GENETIC-LINKAGE OF THE PHYTOPHTHORA RESISTANCE GENE RPS2 AND THE NODULATION RESPONSE GENE RJ2 IN SOYBEAN

Genome mapping of the agronomically important genes in soybean, Glycine max (L.) Merr., is as yet in a primitive stage. In this study we tested the Rps2 locus controlling resistance to phytophthora root rot (induced by Phytophthora megasperma Drechs. f. sp. glycinea T. Kuan & D.C. Erwin) for genetic linkage to the Rj2 locus controlling ineffective nodulation with certain strains of Bradyrhizobium japonicum. The line BARC-4 (Rj2) carrying the Rj2 and Rps2 alleles was hybridized with 'Clark 63' carrying the rj2 and rps2 alleles. The F2 plants were characterized for the Rj2 vs. rj2 phenotypes by inoculating seeds in growth-tray culture with a strain (USDA 7) defining for the Rj2 locus. The F3 progeny derived from F2 plants assayed for the Rj2 gene were screened to determine the genotype at the Rps2 locus. The F3 seedlings were evaluated in hydroponic culture inoculated with Race 4 of P. megasperma. Analysis of data indicated a significant (P < 0.0001) chi-square for linkage. The maximum likelihood estimate for linkage indicated a very tight linkage with 2.7 +/- 1 genetic map units separating these loci. The Rps2-Rj2 linkage is designated Linkage Group 19.