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25 The Fungi (Eumycota), Slime Molds, and Water Molds

 

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 protistsCthe slime molds and water molds, which resemble fungiCare also presented in this chapter.

 

CHAPTER OBJECTIVES

After reading this chapter, you should be able to:

! discuss the distribution of fungi and their roles in the environment

! discuss the morphological characteristics of the fungi

! describe the external digestion of organic matter by fungi

! explain the formation of both asexual and sexual spores for reproduction

! discuss the five major types of organisms in this kingdomCzygomycetes, ascomycetes, basidiomycetes, deuteromycetes, and chytridsCand the basis upon which fungi are assigned to these categories

! discuss the slime molds and water molds, and their resemblance to fungi, even though they are phylogenetically distinct

 

CHAPTER OUTLINE

I. Introduction

A. FungiCeucaryotic, spore-bearing organisms with absorptive metabolism and no chlorophyll that reproduce sexually and asexually

B. MycologistsCscientists who study fungi

C. MycologyCthe study of fungi

D. MycotoxicologyCthe study of fungal toxins and their effects on various organisms

E. MycosesCdiseases in animals caused by fungi

F. Belong to the kingdom Fungi within the domain Eucarya

G. The kingdom is a monophyletic group known as the eumycota (true fungi)

II. Distribution

A. Primarily terrestrial with a few freshwater and marine organisms

B. Many are pathogenic in plants or animals

C. Form associations with plant roots (mycorrhizae) or with algae or cyanobacteria (lichens)

III. Importance

A. Beneficial

1. DecomposersCbreak down organic material and return it to environment

2. Industrial fermentationCbread, wine, beer, cheese, tofu, soy sauce, steroid manufacture, antibiotic production, and the production of the immunosuppressive drug cyclosporine

3. ResearchCfundamental biological processes can be studied in simple eucaryotic organism

B. Detrimental

1. Major cause of plant diseases

2. Cause of many animal and human diseases

IV. Structure

A. ThallusCbody or vegetative structure of a fungus

B. ChitinCnitrogen-containing polysaccharide consisting of N-acetyl glucosamine residues; found in the cell wall

C. YeastCunicellular 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

D. MoldCa fungus with long, branched, threadlike filaments

E. HyphaeCthe filaments of a mold; may be coenocytic (i.e., have no cross walls within the hyphae) or septate (i.e., have cross walls)

F. MyceliaCbundles or tangled masses of hyphae

G. DimorphismCa 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

V. Nutrition and Metabolism

A. Most fungi are saprophytes, securing nutrients from dead organic material

B. Fungi secrete hydrolytic enzymes that promote external digestion

C. They are chemoheterotrophicCuse organic materials as sources of carbon, electrons, and energy

D. Glycogen is the primary storage polysaccharide

E. Most are aerobic (some yeasts are facultatively anaerobic); obligate anaerobic fungi are found in the rumen of cattle

VI. Reproduction

A. Asexual reproductionCthere are several possible mechanisms, including:

1. Transverse fission

2. Budding of vegetative cells or spores

3. Direct spore production

a. Hyphal fragmentationCcomponent cells behave as arthrospores or chlamydiospores (if enveloped in thick cell wall before separation)

b. Sporangiospores are produced in sporangium (sac) at the end of an aerial hypha (sporangiophore)

c. Conidiospores are unenclosed spores produced at the tip or on the sides of aerial hypha

d. Blastospores are produced when a vegetative cell buds off

B. Sexual reproduction

1. Involves the union of compatible nuclei

2. Some 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

a. Fusion of gametes (haploid)

b. Fusion of gamete-producing bodies (gametangia)

c. Fusion of hyphae

d. Immediate fusion of nuclei and cytoplasm

e. Delayed fusion of nucleiCone cytoplasm with two haploid nuclei (dikaryotic stage)

4. Zygotes can develop into spores (zygospores, ascospores, or basidiospores)

VII. Characteristics of the Fungal Divisions

A. Division ZygomycotaCzygomycetes

1. Most are saprophytes; a few are plant and animal parasites

2. Coenocytic hyphaeCno crosswalls

3. Haploid nuclei

4. Sporangiospores (asexual)

5. Zygospores (sexual)Ctough, thick-walled zygotes that can remain dormant when the environment is too harsh for growth

6. Representative member: Rhizopus stoloniferCbread mold (also grows on fruits and vegetables)

a. Normally reproduces asexually

b. Reproduces sexually by fusion of gametangia if food is scarce or environment is unfavorable

c. Zygospores (diploid) are produced and remain dormant until conditions are favorable

d. Meiosis often occurs at time of germination

7. These are used in the production of foods, antibiotics, coloring agents, and other useful products

B. Division AscomycotaCascomycetes

1. Members of this division cause food spoilage, mildew, chestnut blight, and Dutch elm disease

2. They include many yeasts, edible morels, and truffles, as well as the pink bread mold Neurospora crassa

3. The mycelia are septate

4. They produce multinucleate conidiospores (asexual)

5. Ascospores (sexual) haploid spores are located in a sac (ascus)

6. Thousands of asci may be packed together in a cup-shaped ascocarp

C. Division BasidiomycotaCbasidiomycetes

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. UsefulnessCmany are decomposers; some mushrooms serve as food (some are poisonous); one is the causative agent of cryptococcosis; and some are plant pathogens

D. Division DeuteromycotaCdeuteromycetes (commonly called Fungi Imperfecti)

1. This is a classical division based on 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 dividion

2. Most are terrestrial; a few are freshwater or marine organisms

3. Most are saprophytes or plant parasites; some are parasitic on other fungi; some trap and consume nematodes; and some are human parasites, causing ringworm, athlete=s foot, histoplasmosis, and other diseases

4. Useful activities include producing antibiotics, giving aromas to cheeses, and fermenting soy sauce

E. Division ChytridiomycotaCsimplest of true fungi

1. Do not form true mycelia

2. Consist of a multinucleate mass that resembles mycelia

3. Form a thallus containing flagellated zoospores

4. Parasites and pathogens of algae, other fungi, terrestrial and aquatic plants

VIII. Slime Molds and Water MoldsCresemble fungi in appearance and life-style, but their cellular organization, reproduction, and life cycles are more closely related to protists

A. Division MyxomycotaCplasmodial slime molds

1. The multinucleated protoplasm (plasmodium) lack a cell wall and can, therefore, exhibit amoeboid movement

2. Feed by phagocytosis

3. Form ornate fruiting bodies when food and/or moisture are in short supply

4. Form spores with cellulose cell walls that are environmentally resistant

5. Germination produces either

a. MyxamoebaCnonflagellated

b. Swarm cellsCflagellated

6. These will fuse to form a diploid zygote to begin cycle again

B. Division AcrasiomycotaCcellular slime molds

1. The vegetative stage is amoeboid cells called myxamoeba

2. Form pseudoplasmodia when food is scarce 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, which produce spores

5. Released spores will later germinate to form haploid amoebae to begin the cycle again

C. Division OomycotaCoomycetes (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

3. Usually saprophytic in freshwater environments

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