By Susan Milius
On rare occasions chickens don’t cross the road; they get stuck right in the middle, between male and female. A new study of these odd birds sheds light on the developmental processes that determine sex.
Contrary to an old view of sexual development, Michael Clinton and his colleagues at the Roslin Institute in Edinburgh say in the March 11 Nature, individual chicken cells can maintain their own strong male or female identities during development instead of being directed by hormones.
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Clinton says his research group ended up considering hormones and sexual identity in the course of studying three peculiar chickens donated to the Roslin Institute. Each bird looked like a rooster on one side, with a long wattle jiggling under its chin, robust legs and bulging muscles. The other half of the same bird — the right side on two birds and the left on the third — had the darker plumage, reduced wattle and dainty ankles of a hen.
Such male-female mashups, called gynandromorphs, have turned up spontaneously in zebra finches, pigeons and parrots as well as in other kinds of animals, Clinton says. These cases challenge the traditional view that genetics takes a back seat to hormonal signals in guiding vertebrate sexual differentiation.