Molecular breeding for grain yield in barley: an evaluation of QTL effectsin a spring barley cross

We report results from a breeding strategy designed to accumulate favorable QTL alleles for grain yield identified in the Steptoe x 'Morex' (SM) barle y germplasm. Two map lines (SM73 and SM145) from the original mapping popul ation were selected based on their marker genotype and QTL structure. When crossed. these lines would be expected to produce progeny with most favorab le QTL alleles. One hundred doubled haploid (DH) lines from the F-1 hybrid of this cross were enotyped with ten RFLP markers and one morphological mar ker defining grain yield to monitor QTL segregation. A subset of 24 lines r epresenting various combinations of putatively favorable and unfavorable QT L alleles, together with Steptoe,'Morex', SM73, and SM145, were phenotyped for grain yield in five environments. Multiple regression procedures were u sed to explore phenotype and genotype relationships. Most target QTLs showe d significant effects. However, significance and magnitude of QTL effects a nd favorable QTL allele phase varied across environments. All target QTLs s howed significant QTL-by-environment interaction (QTL x E)? and the QTL on chromosome 2 expressed alternative favorable QTL alleles in different envir onments. Digenic epistatic effects were also detected between some QTL loci . For traits such as grain yield, marker-assisted selection efforts may be better targeted at determining optimum combinations of QTL alleles rather t han pyramiding alleles detected in a reference mapping population.