Diversity And Population Structures Of Rhizobia Nodulated With Phaseolus Vulgaris In Two Ecoregions Of China

To reveal the diversity and geographic distribution of bean(Phaseolus vulgaris L.) rhizobia in China, a total of 226 strains of nodule bacteria were isolated from bean plants grown in fields of nine sites in northern(ecoregion I) and southern(ecoregion II) regions of China, with soil pH ranged from 8.06 to 4.97. Based upon the phylogenies of housekeeping and symbiotic genes, 96.9 % of the isolates were Rhizobium corresponding to four defined species and four unnamed genospecies, while the remaining were five Bradyrhizobium genospecies. Rhizobium leguminosarum, Rhizobium etli and Rhizobium sp. II or IV were dominant in different ecoregions, with varied relative abundances. Therefore, diverse and distinctive bean rhizobial communities exist in different ecoregions of China, demonstrating that the introduction of bean plants has driven the evolution of rhizobia to fit the necessity to nodulate the host plant under the local conditions, mainly the soil pH and the nutrient availability. The predominance of R. etli and the high similarities in the symbiotic genes(nodC and nifH) of all the Rhizobium genospecies with R. etli evidenced that symbiotic genes were transferred from R. etli, possibly introduced to China accompanying the seeds, to the related indigenous rhizobia. These findings enlarged the diversity of bean rhizobia, evidenced the biogeographic patterns of these rhizobia, and demonstrated the possible evolution and emerging of novel rhizobia under the combined selection of host plant and soil conditions.Rhizobium etli populations have distinct biogeographic patterns in different ecoregions of China and are affected by local soil pH. To reveal the genetic diversity and population structures of R. etli-like strains isolated in northern(ecoregion I) and southern(ecoregion II) regions of China which exist the difference of pH and the relationship of R. etli from China and the origin, eight genes corresponding to pH stress were involved in as molecular markers. Different levels of haplotype diversity and nucleotide diversity were observed among these genes in different sampling sites. Seletion analyses showed that in addition to actR(D = 2.15383, p < 0.05), actS(D = 2.30857, p < 0.05) and grpE(D = 1.94053, p < 0.05) in the population of the alkaline environment were affected by natural selection, the rest of genes evolved under the neutral selection. Fu and Li’s Fs was positive and significant inferred that populations may experience the bottleneck effect or overdominant selection. Only one positive selection site was detected in the amino acid sequence of actS gene as well as otsA gene, however, it was not significant, which indicated that the positive selection signal was weak. There was high genetic differentiation among pre-defined populations in different sampling sites in China and vary degree of differentiation with R. etli isolated from the Americas. Population structure analysis showed that these isolates were traced to four ancestral lineages. The isolates in Jiangkou, Shenyang, Nanchang, Mazuyan, Sanjiang and Yudu were admixture contained the same ancestral lineages with the R. etli isolated from the Americas, while the isolates in Chutan and Daqing formed the independent genetic clusters which were unrelated with R. etli isolated from the Americas. Mantel test showed that to some extent the divergence among R. etli populations had affected by geographic distances but not the main. Environmental factors and host selection may also take the contribution. Moreover, recombination could influence the evolution of R. etli isolates more than mutation.