Introgression line libraries in melon (Cucumis melo L.): development, genetic insights, and applications in trait dissection and crop improvement

Introgression lines (ILs) are powerful genetic resources for dissecting the architecture of complex traits in crops. This review summarizes the development and characterization of six IL libraries and two additional ILs in melon ( Cucumis melo L.), generated from diverse donor genotypes, including wild accessions, landraces, traditional and elite cultivars, introgressed into Mediterranean elite backgrounds, such as ‘Piel de Sapo’ and ‘Védrantais.’ Together, these libraries capture a wide representation of melon genetic diversity and have revealed numerous stable quantitative trait loci (QTLs) for key agronomic traits, including fruit shape, weight, color, firmness, soluble solids content, volatile composition, climacteric ripening, and resistance to Cucumber Mosaic Virus (CMV). Notably, large-effect QTLs were recurrently identified across independent IL libraries. This suggests that targeted improvement of complex traits may be achieved by manipulating a limited number of loci. Notably, several ILs displayed transgressive phenotypes relative to their donor parents. These results highlight their ability to uncover cryptic genetic variants relevant to modern breeding. Beyond QTL discovery, ILs have facilitated fine mapping and gene identification, leading to the cloning of genes such as CmOFP13 (fruit shape), CmNAC-NOR , CmERF024 (ripening), and CmVPS41 (CMV resistance). Overall, this review highlights the value of ILs as tools for both breeding and fundamental genetic research and outlines future directions, including integrating transcriptomics and metabolomics approaches, developing ILs for abiotic stress tolerance, and extending IL resources to oriental melon germplasm, thereby enhancing their utility for global melon improvement.