Sexual differentiation of the zebra pinch song system parallels genetic, not gonadal, sex

Mechanisms regulating sexual differentiation of the zebra finch song system present an intriguing puzzle. Masculine development of brain regions and b ehavior can be induced in genetic females by posthatching estradiol treatme nt. That result is consistent with the hypothesis that estradiol, converted within the brain from testicular androgen via the aromatase enzyme, mascul inizes neural structure and function. In contrast, treatment during specifi c stages of development with the aromatase inhibitor Fadrozole has not prev ented masculine development, and the presence of testicular tissue in genet ic females did not induce masculine organization of neuroanatomy or singing behavior. Fadrozole treatments in those previous studies were limited, how ever, and most genetic females had both ovarian and testicular tissue. The present experiments were designed to provide increased aromatase inhibition and to reliably produce genetic females with only testicular tissue. Eggs received a single injection at a later age or with higher doses of Fadrozol e than had been used previously. Some embryos were exposed to Fadrozole mor e frequently by either injecting eggs on 2 days of development or dipping t hem for 10-12 days in Fadrozole. Finally, in some individuals from Fadrozol e-treated eggs, the left gonad was removed, leaving each genetic male and f emale with a single right testis. None of these treatments significantly af fected development of the song system compared to appropriate control group s. These results suggest that sexual differentiation of the zebra finch son g system is not regulated by embryonic aromatase activity or by gonadal sec retions and instead involves events that need not be mediated by steroid ho rmones. (C) 1999 Academic Press.