Abstract
In vertebrate the development of gonad into testis or ovary requires the combined effect of genes and hormones to trigger differentiation from a bipotential gonadal primordium. Knowledge of the gonad development process is vital in studies on fish sex determination and differentiation. The spotted scat (Scatophagus argus) is an economically important marine fish with an XX/XY sex-determining system and exhibits sexual growth dimorphism in favor of females. Meanwhile, information on gonad development and gene expression patterns in the early gonads is unknown. This study explores the pattern of spotted scat early gonad development by morphological and molecular means. In the undifferentiated gonads, the number of germ cells in females is not significantly different from males. At age 3 mph (months post-hatch), the number of germ cells in females was significantly higher than in males. The sex differentiation began in females before males with the proliferation of primary oocyte and spermatocytes at 3.5 and 4 mph, respectively. In the early gonads, dmrt1 and amh were significantly higher in XY-fish, while cyp19a1a and foxl2 were higher in the XX-fish. Within the critical period of sex differentiation, estradiol treatment was able to sex-reverse XY-male to XY-female. In the sex-reversed group, male (dmrt1 and amh) and female (cyp19a1a and foxl2) related genes were suppressed and increased, respectively. 17α-Methyltestosterone or aromatase inhibitor (letrozole) could not induce sex reversal in XX-fish due to the absence of the male-specific dmrt1 required for testes development. Conclusively, this study identifies the critical window for sex differentiation and significantly higher germ cell numbers in future females than males. We provide evidence that the phenotypic XY-male is reversible, while the phenotypic XX-female is irreversible. Dmrt1 is necessary for testis differentiation, amh in males, and cyp19a1a and foxl2 in females could also be used as molecular markers in early gonads. This study explicitly presents valuable information for research development on sex determination and differentiation and sex control in spotted scat.
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