THE FUNGI

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Chapter Overview

This chapter discusses the characteristics of the members of the kingdom Fungi. The diversity of these organisms is described, and their ecological and economic impact is discussed. In addition, certain protists-the slime molds and water molds, which resemble fungi-are also presented in this chapter.

Chapter Objectives

After reading this chapter you should be able to:

1. discuss the distribution of fungi and their roles in the environment
2. discuss the morphological characteristics of fungi
3. describe the external digestion of organic matter by fungi
4. explain the formation of both asexual and sexual spores for reproduction
5. discuss the five major types of true fungi: zygomycetes, ascomycetes, basidiomycetes, deuteromycetes, and chytrids
6. discuss the criteria upon which fungi are categorized
7. discuss the slime molds and water molds and their resemblance to fungi

These are the most important concepts you are learning in this chapter:

1.       Fungi are widely distributed and are found wherever moisture is present. They are of great importance to humans in both beneficial and harmful ways.

2.       Fungi exist primarily as filamentous hyphae. A mass of hyphae is called a mycelium.

3.       Like some bacteria, fungi digest insoluble organic matter by secreting exoenzymes, then absorbing the solubilized nutrients.

4.       Two reproductive structures occur in the fungi: (1) sporangia form asexual spores, and (2) gametangia form sexual gametes.

5.       The zygomycetes are characterized by resting structures called zygospores—cells in which zygotes are formed.

6.       The ascomycetes form zygotes within a characteristic saclike structure, the ascus. The ascus contains two or more ascospores.

7.       Yeasts are unicellular fungi—mainly ascomycetes.

8.       Basidiomycetes possess dikaryotic hyphae with two nuclei, one of each mating type. The hyphae divide uniquely, forming basidiocarps within which club-shaped basidia can be found. The basidia bear two or more basidiospores.

9.       The deuteromycetes (Fungi Imperfecti) have either lost the capacity for sexual reproduction, or it has never been observed.

Study Outline

1. Introduction
1. Fungi-eucaryotic, spore-bearing organisms with absorptive metabolism and no chlorophyll; reproduce sexually and asexually
2. Mycologists-scientists who study fungi
3. Mycology-the study of fungi
4. Mycotoxicology-the study of fungal toxins and their effects on various organisms
5. Mycoses-diseases in animals caused by fungi
6. Belong to the kingdom Fungi within the domain Eucarya; is a monophyletic group known as the eumycota (true fungi)
2. Distribution
1. Primarily terrestrial with a few freshwater and marine organisms
2. Many are pathogenic in plants or animals
3. Form beneficial associations with plant roots (mycorrhizae) or with algae or cyanobacteria (lichens)

3. Importance
1. Decomposers-break down organic material and return it to environment
2. Major cause of plant disease; also cause disease in animals, including humans
3. Industrial fermentation-bread, wine, beer, cheese, tofu, soy sauce, steroid manufacture, antibiotic production, and the production of the immunosuppressive drug cyclosporine
4. Research-fundamental biological processes can be studied in these simple eucaryotic organisms
4. Structure
1. Thallus-body or vegetative structure of a fungus; fungal cell walls are usually composed of chitin, a nitrogen-containing polysaccharide consisting of N-acetyl glucosamine residues
2. Yeast-unicellular fungus with single nucleus; reproduces asexually by budding, or sexually by spore formation; daughter cells may separate after budding or may aggregate to form colonies
3. Mold-a fungus with long, branched, threadlike filaments
1. Hyphae-the filaments of a mold; may be coenocytic (i.e., have no cross walls within the hyphae) or septate (i.e., have cross walls)
2. Mycelia-bundles or tangled masses of hyphae
4. Dimorphism-a property of some fungi, which change from the yeast (Y) form (within an animal host) to the mold (M) form (in the environment); this is referred to as the YM shift; the reverse relationship exists in plant-associated fungi
5. Nutrition and Metabolism
1. Most fungi are saprophytes, securing nutrients from dead organic material (chemoorganoheterotrophs); fungi secrete hydrolytic enzymes that promote external digestion
2. Glycogen is the primary storage polysaccharide
3. Most are aerobic (some yeasts are facultatively anaerobic); obligate anaerobic fungi are found in the rumen of cattle
6. Reproduction
1. Asexual reproduction-occurs by several mechanisms
1. Transverse fission
2. Budding
3. Direct spore production
1. Hyphal fragmentation-component cells behave as arthrospores or chlamydiospores (if enveloped in thick cell wall before separation)
2. Sporangiospores are produced in sporangium (sac) at the end of an aerial hypha (sporangiophore)
3. Conidiospores are unenclosed spores produced at the tip or on the sides of aerial hypha
4. Blastospores are produced when a vegetative cell buds off
2. Sexual reproduction
1. Involves the union of compatible nuclei
2. Some fungi are self-fertilizing (male and female gametes produced on the same mycelium (homothallic), while others require outcrossing between different but sexually compatible mycelia (heterothallic)
3. Zygote formation proceeds by one of several mechanisms
1. Fusion of gametes
2. Fusion of gamete-producing bodies (gametangia)
3. Fusion of hyphae
4. Sometimes there is immediate fusion of nuclei and cytoplasm; however, more common is a delayed fusion of nuclei, resulting in the formation of a cell with two haploid nuclei (dikaryotic stage)
4. Zygotes can develop into spores (zygospores, ascospores, or basidiospores); spores are used for identification purposes and also aid fungal dissemination
7. Characteristics of the Fungal Divisions
1. Division Zygomycota-zygomycetes
1. Most are saprophytes; a few are plant and animal parasites
2. Coenocytic hyphae (no crosswalls), with many haploid nuclei
3. Asexual reproduction leads to the formation of sporangiospores
4. Sexual reproduction leads to the formation of zygospores; these are tough, thick-walled zygotes that can remain dormant when the environment is too harsh for growth
5. Representative member: Rhizopus stolonifer (commonly known as bread mold, but also grows on fruits and vegetables)
1. Normally reproduces asexually
2. Reproduces sexually by fusion of gametangia if food is scarce or environment is unfavorable
3. Zygospores (diploid) are produced and remain dormant until conditions are favorable
4. Meiosis often occurs at time of germination
6. Zygomycetes are used in the production of foods, anesthetics, coloring agents, and other useful products
2. Division Ascomycota-ascomycetes
1. Members of this division cause food spoilage, a number of plant diseases (e.g., powdery mildew, chestnut blight, ergot,and Dutch elm disease)
2. Include many types of yeast, edible morels, and truffles, as well as the pink bread mold Neurospora crassa
3. Mycelia are septate
4. Produce conidiospores when reproducing asexually
5. Ascospores (haploid spores located in a sac called an ascus) are formed when reproducing sexually
6. Thousands of asci may be packed together in a cup-shaped ascocarp
3. Division Basidiomycota-basidiomycetes (club fungi)
1. Includes smuts, jelly fungi, rusts, shelf fungi, stinkhorns, puffballs, toadstools, mushrooms, and bird's nest fungi
2. Basidia are produced at the tips of the hyphae, in which the basidiospores will develop
3. Basidiospores are held in fruiting bodies called basidiocarps
4. Usefulness-many basidomycetes are decomposers; some mushrooms serve as food (some are poisonous); one is the causative agent of cryptococcosis; and some are plant pathogens
4. Division Deuteromycota-deuteromycetes (commonly called Fungi Imperfecti)
1. This is a classical division grouping together fungi that lack a sexual reproductive phase or fungi for which a sexual reproductive phase has not been observed; more recently molecular systematics places the Deuteromycota among their closest relatives in the Eumycota and eliminates the Deuteromycota as a separate division
2. Most are terrestrial; a few are freshwater or marine organisms; most are saprophytes or plant parasites; some are parasitic on other fungi
3. Human impact
1. Some are human parasites (e.g., causing ringworm, athlete's foot, histoplasmosis)
2. Some are used industrially to produce antibiotics, cheese, soy sauce, and other products
3. Some produce substances that are highly toxic and carcinogenic to animals (e.g., aflatoxin and trichothecenes)
5. Division Chytridiomycota-chytrids (simplest of true fungi)
1. Terrestrial and aquatic fungi that reproduce asexually by forming motile zoospores
2. Microscopic in size; may consist of single cells, a small multinucleate mass, or a true mycelium
3. Reproduce asexually or sexually
4. Some saprophytic; others are parasites of algae, other true fungi, and plants
8. Slime Molds and Water Molds
1. Resemble fungi in appearance and life-style, but their cellular organization, reproduction, and life cycles are more closely related to protists
2. Division Myxomycota-plasmodial (acellular) slime molds
1. The multinucleated protoplasm (plasmodium) moves by amoeboid movement as it phagocytizes organic matter
2. Form ornate fruiting bodies when food and/or moisture are in short supply; fruiting bodies form spores with cellulose cell walls that are resistant to environmental extremes
1. Spores germinate to produce myxamoeba or flagellated swarm cells
2. Myxamoeba and swarm cells are initially haploid, but eventually fuse to form a diploid zygote
3. Zygote feeds, grows and carries out multiple nuclear divisions, giving rise to a plasmodium
3. Division Acrasiomycota-cellular slime molds
1. During the vegetative stage, amoeboid cells called myxamoeba feed phagocytically on bacteria and yeasts
2. When food is scarce, myxamoeba form pseudoplasmodia by aggregating and secreting a slimy sheath around themselves
3. Become sedentary and differentiate into prestalk and prespore cells
4. Form sorocarps that mature to sporangia; sporangia produce spores
5. Released spores will later germinate to form haploid amoebae to begin the cycle again
4. Division Oomycota-oomycetes (water molds)
1. Resemble fungi, but cell walls are composed of cellulose, not chitin
2. Produce a relatively large egg cell that is fertilized by a small sperm cell or an even smaller antheridium; zygote germinates forming asexual, flagellated zoospores
3. Usually saprophytic in freshwater environments; some parasitic in fish and plants