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Chapter 31: Fungi - Mighty Mushrooms
•
Fungi are diverse and widespread
•
They are essential for the well-being of most terrestrial ecosystems
because they break down organic material and recycle vital nutrients
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Outline for fungi:
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How do they obtain organic materials?
•
Body structure
•
Asexual vs. sexual life cycles
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Fungal diversity – we will see examples of fungi
•
Relationships between fungi and other organisms
•
Lichens – fungi living along with bacteria or algae
•
Pathogenic fungi and uses of fungi
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Fungi are heterotrophs that feed by absorption
•
Despite their diversity, fungi share key traits, most importantly the way
in which they derive nutrition
•
Fungi are heterotrophs but do not ingest their food
•
They secrete exoenzymes that break down complex molecules, and
then they absorb the smaller compounds
•
Fungi exhibit diverse lifestyles:
–
Decomposers
–
Parasites
–
Mutualistic symbionts
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Representation of a saprobe and its mode of action to get
energy source from the environment.
Extracellular digestion in a saprobe with a cell wall.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Body Structure
•
The morphology of multicellular fungi enhances their ability to absorb
nutrients
•
Fungi consist of mycelia, networks of branched hyphae adapted for
absorption
•
Most fungi have cell walls made of chitin
•
Some fungi have hyphae divided into cells by septa – called septate
hyphae, with pores allowing cell-to-cell movement
•
Coenocytic fungi are fungi whose hyphae lack septa
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The body structure
of fungi
Reproductive structure
Hyphae
Spore-producing
structures
20 µm
Mycelium
The two types of hypha in a fungus
Cell wall
Cell wall
Nuclei
Pore
Septum
Septate hypha
Nuclei
Coenocytic hypha
Porous septa in septate hyphae functions similar to
gap junctions in animal cells and plamodesmata in plant cells – they
allow easy transfer of molecules.
In this case, it is transfer of molecules throughout the hypha.
Specialized hyphae
Nematode
Hyphae
25 µm
•Some unique fungi have
specialized hyphae that allow
them to penetrate the tissues of
their host
•These hyphae then secrete
exoenzymes in order to digest
outside food sources to absorb
the released nutrients
Hyphae adapted for trapping and killing prey
Plant
cell
wall
Fungal hypha
Plant cell
Haustorium
Haustoria
Plant cell
plasma
membrane
Fungi produce spores through sexual or asexual life cycles
•Fungi propagate themselves by producing vast numbers of spores, either sexually
or asexually
Key
Heterokaryotic
stage
Haploid (n)
Heterokaryotic
(unfused nuclei from
different parents)
PLASMOGAMY
(fusion of cytoplasm)
Diploid (2n)
KARYOGAMY
(fusion of nuclei)
Spore-producing
structures
Zygote
Spores
ASEXUAL
REPRODUCTION
Mycelium
SEXUAL
REPRODUCTION
MEIOSIS
GERMINATION
GERMINATION
Spore-producing
structures
Spores
Sexual Reproduction
•
Plasmogamy is the union of two parent mycelia
•
In many fungi, the haploid nuclei from each parent do not fuse right
away; they coexist in the mycelium, called a heterokaryon
•
In some fungi, the haploid nuclei pair off two to a cell; such a mycelium
is said to be dikaryotic
•
Hours, days, or even centuries may pass before the occurrence of
karyogamy, which is nuclear fusion
•
During karyogamy, the haploid nuclei fuse, producing diploid cells
•
The diploid phase is short-lived and undergoes meiosis, producing
haploid spores
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Asexual Reproduction
•In addition to sexual reproduction, many fungi can reproduce asexually
•Many of these species grow as mold, sometimes on fruit, bread, and other foods
Asexual Reproduction
•Other fungi that can
reproduce asexually
are yeasts, which
inhabit moist
environments
•Instead of producing
spores, yeasts
reproduce asexually
by simple cell
division
Many molds and
yeasts have no
known sexual stage
Mycologists have
traditionally called
these
deuteromycetes, or
imperfect fungi
Parent
cell
Bud
Fungi descended from an aquatic, single-celled,
flagellated protist
•
Systematists now recognize Fungi and Animalia as sister kingdoms
•
In other words, fungi and animals are more closely related to each
other than they are to plants or other eukaryotes
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Molecular evidence supports the hypothesis that fungi and animals
diverged from a common unicellular, flagellated ancestor
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Fungi probably evolved before the colonization of land by multicellular
organisms
•
The oldest undisputed fossils of fungi are only about 460 million years
old
•
Fungi were among the earliest colonizers of land, probably as
symbionts with early land plants
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•Molecular analysis has
helped clarify evolutionary
relationships between
fungal groups, although
areas of uncertainty
remain
Arbuscular
mycorrhizal
fungi
Sac
fungi
Club
fungi
Zygomycota
Glomeromycota
Ascomycota
Basidiomycota
•Fungi phylogeny is the
subject of much research
Chytridiomycota
Fungi have
radiated into
a diverse set
of lineages
Chytrids
Zygote
fungi
Chytrids
•
Chytrids (phylum Chytridiomycota) are found in freshwater and
terrestrial habitats
•
They can be saprobic or parasitic
•
Molecular evidence supports the hypothesis that chytrids diverged
earliest in fungal evolution
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Chytrids are unique among fungi in having flagellated spores, called
zoospores
Flagellum
Hyphae
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Zygomycetes
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The zygomycetes (phylum Zygomycota) exhibit great diversity of life
histories
•
They include fast-growing molds, parasites, and commensal symbionts
•
The zygomycetes are named for their sexually produced
zygosporangia
•
The life cycle of black bread mold (Rhizopus stolonifer) is fairly typical
of the phylum
•
Zygosporangia, which are resistant to freezing and drying, can survive
unfavorable conditions
•
Some zygomycetes, such as Pilobolus, can actually “aim” their
sporangia toward conditions associated with good food sources
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Life cycle of black bread mold (Rhizopus stolonifer)
Key
Haploid (n)
Heterokaryotic (n + n)
Diploid (2n)
PLASMOGAMY
Rhizopus
growing
on bread
Mating
type (+)
Gametangia with
Mating haploid nuclei
type (–)
100 µm
Young
zygosporangium
(heterokaryotic)
SEXUAL
REPRODUCTION
Dispersal and
germination
Sporangia
Sporangium
ASEXUAL
REPRODUCTION
MEIOSIS
Dispersal and
germination
50 µm
Mycelium
KARYOGAMY
Diploid
nuclei
Zygosporangium
(heterokaryotic)
Pilobolus can “aim” their sporangia toward
conditions associated with good food sources
•Microsporidia are unicellular
parasites of animals and
protists
•They are now classified as
zygomycetes
Host cell
nucleus
Developing
microsporidian
Spore
10 µm
Microsporidia
Glomeromycetes
•The glomeromycetes (phylum Glomeromycota) were once considered
zygomycetes, but are now classified in a different phyla
•Glomeromycetes form a distinct type of endomycorrhizae called arbuscular
mycorrhizae
Ascomycetes
•
Ascomycetes (phylum Ascomycota) live in marine, freshwater, and
terrestrial habitats
•
The phylum is defined by production of sexual spores in saclike asci,
usually contained in fruiting bodies called ascocarps
•
Ascomycetes vary in size and complexity, from unicellular yeasts to
elaborate cup fungi
•
Ascomycetes reproduce asexually by enormous numbers of asexual
spores called conidia
•
Conidia are not formed inside sporangia; they are produced asexually
at the tips of specialized hyphae called conidiophores
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Ascomycetes
The cup-shaped ascocarps (fruiting bodies) of
Aleuria aurantia give this species its common
name: orange peel fungus.
The edible ascocarp of
Morchella esculenta, the
succulent morel is often
found under trees in
orchards.
10 µm
Tuber melanosporum is a truffle, an ascocarp that grows
underground and emits strong odors. These ascocarps
have been dug up and the middle one sliced open.
Neurospora crassa feeds as
a mold on bread and other
food (SEM).
Ascomycetes life cycle
Key
Haploid (n)
Dikaryotic (n + n)
Conidia;
mating type (–)
Diploid (2n)
Dispersal
Germination
ASEXUAL
REPRODUCTION
Mating
type (+)
Mycelium
PLASMOGAMY
Ascus
(dikaryotic)
Mycelia
Conidiophore
Dikaryotic hyphae
extended from
ascogonium
SEXUAL
REPRODUCTION
Germination
Dispersal
Asci
Diploid nucleus
(zygote)
Eight
ascospores
Four
haploid
nuclei
Ascocarp
KARYOGAMY
MEIOSIS
Basidiomycetes
•Basidomycetes (phylum
Basidiomycota) include
mushrooms and shelf fungi
Maiden veil fungus (Dictyphora), a fungus
with an odor like rotting meat
•The phylum is defined by a
clublike structure called a
basidium, a transient diploid
stage in the life cycle
•The life cycle of a
basidiomycete usually
includes a long-lived
dikaryotic mycelium
•In response to environmental
stimuli, the mycelium
reproduces sexually by
producing elaborate fruiting
bodies call basidiocarps
Fly agaric (Amanita muscoria), a
common species in conifer forests
in the northern hemisphere
Puffballs emitting spores
Shelf fungi, important decomposers of wood
Mushrooms are examples of basidiocarps
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Dikaryotic
mycelium
PLASMOGAMY
•The numerous basidia in a
basidiocarp are sources of
sexual spores called
basidiospores
Mating
type (–)
•Asexual reproduction is
much less common in
basidiomycetes than in
ascomycetes
Haploid
mycelia
Mating
type (+)
SEXUAL
REPRODUCTION
Gills lined
with
basidia
Basidiocarp
(dikaryotic)
Dispersal and
germination
Basidiospores
Basidium with
four appendages
Basidium
Basidia
(dikaryotic)
Basidium containing
four haploid nuclei
KARYOGAMY
MEIOSIS
Key
Diploid
nuclei
1 µm
Basidiospore
Haploid (n)
Dikaryotic (n + n)
Diploid (2n)
Fungi have a powerful impact on ecosystems and
human welfare
•
Fungi are efficient decomposers - They perform essential recycling of
chemical elements between the living and nonliving world
•
Fungi form symbiotic relationships with plants, algae, and animals
•
All of these relationships have profound ecological effects by helping
each other survive
•
Mycorrhizae are enormously important in natural ecosystems and
agriculture
•
They increase plant productivity
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Ecological associations of fungi – their lifestyles
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Influence fungi can have on other living organisms in the environment
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Symbiotic relationship: Organisms that live in close nutritional relationship,
required by one or both organisms
–
Types
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Mutualism – both organisms benefit (mutualistic symbionts)
•
Commensalism – one organism benefits, other not harmed
•
Parasitism – host/microbe relationship, microbe benefits, host is harmed
–
Non-symbiotic relationship: Organisms are free-living, and do not rely on
each other for survival
–
Types
•
Synergism – shared metabolism between members, not required
•
Antagonism - competition between microorganisms where some
organisms growth is inhibited or they are destroyed by others
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mycorrhizae
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Mycorrhizae are mutually beneficial relationships between fungi and
plant roots
•
Ectomycorrhizal fungi form sheaths of hyphae over a root and also
grow into the extracellular spaces of the root cortex
•
Endomycorrhizal fungi extend hyphae through the cell walls of root
cells and into tubes formed by invagination of the root cell membrane
•
The fungi aid the plant’s uptake of nutrients and increase the surface
area for water uptake. (root hairs also increase the total surface area
of a plant’s roots!)
•
These relationships between the fungi and the plant roots allow
vascular plants to grow tall. Without the fungi, the plants wouldn’t grow
as tall as they would not take up nutrients as easily.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Fungi have a powerful impact on ecosystems and
human welfare
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Fungus-Animal Symbiosis
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Some fungi share their digestive services with animals
•
These fungi help break down plant material in the guts of cows and
other grazing mammals
•
Many species of ants and termites use the digestive power of fungi by
raising them in “farms”
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lichens
A fruticose (shrub-like) lichen
A foliose (leaf-like) lichen
Crustose (crust-like) lichens
•Lichens are a symbiotic association of millions of photosynthetic
microorganisms held in a mass of fungal hyphae
Ascocarp of fungus
Soredia
Lichens
Fungal
hyphae
Algal
layer
•The fungal component of a
lichen is most often an
ascomycete
•Algae or cyanobacteria
occupy an inner layer below
the lichen surface to
complete the lichen
formation with the fungus
Algal cell
10 µm
Fungal hyphae
Pathogens
•About 30% of known fungal species are parasites, mostly on or in plants
•Animals are much less susceptible to parasitic fungi than are plants
•The general term for a fungal infection in animals is mycoses
Corn smut on corn
Tar spot fungus of sycamore leaves
Ergots on rye
Practical Uses of Fungi
•Humans eat many fungi and use others to make many different cheeses, alcoholic
beverages, and bread.
•Genetic research on fungi is leading to applications in biotechnology
•Antibiotics produced by fungi treat bacterial infections
Staphylococcus
Penicillium
Zone of
inhibited
growth