International Journal of Molecular Zoology, 2025, Vol.15, No.1, 38-47 http://animalscipublisher.com/index.php/ijmz 40 2023). This shows that growth traits are clearly influenced by genes. These results support the idea that selective breeding and genomic selection can work well to improve grouper growth. 3 Genetic Architecture of Fast Growth in Groupers (Epinephelus) 3.1 Growth-related QTL mapping inEpinephelus Researchers have constructed high-density genetic linkage maps using thousands of SNP markers, covering hybrids and purebred strains of grouper, thereby achieving precise QTL positioning for growth traits (Liu et al., 2022; Wu et al., 2023). In hybrid strains, like Yunlong grouper (E. moara ×E. lanceolatus), linkage maps revealed multiple growth-related QTLs, which provided a basis for marker-assisted selection and hybrid vigor research (Liu et al., 2022). Similarly, purebred strains such as giant grouper (E. lanceolatus) and spotted grouper (E. coioides) have also been mapped with linkage maps, and important genomic regions closely related to growth have been identified (Wu et al., 2019; Ai et al., 2023). Multiple QTLs associated with growth and body shape traits (including weight, length, height, and thickness) have been identified on multiple linkage groups and chromosomes (Yang et al., 2020; Liu et al., 2022). For instance, in giant grouper, six growth-related QTLs can explain 4.65% to 12.56% of the phenotypic variation; and in brown grouper, important QTLs associated with weight, length, and height were also identified (Yang et al., 2020; Wu et al., 2023). 3.2 Key candidate genes regulating fast growth Near multiple key QTL regions, the study identified a group of candidate genes closely related to growth regulation, mainly concentrated in the GH-IGF axis and its downstream pathways. Core genes such as ghr (growth hormone receptor) are expressed at significantly higher levels in fast-growing grouper individuals. At the same time, related genes in signaling pathways such as PI3K/AKT/mTOR and MAPK also showed similar expression trends, indicating that they may be directly involved in the molecular process of regulating growth rate (Wang et al., 2023b; Cao et al., 2024). Except that, some genes related to hormone signaling and metabolic regulation also showed traces of positive selection or accelerated evolution, further emphasizing the role of these genetic factors in the formation of growth advantage in grouper (Zhou et al., 2019). It has been confirmed that some genes related to muscle development, such as mustn1, bmp7, tnni2 and bmp2k, as well as metabolism-related genes prkcd, acyp2 and lacs5, play an important role in the rapid growth of grouper (Yang et al., 2020; Ai et al., 2023; Wang et al., 2023a). These genes are involved in multiple key processes such as the regulation of the actin skeleton, protein folding, and energy metabolism, directly affecting the formation and deposition of muscle tissue and improving overall growth efficiency (Yang et al., 2020; Wang et al., 2023a). 3.3 Molecular mechanisms underlying trait variation Studies using transcriptomes and QTLs show that both nearby DNA elements (cis) and far-away regulators (trans) play a role in how groupers grow. Some genes that are turned on or off differently are found in QTL regions. This means that local control and long-distance signals work together to affect how growth-related genes behave (Wu et al., 2023; Cao et al., 2024). In groupers that grow faster, the PI3K/AKT/mTOR pathway is more active. This shows that the pathway might help the fish use nutrients better and build more muscle (Wang et al., 2023a; Cao et al., 2024). Structural genomic variation, including chromosome rearrangement and gene family expansion, has been observed in fast-growing groupers, which may affect the expression of growth-related genes. Zhou et al. (2019) constructed the chromosome-level genome of giant grouper (E. lanceolatus) for the first time, revealing the genetic basis of its rapid growth and innate immune mechanism. The study successfully located 24 chromosomes and annotated 24 718 protein-coding genes. By comparing with 11 other bony fish species, it was found that its immune-related gene family, such as NLRP1, NLRP12, ASC and CARD8, was significantly expanded, enhancing its NOD-like receptor signaling system (Figure 1). In addition, 416 rapidly evolving genes are widely involved in signaling pathways such as insulin, JAK-STAT, and PI3K-Akt, supporting its rapid growth characteristics.
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