Mendelian Inheritance refuted!! "weedy cress defies the rules"

" MENDELIAN inheritance, the central tenet of genetics, is under attack from a few scrawny weeds that haven't read the textbooks. The weeds are somehow inheriting DNA sequences from their grandparents that neither of their parents possessed - which is supposed to be impossible. "

The orthodox view is that genes are passed down in the form of DNA, and all organisms have to make do with this parental DNA inheritance, mutations and all. Chemical or structural modifications to DNA can switch off genes, and these changes can pass from generation to generation, a phenomenon called epigenesis. But epigenetic changes do not alter the actual sequence of DNA.

Yet that is what seems to occur in the weedy cress Arabidopsis thaliana, the workhorse of plant biologists. Cress with two mutant copies of one gene seem to be able to correct the DNA they pass on, ensuring that at least a few of their offspring revert to normal.

Robert Pruitt, whose team at Purdue University in West Lafayette, Indiana, made this extraordinary discovery, thinks that the mutant genes are being repaired using RNA templates inherited from earlier generations.

Genetic mystery

It is possible that the phenomenon is limited to this one plant. But in Nature (vol 434, p 505), Pruitt's team speculates that it might be a more widespread mechanism that allows plants to "experiment" with new mutations while keeping RNA spares as a back-up. If the mutations prove harmful, some plants in the next generation revert to their grandparents' DNA sequence with the help of the RNA. "It does make sense," Pruitt says.
"It was our view that it was heresy when we started working on it, but we've had time to get used to the idea now," he says. "I'd say I've been the biggest sceptic all the way along, but every experiment has been done to find a conventional explanation and it's as foolproof as we can make it. I have every confidence in the data, but I'll feel better about it when other people have seen similar things."

The team has also found that in hothead mutants, other faulty genes mysteriously revert to the sequence of earlier generations too. It may be that the phenomenon is caused by the hothead mutation and restricted to plants that carry it, says Ottoline Leyser, who studies plant developmental genes at the University of York in the UK. "People have been working on mutants for years, and they all behave in a Mendelian way," she says.

Pruitt's team is now trying to find the stash of RNAs from earlier generations that might provide the templates for repair, and work out how it is passed down. "My guess is that it is in the nucleus somehow, or hitchhikes on chromosomes, but that's just speculation," he says.

While the search goes on, Pruitt hopes other biologists will hunt for evidence of the phenomenon in plants, animals and even humans. "If we can understand how these templates are used, we might be able to make our own to order," he says. That might help improve existing methods for repairing genes, which are not yet efficient enough to be used to treat genetic diseases.

{ Rogue weeds defy rules of genetics: From issue 2492 of New Scientist magazine, 26 March 2005, page 8 }