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Science Issue May 2018: Recent Asian origin of chytrid fungi causing global amphibian declines

Research from Fisher Lab published in the May 2018 Science issue has identified East Asia as the likely origin of Batrachochytrium dendrobatidis (Bd), the fungal pathogen that has caused devastation to amphibian populations around the world. The work was led by Dr. Simon O'Hanlon and Prof. Matthew Fisher, with a huge international network of collaborators.

Research Significance

Bd is a highly successful pathogen, capable of infecting at least 700 species of amphibian, and has wiped out at least 200 amphibian populations.

This research highlights that the Bd panzootic has been driven by global trade and poor biosecurity measures. Irreplaceable amphibian biodiversity will continue to be placed recklessly at risk until the ongoing trade in infected amphibians is halted.

Key Findings

1. All modern lineages of the amphibian killing chytrid fungus, Bd, are descended from a population that originated in East Asia.

2. BdGPL, the killer lineage behind all confirmed mass amphibian mortalities to date, diverged from its most recent common ancestor between 50 and 120 years ago, which coincides with a huge expansion in the international trade network and human population size

3. Four genetically distinct Bd lineages are identified in this study: BdGPL; BdAsia-1; BdASIA-2/BdBRAZIL; BdCAPE. All except BdASIA-1 are distributed globally.

Research Summary

Over 200 Bd isolates from across the globe were whole genome sequenced for this study, which has identified four genetically distinct Bd lineages. One of those lineages, BdASIA-1, was only found on native frogs on the Korean peninsula and harboured a higher level of genetic diversity than any other lineage. Furthermore, there is no evidence of a global outbreak in the genomes of any BdASIA-1 isolates which suggests that this strain is native to East Asia, and thus is likely to most closely resemble the ancestor of all modern Bd.

It is not currently possible to posit a more specific origin location than East Asia, but it is notable that isolates from Korea bear all the hallmarks of a population that has been co-evolving with its amphibian hosts for a substantial period of time. To further narrow down Bd’s location of origin will require the collection and analysis of more isolates from elsewhere in East Asia and the Indian subcontinent.

Analysis of isolates belonging to the BdGPL lineage has dated the killer lineage’s emergence to between 50 and 120 years ago, much more recently than previous estimates which suggested emergence dates in the range of thousands of years ago.

Importantly, this revised divergence date coincides with a rapid global expansion in international trade, including that of wildlife, and of an expansion in human population size. This new divergence date adds further weight to strong evidence that human movement of amphibians, such as through the pet trade, has directly contributed to the spread of this pathogen around the world.

Identifying the origin of any pathogen has important policy implications for managing pathogen spread by indicating which regions need to be monitored closely for current and future panzootic and pandemic pathogen exports. We can certainly expect that there are other microbes, including as yet unknown chytrids, that could result in another highly virulent pathogen if they escape the confines of their environmental niches. Global trade is the fastest route for that to happen.

Additionally, study of Bd-amphibian interactions in East Asia should now be considered a research priority in order to understand how these differ from Bd-amphibian interactions in susceptible populations. Understanding how Bd has become such a successful pathogen is the ultimate goal.

It is clear that we are only just beginning to scratch the surface of the vast levels of chytrid diversity in Asia and as such, the threat to global amphibian diversity from novel pathogenic chytrids will continue until biosecurity measures to prevent infected animals being transported though the international trade network are effectively established.

This work is the product of a long term international collaborative research effort, contributed to by researchers from 38 institutions and 22 countries. It underscores the great achievements that can be made through large scale international scientific collaborations.

The full article can be accessed here.

Please contact us on twitter via @fisher_lab or @ChytridCrisis, or by emailing with any queries regarding this research.