Genetic variation and Rht gene distribution across a century of breeding in North American spring wheat

Hard red spring wheat (HRSW) is an important market class of wheat in North America. We genotyped 1013 HRSW lines representing a century of breeding using single-nucleotide polymorphism (SNP) array and targeted Kompetitive Allele-Specific PCR (KASP) genotyping to assess changes in genetic diversity and Rht genes. Our results suggest that early breeding efforts broadened an initially narrow gene pool derived from a few founders and subsequently maintained genetic diversity. Analysis of Rht genes revealed that Rht-D1b was the predominant semi-dwarf allele following the introduction of gibberellic acid (GA)-insensitive alleles (Rht-B1b and Rht-D1b) during the Green Revolution of the 1960s. However, wheat scab epidemics of the 1990s coincided with most breeding programs shifting to Rht-B1b. Adoption of Rht-B1b and Rht-D1b remains low in some regions of the Northern Plains, where breeding programs have utilized alternative dwarfing GA-sensitive alleles at Rht24 and Rht25 loci. We analyzed the effect of Rht genes on plant height in the HRSW-growing region and found that the effects of Rht-B1b and Rht-D1b diminish in drier environments. Combining dwarfing alleles at Rht24 and Rht25 was associated with a significant reduction in plant height even in the absence of Green Revolution genes. This study offers insights to guide future breeding strategies for HRSW in North America.