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Fungi

Chapter 30

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Outline

• Shared Characteristics• The Body of a Fungus• How Fungi Reproduce• How Fungi Obtain Nutrients• Ecology of Fungi• Five Major Groups of Fungi• Lichens• Mycorrhizae• Endophytes• Symbioses

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Shared Characteristics

• Distinctive fungal features– Fungi are heterotrophs.– Fungi have several cell types.– Some fungi have a dikaryon stage.– Fungi have cell walls that include chitin.– Fungi undergo nuclear mitosis.

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The Body of a Fungus

• Fungi exist mainly in the form of slender filaments (hyphae).

– long chains of cells joined end-to-end divided by cross-walls (septa)

rarely form complete barrier cytoplasm freely streams in hyphae

– mycelium - mass of connected hyphae grows through and penetrates substrate

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The Body of a Fungus

• Fungi cell walls are formed of polysaccharides and chitin.

– not cellulose like those of plants• Mitosis is unique.

– nuclear envelope does not break down and re-form

spindle apparatus formed within spindle plaques take place of centrioles

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How Fungi Reproduce

• Differ from most animals and plants in that each compartment of hypha can contain one, two or more nuclei

– monokaryotic - each compartment has a single nucleus

– dikaryotic - two distinct nuclei within each hyphae compartment

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How Fungi Reproduce

• Possible for many nuclei to intermingle in common cytoplasm of fungal mycelium which can lack distinct cells

– heterokaryotic – dikaryotic or multinucleate hypha has nuclei from genetically distinct individuals

– homokaryotic – hyphae whose nuclei are genetically similar to one another

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How Fungi Reproduce

• Fungi are capable of both sexual and asexual reproduction.

– Fungi reproduce sexually after two hyphae of opposite mating type fuse.

in some fungi fusion two haploid cells immediately results in diploid cell (2n)

basidiomycetes and ascomycetes have dikaryotic stage (1n + 1n) before parental nuclei fuse to form diploid nucleus

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How Fungi Reproduce

• Spores most common means of reproduction

– may form from asexual or sexual processes

– most often dispersed by wind but some spread by insects or other small animals

– chytrids only group to retain ancestral flagella and motile zoospores

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How Fungi Obtain Nutrients

• All fungi obtain food by secreting digestive enzymes and then absorbing the organic molecules produced (external digestion).

– extensive hyphae network provides enormous surface area for absorption

– many fungi able to break down cellulose in wood

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Metabolic Pathways

• Anaerobic fermentation provides flavor for wine and cheese.

• Biochemical manufacturing of organic substances

– food– pharmaceuticals

• Yeasts break down carbon-containing products.

– bioremediation

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Ecology of Fungi

• Fungi and bacteria are the principal decomposers in the biosphere.

– mineral cycling Fungi are virtually the only organisms

capable of breaking down lignin.• Fungi often act as disease-causing

organisms for both plants and animals.– agricultural damage– human health

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Ecology of Fungi

• Mutualistic associations– lichens - fungi and green algae– mycorrhizae - fungi and plant roots

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Four Major Groups of Fungi

• Four major groups– Chytridiomycota– Zygomycota– Basidiomycota– Ascomycota

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Chytridiomycota

• aquatic, flagellated fungi– most closely related to ancestral fungi

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Zygomycota

• includes common bread molds• produces temporarily dormant zygosporangia• sexual reproduction occurs by fusion of

gametangia• asexual reproduction most common

– hyphae produce clumps of erect stalks - sporangiophores

form sporangia

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Zygomycota

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Basidiomycota

• Most familiar fungi (mushrooms, toadstools, puffballs, rusts, and smuts)

– named for characteristic sexual reproductive structure, basidium

• Four haploid products of meiosis incorporated into basidiospores

• Mycelium made up of monokaryotic hyphae is called primary mycelium.

– fusion of different mating types forms dikaryotic, secondary mycelium.

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Basidiomycota

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Ascomycota

• Very large group including yeasts, common molds, and morels

• Named for reproductive structure ascus– haploid zygotic nucleus formed within– asci differentiated with ascocarp

• Asexual reproduction takes place in conidia spores at the end of conidiophores.

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Ascomycota

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Ascomycota

• Yeasts– unicellular - most reproduction is asexual

and takes place by cell fission or budding ferment carbohydrates play a leading role in genetic research

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Lichens

• Lichens are symbiotic associations between a fungus and a photosynthetic partner.

– usually ascomycetes Specialized fungal hyphae penetrate

photosynthetic cells and transfer nutrients to fungal partner.

Durable fungus, combined with photosynthetic properties, has enabled lichens to invade harsh climates.

– extremely sensitive to pollutants

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Mycorrhizae

• Roots of about 90% of all kinds of vascular plants are involved in mutualistic symbiotic relationships (mycorrhizae).

– arbuscular mycorrhizae - fungal hyphae penetrate outer cells of plant root

most common– ectomycorrhizae - hyphae surround, but

do not penetrate, cell walls of roots

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Mycorrhizae

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Endophytes

• Endophytic fungi live inside plants in the intercellular spaces.

– some may protect their hosts from herbivores by producing chemical deterrents

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Mutualistic Animal Symbioses

• A range of mutualistic fungal-animal symbioses has been identified.

– ruminants– leaf-cutter ants

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Fungal Parasites and Pathogens

• Chytridiomycosis - emergent infectious disease in amphibians

– chytrid Batrachochytrium dendrobatidis• Aflatoxins - carcinogenic compounds

produced by strains of Aspergillus flavus– grows on corn, peanuts, cotton seeds

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Summary

• Shared Characteristics• The Body of a Fungus• How Fungi Reproduce• How Fungi Obtain Nutrients• Ecology of Fungi• Five Major Groups of Fungi• Lichens• Mycorrhizae• Endophytes• Symbioses

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