Molecular characterization and differential regulation of sex steroids on the gonadotropin ligand-receptor system in mandarin fish (Siniperca chuatsi).

The gonadotropin system plays a fundamental role in regulating teleost reproduction. This study characterizes the molecular properties and physiological functions of the Gth system in the mandarin fish (Siniperca chuatsi). Sequence analysis reveals that both fshb and lhb subunits possess a single N-glycosylation site. Phylogenetic evidence indicates that the lhcgr duplication occurred prior to the teleost specific whole genome duplication event. Tissue distribution analysis demonstrates that lhcgr2 is strictly gonad-specific, while lhcgr1 exhibits a broad distribution across multiple organs, including the brain and liver. Furthermore, the spatiotemporal expression profiles delineate a clear regulatory sequence during gonadal development. At the molecular differentiation stage (15 days post hatching, dph), the significant upregulation of fshr and lhcgrs in males precedes histological gonadal differentiation at 30 dph. During the initiation of gametogenesis at 60 dph a distinct sexual dimorphism emerges. Female development is characterized by a significant increase in pituitary fshb to drive follicle recruitment. In contrast, male development relies on a receptor sensitization mechanism, where early endogenous androgens upregulate gonadal fshr and lhcgrs to sustain spermatogenesis through to sexual maturity at 360 dph. Additionally, experimental hormonal treatments confirm that androgens induce a male-biased receptor expression pattern, whereas estrogens suppress it. Pharmacological assays further identify a biased preference of Fshr for the MAPK pathway and prominent constitutive activity of Lhcgr2. Collectively, these findings provide a comprehensive model for the endocrine regulation of the male developmental trajectory in the mandarin fish.