Tasmanian Tiger Genome Sequenced

Scientists have produced the first high-quality genomic sequence for the Tasmanian tiger (Thylacinus cynocephalus), also known as the thylacine.

Tasmanian tigers (Thylacinus cynocephalus) in captivity. Image credit: University of Melbourne / Museums Victoria.

Tasmanian tigers (Thylacinus cynocephalus) in captivity. Image credit: University of Melbourne / Museums Victoria.

The Tasmanian tiger was a carnivorous marsupial about the size and shape of a medium-to-large size dog: a fully grown animal could measure 6 feet (180 cm) from the tip of the nose to the tip of the tail and stand 2 feet (58 cm) high.

It had tiger-like stripes running down its lower back and an abdominal pouch, and is one of only a few marsupials to have a pouch in both sexes.

Historically the Tasmanian tiger was broadly distributed across Australia before becoming extinct on the mainland around 3,000?years ago.

A Tasmanian population became isolated by rising sea levels approximately 14,000?years ago and persisted until the early 20th century.

European settlers deemed the animal a threat to the Tasmanian sheep industry and the government aggressively targeted it for eradication by offering a £1.00 bounty for each animal killed.

Consequently, the remaining population was rapidly exterminated and the last known Tasmanian tiger died at the Hobart Zoo in 1936. The species was eventually declared extinct in 1982.

The complete genome sequence of the Tasmanian tiger — obtained from one of the best-preserved specimens in the world — a 106-year-old pouch-young held in Museums Victoria’s Collection — provides crucial new information on the biology of this unique marsupial and how it evolved to look so similar to the dingo, despite being very distantly related.

“Our results provide the first full genetic blueprint of the largest Australian apex predator to survive into the modern era,” said project leader Dr. Andrew Pask, from the School of BioSciences at the University of Melbourne, Australia.

“The genome allows us to confirm the Tasmanian tiger’s place in the evolutionary tree. This species belongs in a sister lineage to the Dasyuridae, the family which includes the Tasmanian devil and the dunnart.”

Importantly, the Tasmanian tiger genome has also confirmed the poor genetic health, or low genetic diversity, that the animal experienced before it was over-hunted.

This is a similar fate facing the Tasmanian devil, which was predicted to be due to their genetic isolation from Australia for the last 10,000 to 13,000 years. However, the genome analyses suggest that both animals were experiencing low genetic diversity before they became isolated on Tasmania.

Hence, Tasmanian tigers may have faced similar environmental problems to the Tasmanian devils, had they survived, such as difficulty overcoming disease.

“Our hope is that there is a lot the Tasmanian tiger can tell us about the genetic basis of extinction to help other species,” Dr. Pask said.

“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.”

“We would still need to develop a marsupial animal model to host the Tasmanian tiger genome, like work conducted to include mammoth genes in the modern elephant. But 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.”

“However, the genome does provide important new insights into the biology of this truly unique marsupial apex predator.”

Researchers consider the Tasmanian tiger and the dingo as one of the best examples of convergent evolution, the process where organisms that are not closely related independently evolve to look the same as a result of having to adapt to similar environments or ecological niches.

They found that because of their hunting technique and diet of fresh meat, their skulls and body shape became similar through a process called ‘convergent evolution.’ This is where, despite not being closely related, adaptation to their environment causes their appearance to become identical.

Dr. Pask and colleagues analyzed the characteristics of the Tasmanian tiger’s skull — such as eye, jaw and snout shape.

“We found the Tasmanian tiger had a more similar skull shape to the red fox and gray wolf than to its closest relatives,” said co-authors Dr. Christy Hipsley, from the University of Melbourne and Museums Victoria.

“The fact these groups have not shared a common ancestor since the Jurassic makes this an astounding example of convergence between distantly related species.”

“The appearance of the Tasmanian tiger is almost a dingo with a pouch,” Dr. Pask noted.

“And 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.”

“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.”

“In this case, it seemed the need to hunt led the Tasmanian tiger to transform its appearance into one similar to the wolf over the past 160 million years, and we can now start to understand the genetics that has driven this process and uncover more about the biology of this unique marsupial apex predator.”

The findings were published in the January 2018 issue of the journal Nature Ecology Evolution.

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Charles Y. Feigin et al. 2018. Genome of the Tasmanian tiger provides insights into the evolution and demography of an extinct marsupial carnivore. Nature Ecology Evolution 2 (1): 182-192; doi: 10.1038/s41559-017-0417-y

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