Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidis (/bəˌtreɪkoʊˈkɪtriəm ˈdɛndroʊbətaɪdɪs/ bə-TRAY-koh-KIT-ree-əm DEN-droh-bə-ty-dis), also known as Bd or the amphibian chytrid fungus, is a fungus that causes the disease chytridiomycosis in amphibians. In the decade after it was first discovered in 1998, the disease devastated amphibian populations around the world, in a global decline towards multiple extinctions, part of the Holocene extinction. A recently described second species, B. salamandrivorans, also cause chytridiomycosis and death in salamanders. Some amphibian species appear to have an innate capacity to withstand chytridiomycosis infection due to symbiosis with Janthinobacterium lividum. Even within species that generally succumb, some populations survive, possibly demonstrating that these traits or alleles of species are being subjected to evolutionary selection. The generic name is derived from the Greek words batrachos (frog) and chytra (earthen pot), while the specific epithet is derived from the genus of frogs from which the original confirmation of pathogenicity was made (Dendrobates), dendrobatidis is from the Greek dendron, 'tree' and bates, 'one who climbs', referring to a genus of poison dart frogs. Batrachochytrium dendrobatidis was until recently considered the single species of the genus Batrachochytrium. The initial classification of the pathogen as a chytrid was based on zoospore ultrastructure. DNA analysis of the SSU-rDNA has corroborated the view, with the closest match to Chytridium confervae. A second species of Batrachochytrium was discovered in 2013: B. salamandrivorans, which mainly affects salamanders and also causes chytridiomycosis. B. salamandrivorans differs from B. dendrobatidis primarily in the formation of germ tubes in vitro, the formation of colonial thalli with multiple sporangia in vivo, and a lower thermal preference. B. dendrobatidis infects the keratinized skin of amphibians. The fungus in the epidermis has a thallus bearing a network of rhizoids and smooth-walled, roughly spherical, inoperculate (without an operculum) sporangia. Each sporangium produces a single tube to discharge spores. Zoospores of B. dendrobatidis, which are typically 3–5 µm in size, have an elongate–ovoidal body with a single, posterior flagellum (19–20 µm long), and possess a core area of ribosomes often with membrane-bound spheres of ribosomes within the main ribosomal mass. A small spur has been observed, located at the posterior of the cell body, adjacent to the flagellum, but this may be an artifact in the formalin-fixed specimens. The core area of ribosomes is surrounded by a single cisterna of endoplasmic reticulum, two to three mitochondria, and an extensive microbody–lipid globule complex. The microbodies closely appose and almost surround four to six lipid globules (three anterior and one to three laterally), some of which appear bound by a cisterna. Some zoospores appear to contain more lipid globules (this may have been a result of a plane-of-sectioning effect, because the globules were often lobed in the zoospores examined). A rumposome has not been observed. A nonfunctioning centriole lies adjacent to the kinetosome. Nine interconnected props attach the kinetosome to the plasmalemma, and a terminal plate is present in the transitional zone. An inner ring-like structure attached to the tubules of the flagellar doublets within the transitional zone has been observed in transverse section. No roots associated with the kinetosome have been observed. In many zoospores, the nucleus lies partially within the aggregation of ribosomes and was invariably situated laterally. Small vacuoles and a Golgi body with stacked cisternae occurred within the cytoplasm outside the ribosomal area. Mitochondria, which often contain a small number of ribosomes, are densely staining with discoidal cristae.