[PCA] NEWS: Plant Fixes Its Flawed Gene (fwd)

[Plant Conservation]

---------- Forwarded message ----------
Date: Wed, 23 Mar 2005 08:55:31 -0500
From: Patricia_Ford at fws.gov
To: Plant Conservation <plant at plantconservation.org>
Subject: Plant Fixes Its Flawed Gene

The New York Times
March 23, 2005

Startling Scientists, Plant Fixes Its Flawed Gene

By NICHOLAS WADE

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
Tübingen, 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.

Dr. Haig, the evolutionary biologist, said that the finding was fascinating
but that it was too early to try to interpret it. He noted that if there
was a cryptic template, it ought to be more resistant to mutation than the
DNA it helps correct. Yet it is hard to make this case for RNA, which
accumulates many more errors than DNA when it is copied by the cell.

He said that the mechanism, if confirmed, would be an unprecedented
exception to Mendel's laws of inheritance, since the DNA sequence itself is
changed. Imprinting, an odd feature of inheritance of which Dr. Haig is a
leading student, involves inherited changes to the way certain genes are
activated, not to the genes themselves.

The finding poses a puzzle for evolutionary theory because it corrects
mutations, which evolution depends on as generators of novelty. Dr.
Meyerowitz said he did not see this posing any problem for evolution
because it seems to happen only rarely. "What keeps Darwinian evolution
intact is that this only happens when there is something wrong," Dr.
Surridge said.

The finding could undercut a leading theory of why sex is necessary. Some
biologists say sex is needed to discard the mutations, almost all of them
bad, that steadily accumulate on the genome. People inherit half of their
genes from each parent, which allows the half left on the cutting room
floor to carry away many bad mutations. Dr. Pruitt said the backup genome
could be particularly useful for self-fertilizing plants, as arabidopsis
is, since it could help avoid the adverse effects of inbreeding. It might
also operate in the curious organisms known as bdelloid rotifers that are
renowned for not having had sex for millions of years, an abstinence that
would be expected to seriously threaten their Darwinian fitness.

Dr. Pruitt said it was not yet known if other organisms besides arabidopsis
could possess the backup system. Colleagues had been quite receptive to the
idea because "biologists have gotten used to the unexpected," he said,
referring to a spate of novel mechanisms that have recently come to light,
several involving RNA.