Download Chapter 31: Fungi

Chapter 31: Fungi
1. General Features of Fungi
2. Survey of the Fungi
A. Chytrids
B. Zygomycetes
C. Glomeromycetes
D. Ascomycetes
E. Basidiomycetes
3. Ecological Roles of Fungi
1. General Features of Fungi
General Characteristics of Fungi
Fungi are eukaryotic absorptive heterotrophs.
• secrete enzymes externally to digest, absorb nutrients
Fungi are unicellular (yeasts) or multicellular.
Fungi reproduce asexually OR sexually.
Fungi typically consist of haploid cells with cell walls made
of chitin.
Most fungi are terrestrial.
**The study of fungi is
known as mycology. **
Reproductive structure
Fungal Body
Structures
Hyphae
The thallus (“body”) of a
fungus consists largely
of filamentous chains of
cells called hyphae.
Spore-producing
structures
• vegetative (nonreproductive) or aerial
(reproductive)
60 μm
Mycelium (a mass of hyphae)
• most multicellular
fungi have a mass of
branched hyphae called
a mycelium
Fungal Hyphae
Fungal hyphae can be septate or ceonocytic:
• septate hyphae have porous septa between cell nuclei
• coenocytic hyphae lack septa and thus nuclei distributed throughout a
continuous cytoplasm
Cell wall
Cell wall
Pore
Septum
Nuclei
Nuclei
Septate hypha
Coenocytic hypha
Specialized Hyphae
Nematode
Hyphae
25 μm
Some unique fungi have specialized
hyphae adapted for capturing prey.
(a) Hyphae adapted for trapping and killing prey
Some unique fungi have specialized
hyphae called haustoria that allow
them to penetrate the tissues of their
plant host.
Plant
cell
wall
Fungal hypha
Plant cell
Haustorium
(b) Haustoria
Plant cell
plasma
membrane
Mycorrhizal Fungi
Mycorrhizae are fungi with a mutually beneficial symbiotic
relationship with plants via the roots.
•
ectomycorrhizal fungi form
sheaths of hyphae over a root
& grow into extracellular
spaces of the root cortex
arbuscular mycorrhizal fungi
extend hyphae through the cell
walls of root cells and into tubes
formed by invagination of the
root cell membrane
Mantle 1.5 mm
(fungal sheath)
Mantle (fungal sheath)
Epidermal
cell
Endodermis
(a) Ectomycorrhizae
Epidermis
Cortex
Fungal
hyphae
Root
hair
(b) Arbuscular mycorrhizae
Fungal
hyphae
between
cortical
cells
(LM)
50 μm
Cortical cell
Endodermis
Fungal
vesicle
Casparian
strip
Arbuscules
Plasma
membrane (LM)
10 μm
•
Cortex
(Colorized SEM)
Epidermis
Fungal Reproduction
Haploid (n)
Heterokaryotic
Another
mycelium
Diploid (2n)
PLASMOGAMY
(fusion of cytoplasm)
Heterokaryotic
stage
Sporeproducing
structures
Spores
ASEXUAL
REPRODUCTION
GERMINATION
Mycelium
SEXUAL
REPRODUCTION
GERMINATION
KARYOGAMY
(fusion of nuclei)
Zygote
MEIOSIS
Spores
Fungi reproduce by
generating haploid
spores which
become airborne
once released.
Haploid spores can
be produced sexually
or asexually.
Sexual Reproduction in Fungi
Sexual spores in fungi require 3 phases not seen in the
production of asexual spores:
1) Transfer of a haploid nucleus to a cell of the opposite mating
type: plasmogamy
• opposite mating types recognize each other via pheromones
2) Fusion of the haploid nuclei to form a diploid zygote nucleus:
karyogamy
• haploid nuclei may reproduce by mitosis before fusing
• this is the ONLY occasion when fungal cells are diploid
3) Meiosis to produce genetically unique haploid sexual spores
Asexual Spore Production in Fungi
Asexual spores are produced by mitosis at the tips of aerial
hyphae.
• derived from a single parent fungus
1.5 μm
• no genetic diversity
(i.e., spores are genetic
clones of the parent
fungus)
2A. Survey of the Fungi:
Chytrids
Chytrids
Zygomycetes
Glomeromycetes
Ascomycetes
Basidiomycetes
The Evolutionary History of Fungi
Genomic studies have identified chytrids as an early diverging
fungal lineage, a basal taxon in the fungal clade.
Animals (and their close
protistan relatives
Nucleariids
Chytrids
Fungi
Other fungi
Opisthokonts
UNICELLULAR,
FLAGELLATED
ANCESTOR
The Fungal Clade
Chytrids (1,000 species)
Hyphae
25 μm
Zygomycetes (1,000 species)
• fungi are most
closely related to
the unicellular
nucleariids
Glomeromycetes (160 species)
Fungal
hypha
25 μm
Ascomycetes (65,000 species)
Basidiomycetes (30,000 species)
• the oldest fossil
evidence of fungi
dates to 450 million
years ago
General Characteristics of Chytrids
Chytrids (phylum Chytridiomycota) are found in terrestrial,
freshwater, and marine habitats including hydrothermal vents.
• chytrids can be decomposers, parasites, or mutualists
• chytrids are unique
among fungi in having
flagellated spores called
zoospores
4 μm
Flagellum
• chytrids reproduce
asexually
California
Sixty
Lake
Basin
Chytrid Pathogens
2
N
2
▲ Yellow-legged frogs killed by
B. dendrobatidis infection
Key
Boundary of chytrid spread
Lake status in 2009:
Frog population extinct
07
Treatment lake: frogs
treated with fungicides
and released
• a chytrid species is
thought to be the
cause of the recent
decline in amphibians
worldwide
2B. Survey of the Fungi:
Zygomycetes
Chytrids
Zygomycetes
Glomeromycetes
Ascomycetes
Basidiomycetes
General Characteristics of
Zygomycetes
Sporangia
Zygomycetes (phylum “Zygomycota”) have
coenocytic hyphae and produce sporangiospores
(“sporangia”) in a sac-like sporangium.
100 μm
50 μm
Sexual reproduction in zygomycetes involves
the production of a zygosporangium.
Zygosporangium
Haploid (n)
Heterokaryotic (n + n)
Diploid (2n)
PLASMOGAMY
Mating
type (−)
Mating
type (+)
Rhizopus
growing on bread
New
mycelia
ASEXUAL
REPRODUCTION
50 μm
Gametangia with
haploid nuclei
Mycelium
100 μm
Young
zygosporangium
(heterokaryotic)
SEXUAL
REPRODUCTION
Dispersal and
germination
of spores
Sporangia
Zygomycete
Life Cycle
KARYOGAMY
Sporangium
MEIOSIS
Diploid
nuclei
Zygosporangium
Examples of Zygomycetes
Some zygomycetes, such as Pilobolus, can actually “aim” and shoot
their sporangia toward bright light.
Rhizopus, a
common bread
mold, is a classic
example of a
zygomycete
0.5 mm
Pilobolus
Rhizopus growing on bread
2C. Survey of the Fungi:
Glomeromycetes
Chytrids
Zygomycetes
Glomeromycetes
Ascomycetes
Basidiomycetes
General Characteristics of Glomeromycetes
Glomeromycetes (phylum Glomeromycota) form
arbuscular mycorrhizae.
• the glomeromycetes (phylum
Glomeromycota) were once
considered zygomycetes but
due to molecular analyses they
now belong in their own clade
2.5 μm
2D. Survey of the Fungi:
Ascomycetes
Chytrids
Zygomycetes
Glomeromycetes
Ascomycetes
Basidiomycetes
General Characteristics of Ascomycetes
Ascomycetes (phylum Ascomycota)
have septate hyphae and produce
asexual conidiospores (“conidia”)
that are not enclosed in any sac.
Sexual reproduction in ascomycetes
yields 4 or 8 ascospores in a sac-like
ascus many of which may be contained
in a larger fruiting body – an ascocarp.
Conidia; mating type (−)
Dispersal
Mating
type (+)
Germination
ASEXUAL
REPRODUCTION
Haploid (n)
Dikaryotic (n + n)
Diploid (2n)
Ascomycete
Life Cycle
PLASMOGAMY
Hypha
Ascus
(dikaryotic)
Conidiophore
Dikaryotic
hyphae
Mycelia
Mycelium
Germination
Dispersal
Asci
Ascocarp
SEXUAL
REPRODUCTION
KARYOGAMY
Diploid nucleus
(zygote)
Eight
ascospores
Four
haploid
nuclei
MEIOSIS
Examples of Ascomycetes
Ascomycetes include a wide variety of fungi such as molds
(Neurospora, Penicillium), morels, truffles and the unicellular yeasts.
Saccharomyces
Parent
cell
Morchella
esculenta
Tuber melanosporum
Bud
10 μm
2E. Survey of the Fungi:
Basidiomycetes
Chytrids
Zygomycetes
Glomeromycetes
Ascomycetes
Basidiomycetes
General Characteristics of Basidiomycetes
Basidiomycetes (phylum Basidiomycota)
are characterized by having microscopic
club-shaped basidia from which
basidiospores are produced.
Basidium
• these structures are produced large
fruiting bodies called basiocarps
(e.g., a mushroom) that result from
sexual reproduction
1 μm
Basidiospore
Examples of Basidiomycetes
Shelf fungi
Puffballs
Maiden veil fungus
Mushroom “fairy ring”
PLASMOGAMY
Dikaryotic
mycelium
Mating
type (−)
Mating
type (+)
Basidiomycete
Life Cycle
Gills lined
with basidia
Haploid
mycelia
SEXUAL
REPRODUCTION
Basidiocarp
(n + n)
Dispersal and
germination
Basidiospores (n)
Basidium
Basidium with
four basidiospores
Basidium containing
four haploid nuclei
Basidia
(n + n)
KARYOGAMY
MEIOSIS
1 μm
Haploid (n)
Dikaryotic (n + n)
Diploid (2n)
Basidiospore
Diploid nuclei
Fungal Phylum
Distinguishing Features
Chytridiomycota
(chytrids)
Flagellated spores
Zygomycota
(zygomycetes)
Resistant zygosporangium
as sexual stage
Glomeromycota
(arbuscular
mycorrhizal fungi)
Arbuscular mycorrhizae
formed with plants
Ascomycota
(ascomycetes)
Sexual spores (ascospores)
borne internally in sacs
called asci; vast numbers
of asexual spores (conidia)
produced
Basidiomycota
(basidiomycetes)
Elaborate fruiting body
(basidiocarp) containing
many basidia that
produce sexual spores
(basidiospores)
Key Features of
Each Fungal
Phylum
3. Ecological Roles of Fungi
Fungi as Mutualists
Plants harbor harmless symbiotic endophytes, fungi that live inside
leaves or other plant parts, most of which are ascomycetes.
Results
Endophyte not present; pathogen present (E−P+)
Both endophyte and pathogen present (E+P+)
Leaf area damaged (%)
Leaf mortality (%)
30
20
10
0
E−P+
E+P+
• endophytes make
toxins that deter
herbivores and
defend against
pathogens
15
10
5
0
E−P+
E+P+
Some fungi share their digestive services with animals such as those
that help break down plant material in the guts of cows and other
grazing mammals.
Many species of ants use
the digestive power of fungi
by raising them in “farms”.
Lichens
Lichens are actually 2
different organisms in a
mutualistic symbiosis:
•
cyanobacteria or
green algae living
among the hyphae of
typically an
ascomycete
A foliose (leaflike) lichen
A fruticose (shrublike)
lichen
• fungus gets free carbs!
• algae or cyanobacteria protected from
the elements
Crustose (encrusting) lichens
Soredia
Fungal
hyphae
Algal
layer
50 μm
Lichen Structure
Ascocarp of fungus
Fungal hyphae
Algal cell
Lichens are also important pioneers that can grow on inorganic
surfaces, beginning the process of ecological succession.
Fungi as Parasites
About 30% of known fungal species are parasites or pathogens,
mostly on or in plants.
• each year up to 50% of world fruit
harvests are lost due to fungi
• some fungal plant parasites can be
toxic to humans
•
e.g., ergot which causes a variety of
neurological symptoms and contains
lysergic acid, the raw material used to
make LSD
(b) Tar spot
fungus
on maple
leaves
(a) Corn smut on corn
(c) Ergots on rye
Fungal Illnesses in Humans
The general term for a fungal infection in animals is mycosis.
Examples of humans mycoses include:
• ringworm and athlete’s foot
• coccidiomycosis or “valley fever”
• opportunistic “yeast infections” by Candida albicans