Triticeae Genomics and Genetics, 2024, Vol.15, No.4, 206-220 http://cropscipublisher.com/index.php/tgg 212 improvements. These advantages underscore the importance of leveraging genetic diversity from synthetics to ensure the continued progress and sustainability of wheat production in the face of global challenges. 5 Role of Synthetics in Wheat Breeding Programs 5.1 Crossbreeding techniques and strategies The integration of synthetic hexaploid wheat (SHW) into wheat breeding programs has been a pivotal strategy to enhance genetic diversity and improve yield potential. One effective crossbreeding technique involves the use of double top-cross (DTC) strategies. In this method, one parent is a synthetic hexaploid wheat, and the other is an elite wheat cultivar. This approach has been shown to successfully introgress one-eighth of the synthetic hexaploid wheat genome into the progeny, resulting in high-yielding wheat varieties (Hao et al., 2019). The DTC strategy, followed by a two-phase selection process, has led to the development of varieties such as Shumai 580, Shumai 969, and Shumai 830, which exhibit enhanced yield potential compared to those developed through conventional breeding methods (Hao et al., 2019). Another crossbreeding strategy involves the use of synthetic backcross-derived lines (SBLs). These lines are created by backcrossing SHWs with elite bread wheat varieties. This method has been effective in introducing novel alleles from SHWs into the breeding germplasm, thereby broadening the genetic base of elite wheat varieties (Zhang et al., 2004). The use of SBLs has shown significant yield increases and improved performance across diverse environments, particularly in moisture-limited conditions (Ogbonnaya et al., 2013). 5.2 Genomic selection approaches Genomic selection (GS) has emerged as a powerful tool to accelerate the introgression of exotic germplasm into elite wheat varieties. By using whole-genome profiles generated through genotyping-by-sequencing, researchers can apply various prediction models to select desirable traits more efficiently. For instance, in a study involving double haploid and recombinant inbred line populations derived from primary synthetics and the elite cultivar 'Opata M85', several synthetic-derived lines outperformed the elite parent in various environments, indicating the potential of primary synthetics to contribute alleles that increase yield (Dunckel et al., 2017). Although the prediction models had moderate predictive ability, they demonstrated the feasibility of using GS to enhance the speed of introgression of exotic alleles (Dunckel et al., 2017). The BREEDWHEAT project has also contributed significantly to the development of genomic tools and methodologies for implementing GS. This project has provided high-throughput genomic tools, including SNP arrays and high-density molecular marker maps, which are essential for genome-wide association studies (GWAS) and phenomic selection (Paux et al., 2022). These tools facilitate the detection of genomic regions involved in agronomical traits, thereby aiding breeders in the development of new, high-yielding wheat varieties that are more resilient to biotic and abiotic stresses (Paux et al., 2022). 5.3 Integration into elite wheat germplasm The integration of synthetic wheats into elite wheat germplasm has been a critical step in enhancing the genetic diversity and adaptive evolution of modern wheat varieties. Synthetic hexaploid wheats, recreated from the tetraploid wheat Triticum turgidumand the diploid wild relative Aegilops tauschii, have been used to introduce new genes for various productivity traits, including resistance to abiotic and biotic stresses (Ogbonnaya et al., 2013). The use of SHWs has led to the development of high-yielding wheat varieties with improved disease resistance, more spikes per plant, larger grains, and higher grain-yield potential (Li et al., 2014). The mobilization of genetic variation from germplasm banks to breeding programs has also been an important strategy for sustaining crop genetic improvement. For example, the molecular diversity of 1 423 spring bread wheat accessions was investigated using high-quality genotyping-by-sequencing loci and gene-based markers for various adaptive and quality traits. This study revealed that synthetic hexaploids are genetically more diverse than elite and landrace varieties, opening new avenues for pre-breeding by enriching elite germplasm with novel alleles for drought and heat tolerance (Sehgal et al., 2015).
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