Back to the future: Mastodon extends the time limit on DNA sequencing

In a new paper in the open access journal PLoS Biology, Michael Hofreiter from the Max Planck Institute for Evolutionary Anthropology in Germany, and colleagues from Switzerland and the United States, announce the sequencing of the complete mitochondrial genome of the mastodon (Mammut americanum), a recently extinct relative of the living elephants that diverged about 26 million years ago. The sequence was obtained from a tooth dated to 50,000-130,000 years ago, increasing the specimen age for which such palaeogenomic analyses have been done by almost a complete glacial cycle.

The mastodon becomes only the third extinct taxon for which the complete mitochondrial genome is known, joining the superficially similar looking woolly mammoth, and several species of Moa, the giant flightless Australasian bird.

Using the mitochondrial genome sequence, together with sequences from two African elephants, two Asian elephants, and two woolly mammoths (obtained from previous work), it was shown that mammoths are more closely related to Asian than to African elephants. This shows the power of genetic data to clarify interrelationships, even in the case of well studied taxa. Moreover, the researchers used the mastodon data as a calibration point, lying outside the Elephantidae radiation (elephants and mammoths), which enabled them to estimate accurately the time of divergence of African elephants from Asian elephants and mammoths (about 7.6 million years ago) and the time of divergence between mammoths and Asian elephants (about 6.7 million years ago). These dates are strikingly similar to the divergence time for humans, chimpanzees, and gorillas, and raise the possibility that the speciation of mammoths and elephants and of humans and African great apes had a comm on cause. Despite the similarity in divergence times, the substitution rate within primates is more than twice as high as in proboscideans, showing that the molecular clock ticks differently for different taxa.

Citation: Rohland N, Malaspinas AS, Pollack JL, Slatkin M, Matheus P, et al. (2007) Proboscidean mitogenomics: Chronology and mode of elephant evolution using mastodon as outgroup. PLoS Biol 5(8): e207. doi:10.1371/journal.pbio.0050207.

CONTACT:

Michael Hofreiter
Max Planck Institute for Evolutionary Anthropology
Deutscher Platz 6
Leipzig, D-04103
Germany
+49 341 3550 524
hofreiter@eva.mpg.de

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Proboscidean Mitogenomics: Chronology and Mode of Elephant Evolution Using Mastodon as Outgroup (full paper)

We have sequenced the complete mitochondrial genome of the extinct American mastodon (Mammut americanum) from an Alaskan fossil that is between 50,000 and 130,000 y old, extending the age range of genomic analyses by almost a complete glacial cycle. The sequence we obtained is substantially different from previously reported partial mastodon mitochondrial DNA sequences. By comparing those partial sequences to other proboscidean sequences, we conclude that we have obtained the first sequence of mastodon DNA ever reported. Using the sequence of the mastodon, which diverged 24–28 million years ago (mya) from the Elephantidae lineage, as an outgroup, we infer that the ancestors of African elephants diverged from the lineage leading to mammoths and Asian elephants approximately 7.6 mya and that mammoths and Asian elephants diverged approximately 6.7 mya. We also conclude that the nuclear genomes of the African savannah and forest elephants diverged approximately 4.0 mya, supporting the view that these two groups represent different species. Finally, we found the mitochondrial mutation rate of proboscideans to be roughly half of the rate in primates during at least the last 24 million years.