Triticeae Genomics and Genetics, 2024, Vol.15, No.4, 206-220 http://cropscipublisher.com/index.php/tgg 214 2014). This combined approach enables breeders to make more informed selection decisions, accelerating the development of improved wheat varieties. The methodologies of MAS, GS, AB-QTL analysis, and HTP are critical for leveraging the genetic diversity of synthetic wheat. These advanced techniques enable the precise identification and utilization of beneficial alleles, ultimately leading to the development of superior wheat cultivars with enhanced performance and resilience. By integrating these methodologies, breeders can more effectively harness the potential of synthetic wheat to address the challenges of modern agriculture. 7 Case Studies of Synthetic Wheat in Breeding Programs 7.1 CIMMYT mexico The International Maize and Wheat Improvement Center (CIMMYT) has been at the forefront of utilizing synthetic hexaploid wheat (SHW) to enhance the genetic diversity and performance of bread wheat. CIMMYT has produced over 1 000 SHWs by crossing diverse accessions of the D genome donor species, Aegilops tauschii, with tetraploid wheat, Triticum turgidum. These SHWs have shown significant resistance or tolerance to various biotic and abiotic stresses, making them valuable for breeding programs aimed at improving yield and stress tolerance (Dreisigacker et al., 2008). The SHWs were backcrossed with CIMMYT's improved germplasm to produce synthetic backcross-derived lines (SBLs). These SBLs retain the beneficial traits of SHWs while being agronomically similar to the improved parents. Molecular studies have shown that SHWs and SBLs are genetically diverse compared to traditional bread wheat cultivars, with preferential transmission of some alleles from the SHW parent, indicating positive selection. This genetic diversity has been instrumental in increasing yield and enhancing disease resistance in wheat cultivars developed by CIMMYT (Ogbonnaya et al., 2013). CIMMYT's breeding strategy has also focused on targeting the A and B genomes of hexaploid wheat to improve specific traits. For instance, screening the germplasm collection of T. turgidumsubsp. dicoccumfor resistance to Russian wheat aphid and drought tolerance has led to the identification of promising accessions for future SHW production and introgression into elite bread wheat backgrounds (Dreisigacker et al., 2008). This approach has resulted in significant genetic gains for grain yield in semi-arid environments, demonstrating the potential of SHWs to improve wheat productivity under suboptimal conditions (Crespo-Herrera et al., 2018). 7.2 Southwest China In Southwest China, synthetic hexaploid wheat has been extensively utilized to combat rust resistance and improve yield. The primary SHW lines have been used to develop high-yielding wheat varieties with enhanced disease resistance and yield potential. For example, four high-yielding wheat varieties have been developed using primary SHW lines, and 12 new wheat varieties have been created using SHW-derived varieties as breeding parents (Li et al., 2014). The breeding strategy in Southwest China involves pyramiding quantitative trait loci (QTLs) from SHW into new high-yield cultivars. This approach has led to the development of big-spike wheat varieties with improved yield and resistance to stripe rust. The "large population with limited backcrossing method" and the "recombinant inbred line-based breeding method" have been employed to combine phenotypic and genotypic evaluations, resulting in record-breaking high-yield wheat varieties (Wan et al., 2023). The introgressed alleles from SHW lines have contributed significantly to the new wheat varieties, enhancing traits such as disease resistance, more spikes per plant, more grains per spike, larger grains, and higher grain yield potential. These findings highlight the importance of SHW as a genetic resource for breeding high-yielding wheat varieties resistant to biotic and abiotic stresses (Li et al., 2014). 7.3 India In India, the adoption of synthetic hexaploid wheat has been pivotal in improving resistance to pests and pathogens and enhancing yield potential. The genetic diversity introduced by SHWs has been harnessed to
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