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Introduction
Definition of fish sex reversal
Sex reversal in fish refers to their ability to change from their original sex under specific conditions, transitioning from male to female, female to male, or both simultaneously. Although the precise mechanisms and reasons behind fish sex reversal remain not fully understood, it is believed to be influenced by factors like genetics, environment, and hormones. Various fish species, including groupers, snappers, eels, and tilapias, exhibit natural or artificially induced sex reversal. The application of sex reversal techniques in aquaculture and resource conservation offers several advantages, such as improving both fish yield and quality, while also contributing to heightened genetic diversity and adaptability.
Definition of fish sex reversal
The primary goal of fish sex change is to enhance offspring reproduction and stimulate sexual behavior in individuals. Vertebrate experts suggest that the phenomenon of sex change is unique to fish, with increasing differentiation among higher vertebrates. Scientific studies indicate that exogenous hormones can induce sex changes in fish that are typically resistant to such alterations. Through appropriate treatment, both juvenile and adult fish can undergo transformation into monophyletic or primary monophyletic populations. Given that male fish tend to be heavier and grow faster than females, sex control technology is frequently employed to maximize fish yield and variety, while ensuring the safety of exogenous hormones for human consumption.
How does the mechanism of fish sex reversal operate?
Fish exert control over their reproductive function and determine their final phenotype by regulating the development and differentiation of germ cells. Hormone regulation plays a crucial role in this process through the hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-thyroid (HPT) axes.
In the HPG axis, the hypothalamus releases gonadotropin-releasing factor (GnRH), stimulating the pituitary gland to secrete gonadotropin (GTH). GTH then acts on the gonads, promoting the synthesis and secretion of estrogen and hormones, as well as the proliferation and maturation of germ cells. The HPT axis involves the secretion of thyrotropin-releasing factor (TRH) by the hypothalamus, stimulating the pituitary gland to secrete thyroid-stimulating hormone (TSH). TSH, in turn, acts on the thyroid gland to promote the synthesis and secretion of thyroid hormones that regulate systemic metabolism. Feedback from estrogen and thyroid hormones regulates the activity of the hypothalamus and pituitary gland to maintain a dynamic balance.
When fish encounter factors inducing sex reversal, such as environmental or genetic influences, these two systems undergo changes, disrupting the original male-female balance. This disruption triggers a series of processes, including gene expression, cell proliferation, and tissue remodeling, ultimately leading to sex change.
When a fish undergoes sex reversal from male to female, it typically follows these steps:
- Degeneration of male characteristics: The testis begins to shrink, sperm production decreases or stops, testicular stromal cells decrease or disappear, and testosterone levels decrease.
- Induction of female characteristics: Ovarian primordia begin to increase, oocytes proliferate or rejuvenate, ovarian stromal cells appear or increase, and estradiol levels increase.
- Stable female characteristics: Fully developed ovaries, normal ovulation, establishment of the ovulation cycle, and stable estradiol levels, among other features.
Conversely, when a fish undergoes sex reversal from female to male, it goes through the following steps:
- Regression of female characteristics: Ovaries begin to shrink, oocytes decrease or degenerate, ovarian stromal cells decrease or disappear, and estradiol levels decrease.
- Induction of male characteristics: Testicular primordia begin to increase, spermatogonia proliferate or rejuvenate, testicular stromal cells appear or increase, and testosterone levels increase.
- Stable male characteristics: Fully developed testes, normal sperm production, mating behavior, and stable testosterone levels, among other features.
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What factors contribute to the occurrence of fish sex reversal?
Fish sex reversal is a biological phenomenon where the gender of a fish changes from male to female or vice versa. This process can occur naturally but can also be influenced by various environmental and genetic factors.
Fish regulate the development and differentiation of germ cells to control their reproductive function and determine their final phenotype. Hormone regulation, which involves the hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-thyroid (HPT) axes, is a critical component of this process.
Gene:
The genetic sex determination mechanism in animals means that external environmental factors do not influence the direction of sex differentiation. Instead, genetic genes on sex chromosomes determine it. The sex-determining gene controls the “determination process” and initiates a series of sex differentiation events. The genetic sex determination process involves a complex interaction of biochemical processes, where some components or combinations of components in the pathway can become dominant factors that determine the direction of sex determination.
Temperature:
During the hatching process of fish eggs, there is a temperature-sensitive period (TSP) during which the direction of sex differentiation and sex ratio can be altered by artificially raising or lowering the temperature. Gender reversals can even occur, as seen in Nile tilapia, where hereditary females can become physiological males if treated at a high temperature during the TSP.
Exogenous hormones:
Fish exhibit strong plasticity in sex, and the external environment can be manipulated to reverse the fish’s phenotype. Two main methods of inducing sex reversal in fish involve adding exogenous hormones or inhibitors. Exogenous hormones directly alter hormone levels to induce sex reversal, using common drugs such as 17-methyltestosterone, 11-ketotestosterone, 17-estradiol, among others. Inhibitor drugs interfere with hormones and receptors in the body, reducing the level of fish sex hormones.
Fish Sex Reversal Methods:
The primary methods for achieving fish sex reversal include exogenous hormone induction, environmental change, and gene manipulation.
- Exogenous hormone induction: This involves the injection or implantation of male or female hormones into fish, altering their gonads and ultimately their sex. Commonly used hormones include 17-Methyltestosterone, ketotestosterone (11-KT), 17-estradiol (E2), and letrozole.
- Environment change: This method involves manipulating temperature, light, density, nutrition, and other conditions of a fish’s environment to affect hormone levels and gene expression, ultimately inducing sex reversal.
- Gene manipulation: Editing or transferring the chromosomes or genes of fish to have specific sex-determining genes or lack key genes, allowing for sex reversal. This method has the potential to create new strains and traits but is technically difficult and may raise safety and ethical concerns.
Steps of Fish Sex Reversal:
When a fish reverses from male to female or vice versa, it may go through several steps:
Male to Female:
- Degeneration of male characteristics: Testis begins to shrink, sperm production decreases or stops, testicular stromal cells decrease or disappear, and testosterone levels decrease.
- Induction of female characteristics: Ovarian primordia begin to increase, oocytes proliferate, ovarian stromal cells appear, and estradiol levels increase.
- Stable female characteristics: Fully developed ovaries, normal ovulation, ovulation cycle establishment, stable estradiol levels, etc.
Female to Male:
- Regression of female characteristics: Ovaries begin to shrink, oocytes decrease or degenerate, ovarian stromal cells decrease or disappear, and estradiol levels decrease.
- Induction of male characteristics: Testicular primordia begin to increase, spermatogonia proliferate, testicular stromal cells appear, and testosterone levels increase.
- Stable male characteristics: Fully developed testes, normal sperm production, mating behavior, stable testosterone levels, etc.
Factors Influencing Fish Sex Reversal:
Factors that induce fish sex reversal include environmental or genetic factors. When fish encounter such factors, the hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-thyroid (HPT) axes, involved in hormone regulation, change. This disrupts the original male-female balance, triggering gene expression, cell proliferation, and tissue remodeling processes leading to sex change.
Commonly Used Hormones for Fish Sex Reversal:
The commonly used hormones include 17α-methyltestosterone (MT), ketotestosterone (11-KT), estradiol-17β (E2), and letrozole. These hormones are employed in aquaculture to induce sex reversal in fish species such as tilapia, rainbow trout, and Atlantic salmon.
Advantages of Fish Sex Reversal:
- Control of fish gender: Allows farmers to produce fish of the desired gender, enhancing efficiency and profitability.
- Higher yield and profitability: Leads to higher yields and profits by producing fish of the desired gender.
- Reduction of environmental impact: Producing all-male populations reduces the number of fish that need to be culled, reducing waste and environmental impact.
Disadvantages of Fish Sex Reversal:
- Hormone residues in fish products: Presence of hormone residues in fish products may pose health risks and quality concerns.
- Potential negative effects on fish health and behavior: Fish undergoing sex reversal may experience physiological and behavioral changes.
- Risk of hormone contamination in the environment: Hormones used may enter the environment, impacting other organisms and causing environmental issues.
Regulatory Aspects:
Regulations and guidelines for hormone use in aquaculture vary among countries. The European Union and the FDA regulate hormone use to ensure safety for human consumption
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Methods of administering hormones play a crucial role in the common aquaculture practice of inducing sex reversal in fish to produce all-male populations, known for their faster growth and superior feed conversion rates compared to mixed-sex populations. Hormone administration is the predominant method employed for this purpose, with three main approaches: oral administration, injection, and immersion.
(1) Oral Administration: Oral administration entails blending hormones with fish feed and delivering them through oral consumption. While less invasive and stressful than injection, ensuring consistent consumption of hormone-treated feed poses a challenge. Additionally, variations in hormonal doses among individual fish may result in inconsistent outcomes.
(2) Injection: Injection involves directly injecting a hormone solution into the muscle tissue of fish. This method is highly effective and reliable as it delivers hormones directly into the bloodstream.[9] However, proper execution requires skill and expertise, and it may cause stress and tissue damage to the fish.
(3) Immersion: Immersion entails placing fish in a bath containing a hormone solution. This method is straightforward, easy to perform, and requires minimal handling of the fish.[15][6][14] It is suitable for large-scale applications, but careful control of hormonal concentration and exposure duration is necessary to prevent adverse effects on fish health and survival.
In conclusion, the selection of a hormone administration method depends on various factors, including fish species, population size, and available resources. Regardless of the chosen method, hormone administration proves to be a powerful tool for inducing sex reversal in fish, facilitating the production of all-male populations for aquaculture purposes.
