Sex differences in the brain may be genetic and not just hormonal, according to U.S. researchers who gained the insight after studying a rare hermaphrodite finch.

The unique finch had colourful male plumage on its right side and dull female plumage on its left; it also had only one ovary and a single testis. Conclusions drawn from studying it challenge the long-held theory that hormones determine sex differences in the brain function of birds and mammals.

"Our paper shows that the genetic sex of brain cells is partly responsible for determining whether the brain is structurally masculine or feminine." said the team's leader, Professor Art Arnold of the University of California at Los Angeles.

Their work appears in the journal, Proceedings of the National Academy of Sciences.

Sex hormones, produced by ovaries and testis, influence many cells types in the body - from breasts to brains. However, the team wanted to establish if this was the only determinant. If sexual differentiation in the brain depended entirely on hormones, then both sides of the brain would equally contain male and female genes, since both would be exposed to the same hormonal environment.

Arnold and colleagues studied a zebra finch (Taeniopygia guttata) that was born half male and half female. They examined the expression of sex-linked genes in the brain, gonads, muscle and feathers of the finch.

Cells on the right half of the brain and body were genetically male, and those on the left were genetically female. Their results indicate that the sex genes can control the differentiation of cells outside the gonad.

The study has also shed light on the reason why only male zebra finches sing a courtship song. An intricate network of cell nuclei in the brain of birds, called the 'song circuit', is critical for singing.

The development of song until now had largely been attributed to the production of oestrogen in the brain of male birds. However, the researchers found that the cells on the 'masculine' side of the brain's song region were larger than those on the 'feminine' side.

"The importance of this paper, is that up to now we have known that the size of the song nuclei, when they are present, vary with the gonadal hormones," commented Professor Gisela Kaplan, who specialises in animal behaviour at the University of New England in Armidale, Australia.

"This study has shown that it is on-site expression of genes in the song nuclei themselves," she told ABC Science Online.

The work may also have implications for humans, as there are sex differences in some brain diseases, Arnold said in an interview: "If the results on song birds were to hold for humans, we would then have to think that the XX and XY brain cells - and cells elsewhere in the body - are also different, because of the Y or X chromosomes."

In mammals, gender is determined by having either an X or a Y chromosome; females have XX chromosomes while males have XY.

But Kaplan disagreed. She said the results could not be applied to humans because of the specific ability of birds to produce oestrogen in neural cells in the brain. In addition, male birds are XX and the females are effectively XY - the exact opposite of humans.

"As a theoretical suggestion, it is very interesting," Kaplan said. "But they are several light-years away [from extrapolating the results to humans]."

In the paper, Arnold and colleagues noted that further work was needed to resolve how sex hormones and genes interact to produce sex differences in brain function.

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