A discovery could end the global disappearance of amphibians by protecting them from infection
A fungus that is wiping out frogs and toads on nearly every continent may have an Achilles heel. Scientists have discovered a virus that infects the fungus and could be modified to save amphibians from extinction.
The fungus Batrachochytrium dendrobatidis, or Bd, wreaks havoc on the skin of frogs and toads, eventually causing heart failure. Till date has contributed to the decline of more than 500 amphibian species and 90 possible extinctionsincluding the yellow-legged mountain frogs of the Sierras and the Panamanian golden frog.
A new article in the journal Current Biology documents the discovery of a virus that infects Bd disease and could be manipulated to combat the fungal disease. The University of California, Riverside researchers who discovered the virus are excited about the implications of their discovery. Not only does it help them learn how pathogenic fungi emerge and spread, but it also offers hope for an end to what they call a global amphibian pandemic.
“Frogs fight harmful insects, plant pests and mosquitoes. If their populations collapse worldwide, it could have devastating consequences,” says Mark Yacoub, a doctoral student in microbiology at UCR and author of the paper.
“They are also the canary in the coal mine of climate change.” As temperatures rise, UV light increases and water quality deteriorates, frogs respond. If they disappear, we lose an important environmental signal,” explains Yacoub.
Bd wasn’t common until the late 1990s, but then “the frogs suddenly started dying,” Yacoub said.
When they discovered the virus infecting frogs, Yacoub and UCR microbiology professor Jason Stajich had been working on the genetics of the Bd population, hoping to better understand where it came from and how it mutated.
“We wanted to see how different strains of fungi differ in places like Africa, Brazil and the United States, just as different strains of COVID-19 are being studied,” explains Stajich. To do this, the researchers used DNA sequencing technology. As they examined the data, they noticed some sequences that did not match the fungus’ DNA.
“We realized that these additional sequences, when assembled, had the characteristics of a viral genome,” Stajich said.
Researchers had previously looked for Bd viruses but had not found them. The fungus itself is difficult to study because complex procedures are required to keep it alive in the laboratory.
“It is also a difficult fungus to track because it has a vital phase in which it is mobile, has a flagellum similar to a sperm tail, and swims from side to side,” explains Stajich.
In addition, it was difficult to find the virus that infects Bd because most known viruses that infect fungi, called mycoviruses, are RNA viruses. However, this virus is a single-stranded DNA virus. By examining the DNA, the researchers were able to detect the virus trapped in the fungus’s genome.
It appears that only some fungal strains contain the virus in their genome. But those who are infected appear to behave differently than those who don’t have it. “When these strains have the virus, they produce fewer spores, so it spreads more slowly.” But they can also become more virulent and kill frogs more quickly,” explains Stajich.
The virus is currently trapped in the genome of the fungus. Researchers want to clone the virus and see whether a manually infected Bd strain also produces fewer spores.
“Since some strains of the fungus are infected and others are not, this highlights the importance of studying multiple strains of a fungal species,” Yacoub said.
In the future, researchers will try to better understand how the virus works. “We don’t know how the virus infects the fungus or how it gets into the cells,” explains Yacoub. “If we want to manipulate the virus to help amphibians, we need answers to questions like these.”
In some places there appear to be some amphibian species that are developing resistance to Bd. “Just like with COVID, immunity builds slowly. We hope to help nature take its course,” says Yacoub.
REFERENCE
An endogenous DNA virus in an amphibian-killing fungus that is associated with the genotype and virulence of the pathogen
Photo: Panama golden frog threatened with extinction. Photo credit: BRIAN GRATWICKE/US FISH & WILDLIFE SERVICE