Class: Chytridiomycetes(Phylum: Chytridiomycota)

• Members of the division have unicellular rudimentary to mycelial thalli.

• Their cell wall composition is mostly chitin. • Flagellated spores and gametes are produced. Gametes and

zoospores have a single, posterior whiplash flagellum.• Sexual reproduction is variable and may be isogamous,

anisogamous or oogamous.• The ultrastructure of zoospore is a definitive characteristic of

Chytridiomycota. *The division has a single class, Chytridiomycetes, and three

orders : Chytridiales , Blastocadiales, and Monoblepharidales

Distinguishing characteristics of chytridiomycetes

Most closely related to ancestral fungi. Only fungi with flagellated spores called zoospores. Have chitin in their cell walls. Life cycle has both haploid and diploid multicellular stages. Motile cells have a single, posterior flagellum. Reproduce sexually and asexually. They are parasites or decomposers. Chytridiomycetes are the most primitive fungi alive today. Produce flagellate cells at some stage in their life cycle. No other fungal phylum are flagellate.

The thallusThe thallus may consist of a simple spherical structure, which may be holocarpic or eucarpic. From the vegetative part, fine thread-like structures called rhizoids arise which serve to anchor the unicellular thallus to the substratum and also absorb food. Some spores produce a network of rhizoids called rhizomycelium. The rhizomycelium in such fungi functions as the somatic part which bears one or more reproductive structures. •Monocentric thallus: If the thallus consists of rhizomycelium and a single reproductive structure, the thallus is called monocentric. •Polycentric thallus: When there are many reproductive structures with a rhizomycelium, the thallus is called polycentric.

The thallus may be-•Endobiotic: growing inside the host cell, •Epibiotic: growing attached externally, but sending rhizoids inside host cells,•Interbiotic: attached to many hosts through the rhizomycelium.

The thallus

Zoospores

• Initially zoospores encyst – withdraw or lose flagellum, rapidly form a cell wall

• Cyst then germinates to form rhizoids and enlarges

The thallus

Asexual reproduction• initiates with zoosporangium filled with protoplasm and

many nuclei• protoplasm of zoosporangium is then cleaved into

numerous minute section which develop into zoospores• zoospores are released, swim, encyst, then germinate

producing a thallus• operculate zoosporangia always form a well-defined

circular cap, operculum, through which the zoospores emerge

• inoperculate zoosporangia discharge their zoospores through a pore in the wall of the sporangium or discharge tube, formed when the discharge papilla dissolves

• most known species possess inoperculate zoosporangia

Sexual reproduction Modes of sexual reproduction•Planogametic copulation (three forms)

– <isogamous– anisogamous– nonmotile egg fert. by a motile antherozoid

•Gametangial copulation– tranfer of protoplast from one gametangium to

another•Somatogamy

– fusion between rhizoidal filaments

Economic importance• The Chytridiomycetes include many important obligate

parasites in the order Chytridiales, e.g., Synchytrium, Physoderma and Urophlyctis, which cause serious plant diseases. Aquatic chytrids (a common name for fungi belonging to order Chytridiales) cause frequent epidemics of algae (both fresh water and marine) and by this disturb the habitat and food chain.

• Members of some families in the order Blastocladiales include many endoparasites of arthropods.

• The genera Blastocladiella and Allomyces have been used extensively in morphogenic studies.

Order: Chytridiales• The fungi included in this order are usually

referred to as chytrids. They live as parasites or saprophytes in water and soil.

Family: OlpidiaceaeThallus: endobiotic and holocarpicSporangium: inoperculateSexual reproduction: planogametic copulation (isogamy)Zygote: biflagellate diploid, penetrates the host.

Genus: Olpidium• About 30 species of Olpidium are known which

occur in water or wet soils as parasites of algae and roots of some higher plants. An important root parasites of vascular plants is O. brassicae, causing damping off of crucifer seedlings and other unrelated hosts.

• O. brassicae serves as a vector of at least three viruses in soil, such as, lettuce ‘big vein’ virus (BVV), tobacco necrosis virus (TNV) and tobacco stunt virus (TSV). The viruses are disseminated by zoospores and also survive in the resting spores for several months. Whether the viruses multiply also in the zoospores is not yet known.