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Illustration of Thylacinus cenophalus. Photo: Proceedings of the Zoological Society of London 1850

The Tasmanian tiger (Thylacinus cenophalus) was a unique creature: a predatory marsupial, it had tiger-like stripes on its back and otherwise resembled members of the worldwide dog family, despite its relative isolation in Australia and especially on the island of Tasmania. Humanity meant its days were numbered, with bounties and overhunting killing off its population. The last member of the species died in captivity in 1936.

Now, a new sequencing of the nuclear DNA of a single thylacine specimen – a tiny pup preserved in a jar of alcohol for a century – has shed some new light on how the animals declined and disappeared, as described in in the journal Nature Ecology and Evolution.

“We sequenced the thylacine genome to improve our understanding of this enigmatic marsupial predator,” they write.

The mitochondrial DNA of the thylacine had previously been sequenced, by teams from Penn State and elsewhere, using hair samples that had been collected by various museums in the final decades of the Tasmanian Tiger’s existence.

But the jar of alcohol, found at Museums Victoria in Australia, presented new opportunities. The pup had been in the pouch of a thylacine that had died in captivity in 1909.

The scientists, from the University of Melbourne and elsewhere, extracted DNA fragments from the soft tissue of the specimen.

Twenty-five markers called short interspersed elements (SINEs) were compared against dogs – and also a cousin, the Tasmanian devil. The following PCR amplification sequenced an additional 225 loci, bringing the total number of identifying markers to 250.

The Tasmanian tigers, despite their appearance, were not cousins of dogs. In fact, the last shared common ancestor, according to the genetic comparisons, lived approximately 160 million years ago.

“In spite of their extraordinary phenotypical convergence, comparative genomic analyses demonstrated that amino acid homoplasies between the thylacine and canids are largely consistent with neutral evolution,” they write. “Furthermore, their genes and pathways targeted by positive selection differ markedly between these species. Together, these findings support models of adaptive convergence driven primarily by cis-regulatory evolution.”

But the fates of the Tasmanian tiger and the Tasmanian devil were both sealed in their DNA, according to the analysis. The tiger and devil both experienced precipitous genetic diversity decline starting as long as 120,000 years ago – perhaps because of a cooler climate. That means these animals were experiencing serious decline before the first human colonization of Australia around 50,000 years ago, the scientists contend.

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