Feathers in the male Sebright bantam chicken are feminized as the result of a mutation that causes an increase in the capacity for estrogen synthesis (aromatase activity) in skin. To determine when during development the increased aromatase activity is expressed, we measured the enzyme in tissue slices of Sebright bantam and control bantam chickens on days 3, 12, and 18 of embryogenesis, on day 1 after hatching, and in mature birds. An assay was used that measures the release of 3H2O from [1β-3H]testosterone during the aromatization process. Aromatase activity was expressed in the 3-day-old Sebright embryos, but was not detectable in control embryos of the same age. In older embryos and birds, aromatase activity was assayed in multiple tissues. Enzyme activity was detectable in all tissues examined from day 12 and day 18 Sebright embryos. In control embryos, aromatase was detectable in ovarian but not in extraglandular tissues. The enzyme activity in most tissues of the Sebright decreased within 1 day after hatching and continued to decrease thereafter so that in the adult Sebright, aromatase activity was high only in skin (and skin appendages) and ovary. We conclude that the normal allele of this gene acts to limit the rate of estrogen formation in extraglandular tissues.
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