January 4, 2010

The Origin of the Amphibian Chytrid Fungus

One of the most pressing questions related to the amphibian chytrid fungus (Batrachochytrium dendrobatidis - "Bd") is where this pathogen originated from. Unfortunately, we still don't know, but a recently published paper adds an interesting wrinkle to what we do know. Until recently, the available data suggested an African origin. The "Out of Africa" hypothesis was based on the fact that the earliest known record of Bd was from South African amphibians collected in 1938. Under this hypothesis, Bd was spread around the world as a consequence of the large-scale export of African clawed frogs (infected with Bd) from Africa for medical research starting in the 1940s.

Now Goka and colleagues present detailed information on the distribution and genetic structure of Bd in Japan, the first such study for any Asian country. In addition to showing that Bd is widespread in amphibian populations across Japan, the authors also state that Bd was detected on amphibian specimens collected as early as 1902. Furthermore, one of the species infected with Bd is the Japanese giant salamander (Andrias japonicus - shown in photograph), an ancient species that reaches a length of more than 1 m (39") and is apparently not negatively affected by Bd infection. This species was infected with unique Bd strains that were not found on any other Japanese amphibians, suggesting that the Bd-Andrias relationship is the result of a long-term coevolution between pathogen and host. These findings led Goka et al. to suggest that Bd originated in Japan, not Africa.

Additional research will be necessary to validate this conclusion, but the possibility that Bd originated in Asia certainly has shaken up our current thinking on the origin of this pathogen. For now, all we can say with some certainty is that Bd originated somewhere in the world and was subsequently moved around the globe by human activities. 

For details on the Japanese study: Goka, K., J. Yokoyama, Y. Une, T. Kuroki, K. Suzuki, M. Nakahara, A. Kobayashi, S. Inaba, T. Mizutani, and A. D. Hyatt. 2009. Amphibian chytridiomycosis in Japan: distribution, haplotypes and possible route of entry into Japan. Molecular Ecology 18:4757-4774.

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  1. It is quite interesting how important museum specimens become such a long time after they are collected. We have used them to study fluctuating asymmetry in the same pond at various time periods.
    I still wonder at what point in time can one go back to and know that frogs were not brought to differing parts of the world by man. in 1902 had people already been bringing frogs from Japan, to Africa. Chytrid is such an ancient fungus. I would not be surprised if it could be traced to many places of origin. But it seems that the location with most of the chytrid resistant species would most likely the area of origin.

  2. Hi Dale. Thanks for the post. Museum specimens are indeed incredibly important, and their value in tracking the spread of Bd makes this particularly clear. As for human movement of amphibians, it has certainly been occurring for a long time. However, most of this early movement was intra-continental (e.g., bullfrogs moved from the eastern U.S. to the western U.S.). The movement of frogs between continents is a more recent phenomenon that started in the 1930s. This involved primarily bullfrogs moved to Europe, Asia, and South America, and african clawed frogs moved from Africa all over the world.

  3. I must emphasize, that though I work in a lab that does work with some disease ecology with amphibians I myself do not. But, I do keep up with the work. I have always been interested in Chytrid, and other emerging epizootics. I was stunned by the findings presented here. Before this I was aware of Chytrid being found in specimens from Africa, so you can imagine my surprise when reading this. Great stuff. Keep bringing it to us.

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