Genetic interactions among four genes for resistance to stem rust in bread wheat

Abstract

Near-isogenic lines of bread wheat (Triticum aestivum L.) carrying the genes Sr7a, Sr8a, Sr9b, and Sr11 for stem rust (Puccinia graminis f. sp. tritici Eriks, and Henn.) resistance had been developed previously using nine backcrosses to the cultivar Marquis. The lines were intercrossed to produce the six possible two-gene combinations and three of the four possible three-gene combinations. The objective was to test for additive, modifying, or residual effects of the genes. The nine combinations, the four near-isogenic parents, 'Marquis', and three near-isogenic lines with only five backcrosses to 'Marquis' were tested. Seedling tests with five races were done in a growth room and field tests were done with two races. Relatively few interactions among the genes were detected in the seedling tests. All involved small additive effects when Sr7a was combined with either Sr8a or Sr9b. Interactions were much more common in the field tests. With race 56, the three two-gene combinations of Sr8a, Sr9b, and Sr11 showed significant additive effects. With race 15B-1, there were four cases of an ineffective gene modifying the effect of an effective gene. In addition, the data indicated that Sr9b and possibly Sr11 had residual effects against race 15B-1. The reaction of a genotype to stem rust in the field clearly involves a complex interaction of effective and ineffective genes.Key words: rust resistance, Puccinia, wheat, Triticum, modifiers, residual effects, additive effects.