Ultrastructure and androgen analysis during sex inversion of Brycon orbignyanus reared in captivity.

Piracanjuba, Brycon orbignyanus, is a neotropical gonochoristic species of great ecological and commercial interest. When cultivated in captivity, it sexually differentiates into females, primary males, and secondary males that originate from females that undergo sex inversion. Therefore, the objective of this study was to verify the plasma profile of the androgens testosterone (T) and 11-ketotestosterone (11-KT) throughout the sex differentiation of B. orbignyanus maintained in captivity and ultrastructurally verifying the basement membrane in specimens that underwent natural sex inversion. For this purpose, monthly collections (n = 15/month) were carried out from February to December 2018, and samples were collected from two months to one year of age. The specimens were euthanized, and blood, gonads and liver were collected. The plasma was subjected to steroid analysis, and the androgens T and 11-KT were quantified. The gonadosomatic (GSI) and hepatosomatic (HSI) indices were measured to assess the reproductive stages of fish, and the gonads were subjected to histological and ultrastructural analyses. Among all the animals analyzed, undifferentiated specimens, females in early differentiation, intersex specimens, males in early differentiation, and females with functional ovaries were identified, all of which were considered immature, in addition to males with functional testes considered able to produce sperm. No significant differences were found in the plasma T profile, while males able to produce sperm and intersex individuals had higher plasma concentrations of 11-KT than the other groups analyzed. Thus, it is suggested that sex inversion in females in Brycon orbignyanus is driven by the action of the androgen 11-KT.