Fungi

Fungi: Comprehensive Study Notes

Unit Overview

  • Unit 3 focuses on the study of Fungi and their significant roles in ecological systems.

Lecture Outline

  • Classification of Fungi

  • Fungal Forms, Nutrition, and Reproduction

  • Fungal Ecology

  • Fungal Parasites and Pathogens

  • Basidiomycota: The Club (Basidium) Fungi

  • Ascomycota: The Sac (Ascus) Fungi

  • Glomeromycota: Asexual Plant Symbionts

  • Zygomycota: Zygote-Producing Fungi

  • Chytridiomycota and Relatives: Fungi with Zoospores

  • Microsporidia: Unicellular Parasites

Defining Fungi

  • Mycologists estimate approximately 1.5 million fungal species exist.

    • Fungi can be single-celled or multicellular.

    • Reproduction can occur both sexually and asexually.

    • Exhibit an unusual form of mitosis.

    • Specialized for nutrient extraction and absorption from surroundings.

  • Animals and fungi last shared a common ancestor approximately 460 million years ago (MYA).

Phylogenetic Relationships

  • Phylogenetic relationships among fungi are debated.

  • Acceptance of six major monophyletic phyla, alongside one paraphyletic phylum: Zygomycota.

  • Microsporidia sometimes included in fungal classification.

Classification of Fungi (Table 31.1)

  • Chytridiomycota (e.g., Batrachochytrium)

    • Characteristics: Aquatic, flagellated fungi that reproduce sexually (haploid gametes) or asexually (diploid zoospores).

    • Approximate number of living species: 1000.

  • Zygomycota (e.g., Rhizopus, Pilobolus)

    • Characteristics: Multinucleate hyphae lacking septa, zygosporangium forms zygote through hyphal fusion.

    • Approximate number of living species: 1050.

  • Glomeromycota (e.g., Glomus)

    • Characteristics: Form arbuscular mycorrhizae, multinucleate hyphae, primarily asexual reproduction.

    • Approximate number of living species: 150.

  • Ascomycota (e.g., truffles, morels)

    • Characteristics: Ascospores formed in a sac (ascus); common asexual reproduction.

    • Approximate number of living species: >30,000.

  • Basidiomycota (e.g., mushrooms, toadstools)

    • Characteristics: Basidiospores produced on club-shaped structures (basidia); both sexual and asexual reproduction.

    • Approximate number of living species: >30,000.

  • Neocallimastigomycota (e.g., Neocallimastix)

    • Characteristics: Anaerobic fungi lacking mitochondria, degrading cellulose in herbivore guts.

    • Approximate number of living species: 20.

  • Blastocladiomycota (e.g., Allomyces)

    • Characteristics: Exhibit alternation of generations; possess nuclear caps made of membrane-bound ribosomes.

    • Approximate number of living species: <200.

General Features of Fungi

  • Fungi comprise long, slender filaments called hyphae.

    • Hyphae can be continuous or divided by septa.

    • The cytoplasm in hyphae allows for rapid growth under favorable conditions.

Mycelia and Cell Walls

  • Mycelium: A mass of connected hyphae that grows through and digests its substrate.

  • Fungal cell walls primarily made of chitin, a compound also found in arthropod exoskeletons.

Genetics of Fungi

  • Fungal hyphae can be monokaryotic (1 nucleus), dikaryotic (2 nuclei), or possess multiple nuclei.

  • Heterokaryotic: Nuclei from genetically distinct individuals.

  • Homokaryotic: Nuclei are genetically similar.

Unusual Mitosis in Fungi

  • Fungal mitosis is characterized by the nucleus being the relevant unit of reproduction.

    • The nuclear envelope does not break down during division.

    • Spindle apparatus forms internally without centrioles (except in chytrids).

    • Spindle plaques regulate microtubule formation during mitosis.

Fungal Reproduction

  • Fungi can reproduce both sexually and asexually.

  • Sexual Reproduction involves the fusion of two haploid hyphae of compatible mating types.

    • In some fungi, fusion results in a diploid cell immediately, while in others, a dikaryotic stage (n + n) arises before forming a diploid nucleus.

  • Common structures formed during reproduction: mushrooms or puffballs.

General Reproductive Steps

  1. Spores are released into the environment.

  2. Spores germinate to form haploid hyphae/mycelia.

  3. Haploid hyphae fuse to become dikaryotic (and sometimes diploid).

  4. Plasmogamy creates dikaryotic mycelium, which may mature into mushrooms.

  5. Within mushrooms, Karyogamy produces spores via meiosis.

  6. Spores are released, continuing the cycle.

Spores

  • Spores serve as the most common means of fungal reproduction, formed through sexual or asexual processes.

  • Largest dispersal method is by wind.

Nutrition in Fungi

  • Fungi obtain food by secreting digestive enzymes into their surroundings and absorbing the resulting organic molecules.

    • Exhibiting a significant surface area-to-volume ratio.

    • Capable of breaking down cellulose and lignin, contributing to wood decomposition.

    • Some fungi possess carnivorous traits.

Fungal Ecology

  • Fungi are principal decomposers alongside bacteria.

    • Their cultural difficulties make assessing diversity challenging; environmental DNA studies reveal hidden organisms.

  • Fungi decompose wood by breaking down cellulose and lignin, releasing vital elements like carbon, nitrogen, and phosphorus.

Interactions with Other Species

  • Fungal Symbioses

    • Obligate symbiosis: Essential for fungal survival.

    • Facultative symbiosis: Non-essential.

    • Pathogenic interactions damage hosts through disease, while parasites harm without causing specific illness.

    • Commensal relationships benefit one partner without harming the other, whereas mutualistic relationships benefit both.

Endophytic Fungi

  • Reside in intercellular spaces within plants, exhibiting parasitic, commensal, or mutualistic behaviors.

  • Some endophytes produce toxins that protect their hosts, e.g., protecting perennial rye grass from aphids.

Lichens

  • Symbiotic associations occur between fungi and photosynthetic partners such as cyanobacteria or green algae, with most relationships being mutualistic.

  • Fungi in lichens cannot survive independently of their photosynthetic partners, protecting them from extreme conditions (light, dessication).

  • Lichens are sensitive to pollutants.

Mycorrhizae

  • Mutualistic relationships form between fungi and the roots of ~90% of vascular plants.

    • Fungi serve as extensions of the plant root system, increasing soil contact and nutrient absorption.

  • Two principal types are:

    • Arbuscular mycorrhizae: Common in 80% of plant species; glomeromycete partners that do not produce aboveground fruiting structures.

    • Ectomycorrhizae: Primarily associated with forest trees; fungal partners are mostly basidiomycetes that surround roots without penetrating.

Animal Mutual Symbioses

  • Neocallimastigomycete fungi coexist in the guts of ruminant animals.

  • Leaf-cutter ants cultivate basidiomycete fungi in underground gardens, exchanging leaves for fungal nourishment.

Fungal Parasites and Pathogens

  • Fungi can trigger various health issues in humans by:

    • Causing allergic reactions to fungal spores.

    • Direct infections termed mycoses (affecting skin and nails).

    • Producing toxins when ingested, can lead to severe illnesses.

Diseases Caused by Fungi

  • Common ailments include athlete's foot, ringworm, and nail fungus, which are challenging to treat due to the close genetic relationship between fungi and animals.

Notable Fungal Diseases and Organisms

  • Batrachochytrium dendrobatidis: Causes chytridiomycosis, leading to significant declines in amphibian populations.

Fungal Plant Diseases

  • Many fungal species significantly harm plants, acting as pests and spoiling stored products.

Basidiomycota Characteristics

  • Basidiomycota includes familiar fungi such as mushrooms, puffballs, and shelf fungi, noted for their reproductive structure known as basidium.

Basidiomycota Reproduction

  • Karyogamy occurs within basidia, representing the only diploid stage of the lifecycle.

  • This is followed by meiosis resulting in four haploid basidiospores.

Basidiomycota Development

  • Germinating spores produce monokaryotic hyphae leading to a primary mycelium.

  • Different mating types can merge to form a dikaryotic mycelium, ultimately growing into basidiocarps (mushrooms).

Ascomycota Overview

  • Ascomycota contains about 75% of known fungi, including bread yeasts, common molds, and significant plant pathogens.

    • Known for producing penicillin from the genus Penicillium and featuring the sac-like reproductive structure ascus.

Reproduction in Ascomycota

  • Karyogamy takes place within asci; the only diploid nucleus in their cycle.

  • Subsequent meiosis and mitosis produce 8 haploid nuclei that form into walled ascospores.

Asexual Reproduction in Ascomycota

  • Asexual reproduction is common, primarily through the formation of conidia at the ends of modified hyphae (conidiophores).

    • Promotes rapid colonization of new food sources.

Yeast

  • Yeasts are single-celled ascomycetes, mainly reproducing asexually through cell fission or budding.

  • They possess the ability to ferment carbohydrates, converting glucose into ethanol and carbon dioxide, integral in food production.

    • Example: Saccharomyces cerevisiae (baker's yeast).

Yeast in Genetic Research

  • Yeast models have been pivotal in genetic studies, being the first eukaryotes to be genetically engineered.

  • Saccharomyces cerevisiae was the first eukaryote to have its genome sequenced (1996).

Glomeromycota

  • A small group of fungi forming intracellular associations with plant roots known as arbuscular mycorrhizae, vital for plant colonization.

  • No evidence of sexual reproduction exists within this phylum.

Zygomycota Overview

  • Zygomycota is diverse but not monophyletic; it includes common bread molds and a few human pathogens.

Zygomycota Reproduction

  • Sexual reproduction involves the fusion of gametangia resulting in plasmogamy.

    • The diploid zygote nuclei emerge from the fusion of haploid nuclei (karyogamy), ultimately developing into a zygosporangium where meiosis occurs, releasing haploid spores.

  • Asexual reproduction occurs more commonly, with sporangiophores producing spores within sporangia.

Chytridiomycota

  • Aquatic, flagellated fungi that feature motile zoospores, closely relating to ancestral fungi.

    • Example: Batrachochytrium dendrobatidis, which has been implicated in the decline of amphibians.

Blastocladiomycetes

  • Found in both aquatic and terrestrial environments, parasitizing various organisms, demonstrating a haplodiplontic life cycle.

Neocallimastigomycota

  • Rely on digestion of plant biomass within mammalian herbivore guts; featuring reduced mitochondria and multiple zoospore flagella.

Microsporidia

  • Intriguingly small obligate intracellular animal parasites.

  • Classification is unclear; devoid of mitochondria and unable to perform aerobic respiration.

    • Depend on hosts for ATP.

Encephalitozoon cuniculi

  • Causes diseases in immunosuppressed hosts, infecting them through spores with a polar tube, leading to diarrhea and neurodegenerative issues.

Review Questions

  • List key differences between plants and fungi.

  • Identify the basic anatomy of fungi, including fruiting bodies and their reproduction methods.

  • Define and describe mycorrhizal fungi, endophytes, and lichens, alongside their ecological and biotechnological benefits.