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SYDNEY: Strikingly low genetic diversity in the immune genes of Tasmanian devils might be what is allowing an infectious facial cancer to spread like wildfire, according to a new study. The discovery raises questions about how other endangered species will suffer the effects of inbreeding. Disfiguring and fatal tumours started to appear on the faces of 'Tassie' devils in the mid-1990s. Since then, the population of the world's largest remaining marsupial carnivore has plummeted by 50 per cent (See, Tasmanian devils face extinction, Cosmos Online). Spread by bite In 2006, government experts in Tasmania – a large island state off the southeast coast of Australia – discovered that the tumours all shared an unusual rearrangement in their chromosomes, suggesting that in fact they had all come from an ancestral cancer in one individual devil. The researchers predicted that the disease spreads by the transfer of cancerous cells during fights when the animals bite one another around the face and neck. This left an interesting question: how were the tumour cells able to attach themselves to new individuals? Healthy immune systems are good at recognising and rejecting foreign tissue. In human organ transplants for example, many are eventually rejected, even with the use of strong immune-suppressant drugs. To discover more, researchers led by Katherine Belov and Hannah Siddle of the University of Sydney's Faculty of Veterinary Science decided to test the diversity of immune system genes known as MHC class I and II. These genes usually show high variation among different individuals in a population and are involved in recognition of grafts of foreign tissue. "We need to understand why devils don't appear to mount an immune response to these tumours so populations can be managed, and hopefully, the disease controlled," Siddle told a University of Sydney publication. Evading detection They theorised that either the the tumour was somehow able to evade detection by the immune system by altering the expression of MHC genes, or that the genetic diversity of the entire devil population is so low that that their bodies are unable to recognise foreign tissue. As they reveal this week at the Annual Conference of the Genetics Society of Australasia, held at the University of Sydney, their results show that tumours are not somehow 'blocking' the expression of the immune genes, and confirm that it is paltry genetic diversity which is allowing the bizarre tumours to spread so easily.
"It is very unusual for a tumour to be transmitted in this way," commented Hamish McCallum, an expert on the species at the University of Tasmania in Hobart. "This study confirms that cancer isn't hiding, but that there is so little variation in the MHC genes of devils that they don't even recognise it as foreign. "This is a significant, but depressing finding," he added, "because it suggests it will be very difficult for the animals to evolve resistance to it, unless we find individuals with more diversity, which is unlikely." Population bottleneck McCallum said the research is also significant because the scientists discovered that MHC genes are similar throughout different samples of tumour tissue, but different to the MHC genes in each of the host animals they infect. "This is a second line of evidence to back up the claim that cancer is a clonal cell line that originated in a single individual." Tasmanian devils are likely to have gone through a population bottleneck on the island 10,000 years ago during the last Ice Age, said McCallum, explaining their low diversity. Numbers may have plummeted to less than 500 animals at that time, as cool conditions pushed them into a few remaining suitable patches of habitat. McCallum likened their diversity to that of cheetahs in Africa, which also suffered a bottleneck at some time in the past. One study in the 1980s showed that diversity was so low in the big cats – something akin to that between close relatives in humans – that skin grafts between unrelated animals would not be rejected. The low diversity of devils has been further exacerbated by humans in recent years, who are pushing the species towards extinction. McCallum's figures suggest that Tasmanian devil numbers are now down to around 15,000 to 20,000 individuals, from up to 40,000 only a decade ago. "Because the facial tumours themselves are so similar, it means it might be easier to create a vaccine than for other cancers," said McCallum. "But that's still a long shot." Siddle and Belov believe that the lack of immune barriers may mean that the only way to control the disease it to remove infected individuals from the population. Readers' commentsDFTD causeI'd like to pose another theory on the origin of DFTD. If there is no known method of detecting DFTD until tumours appear, how do we know the disease was not present on mainland Australia? Is it conceivable that DFTD was the cause of mainland Australia's near-extinction of the devil? If the disease was somehow carried from mainland to Tasmania, it stands to reason we would be now discovering all tumours trace back to a single origin. When I say "near-extinction", I am referring to the 5 or 6 live and road-kill devils found throughout Victoria between about 1911 and 1991. A half dozen animals being found, collected, submitted to museums and preserved over an eighty year period from locations in excess of 100km apart sounds something too coincidental to be attributable to random escapees of illegally kept specimens, or Tasmanian souvenirs shipped over in some car boot on the ferry only to be dumped in Victoria later. In other words - they're out there. Professor McCallum is quoted as saying it is unlikely we can find significantly genetically different devils, but is it? How much better if they are the remnant population that survived a DFTD wipeout 500 years ago? It was this century, and only a few years after an official wildlife survey in Gippsland that a private researcher established large populations of mountain brushtail possums where, quite simply, none were supposed to exist. Speculation, I'll concede - but who's researching the Victorian connection? Chris. Submitted by Chris on 2 July 2007 - 1:46am.
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Tasmanian devil epidemic: cause isolated?
One thing is a "cause" and another is how it "spreads".
Almost 100,000 native animals were killed by 1080 in Tasmania in the 2003-04 financial year. Tasmanian Devils eat dead animals. This is their diet. Devils not living in Tasmania, such as the Danish ones given to Princess Mary, are healthy. A long term effect of eating carcasses laced with poison could build up and cause this cancer. Our industry or governments won't want to hear this conclusion, but as a lay person, this cause sounds awfully plausible. This disease has been decimating the Devils over the last decade, killing an estimated 50% of them. The whole of Tasmania should be come a native flora and fauna reserve, and there should be no more industrial or domestic developments, a strict control of domestic species and all logging should permanently cease. We are the greatest killers of wildlife in the world in Australia. We need to cater for our wildlife before we lose these iconic and unique animals too.