Materials and Methods
The mapping population consisted of 1,600 F2 progeny derived from a three-way cross between wheat cultivars Arina, Alsen and Ocoroni86 providing the resistance genes Qsng.sfr-3BS, Fhb1, and Sr2, respectively. The F2 population, its parents and cultivar Chinese Spring as negative control, were planted in January, 2013 in a greenhouse at Purdue University in West Lafayette, Indiana, U.S.A.. The primary spike after spike emergence (Feekes growth stage 10.3) of an adult plant of every F2 progeny and the parental lines was phenotyped in a greenhouse using an isolate of S. nodorum obtained from an infected wheat field in Indiana. The disease severity scores were recorded 21 days after inoculation on a 0–9 scale. The F2 plants were screened along with the parental lines using KASPar genotyping technology to identify the recombinant plants. Single-nucleotide polymorphism markers were utilized to identify the recombinants, determine the gene order, to make the genetic map and for quantitative trait loci (QTL) analysis. Linkage analysis was performed and the map was made with JoinMap 3.0.
Results and Discussion
Phenotyping of all 1,600 F2 progeny was performed and the results utilized the full range of the 0–9 disease-rating scale. The frequency distribution of the percent diseased glume tissue of F2 progeny derived from the three-way cross of Arina (containing the Qsng.sfr-3BS SNB resistance QTL), Alsen and Ocoroni86 inoculated with a field isolate of S. nodorum in a greenhouse showed a continuous distribution with a skew towards susceptibility, indicating the presence of one or more QTL for resistance (data not shown).
The preliminary genetic linkage map suggests that the stem rust resistance gene (Sr2) is the most distal, the fusarium head blight resistance gene (Fhb1) is proximal, and with the S. nodorum resistance gene (Qsng.sfr-3BS) is between Sr2 and Fhb1 on chromosome 3BS of wheat (Fig. 36.1). This result supports our hypothesis made
Fig. 36.1 Preliminary linkage map of wheat chromosome 3BS for markers SNP 1, SNP 2, SNP 3, SNP
4, and SNP 5 in the F2 population of 1,600 individuals derived from a three-way cross of wheat cultivars Arina, Alsen and Ocoroni86. The map was generated with JoinMap 3.0 at LOD = 9.0. Numbers to the left of the vertical bar indicate the total distance in centimorgans, and positions of mapped markers are indicated on the right. The approximate positions of disease resistance genes are indicated by brackets
from analyses of previously published marker positions. Knowing the correct order and relative distance between the three resistance genes indicates how a linkage block can be created. Markers for Sr2 and Qsng.sfr-3BS were placed 1 cM apart in our mapping population while the predictive marker for Fhb1 was located 4 cM proximal from Sr2. Thus, recombinant plants having both Sr2 and Qsng.sfr-3BS were recovered at a lower frequency than plants having Fhb1 and Qsng.sfr-3BS in coupling. These plants having two resistance genes in coupling will be used in further crosses to obtain recombinants having all three resistance genes in a linkage block. The tight linkage between Sr2, Qsng.sfr-3BS, and Fhb1 indicates that it should be easy to maintain this linkage block in a breeding program.