Fungi comprises a huge and varied group of organisms that have common characteristics with both the lower plants (algae) and lower animals but are not related to each other(1, a, b,c). They contain true mitochondria and a membrane-enclosed nucleus. They have no chlorophyll and chloroplast. Reproduction is achieved through sexual and asexual means(12). Most fungi grow as branched tubular systems, or mycelia, whose individual filaments or hyphae are surrounded by rigid cell walls containing chitin, cellulose, or both, and other polysaccharides. All fungi lack photosynthetic ability and therefore require preformed organic compounds(2).
Prior to the development of fungi in the microscope in the 1600s, the only fungi described were the higher fungi that have large fruiting structures, such as morels, mushrooms, and puffballs. In 1836, the study of fungi was termed mycology (a branch of botany). The different groups of fungi are classified according to their means of sexual reproduction, life cycle exhibited, growth and developmental stages, and means of asexual propagation(4). Two Division System: Myxomycota and Eumycota. Ainsworth constructed a two-division system for fungi—the kingdom is divided into two groups, namely Division Myxomycota and Division Eumycota.
In Division Myxomycota, the fungi grow as multinucleate amoeboid plasmodia and produce motile uninucleate amobae, as well as biflagellate cells. The so called true fungi are placed in Division Eumycota. The evolutionary relations for this division may be described as a single phyletic series, except for Oomycetes(6). Eumycota and Subdivisions. The Division Eumycota is subdivided into subdivison Mastigomycotina, Basidiomycotina, Deuteromycotina, Zygomycotina, and Ascomycotina. In subdivision Mastigomycotina, the fungi produce motile spores called zoospores, with one or two flagella.
The subdivision Basidiomycotina includes members producing haplontic sexual spores called basidiospores on a specialized cell called the basidium. The subdivision Deuteromycotina includes all fungi that lack known sexual reproductive structures. Most fungi that cannot be classified fall under this subdivision. The subdivision Zygomycotina has a single class, Zygomycetes and this class includes fungi—bread molds or pin molds—that typically produce an abundant or aerial, coenocytic mycelium and are common causes for the decay of of foods and rich sources of organic material.
The subdivision Ascomycotina (formerly the class Ascomycetes) includes all true fungi in which sexual reproduction results in ascospores, produced within specialized structures called an ascus(7). Zygomycotina. The subdivision Zygomycotina has cell walls containing chitin as a primary component. They reproduce asexually by means of non-motile spores called sporangiospores produced in the sporangia formed on branches (sporangiophores) of the mycelium. In some species, such as the Rhizopus nigrificans, the sporangia arise in clusters with rhizoids at the base and hyphal strands or stolons interconnecting the clusters.
The spores are released by the breakdown of the sporangial wall of the sporangial wall and dispersed by air /wind or water current. They germinate by direct outgrowth of hyphal tube to produce a new mycelium. Rhizoids anchor the fungi to the substrate, acting like roots, thus releasing the enzymes necessary to breakdown the substrate and absorbing the broken down nutrients (1). Sexual reproduction may occur between different parts of the mycelium (homothallic mating) or between two sterile strains/cells of opposite mating type. The latter is regulated by a single pair of genes or alleles.
One gene is said to be the plus mating type and the other the minus mating type (8). The gametangia, or the sex organs, fuse to form a dormant, thick-walled pigmented and often sculpted zygote called the zygospore. The mature zygospores eventually germinate to produce the new haploid mycelium. The representatives for this subdivision are members of the Rhizopus species like R. oligosporus and R. stolonifer(2). The genetic regulation of sexual reproduction in fungi was first discovered in Zygomycetes by Albert Blakeslee, who coined the terms homothallism and heterothallism to describe the two opposite mating types.
Both are known to be common in throughout the kingdom. Ascomycotina. The subdivision Ascomycotina have chitinous cell walls. In many ascomycetes, male structures (antheridia) and female structures (ascogonia) are produced. The antheridia donate nuclei to the ascogonia by fusion with a receptive filament, the trichogyne. In others, the same function may be accomplished by conidia (asexual spores that can also serve as fertilizing elements) or by hyphal fusion. The parental nuclei in the ascogonium unite in the ascogonium and enter the hyphal branches that grow out from it within a developing fruiting body called the ascocarp.
The paired parental nuclei divide synchronously (conjugate division) in specialized hyphae with binucleate cells (ascogenous hyphae). The tip cells of the ascogenous hyphae form a hook in which the haploid parental nuclei fuse to produce a diploid zygote nucleus. The zygote nucleus immediately undergoes meiotic division to produce four haploid nuclei in the enlarging cell, called the ascus at this type of development. In most cases the a mitotic nuclear division then doubles the number of nuclei per ascus, after which each nucleus is enclosed in a cell wall to form the ascospores.
The hypahe have simple washer-shaped septa with a central pore; and the asexual reproduction occurs by formation of non-motile spores — conidia, oidia, arthrospores and others — that are usually produced on specialized branches called conidiophores. Several classes of ascomycetes exist. The class Hemiascomycetes includes the unicellular or mycelial yeast. The class Plectomycetes includes several economically important fungi that form their asci in small, simple, closed fruiting structures (cleisthothecia).
The powdery mildews are all obligate parasites of higher plants and are largely host-specific. A second major group of plectomycetes include the commercially utilizable genera Aspergillus and Penicillium,as well as important pathogens of plants and humans. Sexual reproduction is relatively rare among species of Aspergillus (185 species) and Penicillium (14 species). Aspergillus produces chains of pigmented, asexual conidia on the surface of an inflated region of a branch called a conidiophore.
Conidium formation is similar similar in Penicillium, but the conidiopore is branched to form a brushlike structure (penicillus) instead of having an inflated vesicle. The conidia are connected by chains in conidiophores but are readily dispersed by air currents. The green, black, yellow and gray colonies of these common microfungi are the results of the color of the huge number of pigmented conidia produced on the surface. All fungi in class Pyrenomycetes produce asci and ascospore sas an organized hymenial layer in a fruiting body called perithecium.
The perithecium is a small flask-shaped structure with a thin wall that surrounds a basal tuft of asci with the opening at the top called an ostiole. The ascospores are typically discharged from the tips of the asci as they sequentially protrude from the ostiole. The perithecia may form separate structures on the mycelium, or they may lie just below the surface of a larger mass of sterile hyphae called stroma(10). In addition to their roles in the decay of plant and animal residues and in food spoilage, these fungi (h,j) are of great significance to humans in many other ways.
Aspergillus fumugatus, a common inhabitant of the heated compost, can cause respiratory disease in humans, and a number of related species may produce aflatoxin, a tumor inducing alkaloid, in poorly stored moldy grain(9. ) A. flavus and A. parasiticus produces aflatoxin, B1 (f), B2 (g), G1 and G2, the first mentioned being the most toxic (a, b,c). The disease caused by Aspergillus is termed as aspergillosis characterized by allergy and infection-like symptoms (3,4). The potential threat of Aspergillus as biological weapon of mass destruction is still being investigated (d-j).
Species of both Penicillium and Aspergillus are used extensively in commercial fermentations. Camembert cheese derives its flavor from Penicillium camemberti, and Roquefort from P. roqueforti. Soy sauce is fermented with Aspergillus oryzae or A. soyae. This class also includes other species that cause disease in humans, animals and plants (4, j); for example the fungus Ceratocystis ulmi is responsible for the Dutch Elm disease, other species cause a wilt disease in oaks and still others reduce the quality in number (3, l). Antibiotics were first fist produced using penicillin from P.
notatum; the antibiotic activity of this fungus was first described by Alexander Fleming in 1929. Only through a joint effort of British and American scientists during the World War II, however was the industrial-scale production achieved, by using a better strains of P. chrysogenum. A huge antibiotic industry has since developed. . Various microfungi are used to produced a number of organic acids—gluconic, itaconic and citric acids (d,e, m), for example—and in other chemical processes. Citric acid fermentation yields about 99,000 each year.
Penicillium’s uses do not extend to cheese and to antibiotics alone but also in agriculture—serves as soil bioinoculant. Ochratoxin is produced by P. viridicatum and P. verrucosum. Selected Bibliography I. Books 1. Ainsworth, G. C. (1976). Introduction to the History of Mycology. New York: John Wiley & Sons, In 2. Alexopoulas, C. J. Introductory Mycology. (1979). NY: Wentworth Pub, Inc. 3. Bulmer, G. C. (1979). Introduction to Medical Mycology. London: Academic Press. 4. Christensen, M. C. (1975). Molds, Mushrooms and Mycotoxins. NY: Plenum Press 5. Emmons, C. W. , Binford, J. P.
Utz, J. P. , and Kwon-Chung, K. J. , 1977. Medical Mycology. New England: UP. 6. Garraway, M. O. and Evans, R. C. , Fungal Nutrition and Physiology. 1984. London: Academic Press. 7. Gray, W. D. Alexopoulus, C. J.. Biology of Myxomycetes. 1968. NY: Wentworth Pub, Inc. 8. Grolier Encyclopedia. (1993). Grolier International Inc. 9. Raper KB, Fennell DI. , 1965 The genus Aspergillus . Baltimore, Maryland: Williams and Wilkins. 10. Webster, J. Introduction to Fungi. 1980. NY: WMC Inc. II. Journals a. Asan A, Ekmekci S. , 1994 The determination of Penicillium and Aspergillus species in Edirne
soils and their seasonal distribution Tr J Biol 18:291-303 b. El-Said AHM. , 1994 Studies on soil mycoflora of Bahreen Microbiol Res 149:263-269 c. Fresquez PR. , 1990 Fungi associated with soils collected beneath and between pinyon and juniper canopies in New Mexico Great Basin Naturalist 50:167-172 d. Ghildiyal JC. , 1993 Mycoflora of decomposing leaf litter in a subtropical freshwater swamp Proc Nat Acad Sci India 63: (B)H 207-211 e. Joffe AZ. , 1967 The mycoflora of a light soil in a citrus fertilizer trial in Israel Mycopathologia et Mycologia Applicata 32:209-230 f. Kamal Gupta ML, Kumar P.
, 1979 Aspergilli from soils of Gorakhpur 9: Edaphic factors and distribution in 4 soil types against plane cover Indian J Mycol Plant Pathol 9:56-65 gKhallil AM, Abdel-Sater MA. , 1993 Fungi from water, soil, and air polluted by industrial effluents of Manquabad superphosphate factory (Assuit, Egypt) J Basic Microbiol 31:83- 100 h. Klich M. , 1998 Soil fungi of some low-altitude desert cotton fields and ability of their extracts to inhibit Aspergillus flavus Mycopathologia 142:97-100 i. Rutherford JM, Huang LH. , 1994 A study of fungi of remote sediments in West Virginia caves
and a comparison with reported species in the literature NSS Bulletin 56:38-45 j. Steiman R, Guiraud P, Sage L, Seigle-Murandi F, Lafond J-L. , 1995 Mycoflora of soil around the Dead Sea I-Ascomycetes (including Aspergillus and Penicillium), Basidiomycetes, Zygomycetes System Appl Microbiol 18:310-317 l. Sulun Y, Hasenekoglu I. , 1993 A study on Aspergillus Mich. ex. Fr. and Penicillium Link ex. Gray flora of the soils of northeast Anatolia, Turkiye Doga-Tr J Biol 17:49-60 m. Yaguchi T, Someya A, Udagawa SI. , 1994 Fennellia flavipes and Neosartorya stiamenia, two new records from Japan Mycoscience 35:175-178
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