Resurrecting the Extinct Frog with a Stomach for a Womb by Ed Yong
Two years ago, Mike Archer from the University of New South Wales looked down a microscope and saw that a single fertilised frog egg had divided in two. Then, it did it again. And again. Eventually, the egg produced an embryo containing hundreds of cells.
“There were a lot of hi-fives going around the laboratory,” says Archer.
This might seem like an over-reaction. After all, millions of frog eggs divide into embryos every day, as they have done since before dinosaurs walked the earth. But this egg was special. Archer’s team of scientists had loaded it with the DNA of the southern
gastric brooding frog - a bizarre creature that has been extinct for almost 30 years.
The fact that it started to grow into an embryo was a big deal. The fact that it never went further was disappointing, but not unexpected. This is cutting-edge science—cloning techniques put to the purpose of resurrection.
Archer’s goal is simple: To bring the extinct gastric brooding frog back from oblivion and, in doing so, provide hope for the hundreds of other frogs that are heading that way. Getting the embryo was a milestone and Archer is buoyantly optimistic that he’ll cross the finish line soon. Lazarus, he says, will rise again.
Stomach for a womb
The southern gastric brooding frog (
Rheobatrachus silus)was discovered in 1972 in the mountains of Queensland, Australia. But the world only took notice of it in 1974 when Mike Tyler discovered how it reproduced.
Simply put, the mother frog converts her stomachs into a womb. She swallows her own eggs and stops making hydrochloric acid in her stomach to avoid digesting her own young. Around 20 to 25 tadpoles hatch inside her and the mucus from their gills continues to keep the acid at bay. While the tadpoles grow over the next six weeks, mum never eats. Her stomach bloats so much that her lungs collapse, forcing her to breathe through her skin. Eventually, she gives birth to her brood through “propulsive vomiting”, spewing them into the world as fully-formed froglets.
When news broke about this weird strategy, other scientists were incredulous. Tyler provided vivid accounts of a young frog poking its head out of mum’s mouth like an amphibian Russian doll, but even these were insufficient. “It just seemed to many zoologists absolutely impossible,” he later wrote in a book. “There were frequently insinuations that somehow we were wrong.”
It took many years, field surveys and photographs to persuade the naysayers. Tyler eventually published a full description of the frog and its behaviour in 1981. (
Nature rejected the paper because they—wrongly—deemed it uninteresting.) The medical community took notice. If this creature could deliberately stop making acid in its stomach, it might provide new ways of treating stomach ulcers or helping people who go through stomach surgeries to heal more quickly. Several teams started studying the frog.
They didn’t have long. “There was intense interest and all of a sudden it was gone,” says Archer. The last specimen was seen in the wild in either 1979 or 1981 and despite extensive field surveys, none was ever found again. The last captive individual died in 1983, and the species was no more.
Then, good news! A second species—the northern gastric brooding frog (
Rheobatrachus vitellinus)—was discovered in 1984 in Queensland’s Eungella National Park. But a year later, almost before anyone could uncork the celebratory champagne, it too went extinct.
Archer, however, is in the business of de-extinction. He’s going to clone the southern gastric brooding frog back into life.
Cloning Lazarus
Archer has several reasons for trying. There’s the medical potential. There’s the unusual nature of the frog’s life cycle, which no other animal shares. But really, it’s a more “transcendent reason” that drives him. “If we were responsible for the extinction of the species, deliberately or inadvertently, we have a moral responsibility or imperative to undo that if we can,” he says.
But hang on–no one really knows why the frogs disappeared or if we played any role. Human forestry might have contributed. Alternatively, the lethal chytrid fungus that’s currently triggering a global frog apocalypse might have claimed the gastric brooders as early victims. But Archer still holds to his “moral imperative” argument. He also figures that restoring the frogs might help to inform his other efforts, like a project to resurrect the thylacine—Australia’s charismatic “Tasmanian tiger”. “Maybe it would be easier to get something like the frog across the finish line,” he says. “And then people who were so negative might take a deep breath and back off.”
To clone the gastric brooding frog, the team first needed its DNA. Archer called up Mike Tyler, who rummaged through his freezers and found some old tissue samples. They were in shoddy condition - just bits of frog dropped in a container, without any antifreeze to protect them. The cells should have been useless, ruptured sacks but they had somehow stayed intact. “We thought it was worth a try,” Archer says.
The team then needed something to put the DNA into - the egg of another frog. He chose a barred frog - a reasonably close relative that produces large eggs of the right size. The downside - and it’s a big one - is that barred frogs only lay eggs once a year. “We had a few times when we went in and the frogs weren’t laying, and that was that for the year,” says Archer.
Once the team had their surrogate egg, they had to destroy the native nucleus so they could insert one from the frozen gastric brooding frog tissues. They either did the job manually with a very fine instrument, or bombarded the egg with ultraviolet (UV) radiation. They tried both techniques on hundreds of eggs and one of these eventually divided into an early embryo with hundreds of cells.
Every time the team has done this, the ball of cells starts to turn inwards on itself—a crucial moment called gastrulation—and stops. That’s where they are for now. They have the beginnings of a gastric brooding frog, but are a long way from even a simple tadpole.
Still, Archer is hopeful. Whenever he has gone through the same technical motions with a living frog, and inserted the species’ nucleus into its own egg, the resulting embryo also paused at the same point. This suggests that there’s something wrong with the team’s techniques, rather than with Tyler’s frozen gastric brooding frog tissues. Busted tissues would be a deal breaker but technological problems can be fixed, and Archer has brought in stem cell expert Robert Lanza to help him do so. “We retain our vibrant optimism,” he says.
Is it worth it?
Archer has faced his share of naysayers, from those who think that the technological hurdles are too great to others who believe that restoring the dead is a vanity project. “If you were thin-skinned, you’d race to the corner and give up,” he says.
Some of the arguments against de-extinction don’t apply to the gastric brooding frog. Unlike the woolly mammoth or passenger pigeon, the frog isn’t a social creature that would need companions to learn from or travel among. Unlike the mammoth, which would need to be born inside an elephant, the frog doesn’t need a complicated surrogate parent. And unlike many of the candidates for de-extinction, like the moa or saber-toothed cat, the frog is small and can be reared in a laboratory. Archer has so much frozen tissue that once he successfully clones one frog, he could make a practically infinite supply of them.
“It seems like a good choice as a test case for many reasons, and I think is very defendable choice from scientific and ethical viewpoints,” says Karen Lips from the University of Maryland, who works on conserving living amphibians.
Archer believes that the project is not just defensible, but necessary. Frogs are in such a bad way that some conservationists are already trying to preserve tissue samples in a genetic ark, with a view to cloning these species should they ever disappear completely. “Everyone blissfully takes for granted that we’ll be able to do this down the line but no one has shown that you can,” says Archer. “This work will be relevant to the rest of the frogs around the world and possibly to animals of all kinds.”
Let’s assume Archer succeeds. Where would the new generation gastric brooding frogs live? Their habitat in the Queensland mountains is being threatened by feral pigs, invasive weeds and polluted or diverted waters. And then there’s the chytrid fungus, which has spread to almost every part of the world. It would be like releasing Lazarus into an ecological dystopia.
Archer is unfazed. “We can ultimately fix the wild,” he says. “Even if we had to maintain most of the world’s wildlife in artificial environments, that would be a thousand times better than to let them slide off the brink.” The frogs can wait until their homes are ready for them. In the meantime, scientists could perhaps engineer or breed them to be resistant to the chytrid fungus, or carry out experimental releases to see whether they would actually find a niche in this brave, new world.
But Lips argues that this is impractical. “Zoos are extremely limited in space,” she says, and the resurrected frogs would have to compete with the thousands of other amphibian species that are facing extinction—around 40 percent of the 7,000 or so that we know of. “We can’t keep them all in captivity at sufficient numbers to maintain genetic diversity.”
Lips has another concern: Resurrection projects take up a lot of money. Archer concedes that cloning research is initially expensive, but he says that costs will eventually fall. “I can’t think of what cloning Dolly must have cost and now it’s a routine technique,” he says.
Still, funding is a zero-sum game. There’s only so much cash to go around and conservationists need it to monitor animals that are still alive, work out why they are disappearing, and develop ways of saving them. There are plenty of cases where we know how to save a species, but can’t afford to do so. “I can’t help but think that we can’t even take care of what we’ve got, and now we’re going to invest in very expensive techniques to recover a handful of special-interest species that may or may not be able to survive in the wild on their own,” says Lips. As a best-case scenario, she hopes that these high-profile projects will help to drum up interest in saving a broader swathe of imperilled wildlife.
Archer sees it slightly differently—the plight of living species gives him even more recourse to bring back extinct ones. “No matter how many resources we put into looking after the environment, wildlife is no longer safe in the wild,” he says. “If we accept that maintaining biodiversity is important, we can’t assume that if you whack a fence up, everything’s going to be okay. You need to explore lots of parallel strategies.”
