Abstract
Twenty-two rhizobia strains isolated from three distinct populations (North Ossetia, Dagestan, and Armenia) of a relict legume Vavilovia formosa were analysed to determine their position within Rhizobium leguminosarum biovar viciae (Rlv). These bacteria are described as symbionts of four plant genera Pisum, Vicia, Lathyrus, and Lens from the Fabeae tribe, of which Vavilovia is considered to be closest to its last common ancestor (LCA). In contrast to biovar viciae, bacteria from Rhizobium leguminosarum biovar trifolii (Rlt) inoculate plants from the Trifolieae tribe. Comparison of house-keeping (hkg: 16S rRNA, glnII, gltA, and dnaK) and symbiotic (sym: nodA, nodC, nodD, and nifH) genes of the symbionts of V. formosa with those of other Rlv and Rlt strains reveals a significant group separation, which was most pronounced for sym genes. A remarkable feature of the strains isolated from V. formosa was the presence of the nodX gene, which was commonly found in Rlv strains isolated from Afghanistan pea genotypes. Tube testing of different strains on nine plant species, including all genera from the Fabeae tribe, demonstrated that the strains from V. formosa nodulated the same cross inoculation group as the other Rlv strains. Comparison of nucleotide similarity in sym genes suggested that their diversification within sym-biotypes of Rlv was elicited by host plants. Contrariwise, that of hkg genes could be caused by either local adaptation to soil niches or by genetic drift. Long-term ecological isolation, genetic separation, and the ancestral position of V. formosa suggested that symbionts of V. formosa could be responsible for preserving ancestral genotypes of the Rlv biovar.
Highlights
Root nodule bacteria represent a useful model for studying the molecular and ecological mechanisms of evolution of symbiotic bacteria
Nucleotide similarity of each gene between populations did not differ statistically from that observed within populations, so all three populations could be regarded as components of the same metapopulation of symbionts of V. formosa
Symbionts of V. formosa could be regarded as components of the same metapopulation, Mantel analysis shows the influence of geographical origin on gene diversity, which is more pronounced for hkg than sym genes
Summary
Root nodule bacteria (rhizobia) represent a useful model for studying the molecular and ecological mechanisms of evolution of symbiotic bacteria. (sym) genessymbiosis representing accessory parts of co-adapted cross-inoculation groups of rhizobia and legumes, and their coevolution is directed by bacterial genomes, which differ in their natural histories from housekeeping genes (hkg) representing a set of symbiosis-specific genes from each partner [5,6,7]. As a result of co-evolutionary formed tightly co-adapted cross-inoculation groups rhizobia and they legumes, and their is to autonomous horizontal gene transfer than hkgofgenes, because are located on coevolution plasmids—mobile directed bygenome a set of symbiosis-specific genes from eachrecombination partner [5,6,7]. Inofsome sym chromosomal genes are elements of the [3] This results in an intensive hostrhizobia, specific and more susceptible to autonomous gene transfer than hkgof genes, are have located on markers [8]
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