1. Fungi
Chapter 32

2. 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

3. 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

4. (right): © Garry T. Cole/ Biological Photo Service
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dikaryotic
cell
Septa with
pores
Nuclei
Nuclei
Hypha
Pore
Septum
0.2 µm
(right): © Garry T. Cole/ Biological Photo Service

5. Mycelium – mass of connected hyphae Fungal cell walls include chitin
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Hyphae
10 μm
Mycelium
(inset): © Micro Discovery/Corbis; (right): © Michael & Patricia Fogden/Corbis
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

6. 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

7. 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

8. Reproduction Capable of both sexual and asexual 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 puffballs

9. © Eye of Science/Photo Researchers, Inc.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
10 μm
© Eye of Science/Photo Researchers, Inc.
Spores are the most common means of reproduction among fungi
May form from sexual or asexual processes
Most are dispersed by wind

10. Nutrition Obtain food by secreting digestive enzymes into surroundings
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nutrition
Fungal
loop
Fungus
Nematode a.
400 µm b.
© Carolina Biological Supply Company/Phototake
© L. West/ Photo Researchers, Inc.
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

11. Zygomycota Zygomycetes are incredibly diverse
Not monophyletic – still under research
Include the common bread molds
A few human pathogens
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Neocallimastigomycota
Blastocladiomycota
Chytridiomycota
Glomeromycota
Microsporidia
Basidiomycota
Zygomycota
Ascomycota
Fungi

12. Asexual reproduction more common
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

13. a: © Carolina Biological Supply Company/Phototake
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
GERMINATION
Asexual
reproduction
Spores
Spores
Sporangium
Sporangiophore
– Mating strain
Germinating
zygosporangium
Rhizoid
GERMINATION
Hypha
+ Mating strain
(+)
(–)
(Meiosis occurs
during germination) a. n
700 µm
MEIOSIS 2n
n + n
Gametangia
KARYOGAMY
FERTILIZATION
Zygosporangium b.
a: © Carolina Biological Supply Company/Phototake

14. 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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Neocallimastigomycota
Blastocladiomycota
Chytridiomycota
Microsporidia
Glomeromycota
Zygomycota
Basidiomycota
Ascomycota
Fungi

15. 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

16. a. b. Basidiocarp Gills lined with basidia Basidium n + n Secondary
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Basidiocarp
Gills lined
with basidia
Basidium
n + n
KARYOGAMY
Secondary
mycelium
(dikaryotic) 2n n a.
Zygote
FERTILIZATION
Basidium
MEIOSIS
– Mating strain
+ Mating strain
Primary mycelium
(monokaryotic)
Basidiospores
Sterigma b.
a: © Alexandra Lowry/The National Audubon Society Collection/Photo Researchers, Inc.

17. 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

18. 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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Neocallimastigomycota
Blastocladiomycota
Chytridiomycota
Microsporidia
Glomeromycota
Basidiomycota
Zygomycota
Ascomycota
Fungi

19. 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

20. a: © Richard Kolar/Animals Animals; b: © Ed Reschke/Getty Images
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Conidia
Asexual
reproduction
Developing
mycelium
Ascospore
Each haploid
nucleus divides
once by mitosis
Conidia
GERMINATION
Ascogonium
Antheridium
– Mating strain a.
FERTILIZATION
+ Mating strain
MITOSIS n
n + n 2n b.
MEIOSIS
KARYOGAMY
Young
ascus
Dikaryotic hyphae form
From ascogonium
(formation of
young ascus)
Fully developed ascocarp composed
of dikaryotic (ascogenic) hyphae
and sterile hyphae c.
a: © Richard Kolar/Animals Animals; b: © Ed Reschke/Getty Images

21. 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

22. © David Scharf/Photo Researchers, Inc.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Yeast
5 µm
© David Scharf/Photo Researchers, Inc.
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

23. 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

24. 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

25. Fungi symbioses Interactions
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

26. 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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Algal
cells
Fungal
hyphae
40 μm
© Ed Reschke

27. 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