Molecular and Morphological Changes in Nile Tilapia (oreochromis Niloticus) Gonads During High-Temperature-induced Masculinization

Genetic sex of Nile tilapia (Oreochromis niloticus) is determined by genotype (genetic sex determination). However, these fish are temperature-sensitive, and high-temperature-induced sex reversal is a common practice in research and tilapia farming. In this study, we characterized the molecular and morphological sex differentiation of Nile tilapia during the process of sex reversal induced by high temperatures. The offspring of parental pairs of XX and XY Nile tilapia underwent high-temperature treatment for 20 d beginning at 5 dpf (days post fertilization). Gonads were isolated from Nile tilapia at seven time points between 10-180 dpf. The histology results showed that gonads of high-temperature-induced XX pseudomales exhibited morphological differences from XX females during 25-35 dpf. qRT-PCR analysis revealed that the expression levels of cyp19a1a and foxl2 were temperature-sensitive, and were lowly expressed in the gonads of pseudomales from 35 dpf onward. Such findings were also consistent with the histological observations of gonadal morphological differentiation. The expression of the TGF beta superfamily member, gsdf, was increased in the gonads of pseudomales from 12 dpf onward. The transcription of kdm6bb was elevated by high-temperature treatment at 10-25 dpf and displayed sexual dimorphism from 35 dpf onward. Dmrt1 was specifically expressed in phenotypic males from 12 dpf onward. Collectively, this study identified the roles of specific genes involved in the process of high-temperature-induced masculinization in Nile tilapia and established robust molecular markers of phenotypic sex in Nile tilapia that will be useful for ongoing research related to sex control.