Biogeography and specificity of legume rhizobial symbiovars

The majority of legume species (plant family Fabaceae) can fix atmospheric nitrogen (N₂) via symbiotic bacteria (general term 'rhizobia') in root nodules. Symbiovars are defined as strains of rhizobia that can establish an N₂-fixing symbiosis with distinct legume species and have specific symbiosis gene phylogenies. Here, symbiovar occurrence across rhizobial species was related to rhizobia and legume host taxonomy and geographic location of the symbiosis, and then their specificity was assessed in relation to legume species they did or did not nodulate. Sixty-eight rhizobial symbiovars occurred across 183 of the 275 described rhizobial species in all main rhizobial genera in different countries on all continents except Antarctica. Generally, individual symbiovars occurred within one rhizobial genus, but there were exceptions. Symbiovars associated with widely grown crop legume species were of different rhizobial species on different continents. Most other symbiovars occurred within rhizobial species isolated from one legume species, genus or clade/tribe in a single country. Data on legume nodulation are limited for half of the symbiovars, such that judgements on their wider specificity are not possible. However, several of these symbiovars were shown to be specific to particular legume species within a genus. All other symbiovars showed legume specificity, but this was variable. Overall, further nodulation and symbiosis gene phylogenetic studies are required to more clearly define symbiovars.