FUNGI.pdf

LECTURE NOTE ON BIO 102 (GENERAL BIOLOGY 2) SECOND SEMESTER 2024 BY Dr N.V. Elosiuba FUNGI Up until the mid-20th century many Scientists classified fungi as plants. Until recently the biology of fungi was poorly understood. Based on fossil evidence, fungi appeared in the Precambrian era, about 450...

LECTURE NOTE ON BIO 102 (GENERAL BIOLOGY 2) SECOND SEMESTER 2024 BY Dr N.V. Elosiuba FUNGI Up until the mid-20th century many Scientists classified fungi as plants. Until recently the biology of fungi was poorly understood. Based on fossil evidence, fungi appeared in the Precambrian era, about 450 million years ago. Fungi, like most plants are sessile and seem to be rooted to a spot. They possess a stem like structure similar to plants, as well as having a root-like fungal mycelium in the soil. The field of Mycology, which is the scientific study of fungi, brought a lot of progress in field of fungal biology. The word fungus comes from the Latin word for mushrooms. The Kingdom Fungi includes a large group of living organisms collectively referred to as Eumycota (True fungi). About 100, 000 species of fungi have been identified by scientists which are only a fraction of about 1 million species probably present on Earth. Organisms like mushrooms, yeasts, mould of all types are all fungi. Penicillium notatum used in the production of Penicillin drug is also a member of the Kingdom Fungi. Fungi were once considered as plants but, molecular studies have thrown more light and it was discovered that fungi have more of animal characteristics than plants hence they were carved into their own separate kingdom. Fungi play very important roles in the ecosystem. They are decomposers and play great roles in the cycling of nutrients, by breaking down organic materials into simple molecules. They often interact with other organisms to form mutualistic associations. They could also be harmful, causing diseases in plants, animals and humans BASIC CHARACTERISTICS OF Fungi Fungi are eukaryotic organisms with a complex cellular organization. Their nucleus is membrane bound. The DNA in the nucleus is wrapped around with histone proteins (which package DNA into structural units called nucleosomes). Few fungal cells have structures that can be compared to bacterial plasmids (loop of DNA) however there is no horizontal transfer of genetic information as is often the case in plasmids. Fungi contain mitochondria, endoplasmic reticulum and golgi apparatus. They do not have chloroplasts and chlorophyll; however they have other cellular pigments giving them multiple colours ranging from red, green to black. The pigments in fungi are associated with their cell wall and function protectively against U.V. radiations. Some of these pigments are poisonous. Fungal cells have thick cell walls made of chitin and glucans which give structural strength to the cell walls. Chitin is similarly found in the exoskeleton of insects. Fungi have plasma membrane similar to other eukaryotes except that the structure is stabilized by ergosterol a steroid molecule replacing the cholesterol found in animals cell membranes. Fungi are non-motile, it is only the gametes of the primitive phylum Chytridiomycota that produces flagella. Most fungi are multicellular, while some are unicellular example the yeast cells. They display two distinctive morphological stages; - The vegetative stage - The reproductive stage They reproduce by means of spores. They exhibit alternation of generation (they alternate between two different life stages or generations in their life cycle) a haploid stage (gametophyte) a diploid stage (sporophyte). They develop from the spores, they have no embryonic stage. Reproduction can be by sexual or asexual means. Fungi have a filamentous structure except in yeast cells (Yeast cells are single celled). These filamentous/thread-like structures are known as hyphae, which come together to form a mesh-like structure called mycelium. Mycelium is a network of fungal threads or hyphae. It often grows underground. The fruiting bodies of fungi such as mushrooms sprout from mycelium. CLASSIFICATION OF FUNGI: Based on mode of nutrition, fungi can be classed into 3. Saprophytic mode of nutrition: These saprotrophs feed on dead and decaying organic matter. Examples include Rhizopus stolonifer – the black mould of bread. Penicillium chrysogenum – used in the production of penicillin antibiotic. Aspergillus – a spore forming mould fungus. Parasitic mode of nutrition: these live on animals and plants causing harm to them. Examples include Taphrina deformans – causing peach leaf curl disease (phyto-pathogenic fungi). Those that live on animals are called zoo-pathogenic fungi. Examples include ringworm infections, epidermophytosis and Tinea vescicolor, Candida spp. Those that live on insects are called entomopathogenic fungi. Symbiotic mode of nutrition: Interdependent relationship whereby both organisms are mutually beneficial e.g. Lichens (relationship between algae and fungi) and Mycorrhiza (relationship between fungi and plants). Fungi can also be classified based on their spore formation and reproductive strucures. 1. Deuteromycota – The fungi imperfecti This is an artificial class of fungi created to include all those fungi whose sexual stage is either absent or not known. Some are unicellular like yeast cells, but most are multicellular with septate mycelium. They reproduce asexually by spore formation (Conidia formation). They live mainly on land. Most food spoilages are caused by this group of fungi and are also responsible for most human fungal infections. Examples include red-rot of sugar cane caused by Colletotrichum falcatum. Helminthosporium oryzae, the leaf spot of rice, early blight of potato and tomatoes etc. They occur as saprophytes on a wide range of substrates but a large number of them are parasites of plants and animals. 2. Oomycota - (Oogamous fungi) Mycelium is coenocytic (multinucleate and aseptate). Hyphal wall contain cellulose and in some cases chitin. Asexual reproduction involves spore formation contained in sacs or sporangia. In aquatic conditions sporangia produce zoospores, but in terrestrial condition sporangia behave as spores known as conidia hence the name conidiosporangia. Sexual reproduction is by gametangial contact in which the male sex organ or antheridium passes its product into the female sex organ oogonium or archegonium through a fertilization tube. The product of sexual reproduction here is oospore. Examples include blight diseases, white rust disease, damping off, downy mildew, etc. 3. Zygomycota – (The conjugation fungi) Their mycelium is coenocytic. Hyphal wall contain chitin or fungus cellulose. There is absence of motile cells (zoospores and plasmogametes). Mitospores are non-motile and are called sporangiospores. These are terrestrial fungi which are mostly saprotrophic and rarely parasitic. Sexual reproduction occurs through gametangial copulation or conjugation, hence they are called conjugation fungi. Sexual reproduction gives rise to to zygospore (diploid spore). Examples include squirting fungus (Pilobolus crystallins), Rhizopus stolonifer – black bread mould, Mucor caninus – dung mould, both are saprotrophic. 4. Ascomycota – (The sac fungi) This is a class of diverse fungi having over 30000 species. They are saprotrophs, decomposers, coprophilous (living on dung) and parasitic. They have mycelium with septate hyphae, with the exception in yeasts. Cell wall contain chitin and fungus cellulose. No motile structures in their life cycle. In most ascomycetes asexual reproduction are by formation of conidia borne on conidiophores, which maybe branched or unbranched. We find them growing on bread, citrus fruit, old leather etc. Examples are Penicillium and Aspergillus. They also include pigmented moulds (brown, green, blue, pink), powdery mildew, yeasts, morels and truffles. Sexual reproduction can take place in this group also. 5. Basidiomycota – (the club fungi). These are the most advanced and most commonly seen fungi as their fructifications are large and conspicuous. The class contain about 25,000 species. They are the best wood decomposers. They can decompose both lignin and cellulose with the help of secreted enzymes. They have no motile structures or cells. They have two types of mycelia, 10 and 20 mycelium. Sexual reproduction is by Plasmogamy (fusion of protoplasts without fusion of nuclei. Sex organs are not involved. Examples include: Rusts – Puccinia graminic tritici (black rust of wheat), Puccinia glumarun (yellow rust of wheat). Smuts: Ustilago maydis – (smut of corn). Ustilago triciti – (loose smut of wheat). Mushrooms: with umbrella-like basidiocarp – Agaricus campestris, Voluariella volvacea – (paddy straw mushroom). Toadstool: these are non-edible, often poisonous with white spores Amanita acesarea (Caesaer’ mushroom). Amanita phalloides (death cap), Bracket fungi or shelf fungi also known as polypores.