Chapter 32. Defining Fungi Mycologists believe there may be as many as 1.5 million fungal species...

Chapter 32

Defining FungiMycologists believe there may be as many as

1.5 million fungal speciesSingle-celled or multicellularSexual or asexualSpecialized to extract and absorb nutrients

from surroundingsAnimal and fungi last shared a common

ancestor 460 MYASome debate on timing May have 670 MYA ancestor

2

7 monophyletic phylaMicrosporidiaBlastocladiomycotaNeocallismastigamycotaChytridiomycotaGlomeromycotaBasidiomycotaAscomycota

1 polyphyleticZygomycota

3

4

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

c.

Microsporidia Blastocladiomycota Zygomycota Neocallimastigomycota Chytridiomycota Glomeromycota Basidiomycota Ascomycota

Dikarya

Fungi

a. 10 μm b. h.g.300 μm200 μm300 μm e. f.d.300 μm500 μm

a: © Dr. Ronny Larsson; b: Contributed by Don Barr, Mycological Society of America; c: © Carolina Biological Supply Company/Phototake; d: Contributed by Don Barr, Mycological Society of America; e: © Dr. Yuuji Tsukii; f: © Yolande Dalpe, Agriculture and Agri-Food Canada;

g: © inga spence/Alamy; h: © Michael & Patricia Fogden

General Biology of the FungiMulticellular fungi consist of long, slender

filaments called hyphaeSome hyphae are continuousOthers are divided by septa

Cytoplasm flows throughout hyphaeAllows rapid growth under good conditions

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7


Hypha

Nuclei Nuclei

Pore

Septum

0.2 µm

Dikaryoticcell

Septa withpores

(right): © Garry T. Cole/ Biological Photo Service

Mycelium – mass of connected hyphaeGrows through and digests its substrate

Fungal cell walls include chitinAlso found in the hard shells (exoskeletons) of

arthropods

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Hyphae

Mycelium

10 μm

(inset): © Micro Discovery/Corbis; (right): © Michael & Patricia Fogden/Corbis

Hyphae may have more than one nucleusMonokaryotic – 1 nucleusDikaryotic – 2 nuclei

Both genomes transcribed

Sometimes many nuclei intermingle in the common cytoplasm of the fungal myceliumHeterokaryotic – nuclei from genetically

distinct individualsHomokaryotic – nuclei are genetically similar

to one another

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Fungi have an unusual mitosisCell is not relevant unit of reproductionNuclear envelope does not break down and re-

formInstead, the spindle apparatus is formed within

itFungi lack centrioles

Spindle plaques regulate microtubule formation during mitosis

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ReproductionCapable of both sexual and asexual

reproductionSexual reproduction

Fusion of two haploid hyphae of compatible mating types In some fungi, fusion immediately results in a

diploid (2n) cell Others, have a dikaryon stage (1n + 1n) before

parental nuclei form diploid nucleusMay form mushrooms or puffballs

11

Spores are the most common means of reproduction among fungiMay form from sexual or asexual processesMost are dispersed by wind

12


10 μm© Eye of Science/Photo Researchers, Inc.

Nutrition

Obtain food by secreting digestive enzymes into surroundings

Then absorb the organic molecules produced by this external digestion Great surface area-to-volume ratio

Fungi can break down cellulose and ligninDecompose woodSome fungi are carnivorous

13

b.


© L. West/ Photo Researchers, Inc.


a. 400 µm

Fungus

Nematode

Fungalloop

© Carolina Biological Supply Company/Phototake

MicrosporidiaObligate, intracellular, animal parasitesLong thought to be protistsLack mitochondria

Ancestors lost them

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Fungi

Encephalitozoon cuniculiCommonly cause disease in immunosuppressed

patientsInfect hosts with their spores, which contain a

polar tubeInfects intestinal and neuronal cells, leading to

diarrhea and neurodegenerative disease

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0.5 μm

Spore

Polar tube

(left): © Daniel P. Fedorko

Chytridiomycota

Chytridiomycetes or chytridsAquatic, flagellated fungiClosely related to ancestral fungiHave motile zoosporesBatrachochytrium dendrobatidis has been

implicated in amphibian die-offs

16N

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Fungi


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ZoosporeZoospore

Contributed by Daniel Wubah, Mycological Society of America


Chytrid

Oedogoniumfilament

Contributed by Don Barr, Mycological Society of America

BlastocladiomycetesUniflagellated zoosporesAllomyces example

Water moldHaplodiplontic life cycleFemale gametes secrete pheromone to attract

male gametesGiant mitochondria in its zoospores

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19


100 µm

Female gametangiumMale gametangium

Haploid gametes (n)

Sporophyte(2n)

Gametophyte (n)

Mature sporophyte (2n)

Asexual sporangium

Sporophyte

Sporangium (2n)

n

2n

a. b.

Younggametophyte (n)

Haploidzoospore (n)

Young sporophyte (2n)

Youngsporophyte (2n)

Asexual reproduction

Diploidzoospores (2n)

Maturegametophyte (n)

FERTILIZATION

MEIOSIS

MIT

OS

IS

a: © Carolina Biological Supply Company/Phototake

NeocallimastigomycotaDigest plant biomass in mammalian herbivore

rumensMammal depends on fungi for sufficient

caloriesGreatly reduced mitochondria lack cristaeZoospores have multiple flagellaHorizontal gene transfer brought cellulase

gene from bacteria into Neocallimastix genome

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Zygomycota Zygomycetes are incredibly diverseNot monophyletic – still under researchInclude the common bread moldsA few human pathogens

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Fungi

Sexual reproductionFusion of gametangiaHaploid nuclei fuse to form diploid zygote

nuclei – karyogamyDevelops into zygosporangium in which

zygospore developsMeiosis occurs during germination of

zygospore Releases haploid spores

Asexual reproduction more commonSporangiophores have sporangia that release

spores

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a.

b.

Rhizoid

Sporangiophore

Sporangium

– Mating strain

n

n + n

(+) (–)

Zygosporangium

Hypha

Gametangia

Spores

Spores

MEIOSIS2n

700 µm

Asexualreproduction

(Meiosis occursduring germination)

FERT

ILIZ

ATIO

N

KARYOGAMY

GERM

INATIO

N

Germinatingzygosporangium

+ Mating strain

GERMINATION


a: © Carolina Biological Supply Company/Phototake

Glomeromycota Glomeromycetes are a tiny group of fungiForm intracellular associations with plant

roots called arbuscular mycorrhizaeCannot survive in absence of host plant

No evidence of sexual reproduction

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.N

eoc

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Fungi

BasidiomycotaBasidiomycetes are some of the most familiar

fungiMushrooms, toadstools, puffballs, shelf fungi,

etc.Also important plant pathogens like rusts and

smuts

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Fungi


Named for basidium – club-shaped sexual reproductive structure

Karyogamy occurs within basidiaOnly diploid cell in life cycle

Meiosis followsThe four haploid products are incorporated

into basidiospores

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a.

b.

2nn

– Mating strain

n + n

Basidiocarp

Basidium

Basidium

SterigmaBasidiospores

MEIOSIS

Zygote

Primary mycelium(monokaryotic)

Secondarymycelium(dikaryotic)

+ Mating strain

KARYOGAMY

Gills linedwith basidia

a: © Alexandra Lowry/The National Audubon Society Collection/Photo Researchers, Inc.

FERTILIZATION

Spore germination leads to the production of monokaryotic hyphaeResults in a monokaryotic mycelium, or

primary myceliumDifferent mating types of monokaryotic

hyphae may fuseResults in a dikaryotic mycelium, or secondary

mycelium Heterokaryotic myceliumBasidiocarps (mushrooms) are formed entirely

of secondary mycelium

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Ascomycota Contain about 75% of the known fungiIncludes bread yeasts, common molds,

cup fungi, truffles, and morelsSerious plant pathogens – cause of

chestnut blight and Dutch elm diseasePenicillin-producing fungi are in the

genus Penicillium

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Neo

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Zyg

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Fungi

Named for ascus – microscopic, saclike reproductive structure

Karyogamy occurs within asciOnly diploid nucleus of life cycle

Asci differentiate in ascocarpMeiosis and mitosis follow, producing 8

haploid nuclei that become walled ascospores

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b.

a.

Developingmycelium

Asexualreproduction

Conidia

Ascospore

Each haploidnucleus dividesonce by mitosis

Dikaryotic hyphae formFrom ascogonium

Fully developed ascocarp composedof dikaryotic (ascogenic) hyphaeand sterile hyphae

(formation ofyoung ascus)

n + n

+ Mating strain

Youngascus

n

2n

Ascogonium

Conidia

– Mating strain

Antheridium

c.

MEIOSIS

FERTILIZATION

a: © Richard Kolar/Animals Animals; b: © Ed Reschke/Getty Images

GERMINATION

MITOSIS

KA

RYO

GA

MY

Asexual reproduction is very commonConidia formed at the ends of modified hyphae

called conidiophoresAllow for the rapid colonization of a new food

sourceMany conidia are multinucleate

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Yeast

Unicellular ascomycetesMost reproduce asexually by buddingYeasts can ferment carbohydrates

Break down glucose into ethanol and CO2

Used to make bread, beer, and wineSaccharomyces cerevisiae

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© David Scharf/Photo Researchers, Inc. 5 µm

Yeast is a long-standing model system for genetic researchFirst eukaryotes to be manipulated extensivelySaccharomyces cerevisiae first eukaryote to

have genome sequencedYeast two-hybrid system has been an important

component of research on protein interactions

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Ecology of Fungi Fungi, together with bacteria, are the

principal decomposers in the biosphereBreak down cellulose and lignin from wood

Release carbon, nitrogen, and phosphorus

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Fungi symbiosesObligate symbiosis – essential for fungus

survivalFacultative symbiosis – nonessential

InteractionsPathogen – pathogens harm host by causing

diseaseParasites cause harm to host (do not cause

disease)Commensal relationships benefit one partner

but does not harm the otherMutualistic relationships benefit both partners

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Endophytic fungiLive in the intercellular spaces inside plantsSome parasitic, some commensalisticSome fungi protect their hosts from herbivores

by producing toxins

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• Italian rye grass is more resistant to aphid feeding in the presence of endophytes

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Fungal endophyte No endophyte

Fungal Endophyte No Endophyte

Ap

hid

s af

ter

36 d

ays

020406080

100120140

5 aphids 5 aphids

SCIENTIFIC THINKINGHypothesis: Endophytic fungi can protect their host fromherbivory.Prediction: There will be fewer aphids (Rhopalosiphum padi,an herbivore) on perennial ryegrass (Lolium perenne)infected with endophytic fungi than on uninfected ryegrass.Test: Place five adult aphids on each pot of 2-week-old grassplants with and without endophytic fungi. Place pots inperforated bags and grow for 36 days. Count the number ofaphids in each pot.

Result: Significantly more aphids were found on theuninfected grass plants.

Conclusion: Endophytic fungi protect host plants fromherbivory.Further Experiments: How do you think the fungi protect theplants from herbivory? If they secrete chemical toxins,could you use this basic experimental design to testspecific fungal compounds?

(left): © Nigel Cattlin/Alamy; (right): © B. Borrell Casal/Frank Lane Picture Agency/Corbis

LichensSymbiotic associations

between a fungus and a photosynthetic partner Cyanobacteria, green

algae, or sometimes bothMost are mutualisticAscomycetes are found

in all but about 20 of the 15,000 lichen species

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Algalcells

Fungalhyphae

40 μm © Ed Reschke

Fungi in lichens are unable to grow normally without their photosynthetic partners

Fungi protect their partners from strong light and desiccation

Lichens have invaded the harshest habitatsStriking colors play a role in protecting

photosynthetic partnerSensitive to pollutants

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a. b. c.

Fruticose Lichen Crustose LichenFoliose Lichen

a: © Ken Wagner/Phototake; b: © Robert & Jean Pollock/Visuals Unlimited; c: © Robert Lee/Photo Researchers, Inc.

MycorrhizaeMutualistic relationships between fungi and

plantsFound on the roots of about 90% of all known

vascular plant speciesFunction as extensions of root system

Increase soil contact and absorptionTwo principal types

Arbuscular mycorrhizae Ectomycorrhizae

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Arbuscular mycorrhizae By far the most

commonFungal partners are

glomeromycetesNo aboveground

fruiting structuresPotentially capable

of increasing crop yields with lower phosphate and energy inputs

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Hyphae penetrate the root cell wall but not plant

membranes


© Eye of Science/Photo Researchers, Inc.

a.

Root

5 µm

Arbuscular Mycorrhizae

Ectomycorrhizae Most hosts are

forest trees (pines, oaks)

Fungal partners are mostly basidiomycetes

At least 5000 species of fungi are involved in ectomycorrhizal relationships

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Hyphae surround but do not penetrate the

root cells


© Dr. Gerald Van Dyke/Visuals Unlimited

b.

50 µm

Ectomycorrhizae

Animal mutual symbiosesRuminant animals host neocallimastigamycete

fungi in their gutLeaf-cutter ants have domesticated fungi which

they keep in underground gardens Ants provide fungi with leaves Fungi are food for the ants

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Fungal Parasites and Pathogens Fungal species cause many diseases in plants

Among most harmful pests of living plantsCan also spoil harvested or stored food

products

45c.


b: © agefotostock/SuperStock; c: USDA Forest Service Archive, USDA Forest Service, Bugwood.org

b.

46

a.


Courtesy of Ralph Williams/USDA Forest Service

• Fungi may secrete substances making food unpalatable, carcinogenic, or poisonous Fusarium – vomitoxin Aspergillus flavus – aflatoxin

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a. b. 5 μma: © Dayton Wild/Visuals Unlimited; b: © KAGE-Mikrofotografie

Fungi also cause human and animal diseasesCandida – thrush; vaginal infectionsPneumocystis jiroveci – pneumonia in AIDSAthlete’s foot, ringworm, and nail fungus

Fungal diseases are difficult to treat because of the close phylogenetic relationship between fungi and animals

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Batrachochytrium dendrobatidis causes chytridiomycosis

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– Responsible for the worldwide decline in

amphibian populations


Chytrid

10 μm

(left): © School of Biological Sciences, University of Canterbury, New Zealand; (inset): Courtesy of Dr. Peter Daszak

Chapter 32. Defining Fungi Mycologists believe there may be as many as 1.5 million fungal species...

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Transcript of Chapter 32. Defining Fungi Mycologists believe there may be as many as 1.5 million fungal species...