Across the world, entire species of frogs and toads are being wiped out by a relentless fungal infection, commonly known as the chytrid fungus.
For most amphibians, the fungus Batrachochytrium dendrobatidis is a death sentence.
It’s caused more global species extinction events than any other infectious disease.
And yet somehow, high in the Pyrenees, a few populations of common midwife toads have managed to survive an outbreak that has almost entirely wiped out their neighbors.
Now, a team led by researchers from University College London (UCL), the Zoological Society London (ZSL), and Imperial College London has discovered how these toads are defying death.
The key, it turns out, is secreting unique peptides earlier than other toads, stopping the fungus from taking hold.
The midwife toad is already a bit of a weirdo.
There are a few species, but it’s Alytes obstetricans we’re talking about here.
The male toads carry fertilized eggs around on their back and thighs to protect them from getting eaten, hence the name ‘midwife’.
Then, when the eggs are about ready to pop, the toad takes them down to the nearest water, where the tadpoles leap from their daddy’s posterior into the great unknown.
In the case of the Pyrenees midwife toads, that ‘unknown’ is the stunning clear waters of high-altitude mountain lakes: Lac d’Arlet, Puits d’Arious, Lac de Lhurs, and Ibón de Acherito.
Midwife toads across the Western Pyrenees have been nearly wiped out by chytrid infections, ever since the fungus reached the region 19 years ago.
Previously, these mountains were a safe refuge from the fungus. The lakes stay frozen for half the year, and the cold temperatures have kept the fungus at bay.
But as humans continue to burn fossil fuels, climate change has been warming up the mountains. And there’s nothing a fungus loves more than a warm, damp climate.
Somehow, though, a few toad populations have managed to survive, in spite of their B. dendrobatidis infections: toads at Ibón de Acherito, Puits d’Arious, and Lac de Lhurs.
The researchers compared recently metamorphosed toads from these lakes with those from a population that has been doing much worse: the toads at Lac d’Arlet.
In the toad populations that had recovered from fungal outbreaks, they found a particular kind of immune defense matured much earlier, with the toads beginning to secrete antimicrobial peptides from their skin while they were still in the tadpole stage.
That difference is crucial.
The fungus can only survive on skin that contains keratin, which is present in mature toads, but not tadpoles or larvae.
That’s why chytrid fungus is so lethal to these creatures: frogs and toads absorb most of the oxygen and water they need to survive through their skin. If that porous membrane becomes blocked up by the fungus, it leads to a fatal chain reaction.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>Carrying protection against the fungus during the vulnerable transition phase into keratin-laden adulthood could be the key to their survival.
“The next step is to look at what factors prevent these immune systems from maturing early,” says ZSL herpetologist Phillip Jervis, the lead author of the study.
“This could be down to genetics or environmental factors such as temperature or the presence of trout – a major danger for tadpoles that could drive them to develop into adults faster so they can leave the water, meaning less time for their immune system to develop.”
In the meantime, the team also found an enormous amount of peptides in the toads’ skin secretions – 1,152 in total, out of which only seven were previously known.
That diversity of peptides also seems to give toads an edge when it comes to surviving chytrid fungus.
Those that reached a higher diversity in their tadpole phase were thriving, even through outbreaks, while those that did not develop that diversity while young were more likely to die of the disease.
“We discovered a far greater diversity of peptides than we expected. We now need to understand how they work to control pathogens and which ones are anti-microbial,” says chemist Alethea Tabor.
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As well as having potential to help in the fight against the mass extinction of frogs and toads, this discovery could have applications in human medicine.
“A lot of medicines for humans were initially found in the natural world – penicillin came from fungi, for example,” Tabor says.
“These peptides are new leads that could be used to help human health, especially as we have our own problems as a species with the rise of antimicrobial resistance, which is requiring us to find new ways to treat infections.”
The research is published in Nature Chemical Biology.
This article was fact-checked by Rebecca Dyer and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
