3/18/2019 1 Mushrooms, Molds, and Yeasts Fungi
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Mushrooms, Molds, and Yeasts
Fungi
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Learning Objectives
By the end of this unit, a student should be able to…
– List distinguishing characteristics, describe a typical life
cycle, and give examples of each of the following fungal
groups: chytridiomycetes, zygomycetes, glomeromycetes,
ascomycetes, and basidiomycetes
– Describe the body plan of a fungus
– Summarize the ecological significance of fungi as
decomposers
– Describe the important ecological role of mycorrhizae
– Characterize the unique nature of a lichen
– Summarize some specific ways that fungi affect humans
economically
– Summarize the importance of fungal diseases to
agriculture and medicine; giving specific examples
General Characteristics
Heterotrophs
– Fungi cannot make their own food like plants. They depend upon other
organisms for their carbon source.
Feed by Absorption
–Secrete digestive enzymes onto food
– then absorb predigested food
–Saprotrophs: organisms that feed on dead organic matter
Heterotrophic by Absorption
Fungi get carbon from organic sources
Hyphal tips release enzymes
Enzymatic breakdown of substrate
Products diffuse back into hyphae
Product diffuses back
into hypha and is used
Nucleus hangs back
and “directs”
Distinguishing characteristics of Fungi
• Cell walls contain chitin
• Fibrous polysaccharide
• Polymer of a n-acetylglucosamine (derivative of
glucose)
• Also found in exoskeleton of arthropods
KEY CONCEPTS
Fungi are eukaryotic heterotrophs that absorb nutrients from their surroundings
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Learning Objective 2
What is the body plan of a fungus?
General Characteristics
Multicellular (except for yeasts)
Have cell walls made of chitin
Body made up of long, threadlike filaments
called hyphae
Hyphae branch and form a tangled mass
called the mycelium
Fungal Structure
Threadlike filaments (hyphae)
Branch and form a tangled mass (mycelium)
Hyphae
25 µm Mycelium
Fungal Structure
Threadlike filaments (hyphae)
Branch and form a tangled mass (mycelium)
Fungal Structure
Hyphae
– Perforated septa
(cross walls) divide
hyphae into
individual cells
– Some are
coenocytic
form elongated,
multinuclear cell
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Hyphae
In most fungi
–perforated septa (cross walls) divide hyphae into individual cells
In some fungi
–hyphae are coenocytic (form elongated, multinuclear cell)
– zygomycetes and glomeromycetes septate coenocytic
Hyphae
Hyphae
There is cytoplasmic flow between cells of septate hyphae
Movement of nutrients throughout mycelium
Fig. 26-1 (c-e), p. 557
Coenocytic
Dikaryotic
Septate
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KEY CONCEPTS
A fungus may be a unicellular yeast or a filamentous, multicellular mold consisting of long, branched hyphae that form a mycelium
Learning Objective 3
What is the life cycle of a typical fungus, including sexual and asexual reproduction?
Reproduction
Most fungi reproduce sexually and asexually by spores
Spores
–Produced on aerial hyphae
–Produced in a fruiting body
– Land in suitable spot and germinate
Fungal Reproduction • reproduce with Spores
• sexual
• asexual
• produced on aerial hyphae
• produced in a “fruiting
body”
Germination of a Spore Asexual Reproduction
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Yeast
Yeasts are a type of single celled fungus
– highly reduced mycelium
– reproduce asexually by budding
Budding yeast – nucleus divides; new
cell buds off of mother cell
Life Cycles of Fungi
Reproduction
Fungi reproduce by means of spores
–Asexual spores
–Sexual spores
Spores Produced by Divisions
Mitosis
– forms new mycelia when spores
germinate
Meiosis
–produces 4 different haploid (n) nuclei
–each nucleus becomes part of a spore
Asexual Spores
Can be produced by mitosis
–genetically similar
Spore germination
–Hyphal thread grows out of spore
–As it grows and branches, it develops into a mass of hyphae called a mycelium.
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Sexual Spores
Plasmogamy
– Fungi of two different mating types meet, hyphae fuse
Cytoplasm fuses but nuclei remain separate
Fungi enter dikaryotic (n + n) stage
–each new cell has one nucleus of each type
Karyogamy
– Fusion of nuclei
takes place in hyphal tip
results in diploid (2n) zygote nucleus
Meiosis – produces 4 different haploid (n) spores
Fungal Life Cycles
Fungal Life Cycle
n
haploid mycelium
dikaryotic
mycelium
Spores n
n
mitosis
meiosis
n
plasmogamy
n + n
karyogamy
2n
diploid
mycelium
n n
Fig. 26-4, p. 559
Large numbers of
haploid (n) spores are
produced by mitosis.
Spore germinates
and forms mycelium
by mitosis. Asexual
reproduction
1 Spores germinate
and form mycelia
by mitosis. Mycelia
Mycelia of two different
mating types fuse at their
tips, and plasmogamy
(fusion of cytoplasm)
occurs.
2
6 Spores are
released. Spores
Sexual
reproduction
Haploid stage
(n)
Dikaryotic stage
(n + n) 5
Meiosis results in
four genetically
different haploid ( n )
nuclei. Spores
develop around
nuclei.
Diploid
stage (2n) 3
Zygote nucleus
(2n)
Dikaryotic (n + n)
mycelium develops.
Dikaryotic mycelium
4 Karyogamy (fusion of nuclei)
occurs, forming a diploid (2 n )
zygote nucleus.
7 8
Meiosis Karyogamy
Plasmogamy
haploid mycelium
dikaryotic
mycelium
Spores
mitosis
meiosis
plasmogamy
karyogamy
diploid
mycelium
(n)
Fungal Life Cycles
asexual cycle of
haploid mycelium
Spores (n)
KEY CONCEPTS
Most fungi reproduce both asexually and sexually by means of spores
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Fungal Evolution Chytridiomycetes
Zygomycetes
Glomeromycetes
Ascomycetes
Basidiomycetes
Learning Objective 4
Support the hypothesis that fungi are opisthokonts, more closely related to animals than to plants
Opisthokonts
Fungi are opisthokonts
–along with animals and choanoflagellates
–based on chemical and structural characters
Flagellate Cells
Animals and chytrids have flagellate cells
–Example: chytrid gametes and spores
–Example: animal sperm
Flagellate cells propel themselves
–with single posterior flagellum
Platelike Cristae
Like animal cells, fungal cells have platelike cristae in their mitochondria
Flatties
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Classification
& Phylogeny
motile spores
zygosporangia
asci basidia
Fig. 26-5, p. 560
Ch
ytr
ids
Zyg
om
ycete
s
Glo
mero
mycete
s
Asco
mycete
s
Basid
iom
ycete
s
Evolution of
ascospores
Evolution of
basidiospores
Evolution of
dikaryotic stage
Loss of flagellum
Common
flagellate
ancestor
Motile gametes
and spores
Fungal Evolution
5 phyla
–Chytrids (Chytridiomycetes)
–Ascomycota: sexual and asexual reproduction; spores in ascus (sac)
–Zygomycota: sexual and asexual
reproduction; form zygospore
–Glomeromycota: arbuscular mycorrhizae
–Basidiomycota: sexual reproduction only; spores form on basidium
Learning Objective 5
Support the hypothesis that chytrids may have been the earliest fungal group to evolve from the most recent common ancestor of fungi
Chytrids (Chytridiomycetes)
Produce flagellated cells during life cycle
–no other fungi have flagella
Probably earliest fungi to evolve from flagellate protist
– common ancestor of all fungi
Learning Objective 6
List distinguishing characteristics, describe a typical life cycle, and give examples of each of these fungal phyla:
–Chytridiomycota
–Zygomycota
–Glomeromycota
–Ascomycota
–Basidiomycota
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Chytrids (Chytridiomycetes)
Produce flagellate cells
– no other fungi have
flagella
– a shared characteristic of
the opisthokonts
Probably earliest fungi to
evolve from flagellate protist
Chytridium convervae
Chytridiomycota – “chytrids”
Simple fungi
Produce motile spores
Mostly saprobes
and parasites in aquatic habitats
Could just as well be Protists
Chytrids
Reproduce both asexually and sexually
Gametes and zoospores are flagellated
Allomyces
–part of life is multicellular haploid thallus
–part is multicellular diploid thallus
Mature haploid thallus
♂ gametangium
♀ gametangium 2
♂ Gamete Haploid
zoospore
♀ Gamete
SEXUAL REPRODUCTION
HAPLOID (n)
GENERATION
DIPLOID (2n)
GENERATION
3
Meiosis
Plasmogamy
and karyogamy
5 Resting
sporangium Motile zygote
Zoosporangium
4 Zygote germinates
and develops into
diploid thallus.
ASEXUAL REPRODUCTION
(by mitosis) 7
Diploid zoospore
1
6
Resting
sporangium
“Chytrid” and Chytridiomycosis disease
– Massive amphibian die-off
– mainly in tropics
Caused by fungus Batrachochytrium dendrobatidis
Chytrids (Chytridiomycetes) Fungal Evolution
Chytridiomycetes
Zygomycetes
Glomeromycetes
Ascomycetes
Basidiomycetes
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Zygomycota – “zygote fungi”
Sex - zygosporangia
Asexual - common
Hyphae have no cross walls – coenocytic
– Septa form to separate the hyphae from reproductive structures
Grow rapidly
Many species of mycorrhizae
Rhizopus on strawberries
Zygomycota
Zygomycetes
Rhizopus (black bread mold)
– forms haploid thallus
–produces asexual spores and sexual spores
Asexual spores germinate
– form new thalli
Zygomycetes 2
In sexual reproduction
–hyphae of 2 different haploid mating types form gametangia
Plasmogamy occurs
–as gametangia fuse
Zygomycetes 3
Karyogamy occurs
–diploid zygote forms
– from which zygospore develops
Meiosis
–produces recombinant haploid zygospores
Zygomycetes 4
When zygospores germinate
–each hypha develops a sporangium at its tip
Spores are released
–develop into new hyphae
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Zygospore and suspensors
Zygomycota
Sexual reproduction
– hyphae of 2 different mating types form
gametangia
– Plasmogamy occurs as gametangia fuse
– Karyogamy produces zygote (2n)
– zygote develops into zygosporangium
Fig. 26-9b, p. 564
Zygomycota
Microsporidia
– Small, unicellular opportunistic
obligate intracellular parasites that
infect eukaryotic cells
– May be the smallest and simplest
eukaryotes
Zygomycota
Microsporidia
– Previously classified with yeasts, bacteria, and
protista
– Genome studies suggest that microsporidia
descended from a zygomycete ancestor