International Journal of Aquaculture, 2025, Vol.15, No.4, 184-196 http://www.aquapublisher.com/index.php/ija 191 ratio: environmental stress such as high temperature or high density can trigger an increase in cortisol in fish and indirectly lead to an increase in androgen, thus tending toward a higher male production ratio. In Japanese fish such as cuckoo, high temperature-induced male bias has been shown to be closely related to the increase in serum cortisol and 11-KT (Wang et al., 2020). For grouper farming, if you want more fish to turn males as soon as possible (for example, breed whole male groups for breeding), you can consider applying androgens or reducing estrogen levels within the appropriate developmental window to promote sexual reversal. However, the dosage and timing need to be grasped to avoid negative impact on fish growth. In addition to the gonads themselves, changes in hormone levels also have feedback on the central hormone axis: When androgens are elevated, the secretion of hypothalamic GnRH and pituitary gonadotropin may be inhibited to maintain internal homeostasis. This feedback regulation can sometimes affect the thoroughness of sexual reversals, such as excessive androgens may inhibit FSH through negative feedback, causing the gonadal support cells to not be stimulated sufficiently. Therefore, it is often necessary to balance the effects of various hormones during manual operations. If necessary, combined use of gonadotropins and sex steroids can be achieved successfully. 5.3 Functions of hormone receptors and downstream signaling pathways in gender regulation Sex hormones work by binding to their receptors and triggering downstream gene expression, so the role of hormone receptors and related signaling pathway molecules cannot be ignored. Take estrogen receptor ER and androgen receptor AR as examples: ER includes ERα and ERβ, which are expressed in ovarian cells. After binding to E2, it can regulate a series of ovarian development and maintenance genes; AR is expressed in support cells and germ cells, and after binding to androgen, it can activate spermgenesis-related genes (Cheng et al., 2020). Studies have shown that in socially sexual conversion fish such as blue-headed parrot, high levels of cortisol in the brain of female fish may block sexual conversion by binding to glucocorticoid receptor GR, interfering with the path of estrogen receptor; while when dominant female fish lose community pressure and decrease cortisol, the estrogen receptor signal is relatively enhanced, and the motivation to convert to males is reduced. This suggests that GR-mediated stress signals and ER-mediated female maintenance signals affect gender stability or switching. During the maleization of grouper, GR may also be directly involved in inhibiting ovarian genes: Studies have speculated that the elevated cortisol-GR complex can bind to the glucocorticoid response element (GRE) of the cyp19a1a promoter to recruit inhibitory cofactors, thereby directly reducing the transcription of aromatase genes. This molecular mechanism provides clues to understanding how environmental stress (such as high temperature, high density) affects gender through endocrine pathways. In addition, FSH receptors (FSHR) and LH receptors (LHR) mediate the effects of gonadotropins. FSH mainly promotes follicle growth and estrogen synthesis, so FSHR is highly expressed on ovarian granules cells; while during sexual reversal, FSHR is also expressed in newly formed schizophrenia support cells, indicating that FSH may promote schizophrenia development through these receptors. Some experiments with FSH injections were able to induce female fish to turn male, further demonstrating the importance of FSH-FSHR signaling for reversal initiation. In pattern fish such as bluefish, if Amhr2 is dysfunctional, it will lead to abnormal germ cell number and disordered gonad development. For grouper, the AMH-Amhr2 pathway is also likely to play a role in inhibiting female structure and promoting semen formation-as in experiments with oblique band grouper, through overexpressing the amh gene in juveniles, the production of functional male individuals has been successfully induced (Wu et al., 2020). All these show that hormone receptors and downstream signals are indispensable parts of the gender regulatory network, and their mechanism of action often affects gene regulation. 6 The Impact of External Environmental Factors on Gender Decisions 6.1 The relationship between temperature, density, nutrition and gender bias External environmental factors are important variables that affect the gender decision and sexual ratio of fish, and even play a decisive role in some species. For hermaphrodite fish such as grouper, environmental factors mainly affect the timing and rate of gender conversion, and the impact on initial gender differentiation is still unclear. But in many bony fish, temperature has been shown to significantly tilt the sex ratio of progeny, which provides a reference for understanding the gender plasticity of fish. One experiment showed that males accounted for about 63% of the offspring of tilapia bred at 27 °C, while males accounted for as high as 97.8% under high temperature
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