International Journal of Marine Science, 2025, Vol.15, No.5, 268-276 http://www.aquapublisher.com/index.php/ijms 270 2.3 Growth differences and their genetic background among different abalone species/strains Much growth variation occurs between abalone species, strains, and hybrids. Selectively bred "Huangxun No. 1" strains, for instance, grow faster than their wild relatives according to growth measurements. Interspecific and three-way cross hybrids have heterosis, in that hybrids are superior to purebreds in growth and survival. Growth characteristics are polygenic with multiple QTLs and candidate genes identified, with most being age- or environment-dependent. Molecular studies have demonstrated that growth variation is induced by gene expression, allele-specific expression, and microRNAs, and this is now being exploited for marker-assisted and genomic selection for breeding (Li et al., 2023; Huang et al., 2024). 3 Genetic Variation Studies on Abalone Growth Rate 3.1 Estimation of heritability and genetic parameters Heritability estimation is central to revealing genetic control of abalone growth traits and breeding progress prediction. Current methods utilize genomic data and mixed models to estimate heritabilities and genetic correlations even for large samples. These methods blend equations of genomic prediction accuracy and number of independent chromosome segments to provide correct estimates of heritability and genetic correlations across generations and traits. Up-to-date right parameters is required to effective selection indices and avoid overestimation of genetic gain, especially since parameters can change very quickly under genomic selection (Yang et al., 2017; Misztal and Gowane, 2025). 3.2 Family-based selection and population genetic structure analysis Family-based selection remains a cornerstone of abalone breeding, facilitating the identification of better families and individuals for growth rate improvement. Population genetic structure analysis, typically by the use of high-density molecular markers, is used to account for population stratification and relatedness, a requirement for both association studies and selection. Genomic relationship matrices and mixed models are used heavily to account for family structure and relatedness, improving the precision of genetic assessment and reducing false positives in marker-trait association studies (Sallam et al., 2020). 3.3 Molecular marker association analyses (QTL mapping, GWAS studies) Quantitative trait loci (QTL) mapping and genome-wide association study (GWAS) are rich tools for the analysis of the genetic architecture of growth traits in abalone. GWAS capitalizes on dense marker information and recombination history to nominate genomic loci and genes that affect growth. Advances in methodology over the last decade, such as multi-locus and regional heritability mapping, have improved detection power and accuracy of mapping, especially in the analysis of moderately to highly heritable traits. These approaches also facilitate the construction of QTL-allele matrices, which can be applied in genomic selection as well as breeding design (Link et al., 2023). 3.4 Genetic diversity and utilization value of different germplasm resources Genetic diversity within and between abalone germplasm resources is the basis for long-term breeding performance and responsiveness. High-throughput genotyping and diversity analyses expose population structure, levels of heterozygosity, and the presence of outstanding alleles. Diverse germplasm collections are a source of favorable alleles for growth and other traits, and characterization is imperative to broaden the genetic base and avoid inbreeding. Integrating genetic diversity information with association mapping enhances the identification and exploitation of valuable genetic material in breeding (Sallam et al., 2020). 4 Traditional Breeding Strategies for Improving Abalone Growth Rate 4.1 Practical cases of family selection and population selection Family and population selection have been employed in extensive use in conventional abalone breeding programs for growth rate improvement. In family selection, the best growing individuals are chosen from families and utilized as broodstock for the next generation, while in population selection, the top-performing individuals are chosen from the whole population without regard to family. These operations have proved successful in increasing the mean rate of improvement from one generation to the next but depend on accurate phenotypic measurement
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