Isolation Doomed the Tasmanian Tiger

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The last known Tasmanian tigers.
University of Connecticut

THE GIST

- Before the Tasmanian tiger was hunted to extinction by humans, it had extremely low genetic variability.

- The finding could shed light on Tasmanian devils, which are now on the brink of extinction and suffering from a contagious cancer.

- Both species were isolated in Tasmania after a flood separated the island from mainland Australia.

The enigmatic Tasmanian tiger has, in essence, spoken from the grave, revealing why this dog-resembling marsupial went extinct and why Tasmanian devils could suffer the same fate.

A study published today in the journal PLoS ONE shows that the Tasmanian tiger, also known as the thylacine, had extremely low genetic variability right before humans hunted it to extinction. Its living cousin, the Tasmanian devil, displays a similar genetic plight, making it especially vulnerable to a contagious cancer that is decimating the species.

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"Low genetic diversity does not automatically kill species, it simply limits the pool of possible beneficial gene copies within a population," co-author Brandon Menzies from the Leibniz Institute for Zoo and Wildlife Research told Discovery News. "The Tasmanian population of devils has decreased dramatically in just the last 15 years. It is estimated that they have decreased by up to 90 percent in some areas."

For the study, the researchers surveyed pelts, bones and preserved specimens of the Tasmanian tiger from more than 100 years ago. (The last representative of the entire species died in a Tasmanian zoo in 1936.) The scientists found the individuals to be 99.5 percent similar over a portion of DNA that is normally highly variable, and 99.9 percent similar to the tiger's previously published mitochondrial genome.

The data suggests that the genetic diversity of the tiger was extremely limited before its extinction. Australia and Tasmania used to be connected by a land bridge, permitting animals to interbreed and constantly exchange genetic material. But flooding closed the bridge, isolating Tasmania from the mainland 10,000 years ago.

The tiger, devil and other local animals at that point became geographically isolated from their counterparts in Australia, probably leading to the low genetic diversity.

As if that weren't enough, Menzies said that the Tasmanian government in 1888 "placed a formal bounty on the thylacine in response to persuasion by the agricultural lobby." The Woolnorth Company, which owned 350,000 acres of land, had previously introduced sheep, which they claimed were threatened by the Tasmanian tigers.

"In hindsight, it appears that many of the sheep deaths may have been due to feral dog attack or simply the harsh winter climate," he said.

While the Tasmanian tiger is now long gone from the planet, its DNA could help to unravel current problems plaguing the Tasmanian devil and its horrific disease.

"The Tasmanian devil facial tumor disease is an unusual transmissible cancer that is thought to have only recently evolved in Tasmania," lead author Andrew Pask of the University of Connecticut told Discovery News. "It is spread by the devils biting each other during normal social interactions. The disease has 100 percent mortality and only a few resistant animals have been identified."

"Currently, captive breeding of unaffected animals is the best option for trying to save the population," Pask added.

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Katherine Belov, an associate professor in the University of Sydney's Faculty of Veterinary Science, told Discovery News, "Low levels of genetic diversity is bad news for a species. With low levels of genetic diversity, species are less able to adapt to change."

Belov continued, "We need to make sure we don't allow genetic diversity to be lost from other wildlife populations. Once it is gone, the species is susceptible to disease epidemics and ultimately extinction."

Future studies on Tasmanian tiger remains are planned for yet another reason: This marsupial, in terms of its internal body design, looked so much like a dog that even, to this day, most archaeologists cannot tell the two skeletons apart.

Pask explained that the thylacine and dog last shared a common ancestor around 160 million years ago. This ancestor had an opossum-like body. Although the thylacine and dog went down separate life paths, their similar mode of existence, as nomadic hunters eating small mammals, led them to evolve the nearly identical bodies.

"The striking convergent evolution of the dog and the thylacine provides an absolutely unique system in which to examine how evolution occurs at the genome level," Pask said. "Our team is currently sequencing the complete thylacine genome to address this question."