Genetic analysis of durable resistance to yellow rust in bread wheat

Yellow rust, caused by Puccinia striiformis, is one of the most damaging diseases affecting bread wheat in temperate regions. Although resistance to yellow rust is frequently overcome by new virulent races, a durable form of resistance in the French bread wheat Camp Rémy (CR) has remained effective since its introduction in 1980. We used 217 F₇ recombinant inbred lines (RILs) derived from the cross between CR and the susceptible cultivar Récital to identify and map quantitative trait loci (QTLs) involved in durable yellow rust resistance. Six significant QTLs that were stable over a 4-year period were detected. Two QTLs, denoted QYr.inra-2DS and QYr.inra-5BL.2, were located on the short arm of chromosome 2D and the long arm of chromosome 5B, respectively. Each explained on average 25–35% of the observed phenotypic variation and were probably inherited from Cappelle Desprez, a parent of CR that confers durable adult plant resistance to yellow rust. QYr.inra-2DS probably corresponds to the Yr16 gene. The most consistent QTL, designated QYr.inra-2BL, was located on the centromeric region of chromosome 2B and explained 61% of the phenotypic variation in 2003. This QTL was responsible for seedling-stage resistance and may correspond to a cluster of genes, including Yr7. The remaining QTLs were mapped to the short arm of chromosome 2B (R²=22–70%) and to the long arm of chromosomes 2A (R²=0.20–0.40) and 5B (R²=0.18–0.26). This specific combination of seedling and adult plant resistance genes found in CR and CD may constitute the key to their durable resistance against yellow rust.     

Identification of molecular markers for the detection of the yellow rust resistance gene Yr17 in wheat

The Yr17 gene, which is present in many European wheat cultivars, displays yellow rust resistance at the seedling stage. The gene introduced into chromosome 2A from Aegilops ventricosa was previously found to be closely linked (0.5 cM) to leaf and stem rust resistance genes Lr37 and Sr38, respectively. The objective of this study was to identify molecular markers linked to the Yr17 gene. We screened with RAPD primers, for polymorphism, the DNAs of cv. Thatcher and the leaf rust-resistant near-isogenic line (NIL) RL 6081 of cv. Thatcher carrying the Lr37 gene. Using a F2 progeny of the cross between VPM1 (resistant) and Thésée (susceptible), the RAPD marker OP-Y15580 was found to be closely linked to the Yr17 gene. We converted the OP- Y15580 RAPD marker into a sequence characterized amplified region (SCAR). This SCAR marker (SC-Y15) was linked at 0.8 ± 0.7 cM to the Yr17 resistance gene. We tested the SC-Y15 marker over a survey of 37 wheat cultivars in order to verify its consistency in different genetic backgrounds and to explain the resistance of some cultivars against yellow rust. Moreover, we showed that the Xpsr150-2Mv locus marker of Lr gene described by Bonhomme et al. [6] which possesses A. ventricosa introgression on the 2A chromosome was also closely linked to the Yr17 gene. Both the SCAR SC-Y15 and Xpsr150-2Mv markers should be used in breeding programmes in order to detect the cluster of the three genes Yr17, Lr37 and Sr38 in cross progenies. This revised version was published online in June 2006 with corrections to the Cover Date.     

Marker-assisted pyramiding of two cereal cyst nematode resistance genes from <Emphasis Type="Italic">Aegilops variabilis</Emphasis> in wheat

The cereal cyst nematode (CCN) Heterodera avenae, is a significant pathogen of wheat. The wild grass Aegilops variabilis Accession No.1 has been found to be resistant to pathotypes of CCN; at least two genes transferred to wheat, designated as CreX and CreY, are involved in the resistance response. The CreY gene may be the same as Rkn-mn1, which confers resistance to root knot nematode (RKN) Meloidogyne naasi. The objective of this work was to pyramid the two CCN resistance genes in a wheat background through marker-assisted selection. As a first step, molecular markers flanking CreX were identified. The completely linked RAPD marker of Rkn-mn1 (CreY), OpY16-_1065, previously obtained, was converted into a SCAR. All these dominant markers were used to incorporate in the same genotype the two Ae. variabilis chromosome segments carrying the two genes for resistance. CCN bioassays with the Ha12 pathotype showed that the level of resistance of the pyramided line was significantly higher than that of CreX and CreY single introgression lines, but lower than that of Ae. variabilis. This study thus illustrates the utilization of molecular markers in breeding for host resistance.