Hiebert et al (1) reported that the accession of wheat cultivar Webster (RL6201) maintained at the Cereal Research Centre in Winnipeg, Canada, has a locus for resistance to TTKSK (Ug99), temporarily designated SrWeb, that maps near Sr9 on chromosome 2BL. Webster is known to carry the recessive gene Sr30 on chromosome 5DL, which is also present in accession RL6201, conferring resistance to the North American race TPMK, but not to TTKSK. Interestingly, the accession of Webster maintained at the Cereal Disease Laboratory in St. Paul, MN, is not resistant to TTKSK, suggesting that at least these two accessions are genetically different. So far the distribution of SrWeb across Webster collections is not known.
Chromosome 2BL carries three other Sr genes, Sr9, Sr16 and Sr28 (2). After its first report, it was demonstrated that SrWeb is an allele of Sr9, and the new designation for SrWeb is Sr9h (3).
SrWeb is on the long arm of chromosome 2B. The closest proximal marker is Xgwm47-2B (1.4 cM) and is the recommended marker for marker assisted selection. On the distal side the nearest marker, Xwmc332, is 12.4 cM away. These two markers are dominant in the populations used to map SrWeb, and produce PCR amplification products when SrWeb is present (linked in coupling). The allele sizes for WMS47 and WMC332 are 207-bp and 190-bp respectively. Marker Xwmc175, located 5 cM proximal to Xgwm47-2B produces a PCR amplification product when SrWeb is absent (linked in repulsion).Primers sequences:
WMS47-F 5'- TTG CTA CCA TGC ATG ACC AT -3' WMS47-R 5'- TTC ACC TCG ATT GAG GTC CT -3'PCR conditions for WMS47:
Conditions presented here should be considered only as a starting point of the PCR optimization for individual laboratories.
1. Genetics and mapping of stem rust resistance to Ug99 in the wheat cultivar Webster. Hiebert CW, Fetch Jr. TG, Zegeye T. In: Theoretical and Applied Genetics, 2010, 121:65–69. DOI:10.1007/s00122-010-1291-z
2. Wheat rusts: An atlas of resistance genes. McIntosh RA, Wellings CR, Park RF, 1995, CSIRO Publications, East Melbourne, Australia.
3. Characterization of Sr9h, a wheat stem rust resistance allele effective to Ug99. Rouse MN, Nirmala J, Jin Y, Chao S, Fetch TG Jr, Pretorius ZA, Hiebert CW. In: Theoretical and Applied Genetics, 2014, 127:1681-1688. DOI:10.1007/s00122-014-2330-y