Diversity of Fungi

Topic Overview and Objectives

• Subject: BIO102: General Biology, Department of Plant Science, Olabisi Onabanjo University.

• Lecturer: Bup Oyesiku PhD.

• Topic: Diversity of Fungi.

• Learning Objectives:

  • Develop awareness and knowledge concerning the diversity of fungi.

  • Acquire skills necessary to identify poisonous members of the Fungi kingdom.

  • Understand the ecological functions performed by fungi.

  • Recognize the economic values of fungi in human life.

Introduction to Mycology and Fungi

• Definition and Etymology: Mycology is the formal study of fungi, derived from the Greek word Mykes, meaning mushroom. A scientist dedicated to this field is known as a mycologist.

• Biological Standing: Fungi occupy a unique position in biology, being neither green plants nor animals. They share characteristics with both:

  • Plant-like traits: They possess rigid cell walls. These walls are composed of a mixture of polysaccharides, including cellulose.

  • Animal-like traits: Their cell walls contain chitin, a substance similar to the exoskeleton of insects. Additionally, they utilize glycogen as their basic storage polysaccharide.

• Relationship with Environment: Fungi have traditionally been grouped with plants due to their filamentous multicellular growth forms, although they have no direct evolutionary connection to plants.

• Yeast Exception: While most fungi are multicellular and filamentous, yeast exists in a unicellular form.

• Ecological Necessity: Fungi, along with bacteria, are the primary decomposers (saprobic fungi) in terrestrial food webs. They are the only heterotrophic organisms upon which the continued existence of the universe/ecosystem rests.

• Chemical Function: Fungi possess a powerful arsenal of enzymes that break down organic products. This process of decomposition consumes oxygen and releases:

  • Carbon dioxide ($CO_2$) into the atmosphere.

  • Nitrogenous compounds into the soil, which are recycled by plants and humans.

Background and Evolution of Fungi

• Evolutionary History: Fossil records indicate the first fungi were unicellular eukaryotic organisms with no living copies. The oldest fossils resembling fungi date back to $900 \text{ Million Years Ago (MYA)}$.

• Species Identification: There are approximately $100,000$ species of fungi currently discovered and described. It is estimated that double this number (roughly $200,000$) exist in soils awaiting discovery.

• Habitat: Fungi are primarily terrestrial, with only a few known aquatic members.

• Distinction from Protists: Although heterotrophic protists were traditionally grouped with fungi, they are now treated separately. There is little evidence suggesting protists are the likely ancestors of fungi.

Mechanisms of Reproduction in Fungi

Fungi employ three common modes of reproduction: vegetative, asexual, and sexual strategies.

• Vegetative Strategy: This is the primary method for almost all fungi.

  • Fragmentation: The mycelium breaks into small fragments, each developing into a new mycelium.

  • Other Methods: These include fission, budding, sclerotia, and rhizomorphs.

• Asexual Strategy: This involves the production of spores, which can be produced singly or on specialized structures/stalks.

  • Sporangia: Structures where spores are formed.

  • Conidia: Asexual spores found in genera like Aspergillus and Penicillium.

  • Chlamydospores: For example, those found in Ustilago spp.

• Sexual Strategy: Occurs in most fungi except members of Deuteromycetes (Fungi Imperfecti).

  • Alternation of Generations: Cyclic transition between haploid ($n$) and diploid ($2n$) phases.

  • Process: Gametes (always haploid) fuse to produce diploid sexual spores such as zygospores or oospores.

  • Mating Types: Participating gametes may be morphologically similar (isogametes) or dissimilar (heterogametes). They engage via opposite sexes or plus (+) and minus (-) strains.

  • Sequential Sequence: Sexual reproduction involves three specific steps in order:

    1. Plasmogamy: The fusion of gamete plasma.

    2. Karyogamy: The fusion of nuclei.

    3. Meiosis: Meiotic division to return to the haploid state.

Classification of Fungi

Fungi are primarily classified by their reproductive structures.

• Traditional Membership Changes: Protists (molds) like Myxomycota, Oomycota, and Acrasiomycota have been separated from fungi because they produce motile cells (swarm cells and zoospores), a trait absent in true fungi.

• Main Divisions:

  • Division Zygomycota: Reproduction involves the fusion of plus and minus hyphae leading directly to a zygote.

  • Division Ascomycota: Fusion leads to massive hyphae forming zygotes in a sac-like structure called an ascus. Yeast is mostly classified here.

  • Division Basidiomycota: Zygotes are produced within reproductive structures known as basidia.

  • Division Deuteromycota (Fungi Imperfecti): Fungi where sexual reproduction has not been observed.

  • Division Phycomycota: Microscopic members characterized by coenocytic hyphae.

Fundamental Characteristics and Diversity of Forms

• Core Characteristics:

  • All have cell walls with cellulose, hemicellulose, and chitin.

  • Storage polysaccharide is glycogen.g

  • Production of spores (similar to bryophytes and pteridophytes).

  • Non-motile throughout their entire life cycle.

  • Spore dispersal is predominantly by wind.

  • Non-photosynthetic.

  • Feeding modes: Biotrophic, necrophytic, saprophytic, and symbiotic.

• Structural Forms:

  • Coenocytic: Filaments lack partitions (cross walls); many nuclei are contained in an extensive cytoplasm. Only Ascomycetes typically have coenocytic hyphae within the three main divisions.

  • Non-coenocytic (Septate): Filaments have partitions or cross walls creating compartments for nuclei. These walls may or may not be perforated.

Habitat, Anatomy, and Ecology

• Anatomy: The body consists of filaments called hyphae, which form a network called mycelia.

• Scale: Mycelia can be gargantuan; some spread over more than $30 \text{ acres}$ in forest soil, making them among the world's longest living organisms.

• Specialized Hyphae:

  • Rhizoids: Modified hyphae used by saprobic fungi for attachment to substrates.

  • Haustoria: Specialized hyphae used by parasitic fungi to absorb nutrients from living host cells.

• Symbiotic Relationships:

  • Mycorrhiza: An association between the roots of vascular plants and fungi, facilitating nutrient distribution.

  • Lichens: A symbiotic alliance between fungi and algae or cyanobacteria, capable of occupying inhospitable environments.

• Ecological Impact: While they cause the biodeterioration of materials like cloth, paper, leather, and optical lenses, they are also vital for binding soil particles together.

• Pollution Note: It has not been established that fungi are effective biomonitors of environmental pollution.

Economic Importance and Industrial Applications

• Pharmaceuticals:

  • Cyclosporine: A "wonder" drug extracted from soil-dwelling fungus. It suppresses immune reactions following organ transplantation. Unlike previous drugs, it does not kill bone marrow or cause leukemia.

• Cosmetics:

  • Androstenol: Extracted from truffles and added to cosmetics designed to attract the opposite sex.

• Industry:

  • Yeasts: Crucial for the production of ethanol and carbon dioxide ($CO_2$) via fermentation; essential for baking and brewing.

• Agriculture: Fungi are responsible for the majority of plant diseases.

Identification of Poisonous Mushrooms

Of the world's Basidiomycetes (mushrooms), $95\%$ are deadly to higher organisms.

• Field Diagnostic Characters of Poisonous Mushrooms:

  • Fragments of a veil net on the mushroom cap.

  • Caps colored yellowish-green, brownish, or white, featuring shiny radiating stripes.

  • White or greenish gills accompanying white spores.

  • A large, white annular ring (larger than those on edible mushrooms).

  • A long white stem, potentially featuring green patches.

  • A white cuplike or sheath structure called a volva surrounding the base of the stem, often partially underground.

• Warning: Some characters are unstable and may fade. Traditional tests, such as a silver spoon turning black in a cooking pan, are not reliable. The only safe method is the use of a Taxonomic key by an expert.

Questions & Discussion (Test Yourself Questions)

• Are fungi mostly aquatic? No, they are primarily terrestrial.

• Are all fungi non-photosynthetic? Yes.

• Are protists true fungi? No, they have been separated due to motile cells.

• Is the fusion of two nuclei called karyogamy? Yes.

• Is the association of vascular plant roots with a fungus called Mycorrhiza? Yes.

• Is the length of a fungus mycelium among the longest of living organisms? Yes, spreading over $30 \text{ acres}$.

• Are fungi good indicators of pollution? No, this has not been established.

• Does sexual reproduction in fungi involve plus and minus hyphae? Yes.

• Does the existence of the universe rest upon fungi? Yes, alongside bacteria, as the primary decomposers.

• Is Androstenol the soil fungus drug used to suppress immune reactions? No, that drug is Cyclosporine. Androstenol is used in cosmetics.