181 Bradyrhizobium strains sampled across 50° of latitude (Alaska to Panama) obtained from 41 legume genera (17 of 35 tribes). Sequencing of five gene loci revealed high local and regional diversity. Analysis suggested that a locus in the symbiosis island (SI) region (nifD) exhibited substantial phylogenetic conflict with four housekeeping gene loci. For both nifD and housekeeping gene trees, significant phylogenetic clustering in relation to source region and legume taxonomic group was shown. Nevertheless, bacterial lineages had distributions that spanned multiple regions, and associated with multiple legume host taxa. This indicates that migration, horizontal gene transfer, and host interactions have played significant roles in structuring Bradyrhizobium populations on a continental scale. |
