Genetic analysis of disease resistance to all strains of BaYMV in a Chinese barley landrace, Mokusekko 3

A Chinese landrace of barley, Mokusekko 3, is unique in being completely resistant against all strains of barley yellow mosaic virus (BaYMV). The present investigation revealed that the resistance of Mokusekko 3 is governed by two recessive genes. As one of the resistance genes was known to be tightly linked with alleles at the Est complex locus, consisting of the Est1, Est2 and Est4 loci for esterase isozymes, each of the resistance genes could be separated by means of marker-assisted selection using an isozyme allelic combination as a marker. One of the resistance genes, ym1, is linked to K (hooded lemma) and gl3 (glossy leaf 3) with recombination values of 25.3% and 9.7% respectively, and these three genes are located in the order k-gl3-yml on chromosome 4. Another newly designated resistance gene, ym5, is linked to alleles at the Est complex locus and cu2 (curly growth 2), with recombination values of 1.9% and 19.5% respectively, in the order cu2-Est-ym5 from proximal to distal on the long arm of chromosome 3. The complete resistance of Mokusekko 3 is caused by combining two resistance genes, ym1 and ym5. However, almost all the ''resistant'' cultivars derived from crosses with Mokusekko 3 are susceptible to the recently detected strain BaYMV-III in Japan, since they contain only one resistance gene, ym5. Marker-assisted selection to combine resistance genes into a cultivar is discussed for the breeding of stabilizing resistance to BaYMV.