Planet Earth

Another look at mtDNA

Gene ExpressionBy Razib KhanApr 8, 2012 10:55 PM

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The new article in The American Journal of Human Genetics, A “Copernican” Reassessment of the Human Mitochondrial DNA Tree from its Root, is open access, so you should check it out. The discussion gets to the heart of the matter:

Supported by a consensus of many colleagues and after a few years of hesitation, we have reached the conclusion that on the verge of the deep-sequencing revolution...when perhaps tens of thousands of additional complete mtDNA sequences are expected to be generated over the next few years, the principal change we suggest cannot be postponed any longer: an ancestral rather than a “phylogenetically peripheral” and modern mitogenome from Europe should serve as the epicenter of the human mtDNA reference system. Inevitably, the proposed change could raise some temporary inconveniences. For this reason, we provide tables and software to aid data transition. What we propose is much more than a mere clerical change. We use the Ptolemaian geocentric versus Copernican heliocentric systems as a metaphor. And the metaphor extends further: as the acceptance of the heliocentric system circumvented epicycles in the orbits of planets, switching the mtDNA reference to an ancestral RSRS will end an academically inadmissible conjuncture where virtually all mitochondrial genome sequences are scored in part from derived-to-ancestral states and in part from ancestral-to-derived states. We aim to trigger the radical but necessary change in the way mtDNA mutations are reported relative to their ancestral versus derived status, thus establishing an intellectual cohesiveness with the current consensus of shared common ancestry of all contemporary human mitochondrial genomes. Note that the problem is not restricted to mtDNA. Indeed, in the much larger perspective of complete nuclear genomes in which comparisons are often currently made relative to modern human reference sequences, often of European origin, it seems worthwhile to begin considering, as valuable alternatives, public reference sequences of ancestral alleles (common in all primates) whereby derived alleles (common to some human populations) would be distinguished.

Perhaps the first generation or so of human molecular evolutionary genetics might be thought of as a "first draft." A serviceable first draft which rendered in broad strokes the gist of the truth as we understand it, but lacking in some essential details. On a minor note, there are some theoretical reasons why mtDNA did not yield much evidence for archaic admixture, which is clear in the nuclear genomics (e.g., higher rate of change due to lower effective population size, so more rapid extinction of ancient lineages). But perhaps now that the number of complete mtDNA genomes is increasing in size we might start to see "long branches," which reflect the inferences generated from the ancient nuclear genomes.

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