Amphibians in many parts of the world are falling victim to pathogenic chytrid fungus, first discovered in the late 1990s. The fungus colonises the skin of infected individuals, interferes with respiration, and can ultimately cause infected frogs to die of heart attacks. Although the fungus is devastating amphibianpopulations around the world, Asia appears to be the exception.
Researchers at Seoul National University, led by Professor Bruce Waldman in the Biological Sciences department, have found that Korean amphibians are immune to the disease. They isolated numerous distinct strains of the fungus here, more diverse than strains infecting frogs elsewhere in the world. The researchers thus suggested that the fungus first became pathogenic in Korea and then diversified and spread throughout the world. Over hundreds or even thousands of years, Korean species evolved immunological resistance to the disease. Consistent with this theory, they found fungus in the skin of frogs collected in the Korean peninsula in the early 1900s, long before epidemics of the disease affected amphibians elsewhere in the world. The fungus apparently spread through international trade or war-related transport in the 1950s.
A paper appearing this week in the journal Science, involving collaborative research between the Seoul National University team and others around the world, provides compelling support for this theory. Using new genetic sequencing methods to unravel the complete genomes of chytrid strains isolated from Africa, the Americas, Europe, as well as South Korea, the study demonstrates convincingly that worldwide strains of the pathogen all are descended from fungus infecting 무당개구리(fire-bellied toads, Bombina orientalis) in Korea. This shows that the chytrid fungus first evolved in Korea, then spread, and exchanged genetic material with other global strains, to emerge as an infectious disease.
Since its discovery in 1998, this pathogenic fungus has been associated with mass mortality and declines of amphibian populations in North and Central America, Europe and Australia, leading to extinctions of up to 40% of species in affected areas. It is now recognized as an internationally notifiable disease by the World Organization for Animal Health.
The Seoul National University researchers discovered that species around the world can evolve immune resistance to the pathogen. Resistant amphibians in other parts of the world share the same configuration of immune cell receptors that is most common in Korean amphibians. This configuration optimally recognizes the pathogen, allowing the immune system to prevent the fungus from causing disease. Additional studies conducted by the Seoul National University group show that like global strains, Korean strains of the fungus are lethal to frogs from other parts of the world that have yet to evolve resistance to the pathogen. The research was carried out by the Laboratory of Behavioral and Population Ecology in the School of Biological Sciences with support from the National Research Foundation of Korea.