The Amphibian-Killing Chytrid Fungus

© Thomas S. Jenkinson
Original publication: Mycena News, January 2011

Amphibian species around the world have been recently driven to extinction at alarming rates. While several factors may be contributing to the decline of global amphibian species, the disease caused by a chytrid fungus is now known to be a major factor. The chytrid fungi are microscopic fungi that have a motile (swimming) stage at some point in their life cycles. Most chytrid species are aquatic, and produce zoospores that swim with a flagellum. Evolutionary biologists have been interested in chytrid fungi because we now know that most chytrids make up one of the most ancient lineages within the kingdom Fungi. The posterior whiplash tail of their swimming spores is thought to bear evolutionary homology to animal sperm cells. The chytrid fungi can be thought of as living fossils, holding clues to a not-so distant common ancestor between animals and fungi.

The fungus Batrachochytrium dendrobatidis has recently received a lot of scientific and media attention since its discovery as an amphibian pathogen. Batrachochytrium dendrobatidis (abbreviated Bd in the scientific literature) is an emerging pathogen that has only been described to science since 1999. This fungus had previously gone unnoticed, flying under the radar of biologists, until it suddenly began to wreak havoc on certain amphibian species in the last few decades.

Rana

An unresponsive mountain yellow-legged frog, Rana muscosa, infected by the deadly chytrid fungus Batrachochytrium dendrobatidis in the Sierra Nevada.
Photo provided by Dr. Vance T. Vredenburg.

Many mysteries continue to surround the dynamics of this fungal disease, making it a new focus for research mycologists. Scientists are continuing to struggle with the question: why has this fungus become a problem to amphibian populations so recently?

There are two hypotheses that can be made about why these fungi have recently “emerged” or become an ecological problem. One possible explanation is that this fungus has always been endemic to areas where we see disease outbreaks, and that the fungus has only recently become a killer due to changes in the environment that have made certain amphibians more susceptible to infection. The other major hypothesis is that this fungus has recently spread to new geographic areas it has never been before. While neither hypothesis completely explains the disease patterns observed globally, there is now good evidence that the disease outbreak in many geographic areas is due to the recent introduction of the deadly chytrid. In certain regions experiencing amphibian extinctions including Panama, Australia and here in California, the evidence is convincing that the fungus was recently introduced.

New genetic evidence hints that the current pandemic appears to be caused by a strain of Bd that has been rapidly dispersed around the globe. If this is true, the next obvious question becomes: how was this strain of B. dendrobatidis spread so quickly? There is little doubt that such a rapid dispersal of this disease-causing fungus was human mediated, but exactly how, and from where is still a mystery. One very possible mechanism of a rapid global spread is through the amphibian trade. A couple of the most commonly traded amphibian species are known to be carriers of the fungus without suffering symptoms or death.

Rana

Batrachochytrium dendrobatidis sporangium ready to release swimming spores
observed in the skin of an infected blue poison dart frog, Dendrobates sp.
Photo reproduced with permission of Dr. Joyce E. Longcore.

The first possible culprit is the African clawed frog Xenopus laevis. The clawed frog was widely exported to Australia, Europe and the United States for use in pregnancy tests. For decades—before more advanced pregnancy assays were available—the clawed frog was used to test for the presence of pregnancy-associated hormone HCG in women’s urine. Female frogs injected with human urine containing HCG would begin to ovulate within 24 hours. Huge numbers of the African clawed frog were transported around the world for this reason, and for their later popularity as a model organism in research laboratories. The clawed frogs are known to be good at establishing feral populations if they escape from captivity. A feral population of these frogs was even recently documented in San Francisco’s Golden Gate Park. The hypothesis that the global trade in Xenopus laevis was responsible for the spread of the deadly chytrid gained momentum with a 2004 scientific paper. The authors of that study proposed Africa to be the geographic origin of Bd. They also propose the global trade in the African clawed frog as the mechanism for the dispersal of this chytrid. The paper reported the observation of Bd in a preserved museum specimen of Xenopus laevis collected in 1938 from South Africa, which at the time was the earliest report of the occurrence of Bd. Since that 2004 study however, an even older example of Bd has been documented in museum specimens from Japan. Furthermore, new genetic studies have shown that genetic diversity of the deadly chytrid isolated from clawed frogs is relatively low, indicating that African clawed frogs may not harbor the genetic diversity of Batrachochytrium dendrobatidis that we would expect if they were the hosts of the original source populations of Bd. We would expect to see high genetic diversity in the populations of Bd that are source populations of the global pandemic, and low genetic diversity in populations that have only recently dispersed away from source populations.

In the latest studies, genetic diversity is highest in isolates of Bd collected from North American bullfrogs. The North American bullfrog, Rana catesbeiana, is native to Eastern North America, but major production of the species for human consumption is widespread, with a large volume of production in countries as far apart as Brazil and Taiwan. The industrial scale farming of these amphibians creates perfect conditions for the propagation of Bd. The bullfrogs, like the African clawed frogs, are also able to carry the deadly chytrid without disease symptoms, or death. Bullfrogs from these industrial scale farms are sold and shipped around the world. The global trade in the North American bullfrog is a likely vector for the rapid global transmission of the deadly chytrid fungus from its original source population.

It is important to keep in mind that none of these mechanisms have been conclusively proven to be the cause Bd dispersal. It also remains a mystery how the fungus is transmitted from these reservoir populations to remote, pristine habitats where it has caused dramatic population declines and extinctions. In fact, more questions than answers still remain about the biology of this newly described fungus. Whether the fungus can persist in the environment without an amphibian host is still unknown. The role of fungal parasites and pathogens in controlling the populations of their associated species has been finely tuned through a history of co-evolution. The case of Bd may be another example of how human mediated events have disrupted that process, and are changing our ecosystems in a way that has never been seen.

Literature Cited:

Thomas Jenkinson received his master’s degree focusing on systematic mycology from SF State University with Dr. Dennis Desjardin. He has worked as a collaborator in scientific field surveys of fungal biodiversity, and in laboratory studies on fungal evolution. Thomas is currently a lecturer at SF State coordinating and teaching introductory biology laboratory.