Genetic diversity among bradyrhizobium isolates that effectively nodulate peanut (Arachis hypogaea)

Symbiotic gene diversity and other measures of genetic diversity were examined in Bradyrhizobium isolates that form an effective symbiosis with peanut (Arachis hypogaea). Initially, restriction fragment length polymorphism (RFLP) analysis using a nitrogenase (nif) gene probe was performed on 33 isolates along with one Bradyrhizobium elkanii and two Bradyrhizobium japonicum strains. Considerable diversity was observed among the RFLP patterns of many of the isolates, especially those from South America. Some isolates, however, were found to have similar nif and subsequent nod (nodulation) gene RFLP patterns, indicating symbiotic gene relatedness. With some noted exceptions, symbiotic gene relatedness correlated with relatedness based on total DNA homology and ribotyping analyses. Symbiotic gene relatedness also correlated with symbiotic effectiveness. The RFLP and DNA homology analyses indicate that bradyrhizobia effective with peanut are genetically diverse and consist of at least three different species. This diversity, however, was not particularly evident with partial 16S rRNA gene sequencing. Sequences obtained from the isolates were very similar to each other as well as to sequences previously reported for other Bradyrhizobium strains.