Mutation accumulation in transfer RNAs: molecular evidence for Muller's ratchet in mitochondrial genomes |
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Authors: | Lynch M |
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Affiliation: | Department of Biology, University of Oregon, Eugene 97403, USA. |
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Abstract: | The accumulation of deleterious mutations is thought to be a major factor
preventing the long-term persistence of obligately asexual lineages
relative to their sexual ancestors. This phenomenon is also of potential
relevance to sexual species that harbor asexually propagating organelle
genomes. A comparative study of the transfer RNA genes in animal
mitochondrial and nuclear genomes demonstrates that the former accumulate
nucleotide substitutions much more rapidly than do the latter, and several
lines of evidence are consistent with the idea that the excess
substitutions are mildly deleterious. First, the average binding stability
between complementary strands in the stems of mitochondrial tRNAs is less
than half that in nuclear tRNAs. Second, most loop sizes in the
mitochondrial tRNAs have experienced a net reduction in size over
evolutionary time, and they are nearly 50 times more variable in the
mitochondrial than in the nuclear genome. Third, although nearly 20% of the
nucleotides in nuclear tRNA genes (particularly those involved in tertiary
interactions) are invariant across all animal taxa and all tRNA species,
there are no invariant sites in the mitochondrial tRNAs. These
observations, as well as results from recent laboratory experiments, are
consistent with the hypothesis that nonrecombining organelle genomes are
subject to gradual loss of fitness due to the cumulative chance fixation of
mildly deleterious mutations.
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