Strategies for pyramiding resistance genes against the barley yellow mosaic virus complex (BaMMV, BaYMV, BaYMV-2)

Barley Yellow Mosaic Virus disease caused by different strains of BaYMV and BaMMV is a major threat to winter barley cultivation in Europe. Pyramiding of resistance genes may be considered as a promising strategy to avoid the selection of new virus strains and to create more durable resistances. However, this goal cannot be achieved by phenotypic selection due to the lack of differentiating virus strains. For pyramiding of resistance genes rym4, rym5, rym9 and rym11, located on chromosomes 3H and 4H of barley two different strategies have been developed. These strategies are based on doubled haploid lines (DHs) and marker assisted selection procedures. On the one hand F-1 derived DH-plants of single crosses were screened by molecular markers for genotypes being homozygous recessive for both resistance genes. These genotypes were crossed to lines carrying one resistance gene in common and an additional third gene, leading to a DH-population of which 25% carry three resistance genes, 50% have two resistance genes and 25% possess a single resistance gene homozygous recessively. Alternatively, F-1 plants having one resistance gene in common were directly inter-crossed [e.g. (rym4 x rym9) x (rym4 x rym11)] and about 100 seeds were produced per combination. Within these complex cross progenies plants were identified by markers being homozygous at the common resistance locus and heterozygous at the others. From such plants, theoretically present at a frequency of 6.25%, DH-lines were produced, which were screened for the presence of genotypes carrying three or two recessive resistance genes in a homozygous state. Besides DH-plants carrying all possible two-gene combinations, 20 DH-plants out of 107 analysed carrying rym4, rym9, and rym11 and 27 out of 187 tested carrying rym5, rym9, and rym11 homozygously have been detected using the second strategy which is faster but needs co-dominant markers, because in contrast to the first strategy marker selection is carried out on heterozygous genotypes.