Chapter 10 Complex Heterotrophic Eukaryotes (Fungi)




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Chapter 10

Complex Heterotrophic Eukaryotes (Fungi)


Objectives

Kingdom Fungi. Know the characteristics of fungi. Know the ecological and economical importance of fungi. Understand the similarities and differences between fungi and other kingdoms of eukaryotes. Know some of the symbiotic relationships that involve fungi. Know the components of lichens and understand the roles of each component. Know the characteristics and life cycles, in detail, of zygomycetes, ascomycetes, and basidiomycetes.

Kingdom Fungi

The eukaryotic kingdom Fungi is diverse, versatile, and ecologically and economically important1. Fungi occupy diverse habitats (mostly terrestrial) and are important decomposers (saprobes) and participants in symbioses2. For example, the ascomycete Sphacelia can infect fescue grass (common cattle fodder); the fungus produces alkaloids that are poisonous to herbivores, thus protecting the grass. Fungi are ecologically important and essential; however, they can be an economical nuisance for food produces and distributors. Fungi can grow on living material; thus, causing diseases in plants and animals. Over 5000 species of fungi attack economically valuable crop and garden plants and over 175 species infect humans and domestic animals. For example, Claviceps purpurea, an ascomycete type fungus, infects rye plants and produces alkaloid toxins that can be found in breads made with infected rye grains. These alkaloids, in high doses, cause the disease Ergotism (or St. Anthony’s fire) that is accompanied by hallucinations, spasms, and convulsions; however in low doses, these alkaloids are used therapeutically to cause muscle constriction especially in treating high blood pressure.

Fungi are economically important. Certain yeasts produce ethanol and carbon dioxide and therefore are important for baking, brewing, and alcohol production. Fungi provide the distinctive flavors and aromas of some cheeses. Some fungi are delicacies, such as field mushrooms, shiitakes (both of which are grown locally), chanterelles (which are collected locally), truffles, and morels3. Importantly, fungi produce many antibiotics, such as penicillin. Cyclopsorin, derived from a soil inhabiting fungus, suppresses the human immune reactions that cause rejection of organ transplants.

Characteristics of Fungi. Although they share some characteristics with plants and some with animals, fungi have a unique combination of characteristics4. All fungi are heterotrophic eukaryotes that are filamentous (filaments termed hyphae and mass of hyphae termed mycelium), unicellular, or coenocytic (rarely). All fungi have chitin containing cell walls, in contrast to plant cell walls, which contain cellulose (although, a few fungi contain some cellulose in addition to chitin). Most fungi have no motile cells at any stage of their life cycle, in contrast to protists and most animals. All fungi reproduce5 asexually and, sexual reproduction, when present, is by zygotic meiosis; thus, all cells except the zygote are haploid. The most common method of asexual reproduction is by spores, which are usually produced on either sporangia or conidigenous cells. Sexual reproduction in fungi consists of three phases: plasmogamy, karyogamy, and meiosis (to return to the haploid state). Thus, interestingly, as you will see in the laboratory exercises, some fungi have heterokaryotic cells (i.e. two or more have genetically different nuclei).

In this lab, we will examine three major groups6 of fungi, Zygomyceta, Ascomyceta, and Basidiomyceta.



Zygomycetes7

A major ecological role of many zygomycetes is the formation of mycorrhizae. During sexual reproduction of zygomycetes, they produce a zygospore, or zygosporangium, from which the name is derived. Hyphae of zygomycetes are coenocytic (aseptate) meaning the hyphae lack cross walls and thus contain many nuclei within the same filament. The following describes the asexual and sexual reproduction of zygomycetes.



Specimen 1: Prepared slides (may require observation of two slides) of asexual and sexual structures of Rhizopus (bread mold)

  1. Observe the hyphae of Rhizopus at 100X. Notice that the hyphae are aseptate and exist in two common forms, the stolon (portion that traverses the surface of the substratum) and the rhizoids (portion that penetrates the substratum). Use your textbook figures to help you identify these forms. In the space below, draw and label a portion of stolon and a portion of rhizoids.

______X

  1. Observe the sporangia (a.k.a. sporangiophore, bodies for asexual reproduction) that produce the asexual spores at the tips of some hyphae. Asexual spore containing sporangia enlarge and separate from the rest of the hypha, forming a sac. Then, each nucleus in the sac, along with a bit of cytoplasm, is enveloped by a wall resulting in formation of a spore. When mature, the spores are released and germinate into individual hyphae. Draw and label a sporangiophore and spores below.

______X

  1. During sexual reproduction8, portions of two hyphae (now called gametangia) that are morphologically similar, but genetically distinct, form protrusions toward each other. The protrusions meet and the membranes fuse, forming a sac that then separates from the hyphae. The nuclei from the two hyphae fuse in pairs, and form a structure, the zygospore (or zygosporangium), containing many diploid nuclei. The zygospore9 has a thick, protective wall that is resistant to harsh conditions. Under favorable conditions, the nuclei undergo meiosis. The zygospore then “germinates,” giving rise to a sporangium with many haploid spores that then germinate to form new hyphae.

  2. Observe, draw, and label the fusing gametangia and the zygospores.

______X

Specimen 2: Cultures of Rhizopus (bread mold)

  1. Observe your bread mold culture under the dissecting microscope and draw and describe what you see.

______X

  1. Make a wet mount of a portion of the fungal culture on the bread. Observe at 100X. Draw a portion of what you see.

  2. Answer the following questions.

    1. Do you see any hyphae?

    2. Do you see any zygospores?

    3. Do you see any rhizoids?

    4. Do you think you are looking at Rhizopus? Why or why not?


Ascomycetes10

The ascomycetes include many familiar fungi such as truffles, morels, most yeasts, and some molds. During sexual reproduction, ascomycetes form a sac like structure, the ascus, which bears sexual spores. Hyphae of ascomycetes are septate (cross walls present); however, they are perforated, allowing organelles (including nuclei) and cytoplasm to pass through. Each hypha is made of many cells, and each cell contains one or two nuclei, depending of the stage of the life cycle. The following describes the asexual and sexual reproduction of ascomycetes.

Asexual reproduction of ascomycetes occurs by formation of uni  or multi nucleate haploid spores, conidia11, which form on stalks, conidiophores. Unlike zygomycetes, which produce spores within a sporangium, ascomycetes produce spores externally. Sexual reproduction12 of ascomycetes involves genetically different mating types (each of which undergo asexual reproduction also) that developed from different spores. When two compatible hyphae come in contact, large multinucleate sac like gametangia form from each type of hyphae. Plasmogamy of the gametangia occurs producing dikaryotic (each cell has two haploid nuclei) hyphae. The dikaryotic hyphae proliferate by mitosis and organize into a fruiting body termed ascocarp. Dikaryotic cells lining the ascocarp grow into sacs called asci (pl.; ascus, s.)13. The two nuclei in each ascus fuse to form a diploid zygote (the only diploid cell in the life cycle) that undergoes meiosis to produce four haploid nuclei within the ascus. Each of the resultant four nuclei undergoes a mitotic division resulting in eight haploid nuclei. Each nucleus becomes surrounded by a thick wall, forming an ascospore (sexual spores vs. conidia, which are asexual spores). When the ascospores are mature, the asci rupture and release the spores, which germinate into haploid, monokaryotic hyphae.

Specimen 3: Prepared slide of a cross section through the ascocarp of Peziza


  1. Observe the ascocarp (entire structure) under 40X and 100X. This cup like structure is a distinguishing feature of ascomycetes. Refer to the figure of the life cycle of ascomycetes found in your textbook.

  2. The ascocarp is lined by the asci, which each contain 8 (stained red) ascospores.

  3. Draw, at 400X, and label the ascocarp, hyphae that form the ascocarp main structure, asci, and ascospores.

Specimen 4: Yeast (unicellular ascomycetes), Saccharomyces cereviseae

Yeasts14 are economically invaluable for their use in cooking, brewing, and biotechnology. In addition, yeasts are important model organisms, especially for understanding molecular biology. Most yeasts are ascomycetes. Asexual reproduction of yeasts occurs by mitosis by budding (daughter cells of unequal size) or fission (daughter cells of equal size).



  1. Place a very small drop of yeast culture (grown from commercial baker’s yeast in a sucrose solution) on a slide.

  2. Dilute with a drop of water

  3. Observe at 40X and look for dividing cells. When you find dividing cells, observe at 400X and draw below.

  4. Are these yeast undergoing budding or fission? What observation(s) led to that conclusion?

Basidiomycetes

Basidiomycetes15 include mushrooms, puffballs, and some pathogenic fungi like the rust and smut fungi.

During sexual reproduction, basidiomycetes bear basidiospores (sexual spores) on the surface of a club shaped basidium (s.; basidia, pl.) born on a basidiocarp. Refer to the figures of the life cylce in your textbook. Hyphae of basidiomycetes have perforated septa; thus, each hypha is made of many cells and each cell contains one or two nuclei (number of nuclei depends on stage of life cycle).

Asexual reproduction in basidiomycetes is a variable character and less prominent than sexual reproduction. During sexual reproduction16 of basidiomycetes, monokaryotic mycelia of different strains undergo plasmogamy forming a dikaryotic mycelium that forms the basidiocarp (the conspicuous mushroom) through mitosis. Basidia differentiate along the lining of the basidiocarp (e.g. the underside of the mushroom cap). The terminal dikaryotic cell undergoes karyogamy, which is immediately followed by meiosis. Each resulting haploid nucleus migrates into the tips of the basidia and is “cut off,” forming basidiospores17.



Specimen 5: Common commercial mushrooms

  1. Specimen____________________

  2. Obtain a basidiocarp.

  3. Observe the stalk and cap

  4. Observe the veil (think layer on the upper surface) around the cap and the annulus around the stalk. Both the veil and the annulus are derived from a protective covering over very young basidiocarps.

  5. Slice the basidiocarp longitudinally.

  6. Observe the gills on the underside of the cap. The gills bear the basidia (will be seen in the coming specimens).

  7. Draw and label the cap, stalk, gills, veil, and annulus.

Specimen 6: Spore print specimen

  1. Choose one specimen with a spore print.

  2. Describe the spore print. Include shape and color.

  3. Make a wet mount of a sample of the spores.

  4. Observe the spores under 400X and draw and describe them.

  5. Draw and describe the mushroom specimen from which the spore print was made.

Specimen 7: Prepared slide of a cross section of the basidiocarp cap of Coprinus

  1. Observe the slide under the 100X objective.

  2. Observe the stalk and gills.

  3. Observe the layer of basidia lining the gills.

  4. Observe the basidiospores (red).

  5. Draw a basidium with basidiospores at 400X, and label.

Smuts and Rusts. Smut18 and rust19 fungi are plant parasites that do not produce basidiocarps, but do produce basidia and basidiospores. Corn smut20, Ustilago maydis, is a basidiomycete that parasitizes corn. The hyphae penetrate the tissue of the host plant and produce large distended growths in the corn kernels. At maturity, this growth is filled with the dikaryotic spores of the fungus that can overwinter. In the spring, karyogamy occurs in the spores, which then germinate to form a basidium and basidiospores, all within the corn kernel. The monkaryotic basidiospores germinate and the hyphae penetrate corn seedlings. A dikaroytic mycelium forms from plasmogamy of the monokaryotic hyphae and spreads through the corn plant causing formation of tumor like growths.

Rusts have a complex life cycle, which usually involves the obligate, sequential infection of two different plant hosts. For example, the wheat stem rust21, Puccinia graminis, infects barberry plants in the spring, and then form spores, which then infect wheat plants in the summer. An overwintering spore, released from infected wheat plants, then re infects new barberry plants in the spring. The wheat rust is one of the most destructive of all the species of parasitic fungi. Some control has been achieved by eradicating barberry plants near wheat fields, but total eradication of the secondary host is probably not possible. In recent years, crop scientists have concentrated on developing rust resistant wheat. This process is continuous because rusts adapt quickly to new varieties through mutation and natural selection.



Questions

  1. Describe at least one characteristic that fungi share with plants, but not animals, and one that fungi share with animals, but not plants.

  2. What characteristics, or combination of characteristics, of fungi are not found in other kingdoms?

  3. How can you distinguish the three types of fungi (zygomycetes, ascomycetes, and basidiomycetes) that we studied from each other?

  4. How does the life cycle exhibited by fungi differ from that exhibited by plants?

  5. How are fungi beneficial for farming? How are fungi detrimental for farming?




1 pp. 260-261

2 pp. 285-291

3 Fig. 14-3c

4 pp. 262-265

5 pp. 264-265

6 Table 14-1

7 pp. 268-269

8 Fig. 14-11

9 Fig. 14-12

10 pp. 269-272

11 Fig. 14-15

12 Fig. 14-14

13 Fig. 14-16

14 Fig. 14-31

15 pp. 272-274

16 Fig. 14-18

17 Fig. 14-19

18 pp. 279-282

19 pp. 278-279

20 Fig. 14-30

21 Fig. 14-29


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