Differentiation of resistance genes at the M1-a locus in six pairs of isogenic barley lines

Summary Five pairs of lines, nearly isogenic except for genes for resistance and susceptibility to culture CR3 of Erysiphe graminis (DC). Merat hordei Em. Marchal, developed from the barley (Hordeum vulgare L.) varieties Algerian, Franger, Durani, Rupee, and Multan, had five different genes at the M1-a locus on chromosome 5 conditioning resistance to culture CR3. The resistant isogenic lines developed from Durani, Rupee, and Multan each had one additional resistance gene, closely linked in coupling to their resistance gene at the M1-a locus, conditioning resistance to culture 63.5 or 64.54. The sixth pair of isogenic lines, developed from the variety Long Glumes, had three resistance genes: one at the M1-a locus, a second closely linked in coupling to the first, and a third gene. The first and the third genes in Long Glumes appear to be the same as the two genes in Multan. The outstanding resistance to E. graminis hordei of most of the donor varieties for the isogenic lines is explained by their possessing two or more resistance genes. The presence of approximately 20 different genes in and around the M1-a locus suggests that there is a cluster of closely linked resistance loci in that region of chromosome 5.Contribution from Applied Plant Genetics Laboratory, Northeastern Region, Agricultural Research Service, U.S. Department of Agriculture, in cooperation with the Agricultural Research Department, Danish Atomic Energy Commission.