Fungi-eucaryotic, spore-bearing organisms with absorptive metabolism and no chlorophyll; reproduce sexually and asexually
Mycologists-scientists who study fungi
Mycology-the study of fungi
Mycotoxicology-the study of fungal toxins and their effects on various organisms
Mycoses-diseases in animals caused by fungi
Belong to the kingdom Fungi within the domain Eucarya; is a monophyletic group known as the eumycota (true fungi)
Distribution
Primarily terrestrial with a few freshwater and marine organisms
Many are pathogenic in plants or animals
Form beneficial associations with plant roots (mycorrhizae) or with algae or cyanobacteria (lichens)
Importance
Decomposers-break down organic material and return it to environment
Major cause of plant disease; also cause disease in animals, including humans
Industrial fermentation-bread, wine, beer, cheese, tofu, soy sauce, steroid manufacture, antibiotic production, and the production of the immunosuppressive drug cyclosporine
Research-fundamental biological processes can be studied in these simple eucaryotic organisms
Structure
Thallus-body or vegetative structure of a fungus; fungal cell walls are usually composed of chitin, a nitrogen-containing polysaccharide consisting of N-acetyl glucosamine residues
Yeast-unicellular fungus with single nucleus; reproduces asexually by budding, or sexually by spore formation; daughter cells may separate after budding or may aggregate to form colonies
Mold-a fungus with long, branched, threadlike filaments
Hyphae-the filaments of a mold; may be coenocytic (i.e., have no cross walls within the hyphae) or septate (i.e., have cross walls)
Mycelia-bundles or tangled masses of hyphae
Dimorphism-a property of some fungi, which change from the yeast (Y) form (within an animal host) to the mold (M) form (in the environment); this is referred to as the YM shift; the reverse relationship exists in plant-associated fungi
Nutrition and Metabolism
Most fungi are saprophytes, securing nutrients from dead organic material (chemoorganoheterotrophs); fungi secrete hydrolytic enzymes that promote external digestion
Glycogen is the primary storage polysaccharide
Most are aerobic (some yeasts are facultatively anaerobic); obligate anaerobic fungi are found in the rumen of cattle
Reproduction
Asexual reproduction-occurs by several mechanisms
Transverse fission
Budding
Direct spore production
Hyphal fragmentation-component cells behave as arthrospores or chlamydiospores (if enveloped in thick cell wall before separation)
Sporangiospores are produced in sporangium (sac) at the end of an aerial hypha (sporangiophore)
Conidiospores are unenclosed spores produced at the tip or on the sides of aerial hypha
Blastospores are produced when a vegetative cell buds off
Sexual reproduction
Involves the union of compatible nuclei
Some fungi are self-fertilizing (male and female gametes produced on the same mycelium (homothallic), while others require outcrossing between different but sexually compatible mycelia (heterothallic)
Zygote formation proceeds by one of several mechanisms
Fusion of gametes
Fusion of gamete-producing bodies (gametangia)
Fusion of hyphae
Sometimes there is immediate fusion of nuclei and cytoplasm; however, more common is a delayed fusion of nuclei, resulting in the formation of a cell with two haploid nuclei (dikaryotic stage)
Zygotes can develop into spores (zygospores, ascospores, or basidiospores); spores are used for identification purposes and also aid fungal dissemination
Characteristics of the Fungal Divisions
Division Zygomycota-zygomycetes
Most are saprophytes; a few are plant and animal parasites
Coenocytic hyphae (no crosswalls), with many haploid nuclei
Asexual reproduction leads to the formation of sporangiospores
Sexual reproduction leads to the formation of zygospores; these are tough, thick-walled zygotes that can remain dormant when the environment is too harsh for growth
Representative member: Rhizopus stolonifer (commonly known as bread mold, but also grows on fruits and vegetables)
Normally reproduces asexually
Reproduces sexually by fusion of gametangia if food is scarce or environment is unfavorable
Zygospores (diploid) are produced and remain dormant until conditions are favorable
Meiosis often occurs at time of germination
Zygomycetes are used in the production of foods, anesthetics, coloring agents, and other useful products
Division Ascomycota-ascomycetes
Members of this division cause food spoilage, a number of plant diseases (e.g., powdery mildew, chestnut blight, ergot,and Dutch elm disease)
Include many types of yeast, edible morels, and truffles, as well as the pink bread mold Neurospora crassa
Mycelia are septate
Produce conidiospores when reproducing asexually
Ascospores (haploid spores located in a sac called an ascus) are formed when reproducing sexually
Thousands of asci may be packed together in a cup-shaped ascocarp
Includes smuts, jelly fungi, rusts, shelf fungi, stinkhorns, puffballs, toadstools, mushrooms, and bird's nest fungi
Basidia are produced at the tips of the hyphae, in which the basidiospores will develop
Basidiospores are held in fruiting bodies called basidiocarps
Usefulness-many basidomycetes are decomposers; some mushrooms serve as food (some are poisonous); one is the causative agent of cryptococcosis; and some are plant pathogens
Division Deuteromycota-deuteromycetes (commonly called Fungi Imperfecti)
This is a classical division grouping together fungi that lack a sexual reproductive phase or fungi for which a sexual reproductive phase has not been observed; more recently molecular systematics places the Deuteromycota among their closest relatives in the Eumycota and eliminates the Deuteromycota as a separate division
Most are terrestrial; a few are freshwater or marine organisms; most are saprophytes or plant parasites; some are parasitic on other fungi
Human impact
Some are human parasites (e.g., causing ringworm, athlete's foot, histoplasmosis)
Some are used industrially to produce antibiotics, cheese, soy sauce, and other products
Some produce substances that are highly toxic and carcinogenic to animals (e.g., aflatoxin and trichothecenes)
Division Chytridiomycota-chytrids (simplest of true fungi)
Terrestrial and aquatic fungi that reproduce asexually by forming motile zoospores
Microscopic in size; may consist of single cells, a small multinucleate mass, or a true mycelium
Reproduce asexually or sexually
Some saprophytic; others are parasites of algae, other true fungi, and plants
Slime Molds and Water Molds
Resemble fungi in appearance and life-style, but their cellular organization, reproduction, and life cycles are more closely related to protists
The multinucleated protoplasm (plasmodium) moves by amoeboid movement as it phagocytizes organic matter
Form ornate fruiting bodies when food and/or moisture are in short supply; fruiting bodies form spores with cellulose cell walls that are resistant to environmental extremes
Spores germinate to produce myxamoeba or flagellated swarm cells
Myxamoeba and swarm cells are initially haploid, but eventually fuse to form a diploid zygote
Zygote feeds, grows and carries out multiple nuclear divisions, giving rise to a plasmodium
Division Acrasiomycota-cellular slime molds
During the vegetative stage, amoeboid cells called myxamoeba feed phagocytically on bacteria and yeasts
When food is scarce, myxamoeba form pseudoplasmodia by aggregating and secreting a slimy sheath around themselves
Become sedentary and differentiate into prestalk and prespore cells
Form sorocarps that mature to sporangia; sporangia produce spores
Released spores will later germinate to form haploid amoebae to begin the cycle again
Division Oomycota-oomycetes (water molds)
Resemble fungi, but cell walls are composed of cellulose, not chitin
Produce a relatively large egg cell that is fertilized by a small sperm cell or an even smaller antheridium; zygote germinates forming asexual, flagellated zoospores
Usually saprophytic in freshwater environments; some parasitic in fish and plants
