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Gonochoristic fish display an amazing diversity of sex determining systems, ranging from a strict genetic control (genetic sex determination, GSD) to an environmental control (environmental sex determination, ESD) or the joint action of both factors. therefore, our review will focus on gonochoristic fish that represent most of the teleost species. Sex reversal in hermaphroditic fish species has been reviewed by Todd et al. Based upon their sexuality traits, fish species have been categorized into (1) hermaphroditic: individuals in a population produce both male and female gametes simultaneously or successively through a natural sex reversal process (2) gonochoristic: each individual is either a male or a female, and this fixed sex will be kept permanently and more rarely (3) unisexual: populations are composed exclusively of gynogenetic females. Following a brief overview of sex determination and differentiation, the present review will therefore analyze fish sex reversal in both domesticated strains and wild populations, the associated mechanisms, and its possible consequences on the dynamics of wild populations.Īs reported by many authors, sex determination systems are conserved and stable within birds and mammals with a few exceptions, whereas they appear to be highly variable among poikilotherms, including fish species. Although their prevalence has still to be better characterized for most of these species, modelling studies have recently underlined the possible ecological and evolutionary consequences of sex reversal on wild fish populations. Following the identification of sex-linked markers, sex-reversed individuals have been identified more recently in wild populations of various fish species. Because sex-reversed individuals can be used to produce genetic all-male or all-female populations, they have long been applied in aquaculture and for research on sex determination and differentiation. In fish, as well as in other poikilotherm vertebrates, sex reversal can be easily induced by hormonal and sometimes by environmental treatments.
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In vertebrates, sex reversal has been defined as a mismatch between the phenotypic and the genetic sex. Cases of sex reversal in wild fish populations are shown and their fitness and future perspectives are discussed. Manipulations with exogenous factors reverse the primary sex in many fish species under controlled conditions, but several questions arise on whether this can occur under wild conditions and what is the ecological significance. Studies on epigenetics are presented, since it links the environment, gene expression, and sex reversal, notably the association of DNA methylation in sex reversal.
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The large plasticity in fish for sex reversal is also evident in the brain, with a reversibility existing even in adulthood. We also discuss whether cortisol is the universal mediator of sex reversal in fish due to its implication in ovarian meiosis and 11KT increase. The old players of sex differentiation are revisited with transcriptome data and loss of function studies following hormone- or temperature-induced sex reversal. The different periods of fish sensitivity to sex reversal treatments are presented with the mechanisms implicated. This review discusses sex reversal in gonochoristic fish, with the coexistence of genetic and environmental influences. Fish sex reversal is a means to understand sex determination and differentiation, but it is also used to control sex in aquaculture.