Number 5 bio Chapter 31: Fungi Lecture Notes Flashcards

Taxonomic Context and Scale of Fungi

• Position in the Three-Domain System:

  • Domain Bacteria: Single-celled prokaryotic organisms, typically measuring $2\,\mu m$ in size.

  • Domain Archaea: Prokaryotic organisms occupying diverse environments, typically measuring $2\,\mu m$ in size.

  • Domain Eukarya: Organisms composed of complex eukaryotic cells with membrane-bound organelles, typically measuring $100\,\mu m$ in size; includes Kingdom Plantae, Kingdom Fungi, Protists, and Kingdom Animalia.

• Size and Longevity Extremes:

  • Honey mushrooms (Armillaria) demonstrate immense potential for organismal growth, with individual subterranean fungal networks spanning $965\,\text{hectares}$ (equivalent to $1,800$ football fields) and reaching ages of up to $1,900\,\text{years}$.

• Fungal Diversity Statistics:

  • Described Species: Approximately $100,000$ species of fungi have been officially described.

  • Estimated Total Diversity: Global fungal species diversity is estimated to be approximately $1.5 \times 10^6$ ($1.5\,\text{million}$) total species.

General Characteristics and Bioenergetics

• Trophic Strategy:

  • Fungi are strict heterotrophs that acquire nutrients exclusively through absorption.

• Extracellular Digestion Mechanism:

  • Fungi do not ingest food whole. Instead, digestion occurs outside the body.

  • Fungi secrete powerful hydrolytic enzymes into their immediate environment.

  • These enzymes break down complex organic polymers into simpler organic compounds, which are then absorbed across the fungal cell membrane.

• Ecological Trophic Modes:

  • Saprophytic: Fungi feed on non-living organic material. Alongside bacteria, saprophytic fungi function as key decomposers in terrestrial ecosystems, breaking down dead matter and recycling essential chemical nutrients.

  • Parasitic: Fungi absorb nutrients from living host organisms, causing harm or disease.

  • Mutualistic: Fungi form beneficial ecological partnerships with plants, cyanobacteria, algae, or animals.

• Cell Wall Composition:

  • Fungal cell walls contain chitin, a tough nitrogenous polysaccharide, distinguishing them from plant cell walls which contain cellulose.

Fungal Morphology and Anatomy

• Growth Forms:

  • Multicellular Filaments: The primary structural form for most fungal species.

  • Single Cells: Known as yeasts.

  • Dimorphic Fungi: Species capable of growing as either multicellular filaments, single-celled yeasts, or transitioning between both modes depending on environmental conditions (e.g., Candida albicans).

• Structural Components of Multicellular Fungi:

  • Hyphae: Individual, highly branched microscopic filaments that constitute the structural framework of multicellular fungi.

  • Mycelium: An interconnected mass of individual hyphae forming the vegetative body of a fungus.

  • Absorption Efficiency: The branched morphology of the mycelium maximizes the surface-area-to-volume ratio, enhancing nutrient absorption capacity. Via continuous hyphal extension, fungi grow actively toward and within food sources. Typical structural scales of hyphal networks measure around $60\,\mu m$.

• Cellular Hyphal Structural Types:

  • Septate Hyphae: Hyphae divided into distinct cellular compartments by internal cross-walls called septa. These septa possess central pores large enough to allow cell-to-cell movement of cytoplasm, organelles, and sometimes nuclei.

  • Coenocytic (Non-Septate) Hyphae: Hyphae that lack septa completely, consisting of a continuous cytoplasmic mass containing hundreds or thousands of shared nuclei.

• Specialized Hyphal Modifications:

  • Haustoria: Specialized hyphal structures formed by mycorrhizal and parasitic fungi that penetrate the cell walls of plant host cells (while remaining external to the cell plasma membrane) to facilitate nutrient exchange or extraction.

  • Predatory Hyphae: Specialized hyphal adaptations used to capture living prey. For example, Arthrobotrys spp. possess microscopic hyphal loops (measuring $25\,\mu m$) that constrict around nematodes (roundworms) in less than $1\,\text{second}$ to trap and digest them.

Fungal Reproduction and Life Cycles

• Spore Propagation:

  • Fungi propagate by producing vast quantities of haploid spores through either sexual or asexual pathways.

  • Spores are predominantly dispersed via wind currents.

• Sexual Reproduction Pathway:

  • Chemical Signaling: Hyphae from different mating types release and recognize sexual signaling molecules called pheromones to communicate their mating type and initiate directed growth.

  • Plasmogamy: The fusion of the cytoplasm from two parent mycelia of compatible mating types.

  • Nuclear Heterokaryotic States:

  • Heterokaryon: A mycelium in which genetically distinct haploid parental nuclei coexist in the same cytoplasm without fusing immediately.

  • Dikaryon ($n + n$): A specialized heterokaryotic mycelium where parental haploid nuclei pair off two per cell (one nucleus from each parent type).

  • Karyogamy: The nuclear fusion of two haploid parental nuclei, forming a temporary diploid ($2n$) zygote.

  • Meiosis: Following karyogamy, the diploid zygote undergoes meiotic division, giving rise to haploid ($n$) spores that disperse, germinate, and establish new haploid mycelia, thereby introducing genetic variation.

• Asexual Reproduction Pathway:

  • Molds: Filamentous fungi that form visible mycelia and produce haploid asexual spores called conidia from specialized hyphal structures termed conidiophores (conidia measure approximately $1.5\,\mu m$).

  • Budding Yeasts: Single-celled yeasts reproduce asexually through simple cell division or by pinching off small "bud cells" from the parent cell (budding cells measure approximately $10\,\mu m$).

  • Imperfect Forms: Many mold and yeast species have no documented sexual reproduction stage.

Fungal Diversity and Major Phyla

• Taxonomic Lineages and Evolutionary Relationships:

  • Chytrids ($1,000$ species): Early-diverging basal lineage of fungi.

  • Zygomycetes ($1,000$ species): Conjugated fungi.

  • Glomeromycetes ($160$ species): Arbuscular mycorrhizal fungi.

  • Dikarya Clade: Evolutionary clade consisting of Ascomycetes and Basidiomycetes, unified by an extended dikaryotic life cycle phase.

• Phylum Zygomycota (Conjugated Fungi):

  • Named for spherical sexual structures called zygosporangia formed during sexual reproduction.

  • Predominantly saprophytic organisms, including common bread molds.

• Phylum Ascomycota (Sac Fungi):

  • The largest fungal phylum, comprising approximately $65,000$ species.

  • Commercial and Industrial Importance: Essential in baking, brewing, and industrial fermentation (Saccharomyces cerevisiae), as well as commercial cheese production (Penicillium camemberti).

  • Medical Applications: Source of key biological therapeutics and antibiotics, such as penicillin (Penicillium chrysogenum).

  • Ecological and Pathogenic Roles: Form mutualistic mycorrhizae, lichens, and termite fungal gardens; includes human commensals and opportunistic pathogens (Candida albicans).

• Phylum Basidiomycota (Club Fungi):

  • Comprises approximately $30,000$ species.

  • Defined by the basidium, a microscopic club-shaped cell where karyogamy and meiosis occur to produce basidiospores.

  • Includes common field mushrooms, shelf fungi, toadstools, agricultural rusts, and smuts.

  • Encompasses most edible mushrooms as well as numerous highly poisonous mushrooms.

  • Includes yeast forms such as Cryptococcus neoformans, and participants in lichens and termite fungal gardens.

  • Representative Life Cycle of a Basidiomycete:

  1. Plasmogamy between two haploid mycelia produces an extensive dikaryotic mycelium ($n + n$).

  2. Environmental stimuli trigger rapid growth of a fruiting body known as a basidiocarp (mushroom), which can emerge rapidly or form expanding circular growth patterns called "fairy rings".

  3. The underside of the basidiocarp features gills lined with specialized basidia (basidia scale $1\,\text{mm}$).

  4. Karyogamy occurs inside each basidium, fusing haploid nuclei into a diploid zygote ($2n$).

  5. The diploid zygote undergoes meiosis to generate four haploid basidiospores ($n$).

  6. Basidiospores are released, dispersed by wind currents, and germinate into new haploid mycelia ($n$).

Ecological Roles and Symbiotic Relationships

• Nutrient Cycling in Ecosystems:

  • Saprophytic fungi play vital roles in biogeochemical cycling by breaking down dead plant and organic material, returning essential chemical elements to circulation in terrestrial ecosystems.

• Lichen Symbiosis:

  • Definition: A obligate mutualistic association between fungal hyphae (most commonly sac fungi / Ascomycota) and a photosynthetic microorganism (green algae or cyanobacteria).

  • Mutualistic Exchange:

  • Fungal Partner: Offers structural protection, anchorage, and retains water and dissolved minerals.

  • Photosynthetic Partner: Manufactures organic carbon compounds and sugars via photosynthesis and supplies them to the fungus.

  • Lichen Morphological Types:

  • Crustose Lichen: Flattened, encrusting thallus forms (e.g., Xanthoria).

  • Fruticose Lichen: Shrub-like, branching physical structures (e.g., Cladonia).

  • Foliose Lichen: Flattened, leaf-like lobed thallus structures (e.g., Xanthoparmelia).

• Mycorrhizal Symbiosis:

  • Definition: Mutualistic associations formed between specialized fungal hyphae and host plant roots.

  • Functional Dynamics:

  • Plant Cell Interface: Fungal hyphae form haustoria that penetrate the plant cell wall to interface directly with the plant plasma membrane.

  • Resource Delivery: The fungus absorbs water and essential soil minerals and delivers them to the plant root; in return, the plant translocates organic nutrients (photosynthetic carbohydrates) to the fungal partner.

Human Mycoses and Antifungal Pharmacology

• Medical Mycology and Pathogenesis:

  • Tineas: Superficial skin infections caused by dermatophyte fungi; includes tinea pedis (Athlete's foot) and tinea corporis (Ringworm, which is caused by a fungal infection rather than a parasitic worm).

  • Sick Building Syndrome: Environmental toxicity caused by black mold (Stachybotrys chartarum), leading to severe respiratory irritation in contaminated structures.

  • Histoplasmosis: Systemic fungal pulmonary infection caused by Histoplasma capsulatum (noted as Histoplasmosis capsulatum); produces respiratory symptoms that closely resemble tuberculosis.

  • Candidiasis: Opportunistic fungal infection caused by the dimorphic yeast Candida albicans; presents as oral thrush or vaginal yeast infections when normal bacterial microflora are disrupted by antibiotic usage or in immunocompromised patients.

• Pharmacological Challenges of Antifungal Agents:

  • Development Scarcity: Antifungal and antiparasitic medications are relatively uncommon and difficult to design compared to antibacterial drugs.

  • Biochemical Barriers: Fungi and protozoa are eukaryotic organisms, sharing fundamental cellular and metabolic traits with human host cells:

  • Eukaryotic organelle and cellular organization.

  • Highly conserved 80S ribosomes.

  • Similar biological cell envelopes and membrane components.

  • Treatment Complications: Many effective antifungal agents exhibit significant host toxicity toward human tissues or face emerging antimicrobial resistance.