Genome sequence expands on the story of the extinct Tasmanian tiger
Hunted to extinction by early European settlers in Australia, the Tasmanian tiger (or thylacine) tells a cautionary tale of conservation, but according to a new study led by the University of Melbourne, the creature was struggling even before humans intervened. After sequencing the complete genome of the thylacine from a century-old preserved specimen, the researchers were able to investigate the evolution of the carnivorous marsupial and potentially help current endangered species avoid the same fate.
The thylacine was once spread across all of Australia, but it's believed to have gone extinct on the mainland about 3,000 years ago. A population persisted on the island of Tasmania until the time of the first settlers from Europe, who exterminated the creature to stop it killing their livestock. After the last known Tasmanian tiger died in a zoo in 1936, the animal was officially declared extinct – although scattered (but unverified) sightings persist to this day.
In the new study, researchers extracted DNA from one of the best-preserved specimens of the creature: a developing juvenile preserved in alcohol for 106 years. From that the team was able to sequence the thylacine genome, and while it isn't the first time that's been done, the researchers say this latest effort has resulted in one of the most complete genetic blueprints for an extinct species.
"The genome has allowed us to confirm the thylacine's place in the evolutionary tree," says Andrew Pask, the project's lead. "The Tasmanian tiger belongs in a sister lineage to the Dasyuridae, the family which includes the Tasmanian devil and the dunnart."
Although hunting by humans was quite clearly the final nail in the coffin, the genome study revealed that the creature wasn't exactly thriving beforehand. Thylacines were suffering from low genetic diversity, which would have made them vulnerable to disease. The Tasmanian devil is currently experiencing the same problem, with deadly facial tumors threatening the species with extinction.
Previously, it was thought that the isolation on the Tasmanian isle was responsible for the poor genetic health of both species, but the new study suggests the problems began thousands of years ago, when Tasmanian devils and tigers still roamed the Australian mainland.
"Our hope is that there is a lot the thylacine can tell us about the genetic basis of extinction to help other species," says Pask. "The fact that we now know the Tasmanian tiger was facing limited genetic diversity before extinction means it would still have struggled similarly to the Tasmanian devil if it had survived."
The gene sequence also allowed the scientists to study the evolution of the animal. Although the Tasmanian tiger looks remarkably like the dingo, the two are very distantly-related, with their last common ancestor living 160 million years ago. As such, the two creatures are often considered to be one of the most striking cases of "convergent evolution", where distant species evolve similar features independently, shaped by the same environmental forces.
"When we looked at the basis for this convergent evolution, we found that it wasn't actually the genes themselves that produced the same skull and body shape, but the control regions around them that turn genes 'on and off' at different stages of growth," says Pask. "This reveals a whole new understanding of the process of evolution, we can now explore these regions of the genome to help understand how two species converge on the same appearance, and how the process of evolution works."
So with the genome sequence completed, what are the chances that we could resurrect the long-dead Tasmanian tiger? It's possible, the researchers say, but unlikely in the near future.
"As this genome is one of the most complete for an extinct species, it is technically the first step to 'bringing the thylacine back', but we are still a long way off that possibility," says Pask. "We would still need to develop a marsupial animal model to host the thylacine genome, like work conducted to include mammoth genes in the modern elephant."
The research was published in the journal Nature Ecology & Evolution.
Source: University of Melbourne