Chapter 23
Fungi
Albia Dugger • Miami Dade College
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
Sporangia of Wheat Stem Rust
Developing Fungus-Resistant Wheat
23.2 Fungal Traits and Classification
• Fungi are 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
• 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
• 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
Multicelled Fungi
Hyphae
• 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
one cell (part of
one hypha of
the mycelium)
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 dikaryotic stage 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
Sexual Phase of the Fungal Life Cycle
dikaryotic
(n+n) stage
Fusion of nuclei
Fusion of cytoplasm
diploid (2n) zygote
haploid (n) mycelium
Meiosis
spores (n)
spore-producing
structure (n)
Phylogeny and Classification
• Chytrids, zygote fungi (zygomycetes), and glomeromycetes
are not monophyletic groups
• No dikaryotic stage
• Few or no septae between cells of hyphae
• Sac fungi (ascomycetes) and club fungi (basidiomycetes) are
monophyletic groups
• Dikaryotic mycelium
• Regular cross walls between cells of hyphae
Table 23-1 p381
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
• 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
• 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
Chytridiomycosis
Harlequin Frog
Take-Home Message:
What are chytrids?
• Chytrids are fungi that produce flagellated spores
• Most are decomposers in soil, but some live in the animal gut,
and some are parasites
• A parasitic species that kills amphibians is a matter of
concern
23.4 Zygote Fungi and Relatives
• Only zygote fungi (zygomycetes) produce a thick-walled
diploid spore (zygospore) during sexual reproduction
• Zygote fungi form a branching haploid mycelium on organic
material, and inside living plants and animals
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 fluidfilled sacs that blast spores up to 2 meters
mature
zygospore
Diploid (2n)
Fusion of nuclei
Dikaryotic
(n+n)
Haploid (n)
Meiosis
spores
young zygospore
between hyphae of
its parents
mycelium
Cytoplasmic fusion
asexual
reproduction
gametangia
Figure 23-5 p382
ANIMATED FIGURE: Rhizopus
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Spore Bearing Structures of Pilobolus
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
A Microsporidian Spore
thin region of spore
coat though which
polar tube will be
extruded
nucleus
coiled
polar tube
vacuole
Microsporidian with Polar Tube Extruded
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
sporangium
plant root
hypha branching inside a plant cell wall
Figure 23-8a p383
Figure 23-8b p383
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
• 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
• 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
Asexual Reproduction in Sac Fungi
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
Ascocarp: Scarlet Cup Fungus
Ascospores: Scarlet Cup Fungus
haploid
spore in
ascus
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
Morels
Truffles
part of one hypha that forms a nooselike ring
roundworm
Figure 23-12 p385
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
• 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
Nuclear fusion
Diploid (2n) stage
Dikaryotic
(n+n) stage
Meiosis
Haploid (n)
stage
spore (n)
gill
cap
stalk
Cytoplasmic
fusion
Hyphae of
mycelium
Sexual spore (n)
at gill margin
Figure 23-13 p386
ANIMATED FIGURE: Club fungus life cycle
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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
Club Fungus Diversity
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
• Fungi form partnerships with plants and with single-celled
photosynthetic species
• Lichens
• Fungal endophytes
• Mycorrhizae
• Other fungi are pathogens of other species
Lichens
• 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
Leaflike and Pendant Lichens
Encrusting Lichen
Lichen Structure
dispersal fragment
(cells of fungus and of
photosynthetic species)
outer layer
of fungal cells
photosynthetic
species
inner layer of
loosely woven
hyphae
outer layer
of fungal cells
ANIMATED FIGURE: Lichens
Mycorrhizae
• Mycorrhizae are 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
young
root
hyphal
strands
A
Figure 23-16a p389
without fungus
with fungus
Figure 23-16b p389
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)
Rye with Ascocarps of Claviceps
Athlete’s Foot
Table 23-2 p391
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