Objective The growth rate of female large yellow croaker (Larimichthys crocea) is significantly faster than that of males, making all-female fry highly desirable to aquaculturists. A common method to produce all-female fry involves treating genetically female fish (XX) with androgens before gonadal differentiation to develop them into physiological males (neomale, XX ♂), and then crossing them with original females (XX ♀). However, this method is time-consuming and androgen treatment (using methyltestosterone, MT) severely inhibits the growth of L. crocea fry. Directly inducing sex reversal in adult female L. crocea to become physiological males could circumvent these issues. This study aims to explore the possibility and methods of inducing sex reversal in sexually mature female L. crocea to cultivate physiological males, providing an efficient new approach to produce physiological male fish.

Methods A combination of MT and PHTPP was used as an inducing agent. The mixed drug was made into chitosan gel strips or dissolved in soybean oil to create a sustained-release solution. The mixture was injected into the abdominal cavity of mature female L. crocea to induce the sex reversal of the fish. The histological changes in the gonads were observed, and changes in serum sex hormone levels and the expression of estrogen receptor gene in the gonads were measured.

Results The results showed that the MT-PHTPP-soybean oil mixture had a better inducing effect than the chitosan-MT-PHTPP gel strips. Injecting a dose of (40 μg MT + 4 μg PHTPP) per kg of fish body mass of the MT-PHTPP-soybean oil mixture into the abdominal cavity and continuing treatment for 80 days resulted in ovarian degeneration in the experimental group. Morphologically, the ovaries exhibited changes resembling testes. A large number of spermatogonia were formed, serum estradiol levels significantly decreased, and 11-ketotestosterone levels significantly increased (P<0.05). The expression of the estrogen receptor Esr2 gene in the gonads was severely inhibited.

Conclusion The experimental results indicate that it is possible to cultivate physiological male L. crocea by inducing sex reversal in sexually mature females through gonadal differentiation and development.