Pyramiding and validation of quantitative trait locus (QTL) alleles determining resistance to barley stripe rust: effects on adult plant resistance.(Genomics, Molecular Genetics & Biotechnology)

QR) shows continuous variation and it is usually incom- plete in expression but has a higher probability of dura- The use of molecular and quantitative trait locus (QTL) analysis bility (Parlevliet, 1989). Before the advent of QTL anal- tools initially lent support to the idea that relatively few genetic factors were the principal determinants of complex traits, including quantita- ysis, estimates of gene number for QR were derived tive resistance (QR) to plant diseases. However, there are concerns from statistical analyses of phenotypic data collected regarding bias in QTL estimation and reproducibility of QTL effects from segregating crosses and estimates of the number in different genetic backgrounds. We are interested in mapping deter- of "effective factors" determining QR ranged from two minants of QR, and pyramiding resistance alleles at QTL loci may to 10 (Geiger and Heun, 1989). QTL approaches to lead to durable resistance as well as provide independent validation estimating the number of genes determining QR have of QTL effects and estimation of QTL interactions. We used molecular revealed that in some cases, a significant proportion of marker information to validate effects of resistance alleles at three the total variance in QR is attributable to one locus or QTL conferring QR to barley stripe rust (caused by Puccinia strii- a few loci. However, small population sizes and model formis West. f. sp. hordei). Two of the QTL (one on chromosome assumptions may cause estimates of gene number to 4(4H) and one on chromosome 7(5H)) trace to one parent, while another QTL on chromosome 5(1H) traces to a different parent. The be downwardly biased for both statistical analyses of pyramids of these QR alleles provide independent estimates of QTL phenotypic data (Geiger and Heun, 1989) and for QTL effects, influence of genetic background on QTL effects, QTL QTL analyses (Young, 1996; Kover and Caicedo, 2001). interaction, and QTL environment interaction. Our results validate Stripe rust (caused by P. striiformis f. sp. hordei )i s QTL effect estimates, showing that a small number of QTL explained an important disease of barley worldwide. We initiated 94% of the genetic variation in trait expression in a new genetic a collaborative effort (reviewed in Hayes et al., 2001) background. Original QTL estimates were quantitatively biased, but to map and introgress QR to barley stripe rust (BSR) that did not preclude the achievement of selection responses. We also from germplasm that has remained resistant to the spec- confirmed the additive effects of the QTL alleles, as well as the trum of virulence encountered in Mexico, South Amer- consistent effects of QTL alleles across environments.