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UPDATE vk_doks SET out_text = 'Kharkov National Academy of Municipal Economy Department of Urban Economy GENERAL BIOLOGY for the students of bilingual group, studying on speciality “Environmental engineering” Part 4 Kingdom Fungi Author: A. G. Shatrovskiy Kharkov-2006 Objectives After studying this chapter, the reader should be able to: Describe the major features of the kingdom Fungi and its three subkingdoms. List distinguishing characteristics for each major group of fungi. Trace the life cycles of representative fungi. Describe the ecologic and economic importance of fungi. Discuss the mutualistic relationships formed by lichens and mycorrhizae. I. Fungal Structure, Nutrition, and Ecology A. General information Fungi historically have been classified with plants, but major differences between these organisms necessitated the formation of a separate kingdom Fungi are eukaryotic, usually multicellular, multinucleate organisms They obtain nutrients by secreting enzymes into their substrate and absorbing the digested materials Most fungal cell walls contain the polysaccharide chitin The fungal structure is characterized by long slender filaments called hyphae (see Types of Fungal Hyphae for an illustration)  Coenocytic hyphae consist of long cellular strands with many nuclei, which are not contained within individual cells Hyphae with complete septa (barriers) are separated into individual cells; septate hyphae may contain a single nucleus in each cell or two nuclei in each cell (called dikaryotic hyphae) Hyphae with incomplete septa allow cytoplasm to flow freely from cell to cell Some parasitic fungi have haustoria, which are specialized hyphae that extend into individual host cells The mycelium is the mass of hyphae that constitutes the body of a fungus; it is the most recognizable structure of a mushroom (toadstool) B. Modes of nutrition and metabolism Fungi (along with bacteria) are the principal decomposers in every ecosystem They can break down lignin, a major component of wood Some fungi also attack living organic matter, causing agricultural damage and destroying food stores; food contaminated by fungi may be unpalatable or poisonous to humans Warm, moist, dark conditions are most favorable for fungal growth; however, fungi can grow in various other habitats If conditions become too stressful, fungi survive by producing spores, which are resistant to temperature and moisture extremes Parasitic fungi use haustoria to penetrate individual host cells and absorb nutrients directly from the cytoplasm C. Ecology Fungi typically form symbiotic relationships with plants or algae Two types of fungal symbionts are lichens and mycorrhizae Lichens are a combination of fungi (Ascomycetes or Basidiomycetes) and green algae (phylum Chlorophyta) or cyanobacteria (kingdom Monera) The algae portion of the lichen provides food through photosynthesis; the fungal portion acts as a living sponge to improve water retention Some evidence suggests that the fungal partner parasitizes the algae in a controlled fashion and sometimes may even destroy algal cells The fungal component of lichens rarely grows independently, although the algal component may do so Lichens inhabit cold, dry, and generally harsh environments, where they help break rock surfaces and prepare the habitat for other organisms They survive harsh or adverse conditions by becoming dehydrated, which subsequently slows their metabolism When the lichen's water content drops dramatically, the upper portion of the thallus becomes opaque enough to exclude light from the photo-synthetic algae Dehydrated lichen are unaffected by most environmental extremes because they are temporarily dormant and do not engage in photosynthesis Lichens absorb nutrients from rain and air; for this reason, they are very sensitive to air quality and are among the first organisms to perish in a polluted environment Lichens are grouped into three major growth forms Crustose lichens attach to or embed in their substrate and often form brightly colored, crusty patches on bare rocks and tree bark Foliose lichens have leaflike thalli, are weakly attached to their substrate, and have edges that are crinkly or divided into lobes Fruticose lichens resemble miniature upright shrubs or may hang from tree branches; their thalli are usually branched and cylindrical Mycorrhizae are a combination of fungi and plant roots Mycorrhizae enhance the absorption of essential nutrients by plant roots They also may provide protection against the effects of acidic soil and may make a plant more resistant to drought, cold, and harsh conditions They can prevent the accumulation of toxic metals in plants They can help plants to grow better in poor soils They can speed the seed germination of orchids About 90% of all plants have a symbiotic relationship with mycorrhizae Plants with mycorrhizae develop fewer root hairs than those without mycorrhizae; the mycorrhizae perform the same functions as root hairs, making them less necessary Mycorrhizae are highly susceptible to acid rain, which may negatively affect the growth of some plants The two forms of mycorrhizae are endomycorrhizae and ectomycorrhizae Endomycorrhizae, the more common variety, are characteristic of many crop species and develop when the fungal hyphae penetrate the outer root cells Ectomycorrhizae, the less common variety, are characteristic of shrubs and trees and develop when the hyphae surround, rather than penetrate, the root cells II. Reproduction A. General information Fungi reproduce both asexually and sexually by means of spores Fungi also can reproduce asexually by means of binary fission, budding, or fragmentation B. Spore production Spores are unicellular reproductive cells capable of developing into adult organisms without fusing with another cell Spores enable fungi to colonize in new areas Because they typically are resistant to temperature and moisture extremes, spores can survive harsh environmental conditions Spores are produced by cell division during asexual or sexual reproduction Asexual spores are produced directly from the cells of the hyphae Sexual spores are produced in a saclike structure called the ascus Sporangiospores are produced in specialized hyphae called sporangiophores (spore-bearing hyphae), which contain specialized structures for spore formation (sporangia) Some fungal species (Ascomycetes and Basidiomycetes) produce spores in a specialized reproductive structure called a fruiting body C. Asexual reproduction Fungi reproduce asexually by binary fission, budding, or fragmentation In binary fission, individual fungal cells divide to form two identical daughter cells, which in turn grow into new individual organisms In budding, which is usually restricted to single-cell fungi (such as yeast), a new organism is formed from a small, pinched-off portion of a mature cell In fragmentation, strands of hyphae that are mechanically separated from the original mycelium grow and develop independently into new organisms D. Sexual reproduction Not all fungi reproduce sexually Fungi do not have distinguishable male and female sexes; they have mating types, usually designated as "plus" and "minus" strains Gametes are produced by specialized hyphae called gametangia During fertilization, the two haploid gametes fuse to form a diploid zygote After the zygote forms, the nucleus undergoes meiosis to produce new (now haploid) spores, which disperse, settle into favorable habitats, and grow to produce new organisms The largest portion of the fungal life cycle is haploid; only the zygote is diploid III. Taxonomic Classification A. General information Fungi are generally classified according to the characteristics of their spores and fruiting body The kingdom Fungi is divided into three subkingdoms: Mastigomycotineae, Eumycotineae, and Myxomycotineae Authorities do not agree on fungal classification; Myxomycotineae are sometimes considered part of the kingdom Protista B. Subkingdom Mastigomycotineae (water molds) Water molds share several features with brown algae, from which they are thought to have evolved Their cell walls are composed of cellulose Their bodies vary from unicellular to highly branched coenocytic and filamentous forms Their spores are flagellated and require free-standing water for swimming Many water molds are aquatic, commonly growing on dead insects and plant debris in water Mastigomycotineae reproduce sexually, which results in the formation of spores that are dispersed to form new mycelia Examples include Saprolegnia, a parasite of fish that grows on cuts and bruises and is a common aquarium pest, and Phytophthora infestans, the species responsible for the potato blight that devastated Ireland in the mid-1800s C. Subkingdom Eumycotineae (true fungi) True fungi are filamentous organisms that have a cell wall containing chitin but do not possess motile cells The four major phyla in this subkingdom are Zygomyocota, Ascomycota, Basidiomycota, and Deuteromycota Zygomyocota (black bread molds) is a common fungi that grows on bread and other baked goods and is thus responsible for their spoilage When spores land on a suitable substrate, they germinate and produce extensive coenocytic mycelia Once the mycelia is established, asexual reproduction begins in the sporangiophores, which produce black spores As the walls of the sporangia break down, the spores are released and carried away by air currents to germinate and establish the cycle again Sexual reproduction occurs when the gametangia of two different mating strains fuse to form a zygospore The zygospores may remain dormant for months, but eventually undergo meiosis and germination to produce spore-bearing hyphae The spores produced in these hyphae are released to start the cycle again Ascomycota (sac fungi) are the largest class of sexually reproducing fungi and include yeast, powdery mildews, molds, morels, and truffles This class is named for the ascus, a reproductive structure in which spores are produced The hyphae of sac fungi have incomplete septa dividing adjacent cells Some ascomycetes cause plant diseases, including Dutch elm disease, chestnut blight, and ergot Asexual reproduction is accomplished by means of spore production, budding, or binary fission Some ascomycetes produce long chains of asexual spores (conidia) in specialized hyphae called conidiophores Other ascomycetes (such as yeasts) reproduce asexually by binary fission and budding Sexual reproduction occurs when the haploid hyphae of two different mating strains grow together The fusion of cells at the tip of mating haploid hyphae produces new, di-karyotic hyphae whose cells each contain two nuclei (one nucleus from each mating strain); the fusion of the cytoplasm of the hyphal cells is called plasmogamy When dikaryotic hyphae mature, the nuclei in some of the cells fuse to form an ascus; the fusion of the nuclei within the dikaryotic hyphae is called karyogamy Numerous asci form a layer of cells, or ascocarp, within a fruiting body Within individual asci, the diploid nuclei undergo meiosis and subsequent mitosis to form a total of eight haploid ascospores (spores developed from an ascus) The ascus ruptures to release the ascospores, which are dispersed to germinate and establish new hyphae Basidiomycota (club fungi) include mushrooms, puffballs, bracket fungi, rusts, and smuts Asexual reproduction is not common among basidiomycetes; when it occurs, it is usually by means of spore production Sexual reproduction begins with the formation of spore-producing structures, called basidia, within the fruiting body The spores (basidiospores) germinate to produce the primary mycelium, a haploid structure that contains mononucleate cells (those having only one nucleus) Two compatible mating strains of primary mycelia grow together and fuse to form a secondary mycelium, which contains dikaryotic cells (those possessing two haploid nuclei—one from each mating strain) The secondary mycelium grows to form a dense, solid mass called a button or young basidiocarp The burton expands to form the mature basidiocarp, which is the readily recognizable structure of a mushroom The umbrella-like basidiocarp consists of thin gill-like structures that house the basidia As each basidium matures, karyogamy (nuclear fusion) takes place and is immediately followed by meiosis, a process that produces the haploid basidiospores The basidiospores are dispersed to germinate, form a primary mycelium, and complete the cycle Deuteromycota (imperfect fungi) are so named because their life cycle is imperfect, that is, asexual As far as scientists know, imperfect fungi reproduce only asexually, usually by means of spore production (conidia) Deuteromycetes include Penicillium notatum (from which the antibiotic penicillin is extracted), Penicillium roquefortii (which gives flavor to Roquefort cheese), and Aspergillus tamarrii (which is used to produce soy sauce); other members of this phyla cause athlete's foot and ringworm D. Subkingdom Myxomycotineae (slime molds) Slime molds bear little resemblance to other members of the kingdom Fungi During part of their life cycle, slime molds lack cell walls and consist of a multinucleate mass of cytoplasm called a plasmodium Plasmodia often appear white, but can be blue, orange, yellow, or black They flow or creep along substrates much like the protozoan amoeba Plasmodia contain many (sometimes several thousand) diploid nuclei Under stressful environmental conditions (drought or starvation), plasmodia are converted into a stationary structure from which sporangia begin to form Sporangia, tiny spheres held above the substrate on small stalks, produce spores Meiosis occurs in the individual spores, forming four nuclei, three of which disintegrate; the remaining haploid spore is released and later germinates The germination of the haploid spore produces an amoeba-like cell known as a myxamoeba or a flagellated cell called a swarm cell After feeding on bacteria and other small organisms, the myxamoebae or swarm cells fuse to form a diploid zygote, from which a new plasmodium develops, thereby completing the cycle IV. Importance A. General information Naturally occurring fungi are important in the decomposition of dead organisms and also can cause disease Cultivated fungi are used as food and as sources of antibiotics and other valuable substances B. Edible varieties Edible fungi include truffles and morels (sac fungi), Agaricus bisporus or cultivated mushrooms (club fungi), and shiitake mushrooms from China and Japan (also club fungi) The sac fungus yeast (Saccharomyces cerevisiae) is used in baking and fermentation processes C. Decomposer varieties In nature, fungi serve as decomposers, breaking down dead plant and animal matter into their component organic materials As decomposers, fungi play an important role in sustaining ecosystems They permit recycling of nutrients bound up in the tissues of organisms These nutrients would otherwise be unavailable to sustain new growth D. Intoxicating, poisonous, and hallucinogenic varieties The Conocybe and Psilocybe mushrooms have intoxicating and hallucinogenic properties; they were used by the ancient Mayans for religious ceremonies and are still used by the native peoples of Mexico and Central America Amanita phalloides and Amanita verna (collectively known as the destroying angel, death angel, or death cup) are extremely poisonous; both species appear similar to edible varieties, but can be lethal even if ingested in small quantities PAGE  PAGE 4 ' WHERE id = '3656'