Chapter 24
Lecture Outline
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A 460 million year old fossil fungus –
one of the oldest known
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Chapter 24
Fungi
Chapter Outline:

Evolutionary Relationships of the Kingdom Fungi

Fungal Bodies and Feeding

Fungal Asexual and Sexual Reproduction

The Importance of Fungi in Ecology and Medicine

Biotechnological Applications of Fungi
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Evolutionary Relationships
of the Kingdom Fungi

Eukaryote supergroup Opisthokonta


Fungi are most closely related to animals, but diverged
over a billion years ago


Includes certain protists, Kingdom Animalia, and Kingdom
Fungi
Fungi arose from protists related to Nuclearia – an amoeba
that feeds by engulfing cells
True fungi are a monophyletic group of over 100,000
species

Does not include slime molds or oomycetes
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Supergroup Opisthokonta
Basidiomycetes
Ascomycetes
AM fungi
Zygomycetes
Microsporidia
Chytrids
Cryptomycota
Nuclearia
(protist)
Choanoflagellates
(protists)
Metazoa
(animal kingdom)
Kingdom Fungi
Septate hyphae,
dikaryotic hyphae,
fruiting bodies
Beneficial associations with
photosynthetic organisms
Primarily terrestrial habitat
Rigid chitin
cell wall,
osmotrophic
nutrition
KEY
Critical innovations
Single flagellum
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BIOLOGY PRINCIPLE
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All species (past and present) are related
by an evolutionary history
There are more than 15 fungal phyla, but
their relationships and names are still being
determined.
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Fungal cell walls

Rigid cell walls are composed of chitin
A
tough, nitrogen-containing carbohydrate
 Except

for the cryptomycota which lack cell walls
Benefit
 Allows
cells to resist high osmotic pressure resulting
from feeding by absorption of small organic molecules

Drawbacks
 Cells
cannot engulf food due to rigid cell walls
 Restricts
mobility of nonflagellated cells
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Seven main groups of fungi







Cryptomycota
Chytrids
Microsporidia
Zygomycetes
AM fungi
Ascomycetes
Basidiomycetes
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Cryptomycota

The earliest-diverging fungi

Occur in soil and water

Can produce flagella for reproduction

Only fungi to lack a cell wall
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Chytrids

Microscopic
aquatic species

Have cell walls
made of chitin

Produce reproductive cells with flagella
 Only
found in chytrids and cryptomycota
 Loss
of flagella linked to ecological transition from
aquatic habitats to land
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Zygomycetes

Several lineages
of terrestrial fungi

Have distinctive large zygotes called zygospores

ex: Common black bread molds
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Microsporidia

Pathogens that
can only reproduce
inside the cells of
an animal host.

Linked to honeybee decline

Very small size (1–4 μm)

Single-celled, chitin-walled spores
 Strong
chitin wall helps survival in the environment
until they enter the bodies of animals
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AM fungi

Arbuscular mycorrhizal
fungi

Close symbiotic associations with plant roots


Fungus provides plant with minerals

Plant provides food for the fungus
Fossils suggest that even early plants may have
depended on these AM fungal associations
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Ascomycetes

Asci – unique reproductive structures

Some ascomycetes cause disease

Ecologically important as decomposers

example: Edible truffles and morels
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Basidiomycetes

Basidia – club-shaped
reproductive cells

Very important
decomposers
and plant symbionts
 ~30,000

species
Varied reproductive structures
 Mushrooms,
puffballs, stinkhorns, shelves, rusts, smuts
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Fungal Bodies and Feeding

Fungi are most closely related to animals and
share several opisthokont features
 Both
heterotrophic – cannot produce their own food
use absorptive nutrition – secrete enzymes
and absorb small organic molecules
 Both
 Both
store surplus food as the carbohydrate
glycogen
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Unique body form

Most have mycelia composed of hyphae
 Most

of the mycelium is diffuse and inconspicuous
Fruiting bodies – visible reproductive structures
 Mushrooms
 Fruiting

are one type
bodies produce spores
Spores
 Chitin-walled
 An
reproductive cells
adaptation to the terrestrial environment
 Transported
by wind or by animals
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Fruiting body
above the
substrate
Mated hyphae
Spores
Mycelium
within substrate
(such as soil)
Region where hyphae mate,
forming a fruiting body
Unmated mycelium
Different unmated mycelium
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BIOLOGY PRINCIPLE
Living organisms grow and develop
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After a mating process occurs,
mated hyphae produce fruiting bodies
whose form fosters spore production
and dispersal.
In suitable sites, spores
may germinate, producing
new mycelia.
Distinctive growth processes

Mycelia grow quickly when food is plentiful
 Grow
from the edges as hyphae extend their tips

Narrow dimensions and extensive branching
provides high surface area for absorption

Osmosis important in growth – entry of water
produces force for tip extension
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(a) Mycelium growing in liquid
medium
(b) Mycelium growing on flat,
solid medium
a: © Agriculture and Agri-Food Canada, Southern Crop Protection and Food Research
Centre, London ON; b: CDC

Mycelium shape depends on substrate

Long extensions in soil to reach food-rich areas

Spherical in liquid medium

Flat disk in petri dish
Fungal Asexual and
Sexual Reproduction

Many fungi reproduce by microscopic spores
that grow into a new organism
 Spores
may be asexual clones
 Or
from sexual reproduction with new allele
combinations

Asexual reproduction is ideal for rapid spread
 No
need to find a mate
 No
fruiting body
 No
meiosis
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
Conidia
 Asexual
spores grown
at the tips of hyphae

Aspergillus versicolor
 Causes
skin and lung
infections in vulnerable
patients
69 µm
© Dr. Dennis Kunkel Microscopy/ Visuals Unlimited
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
Medically important fungi that reproduce
primarily by asexual means include
 Athlete’s
foot fungus (Epidermophyton floccosum)
 Infectious

yeast (Candida albicans)
Budding yeast, Saccharomyces cerevisiae
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 Can
reproduce
either sexually
or by asexual
budding
Daughter
cell (bud)
Mother cell
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© Medical-on-Line/Alamy
Sexual reproduction

Involves union of gametes, zygote formation and
meiosis

Gametes of most fungi are cells from hyphal
branches
 Mating
types differ biochemically
 Hyphal
branches fuse between compatible
mating types
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
Most sexual organisms have plasmogamy
(fusion of gametes’ cytoplasm) followed by
karyogamy (fusion of gametes’ nuclei)

In fungi, after plasmogamy, nuclei may remain
separate for a long time
 Mycelium

is dikaryotic or heterokaryotic
Some fungi persist as dikaryons, producing
clones that can live for hundreds of years
 Dikaryotic
mycelia are functionally diploid
 Eventually,
dikaryotic mycelia produce fruiting
bodies, the next stage of reproduction
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Fruiting bodies

Mated mycelia will produce a fleshy fruiting body
when conditions are right
 Fruiting
structure disperse haploid spores that grow
into mycelia
 If
a haploid mycelium meets a compatible mating
type, they fuse (mate) and the cycle repeats

Fruiting body structures aid spore dispersal
 Puffballs
puff spores out onto wind currents
 Stinkhorns
stink, and attract flies that carry off spores
are underground – but their scent attracts
animals that dig them up
 Truffles
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Spores
in a
sticky
matrix
(a) Fruiting bodies adapted for
dispersal of spores by wind
(b) Fruiting body adapted for
dispersal of spores by insects
a: © Felix Labhardt/Taxi/Getty Images; b: © Bob Gibbons/ardea.com
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
Many fungi produce substances in the fruiting
body to deter consumption
 Toxins
can cause liver failure requiring a transplant
 Hallucinogenic
or psychoactive substances
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Ergot
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© David Q. Cavagnaro/Getty Images
The Importance of Fungi
in Ecology and Medicine

Decomposer fungi are essential components of
the Earth’s ecosystems

Work with bacteria

Only certain bacteria and fungi can break down
cellulose

Release minerals to the soil and water
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
Some fungi are predators trapping tiny soil
nematodes
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Nematode
Hyphal loop
© N. Allin & G.L. Barron/Biological Photo Service
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Fungal pathogens

Crop diseases caused by 5000 species



Rust spores can be spread on the wind
Human diseases

Dermatophytes – athlete’s foot, ringworm

Pneumocystis jiroveci and Cryptococcus neoformans infect
people with weakened immune systems (as in AIDS)

Blastomyces dermatitidis, Coccidioides immitis, and
Histoplasma capsulatum infect the lungs
In nature, fungal pathogens play important ecological
role in controlling other species
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Wheat leaf tissue
Puccinia
graminis
spores
0.1 mm
(left): © Nigel Cattlin/Photo Researchers, Inc.; (right): © Herve Conge/ISM/Phototake
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BIOLOGY PRINCIPLE
Biology affects our society
Recent analyses indicate that environmental changes
linked to human activities correlate with increases in
the incidence of new fungal pathogen infestations that
threaten human health and agricultural sustainability.
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© Nigel Cattlin/Photo Researchers, Inc.
Beneficial fungal associations

Fungal associations with photosynthetic
partners can be mutualistic
 Symbioses

where both partners benefit
Some animal species farm fungus for food
 Leaf-cutter
ants, termites, beetles, salt marsh snail

Mycorrhizal fungi associate with plant roots

Lichens are partnerships between fungi and
photocynthetic algae or bacteria
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Mycorrhizae

Association between the hyphae of certain fungi and the
roots of most seed plants

More than 80% of terrestrial plants have mycorrhizae

Plants receive increased supply of water and mineral
nutrients

Fungi get organic food molecules from the plants

Two most common types are endomycorrhizae (within
roots) and ectomycorrhizae (on roots)
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Seedling
root
Mycorrhizal
hyphae
© Dr. D.P. Donelley and Prof. J.R. Leake, University of Sheffield, Department of Animal & Plant Sciences
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
Endomycorrhizae
 Fungal
hyphae penetrate the spaces between root
cell walls and plasma membranes and grow along the
outer surface of the plasma membrane
 Arbuscular
mycorrhizae (AM) form highly branched
structures with high surface area
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Hyphae
Arbuscules
Cell wall
Plasma
membrane
49 µm
(a) Micrograph of arbuscular mycorrhizae
Root cells
(b) Hyphae growing between cell walls
and plasma membranes
a: © Mark Brundrett
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
Ectomycorrhizae
 Coat
root surface and grow between cells of roots
 Some
species of oak, beech, pine, and spruce trees
will not grow unless their ectomycorrhizal partners are
also present
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Ectomycorrhizal
hyphae
Root
cells
Ectomycorrhizal
hyphae coating
a root tip
(a) Ectomycorrhizal fruiting body
(b) SEM of ectomycorrhizal hyphae
(c) Hyphae invading intercellular spaces
a: © Jacques Landry, Mycoquebec.org; b: © Courtesy of Larry Peterson and Hugues Massicotte
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EVOLUTIONARY CONNECTIONS
Comparison of genomes reveals how
basidiomycete metabolism diversified

Basidiomycete genomes show diverse metabolic
pathways that help utilize organic carbon from plants

Some decompose cellulose and lignin

Break down dead trees, woody debris, leaf litter

Some break down similar materials from animal dung

Other species evolved ectomycorrhizal associations
with living plants

What genes are different to give species different
capabilities?
EVOLUTIONARY CONNECTIONS


White rot fungi – decompose both cellulose and lignin

Complex enzymatic pathways to break down the many types of
chemical bonds

Energetically expensive but give access to cellulose
White rot fungal metabolism arose 300 mya during the
Carboniferous


Brown rot fungi – break down cellulose, leave lignin


This new set of enzymes is why there are no major
plant carbon deposits since then!
Evolved from white-rot fungi but lost genes to degrade lignin,
saving energy by not producing those enzymes
Ectomycorrhizal fungi evolved in turn from the
brown-rot fungi
Lichens

Partnerships of particular fungi and certain photosynthetic
green algae and/or cyanobacteria

25,000 lichen species

Not all descended from a common ancestor

At least five separate fungal lineages

Three major forms – crustose, foliose, fruticose

Photosynthetic partner provides organic food molecules
and oxygen

Fungal partner provides carbon dioxide, water, and
minerals
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(a) Crustose lichen
(b) Foliose lichen
(c) Fruticose lichen
(d) Microscopic view of a cross
section of a lichen
a: © Joe McDonald/Corbis; b: © Lee W. Wilcox; c: © Ed Reschke/Getty Images; d: © Lee W. Wilcox
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Biotechnological
Applications of Fungi

Fungi convert inexpensive organic compounds into
citric acid, glycerol, and antibiotics

Distinctive flavor of blue cheese

Saccharomyces cerevisiae for bread, beer and wine

Replace chemical procedures that generate harmful
waste products

Wood pulp bleaching
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