Genetic analysis and identification of DNA markers linked to a novel Phytophthora sojae resistance gene in the Japanese soybean cultivar Waseshiroge

The Glycine max (L.) Merr. cultivar Waseshiroge is highly resistant to several races of Phytophthora sojae in Japan. In order to determine which Rps gene might be present in Waseshiroge, 15 differential cultivars were challenged with 12 P. sojae isolates. None had a reaction pattern identical to that of Waseshiroge, indicating that Waseshiroge may contain a novel Rps gene. In order to characterize the inheritance of Waseshiroge resistance to P. sojae isolates, 98 F2 progeny and 94 F7:8 lines were produced from crosses between the susceptible cultivar Tanbakuro and Waseshiroge. Chi-square tests indicated that segregation fit a 3:1 ratio for resistance and susceptibility in two F2 sub-populations of 42 and 56 seedlings. This and a 46.27:1.46:46.27 (or 63:2:63) ratio for resistance: segregation: susceptibility among the 94 F7:8 lines indicated that resistance was controlled by a single dominant gene. DNA analyses were carried out on Tanbakuro, Waseshiroge and the 94 F7:8 lines, and a linkage map was constructed with 17 SSR markers and nine new primer pairs that amplify marker loci linked to Rps1 on soybean chromosome 3 (linkage group N). The closest markers, Satt009 and T000304487l, map to locations 0.9 and 1.6 cM on each side of the estimated position of the Rps gene, respectively. The results showed that the Rps gene in Waseshiroge is either allelic to Rps1, or resides at a tightly linked locus in a gene cluster. A three-way-contingency table analysis indicated that marker-assisted selection with the two flanking markers could be used in the development of new resistant cultivars.