Understanding grain yield: it is a journey, not a destination.

Approximately 20 years ago, we began our efforts to understand grain yield in winter wheat using chromosome substitution lines between Cheyenne (CNN) and Wichita (WI). We found that two chromosome substitutions, 3A and 6A, greatly affected grain yield. CNN(WI3A) and CNN(WI6A) had 15 to 20% higher grain yield than CNN, whereas WI(CNN3A) and WI(CNN6A) had 15 to 20% lower grain yield than WI. The differences in grain yield are mainly expressed in higher yielding environments (e.g. eastern Nebraska) indicating genotype by environment interactions (G × E). In studies using hybrid wheat, the gene action for grain yield on these chromosomes was found to be mainly controlled by additive gene action. In subsequent studies, we developed recombinant inbred chromosome lines (RICLs) using monosomics or doubled haploids. In extensive studies we found that two regions on 3A affect grain yield in the CNN(RICLs-3A) with the positive QTLs coming from WI. In WI(RICLs-3A), we found a main region on 3A that affected grain yield with the negative QTL coming from CNN. The 3A region identified using WI(RICLs-3A) coincided with one of the regions previously identified in CNN(RICLs-3A). As expected the QTLs have their greatest effect in higher-yielding environments and also exhibit QTL × E. Using molecular markers on chromosomes 3A and 6A, the favorable alleles on 3A in Wichita may be from Turkey Red, the original hard red winter wheat in the Great Plains and presumably the original source of the favorable alleles. Cheyenne, a selection from Crimea, did not have the favorable alleles. In studying modern cultivars, many high yielding cultivars adapted to eastern Nebraska have the WI-allele indicating that it was selected for in breeding higher yielding cultivars. However, some modern cultivars adapted to western Nebraska where the QTL has less effect retain the CNN-allele, presumably because the allele has less effect (is less important in improving grain yield). In addition many modern cultivars have neither the WI-allele, nor the CNN-allele indicating we have diversified our germplasm and new alleles have been brought into the breeding program in this region.