Mosaic origins of Bradyrhizobium legume symbionts on the Caribbean island of Guadeloupe

• Origins of Bradyrhizobium sp. legume root-nodule symbionts on the Caribbean island of Guadeloupe were analyzed.
• Strains grouped into clades with 20 distinct sets of non-Guadeloupe relatives.
• Thus, the island was colonized numerous times from multiple source regions.
• Geographic affinities conflicted for different bacterial loci.
• Gene transfer has created novel strains combining genes from disparate source regions.

To analyze geographic affinities of Bradyrhizobium sp. symbionts associated with the diverse legume flora on the Caribbean island of Guadeloupe, 39 isolates from 18 legume genera were compared to a reference set of 269 Bradyrhizobium strains from North America, Central America, Puerto Rico and the Philippines. A multilocus sequence analysis (4192 bp) showed that nucleotide diversity in Guadeloupe equaled or exceeded that found in all other regional Bradyrhizobium populations examined. Bayesian phylogenetic tree analysis grouped the Guadeloupe Bradyrhizobium strains into clades with at least 20 distinct sets of non-Guadeloupe relatives, implying that the island was colonized numerous times from multiple source regions. However, for 18% of the Guadeloupe isolates, inferred geographic affinities for the nifD locus, in the symbiosis island region of the Bradyrhizobium chromosome, conflicted with the source region deduced from a tree based on six concatenated housekeeping genes. Geographic mosaic ancestry was therefore evident among Guadeloupe bradyrhizobia. Horizontal gene transfer subsequent to island colonization appears to have generated strains that carry combinations of genes from disparate source regions.

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