Sex reversal is a biological process whereby the pathway directed towards the already determined-sex fate is flipped towards the opposite sex, creating a discordance between the primary sex fate and the sex phenotype expressed. The process of sex reversal occurs during embryonic development or before gonad differentiation. In GSD species, sex reversal means that the sexual phenotype is discordant with the genetic/chromosomal sex. In TSD species, sex reversal means that the temperature/conditions that usually trigger the differentiation towards one sexual phenotype are producing the opposite sexual phenotype. Sex reversal can occur naturally, by mutations, or can be induced artificially. Sex reversal can be genetically or hormonally induced in laboratory. It can also occur artificially by exposure to endocrine disruptors such as pollutants, including herbicides, which can act as estrogen promoters or inhibitors, for instance by altering aromatase expression.
In fish Gonochoristic Fish In gonochoristic fish, the sex can be determined genetically, environmentally or by a combination of both. In fish, primary sexual fate can be susceptible to alteration by hormones exposure and multiple environmental factors, such as population density, water pH, or temperature. Those conditions can affect the gonad development and differentiation, which can lead to sex reversal. In medaka fish, where sex reversal has been documented show a shared gene related to normal male development, the dmy gene. In wild populations, genetically female fate can be phenotypically reversed to males if they carry the dmy gene or a mutated dmy gene and genetic males can be reversed to females if they lack the dmy gene. Hermaphroditic Fish Within seven teleost fish families, two modes of hermaphrodism can be observed—simultaneous hermaphrodism and sequential hermaphrodism. These cases often arise due to social factors, such as a decline in population or changes in dominance. It is assumed that hermaphroditic sex changing fish have increased plasticity within their gonads compared to the typical gonochoristic fish, allowing for gonadal differentiation. Although it is not a reversal of sex, simultaneous hermaphrodism is beneficial for low-density populations as it allows for an increase in conspecific mating opportunities overall. Simultaneous hermaphrodism occurs when fish are able to produce both mature sperm and eggs within their gonads—while self-fertilization is rare, it has been seen to occur in few low-density populations such as the Kryptolebias marmoratus. Sequential hermaphrodism occurs when gonadal sex redifferentiation takes place, often due to a decrease in the population of a specific sex. This form of hermaphrodism has taken upon three modes—protogyny, protandry, and bi-directional sex change. Protogynous sex change is the female to male gonadal redifferentiation, and it is often found within species that practice haremic polygyny, or one male reproducing with many females. When socially dominant males disappear, larger initial phase female fish will undergo sex change to become terminal phase males. In this, the ovaries are constructed to mature, functional testis. Protogyny sex change has been found within fish species such as the three-spot wrasse (Halichoreres trimaculatas) and the saddleback wrasse (Thalassoma duperrey) with a decline in plasma estrogen contributing to testis development. In contrast, protandrous sex change is the male to female gonadal redifferentiation, but it is also triggered by the disappearance of the conspecific dominant, in this case the female. While protogynous fish are found within species dominated by larger males, protandrous fish are within large female dominant species, such as the clown fish (Amphiprion). Within the clown fish populations, males develop bi-sexual gonads—mature testis and immature ovaries—while the dominant females only possess ovaries. In cases of protandrous sex change, the immature ovarian tissues develop and the testicular tissues deteriorate due to changes in the activity of gonadal steroid hormones such as estrogen. Bi-directional, or serial, sex change occurs when hermaphrodites are capable of changing their sex in either direction, possibly multiple times. Some fish species are capable of this sex change due to the presence of both ovaries and testis, with only one gonad actively producing gametic cells at a time. The activation of a gonad is possible through changes in expression of the gonadotropin receptors. This primarily occurs in cases of male-male and female-female mating pairs, with the smaller male changing to female or the larger female changing to male due to the disappearance of the respective sex. This can be found in the Okinawa rubble gobiid fish (Trimma okinawae) whose mating system consists of a polygynous harem. When the dominant male is removed, the largest female changes its sex to male; if the dominant male is reintroduced, the sex can be reversed back to female.
Induced reversal in aquaculture industry In aquaculture, sex control is important due to the role of sex in growth and reproduction. In fish, growth rates can be different between sexes. These differences can affect their economic value. Producing a monosex fish population can improve product quality and therefore generates higher financial profit. Hormone-induced sex reversal is the most frequent method used in aquaculture. It consists of exposing sexually undifferentiated fish to sex steroids. There are other methods to induced sex reversal in fish such as chromosomal/genetic manipulation, hybridization, or treatments influencing sex determination or gonad differentiation (e.g. temperature, population density, pH, social factors).
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