Genetic engineering could help rid Australia of toxic cane toads

THIS WEEKDNA, between January 18th and 27th, thousands of volunteers in a band of territory stretching across north-eastern Australia from Darwin to Brisbane are venturing into the night with torches and collecting-buckets. They are taking part in the Great Cane Toad Bust, an annual attempt to keep a lid on the population of these invasive, toxic amphibians. Toads thus caught will be killed humanely by being chilled in refrigerators and then frozen.Popular though this toad-busting party is, however, it is not very effective. The toad’s prolific breeding habits soon replace such losses. To do the job properly, other methods are needed. And one which is gaining ground is tadpole trapping.Toads live in dense populations, and their tadpoles are not above cannibalising the eggs of others, attracted by a chemical signal they release. Scientists at the University of Queensland, in Brisbane, have isolated this substance to develop lures for tadpole traps. Six thousand of these traps have now been made and sold by Watergum, a local conservation charity.Cannibalism is one of several weaknesses discovered during years of studying how these Latin American amphibians have adapted to their new home. Combining such knowledge with genetic technologies has brought hope of slowing, or even reversing, the relentless invasion.The problem began in 1935, when 101 cane toads were brought to northern Queensland in a failed attempt to control pesky beetles that were eating the local sugarcane. Tens of thousands of reinforcements were added in subsequent years and, with few natural checks, the animals bred and spread. Well over 200m toads are thought to live in Australia today, hopping determinedly across most of the tropical north and halfway down the east coast.This population explosion has had serious ecological consequences. Cane toads secrete a substance called bufotoxin from glands in their shoulders. This can be lethal to native wildlife, which has evolved no protection. Predatory marsupials, freshwater crocodiles, monitor lizards (known as goannas) and several of Australia’s most venomous snakes suffer as the toads move in. In some places, up to 90% of goannas vanished upon the toads’ arrival. The disappearance of these large predators distorts entire ecosystems. Prey species boom. Smaller predators go unchecked. Carrion is left to rot.Attempts to control the toads have been going on for decades, yet their advance has accelerated. In the tropics, they now travel up to 70km westward every wet season, compared with 10km when they first arrived. They are thus poised to enter some of Western Australia’s most treasured ecological areas.Toad biologists call this acceleration the Olympic Village effect. It is a superb example of evolution in action. Only the most athletic toads make it to the invasion front, where they breed. Over the generations, toads on the front have thus developed larger size, longer legs and even an urge to travel in a single direction.Armed with this knowledge, some propose dropping toads from the core population onto the invasion’s front line. These toads are less physically impressive but much more competitive breeders. The hope is to dilute the athleticism of the front-line toads and thus slow the advance, a process called genetic backburn.Other genetic solutions are in development. Tadpole cannibalism has inspired a team at Macquarie University to engineer “Peter Pan” tadpoles, so called because the genes which allow them to grow up into adults have been disabled. Releasing hungry swarms of these should keep pools clear of toad eggs for years.The genetic changes involved are so cautious that Peter Pan tadpoles are not even recognised as genetically modified organisms under Australian law. The affected genetic material in them is being deactivated, rather than added to. And the fact that the animals do not mature means changes cannot be passed on to a new generation. “We’re very carefully testing reactions of native fauna to our non-metamorphosing tadpoles before we talk about releasing them in the wild,” explains Rick Shine, the team’s leader. “We’re trying not to repeat the folly of 1935.”Turning tadpoles against their own kind is far less labour-intensive than trapping them. However, even Peter Pans die eventually, and must be replaced. So this is not a permanent fix.Thus far, the new tadpoles have been confined to the laboratory. But New South Wales and the Northern Territory have given permission for them to be tested in the field. The first sites are likely to be small isolated ponds in the Northern Territory, where the team already conducts research, with release happening at the end of this wet season, in March or April. Meanwhile, work continues to scale up the production of tadpoles from a few thousand now to the tens of thousands.But it is not only the toad that is ripe for genetic engineering. A team at the University of Melbourne, led by Andrew Pask, has partnered with Colossal Biosciences, a genetics company in Dallas, Texas, to create gene-edited marsupial cells resistant to bufotoxin. In a preprint last year on , the researchers proved they could replace part of a gene in the fat-tailed dunnart, a small marsupial, with a modification found in African and Asian monitor lizards known to be resistant to toad toxins. The results showed a 45-fold increase in resistance to bufotoxin. The team’s hope is that they can replicate this in their target species, the endangered northern quoll.Quolls, which resemble ferrets, are the largest carnivorous marsupials left on the Australian mainland. Northern quolls currently exist in isolated groups either behind or immediately ahead of the toad front line. Though quolls are also threatened by habitat loss and introduced predators such as foxes and feral cats, studies show the arrival of toads crashes their populations. A toxin-resistant quoll would not only survive the toads’ arrival, but might also actively hunt them, thus reducing their numbers. The team hope something similar may also be possible with other predators, such as goannas.Genetics is already widely used in conservation—for example to monitor elusive species or support breeding programmes. But gene modifications have not been employed in the wild before. “This is really the first demonstration of gene editing for wildlife-conservation purposes to target an anthropogenic problem that we’ve created,” says Professor Pask.His team reckon a toxin-resistant quoll could be ready for release in as little as five years, though the exact schedule will depend on approval by regulators. Peter Pan tadpoles already have the green light. But the gene-edited quoll, the of which would be changed in ways that could (and ideally would) be inherited, is likely to face higher hurdles. More sophisticated forms of genetic engineering, in particular ones that allow for traits to spread rapidly through a population, will be an even tougher sell. But desperate times require desperate measures.