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Effects of embryonic treatment with fadrozole on phenotype of gonads, syrinx, and neural song system in zebra finches.

Previous studies have found that treatment of zebra finch embryos with an aromatase inhibitor on Day 5 or 8 of incubation caused partial sex reversal of gonadal phenotype in females. These females possessed both testicular and ovarian tissue, and the development of the neural circuit for song remained feminine. The present study attempted more complete gonadal reversal by treating zebra finch embryos earlier, on Day 3 of incubation, with Fadrozole (CGS 16949A), an aromatase inhibitor, or with saline. We examined the phenotype of the syrinx (androgen-dependent vocal organ), the gonads, and the telencephallic neural song system in 100-day-old birds. Treated females typically possessed a left ovotestis and a right testis, and significantly larger syringes than control females. The histology and steroid synthetic enzyme activity of the testicular tissue in treated females were quite masculine and similar to that of control males. At the time of sacrifice, the plasma concentrations of testosterone and estradiol for fadrozole-treated females did not differ from those of control females, but dihydrotestosterone was lower in treated females. Despite the large amount of functional testicular tissue and a masculine syrinx, the volumes and soma sizes of song system nuclei (HVC, RA) in treated females remained feminine. These results suggest that testicular secretions masculinize the syrinx, but are not sufficient to masculinize the song system in zebra finches.