The relentless swarm of cane toads (Rhinella marina) conquering Australia may soon be hobbled by an enemy from within.
Plans are afoot that could see special toads released that could help stop the spread of the invasive species.
It sounds counterintuitive but researchers think they’ve found a bioengineering solution that gets the pest to limit its own numbers.
By removing a single gene in a toad egg, the researchers have created a hatchling that never progresses past its tadpole stage.
The removed gene controls the production of the hormone thyroxine, which fuels the metamorphosis that occurs when a tadpole transforms into a toad.
Rick Shine, an evolutionary biologist and ecologist at Macquarie University, is one of the scientists behind the “Peter Pan toads”.
He said cane toad tadpoles were known to be voracious cannibals with a preference for snacking on their kin, both in egg and hatchling form.
This is especially true in Australia where rates of cannibalism of hatchlings by tadpoles have been recorded as 2.4 times that of South America — where the cane toad originated.
The gene-edited tadpoles take this cannibalism to a new level.
The modified hatchlings grew a little bigger than regular tadpoles and ate three times as many eggs, according to results of yet peer-reviewed trials conducted by Professor Shine’s team.
“In some of our field trials if there’s older tadpoles in the pond you get no survival of eggs.” he said.
“The toad tadpoles absolutely eradicate them all and they’re not all that interested in frog eggs.
“We seem to have ended up with these sort of quite large, relatively long-lived voracious cannibals that are just ideally suited to control the numbers of their own species.”
While normal cane toad tadpoles turn into tiny toads and leave the water after 22 days, the gene-edited tadpoles remain unaltered for several months before dying.
Scale of toad invasion is challenging
There are currently upwards of 200 million cane toads in northern parts of Australia, and their numbers can multiply quickly — female toads lay up to 30,000 eggs in a single clutch, and can breed twice a year.
Finding a way to control cane toads, which have wiped out swathes of native marsupials and reptiles, has proven difficult.
Toad busting, a term for capturing and removing the amphibian, barely makes a dint in their overall numbers or in stopping its spread.
Efforts have shifted to trying to teach native animals not to eat the toads, with some success with crocodiles and lizards but less so with marsupials like quolls.
Professor Shine hopes targeting toads in their early stages may be the key to reducing their numbers.
But the gene-edited Peter Pan toads present a conundrum: they don’t get old enough to reproduce, so it’s hard to get enough eggs to implement them at a larger scale.
Creating thousands of gene-edited eggs — a process that involves injecting each egg to knock-out the gene producing the metamorphosis hormone — is impractical.
Professor Shine said one way around the problem could be to release the missing hormone into water in order to shift some of the gene-edited tadpoles into adulthood.
Those edited toads could then produce a clutch of tadpoles that have the gene knockout, but could not breed.
“If we could raise those young toads to adult size and breed them then we would have 20,000 or 30,000 Peter Pan tadpoles per clutch,” Professor Shine said.
“It wouldn’t really take very many adult toads with that genetic change for us to have a vast supply of the sorts of tadpoles we’d like.”
However, even with the help of the added hormone, so far those baby toads haven’t been growing too well.
But Professor Shine is confident they will be able to work out the toad rearing kinks.
Field trials in Western Australia
The project — a collaboration between Macquarie University, Minderoo Foundation and state governments — has so far received permits to run experiments in controlled settings in Western Australia and the Northern Territory.
The gene-edited tadpoles have not been tested in the wild yet but field trials are expected in WA later this year after a risk assessment, according to a Department of Biodiversity, Conservation and Attractions (DBCA) spokesperson.
“As this is still an experimental practice, DBCA has not considered the potential applications of non-metamorphosing tadpoles as a method for managing cane toads in WA,” they said.
“The department will await the results of the project before determining its viability as a management tool.”
Any future wild trial would first take place where toads could only breed in artificial water bodies, like pastoral dams.
The NT government was also supportive of measures that could reduce the impact of cane toads on its biodiversity as long as they were cost-effective and had no perverse impacts.
“The release of genetically modified feral organisms into the wild is a complex issue,” a Department of Planning, Lands and Environment spokesperson said.
“Adequate research to demonstrate there is no possibility of negative consequences for native wildlife will be required before any permits for wild release can be considered.”
Professor Shine said they were looking at whether or not the gene-edited toad tadpoles would have any impact on birds, fish and turtles.
Could history repeat itself?
Cane toads were initially released in Queensland in 1935 as a biocontrol for cane beetles, but ended up creating a bigger problem than the one it was trying to solve.
So is it wise to introduce gene-edited cane toad tadpoles?
Professor Shine said he recognised the initial thought of releasing genetically edited toads may sound far fetched.
But he stressed unlike gene drives, where organisms are modified to pass on genes in a way that only males or females are produced, they were deleting genes instead of adding them.
“We’re just producing something that’s exactly the same as a normal mutation,” Professor Shine said.
Curtin University population biologist Ben Phillips, who was not involved in the project, said he thought the Peter Pan toads were a clever idea.
“It builds off basic toad biology to potentially deliver a new tool for managing toads,” he said.
“It has promise as a local control measure in places where breeding sites are limited, so it is worth developing further.”
Professor Phillips said he thought the risks of releasing a species with a gene knockout where toads had already colonised seemed reasonably small.
But he noted it should still be assessed as the risks were more ecological than genetic.
“Essentially the risks hinge around whether a population of large toad tadpoles that is in a water body for years to months might have a larger impact than normal toad tadpoles,” he said.
“The genetic risks are likely minimal given the knockout makes it impossible for these tadpoles to ever reach maturity, and so to breed.”
By environment reporter Peter de Kruijff
What is this….??