Chapter 31 Fungi

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Chapter 31Fungi
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Introduction

• Fungi are eukaryotes
• Nearly all multicellular (yeasts are unicellular)
  
• Distinguished from other kingdoms by
• Nutrition
• Structural organization
• Growth
• Reproduction

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Absorptive nutrition enables fungi to live as
decomposers and symbionts

• Fungi are heterotrophs that acquire nutrients by
  absorption
• Secrete hydrolytic enzymes and acids to decompose
  complex molecules into simpler ones that can be
  absorbed
• Specialized into three main types
• Saprobes- absorb nutrients from dead organic
  material
• Parasitic fungi-absorb nutrients from cells of
  living hosts some are pathogenic
• Mutualistic fungi-absorb nutrients from a host,
  but reciprocate to benefit the host

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Extensive surface area and rapid growth adapt
fungi for absorptive nutrition

• Hyphae makes up the mycelium and visible
  structure we recognize as a mushroom
• Except for yeast, hyphae are organized around and
  within food source
• Composed of tubular walls containing chitin

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Fungal hyphae may be sepatate or asepatate

• Hyphae of septate fungi are divided into cells by
  cross-walls called septa
• Large pores allowing ribosomes, mitochondria,
  nuclei flow from cell to cell
• Hyphae of aseptate fungi lack cross walls
  (coenocytic)
• Parasitic fungi have modified hyphae called
  haustoria, which penetrate the host tissue but
  remain outside cell membrane

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Fungi reproduce by releasing spores that are
produced either sexually or asexually

• Usually unicellular, haploid and of various
  shapes/sizes
• Asexual reproduction
• reproduce asexually by mitotic production of
  haploid vegetative cells called spores
• Spores are dispersed by wind and water creates
  wide distribution of many species of fungi
• Sexual reproduction
• results in greater diversity

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Basic Cycle
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Generalized life cycle of fungi

• Sexual Reproduction
• Plasmogamy fusion of cytoplasm of two parents
• Heterokaryotic mycelium contains nuclei from two
  parents
• Dikaryotic mycelium contains nuclei from two
  parents, but each cell contain two nuclei
• Karyogamy fusion of nuclei of two parents

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Phylogeny of fungi
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Chytrids might provide clues about fungal origins

• Originally places in Kingdom Protista
• Mainly aquatic
• Share many characteristics w/ fungi
• Absorptive nutrition
• Chitin cell-walls
• Hyphae
• enzymes/ metabolism
• Earliest fungi evolved from protists and
  retained flagella

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Zygomycetes

• Characterized by dikaryotic zygosporangia
• Mostly terrestrial-live in soil or decaying
  material
• Some from mycorrhizae, mutualistc associations
  with plant roots
• Hyphae are coenocytic-septa only found in
  reproductive cells

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Life cycle of zygomycete Rhizopus
stolonifer(common bread mold)

• Under favorable conditions, reproduces asexually
• Sporangia develop at tips of hyphae
• Mitosis produces hundred of haploid spores
• In unfavorable conditions of sexual reproduction
  occurs
• Mycelia of opposite mating types for gametangia
  that contain several haploid nuclei walled off by
  the septum
• Plasmogamy of gameangia occurs ? dikaryotic
  zygosporangium ? diploid nuclei immediately
  undergoes meiosis producing haploid spores
• Zygosporangium germinates sporangium which
  releases recombined haploid spores

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Cycle of common bread mold
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Ascomycota

• Includes unicellular yeasts and complex
  multicellular cup fungi
• Hyphae are septate
• In asexual reproduction, tips of specialized
  hyphae form conidia-chains of haploid asexual
  spores
• In sexual reproduction, haploid mycelia of
  opposite mating strains fuse

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Life cycle of ascomycete
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Basidiomycota club fungi

• Important decomposers of wood/ plant material
• Include
• Mycorrhiza- forming mutualists
• Mushroom-forming fungi
• Plant parasites (rusts and smuts)
• Characterized by dikaryotic mycelium that
  reproduced sexually by basidiocarps

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Life cycle in detail

• Haploid basidiospores grow into short-lived
  haploid mycelia under certain conditions
  plasmogamy occurs
• Resulting dikaryotic mycelium grows forming
  mycorrhiza or mushrooms
• Mushroom cap supports protects fills karyogamy
  in the terminal, dikaryotic cells lining the
  gills produces diploid basidia
• Resulting basidium immediately undergoes meiosis
  producing four haploid basidiospores
• Asexual reproduction less common than ascomycetes

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Molds

• Rapidly growing fungus w/ no sexual stages
• May develop into a sexual fungus, producing
  zygosporangia, ascocarps, or basidiocarps
• Molds w/ no known sexual stage are known as
  Deuteromycota or imperfect fungi
• Penicillium
• Flavor of blue cheese

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Yeasts

• Unicellular reproduce
• Asexually by budding
• Sexually by producting asci or basidia
• Saccharomyces cerevisiae is most important
  domesticated fungus
• Baking brewing
• Model organism
• Can cause problems
• rhodotorula shower curtains
• Candida thrush

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Lichens

• Symbiosis of algae w/ fungal hyphae
• The alga
• Provides fungus w/ food
• May fix nitrogen fixation
• Fungus provides good environment for growth
• Hyphal mass absorbs minerals and protects algae
• Produces compounds that
• Shield algae from sunlight
• Are toxic-prevents predation

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Lichen reproduction

• Occurs as a combines unit or independently
• Fungi reproduces sexually (usually ascocarps)
• Algae reproduces asexually by cell division
• Symbiotic units reproduce asexually by
• Fragmentation of parent
• Formation of soredia small clusters of hyphae w/
  embedded algae

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Mycorrhizae

• Specific, mualistic association of plant roots
  and fungi
• Fungi increase absorptive surface of roots and
  exchange soil minerals
• Found in 95 of vascular plants
• Necessary for optimal plant growth

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Ecological impact of fungi

• ecosys depend on fungi as decomposers
  symbionts decompose food, wood, and plastic
• Some fungi are pathogens (athletes foot,
  ringworms)
• Plants particularly susceptible
• Ergot-affects cereal crops causes gangrene,
  hallucinations, and St. Anthonys fire
• Many animals, including humans, eat fungi
• Truffles are underground ascocarps