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Plants May Be Able To Correct Mutated Genes
ddutt writes "NY Times is running a story that talks of an exciting new discovery, which, if confirmed, could represent an unprecedented exception to Mendel's
laws of inheritance. The discovery involves.. '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.'"
New Scientist has coverage. No registration required.
http://www.newscientist.com/article.ns?id=dn7185
J. Wolfgang Goerlich
For the people who, ah, read the paper, if this particular gene (HTH) is mutated, then a whopping 5% of the second-generation genes manage to revert to the wild type. The other 95% are still mutant. So this mechanism (which is normally masked by the presence of a normal HTH gene) provides for a small number of mutant offspring to revert to wild type, so that a deleterious mutation won't completely destroy the population it occurs in. To disprove "micro-evolution", you'd have to show that this mechanism used to be turned on in every organism and operated at ~100% efficiency rather than 5%. Don't bet on it.
Now, this is definitely a pretty cool discovery, and there's going to be a stampede of people hunting around looking for some sort of, say, RNA copy of the genome hiding somewhere in Arabidopsis, and there will be a lot of fun in epigenetics. But it isn't going to destroy evolutionary theory, although I expect creationists (excuse me, "intelligent design theorists") will be running around for decades insisting that because this phenomenon exists, it's impossible for mutations to happen.
What was reported is that although there were mutations in the DNA of the plant, its siblings didn't have them anymore. The researcher said that the best theory at the moment is that the non-mutated DNA was coming from the RNA of the plant. IANAB, but I think RNA usually is though to serve only a functional "middle man" role betweeen the genetic code and the cell machinery, and not actively involved in reproduction...
He did not say that the plant was actively fixing its DNA for its offspring.
The non-mutated RNA was itself directly inherted from the parents. In a way the RNA has become a bad backup copy of the DNA. That's the present theory... I guess this is what they'll start looking for... "Bad backup copy" since still 90% of the offspring of the plant still contained the mutated DNA.
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sigamajig...
Your explanation is fairly on the mark, and I'd mod it up except that I want to participate in the discussion.
The thing that's so remarkable about this case is as you said: BOTH alleles of the gene of the plant were defective as inherited from their parents, and yet they somehow reverted to an allele from the grandparents, across the entire organism. According to current theory, sexual replication causes a kid to inherit one allele of each gene from each parent (and by "theory", I mean you can watch this happen under a microscope). If both alleles received are "faulty" (which is a sticky term to use in many cases), there's no known way for a newly fertilized cell to know this. There's no information about what the correct gene should look like except the two copies of the gene it has. In cancer, as you point out to address the parent post, there is always a source of information used to correct the mutation.
In the case of UV damage, information exists in the form of two fused thimidine molecules (two T's). If a cell sees two fused T's, it has a repair mechanism for correcting them. But, importantly, if this mistake is not corrected before DNA duplication occurs, then random bases are paired with the T's, because they're damaged. Once this happens, each daughter cell has lost the information required to correct the problem, and the mutation persists. If this happens in an unlucky spot, you can get melanoma.
In the case of other more serious damage, like double-stranded breaks, your cell pulls in the other copy of your genes and edits against that. The information needed for repair is the "good" copy of the allele in the sister chromosome.
So you can see why this is so confusing -- in the case in the article the daughter cells, with two bad alleles for the gene they studied, are supposed to have no information pointing them to the gene from the grandparents. And yet they did, since they were able to fix it. The article postulates that this could be because a THIRD copy of the gene exists as RNA that's passed down from the grandparents (third since there are two chromosomes, each with a copy of the gene). If this were true, then the RNA would be the source of information required to fix the problem. Alternatively, there could be a specific protein that hunts down mutations in this gene and somehow fixes it, since it somehow bonds only to the correct version of the gene. But that's just my wild speculation.
A lot of genetically modified plants will be selected against where they escape into the wild. Golden Rice, for example, uses a lot of energy making Beta Carotine, that is, (from the plant's view), wasted. When its seeds get cross fertilized by wild rices the genes tend to be weeded out in the wild areas quite rapidly. Rice has generations lasting a year or less, and it's been estimated that the genes are 99% gone within 10 years. Even in cultivation, farmers have to suplement their seed stock saved from the last harvest with new purchases of fresh Golden Rice every few years to keep the yields up.
That's not mutation as you've described, it's natural and artificial selection, but so long as there are unmodifed plants in the same areas as the GE ones, it tends to work that way, as the vast majority of GE features are disadvantagious under natural selection, and a lot of them are so disadvantagious they require real rigor to preserve via artificial selection. They're like Pekinese dogs in the wild.
Who is John Cabal?