Fungi

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Fungi

• Chapter 30

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Why Do Biologists Study Fungi?
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Decomposers

• Worlds most important decomposers.
• Absorb nutrients from dead organisms
• Absorbing nutrients from living organisms.
• If fungi absorb nutrients without providing any
  benefit are parasites
• Vast majority of fungi that live in association
  with other organisms

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Why Do BiologistsStudy Fungi?

• Fungi nourish the plants that nourish us
• Critical to the carbon cycle on land
• Some species cause diseases in humans and crop
  plants

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Fungi Feed Land Plants

• Mycorrhizal associations between fungi and roots
  of land plants allow faster plant growth
• Association is found in 85 of all plant families
  in the wild, including many crop species such as
  the grains

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Fungi Speed theCarbon Cycle on Land

• Saprophytes are fungi that make their living by
  digesting dead plant material
• A dip in the fossil record of saprophytic fungi
  during the Carboniferous
• Responsible for the buildup of dead plant
  material in that period
• Spike in the number of fungal fossils at the end
  of the Permian, coinciding with mass extinction

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Carbon Fixation

• There are two basic components of the carbon
  cycle
• (1) the fixation of carbon by land plants
• (2) the release of carbon dioxide from plants,
  animals, and fungi as the result of cellular
  respiration
• Fungi connect the two components.

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Fungi HaveImportant Economic Impacts

• Parasitic fungi cause athletes foot, vaginitis,
  diaper rash, ringworm, pneumonia, and thrush in
  humans
• Incidence of fungal infections in humans is very
  low.
• Major destructive impact is on crops

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How Do Biologists Study Fungi?
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How Do BiologistsStudy Fungi?

• 80,000 species of fungi have been described and
  named
• About 1000 more are discovered each year
• Estimated 1.65 million species of fungi

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Analyzing Morphological Traits

• Fungi have very simple bodies
• Two growth forms exist
• (1) single-celled (yeasts)
• (2) multicellular filamentous forms (mycelia)
• Some fungi adopt both life-forms

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The Nature of theFungal Mycelium

• The filaments that make up a mycelium are called
  hyphae
• Most are haploid, or heterokaryotic
• Each filament is separated by cell-like
  compartments called septa

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Hyphae

• Adaptation to the absorptive lifestyle of fungi
• Give a fungus has a high surface-area-to-volume
  ratio
• Makes absorption extremely efficient
• Also makes fungi prone to drying out
• Reproductive are the only thick, fleshy structures

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Reproductive Structures

• There are four major groups of fungi based on
  reproductive structures

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Chytrids

• Phylum Chytridiomycota
• Are found in freshwater and terrestrial habitats
• Can be saprobic or parasitic
• Unique to other fungi in having flagellated
  spores called zoospores

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Zygomycetes

• Considerable diversity of life histories
• Include fast-growing molds, parasites, and
  commensal symbionts
• Are named for their sexually produced
  zygosporangia
• Where karyogamy and meiosis occur

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Zygomycetes

• Some zygomycetes, such as Pilobolus
• Can actually aim their sporangia toward
  conditions with good food sources
• Zygosporangia are resistant to freezing and
  drying
• Are capable of persisting through unfavorable
  conditions
• Can undergo meiosis when conditions improve

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Ascomycetes

• Found in marine, freshwater, and terrestrial
  habitats
• Defined by the production of sexual spores in
  saclike asci, which are contained in fruiting
  bodies called ascocarps
• Commonly called sac fungi
• Vary in size and complexity

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Ascomycetes
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Basidiomycetes

• Mushrooms, shelf fungi, and
• some mycorrhizae and molds
• Some nasty plant parasites, rusts and smuts
• Are defined by a clublike structure called a
  basidium, a transient diploid stage in the life
  cycle
• Club fungus
• Important decomposers of
• wood and other plant material

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Basidiomycetes
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Evaluating Molecular Phylogenies

• Each type of structure is identified a
  monophyletic group
• Chytridiomycota, Zygomycota, Ascomycota, and
  Basidiomycota have traditionally been recognized
  as separate phyla
• Chytrids are the most basal group
• Ascomycetes and are the most recent to evolve

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Evaluating Molecular Phylogenies
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What Are the Relationships among Major Fungal
Groups?

• Ribosomal RNA sequencing shows
• Ascomycetes and basidiomycetes are monophyletic
• Chytrids and zygomycetes are not
• Data also support the conclusion that the
  chytrids are the most basal group of fungi.

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Experimental Studiesof Mutualism

• Fungi and land plants often have a symbiotic
  relationship
• Provides benefits to both the host and the fungus
• Mycorrhizas
• Lichens

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What Themes Occur in the Diversification of Fungi?
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Evolution of Methods for Absorbing Nutrients

• Ectomycorrhizal fungi (EMF) form a network of
  hyphae around roots but do not enter the root
  cell
• Most EMF are basidiomycetes
• Provide nitrogen and phosphate ions to the host
  plant
• Receive sugars and other complex carbon compounds
  in return.

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Arbuscular Mycorrhizal Fungi (AMF)

• Arbuscular mycorrhizal fungi (AMF) grow into the
  cells of root tissue
• Directly contact the plasma membrane of the
  plant cell
• AMF are zygomycetes
• Provide phosphorus to the plants, but they do not
  provide nitrogen
• Extremely common and extremely ancient

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Adaptations That Make Fungi Effective Decomposers

• Fungi seek out large, complex molecules and
  break them down
• Cellulose, lignin, proteins, and nucleic acids
• Only organisms (besides some bacteria) that can
  digest wood completely
• Digest trough lignin to get to cellulose

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Extracellular Digestion

• Fungi secrete enzymes for extracellular
  digestion
• Only sugars, and other small molecules can enter
  the cytoplasm
• Large molecules cannot diffuse across the plasma
  membranes of hyphae
• Lignin and cellulose
• Fungi must digest their food before they absorb it

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Lignin Degradation

• Lignin important in cell walls
• Tracheids and xylem
• fills the spaces in the cell wall between
  cellulose, hemicellulose and pectin components
• Lignin peroxidase- enzyme that catalyzes the
  removal of a single electron from an atom in the
  aromatic rings of lignin
• Oxidation step creates a free radical and leads
  to a series of reactions that end up splitting
  the polymer into smaller units

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Cellulose Digestion

• Primary cell wall structural component of green
  plants
• Fungi secrete cellulases into the extra-cellular
  environment
• Some cleave long strands of cellulose, into
  disaccharides
• Other cellulases work together, eventually
  convert cellulose into glucose

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Variation in Life Cycles

• The spore is the fundamental reproductive cell in
  fungi
• Spores are the dispersal stage in the fungal life
  cycle
• Produced during both asexual and sexual
  reproduction

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Sporolation

• When a spore falls on a food source and is able
  to germinate, a mycelium forms
• Hyphae grow in the direction in which food is
  most abundant
• If food begins to run out, mycelia respond by
  making spores
• Allows starving mycelia to disperse offspring to
  new habitats where more food might be available

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Unique Aspects ofFungal Life Cycles
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Unique Aspects of Fungal Life Cycles

• The sexual life cycle involves
• Cell fusion, plasmogamy
• Nuclear fusion, karyogamy
• An intervening heterokaryotic stage
• Occurs between plasmogamy and karyogamy in which
  cells have haploid nuclei from two parents
• Sometimes can be dikaryotic and have two
  different, separate nuclei
• The diploid phase following karyogamy
• Is short-lived and undergoes meiosis, producing
  haploid spores

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Unique Aspects of Fungal Life Cycles
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