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Studies find how frogs outleap fungus

Staying ahead Two new studies have revealed how some frogs can survive the chytrid fungal disease that is currently devastating amphibian populations worldwide.

Known as chytridiomycosis, the disease is caused by the fungus Batrachochytrium dendrobatidis (Bd), and is more prevalent in some frog populations than others.

To understand why, Dr Kelly Zamudio and PhD candidate Anna Savage, Cornell University in New York examined lowland leopard frogs. They looked for a genetic difference within a key part of the immune system, known as the major histocompatibility complex (MHC).

Their results appear this week in the Proceedings of the National Academy of Sciences.

MHC proteins act as signposts, alerting the immune system to the presence of a pathogen, which then triggers an immune response to clear the infection. Similar to a lock and key, MHC proteins can only recognise certain pathogens.

Every frog inherits two copies of MHC genes, one from each parent. The study found frogs that had two different variations of the MHC (known as heterozygotes) survived at a much higher rate than those that doubled up on a single copy.

One particular code within the MHC, known as allele Q, was strongly linked to survival. Savage says it's an adaptation that may have arisen recently.

"We examined the DNA code that comprises allele Q, and found that a change had recently occurred," says Savage. "This DNA change gives us evidence that frogs may be adapting to chytridiomycosis by evolving new gene variants that are more effective at combating Bd," she says.

Savage says her research has the potential to greatly enhance captive breeding programs.

"Captive breeding programs could screen frogs to determine how many MHC variants they have, and then only use pairs of frogs with different complements of MHC alleles as parents, so that all of the offspring will be heterozygotes," she says.

Professor Ross Alford of James Cook University in Townsville, says it's interesting research. "We're learning an amazing amount about how amphibians and the fungus interact."

Behaviour keeps fungus at bay

Alford was part of an Australian team that recently rediscovered a surviving population of armoured mist frogs, which had not been seen for 17 years and were thought be extinct as a result of the chytrid fungus.

Though infected with the fungus, these frogs were thriving - but not in the wet rainforest of North Queensland considered their core habitat. Instead, they now inhabit the edges of the rainforest, with less canopy cover.

According to Alford, sun-warmed rocks may provide a lifesaving boost in body heat.

"At around 25°C, the fungus doesn't grow very well," he says. "The frogs don't get sick, but they aren't cured either."

It shows the importance of looking for surviving populations even outside of their known habitat, says Alford.

The Australian findings appear in the October issue of Conservation Biology .