Chytrid Fungus: The Global Epidemic

Batrachochytrium dendrobatidis (Bd) has been reported from almost every country in the world, with the largest number of positive samples being found in the United States and Australia. Whether this is due to research bias (it may be easier to sample frogs in the U.S. and Australia, or there may be more more money in research and conservation in these countries) or to the actual incidence of chytrid, this is a disturbing finding. Perhaps the most troubling aspect of this map is that many countries experiencing amphibian decline have not even been sampled for the fungus (for example, India is estimated to have lost 63 out of 350 amphibian species, yet no samples have been recorded on this database). For enigmatic population declines, or those with no visible cause, Bd is usually implicated along with other factors like habitat loss and pollution. But just how did the amphibian chytrid fungus come to colonize the world’s frogs in such a short period of time?

As you may already know, the amphibian chytrid fungus (Batrachochytrium dendrobatidis) differs radically from all other known chytrid fungi in its infection of living animals. The gene sequence of one benign relative to Bd, Homolyaphlyctis polyrhiza (Hp), was compared to that of B. dendrobatidis, and a large number of genes were identified that differed between the two. Any number of these different genes could have contributed to the pathogenic nature of Bd (see the full description of the study in this fascinating article from Scientific American). There are also several localized strains of Bd that have been identified as less virulent, or deadly, to their frog hosts. A recent comparison of  different chytrid fungus lineages, including these milder strains of chytrid, has shown that among the newly emerged pathogenic, or disease-causing, strains are signs of genetic recombination. The researchers concluded that the most likely cause of this genetic recombination would be the “mating” of two or more fungal strains to produce a “hybrid” strain. According to this hypothesis, the killer amphibian chytrid fungus that has been spread all over the world is most likely a “super hybrid”, producing higher rates of mortalities in amphibian hosts than the original parent fungi did.

Typically in nature, such a deadly pathogen would not survive very long due to the extinction (or sometimes adaptation) of its hosts, but Batrachochytrium dendrobatidis has been aided by a high rate of transmission from host to host. The global pet trade has likely helped to make it possible for this killer fungus to emerge and persist, by spreading it across the globe to naive populations of frogs, or those that have never experienced Bd before. This paints a very grim picture for delicate and rare frog species with high susceptibility to the deadly chytrid hybrid, such as the exquisite Harlequin Frogs of the Atelopus genus. However, determining the origins of the amphibian chytrid fungus will help us get one step closer to knowing how to combat this killer, and stop the spread of the amphibian epidemic.

Frog: 0, Fungus: 2

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