This paper explores the relationship between the genetic diversity of rhizobia and the morphological diversity of their plant hosts. Rhizobium galegae strains were isolated from nodules of wild Galega orientalis and Galega officinalis in the Caucasus, the center of origin for G. orientalis. All 101 isolates were characterized by genomic amplified fragment length polymorphism fingerprinting and by PCR-restriction fragment length polymorphism (RFLP) of the rRNA intergenic spacer and of five parts of the symbiotic region adjacent to nod box sequences. By all criteria, the R. galegae bv. officinalis and R. galegae bv. orientalis strains form distinct clusters. The nod box regions are highly conserved among strains belonging to each of the two biovars but differ structurally to various degrees between the biovars. The findings suggest varying evolutionary pressures in different parts of the symbiotic genome of closely related R. galegae biovars. Sixteen R. galegae bv. orientalis strains harbored copies of the same insertion sequence element; all were isolated from a particular site and belonged to a limited range of chromosomal genotypes. In all analyses, the Caucasian R. galegae bv. orientalis strains were more diverse than R. galegae bv. officinalis strains, in accordance with the gene center theory.Rhizobium galegae (14) is a species that forms an effective symbiosis with plants of Galega orientalis and Galega officinalis, the only species in the genus Galega (Fabaceae) that have been studied for symbiosis. This symbiotic system represents a sharply defined cross-inoculation group. However, there are some differences in symbiotic performance. R. galegae strains are able to infect both Galega species, but strains isolated from G. officinalis form effective nodules on that plant and ineffective nodules on G. orientalis, while the converse is true for strains from G. orientalis (16). This finding, along with a wide range of phenotypic and genotypic approaches using numerical taxonomy (15), phage typing, DNA homology (13, 40), lipopolysaccharide and protein patterns (16), plasmid profiling (29), phylogeny of ribosomal genes (25, 35), randomly amplified polymorphic DNA, and repetitive PCR (21, 28), led to the proposal of two biovars for strains forming an effective symbiosis with G. officinalis (R. galegae bv. officinalis) and G. orientalis (R. galegae bv. orientalis) (24). Symbiosis-related genetic traits were found to be the main factor in genetic divergence between the biovars (24). G. orientalis is a good fodder plant with some widely used cultivars, whereas G. officinalis is rather poisonous because of its high alkaloid content and has no agricultural importance.The interesting symbiotic properties, together with the extensive taxonomic knowledge about R. galegae, prompted us to use the species to answer important questions related to the diversity and evolution of rhizobia and their symbiosis with legumes. A recent study indicated that R.galegae bv. orientalis strains have lower genetic diversity than R. galegae ...

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Legume plants can obtain combined nitrogen for their growth in an efficient way through symbiosis with specific bacteria. The symbiosis between Rhizobium galegae and its host plant Galega is an interesting case where the plant species G. orientalis and G. officinalis form effective, nitrogen-fixing, symbioses only with the appropriate rhizobial counterpart, R. galegae bv. orientalis and R. galegae bv. officinalis, respectively. The symbiotic properties of nitrogen-fixing rhizobia are well studied, but more information is needed on the properties of the host plants. The Caucasus region in Eurasia has been identified as the gene centre (centre of origin) of G. orientalis, although both G. orientalis and G. officinalis can be found in this region. In this study, the diversity of these two Galega species in Caucasus was investigated to test the hypothesis that in this region G. orientalis is more diverse than G. officinalis. The amplified fragment length polymorphism fingerprinting performed here showed that the populations of G. orientalis and R. galegae bv. orientalis are more diverse than those of G. officinalis and R. galegae bv. officinalis, respectively. These results support the centre of origin status of Caucasus for G. orientalis at a genetic level. Analysis of the symbiosis-related plant genes NORK and Nfr5 reveals remarkable diversity within the Nfr5 sequence, although no evidence of adaptive evolution could be found.