Männlichkeit: die Lokomotive der Evolution.

deutsche Version der FAZ: siehe hier
From The Times, January 14, 2010


Why the Y chromosome is a hotbed

for evolution

By By Mark Henderson, Science Editor

The Y chromosome is often seen as the rotten corner of the human genome — a place of evolutionary decline that is slowly decaying and threatening the end of man. Reports of its imminent demise, however, have been exaggerated.

Research has indicated that, far from stagnating, the male chromosome is a hotspot of evolution that is changing more quickly than any other part of humanity’s genetic code.

In most mammals the sex of offspring is determined by X and Y chromosomes. Females have two Xs, males have one X and one Y — with the Y making them male. The Y was originally identical to the X, but over 300 million years it has shrunk, and is now the smallest human chromosome.

This is because it occurs on its own, and cannot swap genes to maintain integrity. This decline has led scientists to suggest it would waste away entirely in 125,000 years or so. This would mean the end of men, and probably of humanity.

But the first comparison of the human Y chromosome with its counterpart in chimpanzees has revealed that they differ radically. The findings suggest that the Y chromosomes of both are evolving rapidly and dynamically — probably because of their critical roles in reproduction — and have a vibrant future.

In the new research, which is published in the journal Nature, Dr David Page, of the Massachusetts Institute of Technology — who first sequenced the Y chromosome in 2003 — has now sequenced the Y chromosome of the chimp, humanity’s closest relative, and compared this with the human version.

The scientists expected the two sequences to look very similar. However, while human and chimp DNA generally differ by less than 2 per cent, more than 30 per cent of the Y chromosome differed between the two species.

“This research shows that the Y chromosome isn’t necessarily degrading, but is evolving very fast,” said Professor Robin Lovell-Badge, of the National Institute for Medical Research in London, an expert on the Y chromosome, who was not involved in the study.

Dr Page likened the process to a home that is constantly under renovation. “People are living in the house, but there’s always some room that’s being demolished and reconstructed,” he said. “This is not the norm for the genome as a whole.” Several factors probably account for the rapid evolution of the Y chromosome. First, the trick it uses to repair genes — known as gene conversion — is probably less efficient than the repair mechanisms used elsewhere in the genome. This allows new mutations to arise more often.

These mutations are then subject to greater selective pressure than the rest of the genome — because of the important role of the Y chromosome in sperm production. Any advantageous mutations would be expected to be preserved as they boost fertility, while deleterious ones would be rapidly flushed from the gene pool.

This is supported by the discovery that the parts of the chromosome involved in sperm production are most different between humans and chimps.

Wes Warren, of Washington University in St Louis, Missouri, who contributed to the study, said: “This work clearly shows that the Y is pretty ingenious at using different tools than the rest of the genome to maintain diversity of genes. These findings demonstrate that our knowledge of the Y chromosome is still advancing.”

aus New York Times, January 13, 2010:

Male Chromosome May Evolve



A new look at the human Y chromosome has overturned longstanding ideas about its evolutionary history. Far from being in a state of decay, the Y chromosome is the fastest-changing part of the human genome and is constantly renewing itself.

This is “a result as unexpected as it is stunning — truly amazing,” said Scott Hawley,* a chromosome expert at the Stowers Institute in Kansas City, Mo.

The Y chromosome makes its owner male because it carries the male-determining gene. Boys are born with on have two X’s. The other 22 pairs of chromosomes in which the human genome is packaged are the same in both sexes.

The Y chromosome’s rapid rate of evolutionary change does not mean that men are evolving faster than women. But its furious innovation is likely to be having reverberations elsewhere in the human genome.

The finding was reported online on Wednesday in the journal Nature by a team led by Jennifer Hughes and David Page of the Whitehead Institute in Cambridge, Mass. In 2003, Dr. Page, working with scientists at the Washington University School of Medicine, decoded the DNA sequence of the human Y chromosome. He and the same Washington University genome team have now decoded the chimpanzee Y chromosome, providing for the first time a reference against which to assess the evolutionary history of the human Y.

The chimpanzee and human lineages shared a common ancestor just six million years ago, a short slice of evolutionary time. Over all, the genomes of the two species are very similar and differ in less than 1 percent of their DNA. But the Y chromosomes differ in 30 percent of their DNA, meaning that these chromosomes are changing far faster in both species than the rest of the genome.

In the case of chimps, their mating habits are probably the source of the fierce evolutionary pressure on their Y chromosome. When a female comes into heat, she mates with all the males in the group, setting up competition within her reproductive tract between the sperm of different males.

Many genes that govern sperm production are situated on the Y chromosome, and any genetic variation that improves a chimp’s chances of fatherhood will be favored and quickly spread through the population.

Sperm competition may have been important in the earliest humans, too, for some years after the chimp and human lineages split. Sperm competition could still play a role in human reproduction, some experts think, given the trickle of cases of heteropaternity, the birth of twins with different fathers.

Another reason for the intensity of selective pressures on the Y chromosome in both chimps and humans may be that natural selection sees it as a single unit, so a change in any one of its genes affects the survival of all the rest. On the other chromosomes, selection is more focused on individual genes because chunks of DNA are swapped between the members of each pair of chromosomes before the generation of eggs and sperm.

This DNA swapping process is forbidden between the X and the Y pair, keeping the male-determining gene from being transferred into the X chromosome, creating gender chaos.

But this prohibition has caused most of the genes on the Y chromosome to decay for lack of fitness. In the rest of the genome, a gene damaged by a mutation can be swapped out for the good copy on the other chromosome.

In the Y, which originally had the same set of genes as the X, most of the X-related genes have disappeared over the last 200 million years. Until now, many biologists have assumed either that the Y chromosome was headed for eventual extinction, or that its evolutionary downslide was largely over and it has sunk into stagnation.

Dr. Page’s new finding is surprising because it shows that the Y chromosome has achieved an unexpected salvation. The hallmark of the Y chromosome now turns out to be renewal and reinvigoration, once the unnecessary burden of X-related genes has been shed.

“Natural selection is shaping the Y and keeping it vital to a degree that is really at odds with the idea of the last 50 years of a rotting Y chromosome,” Dr. Page said. “It is now clear that the Y chromosome is by far the most rapidly evolving part of the human and chimp genomes.”

This does not mean that men are evolving faster than women, given that the two belong to the same species, but it could be that the Y’s rate of change drives or influences the evolution of the rest of the human genome in ways that now need to be assessed. It would be “hard to imagine that these dramatic changes in the Y don’t have broader consequences,” Dr. Page said.

Andrew Clark, a geneticist who works on the Y chromosome at Cornell University, said the Y’s fast turnover of DNA could effect the activity of genes throughout the genome, because just such an effect has been detected in laboratory fruit flies.

The decoding of the Y chromosome’s DNA was particularly difficult because the chromosome is full of palindromes — runs of DNA that read the same backward as forward — and repetitive sequences that confuse the decoding systems. Decoding the human Y took 13 years, and the chimp Y took eight years, Dr. Page said.

*)Dass  ein Chromosomenforscher staunt, weil das Männlichkeits-Chromosom nun doch kein aussterbender Kümmerling ist, muss nicht erstaunen, wenn er im Mutterland der political correctness forscht. Darum bilde ich mir auch kaum was darauf ein, dass ich, der ich kein Molelularbiologe bin, ebendiesen Zusammenhang zwischen der Prekarität des Männlichen und seiner vitalen Dynamik für die menschliche Evolution schon vor Jahr und Tag (2001) beschrieben habe.


~ von Panther Ray - Januar 15, 2010.

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