Chicken sex doesn't work like ours. No, not that sex – but the process by which an embryo becomes a recognizably male or female animal.

Unlike mammals, it's not hormones that dictate a chicken's sex. It's a fundamental property of the cells themselves. But this only became apparent when biologists investigated several odd chickens that were half male and half female, as if a line were drawn down the center of their bodies.

"We assumed this was caused by one side of the body having some kind of sex chromosome anomaly," said Michael Clinton, a University of Edinburgh developmental biologist and co-author of the study, described March 10 in Nature. "But when we looked at them closely, they were composed of entirely normal cells. We realized that birds don't follow the mammalian model."

In mammals, there are two types of sex-determining chromosomes, X and Y. Each cell in an embryo has a pair of chromosomes, either XX or XY, but the cells are otherwise identical. Then, early in development, in response to some environmental cue, a group of cells that will someday become ovaries or testes start to produce hormones that cause other cells to develop in male- or female-specific ways. It's the hormones that matter: Exposed to lots of testosterone and deprived of estrogen, cells with female chromosomes will form masculine tissues, and vice versa.

There are a few oddball species such as the duck-billed platypus which has a whopping 10 sex chromosomes, making males XYXYXYXYXY. But the mammalian system was thought to represent a general rule among vertebrate species. And though birds have Z and W chromosomes rather than X and Y, and ZZ is male rather than female, they were thought to follow this rule, too.

That's why Clinton, along with fellow Edinburgh biologists Debiao Zhao and Derek McBrid, expected to find chromosomal malfunction in their half-female, half-male chickens, known as gynandromorphs. But the cells were perfectly normal. They just happened to be organized according to sex: cells with ZZ chromosomes on the male side, and cells with ZW chromosomes on the female side.

As cells on both sides of the body were exposed to the same hormones, it wasn't hormones that mattered to gender, as with mammals. Gender was a fundamental property of the cells.

"These funky chickens, oddities of nature that they are, will provide new perspectives on questions of sexual identity long thought to have been resolved," wrote Duke University cell biologists Lindsey Barske and Blanche Capel in a Nature commentary accompanying the findings.

About one in 10,000 birds is gynandromorphic, but biologists assumed the mammal model applied to all vertebrates, said Clinton.

To test the proposition, the researchers transplanted male cells into a female embryo, and female cells into a male embryo. In both cases, the cells continued to express their sex-specific hormones. Their fate was already set.

The findings expand on earlier research by University of California, Los Angeles biologist Arthur Arnold, who has studied the brains of gynandromorphic zebra finches. They also fit with long-established observations that heavy hormone doses can only change the sex of chicken embryos, but only as long as the dose is maintained. Take the hormones away, and the chickens revert to their intended form.

The big question is whether this kind of cell-based sexual identity will turn out to be a common sex-determining system in other vertebrates, write Barske and Capel.

Clinton suspects it will. "We believe now that certainly all birds, and possibly lower vertebrates, will have a cellular identity," he said. "Remnants of this cellular system may still exist in mammals, but it's overridden by the effects of hormones."

Images: 1) Gyandromorph chicken reflected in mirror; male side white, female side brown./Roslin Institute, University of Edinburgh. 2) Gyandromorph chicken./Roslin Institute, University of Edinburgh.

*Thanks to Ed Yong for "half-cocked."

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See Also:

Citations: "Somatic sex identity is cell autonomous in the chicken." By D. Zhao, D. McBride, S. Nandi, H. A. McQueen, M. J. McGrew, P. M. Hocking, P. D. Lewis, H. M. Sang & M. Clinton. Nature, Vol. 464 No. 7285, March 11, 2010.

"An avian sexual revolution." By Lindsey A. Barske and Blanche Capel. Nature, Vol. 464 No. 7285, March 11, 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.