Early Temporal Dynamics of Gonadal Development and Sexually Dimorphic Gene Expression During Sex Differentiation in Mandarin Fish (Siniperca chuatsi).

The mandarin fish (Siniperca chuatsi) is a significant freshwater aquaculture species in China, exhibiting sexual dimorphism during growth. However, the cellular and molecular mechanisms underlying early gonadal development and sex differentiation remain unclear. This study traces early gonadal development by examining changes in gonadal histology, germ cell morphologic and distributional changes, and the expression of key signaling molecules. H&E staining and immunofluorescence analysis revealed that oogonia and spermatogonia first appeared at 27 and 33 days post-fertilization (dpf), respectively, marking the onset of sex differentiation. By 51 and 57 dpf, oocytes and spermatocytes became more abundant, indicating the completion of sex differentiation. We examined the expression of female-biased (cyp19a, dmrt2 and foxl2a), male-biased (amh, dmrt1, gsdf and sox9), and meiotic genes (dmc1 and sycp3). In male gonads, the expression of amh was detected at 21-42 dpf, and it showed significantly higher expression after 42 dpf compared to females. Meanwhile, the expression of dmrt1 was significantly higher in male gonads than in female gonads at 30 to 33 dpf. The dmrt3 gene had high expression levels in both male and female gonads before 45 dpf, with a further increase observed after 51 dpf. The gsdf gene exhibited higher expression levels in male gonads between 27 and 42 dpf than at other time points. Additionally, Sox9 expression was significantly higher levels observed in males than in females at 33 dpf. In female gonads, cyp19a was consistently expressed from 21 to 42 dpf and was significantly higher than that in males after 42 dpf, dmrt2 expression was higher than that in males at 36-42 dpf, foxl2a expression was higher at 24-30 dpf and significantly higher than that in males at 36-45 dpf. At 27 dpf, the meiosis-related genes dmc1 and sycp3 were detected, with expression patterns aligning with germ cell development and sex differentiation. In conclusion, by characterizing germ cell morphology and analysing expression changes in sex-related genes, we confirmed that sex determination in mandarin fish occurs before 27 dpf. In females, sex differentiation progresses from 27 to 51 dpf, whereas in males, it occurs from 33 to 57 dpf.