Men and women, according to popular self-help books, hail from different planets, but what really separates them are radically different chromosomes -- with two Xs you're a female, with one X and a tiny Y, a male. No other pair of chromosomes is nearly as diverse. How did the sex chromosomes become so dissimilar?
In the October 29 issue of Science, Bruce Lahn, Ph.D., assistant professor of human genetics at the University of Chicago and an expert on the evolution of the sex chromosomes, and his colleague David Page, M.D., of the Whitehead Institute, report that the X and Y chromosomes -- which arose from a pair of identical, non sex-determining chromosomes (known as autosomes) -- diverged from each other over the course of about 300 million years by going through four discrete stages rather than in a smooth transition.
"By fossil digging on the sex chromosomes, we were able to reconstruct the four events that drove sex chromosomes into their distinctive X and Y forms, and to date when these events occurred during evolution," says Lahn. "The farther back in time we look, the more similar X and Y appear, boosting the theory that they arose from a pair of identical autosomes."
Sex was not always determined by DNA. In many reptiles, the temperature at which the eggs are incubated determines the sex of the offspring. But when warm-blooded mammals with internal reproduction arose, sex determination by temperature became problematic. Shortly after mammals branched off from reptiles, approximately 300 million years ago, a regular pair of autosomes began evolving into what would become the modern X and Y chromosomes.
Many genes on the human Y chromosome have homologues (analogous genes) on the X chromosome. The presence of these X-Y genes reinforces the idea that the Y chromosome developed from an X-like ancestor.
To Lahn and Page, these X-Y genes serve as the "fossils" they can use to help
Contact: Sharon Parmet
University of Chicago Medical Center