The Spread of Bradyrhizobium Lineages Across Host Legume Clades: from Abarema to Zygia

To analyze macroevolutionary patterns in host use by Bradyrhizobium root-nodule bacteria, 420 strains from 75 legume host genera (sampled in 25 countries) were characterized for portions of six housekeeping genes and the nifD locus in the symbiosis island chromosomal region. Most Bradyrhizobium clades utilized very divergent sets of legume hosts. This suggests that Bradyrhizobium spread across the major legume lineages early in its evolution, with only a few derived clades subsequently developing a narrower pattern of host use. Significant modularity existed in the network structure of recent host jumps (inferred from cases where closely related strain pairs were found on different legume taxa). This implies that recent host switching has occurred most often within particular subgroups of legumes. Nevertheless, the observed link structure would allow a bacterial lineage to reach almost any of the 75 legume host genera in a relatively small number of steps. However, permutation tests also showed that symbionts from certain host plant clades were significantly more similar than would be the case if bacteria were distributed at random on the trees. Related legumes thus harbored related sets of symbionts in some cases, indicating some degree of phylogenetic conservatism in partner selection.