Maize Genomics and Genetics 2024, Vol.15, No.3, 136-146 http://cropscipublisher.com/index.php/mgg 138 2.3 Synthetic populations and hybrids Creating new genetic combinations by crossing diverse germplasm to develop synthetic populations and hybrids is a strategy used in maize improvement. For instance, synthetic populations derived from crosses of DH lines from landraces, while not showing yield improvement, maintained phenotypic diversity, indicating their potential for association mapping and breeding (Strigens et al., 2013). Large-scale analyses of the combining ability and heterosis of diverse maize germplasm resources have shown that hybrids from temperate and tropical maize inbred lines exhibit higher heterosis and hybrid performance in yield-related traits. This suggests that germplasm from different ecological environments can be used to enhance the yield potential of maize varieties (Yu et al., 2020). Furthermore, genetic variation in maize breeding populations, including F2 populations and synthetic populations, has been shown to affect the effectiveness of yield improvement selection. Broad-based synthetic populations exhibit greater potential for genetic gains (Fountain and Hallauer, 1996). 3 Strategies for Utilizing Genetic Diversity 3.1 Introgression breeding Introgression breeding involves the incorporation of beneficial alleles from donor lines into elite breeding lines. This strategy is particularly useful for introducing traits such as disease resistance, drought tolerance, and other abiotic stress tolerances. For example, the development of maize introgression populations can be optimized using high-throughput marker assays and specific crossing schemes to ensure the effective incorporation of desired traits while maintaining genetic diversity (Herzog et al., 2014). The use of donor chromosome segments and double haploid (DH) crossing schemes has been shown to be effective in creating introgression populations with clearly separated and evenly distributed target segments. 3.2 Heterosis and hybrid breeding Heterosis, or hybrid vigor, is a phenomenon where hybrid offspring exhibit superior traits compared to their parents. This strategy is widely used in maize breeding to enhance yield and stress tolerance. Studies have shown that combining ability and heterosis can be effectively utilized by developing hybrids from diverse germplasm resources, including temperate and tropical maize lines (Yu et al., 2020). The identification of specific combining ability (SCA) and general combining ability (GCA) among parental lines can help predict hybrid performance and optimize breeding strategies (Betrán et al., 2003; Yu et al., 2020). 3.3 Marker-assisted breeding (MAB) Marker-assisted breeding (MAB) leverages molecular markers to select for desirable traits, thereby accelerating the breeding process. For instance, marker-assisted backcrossing (MABC) has been successfully used to improve drought adaptation in maize by introgressing favorable alleles at target regions involved in yield components and flowering traits. This approach has resulted in hybrids with significantly higher grain yield under severe water stress conditions compared to control hybrids (Ribaut and Ragot, 2006). The use of MAB can thus enhance the efficiency of breeding programs by enabling precise selection for complex traits. 3.4 Genome-wide association studies (GWAS) Genome-wide association studies (GWAS) identify genetic variants associated with specific traits across the genome. This approach has been instrumental in understanding the genetic architecture of traits and informing breeding strategies. For example, GWAS has been used in conjunction with genomic selection (GS) to improve the accuracy of predicting breeding values in rice, which can be translated to maize breeding (Spindel et al., 2015). By identifying quantitative trait loci (QTL) associated with important traits, GWAS can guide the selection of superior genotypes and enhance the efficiency of breeding programs (Spindel et al., 2015; Bernardeli et al., 2023). 3.5 Genomic selection (GS) Genomic selection (GS) uses genome-wide marker data to predict the breeding value of individuals, thereby increasing genetic gains with fewer breeding cycles. GS has been shown to be a powerful tool in maize breeding, enabling the development of superior inbreds and hybrids (Rice and Lipka, 2021). Rapid cycling genomic selection (RCGS) has demonstrated significant genetic gains in tropical maize populations, achieving high genetic gains while conserving genetic diversity (Zhang et al., 2017). The integration of high-throughput phenotypic and
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