Targeting quantitative trait loci for adult plant stripe rust resistance in wheat

[摘要] English: Stripe rust, caused by Puccinia striiformis West. f. sp. tritici is one of the most damagingdiseases of wheat (Triticum aestivum L.) globally. The South African wheat cultivar Kariegaexpresses APR and has retained yield levels acceptable for commercial production, which isof great importance to plant breeders. A Kariega x Avocet S partial linkage map has made asignificant contribution to understanding the genetics underlying APR to stripe rust (Yr) inKariega. Two major YrQTL with indications of different resistance mechanisms wereidentified on chromosomes 2B and 7D.In this study we investigated the effectiveness of identifying AFLP markers closely linked tothe YrQTL using a targeted bulk segregant analysis (tBSA) approach in F1 doubled haploid(DH) individuals. Individual Kariega x Avocet S DH lines were characterised and bulkedbased on stripe rust phenotypes and DNA marker allele profiles. In agreement with standardBSA, an extreme resistant bulk (both QTL present: +7D+2B) and extreme susceptible bulk(both QTL absent: -7D-2B) were constructed based on phenotypic data and verified withmarker allele data. Three additional bulks (+7D-2B; -7D+2B and narrow down +7D±2B withmarker recombinations in 7D QTL interval) were constructed based on a combination ofphenotypic and marker data, with a strong emphasis on the presence or absence of markeralleles representing a specific QTL interval as required by a specific bulk.A total of 184 AFLP primer combinations (SseI and MseI) were tested on the two parentallines and five bulks. Thirty-one of these primer combinations detected 32 putative markersthat could discriminate between the extreme resistant and susceptible bulks and that wereputatively linked to either the 7D or 2B QTL regions. After validation of these markers on allindividuals used in the extreme resistant and extreme susceptible bulks, nine markers wereidentified that were present in the extreme resistant and the specific -7D+2B bulk, but absentin the extreme susceptible bulk. Another two markers were identified that were present in theextreme resistant, +7D-2B and narrow down +7D±2B bulks, but absent in the extremesusceptible bulk. These markers were mapped onto the existing Kariega x Avocet S partial linkage map using Map Manager QTXb20. Six AFLP markers mapped within or close to theQYr.sgi.2B and one close to the QYr.sgi.7D QTL regions.The tBSA approach was efficient since 10 of the 11 markers (91%) putatively identified afterscreening of the individuals constituting the bulk samples mapped to either chromosome 2Bor 7D. AFLP analysis in combination with tBSA was shown to be reproducible, faster and amore cost effective approach compared to a traditional BSA since tBSA lead to a reductionof 28.2% of primers that need to be tested. Following the tBSA approach, marker s23m53dmapped 3 cM from marker gwm148 previously shown to be significantly associated withmean host reaction type for final field data as well as leaf area infected of the QYr.sgi-2BQTL region. This resulted in an increase in LOD score from 20.1 to 23.9 using intervalmapping. Even though two markers were added to the 7D chromosome, both mapped outsidethe QYr.sgi-7D QTL region. Marker s20m38b mapped 9 cM from the SSR marker gwm295and 20 cM from the Ltn gene previously shown to be associated with the trait of interest onchromosome 7D.In summary, the combination of AFLP analysis and a tBSA approach has proved to be usefulin the identification of QTL, the placement of closely linked markers to known QTLs andtargeting chromosome areas with low marker numbers. Indications are that a large number ofAFLP primer combinations need to be screened to successfully target a specific QTL intervalfor increased map resolution.

[发布机构] University of the Free State