A Genetic Battle of the Sexes

Opposite sex.
In zebra finches, males (right) express higher levels of sex-chromosome genes than females do.

Arthur Arnold

Sexual inequality is alive and kicking in the bird world. A new study reveals that, unlike humans, birds do little to compensate when one gender gets more of a particular sex chromosome than another. As a result, male birds express sex-linked genes at up to 40% higher levels than females--an imbalance biologists had assumed would be lethal.

Human males get short-changed when it comes to X chromosomes. Stuck with an X and a Y, they inherit half as much X DNA as their XX female counterparts. To even things out, Mother Nature shuts off one X chromosome in each female cell--a phenomenon known as dosage compensation. Other animals, such as flies and nematodes, adopt similar strategies. Previous studies have indicated that birds--where males double up on the "Z" sex chromosome and females are "ZW"--may not follow suit. But the results have been inconsistent.

Now a team of researchers at the University of California, Los Angeles, may have settled the debate--and quite by accident. Physiological scientist Arthur Arnold and colleagues were probing the genetic basis of brain differences between songbird genders when they came upon a puzzling finding: Z-linked genes seemed to be expressed at much higher levels in males than in females. So Arnold's team dug deeper.

The researchers compared the expression of sex chromosome genes in zebra finches, chickens, mice, and humans using gene chips called microarrays. As expected, mammals showed little gender differences in the expression of X-chromosome genes. In contrast, males of both bird species expressed Z-chromosome genes at levels that were up to 40% higher than those in females, the team reports today in the Journal of Biology. Still, some Z genes showed little difference between males and females, notes Arnold, indicating that birds do employ a mild form of dosage compensation. But it's likely a selective process that only affects "the Z genes that would cause big problems," he says.

Biologist James Birchler of the University of Missouri, Columbia, says that the findings are surprising because the Z chromosome comprises about 7% to 10% of the avian genome. "One might think that such a large number of uncompensated genes would be detrimental." Clearly, in birds, it's not. And because the ZW system of chromosomes evolved before the XY, he notes, the study may provide important clues to the evolution of dosage compensation.

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