Australasian Science: Australia's authority on science since 1938

Devil Is in the DNA

Credit: Rodrigo Hamede, School of Zoology, University of Tasmania

Death of the devil usually occurs only 6 months after tumours first appear. This is often caused by starvation as the devils are unable to feed themselves. Credit: Rodrigo Hamede, School of Zoology, University of Tasmania

By Katrina Morris

Ancient DNA is helping shed light on why the Tasmanian devil is being driven to extinction.

Tens of thousands of years ago, devils roamed widely across mainland Australia. However, since this time they have been on a downward spiral, and today this iconic species is heading towards extinction. What is bringing about the downfall of the world largest remaining marsupial carnivore?

Humans first arrived in Australia around 50,000 years ago and rapidly spread across the continent. Rock art depicting devils has been found as far north as the Kakadu National Park.

Later, dingoes were introduced from South-East Asia by humans 5000 to 10,000 years ago. The dingoes dispersed across the mainland of Australia but never reached Tasmania, which was separated from mainland Australia around 10,000 years ago.

About 3000 years ago, devils disappeared from the mainland. The reason for this extinction is a mystery.

The devil’s demise has been largely blamed on the introduction of dingoes, which are able to outcompete devils. However, some have argued that other factors also played a large role in driving the devils from the mainland.

The mainland of Australia was becoming drier, which would have left less suitable habitat for devils. Around the time of the devil’s mainland extinction there were also technological developments in the aboriginal population that may have allowed them to hunt devils more effectively and compete with the devils for prey. Others have asked whether diseases, possibly introduced along with dingoes, may have spread to devils and aided in their extinction on the mainland.

The last sanctuary of the devils was in the island state of Tasmanian, where they appear to have flourished. This good fortune would not last though. Since the arrival of Europeans in Tasmania around 200 years ago, devils have suffered several population crashes.

Early Europeans settlers did not take to Tasmanian devils. They were described as “untameable and savage in the extreme”. Their otherworldly shrieks and screams, and their reputation as stock-destroying vermin, while unwarranted, earned them the name of devils.

They were widely persecuted through the 1800s and early 1900s by settlers intent on eliminating the species. A bounty was provided for devil carcasses for more than 80 years.

In addition to this persecution, a “distemper-like” disease was observed in the early 1900s in both devils and the thylacine, which became extinct soon after. This extinction left the devil as the world’s largest surviving marsupial carnivore.

After the 1950s the devil population was recovering and by the mid-1990s there were around 150,000 devils in the wild. Once again this stability would not last.

In 1996 a tumour that would be later known as the devil facial tumour disease (DFTD) was first witnessed by a wildlife photographer. This disease rapidly spread through the devil population, and since this time has wiped out more than 85% of Tasmanian devils in the wild.

Devils are now an endangered species. Without intervention they are expected to be gone from the wild within 35 years.

The disease manifests as large tumours, mostly around the devil’s face and neck, although metastases to other parts of the body are common. Death of the devil usually occurs only 6 months after the tumours first appear. This is often caused by starvation as the devils are unable to feed themselves.

DFTD has an incredibly high mortality rate. After the appearance of tumours there have been no verified cases of devils surviving the disease.

When we think of cancer we usually think of a disease that occurs in individuals, or of a tumour that is triggered by a disease agent such as a virus. However, DFTD is extremely unusual in that it is a contagious cancer – the cancer cells themselves are the disease agent. When one devil bites into the tumour of another during fighting or mating, cancer cells are transferred to the new devil and can then establish into a new tumour.

The spread of this catastrophic disease has been linked to a lack of genetic diversity, and in particular a lack of diversity at key immune genes behind the major histocompatibility complex (MHC). The MHC is crucial for the recognition of bacteria, viruses, foreign cells and cancerous cells.

Due to their important role in the immune system, the MHC genes are highly diverse in a healthy animal population, allowing the population to recognise and fight a huge range of potential diseases. However, some species have reduced MHC diversity, and this makes them susceptible to disease outbreaks. In fact, MHC diversity in devils is far lower than other marsupial species such as wallabies and possums, and also lower than comparable carnivores such as wolves and cats.

Why modern devils have such low MHC diversity has been a mystery. It has been proposed that the population crashes experienced by Tasmanian devils over the past 200 years may be responsible for a loss of MHC diversity. On the other hand, low MHC diversity may predate European arrival in Tasmania.

In order to investigate this question we looked at the MHC diversity in devil samples collected over the past 200 years for museum collections as well as subfossil bones predating European arrival in both Tasmania and the mainland of Australia. These samples capture the genetic diversity of the past, allowing us to see how MHC diversity has changed over time.

As some of these samples are older than 3000 years, the genes are the oldest MHC genes sequenced to date and are also the oldest genes to have ever been sequenced in marsupials.

In the Tasmanian devils dating from the 1980s back to before European arrival we found gene variants that are very common in the modern devils. No new MHC gene variants were found, even in the oldest Tasmanian samples, and this tells us that MHC diversity was low before European arrival in Tasmania.

Therefore, the recent population crashes were not responsible for the loss of MHC diversity. Interestingly, it was recently discovered that the thylacine also had low genetic diversity prior to its extinction.

Although new MHC gene variants were discovered in the mainland devils studied, these samples still showed low MHC diversity. This indicates that MHC diversity was low in devils even before their extinction on the mainland.

At this point we cannot be certain of precisely when devils lost MHC diversity, but we can speculate that it may have been due to population contractions that occurred more then 10,000 years ago.

Around 10,000 to 50,000 years ago a mass extinction wiped out large carnivores in Australia, leaving only the devil and thylacine. Whether this extinction event was caused by climate change, the arrival of humans to Australia or a combination of these two is a hotly debated topic.

Whatever caused the extinction of large Australian carnivores may have also affected devils and thylacines. These species, which rely more heavily on scavenging and predation of small prey, may have narrowly escaped extinction but at a cost to their genetic diversity. A population crash and population fragmentation may have reduced their genetic diversity to the levels we see today.

Low MHC diversity in the surviving devil population would have made the devils highly prone to disease outbreaks. This may have contributed to their extinction on the mainland.

One intriguing possibility is that dingoes may have brought with them a new disease that was previously not encountered by Australian wildlife. This disease, along with new competition from dingoes, may have been the final straw for both devils and thylacines on mainland Australia.

Tasmania, separated from the mainland before the arrival of dingoes, was a last sanctuary for the world’s largest remaining marsupial carnivores. However, with the arrival of Europeans to Tasmania in the early 1800s came domestic dogs. Once again, these dogs may have introduced new diseases to the devils.

This may explain the disease epidemics observed in the devil population in the 1800s and early 1900s. A disease described by naturalists as “distemper-like” in both devils and thylacines in the early 1900s likely contributed to the tragic extinction of the thylacine soon after, as well as the population crash that was observed in devils at the time.

The ancient loss of MHC diversity in devils has produced a species that is highly vulnerable to the catastrophic effects of novel diseases. This has enabled the spread and devastating impacts of DFTD.

As the disease continues to spread across the state unimpeded, with no preventative or cure in sight, the captive breeding program is currently our best hope at preventing total extinction of the devil.

Our findings have reinforced the essential need to maintain genetic diversity, most importantly immune gene diversity, in the captive population of devils. This will preserve what little diversity is still left in the devils should the Tasmanian devil be lost from the wild.

Monitoring genetic diversity, especially immune gene diversity, from one generation to the next is vital to ensure that the genetics of the original founding animals is maintained. This will ensure that the problems associated with low immune diversity are not further exacerbated.

Tasmanian devils provide a stark reminder that maintaining genetic diversity at immune genes is critical for the long-term survival of species. Many endangered, threatened, or even populous species worldwide have low immune gene diversity. Even in species not considered to be at threat, low immune diversity makes them highly susceptible to devastating disease epidemics.

Our research highlights the need to study species that may not have been considered at risk of population collapse but may have low diversity due to historical effects. Understanding the genetics and carefully monitoring species with low genetic diversity is now necessary to prevent and manage disease epidemics.

With increasing population declines due to loss of habitat, climate change and invasive species, this monitoring will be crucial if we are to tackle biodiversity loss worldwide.

Katrina Morris is a PhD candidate at the University of Sydney.