The results of an international study of the 'amphibian killer fungus' Batrachochytrium dendrobatidis (Bd) have just been published, which point to south-east Asia as the likely origin of this pathogen (disease-causing organism). Scientists have previously suggested that Bd could have originated in other parts of the world, including Africa, North America and South America.

The latest conclusions have been drawn (largely) from genetic evidence. The analysis of samples of Bd collected from wild amphibians around the world has shown that there is a high level of genetic diversity in Bd from the Korean peninsula in south-east Asia. Here, the lead authors conclude that one lineage of Bd (called 'BdASIA-1') shows the "hallmarks of an ancestral population" from which other strains of Bd  (such as the 'global panzootic lineage' or 'BdGPL') may have evolved.

BdGPL is the most widespread lineage of Bd that has so far been discovered. As part of attempts to unravel the mystery of where Bd came from, scientists have been trying to work out how recently different strains of Bd have evolved, or split off from their last common ancestor. They have reached a variety of conclusions, based on genetic methods which estimate rates of change in the DNA of study organisms, over generations. In one study (Rosenblum et al, 2013), the 'time to most recent common ancestor' (TMRCA) for BdGPL was estimated at 26,000 years before present. In this latest study, the TMRCA for BdGPL has been estimated at just 120 – 50 years ago.

Without first-hand experience of the techniques used, it is difficult to determine which of these estimates is the most reliable. Both estimates suggest that BdGPL is a 'relatively' new strain. 'Mass die-offs' of amphibians at sites where Bd has been detected might be the result of the global trade in amphibians, if the amphibians concerned have not had time to evolve immunity against BdGPL.

ARC staff member Dr Pete Minting is a co-author on the new paper, published in Science. He helped by collecting samples or 'isolates' of Bd from natterjack toads in Cumbria, while completing his PhD "An investigation into the effects of Batrachochytrium dendrobatidis (Bd) on populations of natterjack toads in the UK" (2012). At the time, scientists at Imperial College, London confirmed that the strain of Bd collected from natterjack toads in Cumbria was BdGPL.

Pete's PhD research (available as a free download from suggests that natterjack toads are often able to tolerate infection with BdGPL. At several sites in the UK and Spain where BdGPL is present, natterjack toad populations have shown no evidence of a decline. However, some other European species (such as European midwife toads) do suffer badly from BdGPL if they are infected, so the effects can be strongly species-dependent.

Another species of Bd which has recently been discovered (Batrachochytrium salamandrivorans, or Bsal) could also pose a threat to wild amphibians, if it is introduced to new areas. Bsal has been shown to have negative effects on some species of European salamanders and newts, while most Asian species studied appear unaffected by it. Bsal has not yet been discovered in the wild in the UK.

Useful links:

O'Hanlon S.J. et al (2018) 'Recent Asian origin of chytrid fungi causing global amphibian declines'. Science, 11 May 2018

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