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.

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© L. West/ Photo Researchers, Inc.

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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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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)

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KARYOGAMY

GERM

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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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Fungi

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• 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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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

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• 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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