Characterization of Ug99 Stem Rust Resistance and D Genome Variation from a. tauschii in a Hexaploid Wheat NAM Population.

In 1999, isolates of stem rust (Pucccinia graminis f. sp. Tritici) from Uganda were found to be virulent to the globally important stem rust resistance gene Sr31 (TTKSK), and most Sr genes in common wheat (Triticum aestium L.). In subsequent years, variants of Ug99 developed additional virulence to Sr24 and Sr36 (TTKST and TTTSK). Currently, a majority of globally cultivated wheat is susceptible to these virulent races of stem rust. Aegilops tauschii Coss., the D genome progenitor species of hexaploid wheat, has been one source of novel genetic variation. Eight A. tauschii accessions (TA1615, TA1642, TA1662, TA1693, TA1718, TA10171, TA10187, and TA1617) were selected based on characterized resistance to Ug99 races of stem rust and were crossed to stem rust susceptible wheat genotype KS05HW14. Following embryo rescue and two generations of backcrossing to KS05HW14, a nested association mapping (NAM) population was developed, sampling the genomes of the A. tauschii accessions in a common hexaploid background. This NAM population is composed of 19 BC₁F₁-derived families and roughly 1,000 BC₂F_4:5 recombinant inbred lines (RILs). To evaluate stem rust resistance, a total of 30 seeds from each RIL were planted into 2 pots in the greenhouse. After emergence of the second leaf, plants were inoculated with stem rust race QFCSC and seedling infection types were scored using the Stakman scale. Seven stem rust resistance genes are segregating within the RILs. Four of the genes are expected to be Sr33, SrTA1662, SrTA10187, and SrTA10171. The three additional stem rust resistance genes are expected to be novel. Using GBS we intend to construct a genetic map of the D genome and map these stem rust resistance genes and other important agronomic traits, including resistance to leaf rust, strip rust, and pre-harvest sprouting.