Genetic allelism and linkage tests of a soybean gene,Rfg1, controlling nodulation with Rhizobium fredii strain USDA 205

A soybean gene, Rfg1, controlling nodulation with strain USDA 205, the type strain for the fast-growing species Rhizobium fredii, was tested for allelism with the Rj4 gene. The Rj4 gene conditions ineffective nodulation primarily with certain strains of the slow-growing soybean microsymbiont, Bradyrhizobium elkanii. The F2 seeds of the cross of the cultivars Peking, carrying the alleles rfg1, Rj4, i (controlling inhibition of seed coat color) and W1 (controlling flower color), and Kent, carrying the alleles Rfg1, rj4, i-i and w1, were evaluated for nodulation response with strain USDA 205 by planting surface disinfested seeds in sterilized vermiculite in growth trays and inoculating with a stationary phase broth culture of strain USDA 205 at planting. Plants were classified for nodulation response visually after four weeks growth and transplanted to the field for F3 seed production. Flower color, purple (W1) vs white (w1), was determined in the field. The allele present at the i locus was determined by classification of F3 seed coat color. The F3 seeds were planted in growth trays and inoculated with strain USDA 61 of Bradyrhizobium elkanii to determine the genotype for the Rj4 locus. The Rfg1 and Rj4 genes were determined to be located at separate loci. Chi-square analysis for linkage indicated that Rfg1 segregated independently of the Rj4, I and W1 loci.