International Journal of Aquaculture, 2025, Vol.15, No.4, 184-196 http://www.aquapublisher.com/index.php/ija 185 temperature, light, and density, in addition to genetic factors. Studying the gender determination mechanism of fish (especially species with gender-transforming ability) is not only of great theoretical significance for understanding the evolution and plasticity of vertebrate gender development, but also has direct application value for aquaculture practice. In recent years, with the development of high-throughput sequencing and omics technology, many progress has been made in the research on grouper gender decisions. Based on reviewing the latest research progress in various aspects, this study attempts to propose a molecular network model for grouper gender regulation and discusses the current main challenges and future research directions. 2 Biological Basis of Grouper Gender Decision and Differentiation 2.1 Gender type and gender transition in life cycle of grouper The gender types of grouper are mainly hermaphrodites, and most species are typical female premature type and are heteromature. In natural populations, a dominant male usually leads a certain number of females to form a breeding group. If male fish in the group are missing for reasons such as death or removal, females with the highest rank (largest size or highest social status) in the group will be induced to initiate gender reversal and transform into new males within weeks to months (Palma et al., 2019). Experimental observations of oblique grouper showed that in a group of 1 male and 3 females, the dominant female fish can turn from female to male after only about one month. This mechanism of gender change caused by social factors is believed to be closely related to group stability and reproductive success. In addition, there are also differences between "primary male" and "secondary male" in grouper: the so-called primary male refers to individuals who develop directly into males at the time of not going through the female stage (the proportion in grouper is very low), while secondary males are males that are transformed from mature females again. 2.2 Gonadal structure and histological characteristics The histological characteristics of grouper during gender differentiation and conversion are very typical. Groupers during the juvenile period have undifferentiated primitive dual potential glands that can develop into ovaries or sperm. Under normal circumstances, its gonads first differentiate into the ovary, which is histologically manifested as ovarian cavity formation, oocyte proliferation and growth. When an individual matures into a functional female, ovarian tissue predominates, containing oocytes and helper cells at different developmental stages. It is worth noting that in female mature ovaries, a small number of scattered spermatogonia or primary spermatogonia can often be observed to exist in ovarian tissues, which is more common in protogynous fish such as grouper. These potential male cells do not develop into sedentary structures at the female stage, but proliferate and differentiate rapidly at the onset of sexual reversal. During the process of grouper turning from female to male, the gonad undergoes obvious tissue remodeling: oocytes gradually degenerate and absorb, the ovarian cavity is closed, the semen tubular structure begins to appear and develop and mature, and spermatogonia proliferates and forms semen tissue (Wang et al., 2017; Wu et al., 2017). Transitional gonads (also known as intergonadal) are the intermediate stages of grouper's gender conversion. The microstructure is often manifested as having both ovarian and sperm components - both residual oocytes and developing sperm cells can be seen in the same gonad. 2.3 Overview of hormone regulation during gender differentiation Sex steroid hormones play a key role in the gender differentiation of grouper. Similar to many fish, the rise and fall of estrogen and androgen levels directly affect the direction of differentiation of the gonads. During female differentiation, high expression of aromatase (Cyp19a1a) in ovarian tissue converts androgens such as testosterone into estrogen, which increases the level of estradiol, thereby promoting the undifferentiated gonads to ovarian development. Experiments have shown that administration of aromatase inhibitors at critical periods of grouper gender decisions will lead to hindered differentiation of female fish and tend to develop towards male direction (Wang et al., 2018), demonstrating that estrogen signaling is indispensable for ovarian differentiation. In addition, gonadotropins also play an upstream regulatory function in gender differentiation. The follicle-stimulating hormone (FSH) and luteinizing hormone (LH) secreted by the pituitary gland regulate the synthesis and secretion of endogenous hormones in the gonads through receptors (FSHR and LHR) acting on the surface of the gonads, thereby affecting the development direction of gonads. During the female-to-male transition from grouper to male,
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