Biomics

The goat’s rue rhizobia (Neorhizobium galegae) represent a convenient model for studying the evolution and speciation in symbiotic bacteria. This rhizobia species is composed of two biovars (bv. orientalis, bv. officinalis) forming N₂-fixing nodules with different species of goat’s rue (Galega orientalis, G. officinalis) whereas the cross inoculation between these legume plants leads to the formation of nodules that are devoid of nitrogenase activity. We studied the nucleotide polymorphism of 11 strains of N. galegae isolated from the North Caucasus ecosystems, where G. orientalis has a much higher diversity than G. officinalis. At the same time, differences in the level of polymorphism are higher for sym-genes than for functionally more heterogeneous "non-symbiotic" genes, which may be due to the multidirectional selection induced by host plants in the analyzed nod- and nif/fix-genes. The divergence between bv. orientalis and bv. officinalis for nif/fix-genes is more pronounced than in nod-genes and in "non-symbiotic" genes, indicating the key role of host-specific N₂-fixation in the evolution of N. galegae.

nodule bacteria; Neorhizobium galegae (bv. orientalis, bv. officinalis); nucleotide polymorphizm; divergent evolution; core and accessor parts of genome

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