Gonad development examination of high-temperature–treated genetically female Nile tilapia

Abstract

Nile tilapia (Oreochromis niloticus) is a kind of GSD + TE (genotypic sex determination with temperature effects) fish. It is known that in some reptiles with temperature dependent sex determination (TSD), the gonads are morphologically undifferentiated and bipotential at the start of both Male- and Female-producing temperature (MPT and FPT) cultivation and develop into ovaries and testes while incubating at FPT and MPT, respectively. However, we did not know whether the gonads of high-temperature–treated GSD + TE fish developed in the same way as those of some reptiles with TSD. Here, the 9 dpf (days post fertilization) Nile tilapia fry was cultured at 36 °C for 12 days for the induction of XX female sex reversal, and the gonads from 21, 39 and 99 dpf high-temperature–treated Nile tilapia were used for immunohistochemical analysis. VASA immunohistochemistry indicated that there were oogonia in 21 dpf XX and XX + HT (XX with high-temperature treatments) gonads and gonocytes in 21 dpf XY gonads. CYP19A1A was expressed in 21 dpf XX and XX + HT gonads but was not expressed in 21 dpf XY gonads. These results indicate that the gonads of XX + HT fish are ovary-like at 21 dpf. The immunohistochemistry results for VASA, CYP19A1A and DMRT1 showed that 39 dpf XX + HT gonads contained male germ cells and pre-Sertoli cells but did not contain female somatic cells. Therefore, we concluded that XX + HT gonads transformed from ovary-like to testis-like during 21–39 dpf. In 99 dpf XX + HT gonad, germ cells, such as spermatogonia, spermatocytes and spermatids, and male somatic cells, such as Sertoli cells surrounding the spermatogonia, could be observed. The gonads of high-temperature–treated Nile tilapia became true testes at 99 dpf. This is the first report regarding the gonad cell transformation process during high-temperature–induced sex reversal in fish species. The present study may provide an important foundation for understanding the mechanisms of high-temperature–induced Nile tilapia female sex reversal.