Anyway, according to Tizard, it’s too late in the day to be worried about a few genes here and there. “If you look at a native Australian environment, you see eucalyptus trees, koalas, kookaburras, whatever,” he said. “If I look at it, as a scientist, what I’m seeing is multiple copies of the eucalyptus genome, multiple copies of the koala genome, and so on. And these genomes are interacting with each other. Then, all of a sudden, ploomph, you put an additional genome in there—the cane toad genome. It was never there before, and its interaction with all these other genomes is catastrophic. It takes other genomes out completely.
“What people are not seeing is that this is already a genetically modified environment,” he went on. Invasive species alter the environment by adding entire genomes that don’t belong. Genetic engineers, by contrast, alter just a few bits of DNA here and there.
“What we’re doing is potentially adding on maybe ten more genes onto the twenty thousand toad genes that shouldn’t be there in the first place, and those ten will sabotage the rest and take them out of the system and so restore balance,” Tizard said. “The classic thing people say with molecular biology is: Are you playing God? Well, no. We are using our understanding of biological processes to see if we can benefit a system that is in trauma.”
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Formally known as Rhinella marina, cane toads are a splotchy brown, with thick limbs and bumpy skin. Descriptions inevitably emphasize their size. “Rhinella marina is an enormous, warty bufonid (true toad),” notes the U.S. Fish and Wildlife Service. “Large individuals sitting on roadways are easily mistaken for boulders,” observes the U.S. Geological Survey. The biggest cane toad ever recorded was fifteen inches long and weighed almost six pounds—as much as a chubby chihuahua. A toad named Bette Davis, who lived at the Queensland Museum, in Brisbane, in the 1980s, was nine and a half inches long and almost as wide—about the size of a dinner plate. The toads will eat almost anything they can fit in their oversized mouths, including mice, dog food, and other cane toads.
Cane toads are native to South America, Central America, and the very southernmost tip of Texas. In the mid-1800s, they were imported to the Caribbean. The idea was to enlist the toads in the battle against beetle grubs, which were plaguing the region’s cash crop—sugar cane. (Sugar cane, too, is an imported species; it is native to New Guinea.) From the Caribbean, the toads were shipped to Hawaii and from there, to Australia. In 1935, a hundred and two toads were loaded onto a steamer in Honolulu. A hundred and one of them survived the journey and ended up at a research station in sugar-cane country, on Australia’s northeast coast. Within a year, they’d produced more than 1.5 million eggs. The resulting toadlets were intentionally released into the region’s rivers and ponds.
It’s doubtful that the toads ever did the sugar cane much good. Cane grubs perch too high off the ground for a boulder-sized amphibian to reach. This didn’t faze the toads. They found plenty else to eat and continued to produce toadlets by the truckload. From a sliver of the Queensland coast, they pushed north, into the Cape York Peninsula, and south, into New South Wales. Sometime in the 1980s, they crossed into the Northern Territory. In 2005, they reached a spot known as Middle Point, in the western part of the Territory, not far from the city of Darwin.
Along the way, something curious happened. In the early phase of the invasion, the toads were advancing at the rate of about six miles a year. A few decades later, they were moving at twelve miles a year. By the time they hit Middle Point, they’d sped up to thirty miles a year. When researchers measured the toads at the invasion front, they found out why. The toads on the front lines had significantly longer legs than the toads back in Queensland. And this trait was heritable. The Northern Territory News played the story on its front page, under the headline Super Toad. Accompanying the article was a doctored photo of a cane toad wearing a cape. “It has invaded the Territory and now the hated cane toad is evolving,” the newspaper gasped. Contra Darwin, it seemed, evolution could be observed in real time.
Since they were introduced, cane toads have spread across Australia. They’re expected to continue to expand their territory.
Cane toads are not just disturbingly large; from a human perspective, they’re also ugly, with bony heads and what looks like a leering expression. The trait that makes them truly “hated,” though, is that they’re toxic. When an adult is bitten or feels threatened, it releases a milky goo that swims with heart-stopping compounds. Dogs often suffer cane toad poisoning, the symptoms of which range from frothing at the mouth to cardiac arrest. People who are foolish enough to consume cane toads usually wind up dead.
Australia has no poisonous toads of its own; indeed, it has no toads at all. So its native fauna hasn’t evolved to be wary of them. The cane toad story is thus the Asian carp story inside out, or maybe upside down. While carp are a problem in the United States because nothing eats them, cane toads are a menace in Australia because just about everything eats them. The list of species whose numbers have crashed due to cane toad consumption is long and varied. It includes: freshwater crocodiles, which Australians call “freshies”; yellow-spotted monitor lizards, which can grow up to five feet long; northern blue-tongued lizards, which are actually skinks; water dragons, which look like small dinosaurs; common death adders, which, as the name suggests, are venomous snakes; and king brown snakes, which are also venomous. By far the most winning animal on the victims list is the northern quoll, a sweet-looking marsupial. Northern quolls are about a foot long, with pointy faces and spotted brown coats. When young quolls graduate from their mother’s pouch, she carries them around on her back.
In an effort to slow down the cane toads, Australians have come up with all sorts of ingenious and not-so-ingenious schemes. The Toadinator is a trap fitted out with a portable speaker that plays the cane toad’s song, which some compare to a dial tone and others to the thrum of a motor. Researchers at the University of Queensland have developed a bait that can be used to lure cane toad tadpoles to their doom. People shoot cane toads with air rifles, whack them with hammers, bash them with golf clubs, purposefully run them over with their cars, stick them in the freezer until they solidify, and spray them with a compound called HopStop, which, buyers are assured, “anesthetizes toads within seconds” and dispatches them within an hour. Communities organize “toad busting” militias. A group called the Kimberley Toad Busters has recommended that the Australian government offer a bounty for each toad eliminated. The group’s motto is: “If everyone was a toad buster the toads would be busted!”
An Australian girl with her pet cane toad, Dairy Queen
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At the point Tizard got interested in cane toads, he’d never actually seen one. Geelong lies in a region—southern Victoria—the toads haven’t yet conquered. But one day at a meeting, he was seated next to a molecular biologist who studied the amphibian. She told him that, despite all the busting, the toads kept on spreading.
“She said, it was such a shame, if only there was some new way of getting at it,” Tizard recalled. “Well, I sat down and scratched my head.
“I thought: Toxins are generated by metabolic pathways,” he went on. “That means enzymes, and enzymes have to have genes to encode them. Well, we have tools that can break genes. Maybe we can break the gene that leads to the toxin.”
Tizard brought on a post-doc named Caitlin Cooper to help with the mechanics. Cooper has shoulder-length brown hair and an infectious laugh. (She, too, is from somewhere else—in her case Massachusetts.) No one had ever tried to gene edit a cane toad before, so it was up to Cooper to figure out how to do it. Cane toad eggs, she discovered, had to be washed and then pierced just so, with a very fine pipette, and this had to be done quickly, before they had time to start dividing. “Refining the micro-injection technique took quite a while,” she told me.
As sort of a warm-up exer
cise, Cooper set out to change the cane toad’s color. A key pigment gene for toads (and, for that matter, humans) codes for the enzyme tyrosinase, which controls the production of melanin. Disabling this pigment gene should, Cooper reasoned, produce toads that were light-colored instead of dark. She mixed some eggs and sperm in a petri dish, micro-injected the resulting embryos with various CRISPR-related compounds, and waited. Three oddly mottled tadpoles emerged. One of the tadpoles died. The other two—both males—grew into mottled toadlets. They were christened Spot and Blondie. “I was absolutely rapt when this happened,” Tizard told me.
With CRISPR, guide RNA is used to target the stretch of DNA to be cut. When the cell attempts to repair the damage, often mistakes are introduced and the gene is disabled. If a “repair template” is supplied, a new genetic sequence can be introduced.
Cooper next turned her attention to “breaking” the toads’ toxicity. Cane toads store their poison in glands behind their shoulders. In its raw form, this poison is merely sickening. But toads can, when attacked, produce an enzyme—bufotoxin hydrolase—that amplifies the poison’s potency a hundredfold. Using CRISPR, Cooper edited a second batch of embryos to delete a section of the gene that codes for bufotoxin hydrolase. The result was a batch of detoxified toadlets.
After we’d talked for a while, Cooper offered to show me her toads. This entailed penetrating deeper into AAHL, through more air-lock doors and layers of security. We all put scrubs on over our clothes and booties over our shoes. Cooper spritzed my tape recorder with some kind of cleaning fluid. Quarantine Area, a sign said. Heavy Penalties Apply. I decided it would be better not to mention The Odin and my own rather less secure gene-editing adventures.
Beyond the doors was a sort of antiseptic barnyard, filled with animals in various sized enclosures. The smell was a cross between hospital and petting zoo. Near a block of mouse cages, the detox toadlets were hopping around a plastic tank. There were a dozen of them, about ten weeks old and each about three inches long.
“They’re very lively, as you can see,” Cooper said. The tank had been outfitted with everything a person could imagine a toad would want—fake plants, a tub of water, a sunlamp. I thought of Toad Hall, “replete with every modern convenience.” One of the toads stuck out its tongue and nabbed a cricket.
“They will eat literally anything,” Tizard said. “They’ll eat each other. If a big one encounters a small one, it’s lunch.”
Let loose in the Australian countryside, a knot of detox toads presumably wouldn’t last long. Some would become lunch, either for freshies or lizards or death adders, and the rest would be outbred by the hundreds of millions of toxic toads already hopping across the landscape.
What Tizard had in mind for them was a career in education. Research on quolls suggests that the marsupials can be trained to steer clear of cane toads. Feed them toad “sausages” laced with an emetic, and they will associate toads with nausea and learn to avoid them. Detox toads, according to Tizard, would make an even better training tool: “If they’re eaten by a predator, the predator will get sick but not die, and it will go, ‘I’m never eating a toad again.’ ”
Before they could be used for teaching quolls—or for any other purpose—the detox toads would need a variety of government permits. When I visited, Cooper and Tizard hadn’t started in on the paperwork, but they were already contemplating other ways to tinker. Cooper thought it might be possible to fiddle with the genes that produce the gel coat on the toads’ eggs in such a way that the eggs would be impossible to fertilize.
“When she described the idea to me, I was, like, brilliant!” Tizard said. “If we can take steps to knock down their fecundity, that’s absolute gold.” (A female cane toad can produce up to thirty thousand eggs at a go.)
A few feet away from the detox toads, Spot and Blondie were sitting in their own tank, an even more elaborate affair, with a picture of a tropical scene propped in front for their enjoyment. They were almost a year old and now fully grown, with thick rolls of flesh around their midsections, like sumo wrestlers. Spot was mostly brown, with one yellowish hind leg; Blondie was more richly variegated, with whitish hind legs and light patches on his forelimbs and chest. Cooper reached a gloved hand into the tank and pulled out Blondie, whom she’d described to me as “beautiful.” He immediately peed on her. He appeared to be smiling malevolently, though I realized, of course, that wasn’t actually the case. He had, it seemed to me, a face only a genetic engineer could love.
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According to the standard version of genetics that kids learn in school, inheritance is a roll of the dice. Let’s say a person (or a toad) has received one version of a gene from his mother—call it A—and a rival version of this gene—A1—from his father. Then any child of his will have even odds of inheriting an A or an A1, and so on. With each new generation, A and A1 will be passed down according to the laws of probability.
Like much else that’s taught in school, this account is only partly true. There are genes that play by the rules and there are also renegades that refuse to. Outlaw genes fix the game in their favor and do so in a variety of devious ways. Some interfere with the replication of a rival gene; others make extra copies of themselves, to increase their odds of being passed down; and still others manipulate the process of meiosis, by which eggs and sperm are formed. Such rule-breaking genes are said to “drive.” Even if they confer no fitness advantage—indeed, even if they impose a fitness cost—they’re handed on more than half of the time. Some particularly self-serving genes are passed on more than ninety percent of the time. Driving genes have been discovered lurking in a great many creatures, including mosquitoes, flour beetles, and lemmings, and it’s believed they could be found in a great many more, if anyone took the trouble to look for them. (It’s also true that the most successful driving genes are tough to detect, because they’ve driven other variants to oblivion.)
Since the 1960s, it’s been a dream of biologists to exploit the power of gene drives—to drive the drive, as it were. This dream has now been realized, and then some, thanks to CRISPR.
In bacteria, which might be said to hold the original patent on the technology, CRISPR functions as an immune system. Bacteria that possess a “CRISPR locus” can incorporate snippets of DNA from viruses into their own genomes. They use these snippets, like mug shots, to recognize potential assailants. Then they dispatch CRISPR-associated, or Cas, enzymes, which work like tiny knives. The enzymes slice the invaders’ DNA at critical locations, thus disabling them.
Genetic engineers have adapted the CRISPR-Cas system to cut pretty much any DNA sequence they wish. They’ve also figured out how to induce a damaged sequence to stitch into itself a thread of foreign DNA it’s been supplied with. (This is how my E. coli were fooled into replacing an adenine with a cytosine.) Since the CRISPR-Cas system is a biological construct, it, too, is encoded in DNA. This turns out to be key to creating a gene drive. Insert the CRISPR-Cas genes into an organism, and the organism can be programmed to perform the task of genetic reprogramming on itself.
In 2015, a group of scientists at Harvard announced they’d used this self-reflexive trick to create a synthetic gene drive in yeast. (Starting with some cream-colored yeast and some red yeast, they produced colonies that, after a few generations, were all red.) This was followed three months later by an announcement from researchers at UC–San Diego that they’d used much the same trick to create a synthetic gene drive in fruit flies. (Fruit flies are normally brown; the drive, pushing a gene for a kind of albinism, yielded offspring that were yellow.) And six months after that, a third group of scientists announced they had created a gene-drive Anopheles mosquito.
If CRISPR confers the power to “rewrite the very molecules of life,” with a synthetic gene drive, that power increases exponentially. Suppose that the researchers in San Diego had released their yellow fruit flies. Assuming
those flies had found mates, swarming around some campus dumpster, their offspring, too, would have been yellow. And assuming those offspring survived and also successfully mated, their progeny would, in turn, have been yellow. The trait would have continued to spread, ineluctably, from the redwood forest to the Gulf Stream waters, until yellow ruled.
With a synthetic gene drive, the normal rules of heredity are overridden and an altered gene spreads quickly.
And there’s nothing special about color in fruit flies. Just about any gene in any plant or animal can—in principle, at least—be programmed to load the inheritance dice in its favor. This includes genes that have themselves been modified or borrowed from other species. It should be possible, for example, to engineer a drive that would spread a broken-toxin gene among cane toads. It may also be possible one day to create a drive for corals that would push a gene for heat tolerance.
In a world of synthetic gene drives, the border between the human and the natural, between the laboratory and the wild, already deeply blurred, all but dissolves. In such a world, not only do people determine the conditions under which evolution is taking place, people can—again, in principle—determine the outcome.
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The first mammal to be fitted out with a CRISPR-assisted gene drive will, almost certainly, be a mouse. Mice are what’s known as a “model organism.” They breed quickly, are easy to raise, and their genome has been intensively studied.
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