Sex steroids, sex chromosome genes, and a gynandromorphic finch: Novel mechanisms of brain sexual differentiation.

摘要

Sexual differentiation of the neural song system in zebra finches is influenced by the actions of the sex steroid hormone estrogen. The source of this estrogen may not be the gonads, but the brain itself where the estrogen synthesizing enzyme, aromatase, is abundantly expressed. Steroidogenic factor 1 is a major regulator of aromatase in the gonads and is likely involved in gonadal sexual differentiation. We have examined its expression in the brain to see if it may also be involved in regulating aromatase in this tissue, possibly in a sexually dimorphic manner. We found SF1 expression only in the hypothalamus and not in the telencephalon where several song nuclei reside and where most aromatase is expressed. Thus, sex differences in estrogen production likely do not result from differences in SF1 regulation of aromatase.; Sex differences in the brain may also arise because of inherent genetic differences in the cells of males and females. The sex chromosomes in males are ZZ and females ZW. Therefore, males have twice as many Z genes as females, and females have W genes that males do not. We have examined the first W sex chromosome gene discovered, CHDW, and its closely related homologue on the Z sex chromosome, CHDZ, to determine if sex differences in their properties have evolved. A comparison of the mRNA sequence, expression, and regulatory regions of these genes indicates they have diverged substantially and may have evolved functional differences, possibly for sex-specific roles.; Inherent genetic differences between males and females do appear to influence sexual differentiation of song, based on our examination of a bilateral gynandromorphic zebra finch that was genetically male on its right side and genetically female on its left. This genetic lateralization included the brain. The finch sang a normal male song and had a masculine song system on both sides, but some nuclei on the genetically male side were more masculine than on the genetically female side. This difference was much larger than normal variation in control males. Because both sides of the brain developed within the same hormonal milieu, the lateral differences indicate that sexual differentiation of the brain was controlled by genetic factors intrinsic to the brain. Instead, genetic differences such as sex chromosome gene expression, is suggested.