Photo: The Southern Corroboree Frog (Pseudophryne corroboree) © Tiffany Kosch
Australia’s most iconic amphibian, the Southern Corroboree Frog (Pseudophryne corroboree), may soon be famous for more than its unusual and brilliant yellow and black color combo. One Health Research Group lab at James Cook University in Queensland is investigating two innovative methods to genetically bolster the species’ immunity to the fungal disease chytridiomycosis, with the aim of essentially creating the first amphibian species with the superpowers to ward off the deadly foe.
“The zoos that we work with put a lot of money and resources into raising Southern Corroboree Frogs in captivity and releasing more than 1,000 eggs every year back into the wild,” says Dr. Tiffany Kosch, an adjunct research fellow at James Cook University, who is leading the new genetics research along with Drs. Lee Skerratt and Lee Berger. “We want to make sure that these efforts are truly effective, and the only way that we can do that is by re-introducing frogs that are no longer susceptible to chytrid, which is still in the environment and still a primary threat. We are trying to develop a sustainable solution where the frogs can be released and we don’t ever have to worry about them again.”
Ultimately Dr. Kosch and her colleagues will be looking at two possible methods for improving chytrid resistance in Southern Corroboree Frogs that have been captive-bred in Australian Zoos, including the Taronga Zoo and the Melbourne Zoo: selective breeding and genetic engineering. Selective breeding would involve scoring the frogs on their likely ability to fight chytrid based on their genetic make-up, and then pairing frogs likely to pass on resistance to their offspring.
The other method, which has a science fiction overtone to it but is entirely possible with new tools, is to use the cutting-edge genome editing technology CRISPR-Cas9 to take multiple genes related to chytrid immunity and “knock” them into frog eggs immediately after they’ve been fertilized. While researchers haven’t yet tried this method in Corroboree Frogs, it has been done successfully in pigs, fish and even other frog species, including the African Clawed Frog (Xenopus laevis) and the Tropical Clawed Frog (Silurana tropicalis).
“No one method of disease mitigation is likely to be the silver bullet for all species in all cases, so it makes sense to explore multiple methods to ensure we have a number of approaches in the tool box,” said Reid Harris, a professor of biology at James Madison University and the Amphibian Survival Alliance’s director of international disease mitigation. “Vaccinations and probiotics have had mixed success, and we’re eager now to see the results of Tiffany’s approach to use some cutting-edge genetic techniques to impart resistance to chytrid.”
Which of the techniques will work best will depend on the answer to one of a number of basic questions Kosch’s team needs to answer first: How many genes are involved in providing resistance to chytrid? If just a handful, genetic engineering will be the likely candidate. If the number is too high for current gene knock-in technology, selective breeding will be the better solution. The team also aims to answer: Which specific genes are involved in providing resistance? How likely are the effects of these to be passed on to offspring?
The researchers did a pilot study with 70 frogs to start to answer these questions, but will be expanding that work to a much larger study of about 1,000 frogs from captive breeding programs at partner zoos in Australia. The answers to these questions will provide the first comprehensive understanding of amphibian resistance to chytrid. Once the researchers have these answers, which Kosch predicts will take no longer than a year, they will start the genotyping process, providing the first sequenced genome of an Australian frog, and the fourth-ever sequenced genome of any frog species.
Chytrid was first introduced in Australia in 1978 and reached Southern Corroboree Frog habitat around 1984. Biologists predict that today there are likely fewer than 50 individuals of the Critically Endangered species left in the wild. The Southern Gastric Brooding Frog and five other Australian species have gone extinct—and many others have suffered precipitous declines—primarily as the result of chytrid. If Kosch is successful, other conservationists may be able to use these tools on species threatened not only in Australia, but in Central and South America, where chytrid has also hit amphibians hard.
We’re hoping that the same genes that confer resistance in the Corroboree Frogs would confer resistance in other species,” Kosch says. “What makes this project so ideal is that this approach could be used to save amphibians threatened by chytrid or even other emerging diseases. We still have lots of work ahead of us before we get there, but we are hopeful that this path leads to the return of the beloved and charismatic Southern Corroboree Frog to the wild.”
By Lindsay Renick Mayer