Triticeae Genomics and Genetics, 2024, Vol.15, No.3, 162-171 http://cropscipublisher.com/index.php/tgg 164 2.2.2 Key genetic markers and QTLs Key genetic markers and quantitative trait loci (QTLs) are essential for identifying and selecting desirable traits in wheat breeding programs. Synthetic hexaploid wheat (SHW) has been utilized to introduce favorable genes from tetraploid and diploid donors, leading to the development of high-yield and disease-resistant cultivars (Brammer et al., 2021; Wan et al., 2023). For instance, QTLs related to stripe rust resistance and big-spike traits have been successfully pyramided into new wheat varieties using SHW (Wan et al., 2023). Moreover, advanced technologies such as genome-wide association studies (GWAS) and high-throughput genotyping have facilitated the identification of critical genetic markers, enabling precision breeding and genetic improvement of wheat (Li et al., 2021). 3 Breeding Strategies in Hexaploid Wheat 3.1 Traditional breeding methods 3.1.1 Selection and crossbreeding Traditional breeding methods in hexaploid wheat primarily involve selection and crossbreeding. These methods rely on phenotypic selection of desirable traits such as yield, disease resistance, and stress tolerance. For instance, synthetic hexaploid wheat (SHW) has been developed by crossing durum wheat with wild relatives like Aegilops tauschii to introduce beneficial traits into modern wheat varieties (Aberkane et al., 2020). This approach has led to the creation of high-yield cultivars with improved resistance to diseases and environmental stresses (Aberkane et al., 2020; Wan et al., 2023). 3.1.2 Limitations and challenges Despite their successes, traditional breeding methods face several limitations. One major challenge is the time-consuming nature of these methods, as multiple generations are often required to stabilize desirable traits. Additionally, the genetic diversity available within the primary gene pool of wheat is limited, which can restrict the potential for further improvements (Aberkane et al., 2020; Okada et al., 2020). Moreover, traditional methods may not always effectively address complex traits controlled by multiple genes, such as drought tolerance and heat resistance (Liu et al., 2020; Alghabari et al., 2023). 3.2 Modern breeding techniques 3.2.1 Marker-assisted selection (MAS) Marker-Assisted Selection (MAS) is a modern breeding technique that uses molecular markers to select for desirable traits at the seedling stage, thereby accelerating the breeding process. MAS has been particularly effective in identifying and incorporating quantitative trait loci (QTLs) associated with important traits such as drought tolerance and disease resistance (Liu et al., 2020). For example, QTL analysis in SHW has identified several markers linked to root traits under drought stress, facilitating the development of drought-tolerant wheat varieties (Liu et al., 2020). 3.2.2 Genomic selection (GS) Genomic Selection (GS) is another advanced technique that uses genome-wide markers to predict the breeding value of individuals. This method allows for the selection of superior genotypes based on their genetic potential rather than just their phenotypic performance. GS has been shown to be effective in improving complex traits such as yield, quality, and stress tolerance in hexaploid wheat (Walkowiak et al., 2020). The use of multiple wheat genomes has revealed extensive genetic diversity, which can be harnessed through GS to develop next-generation wheat cultivars (Walkowiak et al., 2020). 3.3 Advanced genetic engineering 3.3.1 CRISPR/Cas9 and gene editing CRISPR/Cas9 and other gene-editing technologies have revolutionized wheat breeding by enabling precise modifications of specific genes. These techniques allow for the targeted introduction or deletion of genes to enhance desirable traits such as disease resistance, yield, and stress tolerance. For instance, gene editing has been used to improve the heat tolerance of SHW by rapidly expressing heat shock proteins under stress conditions (Truong et al., 2020). This approach offers a faster and more efficient way to develop improved wheat varieties compared to traditional breeding methods.
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