Low genetic diversity may be Tasmanian devils’ bane
Lack of genetic diversity among Tasmanian devils may be making the species particularly vulnerable to an infectious facial cancer that has devastated the population in Australia, according to scientists who have sequenced the animals’ genome.
Since 1996, devil facial tumor disease — or DFTD — has wiped out at least 60% of the animals in the wild, and close to 90% in some areas. The cancer is thought to have originated as a nerve tumor and became a parasite of the devils, jumping from animal to animal through bites. Experts estimate the devils may face extinction within 25 years unless humans intervene with a cure or other measures.
After decoding the genomes of two devils from opposite corners of the Australian island of Tasmania, researchers found that the devils shared 47% of the genetic markers identified, they reported this week in Proceedings of the National Academy of Sciences.
Experts think the low genetic diversity is a consequence of widespread killing by bounty hunters, who drove the devils to the brink of extinction before laws were passed to safeguard them in 1941. With all devils alive today descended from a relatively small population, there isn’t much variation in their immune system genes. That could help explain the rapid spread of DFTD and strongly suggests that most of the animals — if not all of them — are at risk, scientists said.
“The key was to measure this while there is still time to respond,†said Stephan Schuster, a molecular biologist at Pennsylvania State University and the senior author of the study.
The team began by analyzing the genomes of two devils: Spirit, who hailed from southeast Tasmania, and Cedric, who was from the northwest. Spirit died of DFTD, but Cedric was able to mount an immune response and survived multiple infections from the cancer before he succumbed to it in 2010.
“Cedric was the hope that devils would be able to get rid of the disease,†said Katherine Belov, a geneticist at the University of Sydney who was not involved in the study.
Schuster found that Cedric expressed a gene involved in tumor suppression, which may have helped him fight DFTD for so long. Spirit, on the other hand, had a mutation in this gene that presumably made it function poorly.
Scientists do not yet know how common Spirit’s mutation is in the devil population or whether Cedric’s version of the gene was responsible for his cancer-fighting ability. They plan to analyze the genomes of healthy and sick devils — including Cedric’s half-brother, Klinky, who died soon after contracting DFTD — to try to answer these questions.
The scientists also reported that Cedric’s and Spirit’s genetic codes were different at about 1 million base pairs — the chemical building blocks of DNA — out of a total of 3 billion base pairs in the entire devil genome.
With the devil genome in hand, scientists are now trying to develop better conservation strategies to make sure they don’t create another genetic bottleneck and make the animals even more vulnerable to natural and man-made threats. Breeding programs in captivity may be conservationists’ best hope for preserving the species.
“This work provides us with another tool through which we can ensure in the long term that we retain as much genetic diversity as possible,†said Andrew Sharman, manager of the Save the Tasmanian Devil Program in Australia, who wasn’t involved in the study.
Schuster said he hopes this strategy can be used to protect other at-risk species such as the California condor or the American bison in Yellowstone Park, which, like the Tasmanian devil, has been the victim of human hunters.
