MOLECULAR DIVERSITY OF RHIZOBIA OCCURRING ON NATIVE SHRUBBY LEGUMES IN SOUTHEASTERN AUSTRALIA

The structure of rhizobial communities nodulating native shrubby legum es in open eucalypt forest of southeastern Australia was investigated by a molecular approach. Twenty-one genomic species were characterized by small-subunit ribosomal DNA PCR-restriction fragment length polymo rphism and phylogenetic analyses, among 745 rhizobial strains isolated from nodules sampled on 32 different legume host species at 12 sites. Among these rhizobial genomic species, 16 belonged to the Bradyrhizob ium subgroup, 2 to the Rhizobium leguminosarum subgroup, and 3 to the Mesorhizobium subgroup. Only one genomic species corresponded to a kno wn species (Rhizobium tropici), The distribution of the various genomi c species was highly unbalanced among the 745 isolates, legume hosts, and sites. Bradyrhizobium species were by far the most abundant, and R hizobium tropici dominated among the Rhizobium and Mesorhizobium isola tes in the generally acid soils where nodules were collected. Although a statistically significant association occurred between the eight mo st common genomic species and the 32 hosts, there was sufficient overl ap in distributions that no clear specificity between rhizobial genomi c species and legume taxa was observed. However, for three legume spec ies, some preference for particular genomic species was suggested, Sim ilarly, no geographical partitioning was found.