RAVEN 9/e
CHAPTER 31: FUNGI
WHERE DOES IT ALL FIT IN?
Chapter 31 is consistent with type of coverage provided in Chapters 27 through 30 and highlights the fungal diversity. Students should be encouraged to recall the principles of eukaryotic cell structure and evolution associated with the particular features of fungal cells. The information in chapter 31 does not stand alone. Students should know that fungi and other organisms are interrelated and originated from a common ancestor of all living creatures on Earth.
SYNOPSIS
Even though fungi have been traditionally classified with plants, the two kingdoms have little in common. In fact, molecular data show that fungi are more closely related to animals than plants. But, in comparing plants versus fungi, we see that plants are photosynthetic, while fungi are heterotrophs obtaining nutrients by secreting enzymes into their substrate and absorbing the digested materials. Plants are composed of several types of cells organized into tissues and organs, while fungi are basically filamentous, composed of hyphae packed into complex structures like mushrooms. Plant cell walls contain cellulose, fungi cell walls are mostly polysaccharides impregnated with chitin. Both plants and fungi reproduce sexually and asexually. Plants and fungi are also similar in their immobility and their linear reproductive structures. Fungi are also different from either plants or animals in their cell division. The nuclear envelope remains intact during mitosis, the spindle apparatus forms within the nucleus and is regulated by spindle plaques. Centrioles are found in animals, but not in fungi or plants. Fungi are similar to prokaryotes in their ecological and commercial value as decomposers and in food production. They are among the few organisms that are able to decompose lignin, a major constituent of wood. Fungi made it possible for plants to colonize on land through mutualistic associations between fungal organisms and plant roots.
Fungi are composed of filamentous hyphae, which are slender filaments; a mass of hyphae is called the mycelium. Hyphae can be either non-septate where there is no separation between individual cells except where reproductive cells form; or, hyphae are septate, but the barrier between vegetative cells can be partially complete allowing cytoplasm to flow freely from one cell to the next or the septae are complete where reproductive cells are formed. All fungal nuclei are haploid, except those of the zygote. Homokaryotic hyphae contain genetically similar nuclei while heterokaryotic hyphae are derived from two genetically distinct individuals. In monokaryotic hyphae each cellular compartment possesses only one haploid nucleus while dikaryotic hyphae possess two genetically distinct haploid nuclei per cellular compartment. Spores are the most common means of reproduction and are formed by sexual or asexual processes.
The four major groups of fungi are: Chytridiomycota, Zygomycota, Basidiomycota, and Ascomycota. Chytridiomycota are aquatic, flagellated fungi that produce motile, flagellated zoospores and play a symbiotic role in the ecosystem. Phylum Zygomycota contain about 1050 zygomycetes species. In the Zygomycota, hyphae fuse and produce a hard-walled zygote that undergoes meiosis at germination. The group is named after a feature of the sexual phase of the life cycle called a zygosporangium. Zygomycota contains decomposers, parasitic forms and human pathogens, with the most common species, Rhizopus, the bread mold. The group Basidiomycota includes the more familiar fungi including the mushrooms, toadstools, puffballs, jelly fungi and shelf fungi as well as many plant pathogens, rusts and smuts. This group is named for their characteristic reproductive structure, the basidium. In both the Ascomycota and the Basidiomycota, reproductive cells are produced from dikaryotic hyphae with meiosis immediately following syngamy. Basidiomycetes produce four haploid spores on the tip of a club-shaped basidium. Phylum Ascomycota contain about 75% of all known fungi, including: bread yeasts, common molds, morels and truffles, chestnut blight fungi and Dutch elm disease causing fungi, and the penicillin producing ascomycetes. Ascomycetes produce eight haploid spores within a sac-like ascus. Asexual reproduction is common in Ascomycota, and also occurs in the Zygomycota, but is rare in the Basidiomycota. Yeasts are important in the production of bread, beer, wine, and in genetic research.
Fungi form important symbiotic associations with plants and animals including: lichens, mycorrhizae, endophytes, mutualistic animal symbiosis and fungal parasites and pathogens symbioses. Lichens are a mutualistic association between a fungus and a photosynthesizer. They generally inhabit cold, dry inhospitable environments and help prepare the habitat for other organisms. They are extremely sensitive to pollution. Mycorrhizae form mutualistic associations between the fungi and plant roots. There are two types of mycorrhizae: (1) arbuscular mycorrhizae, the most common, in which the fungal hyphae penetrate the outer cells of the plant root and, (2) ectomycorrhizae where the hyphae surround but do not penetrate the cell walls of the roots. They likely increase crop yields with less input of energy, provide better growth in poor soils, and may have added plants in their initial invasion of the land. Arbuscular mycorrhizae helped early plants succeed on poor soils and are being studied for their potential of increasing crop yields. Ectomycorrhizae are symbionts of temperate tree and shrub roots. Endophytes are plants that have fungi living inside them in intercellular spaces and may provide protection against herbivores by producing toxins and/or other deterrents. A mutualistic fungal-animal symbiosis has been identified in ruminant animals that host fungal organisms in their gut. Another example includes the leaf cutter ants that grow fungi in underground gardens. Yet, another symbioses relation exists between fungal parasites and pathogens. One example would be a parasitic fungal-animal symbiosis, such as that found in the disease thrush caused by Candida species. Still other parasitic/pathogen relationships exist in plants as fungal pathogens of plants. In addition, animals can be ill from consuming the plants. An example of this is toxic compounds produced by some Aspergillus species growing on corn, peanuts and cotton seed. When consumed by people, damage can occur to the kidneys and the nervous system.
LEARNING OUTCOMES
31.1 Defining Fungi
· Identify characteristics that distinguish fungi from other eukaryotes.
· Compare mitosis in fungi and animals.
· Explain why fungi are useful for bioremediation.
31.2 Microsporidia: Unicellular Parasites
· Explain characteristics that led microsporidians to be classified as protists.
· Describe evidence for placing microsporidians with fungi.
31.3 Chytridiomycota and Relatives: Fungi with Flagellated Spores
· Distinguish between blastocladiomycetes and microsporidians.
· Explain the meaning of “chytrid.”
· Discuss possible uses of neocallimastigamycetes.
31.4 Zygomycota: Fungi that Produce Zygotes
· Describe the defining feature of the zygomycetes.
· Explain the advantage of zygospore formation.
31.5 Glomeromycota: Asexual Plant Symbiotes
· Explain why glomeromycota is now considered separate from zygomycetes.
31.6 Basidiomycota: The Club (Basidium) Fungi
· Explain which cells in the life cycle of a basidiomycete are diploid.
· Distinguish between primary and secondary mycelium in basidiomycetes.
31.7 Ascomycota: The Sac (Ascus) Fungi
· Compare the ascomycetes and the basidiomycetes
· List the ways ascomycetes affect humans.
31.8 Ecology of Fungi
· Identify a trait that contributes to the value of fungus in symbiotic relationships.
· Describe the living components of a lichen.
· List examples of fungal associations with different organisms.
31.9 Fungal Parasites and Pathogens
· Review the pathogenic effects of fungi and the targets they affect.
· Explain why treating fungal disease in animals is particularly difficult.
COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information will inhibit the learning of concepts related to the misinformation. The following concepts covered in Chapter 31 are commonly the subject of student misconceptions. This information on “bioliteracy” was collected from faculty and the science education literature.
· Students do not believe that fungi contain organelles
· Students believe that fungi are a type of plant
· Students believe that fungi are single celled
· Students do not know that fungi carry out sexual reproduction
· Students are unaware that fungi produce eggs
· Students do not understand that fungi can produce zygotes which they think are only found in animals
· Students do not classify yeast as fungi
· Students are unaware that fungi grow in aquatic environments
· Students believe that all fungi produce mushrooms
· Students believe that the body of the fungus are the above-ground reproductive structures
· Students are unaware of fungal diseases
· Student believe that fungi only consume decaying matter
· Students confuse the function of hyphae with roots
· Students do not realize that fungi undergo meiosis
· Students do not realize that hyphae grow my mitosis
· Students believe that fungi only grow in the dark
· Students believe that lichens are self-sufficient and have no environmental needs
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
It is important for the students to understand the uniqueness of fungal genetics and reproduction and the terminology associated with them. Fungi are unique in that even the largest mushrooms are no more than a mass of filaments. They do not exhibit extensive tissue differentiation or specific organs and organ systems as do plants and animals. For the most part, the only specialized cells in fungi are those associated with sexual or asexual reproductive processes.
Nearly all fungi fruit above ground so that their spores are readily dispersed by the wind and rain. Truffles are an exception in that they fruit underground. They also produce chemicals similar to certain animal pheromones. Animals, especially pigs, are attracted to the odor of mature truffles, dig them up, and distribute the spores.
Discuss particularly common mushrooms found in your area. Stress the importance of knowing the identity of a mushroom before eating it. Some are hallucinogenic; others cause mild to serious gastrointestinal upsets. A few, the amanitas in particular, are deadly poisonous as they produce toxins that degrade RNA in the liver as it attempts to metabolize them. Symptoms of poisoning by these mushrooms do not show until four to five hours after ingestion, frequently too late for blood dialysis. Blood dialysis is currently the only treatment other than a complete liver transplant!
The caps of gill and pore fungi are always situated so that the basidial layer is perpendicular to the ground. Basidiospores are dependent on gravity to fall out of the gills or pores. Any intervening fungal tissue would defeat the reproductive process. Predaceous fungi are extremely interesting. One species possesses a noose derived from specialized haustorium cells. When a nematode enters the noose and touches the cell wall, the noose contracts and traps it. The fungus then digests the nematode. Another form has cloverleaf-shaped sticky pads that attach to various soil organisms. Attempts are being made to culture such fungi on an agricultural level because soil nematodes destroy enormous amounts of commercial crops each year.
Discuss wheat rust and its primary and secondary hosts. Eradicating barberry bushes eliminates the second host and, therefore, the infection on the wheat crop. Several naturally rust-resistant strains of wheat have also recently been developed.
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the lecture and the textbook. A complete understanding of biology content provides students with the tools to synthesize new hypotheses and knowledge using the facts they have learned. The following table provides examples of assessing a student’s ability to apply, analyze, synthesize, and evaluate information from Chapter 31.
Application / · Have students describe similarities between the nutritional needs of bacteria and fungi.· Have students explain if all fungi can be inhibited by chemicals that prevent the successful completion of meiosis.
· Ask students to distinguish between the mushrooms of Ascomycetes and Basidiomycetes.
Analysis / · Have students compare and contrast fungi and plants.
· Have students compare and contrast fungi and protists.
· Ask students to explain why antibiotic treatments of humans usually lead to an increase in fungal infections.
Synthesis / · Ask students explain a way that mycorrhizae can be exploited in agriculture.
· Have students design a strategy to classify a newly discovered fungus that was found dried and dead in the body of a 7000 year old mummified body.
· Ask the students come up with a strategy using fungi to reduce the amount of household wastes entering landfills.
Evaluation / · Ask students evaluate the effectiveness and safety of a home remedy that recommends eating moldy oranges as a way of fighting off bacterial infections.
· Ask students to evaluate the strengths and weaknesses of using yeast as a model for human genetics.
· Ask students to evaluate the effectiveness and safety of any medical treatment used to kill pathogen fungi in humans.
VISUAL RESOURCES
Show slides, lots of them, illustrating the variety in shapes, forms, and color.
Bring in as many examples of fungi as you can. A greater variety is now available in grocery stores than ever before. These may include the Japanese Shiitake (black forest mushroom), Pleurotus ostreatus (oyster mushroom), Auricularia (ear fungus), dried morelles, and many chanterelles as well as the ever present Agaricus campestris bisporus (have two rather than four basidiospores).
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Computer Modeling of Fungal Hyphae.
Introduction
Few students realize that researchers use computerized mathematical modeling systems to study the growth of fungi. This demonstration provides a visual way of demonstrating the growth of a fungal mycelium. It gets students thinking about the ways biological growth patterns obey the laws of physics.
Materials
· Computer with Media Player and Internet access
· LCD hooked up to computer
· Web browser linked to the Lindenmayer systems in microbiology website hosted by La Trobe University, Bendigo, Australia at http://coco.ccu.uniovi.es/malva/sketchbook/lssketchbook/examples/fungal/fungal.htm.
Procedure & Inquiry
- Review the stages of fungal germination and growth to the class.
- Then explain to students that mathematic modeling is often used to better study the growth of fungi. Tell them that the information is often useful in understanding how to control fungal growth for commercial and medical applications.
- Start the first animation showing the rapid fungal growth.
- Ask the students to explain any growth patterns including identifying older and younger parts of the mycelium.
- Start the first animation showing the slowed down fungal growth.
- Again, ask the students to explain any growth patterns including any uniformity or irregularities in the growth.
- Ask the students to explain if the growth model shown here is applicable to understanding plant growth.
LABORATORY IDEAS