Different QTLs are associated with leaf rust resistance in wheat between China and Mexico

The wheat line ‘Chapio’ is resistant to leaf rust, caused by Puccinia triticinia, and was derived from a breeding programme that focuses on multi-genic resistance to provide durability. This line was crossed with the susceptible ‘Avocet’ to develop an F6 recombinant inbred line population. The population was phenotyped for leaf rust severity in two environments each in Mexico and China. There were significant differences in the loci providing resistance between the two intercontinental regions. The Lr34 locus had large effects in both Mexico and China, highlighting its importance in providing a basis for broad-spectrum resistance. The Lr46 locus on chromosome 1BL and a 3D locus had effects in Mexico but not in China. Presence of Sr2 was determined by the phenotypic marker of pseudo-black chaff and was mapped to chromosome 3BS. This region was associated with a QTL that had strong effects in China but no significant effect in Mexico, as did a locus on chromosome 4B. Seedling tests on the parents indicated that the 3B locus was not the complimentary gene Lr27, but the 4B locus was in the same position as Lr31 (or Lr12). Further investigations showed that these loci worked independently and additively in adult plants. Chapio was bred for quantitative, non-race-specific resistance under strong phenotypic selection for leaf rust in Mexico. It is interesting that different QTLs contribute to this resistance in another country, and these results suggest that environmental effects, as well as race specificity, can play a role in expression of resistance.