International Journal of Horticulture, 2024, Vol.14, No.5, 319-332 http://hortherbpublisher.com/index.php/ijh 324 The application of CRISPR/Cas9 in fruit crops has also focused on increasing yield by modifying genes that control plant architecture and fruit development. In kiwifruit, the identification of genes involved in stress responses, such as the R1R2R3-MYB transcription factor AcMYB3R, which enhances tolerance to environmental stresses, can be edited to improve overall plant health and yield (Zhang et al., 2019). Additionally, the integration of CRISPR/Cas9 in breeding programs can accelerate the development of high-yielding kiwifruit varieties (Zhou et al., 2020). 4.3 Potential for developing new kiwifruit varieties Breeding programs that integrate CRISPR/Cas9 technology can significantly enhance the development of new kiwifruit varieties. The high-quality reference genome of Actinidia chinensis provides a robust foundation for identifying and editing genes associated with desirable traits (Wu et al., 2019). By combining traditional breeding methods with CRISPR/Cas9, it is possible to introduce specific genetic modifications more efficiently and accurately (Zhou et al., 2020). There have been successful case studies in other fruit crops where CRISPR/Cas9 has been used to develop new varieties with improved traits. For example, the use of CRISPR/Cas9 to edit genes related to fruit quality and yield in crops like tomatoes and strawberries has shown promising results, which can be applied to kiwifruit (Zhou et al., 2020). The development of new kiwifruit varieties through CRISPR/Cas9 can also benefit from the detailed genetic and metabolic maps available, which provide insights into the regulatory networks governing key traits (Shu et al., 2023). 5 Genetic Mapping and QTL Analysis 5.1 SNP Genotyping Array The development of SNP arrays for kiwifruit has been a significant advancement in genetic studies and breeding applications. A high-density SNP genotyping array was developed by performing genome-wide DNA sequencing of 40 kiwifruit genotypes. This process identified 134,729 unique SNPs, which were stringently filtered for sequence quality, predicted conversion performance, and distribution over the available Actinidia chinensis genome. The array was evaluated by genotyping 400 kiwifruit individuals, demonstrating its effectiveness in distinguishing kiwifruit accessions and facilitating genetic studies (Wang et al., 2022). The SNP genotyping array has been utilized in various genetic studies, including the construction of an integrated linkage map and QTL analysis. Research indicates that SNP arrays and resequencing technologies are complementary in detecting quantitative trait loci (QTL). By combining different SNP distributions and densities, the study found that the integration of these two technologies can enhance the ability to detect QTLs and potentially more precisely pinpoint causal polymorphisms (Negro et al., 2019). For instance, using a tetraploid F1 population, researchers constructed a linkage map covering 3060.9 cM across 29 linkage groups. This map was instrumental in performing QTL analysis for the sex locus identified on Linkage Group 3 (LG3) in Actinidia arguta. The array's comprehensive design makes it a valuable tool for genetic studies and breeding applications in kiwifruit (Wang et al., 2022). 5.2 QTL Analysis Quantitative Trait Loci (QTL) analysis has been pivotal in identifying loci linked to important traits in kiwifruit. The high-density SNP genotyping array facilitated the identification of QTLs, such as the sex locus on LG3 in Actinidia arguta. This identification is crucial for understanding the genetic basis of key traits and for the development of marker-assisted selection strategies (Wang et al., 2022). Another study on kiwifruit constructed a high-density linkage map of hexaploid kiwifruit through genotyping-by-sequencing (GBS) and used this map for QTL analysis of sex loci and fruit traits. This is the first time that QTLs have been discovered and reported in hexaploid kiwifruit with complex traits, providing a foundation for molecular marker-assisted selection in kiwifruit (Popowski et al., 2021).
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