Batrochochytrium dendrobatidis gen. et sp. nov.,
a chytrid pathogenic to amphibians.

Joyce E. Longcore1, Alan P. Pessier2 and Donald K. Nichols2

1 Department of Biological Sciences, University of Maine, Orono, Maine 04469-5722
2 Department of Pathology, National Zoological Park, Smithsonian Institution, Washington, DC 20008.

Mycologia 1999; 91: 219-227.

Abstract

Captive and wild frogs from North and Central America and Australia recently have died with epidermal infections by chytridiomycete fungi. We isolated a chytridiomycete into pure culture from a captive, blue poison dart from that died at the National Zoological Park, Washington, D.C. Using this isolate we photographed developmental stages on nutrient agar, examined zoospores with transmission electron microscopy, and inoculated test frogs. This inoperculate chytrid develops either monocentrically or colonially and has threadlike rhizoids that arise from single or multiple areas on the developing zoosporangium. The taxonomically important features of the kinetosomal region of the zoospore indicate that the chytrid is a member of the Chytridales, but differs from other chytrids studied with transmission electron microscopy. Its microtubule root, which begins at kinetosome triplets 9-1 and extends parallel to the kinetosome into the aggregation of ribosomes is distinctive. Histologic examination of test frogs revealed that the pure culture infected the skin of test frogs, whereas the skin of control frogs remained free of infection. The fungus is described as Batrochochytium dendrobatidis gen. et sp. nov.

Interpretation of the Key Details

In the section that follows I have taken the taxonomic definition of the new genus and species, and tried to express it in non-specialist language. I apologise if chytrid taxonomists find this somewhat inadequate.

The elements of the formal taxonomic description from Longcore et al are in bold with the "interpretation" non-bolded.

Some of the features discussed below can be seen in the slide series on chytridiomycosis in Australia. In addition I have placed links to some of the relavant slides. These will open in separate windows and can be closed without losing this file.

RS

From Longcore et al (1999) p.220:

The type species is isolate L-197 from a blue poison dart frog, Dendrobates azureus.

Generic description for Batrachochytrium is:

  • Thalli monocentric or colonial.
    Means the spherical fungi can exist as a single unit or as several organisms contained within the one "shell".
  • One to several rhizoidal axes; rhizoids are thread-like.
    Rhizoids are root-like fine fibres that grow from the "shell" of the fungus. In this species the rhizoides are fine thread-like structures and they can grow from one or many points on the "shell".
  • Each segment of colonial thallus forms a sporangium.
    In colonial forms each separate organism in the colony grows into a sporangium.
  • Zoosporangia with one of more inoperculate discharge papillae;
    The zooporangium is the structure in which zoospores develop. They are roughly sperical with one or more small projections. These are the discharge papillae. The top of each discharge papilla has a "plug" which lyses to release the zoospores contained in the zoosporangium. "Inoperculate" means there is not a definite cap at the top of the discharge papilla, just a plug. m
  • swimming zoospores spherical or slightly ovate.
    As it says - shape of zoospore when swimming is spherical or slightly oval. Zoopores have a posterior flagella.
  • Ribosomes aggregated.
    Ultrastructural feature (i.e., detected using a transmission electron microscope): In the zoopspores the ribosomes are placed together into a large group.
  • Numerous lipid gloules; associated with sheet of microbody and nested in periphery of ribosomal mass.
    Ultrastructural feature: The zoospore has many lipid globules localised to specific areas.
  • Kinetosome root consisting of a group of microtubules; arising near triplet 9-1 and extending parallel to kinetosome into ribosomal core.
    Ultrastructural feature: Now dealing with very fine details at the posterior end of zoopore near where the flagella arises. These features are so specialised that they are difficult to describe in simple language.
  • Kinetosome attached to non-flagellated centriole with overlapping fibers.
    Ultrastructural feature - same region as for previous point: See previous comment.
  • Lacking rumposome and transition zone plug.
    Ultrastructural feature - same region as for previous 2 points: See penultimate comment.
  • Habit. Forming monocentric and colonial sporangia in the epidermis of amphibians.
    "Habit" means how and where the organism lives.

    • Species definition of Batrachochytrium dendrobatidis Longcore, Pessier et Nichols sp. nov.

    Description as for genus. Sporangia on nutrient agar up to 40 micrometre in diameter with one-several discharge papillae. Flagellum (of zoospore) 19-20 micrometre long.


    Comment

    This is the formal description of the chytrid fungus reported in Pessier et al (1999). The Mycologia paper contains host records for 4 species of captive amphibians, Dendrobates azureus, D. auratus, Litoria caerulea, and Dyscophus guineti. The last record is new to the literature.

    Batrachochytrium dendrobatidis appears to be similar to the chytrid described by Berger et al (1998) in amphibians from Australia and Panama. Both groups had decided independently that the chytrid was a new agent and pathogenic to amphibians. Berger et al (1998) had data from both wild and captive amphibians in Australia and from wild amphibians in Panama. Pessier et al (1998) had data from captive amphibians in USA.

    Don Nichols was the first to recognise the chytrid as a new agent in 1990 and subsequently worked with Joyce Longcore to characterise it. In Australia the agent was detected in 1993 (see history in Australia), and recognised as a significant pathogen in 1997. Both groups started collaborating in 1998 and attempted to publish their papers together, but this was not permitted.

    The chytrid was possibly described as Basidiobolus ranarum by Graf et al (1989) in dwarf African frogs sold in the aquarium trade in USA, but the true identity of the agent described in this report has yet to be confirmed. If Graf's Basidiobolus ranarum is B. dendrobatidis, the epidemiology of the chytrid in USA will be very difficult to interpret since it may have been distributed to many sites on infected dwarf African frogs.

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    Updated 2 May 1999
    Rick Speare