Two Ways Frogs Beat a Fatal Fungus

The chytrid fungal disease chytridiomycosis is a major player in the worldwide amphibian decline. The disease, and the fungus that causes it (Batrachochytrium dendrobatidis), are more prevalent in some frog species and populations than others. Two recent studies reveal how some frogs gain resistance.Genetic Differences in ImmunityResearchers from Cornell University looked at the genetic basis of natural variation in susceptibility to the fungus in lowland leopard frogs. Their findings were published last week in the Proceedings of the National Academy of Sciences ('MHC genotypes associate with resistance to a frog-killing fungus').The authors focused on differences in the major histocompatibility complex (MHC), a key part of the immune system. MHC proteins only recognize certain pathogens. Their job is to alert the immune system to the presence of these pathogens so that a response against them can be mounted. Every frog inherits one copy of MHC genes from each of its parents. In this study, frogs that had two different variations of the MHC gene survived at much higher rates than those that had inherited two of the same copies. Frogs with a particular MHC allele, called Q, were also more likely to survive. The authors say that this is a genetic change that may have arisen fairly recently, indicating that frogs may be be adapting to the fungal scourge by evolving new gene variants that offer them better resistance.Behavioral AdaptationsAn Australian research group rediscovered a surviving population of armored mist frogs, a species that had not been seen for 17 years and was thought to be a casualty of chytridiomycosis. The surprising story of where these frogs were found is reported in the October issue of Conservation Biology ('Environmental refuge from disease-driven amphibian extinction').The frogs were discovered not in their normal wet rainforest habitat, but at the drier edges of the rainforest, with less canopy cover. Interestingly, most of the frogs and tadpoles sampled tested positive for the fungus - but no frogs showed any clinical signs of the disease and the population was going strong. The chytrid fungus thrives in cool, moist environments. The authors suggest that in the frogs' new habitat, the lack of canopy cover allows the rocks on which the frogs perch to warm up to a temperature that is inhospitable to the fungus. Although the frogs may still host the fungus, its growth and reproduction are slowed down enough to prevent the frogs from getting sick. The takeaway lesson is that adaptable species, tolerant of a broad range of environments, may be less vulnerable to outbreaks of disease.