International Journal of Aquaculture, 2025, Vol.15, No.6, 298-307 http://www.aquapublisher.com/index.php/ija 300 such as excessively high temperature sometimes causes genetic female fish to become "pseudo-males", thereby disrupting the normal sex ratio. Moreover, the reproductive capacity of these XX pseudo-males is usually inferior to that of true XY male fish. Model analysis also suggests that once the degree of masculinization exceeds a certain limit, the stability of the entire population will be threatened (Chen et al., 2022). 3 Endocrine Regulation Mechanisms and the Role of Reproductive Hormones 3.1 The key role of the HPG axis in yellow catfish reproduction During the breeding process of the yellow catfish (Pelteobagrus fulvidraco), the endocrine system acts like an invisible "command center", and the hypothalamic-pituitary-gonadal (HPG) axis is the main thread of the entire system. It is not merely a chain of command transmission, but more like an information integration hub. After summarizing environmental signals and an individual's physiological state, it regulates the secretion of various reproductive hormones. Like other bony fish, the hypothalamus of the yellow catfish secretes gonadotropin-releasing hormone (GnRH), which prompts the pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones further stimulate the gonads, promoting the synthesis of sex steroids and the development of gametes (Chen et al., 2021). However, this set is not a simple linear reaction. The latest molecular-level research has found that some steroid-related genes (such as TSPO and SMAD4) play the role of "fine regulation" in the response of gonadal tissue. Their specific expression indicates that the HPG axis has undergone long-term evolutionary optimization and differentiation in yellow catfish, enabling reproductive activities to respond more accurately to seasonal and internal and external environmental changes. 3.2 Dynamic changes of major sex hormones The several major sex hormones in the yellow catfish, namely estrogen, androgen and progesterone, are not stable. They fluctuate with the stage of gonadal development and the external environment. When the ovaries are developing, estrogen levels rise, promoting the growth of oocytes. Male fish rely on androgens to maintain sperm production (Chen et al., 2021). But the situation can also be artificially disrupted. For instance, 17α -estradiol (EE2) used in the experiment interfered with the normal development of the testicles of male fish, causing disorders in cell division. This also indicates that the reproductive system of the yellow catfish is highly sensitive to hormone balance. On the other hand, the involvement of microRNAs (miRNAs) makes things more complicated. These small molecules do not directly "produce" hormones, but can regulate related genes at the post-transcriptional level and even show gender-specific expression (Jing et al., 2014). In other words, the reproduction of yellow catfish is not driven by a single hormone, but is a dynamic result of the combined effects of genes, epigenetics and hormones. 3.3 Hormonal feedback and environmental adaptation regulation The ability of the yellow catfish to reproduce rhythmically seems natural, but in fact, it is all thanks to the hormone feedback system within its body that maintains stability. The positive and negative feedback mechanism in the HPG axis enables sex steroids to control the release of GnRH and gonadotropins in turn, ensuring that reproduction does not disrupt the rhythm. Unfortunately, this balance is not always maintained. A slight deviation in the environment, such as an increase in water temperature, may cause the entire system to be out of balance. The sex ratio begins to be skewed and reproductive capacity is also affected accordingly. Especially during the critical period of sex differentiation, if high temperatures persist, so-called "pseudo-males" will emerge, fish that should originally be female are forced to grow into males, but their reproductive capacity is often very poor (Chen et al., 2022). Such a situation becomes even more intractable against the backdrop of global warming. Although the yellow catfish has some ability to adapt to the environment, its endocrine system is overly dependent on external signals. Once the ecological conditions change too rapidly, this coupling becomes a weakness instead. Understanding the details of this feedback and adaptation is not only a scientific research issue, but also related to how future aquaculture should be adjusted and how to keep the reproduction of this species stable and sustainable (Chen et al., 2022).
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