mtDNA varies in a single individual 4 Mar 2010 It turns out, according to a recent study, that mitochondrial genomes vary within a single normal individual (human, but we should be able to generalise). What, I wonder, does this mean for the use of DNA barcoding? Biology Genetics Species and systematics Species concept Systematics
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…and then, of course, we have: Chromosome number and plant morphology in some ecotypes of Poa pratensis L. (Euphytica 21(2):171-180) …showing variation in chromosome number in somatic cells within the same individual. Study published in 1972. I sometimes wonder about the dearth of any estimation of statistical error (of various sorts) in the reports of things like bar codes.
I was under the impression that this was common knowledge? I have followed several courses on mitochondrial disorders, and heteroplasmy has long been recognized as an import factor in mitochondrial inheritance, like here for example. I’ll look into the original paper, but the statement that mtDNA can vary within an individual is nothing new. That it now becomes possible to determine the degree of heteroplasmy is of course interesting, but not the novelty as it is proclaimed by TheScientist.
As the announcement makes clear – what is new is that heteroplasmy is not just a pathological condition, but the normal condition. If it is normal, and the assumption of forensic genetics and barcoding is that there is a stability or universality of mt genomes, then the conclusions drawn are suspect. They may still pan out, or it may turn out that the degree of heteroplasmy in different species – indeed in different populations – may vary wildly. These are empirical questions.
Some colleagues found variation within an individual wheat plant about 15 years ago. I think that was nuclear DNA, but it doesn’t surprise me that it’s in mtDNA too: there’s a much larger population of it in a body.
What does this means for DNA-barcoding? It means that we will see a tremendous surge in the number of new species to science, even for mammals!! Hurray!
MtDNA is not used for tracing individuals, except in maternity disputes, which are much rarer than paternity disputes for obvious reasons; and King Solomon had a good way of solving those.
Ahhh…. http://www.dna.gov/research/mitochondrial_research/ http://findarticles.com/p/articles/mi_m2194/is_8_71/ai_90819640/ http://www.ornl.gov/sci/techresources/Human_Genome/elsi/forensics.shtml in particular, in the last one, this: “Mitochondrial DNA analysis (mtDNA) can be used to examine the DNA from samples that cannot be analyzed by RFLP or STR. Nuclear DNA must be extracted from samples for use in RFLP, PCR, and STR; however, mtDNA analysis uses DNA extracted from another cellular organelle called a mitochondrion. While older biological samples that lack nucleated cellular material, such as hair, bones, and teeth, cannot be analyzed with STR and RFLP, they can be analyzed with mtDNA. In the investigation of cases that have gone unsolved for many years, mtDNA is extremely valuable. All mothers have the same mitochondrial DNA as their offspring. This is because the mitochondria of each new embryo comes from the mother’s egg cell. The father’s sperm contributes only nuclear DNA. Comparing the mtDNA profile of unidentified remains with the profile of a potential maternal relative can be an important technique in missing-person investigations.”
What does it mean for barcoding? Unless different species share the same variation, not much. You can cite The Scientist as you like (gleefully, even), but maybe also notice that so far this hasn’t turned up as a problem in any barcoding study that I know of. But why should data from tens of thousands of species affect your conclusion, eh? 🙂
Have barcoders looked at multiple samples from the same individual? If not, how would they pick this up? The paper the Scientist is reporting on was published online, so can be read, and even cited, itself.
In fairness, it might not make a big difference. barcoders want divergence to be above a certain threshold. I doubt that within-individual divergence will often get to this level, but I couldn’t work out from the paper how much divergence they found (this is probably my failure: it isn’t my area of work). What it does mean is that pairs of individuals who are close to the threshold might be pushed one side or the other, depending on which extremity is cut off for sequencing.
I have to agree with Lucas. Mitochondrial heteroplasmy within normal individuals was already well established. I’m actually surprised that this paper 1) made it into Nature, and 2) is generating so much discussion. Most of the observed variants are, as reported in the paper, somatic (post-gemline) mutations, so will just be variations of the maternally supplied mtDNA sequence. Also, most of them are well below 20%, which has been demonstrated to be the Sanger-sequencing detection threshold on controlled mixtures of mtDNA (Genomics 86: 446–461.).