Fungi

Overview of Fungi

• Fungi form one of three main groups of large eukaryotes alongside animals and land plants.

• Fungi moved to land before land plants, 500 million years ago

• Largest fungi = Humongous fungus (9km²)

• First fungi: 800- 1000 million years ago

Structural Characteristics

• Range from single-celled yeasts to huge multicellular structures

• When you think of a fungus, you probably think about a mushroom, but that mushroom is just a reproductive structure of the fungus, whereas the actual body lives under the soil

  • Hyphae: Long and thin filaments that make up the body of the fungi

  • Mycelium: Multiple hyphae interconnecting

• Because hyphae have a large SA and small volume, they can take up high amounts of nutrients from the surrounding soil

  • More contact with nutrients means more efficient nutrient uptake and absorption

  • This is a major adaptation

External Digestion:

• Fungi do not digest food like animals and plants

• They externally digest food by releasing enzymes into their environment, which break down large molecules that are them absorped by the fungi

• For example, a fungus is growing on a tree

  • The fungus grows hyphae into the wood of the tree

  • Large molecules within the tree are too big to be absorbed

  • Therefore, the fungus releases digestive enzymes that break down these molecules

  • The fungus then absorbs these molecules and takes up nutrients through its hyphae

  • Because of their thin and long hyphae, the fungus has more contact with the surrounding molecules and will have more efficient nutrient uptake

  • Fungi are typically found in moist environments because moisture from water helps break down the nutrients that are absorbed by the hyphae

• Saprotrophs: Decomposers that grow on dead organisms and break down dead material and absorb it

• Symbionts: Obtain nutrients while maintaining a symbiosis with other organisms

Example of Importance of Fungi: White-Rot Fungi

• Wood has two components (Lignin and cellulose), and lignin is particularly difficult to break down

• A particular fungus (White-rot fungi) can break down this organic molecule, allowing wood to decompose

• If White-Rot Fungi couldn’t do this, dead wood would accumulate within the environment

Symbiosis Between Fungi and Organisms:

• Lichens: Fungi + Green Algae or cyanobacteria

  • Photosynthesizer: They use the sun to perform photosynthesis and produce carbon compounds. The fungi then absorb this photosynthetic carbon.

  • Fungi: Provide a habitat for the photosynthesizer.

• Mycorrhizae: Fungi + plant roots

  • Plant: The fungus receives sugar from the plant

  • Fungi: The mycellium in the fungal body are thin and long and they come into contact with more soil. Therefore, they can transfer high amounts of nutrients from the soil (Phosphorus) to the plant.

Dispersal and Spores

• A spore is a reproductive cell that fungi produce to spread fungal growth under certain conditions

• Let’s say a fungus is growing on a tree, and over some time the nutrients available from that tree become depleted. A fungus will produce a spore, which will spread fungal growth.

  • The spore will grow a hyphea, which will then branch off into more hyphae, forming a larger mycelium network

• Ways of Dispersal:

  • Airborne Dispersal: Puffballs

  • Animal Dispersal: Stinkhorn fungi

    • Stinkhorn fungus produces strong odours that attract insects

    • When the insects reach the fungus, the insects can carry the spores to different locations

    • Water Dispersal: Chytrids Fungi

      • Some aquatic fungi produce zoospores with flagella that allow the spores to swim through water

    • Host Manipulation: Cordyceps

      • Cordyceps are zombie-like fungi that infect insects and manipulate their behaviour to disperse fungi in the most suitable environments

Asexual Reproduction

• Fragmentation:

  • A hypha grows locally without having to find a mate

  • A hypha can break off, and if it lands somewhere suitable, it can produce a mycelium

• Conidia:

  • Conidia are asexual spores

  • A specialized hypha called a conidiophore can create conidia, which can then disperse and create a new fungal body

Sexual Reproduction

• Very different from reproduction in animals and plants!

• Phase 1: Plasmogamy

  • We begin with two mycelia, each containing a haploid nucleus (n)

  • When they encounter each other, their hyphae can fuse

  • Plasmogamy is the fusion of cytoplasm, NOT the fusion of nuclei

  • This produces a dikaryotic mycelium (n + n)

  • This stage can last decades

• Phase 2: Mushroom Formation

  • The dikaryotic (n + n) mycellium grows into a mushroom

  • On the underside of the mushroom, some gills have basidia

• Phase 3: Karyogamy

  • During this stage, the two haploid nuclei fuse together to form a diploid nucleus

  • This occurs inside the basidium

• Phase 4: Meiosis

  • Meiosis reduces the number of chromosomes in the diploid nucleus to produce four haploid spores called basidiospores

• Phase 5: Spore Dispersal and Germination

  • Once haploid spores are produced that can disperse and germinate into a haploid mycellium

Sexual Reproduction of Fungi VS Plants

Similarities:

• Have a stage where two haploid nuclei fuse to form a diploid nucleus

• Diploid nuclei form 4 haploid cells during meiosis

Differences:

• In plants, fertilization fuses nuclei and cytoplasm in the same event when a diploid nucleus is formed

• In fungi, a dikaryotic cell is formed, which then becomes a diploid nucleus during karyogamy

Sexual Reproduction of Fungi VS Animals

Similarities:

• Genetic variation occurs through meiosis and the fusion of two haploid nuclei

Differences:

• Fungi undergo the plasmogamy stage where a dikaryotic mycellium forms; then, in the basidia, a diploid 2n is formed which then undergoes meiosis

• In animals, the diploid zygote is formed straight away by the fusion of an egg and sperm, and then the cell remains 2n for most of its cycle