The poison dart frog (Dendrobates auratus) of Panama is one of many frog species affected by the pathogenic chytrid fungus, Batrachochytrium dendrobatidis. [Courtesy Louise Rollins-Smith]
The global amphibian population is shrinking fast, and a particularly sneaky fungus is partly to blame. Now, a new study hints at why this fungus has been so fatal to frogs.
"It's been a puzzle," said Vanderbilt University researcher Louise Rollins-Smith. "Amphibians have great immune responses, so we haven't understood why they couldn't clear this fungus and survive." In a paper appearing in the 18 October issue of Science, Rollins-Smith and her colleagues show that this fungus has developed a way to evade part of the amphibian immune response.
Compared to other vertebrate populations, amphibian species are rapidly dwindling. Indeed, the pace of global amphibian decline — attributed to a variety of factors — has been alarming researchers since the 1980s and presenting a distinct threat to biodiversity on our planet.
In the late 1990s, researchers identified a new species of chytrid fungus. The new species is called Batrachochytrium dendrobatidis, or Bd, and scientists believe it has contributed to the extinction of dozens of frog species around the world, though its exact influence on amphibian decline is still a bit murky. According to Rollins-Smith, the best assessment of amphibian survival was published in Science in 2004, at which time it was thought that approximately 30 percent of amphibian species were threatened. "About half of those were in the category designated 'enigmatic decline,'" she explained, "and our fungus is thought to be one of enigmatic decline's most important contributors." Scientists know that the Bd fungus inserts itself into the skin of amphibians, drying out a layer of skin they require to be wet in order to breathe, but why amphibians can't oust the fungus hasn't been clear.
Rollins-Smith's team proposed that Bd follows in the footsteps of other pathogenic fungi, blocking critical immune responses of the host organism. To figure out just which of the fungus' molecules could perform this trick, the researchers mixed fungal cells with amphibian immune cells, including those involved in amphibians' first and second lines of defense (its innate and adaptive immune responses, respectively).
The fungus didn't appear to impair cells involved in the innate immune response, which kicks on at the first sight of a pathogen. "That was a surprise," Rollins-Smith said.
Cross-banded treefrog (Smilisca sila)
[Courtesy Louise Rollins-Smith]
But something in the fungus did inhibit the more tailored, adaptive immune response carried out by cells called B- and T-lymphocytes.
This discovery helps explain why Bd has been so devastating to amphibian populations. Even with the strong innate immune response they're known to exhibit, frogs are vulnerable if their adaptive immune responses can't finish the job of fighting a fungal invader.
The scientists conducted further experiments in mice and human lymphocytes, finding the same result: Only cells associated with adaptive immunity were impaired by Bd.
Because lymphocytes were shut down only when the researchers used mature fungal cells, and not younger fungal cells lacking a cell wall, Rollins-Smith and colleagues propose that the fungal particle causing deadly damage to frogs is a cell wall component. Further studies will be required to pinpoint it exactly.
For now, this effort gets scientists closer to figuring out how amphibians are at risk from the Bd fungus, which may aid in much needed efforts to preserve these species — and keep the world filled with frogs.
Read the abstract, "The Invasive Chytrid Fungus of Amphibians Paralyzes Lymphocyte Responses," by Louise Rollins-Smith and colleagues.