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 1,013 HRSW lines representing a century of breeding using SNP array and targeted 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 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 1990s coincided with most breeding programs shifting to Rht-B1b. Adoption of Rht-B1b and Rht-D1b remains low in some regions of the Great 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 effect of Rht-B1b and Rht-D1b diminishes 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.