Characterization and cellularity of myofibers during early development of Magalobrama amblycephala.

The growth and development of fish rely heavily on skeletal muscle. This study systematically characterized the morphological and developmental features of skeletal muscle in Megalobrama amblycephala during early development (0-90 days post-hatching, dph). It also determined the stage-specific contributions of hyperplasia and hypertrophy, and integrated transcriptomic and 16S rRNA sequencing analyses to preliminarily uncover the molecular regulatory network and gut microbiota adaptations associated with the rapid growth window from 15 to 20 dph. Morphological observations revealed that the length, diameter, number, and area of both red and white muscle fibers continuously increased with developmental age, and these changes were significantly synchronized with those in body length and weight. Myofiber hypertrophy predominated during juvenile growth, whereas mosaic hyperplasia was markedly enhanced at 15-20 dph, suggesting that this period represents a critical window for determining subsequent growth potential. Transcriptomic analysis revealed that upregulated KEGG pathways at 20 dph were significantly enriched in growth-related pathways, including FoxO, insulin, and AMPK signaling. Moreover, fast-twitch-specific genes (e.g., myl1, mylpfa, tnnc2.2, and tnni2a.2) and the myogenic regulatory factor myog were significantly upregulated, indicating active molecular programming toward a fast-twitch phenotype. Gut microbiota analysis showed a marked shift in microbial composition at 20 dph, with decreased Proteobacteria abundance and increased Actinobacteriota abundance. Spearman correlation analysis further revealed that 15 dph-dominant genera (such as Pseudomonas and Sphingomonas) were negatively correlated with fast-muscle genes, whereas 20 dph-enriched genera (such as Rubellimicrobium) showed positive correlations with myogenic marker genes. Functional prediction indicated enhanced metabolism of energy, amino acids, and carbohydrates at 20 dph, with reduced lipid metabolism, which closely mirrored transcriptomic trends. Collectively, the gut microbiota at 15-20 dph was highly synchronized with the activation of muscle developmental genes, constituting a key ecological factor supporting rapid growth. This study provides new insights into the multi-dimensional regulatory mechanisms of early muscle development in fish and offers a theoretical basis for strategies to enhance growth performance through microecological modulation in M. amblycephala aquaculture.