Devil Facial Tumour Disease: First Resistant Individuals Discovered

When I was a kid one of my favourite tv shows was Taz-Mania, Taz’s short temper and weird way of speaking were great and the juxtaposition of his super middle-class family was hilarious. At the time I don’t think I was even aware that the Tasmanian Devil was a real animal or that Tasmania was a real place, but they are. The island of Tasmania can be found tucked up underneath Australia directly south of Melbourne. It is the world’s 26th largest island and has a population of about half a million humans and, sadly, a rapidly decreasing population of Tasmanian Devils.

Unusually, it isn’t the encroachment of that human population that is causing the decline. Nearly a half of the island is protected within nature reserves and up until twenty years ago the population was at historically high levels having been hunted to near extinction earlier in the 20th century. At about the same time as the cartoon was at its most popular in the mid 1990s, though, there was the first reported sighting of a disease that has gone on to wipeout more than 80% of all devils in just twenty short years.

The illness, atypically, is a cancer that acts as an infectious disease; this is only the third known example of such a thing happening. Though not quite as crazy as portrayed by Looney Tunes, devils can be aggressive and will often scratch and bite each other when competing for food or mates. These open wounds have proved the perfect breeding ground for what is called Devil Facial Tumour Disease. Once infected the tumour grows and grows and begins to inhibit the animal’s ability to eat. The cancer was believed to be completely fatal and for some years the expectation was that the species would be extinct in the next decade or so. To try to mitigate this 15 healthy devils were moved to Maria Island in 2012 to try to act as a secure insurance population.

For the first time in twenty years, however, there is a small ray of hope for the devils of Tasmania. A new open access paper in Nature has reported that there are now some devils that appear to be resistant to the tumour and genetic analysis of the resistant individuals have given clues as to why.

Researchers from Washington State University and the University of Tasmania have identified two regions of the devil genome that are significantly different in the population left behind after the disease has wrought its havoc compared to the population previously. The two regions contain 7 genes the homologues of which in humans are genes involved in immune function and cancer risk.

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Photo courtesy of Menna Jones

It is unclear at this point if the resistant devils are completely immune to the tumours or if they merely survive longer than the others allowing them to live long enough to breed and pass on their genes. As an aside, if you ever come across any of those silly people that don’t believe evolution due to natural selection is real then feel free to cite this to them as a great example of evolution at work.

It has been suggested that breeding programs should immediately start to incorporate the resistant individuals into their populations to help spread the version of the genes that confer resilience; it is not that simple, however. In 1941 devils were almost hunted to extinction as they were seen as a pest, indeed a bounty was offered by the government for every one killed. As the population bounced back their numbers increased but their genetic diversity did not. They went through what is known as a genetic bottleneck. This means that devils are genetically very similar to each other, they are an inbred population. We all know that inbreeding rarely turns out well in the long run and it could be this lack of genetic diversity that has left the devil population open to universal susceptibility to the tumour disease.

The devil population is now being forced through another bottleneck which will reduce the diversity still further, to artificially create yet another bottleneck in the captive population is unlikely to prove beneficial in the long term and may leave generations to come even less well equipped to fight off future threats. We must always be weary of the law of unintended consequences.

Heaping more problems onto the poor devils is the identification of a second line of Devil Facial Tumour Disease in 2014. It is not yet known if this will be the final nail in the coffin or if the devils resistant to the first line will also be resistant to this new one.

For now we have to learn more about what is going on, how the devils are coping and safeguard what small, isolated populations we can. It isn’t impossible that they will bounce back. The current population of european bison is derived from just 12 animals and evolutionary genetics has shown that all 7.4 billion of us are descended from only about 1,200 people that left Africa about 30,000 years ago. How many species over the millennia that were randomly wiped out due to disease is not something we’re ever likely to know, but hopefully the Tasmanian Devil will not be added to that list.

1280px-Sarcophilus_harrisii_taranna
Image courtesy of JJ Harrison
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