Laboratory Notes for BIO 1003
© 30 August 1999, John H. Wahlert & Mary Jean Holland
Myco- derives from the Greek word for fungus. Over 70,000 species of living fungi have been identified. Diploidy is transient, haploidy is the norm. Fungi are heterotrophs that are important decomposers; most are saprobes (grow on nonliving organic matter), and some are parasites (grow on organic matter of living organisms). Digestion is extracellular; fungal cells secrete powerful digestive enzymes into their surroundings. Nutrients are then absorbed. Most fungi consist of multicellular filaments called hyphae (hypha is singular); hyphae are tube shaped and have walls made of the nitrogenous polysaccharide chitin, which makes them resistant to heat, cold, and desiccation. Cross walls are present for strength, and these are perforated so that cytoplasm is continuous from cell to cell. The meshwork of hyphae that absorbs nutrients is called a mycelium (pl. mycelia). Fungal spores disperse from the parent and germinate into new mycelia.
There are about 600 species of bread molds, e.g., Rhizopus. They live in soil and on decaying plants; most are saprobes, and a few are parasitic. The hyphae lack cross walls. Examine the live Rhizopus mycelia on nutrient agar in petri dishes; the black structures are sporangiophores.
Rhizopus nigricans: sporanges. In asexual reproduction special hyphae called sporangiophores bear sporangia (s. sporangium) aloft; spores develop inside by mitosis, and are dispersed.
Rhizopus nigricans: zygotes, w.m. In sexual reproduction special hyphae of opposite mating types touch, their ends swell, and the two cytoplasms intermingle. Nuclei from both parents enter this joint bulge, and a thick-walled zygospore (brown and rough) develops. Nuclei fuse (diploid cell), meiosis occurs, and haploid spores are formed and dispersed.
The Ascomycota or sac fungi are a diverse group with huge numbers of species. Examples include yeasts (e.g., Saccharomyces), edible truffles, morels, Dutch elm disease, and ergot on rye, which is poisonous to humans. They live in soil and on decaying organisms; many are plant pathogens. They play an essential ecological role, because they attack and digest resistant molecules such as cellulose and lignin of plants and collagen of animals. Reproduction: All Ascomycota can reproduce asexually. Yeast may simply divide by budding (slide: Saccharomyces: budding).
In sexual reproduction compatible mating types conjugate, a capsule called an ascus forms. Within it nuclei fuse and the cell becomes a diploid zygote; meiosis produces four haploid nuclei; a protective spore wall forms around each nucleus and some cytoplasm; these ascospores are released and are transported long distances, usually by wind. Be aware that the cell size of yeast is so tiny that they can be mistaken for prokaryotes, except they have organelles that you should be able to see.
There are about 25,000 species of Basidiomycota, the club fungi: mushrooms, e.g., Coprinus, shelf fungi, stinkhorns, puffballs, rusts, and smuts. Some species are hallucinogenic, and many are toxic. They live in soil, on decaying plants; symbiotic or pathogenic on plants, some are animal parasites. Reproduction: In sexual reproduction hyphae of opposite mating types conjugate, and ultimately spores are formed. The mushroom form of fruiting body is called a basidiocarp and consists of a stem and cap. The cap has radial gills; the spore producing cells on the sides of the gills are called basidia (s. basidium) and the spores are called basidiospores.
Slide: Coprinus: cross section. Center and right images show gills, basidiospores, basidia.
These so-called imperfect fungi have no known sexual stage, and thus assignment to the divisions above is not possible. Examples: Penicillium (maybe belongs here), and Trichophyton, athletes foot; abut 25,000 species. Diverse environments.
Symbiosis: Lichens and Mycorrhizae
Many kinds of sac and club fungi have a symbiotic association with photosynthetic cyanobacteria or algae. These are called lichen. Lichens are easily damaged by pollution. Here is an example of lichen growing on a wooden fence.
Mycorrhizae are a symbiotic relationship of fungal hyphae around the young roots of trees and shrubs; almost all healthy land plants have these symbiotic fungi. The fungus gets carbohydrates from plants, and it provides huge surface areas for absorbing water and mineral ions; they are easily damaged by acid rain.
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