The Tasmanian devil, popular as the destructive, out-of-control cartoon character from the Warner Brothers’ Looney Tunes show, was classified as an endangered animal in 2009 and predicted to go extinct from its habitat in the Tasmanian islands south of Australia. The reason this time was not climate change, habitat loss or other human inflictions. It was an outbreak of cancer that had gone out of control.
Once prevalent in Tasmania and Australia both, this carnivorous marsupial has only the Tasmanian islands to call its home now, and here, too, matters have been worsening. Over the last two decades, an unusual affliction called the devil facial tumour disease (DFTD) has spread across the population like wildfire, killing 80% of the animals. DFTD is unusual because it is a cancer – and unlike other cancers, it is infectious. It manifests as lumps on the face and neck of an animal that slowly grow into large, full-blown tumours. Infected animals die in three to five months.
The cancer’s spread has been eased by how the devils behave. Specifically, the devils are aggressive, quarrelsome creatures who are often seen snarling, fighting and biting each other. DFTD spreads from one individual to another through these bites. In the case of healthy humans, foreign cells in the blood aren’t tolerated. The immune system’s surveillance mechanism, which involves special molecules on the cell surface called the major histocompatibility complex (MHC), help identify and destroy the foreigners. However, this is mechanism doesn’t work in the devils.
A genius in the genes
In these marsupials, DFTD plays a trick by decreasing the number of MHC molecules on the surface of the cells, making their surveillance system defective. Together with this, another factor, the low genetic variability among MHC molecules hampers the surveilling abilities of the immune system. The result is that the Tasmanian devils can’t easily distinguish their own cells from foreign ones. And when the cancerous cells infect a healthy individual, the immune system doesn’t react appropriately to the infection, enabling the spread of the disease.
Based on the rate at which DFTD was infecting the wild population, it was predicted that the devils would completely disappear from many regions of the islands. However, some of these populations were later found to persist. What was happening? Was it a case of Darwin’s survival of the fittest, where natural selection preferred individuals that were more resilient to the cancer? To answer this question, a team of researchers, lead by biologist Andrew Storfer from Washington State University, Pullman, looked for genetic signatures of natural selection in devil populations. Their results were published in the journal Nature Communications on August 1.
They suggest that all might not be lost for this species because it might be rapidly evolving.
The DNA of 294 individuals from three different sites in Tasmania was analysed at two times: before and after the disease hit the population. The researchers used the relatively new next-generation sequencing technique called restriction-associated DNA (RAD) sequencing. It is useful when the DNA sequence of a wild species has not been characterised before and so there is no reference sequence to compare it with.
When sequencing individual DNA samples, the technique allowed scientists to identify up to 90,000 single base-pair changes in the sequence across the population. These changes then played the role of flag-posts that helped them figure out nearby genes that were showing variations. Variants of some genes were more common in the population after the spread of the disease, indicating that those genes were being affected by the cancer. And of the seven such genes, five were implicated in melanomas and myelomas as well as immune responses and inflammation in other species. The researchers are now trying to understand the mechanism by which these genes might help in fighting the cancer.
What is striking is the small number of generations – four to six – during which the genetic changes occurred; from an evolutionary perspective, this is an extremely short span of time. At the same time, it has been known that how quickly these changes occur depends on what pressures survival is exerting on the animal, and DFTD appears to be imposing a strong pressure indeed. DFTD causes death in less than six months and infects individuals while they are still of reproductive age. The study indicates that the Tasmanian devils population is rapidly evolving to persist despite the deadly cancer.
An apex predator
However, beyond the context of allowing the devils to survive, T.N.C. Vidya, a biologist, thinks the mechanism is neither new nor surprising. “The paper was interesting but rapid evolutionary response to strong selection pressure is well known, at least among evolutionary biologists,” she said. Vidya worked at the Jawaharlal Nehru Centre for Advanced Scientific Research in Bangalore and was not associated with the study.
“It is appropriate that the authors sampled devils both pre- and post-DFTD to examine possible genetic signatures of selection. It is, however, not clear what the extents of decline in population size and density were between these sampling times and how they may have affected the samples collected and, hence, the results,” she added.
Conservation efforts are on in full swing, with some animals being bred in captivity to keep them away from their cancer-infected counterparts. Trials are also being conducted to release vaccinated animals in the wild to prevent the spread of the disease. However, Vidya felt that the study may not have contributed usefully on this count: “directly assessing survival and the ability to produce offsprings in the devils would have been more useful than the genomic research if the main point of the study was to help with conservation.”
The Tasmanian devils are the largest carnivorous marsupial alive, after the Tasmanian tiger went extinct, possibly due to a disease. In a similar vein, the Darwin’s frogs endemic to Chile and Argentina are also believed to have all died off due to the spread of a fungal disease. Species endemic to islands are particularly at risk of extinction because of the isolated environment in which they live; the species can’t move away into newer, safer spaces. Being an apex predator at the top of the food chain, the Tasmanian devils are also particularly important for the ecology of the islands they call their home. Their absence will lead to an increase in the decaying meat that they scavenge and the animals they hunt, like possums and some insects, and so affect the entire ecosystem.
Neelakshi Varma is a masters student in molecular biology at the Jawaharlal Nehru Centre for Advanced Scientific Research, Bengaluru.
Note: It was earlier stated that myeloma and melanoma are both blood cancers. Melanoma is a cancer of the skin and the mistake has been corrected.