Chapter 29: The Fungi

Characteristics of Fungi

• Fungi are eukaryotic heterotrophs that secrete digestive enzymes onto their food source and then absorb the predigested food.
• Fungi are characterized by cell walls that contain chitin.
• A fungus may be a unicellular yeast or a filamentous, multicellular mold.
  • The body of most multicellular fungi consists of long, threadlike filaments called hyphae that branch and form a tangled mass called a mycelium.
• In most fungi perforated septa, or cross walls, divide the hyphae into individual cells.
  • In some fungi the hyphae are coenocytes that form an elongated, multinuclear cell.

Fungal Reproduction

• Most fungi reproduce both sexually and asexually by means of spores.
  • Spores are produced on aerial hyphae.
  • When fungal spores land in a suitable spot, they germinate.
• When fungi of two different mating types meet, their hyphae fuse, a process called plasmogamy.
  • The cytoplasm fuses, but the nuclei remain separate.
  • The fungi enter a dikaryotic (n + n) stage in which each new cell formed has one nucleus of each type.
• Karyogamy, fusion of the nuclei, takes place in the hyphal tip and results in a diploid (2n) zygote nucleus.
• Meiosis produces four genetically different haploid (n) nuclei.
  • Each nucleus becomes part of a spore.
  • When the spores germinate, they form new mycelia by mitosis.
• Genetically similar asexual spores are produced by mitosis.
  • When these spores germinate, they also develop into mycelia.

Fungal Diversity

• Like animals, some fungi have flagellate cells—for example, chytrid gametes and spores—and the flagellate cells propel themselves with a single posterior flagellum.
  • Also like animal cells, fungal mitochondria have platelike cristae.
• Based on chemical and structural characters, fungi are classified, along with animals and choanoflagellates, as opisthokonts.
• Chytrids, or chytridiomycetes, produce flagellate spores at some stage in their life cycle.
  • No other fungi have flagella.
  • Thus, chytrids probably were the earliest fungi to evolve; the most recent common ancestor of all fungi was a flagellate protist.
• Chytrids reproduce both asexually and sexually.
  • Their gametes and zoospores are flagellate.
  • Allomyces, a common chytrid, spends part of its life as a multicellular haploid thallus and part as a multicellular diploid thallus.
  • The haploid thallus produces two types of flagellate gametes that fuse.
  • Both plasmogamy and karyogamy occur, producing a flagellate zygote.
  • The diploid thallus bears zoosporangia that produce diploid zoospores and resting sporangia in which haploid zoospores form by meiosis.
  • The haploid zoospores form new haploid thalli.
• Zygomycetes, such as the black bread mold, Rhizopus, form a haploid thallus that produces both asexual spores and sexual spores.
  • Asexual spores germinate and form new thalli.
  • In sexual reproduction hyphae of two different haploid mating types form gametangia.
  • Plasmogamy occurs as the gametangia fuse.
  • Karyogamy occurs, and a diploid zygote is formed; the zygote develops into a zygospore.
  • Meiosis produces recombinant haploid zygospores.
  • When zygospores germinate, each hypha develops a sporangium at its tip.
  • Spores are released and develop into new hyphae.
• Microsporidia, currently classified as zygomycetes, are opportunistic pathogens that penetrate and infect animal cells with their long, threadlike polar tubes
• Glomeromycetes have coenocytic hyphae.
  • They reproduce asexually with large, multinucleate spores called blastospores.
  • Glomeromycetes are symbionts that form intracellular associations called mycorrhizae with the roots of plants.
  • Because they extend their hyphae into root cells, glomeromycetes are endomycorrhizal fungi.
  • The most common endomycorrhizae are called arbuscular mycorrhizae because the hyphae inside the root cells form branched, tree-shaped structures known as arbuscules.
• Ascomycetes include yeasts, cup fungi, morels, truffles, and blue-green, pink, and brown molds.
  • Some ascomycetes form mycorrhizae; others form lichens.
  • Ascomycetes produce asexual spores called conidia; they produce sexual spores called ascospores in saclike asci.
  • The asci line a fruiting body called an ascocarp.
• In ascomycetes haploid mycelia of opposite mating types produce septate hyphae.
  • Plasmogamy occurs, and nuclei are exchanged.
  • A dikaryotic (n + n) stage occurs in which hyphae form and produce asci and an ascocarp.
  • Karyogamy occurs, followed by meiosis.
  • The recombinant nuclei divide by mitosis, producing eight haploid nuclei that develop into ascospores.
  • When the ascospores germinate, they can form new mycelia.
• Basidiomycetes include mushrooms, puffballs, bracket fungi, rusts, and smuts.
• These fungi produce sexual spores called basidiospores on the outside of a basidium.
  • Basidia develop on the surface of gills in mushrooms; mushrooms are a type of basidiocarp (a fruiting body).
• Hyphae in the basidiomycetes have septa. plasmogamy occurs with the fusion of two hyphae of different mating types.
  • A dikaryotic secondary mycelium forms.
  • Then a basidiocarp develops, and basidia form.
  • Karyogamy occurs, producing a diploid zygote nucleus.
  • Meiosis produces four haploid nuclei that become basidiospores.
  • When basidiospores germinate, they form haploid primary mycelia.

Ecological Importance of Fungi

• Most fungi are decomposers that break down organic compounds in dead organisms, leaves, garbage, and wastes into simpler nutrients that can be recycled.
• Mycorrhizae are mutualistic associations between fungi and the roots of plants.
  • The fungus supplies water and nutrient minerals to the plant, and the fungus obtains organic compounds from the plant.
  • Glomeromycetes form endomycorrhizae with roots.
  • Some ascomycetes and basidiomycetes are ectomycorrhizal fungi that form mycorrhizae when their hyphae coat tree roots, but do not penetrate the root cells.
• A lichen is a combination of a fungus and a photoautotroph (an alga or cyanobacterium).
  • In this symbiotic relationship, the photoautotroph provides the fungus with organic compounds.
  • The fungus may provide the photoautotroph with shelter, water, and minerals.
  • Lichens have three main forms: crustose, foliose, and fruticose.

Economic, Biological, and Medical Impact of Fungi

• Fungi cause huge economic losses by damaging food and crops.
  • On the other hand, some fungi, like mushrooms, are foods; others, like yeasts, are used to produce beer, wine, and bread; and still others are used to produce cheeses or industrial chemicals.
• Biologists use the yeast Saccharomyces cerevisiae and other fungi as model organisms for research in molecular biology and genetics.
  • Fungi are also being investigated for the biological control of insects.
• Fungi are used to make many medications, including penicillin and other antibiotics; they are used in bioremediation and to control pests biologically.
• Fungi are opportunistic pathogens in humans.
  • They cause human diseases, such as histoplasmosis; some fungi produce mycotoxins, such as aflatoxins, which can cause liver damage and cancer.
• Fungal hyphae infect plants through stomata.
  • Hyphal branches called haustoria penetrate plant cells and obtain nourishment from the cytoplasm.
  • Fungi cause many important plant diseases, including brown rot, corn smut, and wheat rust.