Global selection on sucrose synthase haplotypes during a century of wheat breeding

Spike number per unit area, number of grains per spike, and thousand kernel weight (TKW) are important yield components. In China, increases in wheat (Triticum aestivum) yields are mainly due to increases in grain number per spike and TKW. TKW mainly depends on starch content, as starch accounts for about 70% of the grain endosperm. Sucrose synthase catalysis is the first step in the conversion of sucrose to starch, that is, the conversion of sucrose to fructose and UDP-glucose by the wheat sucrose synthase genes (TaSus1 and TaSus2) that are located on chromosomes 7A/7B/7D and 2A/2B/2D, respectively. A total of 1,520 wheat accessions were genotyped at the six loci. Two, two, five, and two haplotypes were identified at the TaSus2-2A, TaSus2-2B, TaSus1-7A, and TaSus1-7B loci, respectively. Their main variations were detected within the introns. Significant differences between the haplotypes correlated with TKW differences among 348 modern Chinese cultivars from the core collection. Frequency changes for favored haplotypes showed gradual increases in cultivars released since beginning of the last century in China, Europe, and North America. Geographic distributions and time changes of favored haplotypes were characterized in six major wheat production regions worldwide. Strong selection bottlenecks to haplotype variations occurred at polyploidization and domestication and during breeding of wheat. Genetic-effect differences between haplotypes at the same locus influence the selection time and intensity. This work shows that the endosperm starch synthesis pathway is a major target of indirect selection in global wheat breeding for higher yield.