O. Nakabayashi et al., DIFFERENTIAL EXPRESSION OF GENES FOR AROMATASE AND ESTROGEN-RECEPTOR DURING THE GONADAL DEVELOPMENT IN CHICKEN EMBRYOS, Journal of molecular endocrinology, 20(2), 1998, pp. 193-202
In birds, differentiation of embryonic gonads is not as strictly deter
mined by the genetic sex as it is in mammals, and can be influenced by
early manipulation with a sex steroid hormone. Thus administration of
an aromatase inhibitor induces testis development in the genetic fema
le, and administration of estrogen induces a left ovotestis in the gen
etic male embryo. Another feature of avian gonadogenesis is that only
the left ovary develops in most species. Molecular mechanisms underlyi
ng these features at the lei el of gene expression have not been eluci
dated. In this paper, we present evidence that a gene for aromatase cy
tochrome P-450, an enzyme required for the last step in the synthesis
of estradiol-17 beta, is expressed in medullae of the left and right g
onads of a female chicken embryo, but not in those of a male chicken e
mbryo, and that an estrogen receptor gene is expressed only in epithel
ium (and cortex later, in the female) of the left, not the right, gona
d of both sexes, but the expression in the male left gonad is temporar
y and restricted to an early stage of des development. Differential ex
pression of these two genes serves well to explain the above features
of gonadal development in birds. Furthermore, in ovo administration of
estradiol-17 beta from the 5th to the 14th day of incubation does not
cause expression of the estrogen receptor gene in the right gonad of
chicken embryos of either sex, suggesting that the absence of expressi
on of the estrogen receptor gene in the right gonad is not the result
of down-regulation, but may be regarded as an important cause of the u
nilateral ovarian development.