Dynamic Transcriptomic Profiling of the HPG Axis Reveals Stage-Specific Regulatory Networks Underlying Natural Sex Change in Orange-Spotted Grouper (Epinephelus coioides).

Protogynous sex change in teleosts involves dramatic gonadal restructuring, governed by the hypothalamic-pituitary-gonadal (HPG) axis. Here, we established the dynamic regulatory atlas of the HPG axis during natural sex change in protogynous orange-spotted grouper (Epinephelus coioides) through multi-tissue transcriptomics. Profiling hypothalamus, pituitary, and gonad tissues across ovarian, intersex, and testicular stages revealed minimal transcriptomic changes in the hypothalamus during ovarian-to-intersex transition but significant differential expression during intersex-to-testis progression. The pituitary showed pronounced activation during the intersex stage featuring fshb and cga upregulation, positioning FSH as a potential trigger of sex change. Gonadal transformation involved extensive transcriptional rewiring during sex change, with induction of male-determination genes (dmrt1, amh) and suppression of female pathways (cyp19a1a, foxl2). Weighted gene co-expression network analysis identified stage-specific modules: an intersex gonad-specific module enriched for p53 signaling and oocyte meiosis pathways may facilitate ovarian regression through apoptosis and cell-cycle arrest where hub genes kpna2 and patl2 providing mechanistic insights into ovarian atresia, while a testis-specific module enriched for nucleic acid metabolism and spermatogenesis. This integrated multi-tissue analysis reveals how HPG axis coordination drives protogynous sex change, providing a foundation for future mechanistic studies.