The Origin of the Amphibian Chytrid Fungus

The amphibian chytrid fungus (Batrachochytrium dendrobatidis: Bd) is the cause of the most spectacular loss of vertebrate biodiversity in recorded history. To date, at least 200 species have been driven extinct and hundreds more have suffered major declines. Even amphibians within the world's best protected ecosystems have been hard-hit, including California's mountain yellow-legged frog (Rana muscosa, Rana sierrae). This amphibian pathogen appears to have emerged in just the last 50 years, subsequently spreading around the world at lightning speed. 

So, where did Bd come from and what allowed its recent emergence? These are questions that researchers have asked since its description in 1999. Using the best available methods, molecular biologists from around the world have slowly but surely been zeroing in on the answers. In 2003 and 2007, studies by Morehouse et al. and Morgan et al., respectively, used evidence that Bd had little genetic variation to suggest that Bd was a recently emerged clone, not a pathogen with a long evolutionary history with amphibians. Results published in 2009 by James et al. supported these interpretations and suggested that the emergence of Bd may have been caused by a single hybridization event.

A just-published paper by Farrer et al. now advances this story even further. Using sequences of entire Bd genomes, Farrer et al. found evidence of multiple distinct Bd strains with apparently non-overlapping distributions. However, they also found a single lineage that was globally distributed, more virulent than the geographically isolated strains, and associated with worldwide frog die-offs. Based on this evidence, they suggest that contact between two previously isolated strains produced a hypervirulent strain that subsequently spread globally, causing amphibian declines and extinctions in its wake. They further postulate that the global amphibian trade was likely responsible for bringing these genetically isolated strains into contact with each other.

Another research group is using similar methods to provide an even more detailed view of the emergence of Bd as an amphibian pathogen, and will hopefully publish their results in the near future. I suspect that we haven't yet heard the final word of this evolving story. Given the likely role of human commerce in driving the emergence of Bd, there are important lessons here for biodiversity conservation in the Anthropocene. Namely, as our increasingly global economy moves goods around the world we will inevitably also move less desirable things, including invasive animals and plants but also invisible things like pathogens. The spread of introduced pathogens from their new introduction points will often be impossible to control, and decimation of naive animal and plant populations into which they come into contact is all but guaranteed. Bd provides a sobering example of what is to come.  

The citation for the latest paper is as follows: Farrer, R. A., et al. 2011. Multiple emergences of genetically diverse amphibian-infecting chytrids include a globalized hypervirulent recombinant lineage. Proceedings of the National Academy of Sciences, USA 108:18732-18736. [link]

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Do High Elevations Provide Amphibians with a Refuge from Disease?

One of the most puzzling aspects about the impact of the amphibian chytrid fungus (Batrachochytrium dendrobatidis: Bd) on amphibian populations is the diversity of disease outcomes. In some landscapes, the arrival of Bd causes the complete extirpation of all populations, but in others it has little or no negative effect on amphibians. There are lots of reasons for these different disease outcomes, including differences between amphibian species in their susceptibility to Bd and different environmental conditions that change Bd virulence and amphibian susceptibility. The environmental condition that has received the most attention is temperature. Bd grows best at temperatures of 15-25° C and growth is greatly reduced at temperatures above and below this optimum range. Importantly, at temperatures above 30° C Bd is killed within a matter of hours, but no lethal effects are known at the low end of the temperature range (i.e., around 0° C). 

This relationship between temperature and Bd growth likely explains several general patterns related to Bd infection intensities on frogs in hot climates (e.g., tropical and subtropical regions). These include lower Bd infection intensities at low versus high elevation, low versus high latitude, and summer versus winter. However, in temperate climates little is known as to whether similar patterns hold. That is, do colder temperatures limit the growth rates of Bd and impacts to amphibians? If so, then we would expect lower Bd infection intensities at higher elevations. 

During the last several years, my colleagues and I have tested this idea using a series of studies on the Sierra Nevada yellow-legged frog (Rana sierrae) in Yosemite National Park. These studies included a park-wide (i.e., low to high elevation) survey of Bd infection intensities, detailed measurements of infection intensity over the entire ice-free period (from the cold temperatures immediately after ice-out to warm temperatures of mid-summer to cold temperatures of late-fall), and frog reintroductions in which we moved frogs from a single Bd-positive R. sierrae population to five nearby lakes that spanned a wide elevation range.

The results provided no support for the idea that the coldest habitats might provide frogs with a refuge from Bd. In the park-wide survey, Bd infection intensity was unrelated to elevation. In the seasonal study, despite temperatures that ranged from 4° C to 25° C, Bd infection intensities remained remarkably constant. And the reintroduction study indicated no changes in infection intensities related to elevation. As a consequence, in the paper that was recently published describing these results (see below) we concluded that in the temperate zone even the coldest habitats are unlikely to provide amphibians with a refuge from Bd. 

Despite this discouraging finding, we did significantly advance our understanding of Bd-frog dynamics and learned a lot about frog reintroductions as a method of reestablishing R. sierrae populations in Bd-positive landscapes. Although three of the five reintroduced populations quickly declined and never showed evidence of reproduction, the remaining two populations did produce tadpoles in the years following reintroduction and at least one of these  populations shows evidence of becoming a self-sustaining population despite ongoing Bd infections. Future reintroductions are planned to allow us to learn more about how best to conduct these reintroductions to maximize the chances of success. 

The citation for the paper described above is as follows: 

Knapp, R. A., C. J. Briggs, T. C. Smith, and J. R. Maurer. 2011. Nowhere to hide: impact of a temperature-sensitive amphibian pathogen along an elevation gradient in the temperate zone. Ecosphere 2:art93.

The paper is available here: http://vesr.ucnrs.org/pages/knapp/publications/publications.html.

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