Study Dispels Theories of Y Chromosome’s Demise:
Stripped-Down Chromosome Retains Key Genes for Fertility
“Y chromosome has lost 90 percent of genes that once shared with the X chromosome, and some scientists have speculated that the Y chromosome will disappear in less than 5 million years ,” said evolutionary biologist Melissa A. Wilson Sayres a Miller Postdoctoral Fellow in the Department of Integrative Biology , University of California, Berkeley, and lead author of the new analysis .
Some mammals have lost their Y chromosome, although they still have men and women and reproduce normally . Researchers reported some shuffling genes in mice to create Y- less males could produce normal offspring , leading some analysts to wonder if the chromosome is superfluous.
“Our study shows that genes that have remained , and those who migrated from X to Y , that are important , and human and will stay for a long time,” he said.
Wilson Sayres and coauthor Rasmus Nielsen, in PLoS Genetics that patterns of variation in the Y chromosome among the 16 men are consistent with the selection naturally acts to maintain gene content there, many of which have been shown to play a role in male fertility. Insignificant size of the Y chromosome – which contains 27 unique genes in front of thousands of people in the other chromosomes – is a sign that is lean and stripped to essentials.
“The results are quite impressive. They show that because there is a lot of natural selection working on the Y chromosome , which has to be much more depending on the chromosome of people previously thought,” Nielsen said.
Variations in the Y chromosomes are used to track human populations moved around the world, and according to Nielsen , the new research will help improve estimates of the evolutionary history of humans.
” Melissa has shown that this strong negative selection – natural selection to eliminate deleterious genes – tends to make us think the dates are older than they are , which gives very different estimates of the history of our ancestors,” Nielsen said. And it has degraded over the past 200 million
Before about 200 million years ago when mammals were relatively new to the Earth, the first versions of the sex chromosomes, X and Y, were like other pairs of chromosomes in each generation , they swapped a pair of genes for the children were a mix of genes from their parents. Fertilized eggs obtained two proto -X became females and eggs with a proto -X and proto -Y became men.
But for some reason , said Wilson Sayres , the gene that triggers the cascade of events that result in male characteristics became fixed on the Y chromosome and attracted other gene specific , such as those that control the development of the testes men , sperm and semen. Many of them proved to be harmful to women , so that the X and Y stopped exchanging genes and the two chromosomes began to evolve separately.
“Now the X and Y do not exchange DNA over most of its length , which means that Y can not be efficiently fix errors , so degraded over time,” he said. “In XX females , the X still has a partner to exchange with and correct mistakes , so we think the X has also degraded . ”
Wilson Sayres was fascinated by the strange story of the sex chromosomes , and in particular , the lack of genetic variation worldwide in the Y chromosome compared to the range observed in the DNA in the non-sex chromosomes. This variation , although used to trace human history, was poorly characterized in whole chromosome Y. .
” Y chromosomes are more similar to each other than we expect ,” said Wilson Sayres . “There has been some debate about whether this is because there are fewer men who contribute to the next generation , or whether natural selection acts to eliminate variation . ”
Did genes contribute fewer males the Y chromosome ?
UC Berkeley researchers showed that if fewer males were the only cause of low variability , mean that less than 1 in 4 men throughout history had happened in his chromosome each generation. Variations in other human chromosomes , including the X chromosome, making it an unlikely scenario . Instead, showed low variation will be explained by intense natural selection , ie , a strong evolutionary pressure to weed out the bad mutations that eventually cut the chromosome to its essential elements.
” We show that a model of purifying selection acting on the Y chromosome to eliminate harmful mutations , in combination with a moderate reduction in the number of men who are going into their chromosomes and may explain low Y diversity , ” said Wilson Sayres .
The researchers also found that 27 genes on the Y chromosome – 17 humans retained after 200 million years , and 10 genes more recently acquired but little known – are likely to be affected by natural selection . Most new gene, called ampliconic genes are present in multiple copies in the chromosome and the loss of one or more copies has been related to male infertility.
“These ampliconic regions that we have not really understood until now are obviously very important and probably should be researched and studied for fertility ,” he said .
Wilson Sayres was able to accurately measure the variable Y for first comparing the variation in the chromosome of a person with the variation in other 22 pairs of the person (called autosomes ) , the X chromosome and the mitochondrial DNA. She used data from the entire genome of 16 men whose DNA was sequenced by the company based in Mountain View , Complete Genomics Inc. , which is the most accurate of the Y chromosome sequences. The company was recently acquired by BGI , the Genome Institute of Bejing .
Comparative studies of populations of the variation in the Y chromosome are in their infancy , said, noting that more than 36 mammalian genomes sequenced to date, the full Y chromosomes are only available for three. Most human genomes sequenced + 1000 no longer have sufficiently accurate coverage And to make this type of comparison among individuals, but advances in technology to better characterize facilitate future DNA analysis of the Y chromosome , said.