Doctor of Philosophy / Genetics Interdepartmental Program - Plant Pathology / Bikram Gill / The diploid D genome species, Aegilops tauschii Coss. (2n=2x=14,DD) has provided numerous genes for resistance to diseases and insect pests that attack bread wheat (Triticum aestivum L. (2n=6x=42, AABBDD). Wheat production is currently threatened by broadly virulent races of the ‘Ug99’ lineage of wheat stem rust caused by the fungus, Puccinia graminis f.sp. tritici Pers. & Eriks. Screening of a large set of Ae. tauschii germplasm for resistance to TTKSK (Ug99) identified potentially novel sources of resistance. To expedite utilization of TTKSK resistance from Ae. tauschii, a direct hybridization approach was established that integrates gene transfer, mapping and introgression into one process. Direct crossing of Ae. tauschii accessions with an elite wheat breeding was used to initiate transfer of resistance. Genetic mapping of resistance was accomplished during gene transfer through development of BC[subscript]2 mapping populations. Bulked segregant analysis of BC[subscript]2F[subscript]1 genotypes at 70 SSR loci across the D genome identified the chromosome locations of stem rust resistance genes and facilitated genetic mapping. Using this approach, TTKSK resistance from CDL4424 and TA1662 was mapped on chromosome arm 1DS flanked distally by Xwmc432 and proximally by Xwmc222 at 4.4 cM, TA10187 on 6DS linked to Xcfd49 at 1.9 cM and TA10171 on 7DS linked Xwmc827 at 0.9 cM. TTKSK resistance from additional Ae. tauschii accessions CDL4366, TA1615, TA1642, TA1693 and TA1718 has been recovered in segregating populations but await mapping. Altogether, TTKSK resistance from eight Ae. tauschii accessions has been introgressed to a hard winter wheat genetic background. Three new stem resistance genes have been tagged with molecular markers for marker assisted breeding and will provide valuable material for stem rust resistance breeding and gene pyramids for effective control of stem rust.

Doctor of Philosophy / Genetics Interdepartmental Program / Bikram S. Gill / Leaf rust, caused by fungus Puccinia triticina, is an important foliar disease of wheat worldwide. Breeding for race-nonspecific resistant cultivars is the best strategy to combat this disease. Aegilops tauschii, D genome donor of hexaploid wheat, has provided resistance to several pests and pathogens of wheat. To identify potentially new adult plant resistance (APR) genes, 371 geographically diverse Ae. tauschii accessions were evaluated in field with leaf rust (LR) composite culture of predominant races. Accessions from Afghanistan only displayed APR whereas both seedling resistance and APR were common in the Caspian Sea region. Seventeen accessions with high APR were selected for production of synthetic hexaploid wheat (SHW), using ‘TetraPrelude’ and/or ‘TetraThatcher’ as tetraploid parents. Six SHWs were produced and evaluated for APR to LR and resistance to tan spot at seedling stage. Genetic analysis and mapping of APR introgressed from accession TA2474 was investigated in recombinant inbred lines (RIL) population derived from cross between SHW, TA4161-L3 and spring wheat cultivar, ‘WL711’. Genotyping-by-sequencing approach was used to genotype the RILs. Maximum disease severity (MDS) for LR was significantly correlated among all experiments and APR to LR was highly heritable trait in this population. Nine genomic regions significantly associated with APR to LR were QLr.ksu-1AL, QLr.ksu-1BS, QLr.ksu-1BL.1, QLr.ksu-1BL.2, QLr.ksu-2DS, QLr.ksu-2DL, QLr.ksu-5AL, QLr.ksu-5DL and QLr.ksu-6BL. Association of QLr.ksu-1BL.1 with marker Xwmc44 indicated this locus could be slow-rusting APR gene, Lr46/Yr29. QTLs detected on 2DS, 2DL and 5DL were contributed by TA4161-L3 and are novel, along with QLr.ksu-5AL. Tan spot, caused by necrotrophic fungus, Pyrenophora tritici-repentis, has recently emerged as a damaging disease of wheat worldwide. To identify QTLs associated with resistance to Race 1 of P. tritici-repentis, F[subscript]2:3 population derived from cross between SHW, TA4161-L1 and winter wheat cultivar, ‘TAM105’ was used. Two major effect QTLs, QTs.ksu-1AS.1 and QTs.ksu-7AS were significantly associated with tan spot resistance and contributed by TA4161-L1. QTs.ksu-7AS is a novel QTL and explained 17% of the phenotypic variation. Novel QTLs for APR to LR and tan spot identified in SHWs add new variation for broadening the gene pool of wheat and providing resources for breeding of durable resistant cultivars.

Synthetic hexaploid wheats, created from a durum (Triticum durum) cross to Aegilops tauschii Coss. (McFadden and Sears, 1946), proved to be an efficient and beneficial source of new genes for common bread wheat (Triticum aestivum L). The purpose of this research was to evaluate the potential and performance of synthetic wheat in Texas. Ten elite primary synthetics from the International Maize and Wheat Improvement Center (CIMMYT), screened for desirable traits, were backcrossed to two Texas cultivars, TAM 111 and TAM 112. Populations were bulked and modified bulked to advance generations. Agronomic traits related to yield were determined on the F4 and F5 Improvement was observed in South Texas and the Blacklands, which have more disease pressure and fewer intermittent dry spells than another two locations at Chillicothe and Bushland in Texas Rolling and High Plains, respectively. Selected bulks were not superior to non-selected bulks. Head number per unit area had the highest correlation with yield and seed weight was the most heritable trait. Synthetic lines combined better with TAM 111 than TAM 112 in high yielding environments. populations across five Texas locations. Similar to crosses with spring wheat, synthetics contributed to yield through an increase in seed weight. Synthetic populations that produced higher grain yield than both TAM 111 and TAM 112 were able to maintain their large seed size and weight while improving their seed per head and head number traits. Poorer performance in environments with harsh winters could be due to a lack of winter-hardiness in the primary synthetics. This clearly demonstrates that improving yield, through utilization of common wheat by synthetic crosses, could result from selecting for larger seed per head and heads per unit area in lines driven from these populations. Introgression of new genes through synthetic backcrosses could contribute to the improvement of wheat in particular regions of Texas. Primary synthetics and recurrent parents combining for superior hybrids were identified.

Doctor of Philosophy / Department of Agronomy / Gina Brown-Guedira / Allan K. Fritz / Aegilops tauschii, the D-genome donor of modern wheat, has served as an important source of genetic variation in wheat breeding. The objective of this research was to characterize and utilize multiple Ae. tauschii-derived pest resistance genes contained in the germplasm KS96WGRC40. Two Ae. tauschii-derived genes, H23 and Cmc4, provide resistance to the Hessian fly (HF) and wheat curl mite (WCM), respectively. A linkage analysis of a testcross population estimated 32.67% recombination between H23 and Cmc4 on chromosome 6DS in wheat indicating that the two genes are not tightly linked as previous mapping reports show. Haplotype data of recombinant lines and physical mapping of linked microsatellite markers located Cmc4 distal to H23. Haplotype data indicated that both KS89WGRC04 and KS96WGRC40 have the distal portion of 6DS derived from Ae. tauschii. Microsatellite primer pairs BARC183 and GDM036 were more useful than the previously published linked markers in identifying lines carrying Cmc4 and H23, respectively. Through phenotypic selection and advancement within the testcross population, three TC1F2:4 lines were identified as homozygous resistant for H23 and Cmc4 and have the complete terminal segment of 6DS from Ae. tauschii. Two lines are more desirable than the original germplasm releases and can serve as a source of resistance to both HF and WCM in an elite background. A linkage analysis of a segregating recombinant inbred line population identified an Ae. tauschii-derived gene of major effect conferring resistance to Septoria leaf blotch (STB) and another Ae. tauschii-derived gene of major effect conferring resistance to soil-borne wheat mosaic virus (SBWMV) in the germplasm KS96WGRC40. The STB resistance gene in KS96WGRC40 is located in the distal 40% of the short arm of chromosome 7D flanked by microsatellite markers Xgwm044 and Xbarc126. Two previously reported STB genes, Stb4 and Stb5, have been mapped to 7DS in the same region as the STB resistance gene in KS96WGRC40. The uniqueness of the STB resistance genes on 7DS is questionable. The SBWMV resistance gene in KS96WGRC40 is located on chromosome 5DL linked to microsatellite marker Xcfd010. The SBWMV resistance gene within KS96WGRC40 was derived from TA2397 via KS95WGRC33.