Introduction to Mycology and Fungal Kingdom

Introduction and Classification of Fungi

• Fungi were originally classified as plants because they are eukaryotic and sessile (they do not move around like animals).

• Scientific investigation revealed that fungi are distinct from plants based on several factors:

  • They lack chloroplasts.

  • They do not perform photosynthesis.

  • They lack the internal structures (roots, stems, leaves) found in plants.

• Major characteristics that unify fungi:

  • Filamentous: Most fungi have tube-shaped, tubular cells that look like strands. The notable exception is yeasts, which are unicellular and typically oval-shaped cells, though they can occasionally be filamentous.

  • Spore-producing: Both plants and fungi produce spores, but the processes differ. Even seed-producing plants have a spore-producing stage in their life cycle.

  • Heterotrophic: Fungi cannot take carbon from the air and sunlight to produce energy. They cannot generate carbon-containing compounds from carbon dioxide ($CO_2$) and sunlight; they are not photoautotrophs.

Nutritional Strategies and Ecological Roles

• Fungi must obtain carbon and energy by consuming organic matter made by other organisms or by parasitizing living organisms.

• Extracellular Digestion: This distinguishes fungi from many other life forms. Instead of ingesting food and then breaking it down internally, fungi secrete digestive enzymes onto their food source, break it down externally, and then absorb the resulting materials.

• Saprobes (Saprophytic): Fungi that feed on dead matter, such as logs in a forest or decaying leaves.

• Parasites: Fungi that feast on living hosts. An example provided is Athlete’s foot.

• Mutualists: Fungi that live in mutually beneficial relationships with other organisms, such as those found in lichens.

• Cellular Composition: Fungal cell walls are composed of chitin, which is a polysaccharide in the same family as starches and cellulose. This contrasts with plant cell walls, which are made of cellulose.

• Ecological Role: Along with bacteria, fungi serve as primary decomposers. Without them, nutrients would not be cycled back through the environment.

Fungal Anatomy and Growth

• Hyphae: The filamentous, tubular cells of fungi. These filaments can be modified for various specialized functions.

  • Hyphae often have cell walls and cross walls (septa) between cells.

  • They are strung together to form long strands that resemble roots.

• Mycelium: A mass of hyphae growing together.

  • Enoki mushrooms: These are small white mushrooms often used in Asian cooking. They are grown under special conditions to produce long stems and tiny caps. The speaker cultivated mycelium from Enoki mushrooms purchased at an Asian market in China Grove.

• Reproduction: Fungi reproduce both asexually and sexually.

  • Asexual Reproduction: Results in offspring that are clones of the parent. Methods include:

    • Spores.

    • Budding: Commonly observed in yeasts, where new cells grow directly off the original cell.

    • Fragmentation: Propagation by breaking off pieces and spreading. This is used in laboratory cultures to amplify fungus growth.

Taxonomy: The Five Phyla of Kingdom Fungi

Taxonomy is often debated, but this curriculum follows the classification of Kingdom Fungi into five phyla:

• Chytridiomycota (Chytrids):

  • Considered the most ancient and simplest group.

  • They appear different under a microscope, often featuring a body called a thallus with stringy, hyphae-like projections.

  • They are important saprobes but also include significant parasites.

  • Enzymes in this group can digest recalcitrant materials (substances tough to break down) like chitin, keratin, and cellulose.

  • Examples:

    • Potato wart: A crop disease.

    • Batrachochytrium dendrobatidis: A chytrid causing chytridiomycosis, a disease responsible for declining amphibian populations worldwide, including North Carolina. It infects the skin, preventing frogs from breathing properly, as amphibians perform up to half of their respiration through their skin.

• Zygomycota (Conjugated Fungi):

  • Includes bread molds and the genus Pilobolus (the "hat-throwing" fungus).

  • Pilobolus: Found in cow manure (coprophilous). It builds internal water pressure to shoot its sporangia (the "hat") away from the manure onto grass, where it is eaten by cows to complete its life cycle.

  • Rhizopus stolonifer: Known as black bread mold. The mycelium is white, but the sporangia turn black as they mature.

• Glomeromycota:

  • Soil-dwelling fungi that form symbiotic relationships with plant roots called mycorrhizae.

  • The plants provide sugars (carbon) from photosynthesis; in exchange, the fungi absorb minerals like phosphates from the soil and transfer them to the plant.

• Ascomycota (Sac or Cup Fungi):

  • The largest and most diverse group.

  • Includes yeasts (Saccharomyces or "sugar fungus"), Penicillium (source of penicillin), morels, and truffles.

  • Reproductive structure: The ascus (plural: ascii), a microscopic tube-like structure that typically produces $8$ spores.

  • Visual structures include the ascocarp, which can be cup-shaped.

• Basidiomycota (Club Fungi):

  • Includes true mushrooms, puffballs, shelf fungi, and coral fungi.

  • Reproductive structure: The basidium, a microscopic club-shaped structure that produces up to $4$ spores.

  • The visible fruiting body is called a basidiocarp.

Detailed Life Cycles

Zygomycete Life Cycle (e.g., Rhizopus)

1. Two compatible strains (designated as $+$ and $-$) meet.

2. They form structures called gametangia.

3. Plasmogamy: The cell walls between the gametangia disappear, and the contents merge.

4. Dikaryotic Stage: The cell contains two separate nuclei ($n+n$ stage).

5. Karyogamy: The nuclei fuse to form a diploid ($2n$) zygote.

6. This develops into a zygosporangium, which eventually produces a sporangium that releases new, genetically unique spores.

Ascomycete Life Cycle

1. Spores germinate into hyphae, forming a mycelium.

2. Specialized structures called antheridia and ascogonia fuse through plasmogamy.

3. Dikaryotic hyphae ($n+n$) grow to form the ascocarp.

4. At the tips of the hyphae, nuclei fuse (karyogamy) to form a $2n$ cell.

5. Meiosis occurs, followed by mitosis, resulting in $8$ unique ascospores inside the ascus.

Basidiomycete Life Cycle

1. Spores germinate into primary mycelium (monokaryotic, meaning one nucleus per cell).

2. Two compatible monokaryotic mycelia fuse (plasmogamy) to create secondary mycelium (dikaryotic, $n+n$).

3. Secondary mycelium can produce a mushroom (basidiocarp).

4. A diagnostic feature of dikaryotic mycelium is the presence of clamp connections.

5. Parts of a mushroom include the stalk, gills, annulus (leftover inner veil), and sometimes a volva (cup-like base).

6. Spores are produced on the basidia through karyogamy and meiosis.

Specialized Fungi and Lichens

• Notable Basidiomycetes:

  • Amanita muscaria (Fly Agaric): The stereotypical "fairy tale" mushroom; contains hallucinogens and poisons.

  • Earth Star: A puffball-like fungus where the outer layer opens like flower petals.

  • Stinkhorn (Phallus impudicus): Smells like rotting flesh to attract flies for spore dispersal.

• Lichens:

  • A symbiotic relationship between a fungus (usually an ascomycete) and a photosynthetic partner (either green algae or cyanobacteria).

  • The fungus provides protection, anchor, and moisture; the photosynthetic partner provides sugars.

  • Essential for primary succession, as they break down rock into soil.

  • Three forms:

    1. Crustose: Flat, crust-like patches.

    2. Foliose: Leafy, like kale or foliage.

    3. Fruticose: Branching, moss-like structures.

Questions & Discussion

• Lab Procedures: Students were instructed to take a $15$-minute break before using compound microscopes to observe live cultures and prepared slides. They were specifically told to look for clamp connections to differentiate between monokaryotic and dikaryotic mycelium. Oil immersion is required for the final procedure.

• Identification: Mention of Coprinus slides and lichen samples.

• Academic Discussion: Students discussed confusion regarding a "Microbiome journal" assignment and whether it was located in the homework folder. There was frustration over the "Choose one" instruction for homework assignments and concerns about how the instructor grades missing versus elective assignments.

• Syllabus and Policies: Students questioned if the instructor accepts late work, referencing a need to check the syllabus after witnessing grades being posted the previous day.