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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. [ABSTRACT FROM AUTHOR]