In this article we will discuss about the origin and classification of class phycomycetes.
Origin of Class Phycomycetes:
The Class Phycomycetes is also termed as ‘algal fungi’, because it resembles certain green algae as regards the structure of their filamentous vegetative body and their modes of reproduction. It includes such common fungi as bread mould (Rhizopus), water mould (Saprolegnia), ‘white rust’ of mustards (Albugo), and the ‘downy mildews’ (Peronospora).
The lower forms are mostly aquatic and may live either as parasites on plants and animals, or as saprophytes) on submerged organic materials. A great majority of them are parasites on terrestrial phanerogamic plants or they live as saprophytes on various decomposed organic materials.
Others are amphibious, occurring as parasites on living organisms, or as saprophytes on damp soil. The vegetative body, when present, is a much-branched coenocytic mycelium but its older portions may sometimes become septate.
Asexual reproductive spores, both zoospores and sporangiospores, are produced within unicellular sacs, known as sporangia, which are cut off from the tips of hyphae. Some of them may produce conidia and gonidiospores. Sexual reproduction may be isogamous, anisogamous or oogamous, and the plants may be homothallic or heterothallic. When an entire thallus is capable of producing gametes, it is said to be holocarpic.
But when the gamete-production is restricted to the gametangia only, it is referred to as a eucarpic thallus. Besides, among some higher forms union between gametangia takes place whereby their undifferentiated, multinucleate protoplasts unite to form a zygote. The zygote produced as a result of sexual union, forms a thick wall and passes through a period of rest prior to germination.
Classification of Class Phycomycetes:
The class Phycomycetes has been divided into three subclasses:
Subclass I. Archimycetes:
Characterized in having rounded or lobed non-mycelial thallus formed by the enlargement of zoospores, sometimes possessing rhizoids; higher forms may be develop a rudimentary mycelium.
It includes three orders:
Order 1. Chytridiales:
Thallus is variable in form, mostly holocarpic, producing one or more sporangia or gametangia; zoospores uniflagellate or biflagellate, zoogametes uniflagellate. Common genera are Synchritium, Olpidium, etc.
Order 2. Ancylistales.
Order 3. Protomycetales.
Subclass II. Oomycetes:
Characterized in having well-developed coenocytic mycetium, often become septate; characteristic mode of sexual reproduction is oogamy and the zygote always an oospore; accessory spores are mostly motile zoospores formed in most of them.
It includes five orders:
Order 1. Blastocladiales.
Order 2. Monoblepharidales:
Aquatic saprophytes; cytoplasm foamy; zoospores uniflagellate; resting spores bear warts- union of a motile male gamete with an oosphere contained in an oogonium.
Order 3. Leptomitales.
Order 4. Saprolegniales:
Mostly aquatic sometimes terrestrial, parasitic on animals and plants; mycelium aseptate and branched, hyphae net constricted at intervals; sporangia at the tips of hyphae; zoospores biflagellate, usually exhibit diplanatisue; sexual reproduction oogamous, oogonium contains one to several oospheres without periplasm. Common genera are Saprolegnia, Achlya, etc.
Order 5. Peronosporales:
Mostly parasitic on land plants, some being saprophytic; biflagellate zoospores are liberated from zoosporangia, which often function as conidia depending on external conditions; sexual reproduction is oogamous; oogonium contain solitary oosphere with well-defined periplasm. Common genera are Peronospora, Pythium, Phytophthora, Albugo etc.
Subclass III. Zygomycetes:
Characterized in having a well-developed mycelium, often becoming septate; characteristic mode of sexual reproduction is by conjugation of two gametangia, where undifferentiated contents fuse to form a zygote known as zygospore; accessory spores are non-motile aplanospores, known as sporangiospores, formed within an unicellular sac cabled sporangium.
In includes two orders:
Order 1. Mucorales:
Mostly saprophytes on various substrata, rarely parasitic; mycelium coenocytic, at first aseptate and very regular, but irregular and septate with age; sporangiospores formed inside globose or ovoid sporangia; sexual reproduction by union of multinucleate, morphologically alike, gametangia, mostly heterothallic, may be homothallic also. Common genera are Mucor, Rhizopus, Pilobolus, etc.
Order 2. Entomophthorales.
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