In this article we will discuss about the meaning and classification of fungi.
Meaning of Fungi:
The organisms of this group have eukaryotic, non-chlorophylloUs cells having an absorptive mode of food-intake and a chemoorganotrophic mode of nutrition. Cells are generally invested within a cell wall composed of glucan, mannan or chitin. Some lower forms have also cellulose. Slime molds are without a cell wall and some of them have a multinucleate mass of protoplasm.
Fungi may be unicellular or multicellular. The multicellular fungal body is generally made of thin thread-like hyphae having a diameter of 5 to 10 μm. Hyphae may be septate or aseptate. A septum is usually provided with a pore connecting adjacent cells.
Through the pore, a continuity of the cytoplasm of the cells is maintained. Each cell of a septate hypha contains one (monocaryotic), or two nuclei (dicaryotic). Aseptate hyphae are multinucleate (coenocytic). The hyphal complex constitutes a mycelium.
Fungi occur abundantly in soil and aquatic environments, both fresh-water and marine, where they perform an important role in the natural process of mineralization. Many fungi can elaborate enzymes which are capable of degrading complex organic substances, like cellulose, hemicellulose, lignin and a host of other polysaccharides.
A great number of vascular plants, both dicots and monocots, enter into mycorrhizal association with fungi. Such association proves beneficial to the plants. At the same time, most of the major plant diseases are caused by fungi, some of them having devastating effects, particularly when they affect food crops, like potato and wheat.
Fungi can serve as source of many useful compounds. The first wonder drug, penicillin, was extracted from the culture filtrate of Penicillium notatum and later from P. chrysogenum. Several other clinical antibiotics—like cephalosporin’s and griseofulvin—have been isolated from species of fungi.
Fungi have been exploited for large-scale manufacture of several organic acids, like citric acid, itaconic acid, gallic acid etc. Several fungi, like Rhizopus nigricans, are employed in transformation of steroius. A plant pathogen, Claviceps purpurea, infecting the cereal, rye, serves as the source of a useful alkaloid drug, ergometrine. Some species of Taxomyces have been found to produce an anti-cancer drug, taxol.
Several fungi, like Trichoderma viridae, Verticillium leconii, Colletotrichum gloeosporoides, have been used for biological control of pests and diseases of plants. Candida oleophila is used for preventing fungal deterioration of harvested fruits. Species of Entomophaga can kill gypsy moth which destroys trees by feeding on their leaves. Some fungi can kill plant pathogenic nematodes by trapping and digesting them.
The unicellular fungi, yeasts, comprising many genera and species are of great commercial importance. They are used for manufacturing alcoholic beverages, like beer, wine, as also industrial alcohol. Yeasts also serve as source of several vitamins and amino acids.
Like bacteria, fungi have been used for basic research in genetics, biochemistry and molecular biology. Genetical research using Neurospora crassa led to the concept of one gene-one enzyme. Yeast has become a model organism for molecular biological research on eukaryotic cells. They have been used for c g mammalian genes coding for clinically used proteins.
Fungi also serve as human food. Mushrooms, morels and truffles are highly prized as cooked food for human consumption. Some species, like Agaricus bisporus are cultivated in large-scale. Torulopsis has been used as a source of single-cell protein.
Most fungi, except the obligate parasites, are amenable to artificial culture. Many of them can grow in simple inorganic salts media containing a single organic compound which serves as source of carbon and energy. Fungi generally prefer a slightly acidic pH (pH 5.0 to 6.0). Majority of them are aerobic and some, like yeasts, are facultative anaerobes. Some fungi, specially the human pathogens, exhibit dimorphism i.e. they grow either as yeast-like unicellular form or as mycelium-forming fungus.
Classification of Fungi:
Classically, the fungi were classified as a group in the plant kingdom. They have, in common with plants, a cell wall, an absorptive mode of food intake from the environment and the ability of many of them to grow in simple artificial media. However, the fungi differ from plants in having no photosynthetic pigments and hence they are heterotrophic (chemoorganotrophic).
The developments following use of molecular characteristics for taxonomic purpose and electron microscopy led to the recognition of fungi as a distinct kingdom of organisms, separate from the plant kingdom. Thus, on the basis of r-RNA homology and the fine structure of mitochondria, the fungi were treated as a separate kingdom evolving from three monophyletic lines.
The most ancient line is represented by the slime molds. Another evolutionary branch is that of water molds. Both these lines have in common mitochondria with tubular cristae (tubulocristae). The third and the most advanced evolutionary line includes the so-called true fungi (eumycetes) which are zygomycetes, ascomycetes and basidiomycetes. They have platycristate mitochondria (cristae lamellar) like those of higher plants and animals.
According to the rules of international code of botanical nomenclature (ICBN), the endings of fungal taxa of different taxonomic ranks are as follows:
Generally, fungi are capable of reproducing sexually under appropriate conditions, besides reproducing by various asexual means. The sexual stage of a fungus is known as its perfect stage. But for a good number of fungi, the perfect stage is unknown.
Such fungi are designated as imperfect fungi (Fungi Imperfecti). Because fungi are classified mainly on the basis of their sexual stage, the imperfect fungi cannot be placed in any of the major fungal categories. They are, therefore, assembled in a special group — Deuteromycetes. In case the perfect stage of any member of this group is discovered, it is transferred to its rightful taxon.
An outline of a more or less recent classification of fungi based on that proposed by Alexopoulos (1979) is given below:
Gymnomycota (Slime molds)
Organisms lack a cell-wall in vegetative phase; nutrition ingestive.
Acrasiomycetes (Cellular slime molds)
Organisms are free-living myxamoebae in vegetative phase; they aggregate to build a stalked sorocarp containing spores.
Example – Dictyostelium discoideum
Myxomycetes (Acellular slime molds)
The vegetative stage is represented by a wall less mass of multinucleate protoplasm, known as Plasmodium. In the reproductive phase, the Plasmodium builds well- differentiated coloured sporangia, containing sporangiospores.
Example – Physarium polycephalum.
Mastigomycota (Flagellate lower fungi).
Water molds with motile cells.
Chytridomycetes (Commonly known as chytrids).
Organisms reproducing by asexual motile zoospores possessing a posterior whiplash-type flagellum.
Example – Allomyces macrogynus.
Asexual motile zoospores possessing a single anterior tinsel-type flagellum.
Example – Hyphochytrium catenoides.
Vegetative phase represented by a Plasmodium; asexual motile zoospores with two unequal anterior whiplash-type flagella. Organisms obligately parasitic on plants.
Example – Plasmodiophora brassicae.
Oomycetes (mainly water molds).
Motile cells having two lateral flagella of which one is anteriorly disposed tinsel type and the other is posteriorly disposed whiplash type.
Example – Saprolegnia ferax.
Amastimycota (terrestrial fungi; motile flagellate stage absent).
Zygomycetes (zygospore-forming fungi)
Sexual reproduction by gametangial copulation resulting in zygospore formation. Asexual reproduction by sporangiospores. About 600 species.
Examples – Mucor rouxii, Rhizopus stolonifer.
Ascomycetes (Ascus-forming fungi)
Organisms unicellular, or multicellular with septate hyphae. Sexual reproduction typically by multinucleate ascogonia and multinucleate antheridia. Sexual spores (ascospores) formed within ascus originating from ascogenous hyphae. Asci are usually formed in well-differentiated ascocarps. Asexual reproduction by formation of conidia. About 35,000 species. Examples – Saccharomyces cerevisiae (unicellular), Pyronema confluens, Neurospora crassa.
Basidiomycetes (basidium-forming fungi)
Organisms generally multicellular with well-developed septate hyphae. Plasmogamy occurs through hyphal fusion; sexual spores (basidiospores) are borne on basidia in specialized sporocarps, called basidiocarp. Basidia may be septate or aseptate. About 30,000 species. Examples – Agaricus campestris (mushroom), Polyporus sanguineus (bracket fungus), Puccinia graminis (rust fungus).
Deuteromycetes (Fungi Imperfecti)
Organisms generally multicellular with septate hyphae. Sexual reproduction unknown. Asexual reproduction by conidia which may be unicellular or multicellular borne on conidiophores. About 30,000 species. Examples – Alternaria tenuis, Helminthosporium sativum, Candida albicans.