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In this article we will discuss about the characters and classification of chytridiales.
General Characters of Chytridiales:
This order includes 65 genera and 300 species. They are the most primitive and simplest members of true fungi. They are often called the chytrids. The chytrids include both aquatic and soil inhabiting species. The latter are restricted to the wet soil. A few species are marine. Many chytrids are parasitic and others saprophytic. The fresh water species parasitize algae (Fig. 4.2A) or water moulds.
The common examples of parasitic species which attack vascular seed plants of economic value are Synchytrium endobioticum, Physoderma zeaemaydis and Urophylyctis alfalfae. Some of the parasitic species are endobiotic and holocarpic (Synchytrium). During the early stages of their development they often lack a cell wall.
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They live within the host cell and in some of them reproductive structures are produced within the host cell. The more advanced forms are epibiotic and eucarpic. In these, the reproductive structures are developed outside the host.
The rhizomycelium is embedded in the host tissue. The endobiotic species are considered primitive. The thallus in all the chytrids is a single cell. The cell wall consists of fungal chitin or both fungal chitin and cellulose. In the eucarpic species, the thallus develops a system of rhizoid- like fine hyphal branches constituting rhizomycelium (Fig. 4.2 B). It is an intergral part of the thallus.
In some species, it bears a single reproductive structure (sporangium or resting spore) and in others more than one. The thallus in the former is called monocentric (Fig. 4.2 B) and in the latter polycentric (Fig. 4.2 C). The thallus of monocentric species superficially resembles Botrydium, an alga. Sometimes hypha-like branches develop on the thallus.
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Asexual reproduction takes place by means of posteriorly uniflagellate zoospores produced in globose, cylindrical, flask-shaped or irregularly lobed sporangia. The single flagellum is of whiplash type. On finding a suitable substratum, the zoospore retracts its flagellum and germinates by putting out a small peg-like germtube.
The germination is thus monopolar. Sexual reproduction in the primitive chytrids is isogamous. The isogametes morphologically resemble the zoospores. More advanced members of the order exhibit a tendency towards heterogamy.
Classiflcation of Chytridiales:
Brown (1856) introduced operculation as a criterion for classification of the chytridiales. However, it was Sparrow (1935, 1973) who on the basis of the liberation of zoospores from the sporangium (operculation) established the concept of two groups, one operculatae and the other inoperculatae in the chytrids.
In the former zoospores escape through an opening formed by the separation of a minute, circular cap-like lid or operculum at the end of a discharge tube.
The zoospores in the inoperculate species escape through a pore in the wall of the sporangium or a discharge tube. The operculate species are considered more advanced. On this basis, the order Chytridiales has been divided into nine families. These are: Olpidiaceae, Achlyogetonaceae, Synchytriaceae, Phlyctidiaceae, Rhizidiaceae, Physodermataceae Cladochytriaceae, Chytridiaceae and Megachytriaceae.
Barr (1980) proposed reclassification of the Chytridiales based on fundamental differences in the ultrastructure of zoospores. He splits it into two orders Chytridiales (amended) and a new order Shizellomycetales. Both the orders are divided into nine families on the basis of thallus development.
These are: Olpidiaceae, Achlyogetonaceae, Synchytriaceae, Phlyctidiaceae, Rhizidiaceae, Cladochytriaceae, Physodermaceae, Chytridiaceae and Megachytriaceae. Genera in the families are recognized on zoospore ultrastructure. The importance of operculation has been deemphasized at the species level of classification. Of the families included in the order Chytridiales (amended), Synchytriaceae is described here.
