32.0 Introduction

1. Unique Character of Fungi fig 32.1
1. Multicellular Organisms Capable of Rapid Growth
2. Share Very Few Characteristics with Plants
1. Are multicellular
2. Are sessile and may grown in soil

32.1 Fungi are unlike any other kind of organism

1. A Fungus Is Not a Plant
1. Fungi Are Members of a Distinct Kingdom of Organisms fig 32.2
1. Studied by mycologists
2. Traditionally classified as plants
1. Unlike plants due to lack of chlorophyll
2. Like plants due to linear growth form and immobility

2. Differences Between Fungi and Plants
1. Fungi are heterotrophs
1. Mushrooms are not green, do not photosynthesize like plants
2. Fungi absorb food after secretion of enzymes and extracellular digestion
3. Absorb resulting organic molecules

2. Fungi have filamentous bodies
1. Fungi are composed of long, slender filaments
2. Filaments may be packed into complex structures like mushrooms
3. Plants are composed of boxlike cells

3. Fungi have nonmotile sperm
1. Some plants have flagellated sperm while fungi do not
2. Most fungi reproduce sexually with nuclear exchange

4. Fungi have cell walls made of chitin
1. Fungi cell walls made of polysaccharides and chitin
2. Plant cell walls are made of cellulose

5. Fungi have nuclear mitosis
1. Very different from plants or other eukaryotes
2. Nuclear envelope does not break down and reform
3. Mitosis occurs within the nucleus
4. Spindle apparatus forms inside nucleus, chromosomes are dragged to poles of nucleus

6. DNA studies confirm vast differences from other organisms

2. The Body of a Fungus
1. Filamentous Growth Form
1. Slender filaments called hyphae (hypha, singl.)
2. May be divided into cells by septa (septum, singl.)
1. Barrier incomplete except when separating reproductive cells
2. Cytoplasm flows freely through pores in septa fig 32.3
3. Results in rapid growth with optimum food, water and temperature

3. Mass of hyphae called mycelium (mycelia, pl.) fig 32.4
1. If strung end-to-end would be many meters long
2. Grows through and penetrates substrate
3. All parts are metabolically active and interact with environment
4. Rapid growth conspicuous if visible reproductive structures formed

2. Unique Cellular Composition
1. Cell wall
1. Polysaccharide plus chitin (not cellulose like plants))
2. Same component of arthropod exoskeleton

2. Distinctive form of mitosis
1. Nuclear envelope remains intact
2. Spindle apparatus forms within nuclear envelope
3. Lack centrioles
4. Microtubule formation regulated by spindle plaques

3. Origin from some unknown single-celled eukaryote with similar characteristics

3. How Fungi Reproduce
1. Fungal Genetic Status
1. All nuclei haploid except for zygote nuclei, many nuclei in common cytoplasm
2. Hyphae of two different mating strains meet and fuse
3. Similar sexual reproduction in two of the three phyla
1. After fusion of hyphae, nuclei do not immediately combine
2. Two types of nuclei coexist without fusion for most of the life of the fungus

4. Important terminology
1. Heterokaryotic hyphae derived from forms with two genetically different nuclei
2. Homokaryotic hyphae have genetically similar nuclei
3. Dikaryotic if hyphal compartment has two genetically distinct nuclei
4. Monokaryotic if compartment has a single nucleus

5. Dikaryotic hyphae have some properties of diploids, both genomes transcribed

2. Types of Reproductive Structures
1. Cytoplasm flows freely if no septa, or through perforations in septa
2. Reproductive structures are separated with complete septa, without perforations
3. Sporangia: Involved in spore formation
4. Gametangia: Structures in which gametes are formed
5. Conidiophores: Produce multinucleate asexual spores called conidia
6. Spores are always non-motile
1. Produced by asexual or sexual processes
2. Germinate upon landing on suitable substrate
3. Distribution via wind, over great distances
4. Dispersed by insects, small animals

4. How Fungi Obtain Nutrients
1. Fungi Exhibit External Digestion
1. Secrete digestive enzymes into environment
2. Extensive hyphal network provides great surface for absorption
3. Many can break down cellulose
1. Cleave link between glucose units, absorb glucose as food
2. Fungi often grow on dead trees

2. Some Fungi Are Predators fig 32.5
1. Mycelium of Pleurotus ostreatus attacks nematodes
1. Secretes substance that anesthetizes roundworm that feeds on fungus
2. Hyphae envelop and penetrate bodies, absorb nutrients
3. Bulk of mushroom's glucose absorbed from digestion of wood fibers
4. Nematodes serve primarily as nitrogen source

2. Other fungi are more active predators
1. Snare, trap or fire projectiles
2. Attack nematodes, rotifers, small animals

5. Ecology of Fungi
1. Fungi and Bacteria Are the Principal Decomposers in Biosphere
1. Breakdown organic molecules, return substances to ecosystem
2. Break down lignin, a major part of wood
3. Provide critical building blocks for growth of other organisms
4. Some fungi attack still living organic matter
1. Cause diseases in plants and animals, cause agricultural damage fig 32.6
2. Destroy food stores making them unpalatable or poisonous

2. Economic Value of Fungi
1. Yeasts are used in the manufacture of bread and beer
1. Produce ethanol and carbon dioxide
2. Used to supply protein to enrich animal food
3. Used to flavor cheese, wine and other foods

2. Fungi used in industrial production of acids, antibiotics and chemical syntheses
3. Convert one complex organic compound to another
1. Detoxify environment
2. Three species isolated that combine selenium with harmless volatile chemicals

3. Important Mutualistic Associations
1. Lichens = fungi + green algae or cyanobacteria
2. Mycorrhizae = fungi + plant roots
3. Partners perform specific duties
1. Photosynthetic organism fixes carbon dioxide and provides organic materials
2. Fungal portion enhances existence within a particular habitat

4. Mycorrhizae facilitate absorption of essential nutrients by plant roots

32.2 Fungi are classified by their reproductive structures

1. Three Phyla of Fungi
1. Historical Aspects of Classification
1. Four groups tbl 32.1
1. Phylum Zygomycota: The zygomycetes
2. Phylum Ascomycota: The ascomycetes
3. Phylum Basidiomycota: The basidiomycetes
4. Imperfect fungi

2. Presently differentiated from protist slime molds and water molds (both are protists)
1. Characteristics of oomycetes (water molds)
1. Motile spores
2. Cellulose-rich cell walls
3. Regular patterns of mitosis
4. Diploid hyphae

3. Differentiation of phyla by sexual reproductive structures
1. Zygomycetes
1. Hyphal fusion results in formation of zygote
2. Zygote undergoes meiosis at germination

2. Ascomycetes and basidiomycetes
1. Distinctive reproductive cells formed from dikaryotic hyphae
2. Nuclear fusion immediately followed by meiosis

2. Phylum Zygomycota
1. Growth Form
1. Nonreproductive hyphae lack septa
2. Include common bread molds fig 32.7
3. Produce characteristic zygospores, temporarily dormant structures

2. Typical Life Cycle fig 32.8
1. Sexual reproduction via fusion of multinucleate gametangia
1. Gametangium cut off from hypha by complete septum
2. May occur between same or different mating types

2. Presence of + and - strains in a colony
1. Nuclei of different mating types may fuse
2. Form diploid zygote nuclei
3. Massive haploid zygosporangium forms around diploid zygote nuclei
4. Zygosporangium may contain one or more diploid nuclei
5. Meiosis occurs during germination of zygosporangium
6. Haploid hyphae grow from haploid cells produced during meiosis
7. All nuclei are haploid except for zygote nuclei

3. Asexual reproduction is common
1. Hyphae grow over surface of material like bread
2. Erect hyphae form sporangiophores
3. Sporangium forms at tip, have separating septum
4. Haploid spores produced within sporangia
5. Spores shed above substrate, dispersed by wind

3. Phylum Ascomycota
1. Economically Important Fungi
1. Beneficial forms include yeast, molds, morels and truffles fig 32.9
2. Harmful forms include chestnut blight and Dutch elm disease

2. Typical Life Cycle
1. Characteristic reproductive structure called ascus (asci, pl.)
1. Diploid zygote formed within haploid ascus fig 32.10
2. Asci form on ascocarp of densely interwoven hyphae

2. Asexual reproduction is common
1. Conidia (conidium, singl.) are produced at ends of conidiophores
2. Spores separated from hyphae by septa
3. Hyphae are septate, but septae are perforated and cytoplasm flows through them
4. Septae at reproductive structures are initially perforated but are sealed later

3. Multinucleate hyphae may be homokaryotic or heterokaryotic
4. Multinucleate gametangia are specialized hyphae
1. Ascogonia are female, have trichogyne outgrowth
2. Antheridia are male and fuse with trichogyne
3. Male nuclei travel to ascogonium through trichogyne to pair with opposite nuclei
4. Heterokaryotic, dikaryotic hyphae arise from area of fusion
5. An ascus containing two nuclei forms at hyphal tip, separated by septa
6. Nuclei fuse forming diploid zygote, immediately undergoes meiosis
7. Four haploid daughter nuclei formed
8. Daughter nuclei divide by mitosis forming eight ascospores
9. Ascocarps may burst to release ascospores

3. Yeasts
1. Unicellular, mostly ascomycetes
2. Most reproduction is asexual cell fission or budding fig 32.11
3. Fusion of two cells produces one cell with two nuclei
1. Functions as an ascus, undergoes sexual reproduction
2. Syngamy followed immediately by meiosis
3. Ascospores function as new yeast cells

4. Diverse degenerate fungi derived from filamentous forms
1. Yeasts related only by being single-celled
2. Most are ascomycetes, but other two groups are represented
3. Even ascomycetes are not clearly related to one another

5. Putting yeasts to work
1. Ferment carbohydrates, produce carbon dioxide and ethanol fig 32.12
1. Different strains selected, domesticated for specific purposes
2. Important yeast is Saccharomyces cerevisiae

2. Many are also pathogens, including Candida
3. Important future in genetic engineering
1. Model system for eukaryotic genetics
2. Synthesized a functional artificial chromosome in 1983
3. Whole genome sequenced in 1996

4. Phylum Basidiomycota
1. Typical Fruiting Body Form
1. Include mushrooms, jelly fungi, puffballs, rusts and smuts fig 32.13
2. Include edible and poisonous varieties

2. Typical Life Cycle
1. Characteristic reproductive structure called a basidium (basidia, pl.)
1. Syngamy occurs within basidium; forms diploid zygote fig 32.14
2. Meiosis occurs immediately, forming four haploid basidiospores
3. Four basidiospores borne on one sterigma (sterigmata, pl.)
4. Spore germinates forming homokaryotic hyphae
5. Hyphae initially lack septae

2. Eventually, septa form between nuclei of monokaryotic primary mycelium
3. Dikaryotic, heterokaryotic secondary mycelium forms when hyphae of different mating types fuse
4. Basidiocarps form of completely dikaryotic mycelium
5. Basidia line gills of typical mushrooms

5. The Imperfect Fungi
1. Also Called Deuteromycetes
1. Sexual reproductive stages not observed
1. Mostly ascomycetes, few zygomycetes and basidiomycetes
2. Derivation of fungi determined by comparison of hyphae and asexual features
3. Not officially classified as such due to lack of sexual reproductive structures

2. As many as 17,000 species described fig 32.15

2. May Exhibit Parasexuality
1. Provides a certain amount of genetic recombination
1. Occurs after fusion of two strains forming heterokaryotic hyphae
2. Exchange portions of chromosomes between genetically distinct nuclei within a common hyphae

2. May be responsible for production of new strains of rusts

3. Economic Importance
1. Penicillium species
1. Produce penicillin antibiotic
2. Flavor cheeses like Roquefort and Camembert

2. Aspergillus species
1. Ferment soy sauce and soy paste
2. Produce citric acid under highly acidic conditions

3. Some species of Penicillium and Aspergillus produce ascocarps on rare occasions
4. Many other species are human and plant pathogens
1. Include fungi that cause athlete's foot and ringworm
2. Fusarium found widely on food
1. Produces toxic trichothecenes, supposed agents of chemical warfare
2. Dangerous agent of food spoilage

32.3 Fungi form two key symbiotic associations

1. Lichens fig 32.15,17
1. Symbiotic Association Between Fungus and Photosynthesizer
1. Example of mutualism, benefits both partners
2. Mostly ascomycetes with green alga and/or cyanobacterium fig 32.18
1. Specialized hyphae penetrate or envelop photosynthetic cells
2. Fungal chemical signals direct algal special metabolism

3. Reproduction
1. Via normal fungal sexual processes
2. Photosynthetic cell reproduction generally asexual
3. Asexual reproduction by fragmentation

4. Ecology
1. Inhabit cold, dry, generally harsh environments
2. Help break rock surfaces and prepare habitat for other organisms
3. Lichens with cyanobacteria can fix atmospheric nitrogen
4. Survive adverse conditions by nearly halting metabolism
5. Coloration of lichen protects photosynthetic partner

2. Lichens and Pollution
1. Extremely sensitive to atmospheric pollutants
2. Absorb substances dissolved in rain or dew
1. Sensitive to sulfur dioxide an automobile pollutant
2. Destroys chlorophyll and alters membrane permeability

3. Indicates radioactive pollution

2. Mycorrhizae
1. Association of Plants and Fungi
1. Most plant roots (90%) associated with certain fungi
1. Replace function of root hairs
2. Fungus aids in transfer of soil nutrients into roots
3. Plant provides organic carbon to fungus

2. Two types: Endomycorrhizae and ectomycorrhizae
1. Endomycorrhizae
1. Fungal hyphae penetrate outer cells of plant
2. Form coils, swellings, minute branches
3. Extend out into surrounding soil

2. Ectomycorrhizae
1. Hyphae surround but do not penetrate cell walls of roots
2. Mycelium extends far out into soil

2. Endomycorrhizae
1. More common of the two types
2. Generally a zygomycetes, 100 species associated with 200,000 plants
3. May increase yield of crops with less energy input
4. Early fossil plants exhibit association
1. May have aided plants in invading the land
2. Early soil lacking organic matter
3. Provide better growth in poor soils

3. Ectomycorrhizae fig 32.19
1. Characteristic symbiont of trees and shrubs in temperate regions
2. Less common, mostly basidiomycetes, some ascomycetes
3. Several different basidiomycetes form associations with one plant
4. Different combinations have different physiological effects