A widespread Neotropical Bradyrhizobium lineage associated with Machaerium and Desmodium (Papilionoideae)

Multilocus enzyme electrophoresis and ribosomal RNA gene sequencing were used to compare bacterial strains from the liana Machaerium milliflorum and the herb Desmodium axillare at two lowland forests 440 km apart in Panama and Costa Rica. Out of 27 multilocus genotypes (electrophoretic types or ETs) detected in the isozyme analysis, only two ETs were found in both locations. Nevertheless, these shared ETs were closely related to a cluster including 11 other common ETs that represented 72% and 80%, respectively, of all bacterial isolates sampled in Panama and Costa Rica. Isolates in this group from both locations and from both legume hosts all had identical sequences for a highly polymorphic 5’ portion of 23S rRNA, while nine other ETs outside this cluster all had unique sequences. This indicates that both sites are occupied by one primary lineage of nodule symbionts that is both locally predominant and geographically widespread. However, this group coexists with numerous other rare bacterial lineages whose ecology, distribution, and relationships require further study.