Most Canadian hexaploid cultivars are susceptible to Pgt races from the Ug99 race group and all of the cultivars that are grown on significant acreage are susceptible (Fetch et al. 2012). A screen of Canadian cultivars in Njoro, Kenya, where Ug99 races predominated, revealed that two hard red spring wheat cultivars, Peace and AC Cadillac, showed high levels of resistance. Peace and AC Cadillac are closely related cultivars and were believed to carry the same source of resistance to Ug99. Two populations were generated to study the inheritance and determine the chromosome location of the resistance (Hiebert et al. 2011). A doubled haploid (DH) population from the cross RL6071/Peace and an F2 population from the cross LMPG-6S/AC Cadillac both showed that a single gene was responsible for the Ug99 resistance detected at the seedling stage. Mapping with SSR markers showed that the resistance in both populations mapped distally on chromosome 6DS in the same genetic interval. A gene for common bunt resistance, Bt10, has been mapped to a similar chromosome region (Menzies et al. 2006). DNA marker FSD_RSA, a dominant marker that was developed to select Bt10 (Laroche et al. 2000), was added to the genetic maps of the Peace and AC Cadillac derived populations (Hiebert et al. 2011). FSD_RSA showed tight linkage to the Sr gene, which was given the temporary designation SrCad. Both AC Cadillac and Peace carry Bt10 which was inherited from the breeding line BW553. In the Canadian Prairie Spring (CPS) class of hexaploid wheat, BW553 was a parent to several cultivars and used as a donor of Bt10. A survey of CPS cultivars revealed that all of the cultivars that carried the positive allele (the allele found in BW553, Peace and AC Cadillac) were also resistant to Ug99 while all other cultivars were susceptible (Hiebert et al. 2011). In the germplasm screened with FSD_RSA, the positive allele is rare and has been linked to Ug99 resistance in every instance to date (Hiebert et al. 2011; Ghazvini et al. 2012). Thus, it appears that the presence of FSD_RSA is indicative of SrCad though it would be too large an assumption to deem the marker diagnostic as recombinants between FSD_RSA and SrCad were recovered in mapping populations (Hiebert et al. 2011; Hiebert et al. unpublished data). Interestingly, the CPS cultivars that carry SrCad showed a lower level resistance to Ug99 in field nurseries in Njoro, Kenya compared to Peace and AC Cadillac. Both Peace and AC Cadillac carry Lr34 while none of the CPS cultivars carrying SrCad carry Lr34. Hiebert et al. (2011) showed the combination of SrCad and Lr34 improved the level of Ug99 resistance in field conditions.
Given the map location of SrCad, there was interest in determining the relationship between SrCad and Sr42. Sr42 was reported to be carried on chromosome 6DS however there were no published genetic maps. Sr42 was found in Norin 40, a Japanese winter wheat (McIntosh et al. 1995). Using a DH population from the cross LMPG-6S/Norin 40, Ghazvini et al. (2012) mapped Sr42 using SSR markers and FSD_RSA. Sr42 mapped to the same genetic interval as SrCad. Norin 40, like Peace and AC Cadillac, carries the positive allele of FSD_RSA. Given that this marker allele is rare, it seems likely that Norin 40 and BW533, the donor of SrCad to all Canadian carriers, have a parent in common however no such parent could be traced. The sizes of the DH populations used to map SrCad (n = 295; Hiebert et al. 2011) and Sr42 (n = 248; Ghazvini et al. 2012) yielded good map resolution and their respective map positions are not significantly different (Fig. 20.1). Both SrCad and Sr42 conferred resistance to Pgt races Ug99 and RTQSC. While very limited, this does show an additional similarity between these resistances. New markers developed in ongoing fine-mapping experiments were unable to show differences in map position between SrCad and Sr42 (unpublished data). It is likely that SrCad and Sr42 are allelic and probably represent the same allele.
Fig. 20.1 Ug99 resistance on chromosome 6DS mapped in the RL6071/Peace (Hiebert
et al. 2011) and LMPG-6S/ Norin 40 (Ghazvini et al. 2012) DH populations.
Genetic distances are in cM
Other Sr Genes on Chromosome 6DS that Confer Resistance to Ug99 Stem Rust
There have other Sr genes mapped to 6DS in a similar chromosome region as SrCad and Sr42 that confer resistance to Ug99. One such example was reported by Olsen et al. (2013) where an Sr gene, temporarily designated SrTA10187, identified in Aegilops tauschii was transferred to common wheat and was mapped to chromosome 6DS. In that study, few DNA markers were mapped, however two of the three markers were also used to map SrCad (Hiebert et al. 2011). The map positions of SrCad and SrTA10187 cannot be differentiated based on the data published to date. Recently, seedling resistance to Ug99 was mapped in five CIMMYT spring wheat cultivars and one US winter wheat cultivar (Lopez-Vera et al. 2014). In all six populations the resistance mapped to a similar region of chromosome 6DS however the map order varied slightly between the various maps. Whether the map differences are the result of experimental error such as misclassified progeny, or the detection of different genes is unknown presently. The authors postulate that two different genes were mapped, one that is near or allelic to SrCad/Sr42 and one that is distal to these genes but closely linked. It was suggested that the resistance postulated to be allelic to SrCad/Sr42 was SrTmp derived from the cultivar Triumph 64 and our preliminary map of SrTmp that would agree with this assertion. While Lopez-Vera et al. (2014) are basing their hypothesis on gene postulation experiments, we used a DH population from the cross LMPG-6S/Triumph 64 to generate a population to directly determine the map position of SrTmp. Using the LMPG-6S/Norin 40 and LMPG-6S/Triumph 64 DH populations we were able to show that Sr42 and SrTmp differ in the breadth of resistance they confer. Thus, even if the two are allelic, they represent different allelic forms.
Resistance to Ug99-type stem rust races has been mapped to chromosome 6DS in a number of studies from a number of sources. Understanding the relationships between these genes is important for developing strategies to deploy these genes. If the genes are allelic it is prudent to determine which allele is the most relevant for a given growing region and the local Pgt population. If the genes are non-allelic it could be worthwhile to attempt to recover recombinants carrying gene combinations to generate a gene stack carried in a tight linkage group. The DNA markers developed from the fine-mapping of SrCad (unpublished) could be useful in delineating the Sr genes mapped to this chromosome region. To further characterize these genes phenotypically, near-isogenic lines (NILs) should be produced to allow an accurate comparison of their range of effectiveness against a panel of Pgt races.