Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidisBatrachochytrium dendrobatidis
Zoosporangia of Batrachochytrium dendrobatidis growing on a freshwater arthropod (a) and on algae (b). The scale bars represent 30 µm.
Scientific classification
Kingdom: Fungi Division: Chytridiomycota Class: Chytridiomycetes Order: Rhizophydiales[1] Genus: Batrachochytrium Species: B. dendrobatidis
Binomial name
Batrachochytrium dendrobatidis Longcore, Pessier & D.K. Nichols (1999)

Batrachochytrium dendrobatidis (or Bd for short) is a chytrid fungus that causes the amphibian disease chytridiomycosis. In the first decade after it was first discovered in amphibians in 1998,^[2] the disease devastated amphibian populations around the world, in a global decline towards multiple extinctions, part of the Holocene extinction event.

Some amphibian species appear to have an innate capacity to withstand chytridiomycosis infection. Even within species that generally succumb, some populations survive, possibly demonstrating that these traits or alleles of species are being subjected to evolutionary selection. Another explanation for such occurrences, explained below, could be that some forms of the fungus are not pathogenic.

Contents 1 Etymology 2 Systematics 3 Physiology 4 Morphology 4.1 Zoospore structure 4.2 Flagellum structure 5 Life cycle 5.1 Varying forms 5.2 Habitat and relationship to amphibians 6 Disease prevalence 7 Genomics 8 Immunity hypotheses 9 Effects of pesticides 10 2008: Year of the Frog 11 References 12 See also 13 External links

Etymology

The generic name is derived from the Greek words batracho (frog) and chytr (earthern pot), while the specific epithet is derived from the genus of frogs from which the original confirmation of pathogenicity was made (Dendrobates).^[3]

Systematics

B. dendrobatidis is a monotypic 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.

Physiology

B. dendrobatidis can grow within a wide temperature range (4-25°C), with optimal temperatures being between 17-25°C.^[4] The wide temperature range for growth, including the ability to survive at 4°C gives the fungus the ability to overwinter in its hosts, even where temperatures in the aquatic environments are low. The species does not grow well above temperatures of 25°C, and growth is halted above 28°C.^[4] Infected red-eyed treefrogs (Litoria chloris) recovered from their infections when incubated at a temperature of 37°C.^[5]

Morphology

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.

Zoospore structure

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.

Flagellum structure

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.^[3]

Life cycle

Bd has two primary life stages - a sessile, reproductive zoosporangium and a motile, uniflagellated zoospore released from the zoosporangium. The zoospores are known be be active only for a short period of time, and can travel short distances.^[6] However, the zoospores are capable of chemotaxis, and can move towards a variety of molecules that are present on the amphibian surface, such as sugars, proteins and amino acids.^[7] Bd also contains a variety of proteolytic enzymes and esterases that help it digest amphibian cells and use amphibian skin as a nutrient source.^[8] Once the zoospore reaches its host, it forms a cyst underneath the surface of the skin, and initiates the reproductive portion of its life cycle. The encysted zoospores develop into zoosporangia, which may produce more zoospores that can reinfect the host, or be released into the surrounding aquatic environment.^[9]

Varying forms

Bd has occasionally been found in forms distinct from its traditional zoospore and sporangia stages. For example, before the 2003 European heatwave that decimated populations of the water frog Rana lessonae through chytridiomycosis, the fungus existed on the amphibians as spherical, unicellular organisms, confined to minute patches (80-120 micrometers across). These organisms, unknown at the time, were subsequently identified as Bd. Characteristics of the organisms were suggestive of encysted zoospores; they may have embodied a resting spore, a saprobe, or a parasitic form of the fungus that is conditionally non-pathogenic ^[10]. Once the heatwave began, the organisms assumedly changed into the more familiar disease-causing zoospores. This suggests that some populations afflicted with Bd may be free of chytridiomycosis not because of some inherent immunity, but because environmental conditions have not altered the organism into its more common, pathogenic form.

Habitat and relationship to amphibians

The fungus grows on amphibian skin and produces aquatic zoospores ^[11]. It is widespread and ranges from deserts and lowland forests to cold mountain tops. It is sometimes a non-lethal parasite and possibly a saprophyte. The fungus is associated with host mortality in highlands or during winter, and becomes more pathogenic at lower temperatures ^[12].

Disease prevalence

It has been suggested that Bd originated in Africa and subsequently spread to other parts of the world by trade in African clawed frogs (Xenopus laevis).^[13] In this study, 697 archived specimens of three species of Xenopus, previously collected from 1879 to 1999 in southern Africa were examined. The earliest case of chytridiomycosis was found in a X. laevis specimen from 1938. The study also suggests that chytridiomycosis had been a stable infection in southern Africa from 23 years prior to finding any infected outside of Africa.^[13]

Bullfrogs (Rana catesbiana), also widely distributed, are also thought to be carriers of the disease due to their inherent low susceptibility to Bd infection.^[14][15] The bullfrog often escapes captivity and can establish feral populations where it may introduce the disease to new areas.^[6] It has also been shown that Bd can survive and grow in moist soil and on bird feathers, suggesting that Bd may also be spread in the environment by birds and transportation of soils.^[16] Infections have been linked to mass mortalities of amphibians in North America, South America, Central America, Europe and Australia.^[17][18][19] Bd has been implicated in the extinction of the sharp-snouted day frog (Taudactylus acutirostris) in Australia.^[20]

A wide variety of amphibian hosts have been identified as being susceptible to infection by Bd, including wood frogs (Rana sylvatica)^[21], the mountain yellow-legged frog (Rana mucosa)^[22] the southern two-lined salamander (Eurycea cirrigera)^[23], Ambystoma jeffersonianum^[24], the western chorus frog (Pseudacris triseriata), the southern cricket frog (Acris gryllus), the eastern spadefoot toad (Scaphiopus holbrooki), the southern leopard frog (Rana sphenocephala)^[25], the Rio Grande Leopard frog (Lithobates berlandieri)^[26], and the Sardinian newt (Euproctus platycephalus).^[27]

Genomics

In 2008, the genomes of two Bd isolates were sequenced, and scientists have begun using this genetic information to help understand the molecular basis of the Bd life cycle and amphibian pathogenicity. Analysis of global gene expression using whole-genome arrays has revealed that greater than 55% of the approximately 9000 genes in the Bd genome undergo differential expression between the sessile sporangium stage and the infectious zoospore stage.^[28] Expression of a variety of metalloproteases (enzymes that can break down keratin-containing tissue, like amphibian tissue) is believed to contribute to pathogenicity by enabling cutaneous infection.^[29]

Immunity hypotheses

Because of the fungus' immense capacity for effecting amphibian population declines, considerable research exists for methods to combat its proliferation. Among the most promising is the revelation that amphibians in colonies that survive the passage of the chytrid epidemic tend to carry higher levels of the bacteria Janthinobacterium lividum.^[30] This bacteria produces antifungal compounds, such as indole-3-carboxaldehyde and violacein, that inhibit the growth of Bd even at low concentrations.^[31] Similarly, the bacteria Lysobacter gummosus found on the red-backed salamander (Plethodon cinereus), produces the compound 2,4-diacetylphloroglucinol that is inhibitory to the growth of Bd.^[32]

Effects of pesticides

The hypothesis that pesticide use has contributed to declining amphibian populations has been suggested a several times in the literature.^[33][34][35] In 2007, this hypothesis was corroborated, as it was shown that sublethal exposure to the pesticide carbaryl (a cholinesterase inhibitor) increase susceptibility of foothill yellow-legged frogs (Rana boylii) to chytridomycosis. In particular, the skin peptide defenses were significantly reduced after exposure to cabaryl, suggesting that pesticides may inhibit this innate immune defense, and increase susceptibility to disease.^[36]

2008: Year of the Frog

Due to recent studies that suggest a possible Bd-related mass extinction of frogs, the year 2008 was declared the "Year of the Frog" by the Association of Zoos and Aquariums.^[37] Beginning on leap day (February 29) and extending through December, member zoos were encouraged to promote awareness of amphibians, especially frogs. Many zoos conducted captive breeding programs in attempt to help save critically endangered species, particularly those threatened by Bd.

References

See also

• Decline in amphibian populations#Chytridiomycosis
• Ranavirus

External links

Wikispecies has information related to: Chytridiaceae

Wikimedia Commons has media related to: Chytridiomycota

• Chytrid Fungi Online at University of Alabama
• Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America
• Emerging Infectious Diseases and Amphibian Population Declines
• Survival of Batrachochytrium dendrobatidis in Water: Quarantine and Disease Control Implications
• Index Fungorum
• Batrachochytrium dendrobatidis at the Encyclopedia of Life