Distribution of Rht Genes in Modern and Historic Winter Wheat Cultivars from the Eastern and Central USA

Over 70% of wheat (Triticum aestivum L.) cultivars grown worldwide have a semidwarf phenotype controlled by the major genes Rht-B1, Rht-D1, and Rht8c. The objective of this study was to determine their frequency in a set of historic and modern soft and hard winter wheat cultivars grown in the central and eastern USA. Three hundred sixty-two cultivars that were developed from 1808 to 2008 were evaluated with molecular markers for Rht-B1, Rht-D1, and Rht8c. All cultivars released before 1964 (41 soft winter wheat and 6 hard winter wheat) had wildtype (tall) alleles at all three loci. After introduction of the dwarfing genes, the percentage of tested lines carrying either Rht-B1b or Rht-D1b increased rapidly to greater than 90% of modern varieties. Among soft winter wheat cultivars, the Rht-D1b dwarfing gene was the most frequent being present in 45% of all lines tested and Rht-B1b was present in 28%, while in the hard winter wheat cultivars the Rht-B1b allele is the most prevalent in 77% of lines. Only 8% of the hard cultivars tested had the Rht-D1b allele. The presence of the 192-base pair (bp) allele of the microsatellite marker Xgwm261 indicated that Rht8c was less frequently used as a source of dwarfing in U.S. winter wheat germplasm, being present in 8 and 3% of the soft winter wheat and the hard winter wheats, respectively. A number of modern cultivars were identified that did not carry any of the dwarfing genes assayed and may possess alternative reduced height genes.