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

## TL;DR

This study identifies genes in the eye fat of bighead carp that are linked to growth differences at different life stages, offering insights for selective breeding.

## Contribution

The study reveals stage-specific gene expression patterns in orbital fat that correlate with growth variation in bighead carp.

## Key findings

- Juvenile slow growth is linked to stress-related genes like sgk1 and fkbp5.
- Market-size fast growth correlates with lipid-storage genes like dgat2.
- Slow growth at market size is associated with lipid-oxidation genes like cpt1b.

## Abstract

Bighead carp is a valuable farmed fish, and the fat behind its eyes, known as orbital fat, is a prized part of the head. While developmental changes in orbital fat have been described, the molecular basis for growth variation among individuals of the same age remains unclear. Transcriptional profiles were compared in orbital fat from extreme growth phenotypes at juvenile (6-month) and market-size (18-month) stages. In juveniles, slow growth was associated with upregulation of stress-related genes (e.g., sgk1, fkbp5), whereas in market-size fish, fast growth correlated with lipid-storage genes (e.g., dgat2), and slow growth with lipid-oxidation genes (e.g., cpt1b). These findings suggest stage-dependent regulatory roles of orbital fat in growth. The candidate genes identified provide potential molecular markers for selective breeding aimed at improving growth performance in bighead carp.

Bighead carp (Hypophthalmichthys nobilis) 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 (sgk1, fkbp5, lipg), while fast growth correlated with higher expression of stress-buffering (crhbp) and nutrient-signaling (rbp2, mgea5) genes. At 18 months, divergent growth aligned with opposing lipid metabolic states: a pro-anabolic profile (dgat2, fads2) supported fast growth, whereas a catabolic profile (cpt1b, ppargc1a) 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.