Comparative Transcriptome Analysis of Orbital Fat Reveals Stage-Specific Gene Expression Associated with Growth Variation in Bighead Carp (Hypophthalmichthys nobilis)

Bighead carp (<i>Hypophthalmichthys nobilis</i>) is a key aquaculture species, with the head and its orbital fat being a commercially valuable product. To elucidate the molecular basis of growth variation, we performed comparative transcriptome analysis of orbital fat from extreme growth phenotypes at juvenile (6 months) and market-size (18 months) stages. In juveniles, slow growth was linked to upregulation of stress-responsive genes (<i>sgk1</i>, <i>fkbp5</i>, <i>lipg</i>), while fast growth correlated with higher expression of stress-buffering (<i>crhbp</i>) and nutrient-signaling (<i>rbp2</i>, <i>mgea5</i>) genes. At 18 months, divergent growth aligned with opposing lipid metabolic states: a pro-anabolic profile (<i>dgat2</i>, <i>fads2</i>) supported fast growth, whereas a catabolic profile (<i>cpt1b</i>, <i>ppargc1a</i>) was associated with slow growth. These results demonstrate stage-specific transcriptional reprogramming in orbital fat underlying growth variation. This study provides a molecular framework for orbital fat-mediated growth regulation and highlights potential candidate genes for molecular breeding in bighead carp.