Flow velocity regulates gonadal development via actin cytoskeleton and lipid metabolic pathways: A transcriptomic study in juvenile Acrossocheilus fasciatus (Steindachner, 1892).

Flow regimes strongly influence fish reproduction, yet how water velocity shapes gonadal development at the molecular level remains unclear. We examined juvenile Acrossocheilus fasciatus, a stream cyprinid, under static (0 BL s-1) and high-flow (4 BL s-1) conditions for 30 days in a recirculating system. Growth and gonadosomatic indices showed upward trends in flow but no significant differences, and histology revealed comparable gonadal stages. In contrast, transcriptome analysis revealed pronounced sex-specific responses. In ovaries, differentially expressed genes were enriched in cytoskeletal remodeling, adhesion, and lipid metabolism, with activation of PPAR signaling and regulators such as e2f8, notably upregulation of fabp10a and fads2 indicating reinforced fatty acid transport and synthesis, suggesting enhanced oocyte provisioning. In testes, gene shifts indicated actin remodeling, fatty acid elongation, steroidogenesis, and spermatogonial stem cell regulation, with upregulation of zbtb16a, etv5b, and steroid receptors pgr and esr1, reflecting a pro-spermatogenic state. These findings show that sustained flow does not alter gonadal morphology in the short term but drives molecular reprogramming that may prime reproductive readiness. This work provides new insight into hydrodynamic regulation of fish reproduction.