[Paleopsych] Startling Scientists, Plant Fixes Its Flawed Gene

[Steve Hovland]

By NICHOLAS WADE

Published: March 23, 2005


In a startling discovery, geneticists at Purdue University say they have 
found plants that possess a corrected version of a defective gene inherited 
from both their parents, as if some handy backup copy with the right 
version had been made in the grandparents' generation or earlier.




The finding implies that some organisms may contain a cryptic backup copy 
of their genome that bypasses the usual mechanisms of heredity. If 
confirmed, it would represent an unprecedented exception to the laws of 
inheritance discovered by Gregor Mendel in the 19th century. Equally 
surprising, the cryptic genome appears not to be made of DNA, the standard 
hereditary material.

The discovery also raises interesting biological questions - including 
whether it gets in the way of evolution, which depends on mutations 
changing an organism rather than being put right by a backup system.

"It looks like a marvelous discovery," said Dr. Elliott Meyerowitz, a plant 
geneticist at the California Institute of Technology. Dr. David Haig, an 
evolutionary biologist at Harvard, described the finding as "a really 
strange and unexpected result," which would be important if the observation 
holds up and applies widely in nature.

The result, reported online yesterday in the journal Nature by Dr. Robert 
E. Pruitt, Dr. Susan J. Lolle and colleagues at Purdue, has been found in a 
single species, the mustardlike plant called arabidopsis that is the 
standard laboratory organism of plant geneticists. But there are hints that 
the same mechanism may occur in people, according to a commentary by Dr. 
Detlef Weigel of the Max-Planck Institute for Developmental Biology in 
Tubingen, Germany. Dr. Weigel describes the Purdue work as "a spectacular 
discovery."

The finding grew out of a research project started three years ago in which 
Dr. Pruitt and Dr. Lolle were trying to understand the genes that control 
the plant's outer skin, or cuticle. As part of the project, they were 
studying plants with a mutated gene that made the plant's petals and other 
floral organs clump together. Because each of the plant's two copies of the 
gene were in mutated form, they had virtually no chance of having normal 
offspring.

But up to 10 percent of the plants' offspring kept reverting to normal. 
Various rare events can make this happen, but none involve altering the 
actual sequence of DNA units in the gene. Yet when the researchers analyzed 
the mutated gene, known as hothead, they found it had changed, with the 
mutated DNA units being changed back to normal form.

"That was the moment when it was a complete shock," Dr. Pruitt said.

A mutated gene can be put right by various mechanisms that are already 
known, but all require a correct copy of the gene to be available to serve 
as the template. The Purdue team scanned the DNA of the entire arabidopsis 
genome for a second, cryptic copy of the hothead gene but could find none.

Dr. Pruitt and his colleagues argue that a correct template must exist, but 
because it is not in the form of DNA, it probably exists as RNA, DNA's 
close chemical cousin. RNA performs many important roles in the cell, and 
is the hereditary material of some viruses. But it is less stable than DNA, 
and so has been regarded as unsuitable for preserving the genetic 
information of higher organisms.

Dr. Pruitt said he favored the idea that there is an RNA backup copy for 
the entire genome, not just the hothead gene, and that it might be set in 
motion when the plant was under stress, as is the case with those having 
mutated hothead genes.

He and other experts said it was possible that an entire RNA backup copy of 
the genome could exist without being detected, especially since there has 
been no reason until now to look for it.

Scientific journals often take months or years to get comfortable with 
articles presenting novel ideas. But Nature accepted the paper within six 
weeks of receiving it. Dr. Christopher Surridge, a biology editor at 
Nature, said the finding had been discussed at scientific conferences for 
quite a while, with people saying it was impossible and proposing 
alternative explanations. But the authors had checked all these out and 
disposed of them, Dr. Surridge said.

As for their proposal of a backup RNA genome, "that is very much a 
hypothesis, and basically the least mad hypothesis for how this might be 
working," Dr. Surridge said.