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 n odulation primarily with certain strains of the slow-growing soybean m icrosymbiont, Bradyrhizobium elkanii. The F2 seeds of the cross of the cultivars Peking, carrying the alleles rfg1, Rj4, i (controlling inhi bition of seed coat color) and W1 (controlling flower color), and Kent , carrying the alleles Rfg1, rj4, i-i and w1, were evaluated for nodul ation response with strain USDA 205 by planting surface disinfested se eds in sterilized vermiculite in growth trays and inoculating with a s tationary phase broth culture of strain USDA 205 at planting. Plants w ere classified for nodulation response visually after four weeks growt h and transplanted to the field for F3 seed production. Flower color, purple (W1) vs white (w1), was determined in the held. The allele pres ent at the i locus was determined by classification of F3 seed coat co lor. The F3 seeds were planted in growth trays and inoculated with str ain USDA 61 of Bradyrhizobium elkanii to determine the genotype for th e Rj4 locus. The Rfg1 and Rj4 genes were determined to be located at s eparate loci. Chi-square analysis for linkage indicated that Rfg1 segr egated independently of the Rj4, I and W1 loci.