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A ProMED-mail post
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International Society for Infectious Diseases
Date: Mon 27 Jun 2011
Source: New Scientist [edited]
Despite its ferocious nature, the Tasmanian devil is a creature faced
with extinction, the victim of a gruesome facial tumor disease. Now
the 1st genetic sequencing of these carnivorous marsupials has
revealed that we had a hand in their decline: centuries of human
interference left the devils stripped of genetic diversity and
vulnerable to disease.
This meant that when the parasitic face cancer dubbed "devil facial
tumor disease" appeared in 1996 it rapidly spread through the entire
population. As a result, the Tasmanian devil, or _Sarcophilus
harrisii_, population has fallen over 60 per cent since 1996.
The disease is transmitted by physical contact, mostly biting during
sex. It is almost always fatal and has spread across most of
Tasmania.
Due to the species' sharp decline, the International Union for the
Conservation of Nature (IUCN) has classed it as endangered. Some
studies estimate the marsupials could be wiped out within decades.
Marsupial sequencing
--------------------
To find out why they cannot fight the cancer, Stephan Schuster of
Pennsylvania State University in University Park and colleagues
sequenced the genomes of 2 Tasmanian devils, the 1st Australian
marsupials to be sequenced.
Schuster chose animals from opposite ends of the island, which should
have been as genetically distinct as devils get. But he found they
were pretty similar: their genomes only differed at 915 000 sites. A
similar comparison showed that 2 humans from China and Japan differ at
3 257 000 sites, says Schuster.
Genetic analysis of a further 175 wild animals and 7 museum specimens
showed that devils have had a low genetic diversity for over 100
years.
Humans had a heavy hand in this. First the devils were wiped out in
mainland Australia by dingoes brought in by settlers, then those that
remained in Tasmania were hunted as pests, causing several population
crashes. As their genetic diversity was slashed, the devils were left
vulnerable to disease.
This is clearly at the root of their problems, says Katherine Belov
of the University of Sydney, Australia. "Devils are essentially
immunological clones, so tumors pass between them without triggering
an immune response," she says.
The Australian and Tasmanian governments have set up a conservation
programme, capturing disease-free devils to establish an "insurance
population". Housed at sites like the Devil Ark in Somersby, New South
Wales, these devils could re-populate Tasmania if the wild population
is wiped out.
Missing diversity
-----------------
But Schuster says the captive population is missing some of the wild
population's diversity, such as it is. He found 7 subtly distinct
populations on the island and says individuals from each must be
included. But as nobody has genotyped the captive population we don't
know what is missing. Based on partial information on where the
individuals were collected, Schuster suspects the captive genotype is
not as diverse as it could be.
Having a large genetic diversity is essential for any captive
breeding programme, agrees Craig Hilton-Taylor of the IUCN, based in
Cambridge, UK. Schuster says conservationists need to look at genetic
diversity when they assess a species' wellbeing. He points out that
before the facial tumour disease appeared, the IUCN considered devils
to be of "least concern" because their population and habitat were
reasonably large. In fact they were genetic sitting ducks, says
Schuster.
The IUCN doesn't require its scientists to do genetic testing on
species they are assessing, although the information is used if they
do. Hilton-Taylor agrees the information can be invaluable, but says
systematic testing would be difficult and expensive.
Journal reference: Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.1102838108 [abstract available at
[Byline: Michael Marshall]
--
Communicated by:
ProMED-mail
[This article does not mention if there were any sequences of
Tasmanian devils in some other zoo in the world that might lend
genetic diversity or be a tumor free source of genetic material. But I
cannot remember if the Tasmanian devil has ever been sent to other
zoos - Mod.TG]
[Photos of Tasmanian devils can be seen at
Facial tumor at:
Tasmania can be located on the HealthMap/ProMED-mail interactive map
of Australia at
[see also:
2010
----
Devil facial tumor disease - Australia (TS) 20100102.0019
2008
----
Devil facial tumor disease - Australia: (TAS) 20080718.2174
2007
----
Devil facial tumor disease - Australia (TAS) (02) 20071004.3289
Devil facial tumor disease - Australia (TAS) 20070218.0616
2006
----
Devil facial tumor disease - Australia (TAS) (02) 20061024.3051
Devil facial tumor disease - Australia (TAS) 20060201.0328]
.................................................sb/tg/mj/jw
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