Gonadal transdifferentiation in teleosts during normal and altered physiology-a review

Teleost species display a diverse range of reproductive strategies, consisting mainly of gonochorism and hermaphroditism, either sequentially or simultaneously. This leads to species-specific sex determination processes. Owing to the vast range of teleost species, sex determination and differentiation mechanisms vary in their sexual patterns; therefore, sexual plasticity in gonadal development is inevitable. Under certain environmental conditions, such as fluctuations in temperature, pH, density, social interaction, and hypoxia, gonadal fate is liable to change through a phenomenon referred to as gonadal transdifferentiation. This is a reprogrammable mechanism that can occur either naturally or be induced artificially in certain species at various life stages. Transdifferentiation results from the differential expression of steroidogenic enzyme genes or transcription factors that confer phenotypic, molecular, or morphological changes, such as sexual dimorphism. In addition, many endocrine disruptors manipulate the endocrine system of various fish species by interfering with their genetic pathways or with the profiles of endogenous steroid hormones, causing a shift in the sex ratio of a given population. In the current modern chemical environment that includes endocrine disrupting chemicals, fish can undergo gonadal transdifferentiation, which can lead to functional sex reversal. This review summarizes gonadal transdifferentiation and provides evidence of diverse strategies found in teleosts, from molecular-level changes to feminization or masculinization of gonads to sex reversal, induced either naturally or unnaturally. We end with a discussion of the adverse effects of endocrine disruptors and summarize areas for future research.