Molecular Plant Breeding 2024, Vol.15, No.5, 247-258 http://genbreedpublisher.com/index.php/mpb 248 traditional marker-assisted selection, GS considers the effects of all markers across the genome, making it more efficient for selecting complex traits. Integration GS into E. ulmoides breeding can significantly accelerate the breeding cycle and enhance the accuracy of selection for desirable traits (Liu et al., 2022). This approach is particularly advantageous for perennial species like E. ulmoides, where long generation times can impede breeding progress. This study explores the application of combining QTL mapping with GS in the breeding and genetic improvement of E. ulmoides. By integrating and analyzing existing research, we sort out candidate genes and loci linked to key growth traits and assesses the potential of genomic selection to accelerate the breeding process and enhance the accuracy of selecting complex traits. This study aims to provide theoretical support for the genetic research and breeding strategies of E. ulmoides, contributing to the development of superior varieties with higher economic and ecological value. 2 Genetic Linkage Maps inEucommia ulmoides 2.1 Development of genetic linkage maps The construction of genetic linkage maps in E. ulmoides has employed various molecular marker technologies, including genotyping-by-sequencing (GBS) and traditional marker systems such as sequence-related amplified polymorphism (SRAP), amplified fragment length polymorphism (AFLP), inter-simple sequence repeat (ISSR), and simple sequence repeat (SSR) markers. For instance, one study utilized GBS to construct a high-density genetic map by sequencing an F1 population and identifying single-nucleotide polymorphism (SNP) markers (Liu et al., 2022). Another study combined SRAP, AFLP, ISSR, and SSR markers to develop a genetic linkage map from a full-sib family, which encompasses approximately 89% of the estimated E. ulmoides genome (Li et al., 2014). There are four types of markers employed in the construction of genetic linkage maps for E. ulmoides. SSR markers, AFLP markers, SNP (single nucleotide polymorphism) markers, and GBS. SSR markers are highly polymorphic and have been widely used due to their co-dominant inheritance and reproducibility (Jin et al., 2020; Li et al., 2014; Wang et al., 2014). AFLP markers are effective for detecting a large number of polymorphisms and have been utilized in several studies (Aabidine et al., 2010; Li et al., 2014). The first genetic linkage map of Eucommia ulmoides was developed using a pseudo-testcross strategy with AFLP markers from an F1 population comprising 122 plants, laying the groundwork for mapping quantitative trait loci and facilitating breeding efforts (Wang et al., 2014). Furthermore, through the application of SRAP, AFLP, ISSR, and SSR analyses, a comprehensive genetic linkage map was established, spanning 2 133 cM and encompassing 25 linkage groups. This map accounts for nearly 89% of the genome, serving as a valuable resource for identifying quantitative trait loci (QTL) associated with growth-related characteristics and supporting marker-assisted selection as well as genomic research. SNP markers have gained popularity due to their abundance and the high resolution they provide in genetic mapping. The use of GBS has facilitated the identification of a substantial number of SNPs, which are crucial for developing high-density genetic maps (Jin et al., 2020; Liu et al., 2022). The coverage and resolution of genetic maps in E. ulmoides have significantly improved through the utilization of high-density SNP markers. Previous study reported a genetic linkage map covering approximately 89% of the estimated genome, with an average marker interval of 3.1 cM (Li et al., 2014). Recently, using SNP markers and GBS technology, Liu et al. (2022) constructed the first high-density genetic map of E. ulmoides, revealing 10 103 SNP markers distributed across 17 linkage groups. The genetic map constructed using GBS covered 90% of the genome, with a total genetic distance of 4 051.11 cM and an average distance between markers of 0.45 cM (Liu et al., 2022). These high-density maps offer enhanced resolution, facilitating the precise localization of QTLs. 2.2 Applications in breeding Genetic linkage maps are invaluable tools for marker-assisted selection (MAS) in the breeding of E. ulmoides. These maps enable the identification of markers linked to desirable traits, enabling the selection of individuals
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