PARIS - A dreary weed that grows in the cracks of parking lots has caused an uproar amongst biologists by challenging one of the basic laws of genetics.
US researchers, reporting Wednesday in the British science journal Nature, say their lab samples of thale cress (Arabidopsis thaliana) appear to bust a fundamental rule of inheritance.
Convention dictates that all of one's genetic toolbox, for better or for worse, is handed down from one's parents.
But in the bizarre case reported in Nature, the weeds seem to have genes that existed in their grandparents -- but not in their parents.
A team led by Robert Pruitt at Purdue University in West Lafayette, Indiana, took an interest in a gene dubbed "hothead," focussing in particular on a flawed version of this gene that causes petals and leaves to stick together.
In the first generation of plants they studied, the cress had one flawed "hothead" gene out of a set of two.
These plants were then interbred, and as expected one-quarter of the offspring had two mutant genes, while three quarters had only one or none.
Those with the two mutant genes all had the petal and leaf abnormality.
But when the plants with the two flawed genes were then interbred, something odd happened.
Under the rulebook, all of the plants of this third generation should have been abnormal, since the only genes they had both came from flawed parents.
Yet as many as 10 percent of this batch were normal. Where had the normal "hothead" genes come from?
At first, Pruitt wondered if seeds or pollen were contaminating his plants.
But repeated experiments ruled this out, as well as the possibility that lurking on the A. thaliana genome -- the most closely-studied DNA blueprint of any plant -- there might be another gene which duplicates the "hothead" gene and was craftily muscling out the flawed one.
Exasperated, Pruitt eventually came up with an iconoclastic idea: that the normal offspring had acquired genetic data from another source than their parents.
His theory: the right genes were acquired not from DNA -- the genetic code -- but a closely similar molecule called RNA, a cellular intermediary that reads the DNA instructions to transcribe them into proteins.
The heretical idea is that ancestral information is stored on the RNA like a molecular template, and used to fix dangerous flaws.
If this is true, the remarkable conclusion is that our genetic blueprints have a hidden toolkit, bequeathed from our grandparents and possibly beyond, to heal the DNA mutations that cause disease.
In a commentary also carried in Nature, German molecular biologists Detlef Weigel and Gerd Juergens paid tribute to a "spectacular discovery" but also urged caution.
"Many experiments to explore this mechanism of unorthodox inheritance, which can skip several generations, come to mind," they said.
Still unclear is whether the self-healing quality is unique to this particular gene, or to thale cress more generally -- or whether some laboratory condition may be to blame.
In France, Ian Small, deputy director of science at the Unit of Research in Plant Genomics in Evry, near Paris, said the study had mind-boggling implications.
"If this had been published on April 1, I would have immediately said it was an April Fool's joke," Small told the French daily Liberation.

03/23/2005 19:14 GMT