Cloning of brain aromatase gene and expression of brain and ovarian aromatase genes during sexual differentiation in genetic male and female Nile tilapia Oreochromis niloticus

A brain aromatase gene was identified from the Nile tilapia Oreochromis nil oticus. The cDNA sequence of this gene differed from that of the ovarian ar omatase gene previously reported from this species. Tissue specific express ion for both brain and ovarian aromatase genes was examined in the tissues of adult tilapia. Brain aromatase mRNA was expressed in the brain, kidney, eye, ovary, and testis, but not in the liver and spleen. Ovarian aromatase mRNA was expressed in the brain, spleen, ovary, and testis but not in the e ye, kidney, and liver. Differential aromatase gene expression between the s exes was investigated in all-male (XY) and all-female (XX) groups of tilapi a fry from fertilisation throughout the sexual differentiation period. Semi -quantitative RT-PCR analysis revealed that the initiation of expression of both aromatase genes lay between 3 and 4 dpf (days post fertilisation) in both sexes. The level of brain aromatase mRNA gradually increased throughou t the period studied with little difference between the sexes. This contras ted with marked sexual dimorphism of ovarian aromatase mRNA expression. In females, the expression level was maintained or increased gradually through out ontogeny, while the level in males was dramatically down-regulated betw een 15 and 27 dpf. Subsequently, the level of ovarian aromatase mRNA expres sion fluctuated slightly in both sexes, with the expression in females alwa ys being higher than in males. These findings clearly suggest that ovarian aromatase plays a decisive role in sexual differentiation in this species a nd that this is achieved by downregulation of the expression of this gene i n males. (C) 2001 Wiley-Liss, Inc.