Study reveals sex chromosomes 'not as isolated as first thought'
14 September 2009
Researchers from the University of Leicester have shown that the X and Y chromosomes exchange DNA in regions previously thought to be inactive during the process of genetic recombination, which takes place during cell division.
They suggest this may be a way for the Y chromosome to fix harmful mutations that might affect male fertility.
In cells, our genetic material usually exists as chromosomes - condensed bodies of DNA. During cell division, the chromosomes match up in identical pairs, which facilitates an exchange of DNA (known as recombination) that helps make each person distinct. But the sex chromosomes are different.
"Long ago, before mammals evolved, the X and Y chromosomes were an ordinary pair of identical chromosomes, exchanging DNA in a companionable way through the process of genetic recombination," says Professor Mark Jobling, Wellcome Trust Senior Research Fellow at the University of Leicester, who led the study.
"However, once the Y chromosome took on the job of determining maleness, they stopped talking to each other. The X remained much the same, but the Y set out on a path of degeneration that saw it lose many of its genes and shrink to about one-third the size of the X."
Because of the difference, genetic recombination was thought to be restricted to two specialised regions at the ends of the X and Y chromosomes. Most of the Y chromosome was thought to be completely isolated from any other part of the genome.
"Some scientists have predicted that the Y chromosome will eventually vanish altogether," said Professor Jobling. "But our new findings challenge this interpretation of the Y chromosome's fate."
Professor Jobling's team looked at the X and Y chromosomes of 12 men of different heritages, analysing particular regions and comparing them with the reference sequences from the original Human Genome Project.
Most of the men's Y chromosome sequences were identical; however, one from a Namibian man showed differences that could only be explained by genetic exchange with a matching region from his X chromosome - a region previously thought to be incapable of taking part in the recombination process.
Recent studies have shown that the Y chromosome can swap bits of DNA from one region to another within itself, which could allow it to fix mutations that might affect male fertility.
"The Y chromosome actually maintains a genetic conversation with the X chromosome, potentially giving it a way to fix other kinds of mutations too," said Professor Jobling.
However, the researchers say that because the conversation goes both ways - eight of the 12 X chromosome sequences studied showed evidence of genetic exchange - mutations arising on the Y chromosome can also be passed to the X, with possibly harmful effects.
The team now aims to assess how widespread X-Y exchanges have been during evolution, and what the likely functional effects might be.
Image: Linear array of chromosomes showing the variety of their structure and size. Credit: Kate Whitley, Wellcome Images
Reference
Rosser ZH et al. Gene conversion between the X chromosome and the male-specific region of the Y chromosome at a translocation hotspot. Am J Hum Genet 2009;85:130-4.