Evolutionary Divergence

Bruce T. Lahn worked with David Page to determine Y's curious evolution.
Photo: Mark Segal

While tracking genes in human offspring, Page and other investigators are also pondering where Y itself got its genes. To trace Y's lineage, researchers could not ignore this chromosome's heftier counterpart, the X. The two chromosomes have quite a history, coexisting as equal partners without specialty some 240-300 million years ago, only to part ways around 170 million years ago—dates arrived at by Page and Lahn. The Y chromosome map that Page's lab released in 1992 aided the investigators' trip back in time. An article in Science late last year detailed how they determined the time intervals and the incidents that caused two autosomes—chromosomes not involved in sex determination—to differentiate into the mismatched set of sex chromosomes we have today.

The project originally started in a different direction, Lahn recalls. He was looking at the degree of sequence similarity and dissimilarity between genes on the Y chromosome and those on the X and other chromosomes. The high degree of correlation between X and Y got his attention. "Lo and behold, a striking pattern emerged that could readily explain the progression of X-Y divergence," Lahn says.

The split between X and Y took place over millions of years. The chromosomes became less and less alike in four instances of genetic reshuffling on the Y. Most of the genes on our modern X existed before the breakup, but their counterparts on Y gradually degenerated. In mammals, the 22 other pairs of chromosomes swap genetic material, through the process called recombination which occurs during meiosis, the cell division that takes place in ovaries and testes before half of each pair is sequestered into either a sperm or an egg. In human females, XX pairs do this without problem. During sperm formation in males, X and Y still exchange some DNA, but with only a few genes. These reside in Y's "pseudoautosomal" region—the address for genes that Y has in common with X. The rest of Y's genes belong to the genetic equivalent of a boys-only club.

Proof exists that genes came from elsewhere to join this all-male chromosome. One such gene is DAZ. In the same study that identified DAZ, the Page group identified a cousin called DAZL on chromosome 3. Evidence points to DAZL as the original gene, a copy of which made its way to Y. Page and the other investigators wrote in their research report that the existence of a Y-gene precursor challenges theories that X was the source for most or all of Y's genes. While many of Y's genes do have homologs on X, the researchers suggest that Y's evolution had other outside influences.

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