Chytridiomycetes

Chytridiomycetes are water- or soil-dwelling fungi often referred to as chytrids. These fungi are relatively poorly known and understudied, in part due to their microscopic size and minimal effects on humans. There are currently over 1,000 identified species of Chytridiomycetes. Many are parasitic on algae, water molds, and plants. A few parasitize animal eggs and protozoa. They lack true mycelium and live entirely within host cells. When they mature, the vegetative body is transformed into one or many thick-walled resting spores. Usually chitin is the major constituent of chytrid cell wall. The resting spores germinate to produce one or many zoospores which have flagella and are motile. Chytridiomycetes are the only members of the kingdom Fungi that produce motile cells. For this reason they are thought to have derived from a protozoan ancestor having similar flagellation.1,4

Growth and Reproduction

In the asexual life cycle, a chytridiomycete zoospore settles on a solid surface and forms a special branching system of hyphae called rhizoids. Rhizoids anchor the fungus to a land environment and aid in absorption of nutrients. Sexual reptoduction in the Chytridiomycetes involves male and female gametes that unite and form a sexual spore called oospore.2


An alpine tree frog (Litoria verreauxii alpina) with severe chytridiomycosis, showing skin reddening and an inability to maintain normal upright posture.
Source: CDC

The class Chytridiomycetes contains three groups of fungi that cause diseases in plants:

  • Members of Olpidium infect the roots of plants.
  • Members of Synchitrium cause black wart of potato.
  • Physoderma species cause the crown wart of alfalfa ad the brown spot disease of corn.

Pathogenic Chytridiomycetes

These days frogs and other amphibians are falling victim to a global epidemic of fungal diseases. The culprit is a pathogenic cytrid Batrachochytrium dendrobatidis. Many species are already feared extinct because of the deadly infection, others may face this fate soon. The fungus attacks keratin, a vital component of the skin of frogs. In frogs, keratin forms a protective layer in their soft, permeable skin. Once infected, the animals become susceptible to UV, pollution, and parasites, but it appears that the fungus or its toxin is capable to kill a frog as a sole infectious agent.5 Frogs with chytridiomycosis display lethargy, skin sloughing, abnormal posture, and, in severe cases, skin ulceration. For susceptible frog species, chytridiomycosis results in death in typically less than 1 month. Chytridiomycosis can infect many species of frogs and even salamanders.6

References

  1. Plant pathology. George N. Agrios
  2. Schaum's Outline of Microbiology. I. Edward Alcamo, Jennifer M. Warner
  3. Westcott's Plant Disease Handbook. Cynthia Westcott
  4. Textbook of Environmental Microbiology. Mohapatra
  5. The smaller majority: the hidden world of the animals that dominate the tropics. Piotr Naskrecki
  6. Molecular principles of fungal pathogenesis. Joseph Heitman

 

 

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