22
The Evolution
and Diversity of Fungi
Chapter 22 The Evolution and Diversity of Fungi
Key Concepts
• 22.1 Fungi Live by Absorptive Heterotrophy
• 22.2 Fungi Can Be Saprobic, Parasitic,
Predatory, or Mutualistic
• 22.3 Major Groups of Fungi Differ in Their
Life Cycles
• 22.4 Fungi Can Be Sensitive Indicators of
Environmental Change
Chapter 22 Opening Question
Have antibiotics derived from fungi
eliminated the danger of bacterial
diseases in human populations?
Concept 22.1 Fungi Live by Absorptive Heterotrophy
Fungi live by absorptive heterotrophy:
Digestive enzymes are secreted to break down
large food molecules in the environment.
The smaller molecules are absorbed into the
cells.
Concept 22.1 Fungi Live by Absorptive Heterotrophy
Saprobes absorb nutrients from dead organic
matter.
Parasites absorb nutrients from living hosts.
Mutualists live in intimate associations with other
organisms that benefit both partners.
Concept 22.1 Fungi Live by Absorptive Heterotrophy
Modern fungi probably evolved from a unicellular
protist with a flagellum.
Current evidence from gene sequencing
suggests that fungi, choanoflagellates, and
animals share a common ancestor.
Collectively called opisthokonts, if flagella are
present, they are posterior.
Figure 22.1 Fungi in Evolutionary Context
Concept 22.1 Fungi Live by Absorptive Heterotrophy
Most fungi are multicellular, but single-celled
species (yeasts) occur in most groups.
“Yeast” refers to a lifestyle that has evolved
several times.
Yeasts are used in the laboratory as model
organisms for eukaryotes.
Figure 22.2 Yeasts
Concept 22.1 Fungi Live by Absorptive Heterotrophy
Multicellular fungi:
Body is a mycelium, a mass of individual tubular
filaments called hyphae.
The cell walls are strengthened by the
polysaccharide chitin.
Figure 22.3 Mycelia Are Made Up of Hyphae (Part 1)
Figure 22.3 Mycelia Are Made Up of Hyphae (Part 2)
Concept 22.1 Fungi Live by Absorptive Heterotrophy
Septate species—hyphae are subdivided by
incomplete crosswalls called septa. Organelles
can move between compartments.
Some species are coenocytic—no septa, but
many nuclei (from mitosis without cytokinesis).
Figure 22.3 Mycelia Are Made Up of Hyphae (Part 3)
Concept 22.1 Fungi Live by Absorptive Heterotrophy
Mycelia can grow very fast, and may cover a
wide area to forage for nutrients.
Some species produce sexual spores in fruiting
structures (e.g., mushrooms).
The mycelial mass is often far larger than the
mushroom.
Rhizoids—modified hyphae; anchor some fungi
to their substrate.
Concept 22.1 Fungi Live by Absorptive Heterotrophy
Mycelia have very large surface area-to-volume
ratio—excellent for absorptive heterotrophy.
But they can dry out rapidly. Fungi are more
common in moist areas.
Some fungi can live in hypertonic environments,
and some can tolerate temperature extremes.
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Fungi are very important to ecosystem
functioning.
They decompose dead organisms and wastes
and recycle mineral nutrients.
Fungi are the main decomposers of cellulose
and keratin.
Without fungi, the carbon cycle would fail. Most
carbon would be buried instead of being
returned to the atmosphere as CO2.
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
During the Carboniferous period, saprobic fungi
declined dramatically.
Dead plants in the swamps built up into peat,
which eventually formed coal deposits.
But fungi flourished through the extinctions that
marked the end of the Permian.
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
When food becomes scarce, fungi produce
spores that can remain dormant until conditions
improve or be dispersed.
Spores are extremely tiny and easily spread by
wind or water.
They can spread over great distances, and at
least some will find conditions suitable for
growth.
Figure 22.4 Spores Galore
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Parasitic fungi:
Facultative parasites can grow on living
organisms, or by themselves.
Obligate parasites can grow only on their
specific living host.
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Hyphae are well suited to absorbing nutrients
from living plants.
Hyphae can enter through stomata, wounds, or
by direct penetration of epidermal cell walls.
Some produce haustoria, branching projections
that push through cell walls, invaginate into the
cell membrane, and absorb nutrients.
Figure 22.5 Invading a Leaf (Part 1)
Figure 22.5 Invading a Leaf (Part 2)
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Pathogens:
Fungi are especially lethal to people with
comprised immune systems, such as AIDS
patients.
Fungi also cause less threatening problems such
as ringworm and athlete’s foot.
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Amphibian species around the world have been
attacked by a chytrid fungus.
Originating in South Africa, it may have spread
worldwide with the African clawed frog, which
was once used in human pregnancy tests.
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Fungi are the most important plant pathogens,
causing crop losses amounting to billions of
dollars.
Example: black stem rust of wheat
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Predatory fungi:
Some fungi trap microscopic protists or animals.
They secrete sticky substances and hyphae
quickly grow into trapped prey.
Some soil fungi can form a ring that a nematode
enters, then the cells of the ring swell and trap
the nematode.
Figure 22.6 Fungus as Predator
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Symbiotic relationships—the partners live in
close, permanent contact with each other.
Mutualistic—the relationship benefits both
partners.
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Lichens: associations of a fungus with a
cyanobacterium, a photosynthetic alga, or both.
“Species” are assigned the name of the fungal
component. Most have never been found in
nature without the photosynthetic partner.
Lichens can grow on exposed surfaces such as
rocks and can live in harsh environments.
Figure 22.7 Lichen Body Forms (Part 1)
Figure 22.7 Lichen Body Forms (Part 2)
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Fungal hyphae of the lichen absorb mineral
nutrients and provide a moist environment for
the photosynthetic cells.
The fungi receive fixed carbon.
If cultured, the algal cells grow more quickly on
their own, but in the environments where
lichens are found, the algae would not survive
on their own.
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Lichens can reproduce by fragmentation of the
vegetative body;
Or by soredia—one or a few photosynthetic cells
surrounded by hyphae—that disperse on air
currents.
The fungal partner may undergo sexual
reproduction; the spores disperse alone.
Figure 22.8 Lichen Anatomy
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Lichens are often the first colonists on bare rock.
Lichens grow very slowly.
They acidify the environment slightly, which
contributes to rock weathering.
When dry, they become highly insensitive to
extremes of temperature.
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Mycorrhizae—associations of fungi and plant
roots.
Ectomycorrhizae—the fungus wraps around
individual cells in the root but doesn’t penetrate
the cells.
An extensive web of hyphae penetrates the soil
around the root.
The hyphae expand surface area for absorption
of water and minerals.
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Arbuscular mycorrhizae penetrate root cell walls
forming arbuscular (treelike) structures inside
the cell wall but outside the plasma membrane.
The web of hyphae in the soil around the root
increases the absorptive area.
Figure 22.9 Mycorrhizal Associations (Part 1)
Figure 22.9 Mycorrhizal Associations (Part 2)
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
The mycorrhizal fungus obtains organic
compounds from the plant.
The plant’s ability to absorb water and mineral
nutrients is enhanced.
The fungus may also provide some growth
hormones and protect roots from pathogenic
microorganisms.
Many plants grow poorly or not at all without
their mycorrhizal partners.
Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or
Mutualistic
Endophytic fungi live in aboveground parts of
plants, but don’t harm the plant.
The fungi produce alkaloid compounds that are
toxic to animals, which helps protect plant from
herbivores.
Some may not benefit the plant or harm it.
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
There are six major groups of fungi.
Chytrids and zygospore fungi may not be
monophyletic.
Figure 22.10 A Phylogeny of the Fungi
Table 22.1 Classification of the Fungi
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Forms of asexual reproduction:
• Haploid spores produced in sporangia
• Haploid spores (conidia) form at tips of hyphae
• Cell division or budding by yeasts
• Simple breakage of the mycelium
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Sexual reproduction is rare or unknown in some
groups, common in others.
There is no morphological distinction between
female and male individuals, but mating types
are genetically distinct.
Figure 22.11 A Fungal Life Cycle
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Microsporidia:
• Unicellular; obligate intracellular parasites of
animals
• No true mitochondria, but have mitosomes
derived from mitochondria
• Infect insects, crustaceans, fishes, and
mammals, including humans.
• A polar tube grows from the spore, and the
contents of the spore are injected into the host
cell.
Figure 22.12 Invasion of the Microsporidia Spores
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Chytrids:
• Only fungi with flagella at any life stage
• Reproduce both sexually and asexually; some
species have alternation of generations.
• Flagellated spores and flagellated gametes
• May be parasitic or saprobic
Figure 22.13 Sexual Life Cycles of Chytrids and Zygospore Fungi (Part 1)
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
The other four fungal groups are mostly
terrestrial.
No motile gametes.
Cytoplasm of individuals of different mating types
fuse (plasmogamy) before their nuclei fuse
(karyogamy).
Liquid water is not required for fertilization.
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Zygospore fungi:
• Sexual reproduction occurs when adjacent
hyphae of different mating types release
chemical signals (pheromones) and grow
toward each other.
• Fusion results in a unicellular zygospore with
diploid nuclei (a resting stage).
• Zygospore is the only diploid cell in the life
cycle.
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Zygospore nuclei undergo meiosis; a stalked
sporangiophore sprouts, bearing haploid
spores.
Hyphae are coenocytic.
Rhizopus stolonifer, black bread mold, produces
many stalked sporangiophores.
Figure 22.13 Sexual Life Cycles of Chytrids and Zygospore Fungi (Part 2)
Figure 22.14 Zygospore Fungi Produce Sporangiophores
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Arbuscular mycorrhizal fungi:
• Have symbiotic, mutualistic relationship with
80–90% of all plants.
• Hyphae are coenocytic
• Use glucose from plant partners as primary
energy source
• Asexual reproduction only
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
The two remaining fungal clades have a life
stage called a dikaryon.
Plasmogamy (fusion of cytoplasm) results in the
dikaryon stage with 2 genetically different
haploid nuclei within each cell (n + n).
Karyogamy (fusion of nuclei) occurs long after
plasmogamy, in fruiting structures, to give rise
to zygotes.
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Zygote is the only true diploid stage, but genes
in both nuclei of the dikaryon stage can be
expressed.
Dikaryotic hyphae often have characteristics that
are different from their n or 2n products.
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Sac fungi (Ascomycota):
• Many are the fungal partners in lichens
• Hyphae with septa
• Produce haploid spores in sacs called asci
In some species, asci are in a fruiting structure
(ascoma)
Figure 22.16 Sac Fungi (Part 1)
Figure 22.16 Sac Fungi (Part 2)
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Some sac fungi are unicellular yeasts, including
baker’s, or brewer’s, yeast (Saccharomyces
cerevisiae).
They metabolize glucose into ethanol and CO2
by fermentation.
Reproduce by budding and sexual reproduction;
but have lost the dikaryon stage.
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Cup fungi:
Ascomata are cup-shaped
Many are edible; truffles are underground
ascomata—the odor attracts pigs, which eat
them and disperse the fungus.
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Molds:
Many are parasites of flowering plants (e.g.,
Chestnut blight, Dutch elm disease, and
powdery mildew).
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Aspergillus species:
A. tamarii acts on soybeans in production of soy
sauce.
A. oryzae is used in brewing the Japanese
alcoholic beverage sake.
Species that grow on grains and nuts produce
extremely carcinogenic aflatoxins.
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Penicillium species:
Produce the antibiotic penicillin
P. camembertii and P. roquefortii are responsible
for the characteristic flavors of Camembert and
Roquefort cheeses.
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Filamentous sac fungi reproduce asexually by
conidia that form at the tips of specialized
hyphae.
Conidia give molds their characteristic colors.
Figure 22.17 Conidia
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Sexual reproduction includes formation of a
dikaryon.
The dikaryotic mycelium typically forms the cupshaped ascoma, which bears the asci.
Figure 22.15 Sexual Life Cycles among the Dikarya (Part 1)
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Club fungi (Basidiomycota):
The fruiting structures (basidiomata) include
puffballs and mushrooms.
Bracket fungi are saprobic and are important in
the carbon cycle.
Some are plant pathogens, including rusts and
smuts.
Others are fungal partners in ectomycorrhizae.
Figure 22.18 Club Fungus Basidiomata (Part 1)
Figure 22.18 Club Fungus Basidiomata (Part 2)
Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles
Hyphae have septa.
The basidium is a cell at the tip of a specialized
hypha; site of nuclear fusion and meiosis.
The dikaryon stage may persist for years. Some
club fungi live for decades or even centuries.
Figure 22.15 Sexual Life Cycles among the Dikarya (Part 2)
Concept 22.4 Fungi Can Be Sensitive Indicators of Environmental
Change
Lichens are highly sensitive to air pollution—they
can’t excrete toxic substances they absorb.
Lichens are not found in large cities or heavily
industrialized areas.
They can be used to gauge air pollution around
cities and to track pollutants and their effects.
Figure 22.19 More Lichens, Better Air (Part 1)
Figure 22.19 More Lichens, Better Air (Part 2)
Concept 22.4 Fungi Can Be Sensitive Indicators of Environmental
Change
Museum collections of fungi provide a record of
air pollutants over decades or centuries.
They can provide information on pollutants that
existed before people were taking direct
measurements.
Also, effectiveness of cleanup efforts and
regulatory programs for controlling air
pollutants can be monitored.
Concept 22.4 Fungi Can Be Sensitive Indicators of Environmental
Change
When deforestation removes trees, the
populations of mycorrhizal fungi decline
quickly.
Reforestation projects must also restore the
mycorrhizal community.
A planned succession of plant growth and soil
improvement is often necessary before forest
trees can be replanted.
Answer to Opening Question
Many antibiotics are losing their effectiveness as
pathogenic bacteria evolve resistance.
Mutations that allow bacteria to survive
antibiotics are favored by selection whenever
an antibiotic is used.
To reduce rate of evolution of resistance,
antibiotics should be used only for the
treatment of appropriate bacterial diseases.
Figure 22.20 Penicillin Resistance