The Tasmanian devil (Sarcophilus harrishii) is the largest remaining marsupial carnivore. They are found throughout the Australian island of Tasmania and can be found in all native terrestrial habitats. Devils are nocturnal, highly social marsupials but are extremely aggressive towards each other.

Tasmanian devil numbers have dramatically reduced since the late 1990s and devil facial tumour disease (DFTD) is the primary cause of this decline. DFTD is a contagious cancer which is passed from devil to devil through biting during social interactions. Tumours grow on the faces and mouths of the infected devil and individuals die within months of infection. DFTD was first seen in 1996 in Mount William in north eastern Tasmania and has since spread to over 65% of Tasmania, with only the populations on the west coast and the far north-west remaining DFTD-free. Over the last 20 years, it is estimated that there has been localised declines in devil populations by as much as 90%, with an overall species decline of more than 80%. In 2008 the Tasmania devil was listed as endangered on the IUCN red list of threatened species.

DNA sequencing of the devils genome has shown that they have relatively low genetic diversity compared to other Australian marsupials. In particular, the lack of genetic diversity at the major histocompatibility complex (MHC) together with downregulation of the host MHC by the DFTD, is thought to be the major cause of the rapid spread and universal susceptibility to DFTD.

But there is hope for the Tasmanian devils. Despite epidemiological studies suggesting that long-infected populations of devils should have become extinct by now, there are still small numbers remaining. Andrew Storfer, of Washington State University, and a team of international researchers have discovered genomic evidence to suggest that the Tasmanian devil is evolving resistance to DFTD.

Using tissue samples collected between 1999 and 2014, the team of researchers were able to sequence and analyse the genomes of 294 Tasmanian devils from three locations across Tasmania. Genomes were compared before and after the emergence of DFTD at each location. The team identified two genomic regions (containing genes related to immune function and cancer risk in mammals) which showed evidence of positive selection in response to DFTD. In short, the Tasmanian devil is evolving resistance to DFTD.

Given the relatively recent appearance of this deadly disease, the Tasmanian devil has exhibited an extremely rapid evolutionary response to the selection pressure imposed by DFTD. Vaccine development is ongoing but it may take several more years to develop a working vaccine. Meanwhile, programs to remove sick devils and quarantine healthy devils to build up a healthy DFTD-free captive population continue, to ensure the continued survival of the species.

Vickie Flint, PhD

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Epstein et al. Rapid evolutionary response to a transmissible cancer in Tasmanian devils. Nature Communications, doi:10.1038/ncomms12684, 2016.

Photo credit: https://www.zoo.org.au/healesville/animals/tasmanian-devil

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