RETURN OF THE LIVING THYLACINE
Few extinct animals capture the imagination like the Tasmanian tiger. Geneticists have taken the first steps to bring it back from the dead. JOHN PICKRELL explains what comes next.
ON THE ISLANDS OF the Dampier Archipelago, just off the coast of north-west Western Australia, giant piles of rusty, iron-rich boulders tumble into the brilliant turquoise waters of the Indian Ocean. Six thousand years ago, these islands were hilltops emerging from a wide coastal plain teeming with life. Aboriginal people recorded these animals by carving petroglyphs into the deep-red rocks.
AMONG THE IMAGES are more than 20 thylacines, also known as Tasmanian tigers. These wolf-like, carnivorous marsupials carried their young in a pouch like kangaroos, sported tiger-like stripes on their backs and had jaws capable of an impressive 120-degree gape. They were once common across much of Australia and New Guinea.
The thylacine vanished from the Australian mainland about 3,000 years ago, probably as a result of a drying climate and the loss of dense vegetation. It maintained a toehold in forested Tasmania, only to be hunted to extinction by Europeans from the 1800s. The last known tiger died in Hobart Zoo in 1936.
Australia’s roll call of extinct species includes carsized relatives of the wombat, lion-like predators and giant flightless birds. But the thylacine holds a special place in the public consciousness. Frequent ‘sightings’ and quests to find evidence of a living thylacine manifest hopes it might not really be lost.
In recent times, that hope has translated into possible ‘de-extinction’ through cloning.
Specimens from 450 thylacines are in museums around the world. Most are skin and bones, but 13 pouch young (joeys) were preserved in alcohol or formaldehyde. The Melbourne Museum has one so well-preserved that a team led by Andrew Pask at the University of Melbourne announced, in 2017, the successful sequencing of its entire genome. It is the most intact genome obtained for an extinct species.
The Melbourne joey’s own life might have been cut short, but its DNA may be a blueprint to resurrect the entire species. No one thinks it will happen soon but, as University of New South Wales palaeontologist and incurable ‘de-extinction’ champion Michael Archer puts it: “It’s a brave geneticist these days who’ll say what’s impossible in the next decade or two.”
ARCHER WAS PERHAPS the first person to dare to dream of cloning the thylacine. In 1996, when Dolly the sheep made history as the first mammal to be cloned, he declared doing the same with a thylacine was “a matter of not if but when”.
Dolly’s DNA originated from the mammary cell of an adult ewe. The cell’s nucleus, containing the DNA, was sucked out and transferred into a sheep egg whose own nucleus had been removed. The transferred nucleus ‘rebooted’ the egg’s development, creating a clone of the original ewe.
There is no chance of doing the same with a thylacine. Museum specimens can deliver thylacine DNA but not a viable nucleus or egg. So how do you clone something without these seemingly essential ingredients? Geneticist George Church, at Harvard University, has pioneered a way.
It is somewhat like the cloning strategy imagined in Jurassic Park. The fictional genetic engineers source dinosaur DNA from amber-preserved mosquitoes that dined on dinosaur blood. Gaps in the dinosaur DNA are filled by reptilian, bird or amphibian DNA.
In a similar manner, Church is heading an effort to clone the mammoth by using the DNA of its closest living relative, the Asian elephant, to fill in the missing bits of mammoth DNA.
What takes the scenario from fiction to reality is CRISPR. This latest tool in the genetic engineer’s kit is a set of enzymes used by bacteria to target and destroy foreign DNA. In 2015 genetic engineers co-opted CRISPR to target and alter DNA within living cells. Church’s goal is to ‘edit’ key tracts of elephant code to convert them into mammoth code, rather like turning a modern novel into medieval-era prose.
Church’s team have identified 1642 genes that differ between the species. In February 2017 Church announced the successful conversion of 45 of those genes. “We already know about the ones to do with small ears, subcutaneous fat, hair and blood,” he said, predicting a hybrid elephant-mammoth embryo “could happen in a couple of years”.
Once an edited facsimile of a mammoth nucleus has been created, it could be placed into an Asian elephant egg and then into a womb. Church is also looking into technologies for artificial wombs.
BY THE TIME DOLLY the sheep was cloned, acquiring a thylacine’s DNA blueprint from a museum specimen was a tantalising possibility. Short sequences of DNA were already being extracted from mammoths and other long-dead specimens. Archer, then at the Australian Museum in Sydney, attempted to extract DNA from a thylacine in the museum’s collection – a six-month-old pup preserved in alcohol in 1886 – but the DNA was too fragmented to be useful.
Given those difficulties, Pask in Melbourne thought sequencing the thylacine genome would be impossible. His team focused instead on sequencing the genomes of living species – the platypus, tammar wallaby and
dunnart. The goal was to compare their blueprints to placental mammals like us and trace how genes had evolved since these mammalian relatives had diverged.
Success at reading marsupial genomes emboldened the scientists to take another shot at the thylacine. In 2008 they reported a milestone: isolating a fragment of thylacine DNA so intact its code was still readable. A computer program recognised the DNA as the code for a gene – Col 2A1 – that directs the development of cartilage and bone. The researchers inserted the gene fragment into a mouse embryo, together with a chemical tag that made the gene glow blue wherever it was active. Blue patterns appeared in the embryo’s developing skeleton, meaning the code was good enough to work in a living creature.
The finding was encouraging. Even if scientists could never read a complete thylacine genome, they might glean important information from studying its genes – such as clues about how this cousin of the kangaroo evolved the body shape of a wolf.
Pask’s team spent 10 years taking samples from 40 thylacine specimens worldwide. “Most of the museum samples had really, really badly damaged DNA,” he says. He had almost given up hope when, in 2010, he came across a specimen on his doorstep. In a dusty cabinet in the bowels of the Melbourne Museum, preserved in a jar of ethanol, was a four-week-old joey taken from its dead mother’s pouch in 1909.
Pask’s team sampled its DNA. Unlike all the other specimens, the joey retained strings of DNA 1,000 letters in length – long enough to mean the entire three-billion-letter genome might be puzzled back together. Pask believes the DNA’S good condition might be due to the specimen missing the standard formalin fixation, instead going straight into ethanol.
The sample not only yielded long strings of DNA but plenty of them. Crucially that allowed Pask’s team to read every bit of the DNA sequence 60 times over using different strands. This enabled them to correct inevitable errors in the century-old material.
Imagine finding an old car manual with many pages missing. You would struggle to make use of it. But with 60 tattered incomplete copies you could probably compile a whole manual. Pask is similarly confident the blueprint is accurate enough to instruct the building of a thylacine. So too is Archer, who has lost none of his enthusiasm for bringing back extinct species. “It’s the roadmap for getting a thylacine back,” he says.
Keeping hopes alive: Andrew Pask reconstructed a thylacine genome from the pup in the bottle in what may be the first step in resurrecting the species.
04 | Thylacine DNA is so intact it can function in a mouse embryo. The blue pattern shows where the DNA is trying to direct the development of the skeleton.