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Devil Facial Tumour Evolves to a More Deadly Form

The tumours of the Devil Facial Tumour Disease (DFTD) at a population in north-western Tasmania have been changing and competing over the years to increase infection rates.

The findings, published in Proceedings B, indicate that future research efforts to fight the disease that is decimating the Tasmanian devil population will need to focus on the tumour and its ability to change, as well as on the devils and their genetics.

Researchers have been monitoring devils in north-western Tasmania for a decade, regularly taking tumour and blood samples every 3 months. It is the only site where the researchers have a long-term genetic and immunological data set of devils and tumours from the beginning of the DFTD epidemic outbreak.

Lead author Dr Rodrigo Hamede of The University of Tasmania said this research is the first solid evidence that tumour lineages are competing and having an effect on transmission. “The tumour is subject to changes for its own benefit rather than the devil’s benefit. The tumour is a living organism and wants to do whatever is best for itself.”

Hamede said that only 3 years ago the devil population they had been sampling had not declined, the disease prevalence was very low, and animals were surviving for quite a long time and dying from old age, not from DFTD. What changed recently?

“We were looking at the devils from different angles but we couldn’t associate the reduced effects of DFTD with devils’ genotypes or immune responses in this population.

“Then we started looking at the tumour and we realised that the tumour in this population was tetraploid. That means it has four copies of chromosomes rather than two. So it was an unusual tumour strain.”

Since then the tumour strain has changed again and become diploid, a more normal and stable genotype. This coincided sharply with a large and rapid population decline, higher infection rates and younger devil deaths.

“We began seeing basically the same patterns in that population that happened everywhere else,” Hamede said. “Our unique population was not unique anymore because the tumour had changed.

“The tumour used to allow devils to survive longer and the population to sustain itself. Then the diploid strain arrived and out-competed the more benign tumour strain, and has caused a severe population decline. The diploid tumour at this site is the older and most common tumour type which is spread over most of Tasmania.

“This is the first evidence since we’ve been studying this disease that the tumour strain can have an effect on the epidemic outcome and population impacts in devils.”