Combined estrogen and elevated temperature treatments induce feminization in Nile tilapia, Oreochromis niloticus

Nile tilapia has proven to be sensitive to change phenotypic sex upon exposure to steroid hormone and high temperature during the critical period of gonad differentiation. It is more difficult to feminize XY males than to masculinize XX females. High temperature feminization has also been observed in very few cases in Nile tilapia. Therefore, this study was designed to increase the feminization rates using combined treatments of estrogen hormone (either diethylstilbestrol, DES, or 17α-ethynylestradiol, EE2) and high temperature (36 oC) during the sensitive period of gonad differentiation in Nile tilapia. The parents genotype of the crosses was XX♀ × XY♂; XX♀ × YY♂ and XX♀ × XX♂. Progenies from each cross were exposed to one of six types of treatments: (i) combined treatment of estrogen hormone (DES or EE2) and high temperature (36 oC) (ii) hormone (DES or EE2) alone as positive control (iii) 36 oC as positive control and (iv) 28 oC as negative control. The sexual genotype was confirmed using tightly sex-linked markers located in linkage groups (LG) 1, LG20 and LG23. Both of the combined treatments of estrogen hormone (either DES or EE2) and high temperature induced significant feminization compared with hormone alone in all the crosses except XX (all female progeny). The combined hormone and high temperature treatment using DES showed 100% feminization rate in mixed sex group. On the other hand, high temperature treatment with EE2 showed 75% (100 mg/kg feed) and 65% (150 mg/kg feed) feminization rate in mixed sex group whereas in XY progeny group c. 19% (100 mg/kg feed) and c. 22% (150 mg/kg feed) feminization percentage were observed. However, the survival rate in the negative control ranged from 79% to 96% whereas the EE2-high temperature combined treatment showed higher survival rate (ranged from 79% to 94%) than in the DES-high temperature combined treatment (46.88% in mixed sex group). Similarly, survival rate was also higher in EE2 alone (ranged from 73% to 96%) than in DES alone treatment (c. 66%). DES hormone alone showed higher percentage of feminization (57.14%) than EE2 whereas EE2 hormone alone showed variable results and induced feminization (38.89%) in only one of the batches produced (mixed sex progeny treated with 150 mg/kg feed). High temperature masculinization was observed in one of the mixed sex crosses produced (40%) and in the XX cross (30%). This is the first study demonstrating that combined treatment of estrogen hormone and elevated temperature induced higher feminization rates in Nile tilapia than hormone alone, despite the generally opposite effect of the estrogen hormone and high temperature on gonad differentiation pathways in Nile tilapia.