Batrachochytrium dendrobatidis - The Link Between Climate Change and Amphibians

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Introduction

Fig 1. Illustration shows a cross-sectional view of a Batrachochytrium dendrobatidis zoospore. The spore type responsible for the infectious disease called chytridiomycosis.

By Scott Upton

Batrachochytrium dendrobatidis(Bd) is a pathogen primarily found in amphibians, and it’s presence can have devastating effects on amphibian populations. Bd is responsible for causing chytridiomycosis, or chytrid, in amphibians all across the world in response to rising global temperatures (Fig 1) . Bd can be found on all continents in which amphibians are present and as of 2019, Bd has been found to infect 1,015 of 1,854 species (54%) and has lead to the decline of 6.5% of all amphibian species and the extinction of 90 species . Bd has been given the nickname “the doomsday fungus” in response to its incredible ability to reduce biodiversity and its ability to produce mass extinction of species. At the time, Bd is said to have led to the largest reduction of biodiversity of any species of vertebrates in the world . Unfortunately, this fungus is not slowing down any time soon and its effects will continue to devastate amphibian populations as long as temperatures continue to fluctuate .

Recent studies on climate change have suggested that the frequency of temperature fluctuations will continue to rise without warning . Global climate change and its effects on infectious diseases has proven to be one of the most formidable ecological challenges in modern history. With that being said, there has been controversy over whether or not climate change and the increase in infectious diseases are directly linked without the addition of other factors. Despite controversy of the two being directly linked, the combination of climate change and infectious diseases are creating significant declines in biodiversity as well as extinction across amphibian populations. Both Bd and amphibians are sensitive to the results of climate change, but Bd is able to outperform amphibians when the performance gap between the pathogen and host is at its greatest; this can be called the thermal mismatch hypothesis (Jeremy Cohen). This hypothesis suggests that hosts who are adapted to cooler environments will be more susceptible to chytridiomycosis in unusually warm conditions. Similarly, hosts who are adapted to warmer temperatures are more susceptible to chytridiomycosis in unusually colder temperatures. Scientists may be able to mitigate the effects of chytridiomycosis on amphibian populations by further studying Bd in terms of host interactions and comparing it to temperature fluctuations. With that being said, mother nature has a strange way of correcting human influence on the environment and amphibians may be able to avoid the risks Bd poses by adapting to unusual environments.

Global Distribution and Origin of Batrachochytrium dendrobatidis

Fig 2. Global distribution of differnt Batrachochytrium dendrobatidis(Bd) lineages including Batrachochytrium salamandrivorans (Bsal), another sever fungus that causes chytridiomycosis.

There has been debate on how Bd has risen to the forefront of amphibian declines, but two competing hypotheses have shed light on how Bd has become a global issue among amphibians. The first hypothesis, the “endemic pathogen hypothesis” (EPH), suggests that Bd led to an outbreak in mutualistic amphibians in which the virulence of the pathogen created imbalanced infection rates as a result of global climate change . The second hypothesis, the “novel pathogen hypothesis” (NPH), suggests that chytridiomycosis consequentially emerged from global trade routes and invaded weak ecosystems . Africa was initially thought to be the origin of chytridiomycosis due to the fact that skin samples of African clawed frogs (Xenopus laevis) revealed the presence of Bd in 1938 . The NPH is thought to have led to the introduction of Bd into amphibian populations due to decades of global trading of African clawed frogs. Recent genomic studies have revealed that genetic diversity has significantly decreased in the African clawed frogs, but high allelic diversity has been maintained in North American bullfrogs despite both species being ravished by the chytrid fungus . This suggests that Bd may not have origins in Africa, rather that Bd infections in amphibians can be carried across continents with global trade, supporting the NPH. Early genome sequencing of Bd across globally distributed sites experiencing chytridiomycosis revealed strikingly similar patterns in which several lineages were identified. BdGPL, BdCAPE,BdCH, and BdAsia1-3 strains were all identified and labeled based on geographic distribution. BdGPL was the only globally distributed lineage, and it has been found on four continents (Fig. 2) . BdCAPE was given its name based on its distribution across Cape regions of Southern Africa, and BdCH was found in isolated regions of Switzerland (Fig. 2) . Lastly, BdAsia1-3 strains are obviously found throughout Asian amphibian populations and are now considered to be the origin of chytridiomycosis .

The commercial global trade of amphibians continues to transfer the fungus between populations across the globe, however new studies have revealed Bd’s origins can be dated back to Eastern Asia in the 20th century . Analysis of over 200 hosts insinuated that the species BdAsia1 had originated in Korea and is considered the ancestral population of Bd . BdAsia1 is thought to have emerged approximately 150 years ago due to mitochondrial DNA analyses and dating . Resistance to the fungus has led to little decline in Asain amphibian populations in recent years, suggesting that the amphibians who were exposed 150 years ago have developed stable responses to the pathogen.
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Section 2

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Section 3

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References

Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2021, Kenyon College.