Chapter 17 Overview

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Chapter 17 Overview

• Fungi as Multicellular Organisms
• Fungal Diversity
• The Ecology of Fungi

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Complex Multicellularity

• Complex multicellular organisms
• Individuals are composed of many highly
  specialized cells that coordinate their
  activities
• Three kingdoms exhibit multicellularity
• Plants
• Animals
• Fungi

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Complex Multicellularity

• Two key characteristics distinguish between
  complex multicellular and simple multicellular
  organisms
• Cell specialization
• Different cells use different genes
• They therefore develop in different ways
• Intercell coordination
• Cells adjust their activity in response to what
  other cells are doing

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A Fungus Is Not a Plant

• The study of fungi is called mycology
• Fungi have traditionally been included in the
  plant kingdom
• However, there are significant differences
  between fungi and plants
• Fungi are heterotrophs
• Fungi have filamentous bodies
• Fungi have nonmotile sperm
• Fungi have cell walls made up of chitin
• Fungi have nuclear mitosis

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The Body of a Fungus

• Fungi exist mainly as slender filaments called
  hyphae (singular, hypha)
• Hyphae are strings of cells separated by septa
  (singular, septum)

• Pores in the septa allow for cytoplasmic
  streaming between cells

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The Body of a Fungus

• The main body of a fungus is a mass of hyphae
  termed a mycelium (plural, mycelia)

• All parts of the fungal body are metabolically
  active

• Because of cytoplasmic streaming, many nuclei may
  be connected by shared cytoplasm

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Reproduction and Nutritionof Fungi

• Fungi reproduce both sexually and asexually
• Sexual reproduction is initiated when two hyphae
  of different mating types come in contact and
  fuse
• The two nuclei do not fuse immediately
• Heterokaryon
• Hyphae containing nuclei derived from two
  genetically different individuals
• Homokaryon
• Hyphae containing nuclei derived from two
  genetically similar individuals

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Reproduction and Nutritionof Fungi

• Fungi have three types of reproductive structures

• Gametangia
• Form haploid gametes that fuse to form zygote
• Sporangia
• Produce haploid spores that are dispersed
• Conidiophores
• Produce asexual spores

Puffball spores

• Spores are a common means of fungal reproduction

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Reproduction and Nutritionof Fungi

• Fungi obtain nutrients by external digestion
• They secrete digestive enzymes into their
  surroundings and absorb the resulting organic
  molecules

• Some fungi are active predators

Immobilizes nematodes then eats them!

• Others are even more active predators
• Snare or trap prey

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Kinds of Fungi

• 73,000 species of fungi have been named so far

• There are divided into four phyla

• Zygomycota
• Ascomycota
• Basidiomycota
• Chitrydiomycota

• A fifth group, the imperfect fungi, is artificial
  
• Its a catch-all grouping of fungi in which
  sexual reproduction has not been observed yet!

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Zygomycetes

• Zygomycetes are unique fungi

• Their hyphae lack septa
• Fusion of their hyphae produces a zygote (one 2n
  nucleus)
• The other fungi produce a heterokaryon (two n
  nuclei)

• Zygomycetes make up lt 1 of named fungi

• They include
• Bread molds
• Some pathogens
• Candida

• Zygomycetes typically undergo asexual reproduction

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Life cycle of a zygomycete
Rhizopus
Effective survival mechanism
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Ascomycetes

• Phylum Ascomycota, the ascomycetes, is the
  largest of the four phyla
• Yeasts, morels and truffles
• Many plant fungal pathogens
• Dutch elm disease and chestnut blight
• Reproduction is usually asexual
• Spores, or conidia, are cut off by septa at ends
  of hyphae
• Ascomycetes are named after a characteristic
  sexual reproductive structure, the ascus (plural
  asci)
• The ascus differentiates within the ascocarp

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Life cycle of an ascomycete
Morel
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Basidiomycetes

• Phylum Basidiomycota contains the most familiar
  of the fungi
• Mushrooms, toadstools, puffballs
• Many plant fungal pathogens
• Rusts and smuts
• Asexual reproduction is rare
• Basidiomycetes are named after a club-shaped
  sexual reproductive structure, the basidium
• There, haploid nuclei fuse, forming a diploid
  zygote
• Zygote undergoes meiosis releasing haploid
  basidiospores

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Life cycle of a basidiomycete
Amanita
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Chytridiomycetes, Imperfect Fungi, and Yeasts

• Chytridiomycetes
• Aquatic, flagellated fungi
• Most closely related to ancestral fungi
• Include
• Plant pathogens
• A frog pathogen

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Chytridiomycetes, Imperfect Fungi, and Yeasts

• Imperfect fungi
• Fungi where sexual reproduction has not been
  observed
• Most appear to be ascomycetes
• Include
• Fungi that cause ringworm infections

A pathogen of alfalfa
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Chytridiomycetes, Imperfect Fungi, and Yeasts

• Yeasts
• Generic name given to unicellular fungi
• 250 named species
• Most appear to be ascomycetes
• Reproduction is mostly asexual
• Takes place by budding

Bakers yeast
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Ecological Roles of Fungi

• Fungi, together with bacteria, are the principal
  decomposers in the biosphere
• Fungi are virtually the only organisms that can
  break down lignin

• Fungi cause animal diseases
• Fungi are the most harmful pests of living plants

Coniferous forest
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Ecological Roles of Fungi

• Many commercial products are dependent on the
  biochemical activities of fungi
• Bread
• Beer
• Cheese
• Soy sauce
• Penicillin

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Fungal Associations

• Mutualism is a form of symbiosis in which each
  partner benefits
• Two kinds of mutualistic associations between
  fungi and autotrophic organisms are ecologically
  important
• 1. Mycorrhizae
• 2. Lichens

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• Mycorrhizae

• Symbiotic association between a fungus and the
  roots of plants
• Fungus helps plant absorb minerals and nutrients
• Plant provides fungus with food (organic
  molecules)

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• Mycorrhizae

• Endomycorrhizae vs. Ectomycorrhizae

Far more common type
Hyphae do not penetrate root cells
Hyphae penetrate root cells
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• Lichens

• Symbiotic association between a fungus and a
  green algae or cyanobacterium

• Lichens are found in the harshest of habitats

• Indeed, they are often the first colonists in
  such harsh areas

• Lichens are pollution indicators