In the 1970s, researchers were perplexed by the rapid, unexplained deaths of tree frogs and other amphibians in the cloud forests of Central America. The concern grew as herpetologists realized that populations were declining all around the world, with some species going extinct. Only in 1998 did they discover the culprit: a fungus, given the name Batrachochytrium dendrobatidis (Bd), which damages the skin and causes heart failure. But where and when it originated has remained a mystery.
Four lineages of Bd were already known; Europe, Africa, and Brazil each have one, and the fourth—virulent and easily spread—is found everywhere. A new analysis—which involved sequencing the genomes of 177 samples of Bd from around the world—has revealed a fifth lineage in South Korean frogs. These samples had the greatest genetic diversity, suggesting that this lineage is the ancestor of others, which then spread outside of Asia, the researchers report today in Science. Based on mutation rates, the researchers estimate that the ancestor of globally distributed lineage arose 50 to 120 years ago.
Its descendants have since spread widely, hitchhiking with stowaway amphibians in cargo and others shipped for the pet trade. The Asian lineage, for example, was detected in oriental fire-bellied toads (Bombina orientalis, pictured) that had been exported to Europe for sale as pets.
The team says more sampling is needed in Asia to check for other kinds of Bd that might pose a new threat if they get out, and that biosecurity needs to be tightened. In the past decade, salamanders in northern Europe have fallen victim to a related fungus, B. salamandrivorans. Like Bd, it came from Asia.