News in Science

Human intervention speeds up devil cancer evolution

Selection pressures Efforts to control the spread of Tasmanian devil facial tumour disease (DFTD) have inadvertently sped up the cancer's evolution, research has found.

DFTD is one of only two known transmissible cancers, and is spread between Tasmanian devils (Sarcophilus harrisii) when they bite each other during social interaction. It usually kills the infected animals within six months.

Since it was first observed in 1996, populations in some areas have declined by more than 90 per cent.

In an attempt to combat the spread of the disease on Tasmania's Forestier Peninsula infected devils, comprising about a third of the population, were removed between 2006 and 2010.

"It was hoped that removing the infected animals would stop the disease from spreading, but instead it made the cancer evolve," says study lead author and geneticist Dr Beata Ujvari from the University of Sydney.

Artificial selection

Dr Ujvari, along with other Australian researchers, looked at the chromosome structure of tumour tissue samples taken from 11 DFTD sites around Tasmania.

Their findings appear in the journal Evolutionary Applications.

They found that in tumour samples taken from the Forestier Peninsula the number of chromosomes increased (these are known as tetraploid tumours), while in samples from the other study sites the numbers of chromosomes in the tumours remained the same.

"This is significant because more chromosomes mean the cells divide more slowly," explains Ujvari.

"Devils were removed from the population as soon as lesions appeared, however, cancers that grew more slowly avoided detection, remained in the population and became increasingly prevalent."

Ujvari says that it is likely that both strains were always present in the population, but removing devils with faster growing tumours has artificially selected the slow-growing cancer line.

Wide-reaching implications

"Currently devils taken to insurance populations are kept in quarantine for six months before being introduced to the group," Ujvari says. "This research suggests that we need to reevaluate the time devils are kept in quarantine to ensure neither type of tumour is present."

The research has another implication too: in humans with cancer, medical treatments such as chemotherapy have also been shown to act as selection agents and accelerate the development of aggressive and drug resistant tetraploid cancers.

"Any progress made in combating the disease in devils may also have implications for human cancer research," says Ujvari.

The Devil research community commonly refers to the cancer-causing agent as an obligate parasite — a novel way of referring to transmissible cancers.

"It can't live outside devils," explains Ujvari, "and it spreads like a virus infection, although it isn't caused by one.

"The biological behaviour within devil populations makes DFTD a unique cell pathogen."