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In fish, as in higher animals, the normal ontogenesis of the gonads is the basis for successful reproduction. Gonadal differentiation and development depend on the genetic cascade that programmes the events. Eventually, gonads become the sites for the maturation of gametes, which are originally derived from the primordial germ cells (PGCs) and form the cellular basis of reproduction. In fish hormones of the hypothalamic-pituitary-gonadal (HPG) axis regulate reproduction and fertility by promoting or inhibiting the production of gonadal sex steroids and their function. Gonadal sex steroids not only guide proper gonadal development and function but also play a key role in the activation and regulation of the HPG axis through two feedback control systems. In this review, we discuss the general outline and expression pattern of the major genes involved in testicular development and gonadal steroidogenesis in fish. The review attempts, for the first time, to collect and discuss recent information regarding the regulation of sex differentiation and steroidogenic genes during the stages of testicular development and in response to different external factors. Furthermore, clear insights into the important genes and their regulation involved in the HPG axis and spermatogenesis are presented briefly. This review can serve as a guide for fish breeders on the substances that have a positive impact on reproduction in males.

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