By using human and chimpanzee Y chromosomes as a geneticfossil record to examine our past, scientists have seen a surprisingdifference in the way the male-making chromosomes from the two speciescope with the inexorable pressures of evolution.
After comparingthe Y chromosome sequences of the human and chimpanzee, Howard HughesMedical Institute researcher David C. Page, postdoctoral fellowJennifer F. Hughes and their colleagues have seen evidence that thehuman chromosome has found a way to stabilize itself and survive in theroughly 6 million years since humans and chimpanzees have beenfollowing different evolutionary paths. In other words, the human Ychromosome is not rotting away, as some scientists have argued.
Incontrast, the chimpanzee Y chromosome is not faring quite as well.Studying the same family of single-copy genes shows that the chimp's Yhas been accumulating mutations that are gradually making some of itsgenes useless.
About 6 million years ago humans and chimps shareda common ancestor, Page explained. Since then the chimp genome and thehuman genome have evolved along separate paths. Why natural selectionhas created such differences in their Y chromosomes is not clear.
Page,who is interim director of the Whitehead Institute for BiomedicalResearch, collaborated with genome researchers at Washington UniversitySchool of Medicine in St. Louis. A detailed report on their findingsappears in the September 1, 2005, issue of the journal Nature. Page'sco-authors at the Whitehead Institute included Hughes, Helen Skaletsky,Tatyana Pyntikova and Steve Rozen. Washington University team memberswere Patrick J. Minx, Tina Graves and Richard K. Wilson, who are at theGenome Sequencing Center.
“The big idea has been that the Ychromosome's genes are going away — that it's not long for this world,”Page explained. But these new data — based on comparing the chemicalsequence, the “spelling” of single genes found on the two primatespecies' Y chromosomes — indicates that the chimp Y is falling behind,while the human Y is maintaining the status quo.
“So the bottomline is that the human Y looks to be doing a better job of preservingits single genes than anyone expected. And it looks like naturalselection can work very well in Y chromosomes, even though they cannotswap genes with a counterpart chromosome,” Page said. For the otherso-called autosomal chromosomes, the swapping of genes between matchingpairs from parents is considered important to maintain the health ofthe genome. This process gets rid of mutant genes and provides new genecombinations to improve the overall genetic “vigor” of the species.
TheY chromosome genes the researchers studied occur in single copies, andmake substances used throughout the body to perform basic functions incells.
In 2003, Page and his colleagues reported that another setof genes on the Y, those that are active only in the testes and makethe sperm, come in pairs and somehow repair or replace themselves.
“Atthat time, we were coming to the defense of the Y chromosome,” Pagesaid. “But it turns out that the human Y chromosome is far more able todefend itself because it carries spares — back-up copies — of thesetestis-expressing genes.” As part of the mating game, chimpanzees makemuch more sperm than humans do, so their testes need to be operating athigh capacity.
As a result of the newest research, Page added,“we can also draw some inferences about that common ancestor” shared byhumans and chimps some 6 million years ago. By examining and comparingthe Y chromosomes from both species, “we're getting a gene catalog ofthe common ancestor's Y chromosome” even though that ancestor livedlong ago and cannot be studied directly. Evidence suggests, too, thatthe X and Y sex chromosomes became separate and distinct from theautosomes, the other type of chromosomes, about 300 million years ago.
Unlikethe autosomes, the X and Y do not pair up and exchange genes duringreproduction. As a result, the Y chromosome remains essentially stable- a clone from deep in the past - carried down through eons of time.Yet when damaging mutations do occur, the Y chromosome is stuck withthem, having no means to reshuffle its genes to jettison the junk.
Pageand his colleagues wrote that their new studies suggest the human Ychromosome is able to cleanse itself of genetic errors by a processthey call “purifying selection.” In fact, mathematical models have beenproposed which suggest how this occurs - pointing to a slowing of therate of gene decay for the human Y chromosome late in evolution.
“Thesefindings also suggest that `purifying selection' on the Y chromosomehas been more effective during recent human evolution than previouslysupposed,” the researchers conclude.
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