26The Fungi(Eumycota)

Prescott’s Microbiology, 9th Edition

26The Fungi(Eumycota)

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.

LEARNING OUTCOMES

After reading this chapter, you should be able to:

discuss the importance of fungi in the environment as well as their practical importance
differentiate between a yeast and a mold, and between hypha and mycelium
compare and contrast different spore morphologies
explain how mating types govern sexual reproduction
infer what is meant by a “dikaryotic fungus”
assess the role of fungal decomposition on global carbon flux
explain why chytrids are unique among fungi
identify several habitats where chytrids grow
describe general chytrid morphology
recognize zygomycetes in nature
outline the Rhizopus life cycle
list at least two reasons zygomycetes benefit humans
describe the functional importance of Glomeromycota
differentiate between ectomycorrhizae and arbuscular mycorrhizae
describe ecological, economic, and medical importance of ascomycetes
diagram the Saccharomyces cerevisiae life cycle
outline the mechanisms used by typical filamentous ascomycetes to reproduce, survive unfavorable conditions, and disperse
explain the function of sclerotia
recall the origin of St. Anthony’s fire, LSD and “sick building syndrome”
describe the life cycle of a typical basdiomycete
identify the fungi that cause plant rusts
describe one human and one plant disease caused by a basidiomycete
explain why the microsporidiaare no longer considered protists
describe the microsporidian life cycle and relate its unique morphology to its host invasion
list at least two human diseases caused by microsporidia

CHAPTER OUTLINE

Overview of Fungal Biology

A.Fungi—eukaryotic, spore-bearing organisms with absorptive (osmotrophic) metabolism and no chlorophyll; reproduce sexually and asexually

B.Mycologists—scientists who study fungi

C.Mycology—the study of fungi

D.Mycotoxicology—the study of fungal toxins and their effects on various organisms

E.Mycoses—diseases caused by fungi in animals

F.Belong to the kingdom Fungi (Eumycota)within the domain Eukarya; is a monophyletic group also known as the true fungi

G.Fungi are saprophytes obtaining nutrition from dead organic material

H.Fungal cell walls contain the polysaccharide chitin

I.The body of a fungus is called the thallus and varies amongst members in structure

1. Yeasts are single celled.

2. Multicellular masses are known as molds

3. The thallus of a mold is filaments known as a hyphae.

4. These hyphae can form a tangled mass known as mycelium

J. Reproduction can be sexual or asexual

Chytridiomycota

A.The simplest of the true fungi; not considered monophyletic

B.Terrestrial and aquatic fungi that reproduce asexually by forming flagellated zoospores or sexually with sporangia

C.Microscopic in size; may consist of single cells, a small, multinucleate mass, or a true mycelium (mat of hyphae)

D.Free-living members are saprotrophic

Zygomycota

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

B.Have coenocytic hyphae (no cross walls), with many haploid nuclei

C.Asexual reproduction leads to the formation of sporangiospores

D.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

E.Rhizopus stolonifer—commonly known as bread mold, but also grows on fruits and vegetables

Normally reproduces asexually
Reproduces sexually by fusion of gametangia if food is scarce or environment is unfavorable; zygospores (diploid) are produced and remain dormant until conditions are favorable; meiosis often occurs at time of germination
Rhizopus is an important pathogen in rice, causing seedling blight

F.Zygomycetes are used in the production of foods, anesthetics, coloring agents, and other useful products

Glomeromycota

A.Mycorrhizal fungi form mutualistic associations with plant roots, delivering nutrients to the plant and deriving nutrients from the plant

B.Ectomycorrhizae do not penetrate plant root cells; arbuscular mycorrhizae penetrate plant cells walls but not the plasma membrane

C.Reproduction is asexual with spores that germinate on plant roots, creating mycelia that penetrate the roots;fragmentation is another mode of propagation

Ascomycota

A.Members of this division cause food spoilage, and a number of plant diseases (e.g., powdery mildew, chestnut blight, ergot, and Dutch elm disease); they have also been important research organisms

B.Include many types of yeast, edible morels, and truffles, as well as the bread molds Neurospora crassa and Aspergillus nidulans

C.Mycelia have septate hyphae

D.Many produce conidiospores when reproducing asexually; some produce masses of hyphae (sclerotia) that survive winter and produce more hyphae or conidia

E.Ascospores (haploid spores located in a sac called an ascus) are formed when reproducing sexually; thousands of asci may be packed together in a cup-shaped ascocarp

F.Many yeast genera are classified with ascomycetes with Saccharomyces cereviseae being highly studied; it alternates between haploid and diploid states in response to nutrients

G.Aspergillus is widespread and can be pathogenic in immunocompromised individuals; some species are also used for fermenting saki and soy sauce; several Aspergillus genomes have been sequenced; insights into eukaryotic evolution and the immune system are obtained from comparative analyses

H.Most fungal pathogens in humans are ascomycetes including Candida, Blastomyces, and Histoplasma; sick building syndrome is caused by Stachybotrys; Aspergillus produces aflatoxin

Basidiomycota

A.Includes jelly fungi, rusts, shelf fungi, stinkhorns, puffballs, toadstools, mushrooms, and bird’s nest fungi

B.Basidia are produced at the tips of the hyphae, in which the basidiospores will develop; basidiospores are held in fruiting bodies called basidiocarps

C.Usefulness—many basidiomycetes are decomposers; some mushrooms serve as food (some are poisonous); one is the causative agent of cryptococcosis (Cryptococcus neoformans), a systemic infection involving the lungs and central nervous system

D.Urediniomycota and Ustilaginomycota

Previously placed in the basidiomycetes, even though they don't produce basidiocarps; this group includes important plant pathogens (smuts and rusts) and some human pathogens
Ustilago maydis causes corn smut; it is a dimorphic, yeastlike saprophyte; it penetrates the leaf with a specialized structure called an appresorium and triggers tumor formation, eventually releasing diploid spores (teliospores) that germinate to form haploid sporidia

Microsporidia

A.Includes odd fungi that have been considered as protists; perhaps evolving from endocytic chytrids

B.Lack mitochondria, peroxisomes, and centrioles, but contain mitosomes; genome is reduced in size and complexity

C.Upon germination on host cells, the spores release an organelle called the polar tube with such force that it pierces the host cell membrane allowing entrance; more spores then develop in the host

D.Common human pathology with infection includes diarrhea, pneumonia, encephalitis, and nephritis; can be used as a biocontrol agent for some insect pests

CRITICAL THINKING

© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.

Prescott’s Microbiology, 9th Edition

1. Fungi have complex life cycles that often include haploid and diploid stages, as well as a variety of

sexual and asexual strategies with specialized cells.What potential advantages are conferred through

these complex life cycles?Why would the production of motile cells be valuable?What is the role of

spores in the success of fungi?

Prescott’s Microbiology, 9th Edition

2.When bread or cheese becomes moldy as demonstrated by fruiting body and conidia appearance on the

surface many people cut the moldy parts off and continue to eat the items. Knowing how fungi grow,

why is this not guaranteed to remove the mold entirely?

3. Both bacteria and fungi are major environmental decomposers. Competition naturally occurs in a given

environment, but fungi generally have an advantage. What characteristics specific to the fungi provide

this advantage?

4. Some fungi can reproduce both sexually and asexually. What are the advantages and disadvantages to

each mode?