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The story of X -- evolution of a sex chromosome
Date:4/16/2009

that probably took place about 150 million years ago, while in the fruit fly Drosophila melanogaster, a laboratory favorite, the sex chromosomes arose independently about 100 million years ago. In both humans and fruit flies, the Y chromosome has dwindled from a few thousand genes to a few dozen.

Hence the intense interest in why and how the Y chromosome lost genes once it stopped interacting with the X. Scientists have found that, as the only chromosome pair that doesn't break and recombine every time a cell divides, the XY pair in males is unable to take advantage of the main way deleterious genetic mutations are eliminated. The XX pair in females does recombine, but for the Y, the only way to get rid of a bad mutation in a gene is to inactivate or delete the entire gene. Over millions of years, inactive genes are lost, and the Y shrinks.

"If you have no recombination, natural selection is less effective at removing detrimental genes," said Bachtrog. "Y is an asexual chromosome, and it pays a price for that: It keeps losing genes."

Bachtrog, whose career has revolved mostly around the study of the degeneration of the Y chromosome, decided to focus on the X chromosome several years ago and went about searching for sex chromosome pairs that have arisen more recently and thus might be in the process of adapting to their new role. Her paper centers around study of the three sex chromosomes in a rare western fruit fly, Drosophila miranda, a darker-colored cousin of D. melanogaster. (Many creatures have more than one pair of sex chromosomes; the platypus, for example, has five pairs, all inherited together.)

While one of D. miranda's sex chromosomes is descended from the original sex chromosome that appeared in Drosophila nearly 100 million years ago, a second originated perhaps 10 million years ago, and the third about a million years ago. The older two look much alike, Bachtrog said: The Y chromosome in each pair has lost genes to
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Contact: Robert Sanders
rsanders@berkeley.edu
510-643-6998
University of California - Berkeley
Source:Eurekalert

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