User:Mjsalnic/Sex reversal

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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. Sex reversal process 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 hormonal induced in laboratory, it also can occur artificially by exposure to Endocrine disruptors like pollutants, including herbicides, which can act as estrogen promoters or inhibitors, which alters Aromatase expression

Sex reversal in gonochoristic fishes
In gonochoristic fishes, the sex can be determined genetically, environmentally or by a combination of both. In fishes, primary sexual fate can be susceptible to hormones exposure and multiple environmental cues such as population density, water pH or temperature. Those cues can affect the gonad development and differentiation which can lead to sex reversal. Wild populations where sex reversal has been documented shows 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 genetically males can be reversed to females if they lack the dmy gene.

Induced sex reversal in Aquaculture industry
In aquaculture sex control is important due to is related to reproduction. In fishes, growth rates might be different between sexes. These differences can affect their economic value. Producing 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 fishes to Sex steroid s. There are other methods to induced sex reversal in fishes such as chromosomal/genetic manipulation, hybridization or  treatments influencing sex determination or gonad differentiation (e.g. temperature, population density, pH, social factors).

Sex reversal in Amphibians
Sex is genetically determined in Amphibian s.  Temperature-induced sex reversal has been documented in some species of anuran and caudate amphibians. Temperature only can have an effect on sex differentiation during a window period called thermosensitive period (TSP) which varies among species. Tadpoles or larvae exposed to specific higher or lower temperatures, depending on the temperature thresholds of the species, can differentiate gonads that do not align with their primary sexual destiny.

Amphibian sex reversal can be also induced by sex steroid and pollutants exposure. Endocrine disruptors can also affect gonad differentiation, and therefore induce sex reversal. Exposure to Ethylnylestradiol (EE2) and bisphenol A (BPA) induce feminizing effects. Masculinizing effects can be induced by exposure to the drug trenbolone use in cattle.

Research in wild populations of the North American green frog has demonstrated that sex reversal is common. This work shows that genetic females sex reverse into phenotypic males and that genetic males sex reverse into phenotypic females, providing evidence that sex reversal can be bidirectional in amphibians.

Sex reversal in Reptiles
Sex in reptiles can be determined genetically (GSD), environmentally (ESD). Sex reversal has been documented in detail in wild populations of the central bearded dragon Pogona vitticeps, Eastern and in the three-lined skink Bassiana duperreyi.  In these species their genetically determinated sex is overriden by temperature influence.

Sex reversal in reptiles can be induced by hormonal manipulation, treatments influencing sex determination (e.g. temperature) or by inhibition of the aromatase gene (CYP19A1) which causes sex reversal from female to male phenotype.

Sex reversal in Birds
In birds, sex reversal has been documented in natural and experimental conditions. Sex steroid manipulation can induced sex reversal in birds. Aromatase inhibitors injected in chicken eggs before the gonadal differentiation stage induce testis development in ZW embryos.

Sex reversal in Mammals
Sex reversal in mammals has been documented in domestic species such as cattle, water buffalo, horses, dogs, cats, pigs, goats, etc. Sex reversal in these species relies usually in genetic defects and the resulting phenotype is often associated with gonadal malformation.

Natural sex reversal without disrupting effects on fertility has been documented in Myopus schisticolor, Dicrostonyx torquatus, Akodon species, Mus minutoides, Microtus cabrerae. In these species some individuals genetically determined as males develop typical ovarian structure. In mammals, sex reversal mainly occurs after mutational events.