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Chapter 23 Fungi. 23.1 High-Flying Fungi. Dust carries spores of fungi long distances – even across oceans Puccinia graminis is the fungal pathogen that causes wheat stem rust, a disease that destroys food crops worldwide

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Chapter 23 Fungi

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23 1 high flying fungi
23.1 High-Flying Fungi
  • Dust carries spores of fungi long distances – even across oceans
  • Puccinia graminis is the fungal pathogen that causes wheat stem rust, a disease that destroys food crops worldwide
  • Plant breeding programs have developed rust-resistant strains of wheat – but a mutant form of the fungus (Ug99) is now infecting rust-resistant wheat
23 2 fungal traits and classification
23.2 Fungal Traits and Classification
  • Fungiare heterotrophs that obtain nutrition from their environment by extracellular digestion
  • They have walled cells, spend their lives fixed in place, and disperse by producing spores
  • Most are free-living saprobes that live on the remains of other organisms; others live on or in other organisms
ecology of fungi
Ecology of Fungi
  • Some fungi decompose organic wastes and remains, which helps recycle nutrients in ecosystems
  • Some form beneficial partnerships with plants, photosynthetic cells (lichen), or herbivores
  • Some are parasites or pathogens
structure of fungi
Structure of Fungi
  • Some fungi live as single cells (yeasts)
  • Most are a multicelled (molds, mushrooms)
  • Multicelled fungi grow as a mesh of branching filaments (mycelium) – each filament is one hypha
  • In the oldest fungal lineages (chytrids, zygomycetes, and glomeromycetes) hyphae do not have cross-walls
  • Hyphae divided into compartments by cross-walls (septae) evolved in the common ancestor of sac fungi and club fungi
  • Cross-walls made hyphae stronger, allowing the evolution of larger, more elaborate spore-producing bodies
  • Septate hyphae also are more resistant to desiccation than nonseptate hyphae
hyphae in a mycelium
Hyphae in a Mycelium

one cell (part of one hypha of the mycelium)

life cycles
Life Cycles
  • Asexually produced spores form by mitosis in sporangia at the tips of specialized haploid hyphae
  • Sexual reproduction begins with cytoplasmic fusion of haploid hyphae to produce a dikaryoticstage having two genetically distinct types of nuclei (n+n) in each cell
  • Fusion of two haploid nuclei produces a diploid zygote that undergoes meiosis, restoring the haploid state
take home message what are characteristics of fungi
Take-Home Message:What are characteristics of fungi?
  • Fungi are heterotrophs that absorb nutrients from their environment
  • Some live as single cells; others live as a multicelled mycelium
  • They disperse by producing spores
23 3 the flagellated fungi
23.3 The Flagellated Fungi
  • Chytrids are the only modern fungi with a life cycle that includes flagellated cells
    • Some feed on organic wastes and remains
    • Some live in guts of herbivores and help digest cellulose
    • Some are parasites
a parasitic chytrid
A Parasitic Chytrid
  • Frog populations are threatened by Batrachochytrium dendrobatidis, a parasitic chytrid that causes chytridiomycosis
  • A mycosis is a disease caused by a fungus
  • Outbreaks of B. dendrobatididis have far-reaching ecological effects because frogs help control populations of insects and serve as food for many other animals
zygote fungi
Zygote Fungi
  • Rhizopus species
    • Include black bread mold, molds that spoil foods, and the fungus that causes zygomycosis
  • Pilobolus
    • Produces specialized spore-bearing hyphae with fluid-filled sacs that blast spores up to 2 meters



Diploid (2n)

Fusion of nuclei




Haploid (n)


young zygospore

between hyphae of

its parents


Cytoplasmic fusion




Figure 23-5 p382

microsporidia intracellular parasites
Microsporidia – Intracellular Parasites
  • Microsporidia are single-celled intracellular parasites
  • Most infect animals, usually fish and insects
  • A microsporidium spore has a tough coat enclosing a long coiled tube that uncoils and perforates a host cell
  • One microsporidium (Nosema ceranae) has been implicated in honeybee colony collapse disorder
glomeromycetes partners of plants
Glomeromycetes – Partners of Plants
  • Glomeromycetes take part in plant root–fungus partnerships called mycorrhizae
  • A glomeromycete hypha grows into a root and branches inside the wall of a root cell
  • The fungus shares nutrients from the soil with its host


plant root

hypha branching inside a plant cell wall

Figure 23-8a p383

take home message what are zygote fungi and their relatives
Take-Home Message: What are zygote fungi and their relatives?
  • Zygote fungi form a thick-walled diploid spore when they reproduce sexually
  • Some spoil food or cause disease
  • Microsporidia are single-celled parasites that invade an animal cell by way of a polar tube
  • Glomeromycetes are mycorrhizal fungi; they partner with plants
23 5 sac fungi ascomycetes
23.5 Sac Fungi—Ascomycetes
  • Sac fungi are the most diverse fungal group
  • Some are single cells (yeasts), but in most a haploid mycelium dominates the life cycle
  • The hyphae have cross-walls at regular intervals and often form elaborate spore-producing bodies
  • Sac fungi are the group that most often causes diseases in humans
asexual reproduction
Asexual Reproduction
  • Sac fungi commonly reproduce by asexual mechanisms
  • Yeasts usually reproduce asexually by binary fission or by budding
  • Multicelled sac fungi grow as a haploid mycelium that produces spores by mitosis at the tips of specialized hyphae
sexual reproduction
Sexual Reproduction
  • Two haploid hyphae undergo cytoplasmic fusion – the dikaryotic cell divides by mitosis to produce dikaryotic hyphae
  • Dikaryotic hyphae intertwine with haploid hyphae to form a spore-producing structure called an ascocarp
  • Spores form by meiosis inside a saclike cell called an ascus
  • Meiosis of the ascus produces spores
human uses of sac fungi
Human Uses of Sac Fungi
  • Food and beverages
    • Morels and truffles
    • Baking yeast and fermentation (Saccharomyces, Aspergillus), blue cheese (Penicillium)
  • Drugs
    • Antibiotics (Penicillium, Cephalosporium)
    • Statins (Aspergillus)
  • Natural herbicides and pesticides
    • Arthrobotrys
take home message what are sac fungi
Take-Home Message:What are sac fungi?
  • Sac fungi are the most diverse group of fungi; some are yeasts, but a haploid mycelium dominates most life cycles
  • Sac fungi that reproduce sexually typically form spores inside an ascus
  • Yeasts reproduce asexually by budding, and multicelled species by formation of conidia
  • We use sac fungi as sources of food and beverages, as drugs, and as control agents for pests
23 6 club fungi basidiomycetes
23.6 Club Fungi—Basidiomycetes
  • Club fungi make the largest and most elaborate fruiting bodies of all fungi
  • They are typically multicelled fungi in which a dikaryotic mycelium dominates the life cycle
  • They form sexual spores inside club-shaped cells that develop on a fruiting body (basidiocarp) composed of interwoven dikaryotic hyphae

Diploid (2n) stage

Nuclear fusion



(n+n) stage

Haploid (n)


spore (n)






Hyphae of mycelium

Sexual spore (n) at gill margin

Figure 23-13 p386

ecology and diversity
Ecology and Diversity
  • Club fungi are the only decomposers capable of breaking down lignin in plants and trees (e.g. shelf fungus, honey mushroom)
  • Jelly fungi form a parasitic gelatinous fruiting body that taps into the mycelia of other fungi that feed on wood
  • Other club fungi include edible mushrooms (chanterelles, puffballs), poisonous mushrooms (death cap), and hallucinogenic psilocybin-containing mushrooms
take home message what are club fungi
Take-Home Message:What are club fungi?
  • Club fungi are a group of mostly multicelled fungi. Multicelled species spend most of their life cycle as a dikaryotic mycelium and have the largest, most complex spore-bearing structures of all fungi
  • Club fungi are the only fungi that can break down lignin, and they are important in forests both as decomposers and as pathogens of trees
23 7 fungal partners and pathogens
23.7 Fungal Partners and Pathogens
  • Fungi form partnerships with plants and with single-celled photosynthetic species
    • Lichens
    • Fungal endophytes
    • Mycorrhizae
  • Other fungi are pathogens of other species
  • A lichen is a composite organism consisting of a sac fungus and a photosynthetic cyanobacteria or green algae
  • A lichen may be a mutualism that benefits both partners, or the fungus may be parasitically exploiting captive cells
  • Lichens disperse by fragmentation, or the fungus alone can release spores
  • Lichens break down rock by releasing acids, and colonize hostile such as bedrock
lichen structure
Lichen Structure

dispersal fragment

(cells of fungus and of

photosynthetic species)

outer layer

of fungal cells



inner layer of

loosely woven


outer layer

of fungal cells

  • Mycorrhizaeare a partnership between fungi and plant roots
    • Some hyphae form a dense net around roots but do not penetrate them (sac fungi and club fungi)
    • Some hyphae penetrate root cells (glomeromycetes)
  • Hyphae increase the absorptive surface area of their partner
  • Both partners benefit
    • Fungus concentrates nutrients for plant
    • Plant supplies sugars to the fungus

without fungus

with fungus

Figure 23-16b p389

fungal pathogens
Fungal Pathogens
  • Some fungi are plant pathogens
    • Wheat stem rust
    • Powdery mildew
    • Ergotism (alkaloid poisoning, Claviceps)
  • A minority of fungi are human pathogens
    • Athlete’s foot
    • “Ringworm”
    • Vaginal yeast infection (Candida)
take home message what close relationships do fungi have with other species
Take-Home Message: What close relationships do fungi have with other species?
  • Fungi form mutually beneficial partnerships with plants, cyanobacteria, and green algae
  • Some ants, termites, and beetles farm fungus and rely on them for food
  • Some fungi are pathogens that invade the tissues of plants and animals