.
Magnified
view of the hair of the thylacine.
Photo:
Nicholas Ayliffe (International Thylacine Specimen Database Project).
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The genome team noted that: "Long-term hair survival occurs in a variety
of natural environments, and large quantities are present in taxonomic
collections representing most extant, and many recently extinct, mammalian
taxa". Further, they state that: "Most hair-based genetic
studies have used roots, instead of shafts, as a DNA source, primarily
because hair shafts consist of dead keratinized cells that contain relatively
low levels of DNA. However, several studies have reported shafts
as a viable source of modern and ancient mitochondrial DNA".
The genome team noted that hair shafts possess several properties that
make them both an attractive and viable source of DNA for sequencing-by-synthesis
(SBS):
I. Their relative abundance (when present) renders them preferable
to bones, because the destructive nature of bone sampling can lead to the
loss of important morphological information.
II. Turnover of keratinocytes in the hair bulb is exceedingly high,
second only to that of cells in the gut epithelium. Therefore, baseline
mitochondrial levels in these cells (and thus the precortical cells that
develop into the bulk of the shaft) may be higher than those in other tissues
commonly used for ancient DNA analysis.
III. Even when degraded, shafts are resistant to contamination from
exogenous DNA from sources such as bacteria and foreign blood and skin
cells. |
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Woolly
mammoths near the Somme River. AMNH mural. Artist: Charles
Knight (1916).
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Miller, Schuster, and their colleagues were the first to report the genome-wide
sequence of an extinct animal, the woolly mammoth (Mammuthus primigenius),
in November 2008. They next collaborated with Anders Goetherstroem,
of the Department of Evolutionary Biology at Uppsala University in Sweden,
to target the thylacine because, like the mammoth, it was a coveted goal
of ancient DNA researchers, who considered its sequencing impractical due
to the inadequate quality of the DNA available from specimens.
Schuster states: "The speculation was that the only reason we were able
to extract DNA from mammoth hair is that the mammoths had remained frozen
in the Arctic permafrost, but our success with the Tasmanian tiger shows
that hair can protect DNA for long periods under a variety of environmental
conditions".
In their new paper published in "Genome Research", Miller, Schuster,
and their colleagues describe the completion of the mitochondrial genome
sequences of two thylacines; one specimen at the Smithsonian Institution's
National Museum of Natural History in Washington, DC, and the other specimen
at the Swedish Museum of Natural History in Stockholm. |
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Thylacine
specimen 1 (GenBank accession number: FJ515780).
Courtesy: National
Museum of Natural History (Smithsonian Institution), Washington, DC.
Photo: International Thylacine
Specimen Database, 5th Revision 2013.
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The Smithsonian specimen is a prepared skin; museum ID number: USNM 125345.
The skin is from a young adult male thylacine that died at the National
Zoo in Washington, DC, in 1905. The male arrived at the National
Zoo on the 3rd September 1902 as one of three
pups with their mother. The family group was purchased for the
zoo by Dr. Frederick Webster Goding, the American Consul in Newcastle (NSW)
1898-1908. The male died at the zoo on the 10th January 1905. |
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"Tasmanian
Wolf and Cubs". Image: Charles R. Knight
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The above painting by Charles Knight (1874-1953), shows the Washington
Zoo's female thylacine and her two surviving pups; one of which is the
young male (specimen 1) in the genome team's study. |
