Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 22
Fungi
(Sections 22.1 - 22.7)
Albia Dugger • Miami Dade College
22.1 High-Flying Fungi
• Fungi produce microscopic spores that winds can disperse
long distances
• As a result, it is difficult to prevent the spread of a fungal
pathogen, such as wheat stem rust
• A new strain of wheat stem rust, Ug99, is able to infect most
wheat varieties that were previously considered resistant
Wheat Stem Rust
• An infection begins
when a spore lands on
a wheat plant and
fungal filaments invade
the plant through its
stomata
22.2 Fungal Traits and Diversity
• Like animals, fungi are heterotrophs
• Unlike animals, fungi have walled cells and digest food
outside their body
• fungus
• Eukaryotic heterotroph with cell walls of chitin
• Obtains nutrients by digesting them outside the body and
absorbing them
Structure and Function
• Most fungi are saprobes that feed on organic remains
• They serve as essential decomposers in most ecosystems
• Other fungi live in or on organisms
• They may be harmless, beneficial, or parasitic
• saprobe
• Organism that feeds on wastes and remains
Structure and Function (cont.)
• Fungi include single-celled yeasts and multicelled species
• They disperse by releasing spores
• In multicelled species, spores germinate and give rise to
filaments (hyphae) which typically grow as an extensive
mesh called a mycelium
Key Terms
• hypha
• Component of a fungal mycelium
• A filament made up of cells arranged end to end
• There may or may not be cross-walls (septa) between
cells of a hypha
• mycelium
• Mass of threadlike filaments (hyphae) that make up the
body of a multicelled fungus
Examples of Multicelled Fungi
• Green mold on
grapefruit
• Scarlet hood mushroom
Mycelium
• Close-up of a mycelium,
showing threadlike
hyphae, each consisting
of cells arranged end to
end
Mycelium
one cell of a hypha
in the mycelium
Fig. 22.2, p. 344
Life Cycles
• In fungi, only the zygote is diploid
• Spores may form by mitosis (asexual spores) or by meiosis
(sexual spores)
• Depending on the group, cells of a hyphae may be haploid (n)
or dikaryotic (n+n)
• dikaryotic
• Having two genetically distinct nuclei
Fungal Diversity
• In early lineages (chytrids, glomeromycetes, zygote fungi),
hyphae have few or no cross-walls (septa)
• More recently evolved lineages (sac fungi, club fungi) have
hyphae with porous crosswalls at regular intervals
• In all groups, water and nutrients move freely between cells of
a hypha
Key Concepts
• Traits and Classification
• Fungi are single-celled and multicelled heterotrophs that
feed by secreting digestive enzymes onto organic matter,
then absorbing the released nutrients
• Multicelled species form a mesh of absorptive filaments
• In the most diverse groups, cross-walls reinforce the
filaments
ANIMATION: Mycelium
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22.3 Chytrids, Zygote Fungi, and
Relatives
• Chytrids, zygote fungi, and glomeromycetes are the oldest
and least diverse groups of fungi
• chytrid
• Fungus that makes flagellated spores
• zygote fungus
• Fungus that forms a zygospore during sexual reproduction
• glomeromycete
• Fungus with hyphae that grow inside the wall of a plant
root cell
Chytrids
• The oldest fungal lineage, the chytrids are a mostly aquatic
group and the only fungi with flagellated spores
• Chytrids include aquatic decomposers, some that digest
cellulose in herbivore guts, and parasites
• Parasitic chytrid infections are pushing some frog species to
extinction
Zygote Fungi (Zygomycetes)
• Zygote fungi include familiar molds that grow on fruits,
breads, and other foods
• Their hyphae are continuous tubes that have no cross-walls
• Sexual reproduction produces a thick-walled zygospore
• Zygote fungi also produce asexual spores on specialized
hyphae
Life Cycle of a Zygote Fungus
1. Black bread mold (Rhizopus) reproduces asexually as it
grows through a slice of bread
2. Sexual reproduction begins when hyphae of two different
mating strains meet
3. Cytoplasmic fusion of hyphal cells yields an immature
zygospore that is dikaryotic
4. Fusion of the zygospore’s haploid nuclei produces the mature
diploid zygospore
5. The zygospore undergoes meiosis, germinates, and produces
an aerial hypha with haploid spores at its tip; each spore can
give rise to a new mycelium
Life Cycle of a Zygote Fungus
Life Cycle of a Zygote
Fungus
4
Diploid (2n)
Fusion of nuclei
Dikaryotic
(n+n)
3
mature
zygospore
Meiosis
Haploid (n)
5
spores
young zygospore
Cytoplasmic fusion
mycelium
1
2
asexual
reproduction
Fig. 22.3, p. 345
Life Cycle of a Zygote
Fungus
4
Diploid (2n)
Fusion of nuclei
Dikaryotic
(n+n)
3
mature
zygospore
Meiosis
Haploid (n)
5
spores
young zygospore
Cytoplasmic fusion
mycelium
1
2
asexual
reproduction
Stepped Art
Fig. 22.3, p. 345
ANIMATION: Zygomycete life cycle
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Glomeromycetes
• Glomeromycetes live in
soil and extend their
hyphae into plant roots
• The hyphae branch
inside plant cell walls
and deliver nutrients to
the cell and take up
plant sugars
Key Concepts
• The Oldest Lineages
• The oldest fungal lineages have filaments with few or no
cross-walls
• The mostly aquatic chytrid fungi make flagellated spores
• Zygote fungi make thick-walled zygotes that release
sexual spores
• Glomeromycetes are fungi that branch inside plant cell
walls
22.4 Sac Fungi
• Sac fungi are the most diverse fungal group
• They include single-celled yeasts and multicelled species
such as cup fungi and morels
• sac fungi
• Most diverse fungal group; sexual reproduction produces
ascospores inside a saclike structure (an ascus)
Life Cycle
• Sac fungi produce sexual spores in asci
• In multicelled species, these saclike structures form on an
ascocarp, a fruiting body composed of haploid and dikaryotic
hyphae that have internal cross-walls at regular intervals
Sac Fungus Fruiting Bodies
• The cup of the scarlet
cup fungus is an
ascocarp
• Asci, each containing
eight ascospores
(sexual spores) form on
the cup’s inner surface
A Sampling of Diversity
• Yeasts that are sac fungi help us make wine, breads, and
alcoholic beverages
• The antibiotic penicillin was originally isolated from a mold
(Penicillium) that is a sac fungus
• Related molds are used to flavor cheeses
Edible Ascocarps
• Morels and truffles are
edible ascocarps
• Morels develop
aboveground
• Truffles form
underground
A Predatory Sac Fungus
• Arthrobotrys captures and feeds on roundworms; rings on the
hyphae constrict and entrap the worms, then hyphae grow
into the captive and digest it
A Predatory Sac Fungus
part of one hypha that forms a nooselike ring
roundworm
Fig. 22.6, p. 346
VIDEO: Predatory Fungus
Key Concepts
• Sac Fungi
• Sac fungi, the most diverse fungal lineage, include single
cells such as yeasts used in baking, and multicelled
species that have filaments reinforced with cross-walls
• When sac fungi reproduce sexually, they produce spores
inside a saclike structure
22.5 Club Fungi
• Like sac fungi, multicelled club fungi have hyphae with crosswalls; they can produce complex fruiting bodies
(basidiocarps) such as mushrooms
• Club fungi are the only fungi that can break down lignin in
wood, and are important as decomposers in forest habitats
• club fungi
• Fungi that produce spores in club-shaped cells
Club Fungus Fruiting Bodies
Club Fungus Fruiting Bodies
Puffball
Fig. 22.7a, p. 347
Club Fungus Fruiting Bodies
Shelf fungus
Fig. 22.7b, p. 347
Club Fungus Fruiting Bodies
Coral fungus
Fig. 22.7c, p. 347
Club Fungus Fruiting Bodies
Chanterelles
Fig. 22.7d, p. 347
Club Fungus Life Cycle
1. Haploid hyphal cells meet; cytoplasm fuses and forms a
dikaryotic (n+n) cell
2. Mitotic cell divisions form a mycelium; produces a mushroom
3. Spore-making cells form at edges of the mushroom’s gills
4. Inside these dikaryotic cells, nuclei fuse, producing diploid
(2n) cells
5. Diploid cells undergo meiosis, forming haploid (n) spores
6. Spores are released; give rise to a new haploid mycelium
Club Fungus Life Cycle
Club
Fungus
Life
Cycle
4
Fusion of nuclei
zygote
Diploid (2n)
Dikaryotic
Haploid (n)
(n+n)
Meiosis
5
spore (n)
3
gill
cap
6
stalk
1
2
Cytoplasmic
fusion
Fig. 22.8, p. 347
Club
Fungus
Life
Cycle
4
zygote
Diploid (2n)
Fusion of nuclei
Meiosis
Dikaryotic
(n+n)
Haploid (n)
55
spore (n)
3
gill
cap
6
stalk
1
2
Cytoplasmic
fusion
Stepped Art
Fig. 22.8, p. 347
ANIMATION: Club fungus life cycle
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Key Concepts
• Club Fungi
• Most club fungi are multicelled
• Like the closely related sac fungi, they have filaments
reinforced with cross walls
• Sexual reproduction involves formation of spores in clubshaped cells
• A mushroom is an example of a club fungus fruiting body
22.6 Fungi as Partners
• Many fungal species are symbiotic with photosynthetic cells in
lichens, while others are symbionts that live in or on plants
• lichen
• Composite organisms, consisting of a fungus and a single
celled alga or a cyanobacterium
• mycorrhiza
• Mutually beneficial partnership between a fungus and a
plant root
Lichens
• A lichen is a composite organism composed of a fungus and
photosynthetic cells of a green alga or cyanobacterium
• A lichen may be a mutualism or the fungus may be
parasitically exploiting captive photosynthetic cells
• mutualism
• Mutually beneficial relationship between two species
Lichens (cont.)
• The fungus makes up the bulk of the lichen body and obtains
a supply of nutrients from its photosynthetic partner
• Lichens disperse by releasing fragments that include cells of
both partners – the fungus also produces spores
• Lichens are important as pioneers in new habitats; they
facilitate the breakdown of rock to form soil
Lichens
Lichens
dispersal fragment
(cells of fungus and of
photosynthetic species)
outer layer
of fungal cells
photosynthetic
species
inner layer of
loosely woven
hyphae
A
B
outer layer
of fungal cells
C
Fig. 22.9, p. 348
Lichens
Fig. 22.9a, p. 348
Lichens
Fig. 22.9b, p. 348
Lichens
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
B
Fig. 22.9b, p. 348
Lichens
Fig. 22.9c, p. 348
ANIMATION: Lichens
Mycorrhizae: Fungus + Roots
• Most plants have mycorrhizae (mutualism between a plant
root and a fungus) and grow better with them
• Fungal hyphae surround or penetrate the roots and
supplement the root’s absorptive surface area
• The fungus shares absorbed mineral ions with the plant and
gets some photosynthetic sugars in return
Mycorrhiza
Mycorrhiza
young
root
hyphal
strands
A
Fig. 22.10a, p. 348
Effects of Mycorrhizae
• Seedlings grown without and with a partner fungus
Key Concepts
• Interactions With Other Organisms
• Some fungi partner with photosynthetic cells as lichens
• Others live in or on plant roots and benefit the plant by
sharing nutrients
• A minority of fungi are parasites
• Some of these cause diseases in plants or in humans
22.7 Fungi as Pathogens
• Some fungi are pathogens that attack living plants or animals
• Fungi that feed on plants can be ecologically and
economically important
• In humans, fungal infections usually involve the skin, or are
inhaled
Plant Pathogens
• Fungal pathogens cause wheat stem rust, powdery mildew of
roses, and killed American chestnuts
• One sac fungus that infects cereal grains makes a toxin that
causes ergotism, a type of food poisoning that causes
hallucinations, hysteria, and convulsions
Ergotism
• Grains of rye with
spore-bearing
structures of Claviceps
purpurea
• Eating grain infected by
this species causes
ergotism
Human Pathogens
• Inhaled fungal spores can cause histoplasmosis or
coccidioidomycosis, which can be fatal to people with a
weakened immune system
• Fungal skin infections include “athletes foot” and “ringworm”
• Yeast infections of the vagina are also common
“Athlete's Foot”
• Fungal skin
infections include
“athletes foot”
High-Flying Fungi (revisited)
• In 2006, sac fungus spores (Fusarium) in contact lens
solution caused eye infections that resulted in cornea surgery
for many patients
• Fusarium species have been found in air samples collected
using remote-controlled model aircraft