Characterization of soybean bradyrhizobia for which serogroup affinities have not been identified

The USDA, ARS National Rhizobium Germplasm Collection contains 143 accessio ns of slow-growing soybean strains among which there are 17 distinct serolo gical groups. However, 11 strains appear to have no serological affinity wi th the 17 serogroups. Therefore, we determined whether these strains were d iverse and examined their phylogenetic placement. Nine strains formed nitro gen-fixing symbioses with soybean indicating that these accessions were not contaminants. We concluded from results of amplified fragment length polym orphism (AFLP) analysis, using 3 selective primers with 8 strains, that the y were genetically dissimilar. Nine strains were examined for their fatty a cid composition using fatty acid methyl ester (FAME) derivatives. The FAME results with 5 strains and serotype strains of Bradyrhizobium elkanii were similar, while results with each of the remaining 2 pairs were either simil ar to the type strain of Bradyrhizobium japonicum (USDA 6) or to USDA 110. Evolutionary history of 9 strains was reconstructed from sequence divergenc e of a combination of the complete 16S rRNA gene, the internally transcribe d spacer region, and about 400 bases of the 5' end of the 23S rRNA gene. Pl acement of 5 strains was nested within B. elkanii, 2 with USDA 110, and the other 2 with USDA 6. We concluded that soybean isolates that cannot be pla ced within one of the 17 established serogroups are phenotypically and gene tically as diverse as the serotype strains.