Australasian Science Magazine

Gene drives could make cane toads non-toxic, enabling predators to consume the toads safely and reduce their numbers.

By Ella Kelly

Gene drives may provide a novel tool to counteract seemingly unstoppable threats to global biodiversity.

The global environment is changing so quickly that many species have been unable to cope. Australia’s biodiversity is being threatened by human-mediated impacts to the landscape, and as a result we now have almost 50 vertebrate species listed as critically endangered by the IUCN Red List. Although some species have begun adapting, often these adaptations have not spread rapidly enough through populations to prevent declines.

In the face of such rapid and irreversible changes to the environment, conventional conservation methods are not always effective. Instead, conservationists are now looking towards new technology and innovative strategies to help combat what appear to be unstoppable threats. For instance, gene drives could be used to help accelerate adaptions to threats or spread favourable genes rapidly through threatened populations. Gene drives could potentially be used to help restore native biodiversity, particularly when alternative methods have been ineffective or too costly.

Invasive species, for example, are almost impossible to eradicate once they take hold. Feral animals are incredibly damaging to Australia’s native fauna, particularly as our ecosystem has been isolated for so long that it’s now ill-adapted to deal with novel threats. Current methods for control generally involve individual removal through techniques such as lethal baiting, or the introduction of diseases such as myxomatosis in rabbits. Even though these methods can be effective on a small scale, invasive species continue to pose a significant threat to native Australian fauna.

Gene drives provide a novel avenue for conservationists, and could be used to either control invasive species or promote adaptations that allow native animals to coexist alongside invaders. Due to the targeted nature of gene drives, conservationists could specifically target feral animals and have little effect on the native fauna (unlike other control methods such as baiting, which run the risk of poisoning native animals that unknowingly consume baits).

The speed at which gene drives work within a population would also be to conservationist’s advantage, as invasive species often spread so quickly that traditional management efforts cannot prevent the damage they cause.

There have been suggestions to use gene drives to reduce populations of invasive rodents in Australia. By targeting the masculinising Sry gene, scientists could artificially increase the number of male offspring born in problem populations. This would reduce the population’s breeding output, ultimately leading to declines in rodent numbers and possibly resulting in their extinction.

Lord Howe Island was invaded by the black rat in 1918, and this invasive rodent has been implicated in the decline of several native bird species and the extinction of five species. Traditional methods for removal, particularly baiting, have so far provided some reprieve for the native ecosystem, but have proved costly and haven’t completely eradicated the rats from the island. The use of gene drives in this instance would help to reduce the number of black rats, perhaps eventually eliminating them with little negative effect on the unique fauna of the island.

Gene drives have also been suggested to help control the poisonous cane toad, which is currently spreading across northern Australia at an alarming rate, leading to local population crashes of native predators that unwittingly attack the poisonous toads. Unfortunately, current methods have not been able to stop the invasion, or effectively remove the toads once they are established. With current control methods mostly ineffective, researchers have highlighted the potential for new molecular technologies to enable specific gene editing of the cane toad. By targeting the pathway to toxicity in the cane toad, researchers could limit the damage the toads have on native populations. This would require a relatively simple synthetic gene drive, with no “cargo” attached. Using gene drives to promote and help spread the gene through the cane toad populations, we could theoretically make all members of the population non-toxic. Then, predators would be able to consume the toads safely, and could help us in bringing down their numbers.

Invasive species are not the only threatening process impacting Australia’s biodiversity, and gene drives may have other potential uses for conservation. There are adaptations occurring in the natural environment in response to many threatening processes, but often they do not spread quickly enough to allow for population survival. Take, for instance, the Tasmanian devil, which is being seriously threatened by the spread of the deadly facial tumor. Although devil populations have reduced by over 80% in the wild since the disease first appeared less than 20 years ago, recently we have had cause for hope. Researchers in Tasmania have discovered genomic regions associated with immune-modulated resistance, which potentially is being selected for and enabling some devils to survive. Although it’s still very early days, there have been some suggestions that gene drives could be used to promote the adaptation in devils and help them return to healthy levels in the wild.

As with any complex and novel conservation strategy, there are inherent risks in using gene drives for conservation benefits. Introducing new genes into wild populations is risky as we cannot predict every outcome. An introduction of new genes that is poorly managed or ill-conceived could have wide-reaching effects on the environment, so any such measure must be considered carefully beforehand. Conservationists must have a thorough understanding of how genes spread between populations prior to the release of any synthetic gene drive.

Understanding the wider implications of using gene drives for conservation benefits is particularly important because of the complexity of the ecosystems in which they are employed. Calculated changes to threatening process could lead to many unintended consequences, not necessary directly related to the original target species. Natural ecosystems exist in balance and involve countless organisms and connections that may be disrupted by a small change.

Take, for example, if we were able to successfully remove feral foxes and cats from the landscape. These invasive predators cause billions of deaths of native animals each year, and have contributed to the extinction of many native Australian species. In Australia we have worked hard at controlling the numbers of foxes and cats, and if we could remove them from the landscape there would be many positive consequences for the environment. However, removing these predators leads to an increase in European rabbit numbers, which also has damaging impacts on the native ecosystem. Similarly, rendering the cane toad non-toxic would lead to a recovery of native predator populations, but this would have flow-on effects for the native species these predators prey upon. Removing any species from the food chain, even an invasive animal, will have trickle-down effects on the larger ecosystem, many of which may be difficult to identify before it’s too late.

Therefore, before gene drives can be considered as a potential conversation method we must carefully consider the broader consequences for any such actions. Detailed case studies should be undertaken prior to any manipulations of the natural environment. If effective management of this strategy can be achieved, however, it may provide us with a valuable tool to use to protect our biodiversity when all other methods have failed.