The Why of Fat X, Lean Y

The X and Y chromosomes are the odd couple of the human genome.

The other 22 pairs of chromosomes are perfectly matched in size and shape. The Y and the X, however, are like Jack Spratt and his wife, different in every way, but somehow perfect for each other. The Y, or male sex chromosome, is slight and underfed (in terms of gene content), while the female X is three times Y's size and loaded with genes.

This pair plays an essential role in human development. The egg and the sperm each carry a single sex chromosome, and their combination determines the sex of the embryo they create. Two Xs and a female develops. An X and a Y--it's a male.

How did this system evolve? Two biologists, writing in the Oct. 29 issue of Science, have a theory.

The X and the Y were originally like the 22 other pairs of chromosomes--virtual identical twins. In the course of evolution, however, their ability to line up and exchange pieces of DNA with each other was traumatically disrupted. Denied the chromosome-to-chromosome communication the other pairs had, the two halves of this pair went separate ways, ultimately developing different identities.

The researchers, David C. Page of the Whitehead Institute for Biomedical Research in Cambridge, Mass., and Bruce T. Lahn of the University of Chicago, began by comparing the locations of the 19 genes X and Y still have in common. Not surprisingly, they no longer line up with each other, and are in a different north-to-south order on the X than they are on the Y.

Page and Lahn then determined how long it had been since each one of the genes had ceased occupying a similar address on the chromosomes. (This is done by counting the number of small changes that have crept into a gene's DNA sequence.) They found that the 19 genes could be divided into four groups, each with a distinct age.

It was as if the four groups were different strata in an archaeological site, and their genes clustered artifacts dating from the same prehistoric period.

The scientists hypothesize that four times during evolution, the Y chromosome broke. Each time, some of the genes took up a new position on the restitched chromosome. (Many were lost entirely.) The first of these rearrangements occurred 240 million to 320 million years ago, soon after birds and mammals diverged from a common reptilian ancestor. The most recent occurred 30 million years ago, during the evolution of primates, which is the evolutionary equivalent of last month.

Boar Semen as Source of Drugs

Semen from wild boars may someday be prized as a cheap and plentiful source of human medicines, if research reported last week continues apace.

Scientists already have made gene-altered cows, sheep and goats that produce human drugs in their milk, in some cases more efficiently than standard laboratory methods allow. But it can take years for those animals to produce their first quart, and some do not lactate year-round.

Michael Dyck and his colleagues at Laval University in Quebec knew that wild boars can ejaculate a full cup of semen three times a week, 52 weeks a year. In their home province alone, 1,800 boars are regularly "milked" for their semen as part of Quebec's swine artificial insemination industry.

To see if it might be practical to make medicines in semen, the team inserted into mice a human gene that directs production of human growth hormone--a drug that can add inches to the adult height of children with congenital dwarfism, and which is today made by gene-altered bacteria in industrial vats. By placing the gene under the control of a biological switch that turns on only in the seminal vesicles, they were able to grow otherwise normal mice whose semen was packed with the medicinal hormone.

Purification of human drugs from boar semen would probably be easier than from milk, which is more chemically complex, the researchers suggest in the November issue of Nature Biotechnology.

Late-Surviving Neanderthals

One of the most enduring scientific mysteries is what exactly happened to the Neanderthals. Some researchers have proposed the highly controversial theory that Neanderthals coexisted with early human ancestors, and perhaps even interbred with them. That camp has some new evidence supporting this idea.

An international team of researchers retested Neanderthal bones found in the 1970s in a cave in Croatia and concluded they date back only 28,000 years, according to a report in the Oct. 26 issue of the Proceedings of the National Academy of Sciences. Previously, it was believed the Neanderthals disappeared from what is now central Europe about 34,000 years ago.

"Neanderthals were late survivors not only in the cul-de-sac of Atlantic Europe, but also in central Europe. This fact indicates that the disappearance of Neanderthals in Europe did not follow a simple geographic pattern from east to west, which in turn implies that the dynamics involved in the disappearance of Neanderthals were more complicated than a gradual retreat of Neanderthal populations into peripheral refugia," they write.

Sleep Deficit Affects Body

Scientists have long known that a lack of sleep adversely affects the human brain. But the body has been another matter.

Now, researchers at the University of Chicago have found evidence that a sleep deficit has negative effects on the body as well. Karine Spiegel and colleagues carefully studied the bodily functions of 11 young men as they got various amounts of sleep. The researchers found changes when the men had little sleep, including levels of glucose in their blood shooting up and concentrations of a key thyroid hormone falling.

"Sleep debt has a harmful impact on carbohydrate metabolism and endocrine function," the researchers wrote in the Oct. 23 issue of The Lancet. "The effects are similar to those seen in normal aging and, therefore, sleep debt may increase the severity of age-related chronic disorders."