Biology XI:2. Biological Classification(3 Kingdom Fungi)

Kingdom Fungi General Characters Constitute a unique kingdom of heterotrophic organisms Show great diversity in morphology and habitat Some are parasitic while most are saprophytic (feed on dead organisms). Except Yeasts (which are unicellular), other are multicellular and filamentous. The body consists of long, slender, thread-like structures called hyphae. A network of hyphae is known as mycelium. Some hyphae are continuous and multinucleate known as coenocytic hyphae while others have sepatae (cross walls). Cell wall is composed of chitin and polysaccharides. Shows symbiotic association with algae in lichens with roots of higher plants in mycorrhiza Sexual reproduction involves plasmogamy − fusion of protoplasm of two gametes karyogamy − fusion of two nuclei meiosis in zygote resulting in haploid spores During sexual reproduction in fungus, fusion of two haploid hyphae of compatible mating types takes place which may result in a diploid cell (2n) in some fungi or an intervening dikaryotic stage (n + n) called dikaryon and a phase called dikaryophase occurs - mainly in classes Ascomycetes and Basidiomycetes The dikaryotic phase of the fungal life cycle is unique. During this unusual phase, which is common in many species of fungi, cells contain two distinct nuclei. These two nuclei divide simultaneously as the mycelium grows; growth continues until fusion occurs during karyogamy resulting in diploid condition. Formation of fruiting bodies takes place Production of haploid spores occurs in fruiting bodies as a result of meiosis Classes of Fungi Phycomycetes Found in aquatic as well as damp habitats Some are obligate parasites on plants Mycelium − aseptate and coenocytic Asexual reproduction − by motile zoospores or non-motile aplanospores Spores produced endogenously in sporangium Zygospores formed by fusion of two gametes which may be isogamous, anisogamous, or oogamousExample: Mucor, Rhizopus (Bread mould), Albugo (Plant parasite) Ascomycetes Commonly called sac-fungi Unicellular (Yeast) or multicellular They are saprophytic, decomposers, parasitic or coprophilous (growing on dung) Mycelium − branched and septate Asexual spores (conidia) produced exogenously on special mycelium called conidiophores Sexual spores (Ascospores) produced exogenously in sac-like asci, arranged in different types of fruiting bodies called ascocarps Example: Aspergillus, Claviceps, Neurospora − Used extensively in biochemical and genetic work Several members such as morels and baffles are edible (considered as delicacies). Basidiomycetes Commonly known members are mushrooms, puff balls, or bracket fungi Grow in soil, on logs and tree stumps, or as plant-parasites (rusts and smuts) Mycelium − branched and septate Asexual spores absent and most common means of vegetative reproduction is fragmentation Sex organs absent, but plasmogamy takes place through fusion of vegetative cells of different strains Resultant structure is a dikaryon, which gives rise to basidium Basidia are arranged in fruiting bodies called basidiocarpsExample: Agaricus (mushroom), Ustilago (smut), Puccinia (rust) Deuteromycetes Known as imperfect fungi because sexual phase is absent On recognition of sexual phase, the member is shifted to either class Ascomycetes or class Basidiomycetes Reproduce only by asexual spores (conidia) Mycelium is septate and branched. Some are saprophytic while others are parasitic. Being decomposers, these help in mineral cycling.Example: Alternaria, Colletotrichum, Trichoderma