Molecular Plant Breeding 2024, Vol.15, No.4, 187-197 http://genbreedpublisher.com/index.php/mpb 190 In summary, the development and application of genetic markers in Camellia breeding programs have significantly advanced with the use of molecular techniques. These markers facilitate the identification of genetic diversity, construction of linkage maps, and QTL analysis, ultimately improving the efficiency of marker-assisted selection in Camellia breeding. 4 Application of Marker-Assisted Selection inCamellia Breeding 4.1 Marker-assisted backcrossing (MABC) Marker-Assisted Backcrossing (MABC) is a powerful technique used to introduce or enhance specific traits in elite Camellia cultivars. This method involves backcrossing a hybrid progeny with one of its parents while using genetic markers to select for the desired trait and the background genome of the elite parent (Kim et al., 2019). The steps of MABC are to cross the donor parent, which possesses the desired trait, with the recurrent parent, an elite cultivar; to screen the progeny using genetic markers to identify individuals carrying the desired trait; to backcross selected progeny with the recurrent parent; and to continue backcrossing and selecting progeny over several generations to recover the recurrent parent's genome while retaining the desired trait. 4.2 Marker-assisted recurrent selection (MARS) Marker-Assisted Recurrent Selection (MARS) is employed to improve polygenic traits by selecting individuals with the best combination of genetic markers associated with the desired characteristics over multiple generations. The steps of MARS are to cross individuals with desirable traits and screen progeny using multiple genetic markers linked to these traits; to select the best individuals based on marker data and intercross them; and to repeat the selection and intercrossing process over several generations to accumulate favorable alleles (Ali et al., 2020). 4.3 Genomic selection (GS) Genomic Selection (GS) represents a more advanced approach where genome-wide marker data is used to predict the breeding value of individuals. Unlike MABC and MARS, GS considers the cumulative effect of all genetic markers across the genome, providing a more comprehensive selection strategy. The steps of GS are collecting genotype and phenotype data from a reference population; developing predictive models using statistical methods to associate marker data with phenotypic traits; and using the models to predict the breeding values of new individuals and select the best candidates for breeding (Spindel et al., 2015). 4.4 Case studies and success stories inCamellia breeding 4.4.1 Case study 1: enhancing disease resistance in Camellia lutchuensis Kondratev et al. (2020) used Ciborinia camellia to induce rapid infection in the flowers of the susceptible cultivar 'Nicky Crisp' (Camellia japonica × Camellia pitardii var. pitardii), while Camellia lutchuensis showed highly resistant. Genome-wide analysis of antagonistic plant gene expression revealed that after 6 hours of inoculation with ascospores, the host transcriptional activity was rapidly regulated (Figure 1). The research highlighted that the early induction of defense responses in Camellia plants significantly reduced disease development. 4.4.2 Case study 2: improving cold tolerance in Camellia oleifera Breeders identified markers associated with cold tolerance and used them to select and intercross the best-performing individuals over several generations. Wu et al. (2020) utilized Illumina NGS technology to perform transcriptomic analyses of Camellia oleifera 'Huaxin' leaves under long-term cold stress. This study identified a group of cold-responsive genes related to hormone regulation, photosynthesis, membrane systems, and osmoregulation, which are critical for chilling tolerance. Meanwhile, Wu et al. (2020) measured some physiological indicators of 'Huaxin' Camellia oleifera under three temperature conditions, and the results were consistent with molecular sequencing results (Figure 2). 4.4.3 Case study 3: advancing tea quality inCamellia sinensis Genomic selection has been transformative for improving tea quality in Camellia sinensis. By integrating genome-wide marker data with traditional phenotypic evaluations, breeders have significantly accelerated the
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