Fungi Chapter 32. Defining Fungi Mycologists believe there may be as many as 1.5 million fungal...
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Transcript of Fungi Chapter 32. Defining Fungi Mycologists believe there may be as many as 1.5 million fungal...
Fungi
Chapter 32
Defining Fungi• Mycologists believe there may be as many
as 1.5 million fungal species• Single-celled or multicellular• Sexual or asexual• Specialized to extract and absorb nutrients
from surroundings• Animal and fungi last shared a common
ancestor 460 MYA– Some debate on timing – May have 670 MYA ancestor
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General Biology of the Fungi
• Multicellular fungi consist of long, slender filaments called hyphae– Some hyphae are continuous– Others are divided by septa
• Cytoplasm flows throughout hyphae– Allows rapid growth under good conditions
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Hypha
Nuclei Nuclei
Pore
Septum
0.2 µm
Dikaryoticcell
Septa withpores
(right): © Garry T. Cole/ Biological Photo Service
• Mycelium – mass of connected hyphae– Grows through and digests its substrate
• Fungal cell walls include chitin– Also found in the hard shells (exoskeletons) of arthropods
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Hyphae
Mycelium
10 μm
(inset): © Micro Discovery/Corbis; (right): © Michael & Patricia Fogden/Corbis
• Hyphae may have more than one nucleus– Monokaryotic – 1 nucleus– Dikaryotic – 2 nuclei
• Both genomes transcribed
• Sometimes many nuclei intermingle in the common cytoplasm of the fungal mycelium– Heterokaryotic – nuclei from genetically
distinct individuals– Homokaryotic – nuclei are genetically similar
to one another
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• Fungi have an unusual mitosis– Cell is not relevant unit of reproduction– Nuclear envelope does not break down and
re-form– Instead, the spindle apparatus is formed
within it
• Fungi lack centrioles – Spindle plaques regulate microtubule
formation during mitosis
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Reproduction
• Capable of both sexual and asexual reproduction
• Sexual reproduction– Fusion of two haploid hyphae of compatible
mating types• In some fungi, fusion immediately results in a diploid
(2n) cell • Others, have a dikaryon stage (1n + 1n) before
parental nuclei form diploid nucleus
– May form mushrooms or puffballs8
• Spores are the most common means of reproduction among fungi– May form from sexual or asexual processes– Most are dispersed by wind
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10 μm© Eye of Science/Photo Researchers, Inc.
Nutrition
• Obtain food by secreting digestive enzymes into surroundings
• Then absorb the organic molecules produced by this external digestion – Great surface area-to-volume ratio
• Fungi can break down cellulose and lignin– Decompose wood– Some fungi are carnivorous
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b.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© L. West/ Photo Researchers, Inc.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a. 400 µm
Fungus
Nematode
Fungalloop
© Carolina Biological Supply Company/Phototake
Zygomycota
• Zygomycetes are incredibly diverse
• Not monophyletic – still under research
• Include the common bread molds
• A few human pathogens
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Neo
call
imas
tig
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yco
ta
Zy
go
my
co
ta
Ch
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idio
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Glo
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Ba
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As
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Bla
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Mic
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Fungi
• Sexual reproduction– Fusion of gametangia– Haploid nuclei fuse to form diploid zygote nuclei
– karyogamy– Develops into zygosporangium in which
zygospore develops– Meiosis occurs during germination of zygospore
• Releases haploid spores
• Asexual reproduction more common– Sporangiophores have sporangia that release
spores
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a.
b.
Rhizoid
Sporangiophore
Sporangium
– Mating strain
n
n + n
(+) (–)
Zygosporangium
Hypha
Gametangia
Spores
Spores
MEIOSIS2n
700 µm
Asexualreproduction
(Meiosis occursduring germination)
FE
RT
ILIZ
AT
ION
KARYOGAMY
GERM
INATIO
N
Germinatingzygosporangium
+ Mating strain
GERMINATION
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a: © Carolina Biological Supply Company/Phototake
Basidiomycota
• Basidiomycetes are some of the most familiar fungi
• Mushrooms, toadstools, puffballs, shelf fungi, etc.
• Also important plant pathogens like rusts and smuts
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Neo
calli
mas
tig
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ta
Zyg
om
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ta
Ch
ytri
dio
myc
ota
Glo
mer
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ta
Bas
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myc
ota
Asc
om
yco
ta
Bla
sto
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Mic
rosp
ori
dia
Fungi
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Named for basidium – club-shaped sexual reproductive structure
• Karyogamy occurs within basidia– Only diploid cell in life cycle
• Meiosis follows
• The four haploid products are incorporated into basidiospores
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a.
b.
2nn
– Mating strain
n + n
Basidiocarp
Basidium
Basidium
SterigmaBasidiospores
MEIOSIS
Zygote
Primary mycelium(monokaryotic)
Secondarymycelium(dikaryotic)
+ Mating strain
KARYOGAMY
Gills linedwith basidia
a: © Alexandra Lowry/The National Audubon Society Collection/Photo Researchers, Inc.
FERTILIZATION
• Spore germination leads to the production of monokaryotic hyphae– Results in a monokaryotic mycelium, or primary
mycelium
• Different mating types of monokaryotic hyphae may fuse– Results in a dikaryotic mycelium, or secondary
mycelium – Heterokaryotic mycelium– Basidiocarps (mushrooms) are formed entirely of
secondary mycelium
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Ascomycota • Contain about 75% of the known fungi• Includes bread yeasts, common molds, cup
fungi, truffles, and morels• Serious plant pathogens – cause of chestnut
blight and Dutch elm disease• Penicillin-producing fungi are in the genus
Penicillium
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Neo
call
imas
tig
om
yco
ta
Zyg
om
yco
ta
Ch
ytri
dio
myc
ota
Glo
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ta
Bas
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myc
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Asc
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Bla
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Mic
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Fungi
• Named for ascus – microscopic, saclike reproductive structure
• Karyogamy occurs within asci– Only diploid nucleus of life cycle
• Asci differentiate in ascocarp
• Meiosis and mitosis follow, producing 8 haploid nuclei that become walled ascospores
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b.
a.
Developingmycelium
Asexualreproduction
Conidia
Ascospore
Each haploidnucleus dividesonce by mitosis
Dikaryotic hyphae formFrom ascogonium
Fully developed ascocarp composedof dikaryotic (ascogenic) hyphaeand sterile hyphae
(formation ofyoung ascus)
n + n
+ Mating strain
Youngascus
n
2n
Ascogonium
Conidia
– Mating strain
Antheridium
c.
MEIOSIS
FERTILIZATION
a: © Richard Kolar/Animals Animals; b: © Ed Reschke/Getty Images
GERMINATION
MITOSIS
KA
RYO
GA
MY
• Asexual reproduction is very common– Conidia formed at the ends of modified
hyphae called conidiophores– Allow for the rapid colonization of a new food
source– Many conidia are multinucleate
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Yeast
• Unicellular ascomycetes• Most reproduce asexually by budding• Yeasts can ferment carbohydrates
– Break down glucose into ethanol and CO2
– Used to make bread, beer, and wine– Saccharomyces cerevisiae
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© David Scharf/Photo Researchers, Inc. 5 µm
• Yeast is a long-standing model system for genetic research– First eukaryotes to be manipulated
extensively– Saccharomyces cerevisiae first eukaryote to
have genome sequenced– Yeast two-hybrid system has been an
important component of research on protein interactions
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Ecology of Fungi
• Fungi, together with bacteria, are the principal decomposers in the biosphere
• Break down cellulose and lignin from wood– Release carbon, nitrogen, and phosphorus
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• Fungi symbioses– Obligate symbiosis – essential for fungus
survival– Facultative symbiosis – nonessential
• Interactions– Pathogen – pathogens harm host by causing
disease– Parasites cause harm to host (do not cause
disease)– Commensal relationships benefit one partner
but does not harm the other– Mutualistic relationships benefit both partners
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• Lichens– Symbiotic associations
between a fungus and a photosynthetic partner
• Cyanobacteria, green algae, or sometimes both
– Most are mutualistic– Ascomycetes are found
in all but about 20 of the 15,000 lichen species
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Algalcells
Fungalhyphae
40 μm © Ed Reschke
• Fungi also cause human and animal diseases– Candida – thrush; vaginal infections– Pneumocystis jiroveci – pneumonia in AIDS– Athlete’s foot, ringworm, and nail fungus
• Fungal diseases are difficult to treat because of the close phylogenetic relationship between fungi and animals
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