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By Maggie Fox

WASHINGTON, June 18 (Reuters) - The closest look yet at the Y chromosome -- which makes men different from women at the most basic level -- shows it is not as puny as scientists believed, researchers reported on Wednesday.

The tiny chromosome in fact carries more genes than mainstream wisdom had dictated. Most seem to be devoted to sperm production, the U.S. researchers reported.

And the chromosome uses an unusual mechanism to repair these genes when they become damaged, the researchers report in this week's issue of the science journal Nature.

"Does analyzing the sequence of the Y chromosome tell us why men are incapable of stopping to ask for directions?" asked Dr. David Page, a Howard Hughes Medical Institute investigator who led the study.

"I don't think we have an answer to that yet. It remains a great mystery."

People carry 23 pairs of chromosomes, which in turn carry the genes. These pairs -- one inherited from the father and one from the mother -- include a pair of X chromosomes in females and an X and a Y in males.

It was believed for years that the Y, a small, fragile chromosome compared to the X, was a genetic wasteland. Scientists believed it was not capable of pairing up with its partner, the X, to repair defects the way other genes can.

"The idea was that in this male-specific region of the Y, genes were singletons. They had no one to swap with and they had no way to rid themselves of ... genetic injuries," Page, who works at the Whitehead Institute at the Massachusetts Institute of Technology, told a news conference.

But to their surprise, the scientists found that the genes in this region are in fact palindromes -- the sequences read the same backwards and forwards, similar to the numbers in the year "2002," only much, much longer.

And the chromosome can form little loops in which the genes at one end can press against the genes at the other end of the palindrome, swapping sequences and thus repairing -- or passing along -- mutations.

"What we infer is that these two arms of genes can actually swap parts," Page said.

"This, we propose, keeps the Y chromosome in business and keeps it stocked with healthy genes for human reproduction," he added. "It is not merely a rotted-out version of an ancient, ordinary, chromosome."
The Y can also do a limited amount of gene swapping with the X, but for the most part manages to repair itself, the researchers, which included a team at Washington University in St. Louis, found.
"This gene says 'I can can do it myself'," joked Dr. Francis Collins, head of the National Human Genome Research Institute.
Scientists once believed that the Y chromosome carried only one gene -- the SRY gene, which instructs the body to make testes instead of ovaries.

"Testes make testosterone. Testosterone makes for male behavior. That is the prevailing view," Page said.

It will take much closer research to determine just how much influence the tiny Y chromosome has on male behavior and biology, the researchers said. It could cast light, for example, on differences between male and female disease patterns.
Collins said the findings could also translate into cures for infertility. "We certainly know mistakes on the Y chromosome are the most common cause of male infertility," he said.

06/18/03 13:12 ET
   
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