GENETIC-LINKAGE TESTS FOR THE RJ4 GENE IN SOYBEAN

The Rj4 allele in soybean, Glycine max (L.) Merr., conditions an ineffective nodulation response primarily with bradyrhizobial strains classified as Bradyrhizobium elkanii, which are less efficient in N2 fixation with soybean, thus protecting the plant from many of these strains. The diminishing frequency of the Rj4 allele with agronomic selection poses the question of possible linkage with genes of negative agronomic selective value. Little is known of the linkage relationships of the Rj4 locus. This report analyzes genetic linkage tests of the Rj4 locus with 12 other loci: F, Fr1, Fr2, I, L1, Ln, P, W1, Y9, Y10, Y12, and Y17. Segregating F2 and F3 progeny of several crosses were used to generate data for analysis. The Rj4 assays were conducted by planting seeds in sterile vermiculite and inoculating with broth cultures of strain USDA 61 from the Beltsville Rhizobium Culture Collection and examining the roots after 2 to 4 wk culture in the greenhouse. Other traits were scored either in the field or in the greenhouse. Chi-square was used to test for linkage, and the product method or method of maximum likelihood was used to estimate recombination. Only the Y17 locus showed evidence of genetic linkage with Rj4. The estimated recombination of Y17 and Rj4 was 46 +/- 2%.