W1L1

Fungi and the Tree of Life

Domains of Life

  • Bacteria

  • Archaea

  • Eukarya

Fungi as Macroorganisms

  • Closer to animals and choanoflagellates than to plants

  • The biggest living organism is monoclonal; occupies an entire valley, but underground.

  • Crown eukaryote: shares terminal stem with plants and animals.

  • Diverged together with them almost simultaneously.

Definition of Fungi

  • Heterotrophic eukaryotes with chitinaceous cell walls incapable of photosynthesis.

  • The term "fungus" comes from Latin meaning "mushroom."

  • Common examples include: yeasts, rusts, smuts, mildews, molds, and mushrooms.

Characteristics

  • Eukaryotic Structures:

    • Hyphae: elongated cellular tubes for growth and reproduction.

    • Yeasts: some containing hyphae, others do not.

    • Cell wall composition: primarily chitin, also may contain glucans (1-6, 1-3, 1-4), sugars, and mannoproteins.

  • Nutritional Mode: Heterotrophic; fungi derive nutrition by secreting substances for degradation of organic material.

  • Nuclear Structure: Haploid nuclei; can have several nuclei in a single cell.

  • Reproductive Strategy: Spores; can fuse; reproduction can be sexual or asexual.

Evolution of Fungi

  • Share a common protozoan ancestor with animals.

  • Chytridiomycota: contain the oldest fossils.

  • Some fungi still possess flagella, indicating their aquatic evolutionary history.

  • Likely evolved from choanoflagellate ancestors.

    • Choanoflagellates: stalked unicellular organisms that serve as an anchor with flagella and collars for feeding.

Association with Plants

  • Nearly 90% of living land plants have associations with fungi (coevolution).

Brief History of Mycology

  • Earliest Documentation:

    • Ancient Egypt (hieroglyphs).

    • Yeast was used for making bread and beer (low alcohol content, 1% or lower).

  • Oldest Mycological Illustrations: Found in Pompeii.

  • Mycological illustrations have a rich history in medicinal texts, albeit simplistic.

  • Fungal Origins:

    • Ancient Greeks, especially Pliny, theorized that fungi originated from thunderstorm phenomena (mixing with heat and penetrating the Earth).

    • Other theories included spontaneous generation and spores.

    • Early classification deemed fungi as imperfect or deficient plants.

  • Lichens: are symbiotic associations of fungi and algae.

Fungal Systematics

  • Historical attempts to understand fungal properties and identity.

  • Edmond and Charles Tulasne provided early, yet accurate, descriptions of fungi amidst ongoing classification challenges.

  • The Five Kingdoms system provides a more accurate classification.

Traditional Fungal Groups

  • Chytridiomycota

  • Zygomycota (bread molds)

  • Glomeromycota (relatively new; associated with mycorrhizal networks)

  • Ascomycota (sac fungi)

  • Basidiomycota (club fungi)

    • mycota: designates phylum

    • mycetes: designates class

Fungal Structure and Ultrastructure

  • Forms include yeast and hyphal (elongated, filamentous).

    • Yeast: reproduce by budding (protuberance → detached).

    • Hyphal: involves moving protoplasm; characterized by tip growth/apical extension.

  • Dimorphism: environmental triggers such as temperature can induce changes in form.

  • Histoplasmosis: caused by airborne spores of H. capsulatum; spores from bird and bat droppings are inhaled, migrate to lungs, grow as yeast, and spread through the lymphatic system.

  • Ultrastructure Variants:

    • Yeast Form: Usually uninucleate; can evolve into yeast when nutrients are abundant.

    • Hyphal Form: Triggered by nutrient scarcity; focus on nutrient acquisition from surroundings.

Yeast Dimorphism

  • Pseudohyphae: appear elongated but consist of a cluster of cells forming a tube.

  • Hyphal: is a true form, composed of single cells.

  • The presence of pseudohyphae can pose medical issues, as they can invade tissues.

  • Dimorphic fungi often grow as yeast-like cells for proliferation in the body fluids but convert to hyphae for invasion of the tissues

    • yeast-like cells = body fluids

    • hyphae = body tissues

Ultrastructure: Hyphal Form

  • Cytoplasm: a gel-like substance.

  • Protoplasm: living components excluding the cell wall.

  • Not all fungi form septa (for cellular compartmentalization) but can employ various structures for growth.

  • Tip Growth: hyphal tips filled with vesicles containing essential materials for growth.

    • Apical vesicular cluster: most fungi

    • Spitzenkörper: specific density-induced structure found in Asco- and Basidiomycota.

  • Growth can halt due to tactile stimuli (detected barriers) or nutrient deprivation affecting protoplasm.

Internal Structures

  • Chitosomes: involved in the synthesis and storage of chitin.

Hyphal Form Types

  • Septate Hyphae: divided by cross-walls (septa); each compartment often contains multiple nuclei, allowing localized damage repair.

    • Dolipore Septum: predominantly observed in Basidiomycota.

    • Septa: advantages include rapid damage containment; disadvantages include slower nutrient transport.

    • Woronin Bodies: serve to block septa, preventing cytoplasm loss; plants possess similar molecules that obstruct xylem and phloem.

  • Coenocytic Hyphae: continuous cytoplasm without septa, allowing for efficient transport of nutrients.

  • Pseudohyphae: intermediate form, elongated yeast cells resembling hyphae but lack true cellular structure; play a role in infection and tissue invasion.

  • Hyphae are typically part of a larger colony, acting in a collaborative manner for survival and growth.