The infectious tumors threaten to wipe out the small, fierce marsupials, but genetic analysis shows rapid evolution may come to the rescue.
It’s not unusual for scientists to study creatures half a world away, but Washington State University geneticist Andrew Storfer never imagined himself working with Tasmanian devils.
His main interest is viruses that attack amphibians. When Storfer scored a Fulbright grant in 2008, he intended to study Australian tree frogs decimated by an epidemic. But it soon became clear he couldn’t finish that project in a year.
So when a colleague invited him to visit Tasmania, Storfer jumped at the chance and became fascinated by the bizarre cancer threatening to wipe out the Australian island’s famous, ferocious marsupials.
Now, Storfer and an international team of researchers say they’ve made one of the first hopeful discoveries since the disease surfaced 20 years ago: Devils appear to be evolving resistance to the cancer.
“This gives us hope for the survival of the Tasmanian devil,” Storfer said.
The results were published Tuesday in the journal Nature Communications.
Researchers have been racing to understand the grotesque facial tumors that began showing up on the small mammals beginning in 1996. The disease is almost always fatal, killing its victims through suffocation or starvation as the tumors grow and spread.
The population of Tasmanian devils on the island — the only place where they occur — has plummeted by about 80 percent. The disease now threatens the few uninfected populations on Tasmania’s West Coast.
The tumors are caused by a rare, infectious form of cancer unlike anything that occurs in humans. Animals pass tumor cells to each other through biting — a common behavior in a species known for its combativeness.
Computer models predicted that many devil populations would be extinct by now — yet small numbers of animals persist in most parts of the island. That’s what prompted Storfer and colleagues from Idaho, Australia and the U.K. to take a closer look at the animals’ genes.
“What motivated our study was trying to figure out why these populations are surviving despite predictions of extinction,” Storfer said.
With access to a collection of 10,000 tissue samples, many collected before the cancer appeared, the scientists were able to compare genes between pre- and post-epidemic populations.
“The idea was to scan the genome of the Tasmanian devil to see if we could find genes that are changing in frequency,” Storfer explained. “Our hope was that we would find some genes that were perhaps associated with resistance to cancer or immune function.”
Using a rapid scanning method, the team examined portions of the genomes of nearly 300 animals, from three separate locations across Tasmania.
“It was actually pretty challenging to do,” said WSU researcher Brendan Epstein. The scientists examined 800,000 sites on the genomes, with no idea at first where changes were most likely.
They zeroed in on seven genes that appear to have changed most dramatically since the start of the epidemic. Five of those genes may play a role in fighting off cancer, possibly directing immune cells to attack tumor cells, Epstein explained.
There’s much more research needed to determine exactly what the genes do and to verify that the changes actually reflect evolution in action. The scientists also want to perform more in-depth genetic analysis on several animals. But if the results hold up, they would show that evolution can happen with blazing speed even in complex animals.
“Twenty years is extremely rapid, especially for a mammal,” said co-author Paul Hohenlohe of the University of Idaho.
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Since there is no sign of widespread immunity to the cancer among Tasmanian devils, it’s possible that the genetic changes simply allow animals to live long enough to reproduce, Epstein added.
In the short term, genetic insights into tumor resistance could help scientists identify the hardiest animals for captive breeding programs launched as a last-ditch effort to ensure the devils’ survival, Storfer said.
But the species’ long-term prospects are complicated by the fact that a second form of infectious cancer was recently discovered in some areas. That supports the idea that Tasmanian devils may be naturally vulnerable, possibly because populations have little genetic variability, Storfer said.
Most of the WSU work was conducted in the lab, but Storfer spent some time in the field with biologists capturing and examining wild animals.
The devils probably got their name from the cacophony of howls and roars and screeches they produce, not to mention their fearsome jaws and teeth. As marsupials, akin to wombats and North American opossums, they raise their young in pouches.
About the size of a Jack Russell terrier, Tasmanian devils may bite and attack each other while mating and jockeying for food or dominance — but they are rarely aggressive to humans, Storfer said.
“It’s really sad to see them with these tumors.”