Since the 1970s, the aquatic fungus Batrachochytrium dendrobatidis has triggered die-offs in hundreds of amphibian species such as the common midwife toad (Alytes obstetricans). These frogs—arranged in rows by researchers documenting the fungus—died in the French Pyrenees. Photo: Matthew Fisher / National Geographic

By Michael Greshko
10 May 2018
(National Geographic) – Many of the world’s amphibians are staring down an existential threat: an ancient skin-eating fungus that can wipe out entire forests’ worth of frogs in a flash.This ecological super-villain, the chytrid fungus Batrachochytrium dendrobatidis, has driven more than 200 amphibian species to extinction or near-extinction—radically rewiring ecosystems all over Earth.“This is the worst pathogen in the history of the world, as far as we can tell, in terms of its impacts on biodiversity,” says Mat Fisher, an Imperial College London mycologist who studies the fungus.Now, a global team of 58 researchers has uncovered the creature’s origin story. A groundbreaking study published in Science on Thursday reveals where and when the fungus most likely emerged: the Korean peninsula, sometime during the 1950s.From there, scientists theorize that human activities inadvertently spread it far and wide—leading to amphibian die-offs across the Americas, Africa, Europe, and Australia.“[The pathogen’s spread] could have happened from any one event, from the cumulative number of events, or maybe some big anthropogenic events like the Korean War,” says Imperial College London researcher Simon O’Hanlon, the study’s lead author. [more]

Ground Zero of Amphibian ‘Apocalypse’ Finally Found

By Ryan O’Hare
10 May 2018
(Imperial College London) – A deadly fungus responsible for the devastation of amphibian populations around the world may have originated in East Asia, new research has found.Batrachochytrium dendrobatidis (Bd), known as chytrid fungus, has long been identified as a cause of the decline and extinction of species of frogs, toads, newts and other amphibians across several continents.Chytrid is distributed around the world but to date it has remained unclear where killer strains of the pathogen first emerged.Now, new research published in the journal Science and led by researchers at Imperial College London alongside partners including ZSL (Zoological Society of London) and UCL, suggests the killer fungus currently ravaging global amphibian populations originated in East Asia.The researchers highlight the need to tighten biosecurity across borders, including a potential ban on trade in amphibians as pets to ensure the survival of vulnerable species.Dr Simon O’Hanlon, from the Department of Infectious Disease Epidemiology at Imperial and first author of the paper, said: “Biologists have known since the 1990s that Bd was behind the decline of many amphibian species, but until now we haven’t been able to identify exactly where it came from.”“In our paper, we solve this problem and show that the lineage which has caused such devastation can be traced back to East Asia.”Chytrid is passed from animal to animal and spreads rapidly in the wild, causing catastrophic mortality and declines in some species, while others are less affected.The fungus causes a disease called chytridiomycosis, which attacks the animal’s skin, affecting their ability to regulate water and electrolyte levels and leading to heart failure.In this latest study, an international team involving 38 institutions gathered samples of the pathogen from around the world. They sequenced the genomes of these samples, combining the data with genomes from previous Bd studies to make a collection of 234 samples.Researchers analysed the data, looking at differences between the genomes. From the samples, they identified four main genetic lineages of the fungus, three of which are distributed globally. A fourth lineage was found only in Korea, on frogs native to the region.

Korean connection

Cultures from this Korean lineage were found to contain much more genetic diversity than any other lineage.Deeper analysis of the Korean Bd showed no history of global outbreaks within their genomes suggesting the Korean chytrid strains were native to the region, and most closely resemble the ancestor of all modern Bd.Using the genetic data, the team estimated when the killer strain of Bd currently plaguing amphibians diverged from its most recent common ancestor.Their findings support the idea that rather than dating back thousands of years, as previously thought, the range of the disease expanded greatly between 50 and 120 years ago, coinciding with the rapid global expansion of intercontinental trade.The team’s finding Asian strains of Bd in pet Oriental fire-bellied toads strongly supported this idea.According to the researchers, human movement of amphibians – such as through the pet trade – has directly contributed to spreading the pathogen around the world.They add that the paper provides strong evidence for a ban on trade in amphibians from Asia, due to the high risk associated with exporting previously unknown strains of chytrid out of this region.The group also highlights the threat of another amphibian pathogen which has also emerged from Asia (B. salamandrivorans or BSal) affecting salamanders in Europe and whose spread is also linked with the global trade in pet amphibians from Asia.Professor Matthew Fisher, from the School of Public Health at Imperial, said: “Our research not only points to East Asia as ground zero for this deadly fungal pathogen, but suggests we have only uncovered the tip of the iceberg of chytrid diversity in Asia. Therefore, until the ongoing trade in infected amphibians is halted, we will continue to put our irreplaceable global amphibian biodiversity recklessly at risk.”The research was supported by funding from the National Environment Research Council.

Genetic clues reveal origins of killer fungus behind the ‘amphibian plague’Genotypes of Bd isolated from infected amphibians in the international trade and phylogenetically linked genotypes from segregated geographic localities. The red diamonds on the phylogeny indicate isolates recovered from traded animals. Their geographic location is displayed by the red diamonds on the map. The red numbers link each trade isolate to the relevant picture of the donor host species atop the figure and their placement in the phylogeny. The arrows on the map link geographically separated isolates that form closely related phylogenetic clades with high bootstrap support (≥90%). Each clade is denoted by a different-shaped point on the map; names of isolates within each clade are displayed on the map. The dates displayed indicate the sampling time frame for each clade. Graphic: O'Hanlon, ewt al., 2018 / Science

ABSTRACT: Globalized infectious diseases are causing species declines worldwide, but their source often remains elusive. We used whole-genome sequencing to solve the spatiotemporal origins of the most devastating panzootic to date, caused by the fungus Batrachochytrium dendrobatidis, a proximate driver of global amphibian declines. We traced the source of B. dendrobatidis to the Korean peninsula, where one lineage, BdASIA-1, exhibits the genetic hallmarks of an ancestral population that seeded the panzootic. We date the emergence of this pathogen to the early 20th century, coinciding with the global expansion of commercial trade in amphibians, and we show that intercontinental transmission is ongoing. Our findings point to East Asia as a geographic hotspot for B. dendrobatidis biodiversity and the original source of these lineages that now parasitize amphibians worldwide.

Panzootic chytrid fungus out of Asia

Species in the fungal genus Batrachochytrium are responsible for severe declines in the populations of amphibians globally. The sources of these pathogens have been uncertain. O’Hanlon et al. used genomics on a panel of more than 200 isolates to trace the source of the frog pathogen B. dendrobatidis to a hyperdiverse hotspot in the Korean peninsula (see the Perspective by Lips). Over the past century, the trade in amphibian species has accelerated, and now all lineages of B. dendrobatidis occur in traded amphibians; the fungus has become ubiquitous and is diversifying rapidly.

Recent Asian origin of chytrid fungi causing global amphibian declines