Chapter 28: Fungi

28-1 Overview of Fungi

28-2 Classification

28-3 Fungi and Humans

I. Characteristics of Kingdom Fungi (i.e., Mycology)• Differ from bacteria and protists in structure, reproduction (haploid for most of their life cycle), and in obtaining nutrition. (i.e., eukaryotic, multicellular heterotrophs)

28-1 Overview of Fungi

(1) Molds (saprophytic)

• Tangled masses of filaments of cells that grow on bread and fruit(2) Yeasts

• Unicellular fungi whose colonies resemble bacteria

(3) Hyphae (hypha)

• Filaments of fungal cells

(4) Chitin

• Polysaccharide component of fungal cell walls NOT found in bacteria, protists, or plants, but found in the exoskeleton of insects.

Critical Thinking(1) The cell walls of fungi and the exoskeletons of insects and crustaceans contain chitin. Would this phenomenon be of phylogenetic significance or not?

(A) Structure of Fungi (vegetative and reproductive)

• Multicellular and designed for extracellular digestion, making fungi excellent recyclers of organic material in nature.(1) Mycelium

• An interwoven mat of hyphae visible to the unaided eye

(2) Septa (Basidiomycota and Ascomycota)

• Cross-sections that divide hyphae (called “septate hyphae”)(3) Coenocytic (Zygomycota and Ascomycota)

• Hyphae that lack septa.

(4) Dimorphism (e.g., human disease, histoplasmosis)

• When a fungus can change its form in response to changes in their environment (i.e., Histoplasma capsulatum grows as a mycelium in the soil, but inside a human host, grows unicellularly as a yeast.)

(B) Asexual Reproduction

• Fungi produce thousands of genetically identical haploid spores, usually on modified cells of the hyphae. (NOTE: A variety of asexual spores are formed by different fungi)(1) Sporangiophores (Zygomycota)

• Specialized hyphae that appear as upright stalks

(2) Sporangium• Enclosed sac that sits on top of the sporangiophore

(3) Sporangiospores (haploid)

• Spores manufactured through meiosis in the sporangia

(4) Conidia (haploid)• Spores that lack the protection of an enclosing sac

(5) Conidiophore (Ascomycota)

• Stalk-like structure that produces conidia (e.g., Penicillium)

(2) Fungi such as Penicillium engage in a kind of biochemical warfare against other microorganisms by producing chemicals that diffuse outward and kill nearby organisms. Suggest how producing antibiotics could be an adaptive advantage for fungi?

Critical Thinking

(3) Long before antibiotics were discovered by Alexander Fleming, it was common practice to place a piece of moldy bread on wounds. Suggest a reason as to why this practice may have helped the wounds to heal.

Critical Thinking

(6) Fragmentation (another form of fungal asexual reproduction)• A septate dries and shatters, releasing individual cells that act as spores (e.g., Tinea pedis, Athlete foot fungus)

(4) Most fungi grow best at temperatures between 50 and 68 degrees F. Aspergillus fumigatus, however, grows well between 96 and 100 degrees F. Knowing this, where would you expect A. fumigatus to grow?

Critical Thinking

(7) Budding (e.g., yeast cells)• Asexual process in which part of a yeast cell pinches itself off to produce a small offspring cell (able to be repeated many times)

(C) Sexual Reproduction (mating strains of all three phyla)• Ensures genetic recombination among offspring, assisting evolution of some fungal species (also called “conjugation”)• When 2 different mating types of the same species encounter each other, the opposing hyphae fuse together, producing a diploid cell (zygospore, basidiospore, ascospore—depending on the phylum).

(D) Evolution (~300 m.y.a, around the same time as early land plants)• Fungi, like other eukaryotes, arose from prokaryotes (endosymbiosis)• Possibly evolved as unicellular organisms that may have clung together after mitosis to form a long filament of cells.

I. Phylum Zygomycota (~600 species)

• Terrestrial, rich soil fungi that have coenocytic hyphae (e.g., Rhizopus stolonifer—bread mold)

28-2 Classification (~100,000 species of fungi)

(1) Rhizoids

• Specialized hyphae that anchor the fungus to the food source and secrete digestive enzymes to break down the source into absorbable nutrients.

(2) Stolons

• Specialized hyphae that grow across the surface of the bread(3) Gametangium (occurs during fungal conjugation)

• Sexual reproductive structure that contains a nucleus of each mating type of hyphae.

(4) Zygosporangium (diploid)

• Fused gametangia forms a thick wall and becomes dormant until environmental conditions promote germination. (this will grow into a new sporangiophore upon germination)

II. Phylum Basidiomycota (~25,000 species, sexual reproduction)• Club Fungi, noted for their basidia or gill-bearing caps. (e.g., portabellas and white-button mushrooms)

(1) Basidia (the “clubs”)

• Small, club-like structures that line the gills and are produced during sexual reproduction.

(2) Basidiocarp (stalk (a.k.a. stipe) and mushroom cap)

• Underside of cap are rows of gills that are each lined with thousands of dikaryotic (diploid) basidia.

(3) Dikaryotic

• Basidia cells contain TWO nuclei and fuse to form a diploid zygote; zygote undergoes meiosis to form FOUR haploid basidiospores.(4) Basidiospores• Released into the air and will germinate to produce haploid mycelia that grow underground (when mating strains fuse, a basdiocarp emerges above ground)

III. Phylum Ascomycota (60,000 species)• Sac Fungi often reproduce asexually but can also reproduce sexually (e.g., Saccharomyces cerevisiae—Baker’s and Brewer’s Yeast)

(1) Ascogonium (female gametangium)

• Produces many hyphae once fertilized, resulting in a ascocarp and many asci.

(2) Antheridium (male gametangium)

• Develops from one of the mating strains of hyphae, seeks ascogonium.NOTE: A tube forms between the two gametangia, and the male nuclei pass into the female ascogonium resulting in dikaryotic hyphae.

(3) Ascocarp (edible truffles and morels)

• Visible, cup-like structure that is the sexual reproductive structure from which asci develop.

(4) Monokaryotic (haploid cells)• Hyphae cells that intertwine to form the ascocarp.

(5) Ascus (Asci)• Sacs developing at the TIPS of the dikaryotic hyphae within the ascocarp.(6) Ascospores

• Haploid spores that upon release, will germinate in a favorable environment.

(A) Deuteromycota (Imperfect Fungi)

• Do not possess sexual reproductive capabilities.; reproduces via conidia during asexual state.

IV. Mycorrhizae and Lichens

• Symbiotic mutualisms involving fungus and a photosynthetic autotroph.(1) Mycorrhiza (fungus and plant roots)

• Fungi increases root absorption (water, nutrients) in exchange for plant sugars synthesized during photosynthesis.(2) Lichens (fungus and cyanobacteria or green alga)

• Autotroph makes sugars for the fungus, while the fungus provides moisture, shelter, and anchorage for the photosynthesizer.

(5) When transplanting a wild plant to garden soil, it is very important to include some of the soil from the original habitat. Suggest a possible reason as to why this would be a wise idea.

Critical Thinking

(3) Crustose Lichens

• Lichens grow close together as a layer on the surface of rocks and trees.(4) Fruticose Lichens

• Lichens are shrub-like and can grow up to 5 feet in length.(5) Foliose Lichens

• Inhabit flat surfaces, forming mat-like growths with tangled bodies.

(A) Fungal Skin Infections• Fungi can infect the skin, hair, nails, and other tissues of the body.• Tinea pedis—Athlete’s Foot• Candida albicans—Yeast Infections

(B) Other Fungal Infections• Internal organs can be attacked as well, especially if poisonous fungi are ingested.

(1) Aflatoxins• Poisons of by species of the Aspergillus genus; have been linked to liver cancer and been found in contaminated peanuts and corn.

(6) Many fungi are fatally poisonous to mammals. What adaptive advantage do toxins give to a fungus? (Amanita genus)

Critical Thinking

(7) Drosophila, the common fruit fly, ingests juices from ripe fruit and yeast. How might the fruit fly help speed the natural decay of fruit?

Critical Thinking

II. Fungi in Industry

• Penicillium—Penicillin, Rhizopus—Cortisone derivatives, Saccharomyces cerevisiae—genetic engineering yeast cells (hepatitis B antigens)

(A) Fungi and Food Industries

• Mushrooms, morels, truffles, blue cheese, limburger cheese, beer, wine, breads.

Revisiting Cell Structure and Function

• During most of the life cycle of a fungus, its cells are similar and able to meet their own needs for survival.

Assessing Prior Knowledge

• Describe the types of organelles found in a eukaryotic cell.• Explain how the chromosome number differs in cells produced by mitosis, versus cells produced by meiosis.

• When fungi reproduce, they produce specialized structures that aid in the fertilization and dispersal of offspring.