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					Fungi: Recyclers, Pathogens,
    and Plant Partners
31        Fungi: Recyclers, Pathogens, and Plant Partners

     • General Biology of the Fungi
     • Diversity in the Kingdom Fungi
     • Fungal Associations
31                              General Biology of the Fungi

     • The fungi live by absorptive nutrition, secreting
       digestive enzymes that break down large food
       molecules and absorbing the breakdown
       products.
     • Some are saprobes (feeding on dead matter);
       others are parasites.
     • A few have mutually beneficial (symbiotic)
       relationships with other organisms.
Figure 31.1 Parasitic Fungi Attack Other Living Organisms
31                               General Biology of the Fungi

     • The production of chitin is a shared derived trait
       for fungi, choanoflagellates, and animals.
     • The presence of chitin in fungi is evidence that all
       fungi are more closely related to animals than to
       plants.
     • The kingdom Fungi consists of four phyla:
       Chytridiomycota, Zygomycota, Ascomycota, and
       Basidiomycota.
     • The four phyla are primarily distinguished by their
       methods and structures of reproduction.
Table 31.1 Classification of Fungi
31                               General Biology of the Fungi

     • Unicellular forms are found in all of the fungal
       phyla.
     • Those of the Zygomycota, Ascomycota, and
       Basidiomycota are called yeasts.
     • Yeasts may reproduce by budding, fission, or
       sexual means.
Figure 31.2 Yeasts Are Unicellular Fungi
31                              General Biology of the Fungi

     • The vegetative body of a multicellular fungus is
       called the mycelium (plural mycelia).
     • The mycelium is composed of threadlike hyphae
       (singular hypha).
     • Within the hyphae of two clades, cell-like
       compartments are formed by incomplete cross-
       walls called septa (singular septum).
     • Pores in septa allow free movement of organelles,
       sometimes even nuclei, and other materials.
     • Some hyphae are coenocytic—they have no
       internal separations into distinct cells.
Figure 31.3 Most Hyphae Are Incompletely Divided into Separate Cells
31                                General Biology of the Fungi

     • The hyphae may be widely dispersed to forage for
       nutrients or they may be clumped together in a
       cottony mass to exploit a rich nutrient source.
     • Sometimes the mycelium becomes reorganized
       into a fruiting (reproductive) structure, such as a
       mushroom.
     • Rhizoids are modified hyphae, which anchor
       Chytridiomycota to a substrate.
     • These rhizoids are not homologous to the rhizoids
       of plants because they are not specialized to
       absorb water and nutrients.
31                               General Biology of the Fungi

     • Some parasitic fungi may have modified hyphae
       that take up nutrients.
     • Fungal parasites can invade wounds on plants
       and grow mycelium throughout the plant.
     • Some hyphae produce haustoria, branching
       projections that push into the living plant cells and
       absorb nutrients within them.
Figure 31.4 A Fungus Attacks a Leaf
31                               General Biology of the Fungi

     • The mycelium has a very high surface area-to-
       volume ratio.
     • Throughout the mycelium (except in fruiting
       structures), all the hyphae are very close to their
       environmental food source.
     • Fungi are tolerant to highly hypertonic
       environments.
     • Many can tolerate temperatures as low as 5–6C
       below freezing. Some can tolerate temperatures
       as high as 50 C or more.
31                              General Biology of the Fungi

     • The majority of fungi are saprobes, living on dead
       organisms.
     • Saprobic fungi (along with bacteria) are Earth’s
       primary decomposers.
     • Fungi are the principal decomposers of cellulose
       and lignin.
     • Many fungi can use ammonium (NH4+) ions or
       nitrate (NO3–) as a sole source of nitrogen.
     • Most are unable to synthesize their own thiamin
       or biotin, but they can synthesize some vitamins
       that animals cannot.
31                              General Biology of the Fungi

     • Facultative parasites can attack living organisms
       but they can also be grown on defined media.
     • Obligate parasites grow only on their specific
       host.
     • Most predatory fungi secrete sticky substances
       from the hyphae. Trapped prey are penetrated by
       hyphae and eventually killed.
     • Some species form a ring with modified hyphae
       that constricts around nematodes.
     • The crawling nematode triggers these rings to
       swell and trap the worm. Hyphae quickly invade
       and digest the worm.
Figure 31.5 Some Fungi Are Predators
31                               General Biology of the Fungi

     • Lichens are symbiotic associations of a fungus
       with a cyanobacterium, a unicellular photosynthetic
       eukaryote, or both.
     • Mycorrhizae are mutualistic associations of fungi
       and plant roots.
     • The fungus obtains organic compounds, while the
       plant is provided with water and soil minerals.
31                              General Biology of the Fungi

     • Asexual reproduction among the fungi includes:
         The production of haploid spores within
          sporangia.
         The production of naked spores at the tips of
          hyphae (not within sporangia) called conidia.
         Cell division by unicellular fungi—either equal
          division (fission) or production of a daughter
          cell (budding).
         Simple breakage of the mycelium.
31                              General Biology of the Fungi

     • Sexual reproduction involves fusion between
       different mating types.
     • Some fungi have more than two mating types.
     • Mating types cannot be distinguished
       morphologically.
     • Mating can occur only between different mating
       types, which prevents self-fertilization.
     • Fungi reproduce sexually when hyphae (or motile
       cells in chytrids) of different mating types meet
       and fuse.
31                                General Biology of the Fungi

     • In many fungi, the zygote nuclei are the only
       diploid nuclei of the life cycle.
     • These nuclei undergo meiosis, producing haploid
       nuclei.
     • Haploid spores divide mitotically to form haploid
       hyphae.
     • This type of life cycle is called a haplontic life
       cycle and is a characteristic of many protists.
31                              General Biology of the Fungi

     • Fungal pathogens are a major cause of death
       among people with compromised immune
       systems.
     • Most patients with AIDS die of fungal diseases
       such as Pneumocystis carinii.
     • Candida albicans and other yeasts also cause
       severe diseases in those with AIDS.
     • Other less severe and common diseases include
       ringworm and athlete’s foot.
     • Plant diseases include black stem rust and others.
Figure 31.6 Phylogeny of the Fungi
31                              Diversity in the Kingdom Fungi

     • The chytrids (phylum Chytridiomycota) are the
       earliest diverging fungal lineage.
     • They are aquatic microorganisms, formerly classed
       with protists but now classed with fungi because of
       the chitin in their cell walls.
     • They are the only fungi that have flagella at any
       stage of the life cycle.
     • Chytrids are parasitic or saprobic, but some are
       found in the rumen of ruminants. Most live in fresh
       water or moist soil; some are marine.
     • Some are unicellular; others have coenocytic
       hyphae.
     • They reproduce both sexually and asexually.
31                             Diversity in the Kingdom Fungi

     • Allomyces displays alternation of generations.
     • A haploid zoospore germinates to form a haploid
       organism, which later forms female and male
       gametangia. Both have flagella.
     • The female gametes produce a pheromone that
       attracts male gametes. The male and female
       gametes fuse to produce a diploid organism, which
       produces numerous diploid flagellated zoospores.
       These disperse and produce more diploid organisms.
     • These eventually produce resistant resting sporangia
       that can survive dry and freezing weather.
     • The nuclei in sporangia eventually undergo meiosis
       to produce haploid zoospores.
Figure 31.7 Reproductive Structures of a Chytrid
31                           Diversity in the Kingdom Fungi

     • Zygomycetes (phylum Zygomycota) have
       coenocytic hyphae; they have only one diploid
       cell, the zygote.
     • Most form occasional stalked reproductive
       structures called sporangiophores.
     • Sporangiophores may have one or many
       sporangia.
     • One group are the fungal species in the most
       common mycorrhizal associations.
     • Black bread mold is Rhizopus stolonifer.
Figure 31.8 A Zygomycete
Figure 31.9 Sexual Reproduction in a Zygomycete (Part 2)
31                            Diversity in the Kingdom Fungi

     • The ascomycetes (phylum Ascomycota) are a
       large and diverse group with septate hyphae, and
       distinguished by the production of asci (singular
       ascus).
     • The ascus contains the products of meiosis.
     • There are two groups of ascomycetes:
         Those with an ascocarp are called
          euascomycetes (―true ascomycetes‖).
         Those without are called hemiascomycetes
          (―half ascomycetes‖).
Figure 31.10 Asci and Ascospores
31                           Diversity in the Kingdom Fungi

     • Most hemiascomycetes are microscopic and
       some are unicellular.
     • Baker’s or brewer’s yeast (Saccharomyces
       cerevisiae) is an ascomycete.
     • Hemiascomycete yeasts reproduce asexually by
       budding or fission.
     • Sexual reproduction occurs when two haploid
       cells of opposite mating types fuse.
     • In some, the zygote immediately undergoes
       meiosis. The entire cell becomes an ascus.
     • Four or eight ascospores are produced
       depending on whether the cells divide once after
       meiosis.
31                           Diversity in the Kingdom Fungi

     • The euascomycetes include some of the fungi
       known as mold. Neurospora is pink bread mold.
     • Many euascomycetes are plant parasites such as
       chestnut blight and Dutch elm disease.
     • Powdery mildews infect cereals, lilacs, roses, and
       other plants.
     • Cup fungi such as morels and truffles are
       euascomycetes. These produce huge numbers of
       spores and can be several centimeters in
       diameter.
Figure 31.11 Two Cup Fungi
31                           Diversity in the Kingdom Fungi

     • Penicillium is a genus of green molds. Some
       species produce the antibiotic penicillin.
     • P. roquefortii and P. camembertii provide the
       flavors to the cheeses Roquefort and Camembert.
     • Aspergillus tamarii is used to ferment soybeans to
       make soy sauce. A. oryzae is used in brewing the
       Japanese alcoholic beverage sake.
     • Some Aspergillus species that contaminate
       peanuts and pecans produce powerful mutagens
       called aflatoxins.
Figure 31.13 The Life Cycle of a Euascomycete
31                            Diversity in the Kingdom Fungi

     • About 25,000 species of basidiomycetes
       (phylum Basidiomycota) have been described.
     • They produce a wide variety of fruiting structures
       (basidiocarps): puffballs, mushrooms, and giant
       bracket fungi.
     • There are more than 3,250 species of
       mushrooms.
     • Agaricus bisporus is the common edible one;
       some Amanita mushrooms are deadly poisonous.
     • Bracket fungi are tree parasites.
     • Smut fungi parasitize cereal grains.
     • Basidiomycetes have septate hyphae.
Figure 31.14 Basidiomycete Fruiting Structures (Part 1)
Figure 31.14 Basidiomycete Fruiting Structures (Part 2)
Figure 31.14 Basidiomycete Fruiting Structures (Part 3)
Figure 31.15 The Basidiomycete Life Cycle
31                           Diversity in the Kingdom Fungi

     • Fungi not yet placed in any existing phyla are
       grouped as imperfect fungi or deuteromycetes.
     • Deuteromycetes currently include 25,000 species.
     • The sexual cycle has yet to be observed in these
       species.
     • DNA sequences can now be used to determine
       actual relationships between deuteromycetes and
       other fungi.
31                                        Fungal Associations

     • Almost all tracheophytes have mycorrhizae, which
       help make water and minerals more available to the
       plant.
     • Ectomycorrhizae are fungi that wrap around the
       root tips and acts as a sponge.
     • Endomycorrhizae infect the interior of the root.
     • The fungi get sugars, amino acids, and some
       vitamins from the plant.
     • The fungi might supply growth hormones as well,
       and protect the plant against attack by
       microorganisms.
     • Fungal–plant root interactions have existed for
       hundreds of millions of years.
Figure 31.16 Mycorrhizal Associations
31                                         Fungal Associations

     • Lichens are a meshwork of two different
       organisms. One is a fungus, and the other is a
       photosynthetic organism.
     • Lichens can survive harsh environments.
     • In spite of this hardiness, lichens are sensitive to
       air pollution because they cannot excrete toxic
       substances. Hence they are good biological
       indicators of air pollution.
     • The fungi of most lichens are ascomycetes.
     • The photosynthetic component may be either a
       cyanobacterium or a unicellular green alga.
31                                       Fungal Associations

     • There are about 13,500 ―species‖ of lichens.
     • The fungal components cannot grow
       independently of their photosynthetic partners.
     • The reindeer ―moss‖ is a lichen that is very
       important in the diet of large mammals in the
       arctic, subarctic, and boreal regions.
     • Lichen growth forms include crustose, foliose,
       and fruticose.
Figure 31.17 Lichen Body Forms (Part 1)
Figure 31.17 Lichen Body Forms (Part 2)
31                                        Fungal Associations

     • The most widely held interpretation is that the
       lichen relationship is a mutualistic one.
     • The algal cells in a lichen ―leak‖ photosynthetic
       products at a greater rate than do similar cells
       growing on their own.
     • On the other hand, the photosynthetic cells from
       lichens grow more rapidly on their own than when
       combined with a fungus.
     • Therefore, we could consider lichen fungi as
       parasitic on their photosynthetic partners.
31                                       Fungal Associations

     • Lichens can reproduce simply by fragmentation of
       the vegetative body called the thallus.
     • They can also reproduce by means of specialized
       structures called soredia (singular soredium).
     • These are composed of fungal hyphae and a few
       photosynthetic cells.
     • These become detached, are dispersed by air
       currents, and then develop into a new lichen.
     • If the fungal partner is an ascomycete or a
       basidiomycete, the fungus may undergo a sexual
       process, but the spores are released alone into
       the environment and fail to reestablish the lichen
       relationship.
Figure 31.18 Lichen Anatomy (Part 1)
Figure 31.18 Lichen Anatomy (Part 2)
31                                        Fungal Associations

     • Lichens are often the first colonists on newly
       exposed areas of rock.
     • They satisfy most of their nutritional needs from
       air, rainwater, and from the minerals absorbed
       from dust.
     • A lichen usually begins to grow shortly after a rain
       event. Eventually, as the lichen grows, its water
       content drops to less than 10 percent of its dry
       weight, and it becomes highly tolerant of
       temperature extremes.

				
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