Tracing the evolution of sex chromosomes

Of the 46 human chromosomes, 44 are members of identical pairs. But two--the X and the Y--stand apart because they have no perfect match. Nevertheless, evolution has charged these two genetic loners with the critical task of sex determination: embryos with two X chromosomes develop into females, while embryos with an X and a Y chromosome develop into males.

The rationale behind this arrangement has long perplexed scientists like David Page, a member of the Whitehead Institute and an expert on the Y chromosome, who wonder how these unique chromosomes first came about. Now research by Dr. Page indicates that sex chromosomes descended from an ordinary pair of autosomes that over hundreds of millions of years evolved into the modern X and Y chromosomes. In a study published in Friday's issue of Science, Dr. Page and Bruce Lahn, then a postdoctoral fellow at the Whitehead Institute, report that they have reconstructed the stages of sex chromosome evolution and the time course over which these chromosomes were built. This research provides a glimpse back in time and a means to understand how sex chromosomes--one of nature's greatest experiments--first emerged.

Millions of years ago sex was probably determined not by sex chromosomes, but by some environmental factor, like the temperature of the water at which the egg incubated. (Sex determination still occurs this way in some animals like crocodiles and sea turtles.) Over the years, a pair of autosomes differentiated into two distinct chromosomes, the X and the Y. Today the X is still home to thousands of genes, but the Y has only a measly few dozen. Of those, 19 are shared between the X and Y.

These 19 genes are essentially living fossils, in that they are able to provide scientists with information about the history of sex chromosomes, says Dr. Page, who is also an investigator at the Howard Hughes Medical Institute. "Sex chromosomes are the only chromosomes with a long intellectual history,

Contact: Stefanie Doebler
Whitehead Institute for Biomedical Research

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