Article: Is RNA inheritance possible?
From: Robert Karl Stonjek (rstonjek_at_bigpond.net.au)
Date: 03/23/05
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Date: Wed, 23 Mar 2005 16:32:52 -0500 (EST)
Is RNA inheritance possible?
Researchers find plant clues to a non-DNA pathway for genetic transmission
By Laura M Hrastar
Evidence for inheritance based on RNA has been found in a particularly
unstable plant gene, researchers suggest in Nature this week. Robert Pruitt
and colleagues at Purdue University in West Lafayette, Ind., found that
Arabidopsis plants homozygous for recessive mutant alleles of particular
gene may inherit genomic information that is not found in the parent plant,
but that existed in earlier generations.
"We can imagine two parallel pathways, one of DNA-based inheritance, and in
the background you have this other which might be RNA-or an odd form of
DNA-based inheritance we don't understand," Pruitt told The Scientist.
Pruitt's team was investigating HOTHEAD, a gene responsible for organ fusion
and wax cuticle production. Previous research has shown unusual point
mutation instability in the gene in Arabidopsis, which results in a high
frequency of wildtype plants from mutant parents.
In the latest study, Pruitt's team used polymerase chain reaction to
determine that 10% of the progeny of mutant plants revert to wildtype, and
those plants were heterozygous for the parent's mutant HOTHEAD (hth) allele.
Measuring directly in pollen, they also found a strong bias for wildtype
changes in the plant's male reproductive system-seen from the creation of
dominant allele (HTH/HTH) wildtype progeny when the male parent plant, not
the female parent, was a homozygous mutant.
The progeny of mutants, Pruitt said, bypass their parents' DNA mutations by
some undetermined mechanism, and the allelic reversions restore the original
DNA sequence of the grandparent plant.
"In this study, you have the fascinating proposal that in the rare cases
when a DNA mutation occurs, RNA descending from previous generations can
lend a hand to assist its superior, more privileged cousin," David Bartel
from the Massachusetts Institute of Technology told The Scientist in an
E-mail. Bartel was not involved in the current study.
Pruitt's group also looked at randomly chosen polymorphisms in F3 generation
progeny from crossed mutant plants homozygous for HOTHEAD and found evidence
of genetic instability for all sequence polymorphisms across the genome.
After tests that found three more independent reversion events within three
different mutant hth alleles, a high rate of random mutation was ruled out.
Data strongly suggested that the genetic changes were the result of a
template-driven process, the authors write.
"We've looked for a DNA template fairly carefully and can't find one, so
then by exclusion, we say it's probably a type of stable RNA," said Pruitt.
However, the phenomenon has only been found in mutants because "in the
wildtype individual, the contribution of this secondary pathway for
inheritance is probably negligible," Pruitt told The Scientist.
Pruitt and his coauthors note that their RNA template hypothesis is
supported by recent work showing that double-stranded RNA can be transmitted
for many generations in Caenorhabditis elegans.
Why mutant progeny revert back to wildtype is unknown, but the authors
suggest that stress linked to the hth gene may be involved. "With the theory
of stress, it's possible that rather than introducing random mutations into
the DNA, this unknown process kicks in, which potentially reverts alleles
back to successful alleles of past generations," said John Bowman, from
University of California at Davis, who was not involved in the study.
Speculating on the implications of a RNA-based inheritance, plant geneticist
Detlef Weigel of the Max Planck Institute for Developmental Biology in
Germany told The Scientist, "If there were some cache of RNA, I would still
think of the DNA as the dominating form. [Possibly] the genome has this kind
of fail-safe mechanism to make use of stored RNA. It's not presently clear."
Continuing their work, Pruitt and colleagues will now focus on which genes
are most directly involved in the process of sequence change and
biochemically identify the location of the putative templates.
Links for this article
S.J. Lolle, et al., "Genome-wide non-mendelian inheritance of extra-genomic
information in Arabidopsis," Nature, 434:505-09, March 24, 2005.
http://www.nature.com
Robert E. Pruitt
http://www.btny.purdue.edu/Faculty/Pruitt/
K.A. Krolikowski et al., "Isolation and characterization of the Arabidopsis
organ fusion gene HOTHEAD," Plant J, 35:501-11, August 2003.
[PubMed Abstract]
David P. Bartel
http://www.wi.mit.edu/research/faculty/bartel.html
A. Fire et al., "Potent and specific genetic interference by double-stranded
RNA in Caenorhabditis elegans," Nature, 391:744-5, February 19,1998.
[PubMed Abstract]
John L. Bowman
http://biosci.ucdavis.edu/BioSci/FacultyAndResearch/DisplayFacu
ltyProfile.cfm?ResearcherID=1507
Detlef Weigel
http://www.weigelworld.org/research
Full Text from TheScientist
http://www.biomedcentral.com/news/20050323/01
Comment:
The impact of this discovery can not be underestimated.
Posted by
Robert Karl Stonjek
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