The P450 aromatase (P450 arom) gene is asymmetrically expressed in a critical period for gonadal sexual differentiation in the chick

Steroid hormones appear to play an important role in gonadal sex differenti ation of birds. Here we studied the steady-state level of the P450 arom mRN A by reverse transcriptase polymerase chain reaction (RT-PCR) in the left a nd the right presumptive ovary and testis of developing chicken embryos. Th e gonads were evaluated every hour during the undifferentiated period, at 1 44-156 h of incubation (h/i), and every 24 h after sexual differentiation a t 168 and 192 h/i. Activity of P450 arom was determined by estrone producti on from [H-3]androstenedione at 144-192 h/i. Moreover, morphological develo pment of the gonad was also examined by light microscopy. Results show that onset of P450 arom mRNA and its protein activity were simultaneously detec ted in the left and the right ovaries at 147 h/i. Asymmetric function of P4 50 arom gene expression was observed at 156 h/i when morphological gonadal differentiation is first recognized. Biotransformation of [H-3]androstenedi one to estrone was also asymmetrically detected between the left and right gonad at 156 h/i and asymmetry was maintained throughout the analyzed stage s. It is proposed that there is a gene in birds that is asymmetrically expr essed in the undifferentiated stage of the female and the male gonad. In th e female this gene could promote P450 arom gene expression, increasing estr ogen production, which in turn could induce ovarian cortex proliferation an d expression of other structural estrogen-regulated genes involved in ovari an sexual determination. (C) 2000 Academic Press.