Science: Update

TWO LEADING virologists have argued against the prevailing view that the main Aids virus - HIV-1 - is a recent infection of human beings. Some researchers believe that HIV-1 jumped from chimpanzees to humans in the past few decades, possibly around the middle of this century, judging by its speed of evolution. However, Jaap Goudsmit and Vladimir Lukashov, of the University of Amsterdam, question one of the underlying assumptions of the argument in this week's Nature. "Evolutionary rates among viruses are unlikely to be equal and constant over time," they write. Previous research on a virus found in stored blood samples taken in Zaire in 1959 suggested that the many subtypes of HIV-1 diverged from one another no longer ago than the Forties or Fifties. However, Goudsmit and Lukashov estimated the year of divergence to be 1913, based on a different interpretation of the "molecular clock" that dictates the speed of viral mutation. The scientists who led the investigation into the 1959 virus, however, remain confident of their interpretation of the recent evolution of the main Aids virus.

NERVE CELLS are known to migrate from one part of the brain to another during early childhood development. Now scientists believe that they have found one of the ways this is controlled. A team led by Yi Rao, assistant professor of neurobiology at Washington University School of Medicine in St Louis, has found a protein produced in the brain that seems to guide nerve cells to their final destination by repelling them into place, much like a dog herding a flock of sheep. The protein, called "slit", could prove to be an invaluable tool in understanding brain disorders. "This is the first demonstration of a diffusible molecule that directs migrating neurons. Such a `repulsive' molecule might be useful for controlling unwanted migration of tumour cells, or for delivering therapeutic cells to specific regions of the brain in patients with Parkinson's or Alzheimer's," Professor Rao said.