The third and youngest sex chromosome is different. The Y is not yet shriveled, though it contains many non-functional genes about half the total that will eventually be lost. The X, which is dubbed neo-X, is undergoing rapid change, however, with about 10 times the normal amount of adaptation seen in the autosomes, according to the researchers.
By adaptation, Bachtrog means that the gene sequences in the X chromosome are becoming fixed as random mutations have finally settled on a few beneficial changes that accommodate the increasingly irrelevant Y chromosome. Between 10 and 15 percent of neo-X genes show adaptation, compared to only 1-3 percent of autosome genes.
"In hindsight, that is not surprising," Bachtrog said. "Neo-X is facing a much more challenging situation than the autosomes because its pair, the Y chromosome, is degenerating. Its genes are no longer producing proteins, so neo-X has to compensate by up-regulating its genes. We find a lot of genes on the X chromosome are involved in dosage compensation."
In humans, for example, all genes on the X chromosome are twice as active to account for the lack of genes on the Y. Women accommodate this by inactivating one entire X chromosome so as not to produce too much protein, Bachtrog said.
Another change in neo-X that Bachtrog suspects is taking place is the elimination of genes that are harmful to females. Biologists have realized recently that some genes have opposite effects in males and females, and evolution is a tug of war between males jettisoning genes that they find detrimental only to have females put them back, and vice versa.
"A good place to put sexually antagonistic genes that are beneficial to one sex but detrimental to the other is on the sex chromosomes," she said. The Y always ends up in the male, she said, so genes on the Y chromosome
|Contact: Robert Sanders|
University of California - Berkeley