Chytridiomycetes

 Chytridiomycetes are water-inhabiting fungi, often parasitic on algae and oomycetes, or soil inhabitants, some of which are parasitic on vascular plants. 

A few chytrids parasitize animal eggs and protozoa, while others are saprobic on the decaying remains of plants. Multigene phylogenetic analyses, new culture techniques, and additional collections of Chytridiomycetes have revealed greater diversity and led to increased numbers of orders in which to classify about 700 species in under 90 genera. 

Today there are 10 described orders of Chytridiomycetes: Chytridiales, Spizellomycetales, Cladochytriales, Rhizophydiales, Polychytriales, Rhizophlyctidales, Lobulomycetales, Synchytriales, Gromochytriales, and Mesochytriales. 

An exemplar life cycle is that of Chytriomyces hyalinus, which forms a well-developed rhizoidal system within its substrate. The sporangium that develops from the encysted zoospore has a saucer-shaped operculum from which zoospores escape into a fibrous vesicle of overlapping filaments where the cells complete their maturation and then escape. The zoospores encyst and germinate to form new sporangia and rhizoids (asexual thalli) or to function as sexual thalli.

This is one of the few members of the Chytridiomycetes in which sexual reproduction has been well documented. The rhizoids of the two thalli come into contact and fuse, and a resting spore forms at the junction of the rhizoidal anastomosis. 

The resting sporangium develops a thick wall and eventually germinates by the production of zoospores, apparently after meiosis.  

Other well-studied members of Chytridiomycetes include Synchytrium endobioticum, a pathogen that causes potato wart disease, and Nowakowskiella, and operculate, a polycentric genus of aquatic saprobes of decaying plant materials. 

Species of Nowakowskiella are often isolated from pond water by using cellulosic baits. The most widely studied chytrid is Batrachochytrium dendrobatidis, the fungus associated with amphibian declines. 

The disease was first detected in Australia and Panama, but it now has been found on all continents except Antarctica. 

Estimates predict that 30% of the world’s amphibian species will be affected by severe population decline or extinction. Zoospores attack the keratinized parts of the amphibian, such as larval mouthparts and the first two layers of adult skin, to cause the infection. 

The flagella are resorbed, and the B. dendrobatidis cells enlarge within the infected host cells. Growth of the somatic cells and their conversion to zoosporangia cause hyperplasia and hyperkeratosis that are symptomatic of the disease. 

Severe infections reduce the efficiency of cutaneous respiration and osmoregulation. Evolutionary analyses of the B. dendrobatidis genome revealed that it has evolved from saprobic ancestors and that its unique ecology of being a vertebrate pathogen is correlated with the lineage-specific expansion of multiple gene families of proteases.

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