Theories that human males will eventually become extinct can now be dismissed. U.S. scientists have discovered that the genetic material that defines men biologically is more durable and adaptable than thought. The researchers have finished decoding the Y chromosome, which is handed down only by fathers to sons, and they suggest that it will not disappear any time soon.
The male Y chromosome has been widely misunderstood, even belittled. It is one of 46 chromosomes in the human body, the long microscopic strands of DNA that contain genes and are tightly coiled to fit within the tiny nucleus of each cell.
In the view of some scientists, the Y chromosome had a bleak future. After all, it had shrunk from about 1,500 genes 300 million years ago to what was believed to be one or at most a few genes. Furthermore, researchers considered it a wasteland because they thought its genes did not contribute anything important to male biology. To make matters worse, the Y chromosome is the only one not to have a twin from which it can draw genetic matter to repair mutations or replace lost material. All the others are paired, one from each parent. Even women have two female X chromosomes because both men and women carry them and pass them down. Many scientists thought this apparent inability to repair itself might be why the Y chromosome had shrunk over the ages. They feared it might die out in a several million more years, taking the human species with it.
"How could such things be said about this wonderful chromosome?," asks David Page of the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts. He is optimistic about the Y chromosome's future. He led a team of researchers from several institutions who have deciphered its physical structure and discovered biological secrets that are likely to earn it some badly needed respect.
"We're here really to defend the honor of the Y chromosome in the face of a century of insults," he said.
Based on an analysis of Y chromosomes in human men and several types of male apes, Dr. Page's team has discovered that this genetic material is much more interesting than thought. The scientist who has overseen the global project to determine the structure of all human genes, Francis Collins of the U.S. government's National Institute's of Health, says the new findings counter the idea of a rotting chromosome.
"This sequence reveals some fascinating and unexpected twists in human biology and genetics, explaining how the solitary Y manages to avoid the evolutionary dustbin that many had predicted it would ultimately land in," he said.
For one thing, the Y chromosome has more genes than believed, 78 of them. As Dr. Page's team reports in the journal Nature, 60 of them are involved in production of sperm. He says many are arranged in pairs along its length and might exchange material for repair purposes the same way the paired chromosomes do.
"This will in many cases correct the mutation and thereby repair the gene," he said. "So we found many sperm production genes on the Y chromosome and almost all of them live as sort of married couples. This, we propose, keeps the chromosome well stocked with healthy, intact genes that are vital to human reproduction."
Rather than think of the Y chromosome as a dying stretch of DNA, Dr. Page offers the notion of an adaptable chromosome once paired with the X chromosome hundreds of millions of years ago. He says it separated, shed itself of unnecessary genes, and adopted only those it needed to carry out its sperm production and other male functions.
As important as the work is for understanding male fertility, it is also expected to shed light on male infertility. But scientists do not yet know what the the rest of the Y chromosome's genes do. How do they contribute to maleness?
Francis Collins at the National Institutes of Health says the question is not trivial. "The comedienne Elayne Boosler wrote, 'When women get depressed, they eat chocolate or go shopping. When men get depressed, they invade countries!' There's a difference here. Now obviously that's a bit of an overstatement of the power of the sequence, but it does reflect some of the myths and the lore that surround this particular part of biology," he said.
The new research will undoubtedly set off a series of new studies to find the answers.