Surprise in the Y
The traditional thinking about the male Y chromosome is that it is a stagnant part of the genome. But the first comprehensive comparison of the Y chromosome in humans and chimpanzees shows that, in fact, it is a hot spot of evolution.
Since sex chromosomes first evolved 200 million years ago, the Y chromosomes have steadily lost genes, mainly retaining only those needed to determine sex and produce sperm. Because humans and chimps have a 98% genetic overlap, researchers assumed that their Y's should be nearly identical.
But when geneticist David Page of the Massachusetts Institute of Technology in Cambridge and colleagues sequenced the male-specific region of the Y (MSY) in chimpanzees (which comprises 95% of the Y) and compared it with the human MSY, they got a big surprise: More than 30% of the DNA differs between the two species.
This suggests that the Y has undergone “extraordinary” remodeling in the 6 million years since humans and chimpanzees split from a common ancestor, says Page. The team found that the chimpanzee Y has lost many genes that are still present in humans—which presumably makes humans closer to the common ancestor than chimps are. What's more, the chimpanzee MSY has acquired twice as many palindromes—blocks of DNA in which the sequence of nucleotides is a mirror image of the sequence on its complementary strand—as the human MSY has. These have led to even greater structural changes in the chimp Y, the team reported online in Nature last week.
The researchers suggest that one cause of these changes is the competitive advantage an animal gains by developing new genes for sperm production. In chimps in particular, many males mate with one fertile female, so natural selection favors those with more (or better) sperm.
The discovery of so much variation in the Y chromosome is stunning researchers. “It's really exciting; it's totally well-documented; it's really dramatic,” says population geneticist Andrew Clark of Cornell University.
Foreign Brains Sticking
Several prominent Chinese-born U.S. scientists have recently announced plans to return to their native country. But contrary to many reports, foreign Ph.D.s are not flocking home in droves, Michael Finn of the Oak Ridge Institute for Science and Education in Tennessee reports in a study prepared for the U.S. National Science Foundation. The study finds that 2-year “stay rates” have hovered at about 70% for the past decade. Chinese and Indians, who make up the largest number of foreign doctoral students in the United States, have the highest stay rates.
The panel at left depicts the Maya ruler Tajchanahk, or “Torch-Sky-Turtle,” seated on a water lily throne. The work, framed by a bubbling stream and foliage, symbolizes the melding of Earth, sea, and cosmos, according to the Peabody Essex Museum in Salem, Massachusetts. The museum will feature 90 works on the theme of the sea in Maya spiritual life. The show, “Fiery Pool: The Maya and the Mythic Sea,” will open on 27 March.
High-energy Doberman pinschers are a breed particularly susceptible to developing compulsive behaviors—such as incessant licking of flanks or sucking on blankets. Now researchers have used Dobermans to make what Nicholas Dodman of Tufts University Cummings School of Veterinary Medicine in North Grafton, Massachusetts, calls “the first discovery of a really robust psychiatric gene.”
Dodman and his colleagues say canine compulsive disorder (CCD)—analogous to human obsessive-compulsive disorder—is seen in 2% to 5% of dogs brought to the vet. The scientists did a genomewide association study of 94 Dobermans with CCD, compared with 73 healthy controls. They report in this month's issue of Molecular Psychiatry a “highly significant” association with a mutation in neural cadherin-2, a gene involved in central nervous system development, on chromosome 7. Sixty percent of the highest-risk dogs had the mutation, almost three times the rate in the unaffected dogs.
Dennis Murphy, a psychiatrist at the National Institute of Mental Health in Bethesda, Maryland, says an informal consortium of researchers plans to explore the gene further. “It gives us a specific target to look at for compulsive behaviors in humans,” he says.