Tree Genetics and Molecular Breeding 2025, Vol.15, No.5, 176-184 http://genbreedpublisher.com/index.php/tgmb 178 mutations). Many improved or local varieties of high-quality grape varieties are obtained through clonal selection and bud mutation screening (Gascuel et al., 2017; Butiuc-Keul and Coste, 2023). Meanwhile, clonal selection also provides a stable genetic background for molecular breeding and precise improvement. 4 Molecular and Genomic Insights 4.1 Quantitative trait loci (QTL) mapping for fruit quality traits QTL (Quantitative trait loci) mapping is an important method for studying the genetic basis of complex fruit quality traits in grapes. In recent years, QTL mapping based on high-density genetic maps has achieved many results in traits such as fruit flavor, firmness, shape and sugar-acid content. For instance, by using high-throughput molecular marker techniques such as SLAF-seq, researchers constructed high-density genetic maps on 19 linkage groups and identified multiple stable QTLS related to musk flavor, fruit firmness and fruit shape. On LG8, new QTLs closely related to fruit firmness and fruit shape were discovered for the first time, and candidate genes were screened out, such as VIT_08s0007g00440 (Jiang et al., 2020; Wang et al., 2020). Furthermore, the QTLs of traits such as fruit firmness and fruit shape showed high stability in different populations and years, providing a theoretical basis for molecular marker-assisted selection (MAS) and precision breeding (Jiang et al., 2020; Wu et al., 2022). 4.2 Genome-wide association studies (GWAS) and genomic resources in grape GWAS conducts correlation analysis using genotype and phenotypic data from large-scale natural populations, significantly enhancing the analytical ability for fruit quality traits. The latest research, based on three years of multi-trait data from 588 grape samples, discovered new QTLS related to fruit diameter, fruit weight and post-ripening qualities (such as rot and weight loss), and identified some key candidate genes, such as Vitvi11g000454, which is involved in jasmonic acid signaling regulation (Guo et al., 2019; García-Abadillo et al., 2024; Liu et al., 2024). The establishment of the Grapepan pan-genome (Grapepan v.1.0) integrates structural variations (SV) and short variations, improving the accuracy of QTL mapping and the estimation of trait heritability. This provides a solid foundation for multi-trait genomic selection and precision breeding (Liu et al., 2024). The results of GWAS and traditional QTL mapping were highly consistent, and new loci could also be discovered, promoting the molecular breeding research of grape quality traits (Guo et al., 2019; García-Abadillo et al., 2024). 4.3 Functional genomics: candidate gene discovery and expression analysis Functional genomics, combined with QTL/GWAS mapping and transcriptome and expression analysis, has accelerated the identification and functional verification of candidate genes. For example, in the study of fruit firmness, GWAS and expression analysis jointly identified key genes such as CesA and VvCslD5 involved in cellulose synthesis, and their functions were verified through transgenic and subcellular localization experiments (Jiang et al., 2020; Hu et al., 2025). In the study of fruit shape, by combining QTL mapping and RNA-seq, researchers screened out candidate genes related to DNA replication, cell wall modification and hormone synthesis, and found that the contents of auxin (IAA) and zeaxanthin (tZ) were significantly different among different fruit shape types (Wu et al., 2022). Furthermore, by using expression analysis methods such as qRT-PCR, the expression differences of candidate genes within the QTL interval (such as VIT_18s0041g02410, VIT_18s0089g00210) in different varieties and developmental stages were further verified. It indicates that they play an important role in the formation of fruit quality (Jiang et al., 2020). 5 Molecular Breeding Strategies 5.1 Marker-assisted selection (MAS) for sugar content, berry size, and anthocyanins Marker-assisted selection (MAS) utilizes molecular markers closely linked to the target trait to efficiently screen complex traits. In grapes, quality traits such as sugar content, grape size and anthocyanin accumulation are mostly quantitative traits and are regulated by multiple genes. Through QTL mapping and association analysis, researchers have identified candidate genes and molecular markers related to grape width, grape weight, glycolic acid metabolism and anthocyanin synthesis, laying a foundation for the application of MAS in the improvement of grape quality. For instance, GWAS studies have found that the Vitvi11g000454 gene is closely related to fruit
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