Chapter 31

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 31
Fungi

• Overview: Mighty Mushrooms
• Fungi
– Are diverse and widespread
– Are essential for the well-being of most terrestrial
ecosystems because they break down organic
material and recycle vital nutrients
Figure 31.1

• Concept 31.1: Fungi are heterotrophs that feed
by absorption
• Despite their diversity
– Fungi share some key traits

Nutrition and Fungal Lifestyles
• Fungi are heterotrophs
– But do not ingest their food
• Fungi secrete into their surroundings
exoenzymes that break down complex
molecules
– And then absorb the remaining smaller
compounds

• Fungi exhibit diverse lifestyles
– Decomposers
– Parasites
– Mutualistic symbionts

Body Structure
• The morphology of multicellular fungi
– Enhances their ability to absorb nutrients from
their surroundings
Hyphae. The mushroom and its
subterranean mycelium are a continuous
network of hyphae.
Reproductive structure.
The mushroom produces
tiny cells called spores.
Spore-producing
structures
20 µm
Mycelium
Figure 31.2

• Fungi consist of
– Mycelia, networks of branched hyphae
adapted for absorption
• Most fungi
– Have cell walls made of chitin

• Some fungi
– Have hyphae divided into cells by septa, with
pores allowing cell-to-cell movement of
materials
• Coenocytic fungi
– Lack septa
Nuclei
Cell wall
Septum
Pore
(a) Septate hypha (b) Coenocytic hypha
Cell wall
Nuclei
Figure 31.3a, b

• Some unique fungi
– Have specialized hyphae that allow them to
penetrate the tissues of their host
Nematode Hyphae 25 µm
(a) Hyphae adapted for trapping and killing prey
(b) Haustoria
Fungal hypha Plant
cell
wall
Haustorium
Plant cell
plasma
membrane
Plant cell
Figure 31.4a, b

• Mycorrhizae
– Are mutually beneficial relationships between
fungi and plant roots

• Concept 31.2: Fungi produce spores through
sexual or asexual life cycles
• Fungi propagate themselves
– By producing vast numbers of spores, either
sexually or asexually

• The generalized life cycle of fungi
Key
Haploid (n)
Heterokaryotic
(unfused nuclei from
different parents)
Diploid (2n)
PLASMOGAMY
(fusion of cytoplasm)
Heterokaryotic
stage
KARYOGAMY
(fusion of nuclei)
SEXUAL
REPRODUCTION
Spore-producing
structures
Spores
ASEXUAL
REPRODUCTION
Zygote
Mycelium
GERMINATION
GERMINATION
MEIOSIS
Spore-producing
structures
Spores
Figure 31.5

Sexual Reproduction
• The sexual life cycle involves
– Cell fusion, plasmogamy
– Nuclear fusion, karyogamy
• An intervening heterokaryotic stage
– Occurs between plasmogamy and karyogamy
in which cells have haploid nuclei from two
parents

• The diploid phase following karyogamy
– Is short-lived and undergoes meiosis,
producing haploid spores

Asexual Reproduction
• Many fungi can reproduce asexually

• Many fungi that can reproduce asexually
– Grow as mold, sometimes on fruit, bread, and
other foods
2.5 µm
Figure 31.6

• Other asexual fungi are yeasts
– That inhabit moist environments
– Which produce by simple cell division
10 µm
Parent cell
Bud
Figure 31.7

• Many molds and yeasts have no known sexual
stage
– Mycologists have traditionally called these
deuteromycetes, or imperfect fungi

• Concept 31.3: Fungi descended from an
aquatic, single-celled, flagellated protist
• Systematists now recognize Fungi and
Animalia as sister kingdoms
– Because fungi and animals are more closely
related to each other than they are to plants or
other eukaryotes

The Origin of Fungi
• Molecular evidence
– Supports the hypothesis that fungi and animals
diverged from a common ancestor that was
unicellular and bore flagella
• Fungi probably evolved
– Before the colonization of land by multicellular
organisms

• The oldest undisputed fossils of fungi
– Are only about 460 million years old
50 µm
Figure 31.8

The Move to Land
• Fungi were among the earliest colonizers of
land
– Probably as symbionts with early land plants

• Concept 31.4: Fungi have radiated into a
diverse set of lineages
• The phylogeny of fungi
– Is currently the subject of much research
• Molecular analysis
– Has helped clarify the evolutionary
relationships between fungal groups, although
there are still areas of uncertainty

• The phylogeny of fungi
Chytrids
Zygote
fungi
Arbuscular
mycorrhizal
fungi
Sac
fungi
Club
fungi
Chytridiomycota
Zygomycota
Glomeromycota
Ascomycota
BasidiomycotaFigure 31.9

• A review of fungal phyla
Table 31.1

Chytrids
• Fungi classified in the phylum Chytridiomycota,
or chytrids
– Are found in freshwater and terrestrial habitats
– Can be saprobic or parasitic

• Chytrids are unique among fungi
– In having flagellated spores, called zoospores
25 µm
4 µm
Hyphae
Flagellum
Figure 31.10

• Until recently, systematists thought that
– Fungi lost flagella only once in their history
• Molecular data
– Indicate that some “chytrids” are actually more
closely related to another fungal group, the
zygomycetes
Some
chytrids
Zygomycetes and other chytrids
Glomeromycetes,
ascomycetes, and
basidiomycetes
Common ancestor
Key
Loss of
flagella
Figure
31.11

Zygomycetes
• Fungi in the phylum Zygomycota, the
zygomycetes
– Exhibit a considerable diversity of life histories
– Include fast-growing molds, parasites, and
commensal symbionts
– Are named for their sexually produced
zygosporangia

Rhizopus
growing
on bread
ASEXUAL
REPRODUCTION
Mycelium
Dispersal and
germination
MEIOSIS
KARYOGAMY
PLASMOGAMY
Key
Haploid (n)
Heterokaryotic (n + n)
Diploid
Sporangium
Diploid
nuclei
Zygosporangium
(heterokaryotic)
100 µm
Young
zygosporangium
(heterokaryotic)
SEXUAL
REPRODUCTION
Dispersal and
germination
Mating
type (+)
Mating
type (−)
Gametangia with
haploid nuclei
50 µm
Sporangia
• The life cycle of Rhizopus stolonifer
– Is fairly typical of zygomycetes
Mycelia have
various mating types
(here designated +,
with red nuclei, and −,
with blue nuclei).
1
Neighboring mycelia of different
mating types form hyphal extensions
called gametangia, each walled off
around several haploid nuclei by a septum.
2
A heterokaryotic
zygosporangium
forms, containing
multiple haploid
nuclei from the two
parents.
3
The sporangium
disperses genetically
diverse, haploid spores.
7
4
This cell develops a
rough, thick-walled
coating that can resist
dry environments and
other harsh conditions
for months.
5
When conditions are favourable,
karyogamy occurs, followed by
meiosis.6
The zygosporangium
then breaks dormancy,
germinating into a
short sporangium.
The spores
germinate and
grow into new
mycelia.
8
9
Mycelia can also reproduce
asexually by forming sporangia
that produce genetically identical haploid spores.
Figure 31.12

• Some zygomycetes, such as Pilobolus
– Can actually “aim” their sporangia toward
conditions associated with good food sources
0.5 mm
Figure 31.13

• Zygosporangia, which are resistant to freezing
and drying
– Are capable of persisting through unfavorable
conditions
– Can undergo meiosis when conditions improve

Microsporidia
• Microsporidia
– Are unicellular parasites of animals and
protists
– Are now classified as zygomycetes
10µm
Host cell
nucleus
Developing
microsporidian
Spore
Figure 31.14

Glomeromycetes
• Fungi assigned to the phylum Glomeromycota
– Were once considered zygomycetes
– Are now classified in a separate clade

• All glomeromycetes
– Form a distinct type of endomycorrhizae called
arbuscular mycorrhizae
2.5 µm
Figure 31.15

Ascomycetes
• Fungi in the phylum Ascomycota
– Are found in a variety of marine, freshwater,
and terrestrial habitats
– Are defined by the production of sexual spores
in saclike asci, which are usually contained in
fruiting bodies called ascocarps

• Ascomycetes
– Vary in size and complexity from unicellular
yeasts to elaborate cup fungi and morels
(a) The cup-shaped ascocarps (fruiting bodies)
of Aleuria aurantia give this species its
common name: orange peel fungus.
(b) The edible ascocarp of
Morchella esculenta, the
succulent morel, is often
found under trees in orchards.
(c) Tuber melanosporum is a truffle, an ascocarp that grows
underground and emits strong odors. These ascocarps
have been dug up and the middle one sliced open.
(d) Neurospora crassa feeds as
a mold on bread and other
food (SEM).
10 µm
Figure 31.16a–d

• Ascomycetes reproduce
– Asexually by producing enormous numbers of
asexual spores called conidia

• The life cycle of Neurospora crassa, an
ascomycete
Dispersal
ASEXUAL
REPRODUCTION
Germination
Mycelium
Conidiophore
Germination
Dispersal
Mycelia
Asci
Eight
ascospores
Ascocarp
Four
haploid
nuclei
MEIOSIS
KARYOGAMY
PLASMOGAMY
SEXUAL
REPRODUCTION
Diploid nucleus
(zygote)
Ascogonium Ascus
(dikaryotic)
Dikaryotic
hyphae
Mating
type (+)
Conidia;
mating type (−)
Key
Haploid (n)
Dikaryotic (n + n)
Diploid (2n)
Ascomycete mycelia
can also reproduce
asexually by producing
haploid conidia.
7 Neurospora can reproduce
sexually by producing specialized
hyphae. Conidia of the opposite
mating type fuse to these hyphae.
1
A dikaryotic
ascus develops.
2
Karyogamy
occurs within the
ascus, producing a
diploid nucleus.
3
The diploid nucleus
divides by meiosis, yielding
four haploid nuclei.
4
The developing asci
are contained in an
ascocarp. The ascospores
are discharged forcibly
from the asci through an
opening in the ascocarp.
Germinating ascospores
give rise to new mycelia.
6
5 Each haploid nucleus divides
once by mitosis, yielding eight
nuclei. Cell walls develop around
the nuclei, forming ascospores (LM).Figure 31.17

Basidiomycetes
• Fungi in the phylum Basidiomycota
– Include mushrooms and shelf fungi
– Are defined by a clublike structure called a
basidium, a transient diploid stage in the life
cycle

• Basidiomycetes
(a) Fly agaric (Amanita muscaria), a
common species in conifer forests in
the northern hemisphere
(b) Maiden veil fungus (Dictyphora),
a fungus with an odor like rotting
meat
(c) Shelf fungi, important decomposers of
wood
(d) Puffballs emitting spores
Figure 31.18a–d

• The life cycle of a basidiomycete
– Usually includes a long-lived dikaryotic
mycelium, which can erect its fruiting structure,
a mushroom, in just a few hours
Figure 31.19

PLASMOGAMY
Dikaryotic
mycelium
Basidiocarp
(dikaryotic)
KARYOGAMY
Key
MEIOSIS
Gills lined
with basidiaSEXUAL
REPRODUCTION
Mating
type (−)
Mating
type (+)
Haploid
mycelia
Dispersal
and
germination
Basidiospores
Basidium with
four appendages
Basidium containing
four haploid nuclei
Basidia
(dikaryotic)
Diploid
nuclei
Basidiospore1 µm
Basidium
Haploid (n)
Dikaryotic (n + n)
Diploid (2n)
• The life cycle of a mushroom-forming
basidiomycete
Each diploid nucleus
yields four haploid
nuclei. Each basidium
grows four appendages,
and one haploid nucleus
enters each appendage
and develops into a
basidiospore (SEM).
6
Two haploid mycelia
of different mating types
undergo plasmogamy.
1
A dikaryotic mycelium forms,
growing faster then, and ultimately
crowding out, the haploid parental mycelia.
2
3 Environmental
cues such as rain or
temperature changes
induce the dikaryotic
mycelium to form
compact masses that
develop into
basidiocarps
(mushrooms, in this
case).
The basidiocarp
gills are lined with
terminal dikaryotic
cells called basidia.
4
Karyogamy in the
basidia produces diploid
nuclei, which then
undergo meiosis.
5
When mature,
the basidiospores
are ejected, fall
from the cap, and
are dispersed by
the wind.
7
In a suitable
environment, the
basidiospores
germinate and
grow into
short-lived
haploid mycelia.
8
Figure 31.20

• Concept 31.5: Fungi have a powerful impact on
ecosystems and human welfare

Decomposers
• Fungi are well adapted as decomposers of
organic material
– Performing essential recycling of chemical
elements between the living and nonliving
world

Symbionts
• Fungi form symbiotic relationships with
– Plants, algae, and animals

Mycorrhizae
• Mycorrhizae
– Are enormously important in natural
ecosystems and agriculture
– Increase plant productivity
RESULTS
Researchers grew soybean plants in soil treated with fungicide (poison that kills fungi) to
prevent the formation of mycorrhizae in the experimental group. A control group was exposed to fungi that formed
mycorrhizae in the soybean plants’ roots.
EXPERIMENT
The soybean plant on the left is typical of the experimental group. Its
stunted growth is probably due to a phosphorus deficiency. The taller, healthier plant on
the right is typical of the control group and has mycorrhizae.
CONCLUSION These results indicate that the presence of mycorrhizae benefits a soybean
plant and support the hypothesis that mycorrhizae enhance the plant’s ability to take up
phosphate and other needed minerals.Figure 31.21
RESULTS

Fungus-Animal Symbiosis
• Some fungi share their digestive services with
animals
– Helping break down plant material in the guts
of cows and other grazing mammals

• Many species of ants and termites
– Take advantage of the digestive power of fungi
by raising them in “farms”
Figure 31.22

Lichens
• Lichens
– Are a symbiotic association of millions of
photosynthetic microorganisms held in a mass
of fungal hyphae
(a) A fruticose (shrub-like) lichen
(b) A foliose (leaf-like) lichen (c) Crustose (crust-like) lichensFigure 31.23a–c

• The fungal component of a lichen
– Is most often an ascomycete
• Algae or cyanobacteria
– Occupy an inner layer below the lichen surface
Ascocarp of fungus
Fungal
hyphae
Algal
layer
Soredia
Algal cell
Fungal hyphae
10µm
Figure 31.24

Pathogens
• About 30% of known fungal species
– Are parasites, mostly on or in plants
(a) Corn smut on corn (b) Tar spot fungus on maple leaves (c) Ergots on rye
Figure 31.25a–c

• Some of the fungi that attack food crops
– Are toxic to humans

Practical Uses of Fungi
• Humans eat many fungi
– And use others to make cheeses, alcoholic
beverages, and bread

• Antibiotics produced by fungi
– Treat bacterial infections
Staphylococcus
Penicillium
Zone of
inhibited
growth
Figure 31.26

• Genetic research on fungi
– Is leading to applications in biotechnology

Chapter 31

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Chapter 31