Fighting like the devil to survive
Few things in nature are as gruesome as devil facial tumor disease, the deadly cancer ravaging Tasmanian devil populations.
The cancer first appears as clusters of lesions and lumps around the little marsupials’ mouths, but then it metastasizes until the creatures’ whole faces are covered in angry red welts. Eventually the tumors become so extensive that the animals can no longer eat, and they starve to death — unless they suffocate first.
Worse still, the disease is contagious. Unlike most cancers, which involve cells that betray their own hosts by multiplying at deadly rates, DFTD originated in a single individual and spread like a virus. In the 20 years since itwas first discovered, DFTD has killed at least 80 percent of the wild Tasmanian devil population. The devils have no immunity; once infected, they are almost certain to die.
But probably just in the nick of time, the devils are starting to put up a fight. In a study in the journal Nature Communications, scientists report that they’ve found new variants in parts of the Tasmanian devil genome associated with the immune system and cancer response — proof, these scientists believe, that the creatures are evolving resistance to the fast-moving disease.
Andrew Storfer, an evolutionary geneticist at Washington State University and a co-author on the study, had been following the sad case of the Tasmanian devils for years. He’d seen images of the creatures, which are the size of a Pekingese and have the ferocity of a pit bull, covered in painful red tumors. He’d also read the studies predicting local extinction of some populations in a matter of years.
Strangely, some of those populations still persisted, despite the epidemiological models that predicted otherwise.
Scientists have been taking DNA samples of Tasmanian devils for decades — long before the current DFT Dout break. Storfer and his colleagues were able to gather up the genomes of more than 250 animals, quickly sequence relevant sections, and compare their samples to figure out where they differed.
Tasmanian devils have little genetic diversity among them, because their population has been through several periods when a sharp decline in population left just a small number of Tasmanian devils to spawn the next generation. This lack of diversity may have contributed to the weakness of their immune systems. If Tasmanian devils had evolved resistance to the cancer that ravages them, Storfer and his colleagues might find evidence of gene variants that didn’t exist before the outbreak.
They found what they sought: two regions of the genome, both associated with immune function and cancer response, where devils from affected communities had variants that didn’t exist in the pre-outbreak population.