Chytridiomycetes is the major class of the phylum Chytridiomycota. The member of the class Chytridiomycetes, commonly called chytrids.
Habitat of Chytridiomycetes.
They also parasitize the terrestrial higher plant (synchytrium endobioticum on a potato). A few chytrids parasitize animal eggs and protozoa, while others are saprobic on the decaying remains of plants.
Classification of Chytridiomycetes.
The analyses of Multigene phylogenetic analyses, new culture techniques, and additional collections of Chytridiomycetes have revealed greater diversity and led to increasing the number of orders in which to classify about 700 species under 90 genera.
Today there are 10 described orders of Chytridiomycetes:
We will discuss each order in separate article.
Phylogenetic classification of Chytridiomycetes.
Life Cycle of Chytridiomycetes.
As we discuss above members of class Chytridiomycetes are parasitic which means it leaves on or in the host cell.
We discuss the life cycle of chytrids with an example of well-studied species Chytriomyces hyalinus. This fungus 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 escaped zoospores encyst and germinate to form new sporangia and rhizoids (asexual thalli) or to function s sexual thalli.
This is one of the few members of Chytridiomycetes in which sexual reproduction has been well documented.
In the sexual reproduction of chytrids, the rhizoids of two thalli come into contact and fuse, and the resting spore forms the junction of the rhizoidal anastomosis.
The resulting sporangium develops a thick wall and eventually germinates by the production of zoospores, apparently after meiosis.
General Characteristics of Chytridiomycetes
1) Flagellated spore
The main characteristic feature of this class is the production of uniflagellate reproductive cells known as zoospores and planogametes.
The single flagellum is of a whiplash-type and is inserted posteriorly. The zoospore with a posteriorly inserted flagellum is called opisthocont.
The flagellum is attached to the blepharoplast within the cell. The motile cells of some species possess a nuclear cap which consists of RNA.
It shielded the nucleus at the anterior end of the cell.
2) Cell wall and Rhizoids
It has no cell wall in the earlier stages (Olpidium). In the more advanced species, the unicellular thallus is drawn out at one point into fine, branching hairs, the so-called rhizoids which aid in anchorage and intake of nutrients (Rhizophidium).
In slightly more complex members, there is a much branched rhizomycelium.
The scanty mycelium consisting of a few short filamentous hyphae is in evidence.
Some members of this class have a mycelium consisting fo typical hyphae woven into a eucarpic mycelial network (Monoblepharis).
The hyphae are coenocytic. Chitin is a main component of the hyphal cell wall. Besides chitin, there is β glucan.
3) Mode of Reproduction
As we discussed above, The asexual reproductive organ is the sporangia, each of which produces numerous tiny, uninucleate, and uniflagellate opisthocont zoospores.
The liberated zoospores swim for a time. Later each retracts its flagellum and undergoes encystment. After a short period of rest, the encysted spore germinates.
The sexual reproduction may be isogamous or anisogamous. In some it is typically oogamous.
Disease caused by Chytridiomycetes
I) Most of the Soil inhabiting Chytridiomycetes attack the underground as well as aerial part of the higher plant and cause diseases that are of great economic significance.
For example, Synchytrium endobioticum causes black wart disease of Potato, Urophlyctis alfalfa crown wort of alfalfa (Medicago), and Physoderma maydis causes brown spot disease of maize (Zea mays).
II) Many chytrids indirectly harm humans and animals. They parasitize and destroy the phytoplanktonic forms of algae that form an important link in the food chain of aquatic ecosystems.
III) Species of Batrachochytium (B. dendrobatidis), this 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 word’s amphibian species will be affected by severe population decline or extinction.