The symbiotic partnerships between legumes and their root-nodule bacteria (rhizobia) vary widely in their degree of specificity, but the underlying reasons are not understood. To assess the potential for host-range evolution, we have investigated microheterogeneity among the shared symbionts of a group of related legume species. Host specificity and genetic diversity were characterized for a soil population of Rhizobium leguminosarum biovar viciae (Rlv) sampled using six wild Vicia and Lathyrus species and the crop plants pea (Pisum sativum) and broad bean (Vicia faba). Genetic variation among 625 isolates was assessed by restriction fragment length polymorphism (RFLP) of loci on the chromosome (ribosomal gene spacer) and symbiosis plasmid (nodD region). Broad bean strongly favoured a particular symbiotic genotype that formed a distinct phylogenetic subgroup of Rlv nodulation genotypes but was associated with a range of chromosomal backgrounds. Host range tests of 80 isolates demonstrated that only 34% of isolates were able to nodulate V. faba. By contrast, 89% were able to nodulate all the local wild hosts tested, so high genetic diversity of the rhizobial population cannot be ascribed directly to the diversity of host species at the site. Overall the picture is of a population of symbionts that is diversified by plasmid transfer and shared fairly indiscriminately by local wild legume hosts. The crop species are less promiscuous in their interaction with symbionts than the wild legumes
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