Fungi Notes

Overview: Mighty Mushrooms

  • Fungi are diverse and widespread.
  • They are essential for the well-being of most terrestrial ecosystems because they break down organic material and recycle vital nutrients.
  • Fungi are heterotrophs and absorb nutrients from outside of their body.
  • Fungi use enzymes to break down a large variety of complex molecules into smaller organic compounds.
  • The versatility of these enzymes contributes to fungi’s ecological success.

Fungal Body Structures and Lifestyles

  • The most common body structures are multicellular filaments and single cells (yeasts).
  • Some species grow as either filaments or yeasts; others grow as both.
  • Fungi exhibit diverse lifestyles:
    • Decomposers / saprophytes
    • Parasites (+ -)
    • Mutualists (+ +)

Fungal Morphology: Hyphae

  • The morphology of multicellular fungi enhances their ability to absorb nutrients.
  • Fungi consist of mycelia, networks of branched hyphae adapted for absorption.
  • Most fungi have cell walls made of chitin.

Septate and Coenocytic Fungi

  • Septate fungi: Some fungi have hyphae divided into cells by septa, with pores allowing cell-to-cell movement of organelles.
  • Coenocytic fungi lack septa.
  • Some unique fungi have specialized hyphae called haustoria that allow them to penetrate the tissues of their host.

Specialized Hyphae in Mycorrhizal Fungi

  • Mycorrhizae are mutually beneficial symbiotic relationships between fungi and plant roots (+ +).
  • Ectomycorrhizal fungi form sheaths of hyphae over a root and also grow into the extracellular spaces of the root cortex.
  • Arbuscular mycorrhizal fungi extend hyphae through the cell walls of root cells and into tubes formed by invagination of the root cell membrane.

Specialized Hyphae

  • Fungi have specialized hyphae.
  • Haustoria penetrate cell walls of plants (+ +) or (+ -).
  • Some hyphae are adapted for trapping and killing prey (e.g., nematodes).

Fungal Reproduction

  • Fungi produce spores through sexual or asexual life cycles.
  • Fungi propagate themselves by producing vast numbers of spores, either sexually or asexually.
  • Fungal nuclei are normally haploid.
  • Sexual reproduction requires the fusion of hyphae from different mating types.
  • Fungi use sexual signaling molecules called pheromones to communicate their mating type.

Fungal Life Cycle

  • Spores germinate into mycelia.
  • Asexual reproduction occurs through spore-producing structures.
  • Sexual reproduction involves plasmogamy (fusion of cytoplasm) and karyogamy (fusion of nuclei).
  • The heterokaryotic stage contains unfused nuclei from different parents.
  • Meiosis occurs in the zygote to produce haploid spores.

Plasmogamy and Karyogamy

  • Plasmogamy is the union of two parent hyphae, mycelia.
  • In most fungi, the haploid nuclei from each parent do not fuse right away; they coexist in the hyphae; this mycelium is called a heterokaryon.
  • In some fungi, the haploid nuclei pair off two to a cell; such a mycelium is dikaryotic.
  • During karyogamy, the haploid nuclei fuse, producing diploid cells: n+n=2nn + n = 2n.

Timing of Karyogamy

  • Hours, days, or even centuries may pass before the occurrence of karyogamy, nuclear fusion.
  • The diploid phase is short-lived and undergoes meiosis, producing haploid spores.

Asexual Reproduction

  • In addition to sexual reproduction, many fungi can reproduce asexually.
  • Molds produce haploid spores by mitosis and form visible mycelia.
  • Other fungi that can reproduce asexually are yeasts, which inhabit moist environments.
  • Instead of producing spores, yeasts reproduce asexually by budding: simple cell division and pinching of “bud cells” from a parent cell.

Fungal Ancestry

  • The ancestor of fungi was an aquatic, single-celled, flagellated protist.
  • Fungi and animals are more closely related to each other than they are to plants or other eukaryotes.
  • DNA evidence suggests that fungi are most closely related to unicellular nucleariids, while animals are most closely related to unicellular choanoflagellates.
  • This suggests that fungi and animals evolved from a common flagellated unicellular ancestor, and multicellularity arose separately in the two groups.
  • The oldest undisputed fossils of fungi are only about 460 million years old.

Move to Land

  • Fungi were among the earliest colonizers of land and probably formed mutualistic relationships with early land plants (+ +).
  • Chytrids (phylum Chytridiomycota) are found in freshwater and terrestrial habitats.
  • They can be decomposers, parasites, or mutualists.
  • Chytrids are unique among fungi in having flagellated spores, called zoospores.

Fungal Diversity

  • Chytrids (1,000 species)
  • Zygomycetes (1,000 species)
  • Glomeromycetes (160 species)
  • Ascomycetes (65,000 species)
  • Basidiomycetes (30,000 species)

Zygomycetes

  • The zygomycetes (phylum Zygomycota) exhibit great diversity of life histories.
  • They include fast-growing molds, parasites, and commensal symbionts.
  • The zygomycetes are named for their sexually produced zygosporangia.
  • Zygosporangia, which are resistant to freezing and drying, can survive unfavorable conditions.

Rhizopus Life Cycle

  • Rhizopus (black bread mold) reproduces sexually and asexually.
  • Plasmogamy leads to the formation of a young zygosporangium (heterokaryotic).
  • Karyogamy and meiosis occur within the zygosporangium.
  • Sporangia release spores through asexual reproduction.

Ascomycetes

  • Ascomycetes (phylum Ascomycota) live in marine, freshwater, and terrestrial habitats.
  • The phylum is defined by the production of sexual spores in saclike asci, usually contained in fruiting bodies called ascocarps.
  • Ascomycetes are commonly called sac fungi.
  • Ascomycetes vary in size and complexity from unicellular yeasts to elaborate cup fungi and morels.

Ascomycete Characteristics

  • Ascomycetes include plant pathogens, decomposers, and symbionts.
  • Ascomycetes reproduce asexually by enormous numbers of asexual spores called conidia.
  • Conidia are not formed inside sporangia; they are produced asexually at the tips of specialized hyphae called conidiophores.
  • Neurospora is a model organism with a well-studied genome.

Neurospora Life Cycle

  • Asexual reproduction occurs through conidia.
  • Sexual reproduction involves plasmogamy, karyogamy, and meiosis within the ascus.
  • Ascospores are formed and dispersed from the ascocarp.

Basidiomycetes

  • Basidiomycetes (phylum Basidiomycota) include mushrooms, puffballs, and shelf fungi, mutualists, and plant parasites.
  • The phylum is defined by a clublike structure called a basidium, a transient diploid stage in the life cycle.
  • The basidiomycetes are also called club fungi.

Ecological Roles of Fungi

  • Fungi play key roles in nutrient cycling, ecological interactions, and human welfare.
  • Fungi interact with other organisms in many ways.
  • Fungi are efficient decomposers. They perform essential recycling of chemical elements between the living and nonliving world.
  • Fungi form mutualistic relationships with plants, algae, cyanobacteria, and animals.
  • All of these relationships have profound ecological effects.

Fungus-Plant Mutualisms

  • Mycorrhizae live symbiotically in plant roots (+ +) and are enormously important in natural ecosystems and agriculture.
  • Plants harbor harmless symbiotic endophytes that live inside leaves or other plant parts (+ +).
  • Endophytes make toxins that deter herbivores and defend against pathogens.

Fungus-Animal Symbioses

  • Some fungi share their digestive services with animals.
  • These fungi help break down plant material in the guts of cows and other grazing mammals.
  • Many species of ants and termites use the digestive power of fungi by raising them in “farms.”

Lichens

  • A lichen is a symbiotic association between a photosynthetic microorganism and a fungus (+ +).
  • Millions of photosynthetic cells are held in a mass of fungal hyphae.
  • The fungal component of a lichen is most often an ascomycete.
  • Algae or cyanobacteria occupy an inner layer below the lichen surface.

Lichen Structure and Reproduction

  • The algae provide carbon compounds, cyanobacteria provide organic nitrogen, and fungi provide the environment for growth.
  • The fungi of lichens can reproduce sexually and asexually. Asexual reproduction is by fragmentation or the formation of soredia, small clusters of hyphae with embedded algae.
  • Lichens are important pioneers on new rock and soil surfaces.
  • Lichens are sensitive to pollution, and their death can be a warning that air quality is deteriorating. Lichen (+ +).

Harmful Fungi

  • About 30% of known fungal species are parasites or pathogens (+ -), mostly on or in plants.
  • Some fungi that attack food crops are toxic to humans.
  • Animals are much less susceptible to parasitic fungi than are plants.
  • The general term for a fungal infection in animals is mycosis.

Practical Uses of Fungi

  • Food: Humans eat many fungi and use others to make cheeses, alcoholic beverages, and bread.
  • Some fungi are used to produce antibiotics for the treatment of bacterial infections, for example, the ascomycete Penicillium.
  • Genetic research on fungi is leading to applications in biotechnology:
    • For example, insulin-like growth factor can be produced in the fungus Saccharomyces cerevisiae.

Review of Fungal Phylums

  • Chytridiomycota (chytrids): Flagellated spores
  • Zygomycota (zygote fungi): Resistant zygosporangium as a sexual stage
  • Glomeromycota (arbuscular mycorrhizal fungi): Form arbuscular mycorrhizae with plants
  • Ascomycota (sac fungi): Sexual spores (ascospores) borne internally in sacs called asci; ascomycetes also produce vast numbers of asexual spores (conidia)
  • Basidiomycota (club fungi): Elaborate fruiting body (basidiocarp) containing many basidia that produce sexual spores (basidiospores)