Chytrids

The one thousand species of chytrids (Phylum Chytridiomycota) are distributed worldwide.
   

Phylum Chytridiomycota

The name is derived from the Greek word "chytridion," or "little pot," which describes the structure containing unreleased spores. 

Chytrids are unique among fungi in having a flagellated stage. They produce zoospores with a single flagellum, making them capable of active movement through water. 
 

Chytrids, the primitive fungi

Because of this, early taxonomists classified them as protists. According to rRNA sequences in ribosomes, the chytrids are the most primitive of the fungal phyla.


However, they have several characteristics in common with other fungi, including cell walls made of chitin, various enzymes, and metabolic pathways. They have coenocytic thalli and usually do not form a true mycelium (having rhizoids instead). Some species are unicellular.
Both zoospores and gametes of the chytrids have a single flagellum, and therefore are mobile. 

The water mold (Allomyces sp.) is a typical example of a chytrid. Thissaprotroph is found in water or wet soil. Its thallus (body) is attached by rhizoids, and its reproductive organs are formed at the end of branches on an erect trunk. 

Sproangium of Allomyces arbuscula

The haploid thallus forms male and female gametangia (singular: gametangium). 

These release flagellated gametes that merge to form zygotes. The zygote produces a diploid thallus with two types of sporangia: thin-walled zoosporangia that release diploid zoospores resulting in diploid thalli, and thick-walled sporangia that undergo meiosis and release haploid zoospores, which then form haploid thalli.

Chytrids fulfill various roles in the ecosystem. Many are found in soils and lakes, where they act as decomposers. Some are parasites. 

A particularly important parasite is Batrachochytrium dendrobatidis, which is responsible for the amphibian disease, chytridiomycosis. It is thought to be a principle cause for the worldwide decline in amphibians. 

 

Batrachochytrium dendrobatidis

Other chytrids have mutualistic relationships with organisms; for example, sheep and cattle have anaerobic chytrids in their digestive tracts, which aid in digestion.