3/18/2019

1

Mushrooms, Molds, and Yeasts

Fungi

3/18/2019

2

Learning Objectives

By the end of this unit, a student should be able to…

– List distinguishing characteristics, describe a typical life

cycle, and give examples of each of the following fungal

groups: chytridiomycetes, zygomycetes, glomeromycetes,

ascomycetes, and basidiomycetes

– Describe the body plan of a fungus

– Summarize the ecological significance of fungi as

decomposers

– Describe the important ecological role of mycorrhizae

– Characterize the unique nature of a lichen

– Summarize some specific ways that fungi affect humans

economically

– Summarize the importance of fungal diseases to

agriculture and medicine; giving specific examples

General Characteristics

Heterotrophs

– Fungi cannot make their own food like plants. They depend upon other

organisms for their carbon source.

Feed by Absorption

–Secrete digestive enzymes onto food

– then absorb predigested food

–Saprotrophs: organisms that feed on dead organic matter

Heterotrophic by Absorption

Fungi get carbon from organic sources

Hyphal tips release enzymes

Enzymatic breakdown of substrate

Products diffuse back into hyphae

Product diffuses back

into hypha and is used

Nucleus hangs back

and “directs”

Distinguishing characteristics of Fungi

• Cell walls contain chitin

• Fibrous polysaccharide

• Polymer of a n-acetylglucosamine (derivative of

glucose)

• Also found in exoskeleton of arthropods

KEY CONCEPTS

Fungi are eukaryotic heterotrophs that absorb nutrients from their surroundings

3/18/2019

3

Learning Objective 2

What is the body plan of a fungus?

General Characteristics

Multicellular (except for yeasts)

Have cell walls made of chitin

Body made up of long, threadlike filaments

called hyphae

Hyphae branch and form a tangled mass

called the mycelium

Fungal Structure

Threadlike filaments (hyphae)

Branch and form a tangled mass (mycelium)

Hyphae

25 µm Mycelium

Fungal Structure

Threadlike filaments (hyphae)

Branch and form a tangled mass (mycelium)

Fungal Structure

Hyphae

– Perforated septa

(cross walls) divide

hyphae into

individual cells

– Some are

coenocytic

form elongated,

multinuclear cell

3/18/2019

4

Hyphae

In most fungi

–perforated septa (cross walls) divide hyphae into individual cells

In some fungi

–hyphae are coenocytic (form elongated, multinuclear cell)

– zygomycetes and glomeromycetes septate coenocytic

Hyphae

Hyphae

There is cytoplasmic flow between cells of septate hyphae

Movement of nutrients throughout mycelium

Fig. 26-1 (c-e), p. 557

Coenocytic

Dikaryotic

Septate

3/18/2019

5

KEY CONCEPTS

A fungus may be a unicellular yeast or a filamentous, multicellular mold consisting of long, branched hyphae that form a mycelium

Learning Objective 3

What is the life cycle of a typical fungus, including sexual and asexual reproduction?

Reproduction

Most fungi reproduce sexually and asexually by spores

Spores

–Produced on aerial hyphae

–Produced in a fruiting body

– Land in suitable spot and germinate

Fungal Reproduction • reproduce with Spores

• sexual

• asexual

• produced on aerial hyphae

• produced in a “fruiting

body”

Germination of a Spore Asexual Reproduction

3/18/2019

6

Yeast

Yeasts are a type of single celled fungus

– highly reduced mycelium

– reproduce asexually by budding

Budding yeast – nucleus divides; new

cell buds off of mother cell

Life Cycles of Fungi

Reproduction

Fungi reproduce by means of spores

–Asexual spores

–Sexual spores

Spores Produced by Divisions

Mitosis

– forms new mycelia when spores

germinate

Meiosis

–produces 4 different haploid (n) nuclei

–each nucleus becomes part of a spore

Asexual Spores

Can be produced by mitosis

–genetically similar

Spore germination

–Hyphal thread grows out of spore

–As it grows and branches, it develops into a mass of hyphae called a mycelium.

3/18/2019

7

Sexual Spores

Plasmogamy

– Fungi of two different mating types meet, hyphae fuse

Cytoplasm fuses but nuclei remain separate

Fungi enter dikaryotic (n + n) stage

–each new cell has one nucleus of each type

Karyogamy

– Fusion of nuclei

takes place in hyphal tip

results in diploid (2n) zygote nucleus

Meiosis – produces 4 different haploid (n) spores

Fungal Life Cycles

Fungal Life Cycle

n

haploid mycelium

dikaryotic

mycelium

Spores n

n

mitosis

meiosis

n

plasmogamy

n + n

karyogamy

2n

diploid

mycelium

n n

Fig. 26-4, p. 559

Large numbers of

haploid (n) spores are

produced by mitosis.

Spore germinates

and forms mycelium

by mitosis. Asexual

reproduction

1 Spores germinate

and form mycelia

by mitosis. Mycelia

Mycelia of two different

mating types fuse at their

tips, and plasmogamy

(fusion of cytoplasm)

occurs.

2

6 Spores are

released. Spores

Sexual

reproduction

Haploid stage

(n)

Dikaryotic stage

(n + n) 5

Meiosis results in

four genetically

different haploid ( n )

nuclei. Spores

develop around

nuclei.

Diploid

stage (2n) 3

Zygote nucleus

(2n)

Dikaryotic (n + n)

mycelium develops.

Dikaryotic mycelium

4 Karyogamy (fusion of nuclei)

occurs, forming a diploid (2 n )

zygote nucleus.

7 8

Meiosis Karyogamy

Plasmogamy

haploid mycelium

dikaryotic

mycelium

Spores

mitosis

meiosis

plasmogamy

karyogamy

diploid

mycelium

(n)

Fungal Life Cycles

asexual cycle of

haploid mycelium

Spores (n)

KEY CONCEPTS

Most fungi reproduce both asexually and sexually by means of spores

3/18/2019

8

Fungal Evolution Chytridiomycetes

Zygomycetes

Glomeromycetes

Ascomycetes

Basidiomycetes

Learning Objective 4

Support the hypothesis that fungi are opisthokonts, more closely related to animals than to plants

Opisthokonts

Fungi are opisthokonts

–along with animals and choanoflagellates

–based on chemical and structural characters

Flagellate Cells

Animals and chytrids have flagellate cells

–Example: chytrid gametes and spores

–Example: animal sperm

Flagellate cells propel themselves

–with single posterior flagellum

Platelike Cristae

Like animal cells, fungal cells have platelike cristae in their mitochondria

Flatties

3/18/2019

9

Classification

& Phylogeny

motile spores

zygosporangia

asci basidia

Fig. 26-5, p. 560

Ch

ytr

ids

Zyg

om

ycete

s

Glo

mero

mycete

s

Asco

mycete

s

Basid

iom

ycete

s

Evolution of

ascospores

Evolution of

basidiospores

Evolution of

dikaryotic stage

Loss of flagellum

Common

flagellate

ancestor

Motile gametes

and spores

Fungal Evolution

5 phyla

–Chytrids (Chytridiomycetes)

–Ascomycota: sexual and asexual reproduction; spores in ascus (sac)

–Zygomycota: sexual and asexual

reproduction; form zygospore

–Glomeromycota: arbuscular mycorrhizae

–Basidiomycota: sexual reproduction only; spores form on basidium

Learning Objective 5

Support the hypothesis that chytrids may have been the earliest fungal group to evolve from the most recent common ancestor of fungi

Chytrids (Chytridiomycetes)

Produce flagellated cells during life cycle

–no other fungi have flagella

Probably earliest fungi to evolve from flagellate protist

– common ancestor of all fungi

Learning Objective 6

List distinguishing characteristics, describe a typical life cycle, and give examples of each of these fungal phyla:

–Chytridiomycota

–Zygomycota

–Glomeromycota

–Ascomycota

–Basidiomycota

3/18/2019

10

Chytrids (Chytridiomycetes)

Produce flagellate cells

– no other fungi have

flagella

– a shared characteristic of

the opisthokonts

Probably earliest fungi to

evolve from flagellate protist

Chytridium convervae

Chytridiomycota – “chytrids”

Simple fungi

Produce motile spores

Mostly saprobes

and parasites in aquatic habitats

Could just as well be Protists

Chytrids

Reproduce both asexually and sexually

Gametes and zoospores are flagellated

Allomyces

–part of life is multicellular haploid thallus

–part is multicellular diploid thallus

Mature haploid thallus

♂ gametangium

♀ gametangium 2

♂ Gamete Haploid

zoospore

♀ Gamete

SEXUAL REPRODUCTION

HAPLOID (n)

GENERATION

DIPLOID (2n)

GENERATION

3

Meiosis

Plasmogamy

and karyogamy

5 Resting

sporangium Motile zygote

Zoosporangium

4 Zygote germinates

and develops into

diploid thallus.

ASEXUAL REPRODUCTION

(by mitosis) 7

Diploid zoospore

1

6

Resting

sporangium

“Chytrid” and Chytridiomycosis disease

– Massive amphibian die-off

– mainly in tropics

Caused by fungus Batrachochytrium dendrobatidis

Chytrids (Chytridiomycetes) Fungal Evolution

Chytridiomycetes

Zygomycetes

Glomeromycetes

Ascomycetes

Basidiomycetes

3/18/2019

11

Zygomycota – “zygote fungi”

Sex - zygosporangia

Asexual - common

Hyphae have no cross walls – coenocytic

– Septa form to separate the hyphae from reproductive structures

Grow rapidly

Many species of mycorrhizae

Rhizopus on strawberries

Zygomycota

Zygomycetes

Rhizopus (black bread mold)

– forms haploid thallus

–produces asexual spores and sexual spores

Asexual spores germinate

– form new thalli

Zygomycetes 2

In sexual reproduction

–hyphae of 2 different haploid mating types form gametangia

Plasmogamy occurs

–as gametangia fuse

Zygomycetes 3

Karyogamy occurs

–diploid zygote forms

– from which zygospore develops

Meiosis

–produces recombinant haploid zygospores

Zygomycetes 4

When zygospores germinate

–each hypha develops a sporangium at its tip

Spores are released

–develop into new hyphae

3/18/2019

12

Zygospore and suspensors

Zygomycota

Sexual reproduction

– hyphae of 2 different mating types form

gametangia

– Plasmogamy occurs as gametangia fuse

– Karyogamy produces zygote (2n)

– zygote develops into zygosporangium

Fig. 26-9b, p. 564

Zygomycota

Microsporidia

– Small, unicellular opportunistic

obligate intracellular parasites that

infect eukaryotic cells

– May be the smallest and simplest

eukaryotes

Zygomycota

Microsporidia

– Previously classified with yeasts, bacteria, and

protista

– Genome studies suggest that microsporidia

descended from a zygomycete ancestor