Transcript Lecture 07, Fungi - Cal State LA

Fungus-like protists
Fungi and animals share a common
ancestor with amoebozoans
(amoebae + slime molds)
Fungus-like protists
1) Plasmodial slime molds (Physarum, below)
2) Cellular slime molds
Dictyostelium…
Plasmodial slime mold life cycle
spores hatch into
2 haploid forms
that switch back + forth,
fuse to form a zygote
Cellular slime mold life cycle
2 cells may fuse into a
“giant cell” (2N) that eats
other amoebae as it grows
thousands of unrelated
haploid cells aggregate
to form a single “slug”
fruiting body forms;
some cells become nonreproductive stalk
Fungi
Heterotrophic decomposers
- feed by absorbing nutrients
Single-celled (yeasts) or
multicellular
May be free-living, parasitic, or
mutualists (= symbionts)
Bodies composed of threadlike
structures called hyphae
Cell walls contain chitin
Key Terms:
Symbiosis – two organisms (or species) living in close association
(e.g. mycorrhizae)
mutualism – both benefit
parasitism – parasite benefits, host suffers
commensalism – one benefits, the other is unaffected
Endosymbiosis – one organism living inside another (host)
Examples: Plastids, mitochondria evolved through endosymbiosis
Rhizobium bacteria living in root nodules – fix nitrogen
Lichens – algal symbionts within a fungal host
Heterotrophs – ingest or absorb an external source of
organic carbon (molecules with CH’s)
- Fungi are heterotrophs that absorb organic carbon
from their surroundings
Decomposers (saprobes) – break down complex organic
molecules into simpler organic molecules
- Fungi and bacteria are important decomposers,
recycling carbon and other nutrients
Biogeochemistry – the (re-)cycling of key elements
O, C, N, P, S

Basic fungal structure: hyphae and mycelia (singular: mycelium)
Hyphae release digestive exo-enzymes
into their surrounding
Hyphae form a web called the mycelium
that greatly increases surface area,
maximizing uptake of dissolved
nutrients from the substrate
(= whatever the fungus is growing on)
Hyphae release digestive exoenzymes
into their surrounding
- break down organic matter into small
molecules that can be absorbed
Produce enzyme that can break down..
1- lignin, a complex polymer that makes
wood tough
2- cellulose, a polymer of glucose (sugar) that animals can’t
break down (hence why termites + cows need gut symbionts)
Fungi grow by extending
the tips of their hyphae
through cytoplasmic
streaming
Hyphae can be divided
into individual cells by
partitions called septa
Some fungi grow by repeated
mitotic divisions of nuclei
without cell division
coenocytic condition
- giant multi-nucleated cells,
similar to slime molds
Some fungi form mutualistic
or parasitic associations
with plants
Use special hyphae called
haustoria to penetrate cell
wall of plants
- push into cell surrounded
by plant plasma membrane
Fungal Life Cycle
Fungi spread by producing huge #’s of spores
- structures that resist harsh environmental conditions
- can disperse long distances
by wind
Produced during
both sexual and
asexual phases
of life cycle
Stage 1: Plasmogamy
Most hyphae contain haploid nuclei
N+N
2 hyphae of different
mating types can
grow together + fuse
= plasmogamy
2N
N
Stage 2: Karyogamy
Eventually, 2 haploid nuclei from
different parents fuse into diploid nuclei
N+N
Zygote quickly undergoes
meiosis, producing
haploid spores
2N
N
Heterokaryotic stage = separate haploid nuclei from different
parents, in the same hyphae
N+N
Karyogamy = the 2 haploid nuclei fuse into 1 diploid nucleus
2N
(like syngamy)
Fungal Evolution
Fungi evolved from an ancestor that was
an aquatic protist with a flagellum
(like sperm cells of animals)
Molecular evidence indicates this ancestor
was also single-celled
- thus, animals & fungi independently
evolved multicellularity
- only primitive fungi have flagellated
spores
phylogeny assuming
flagellae were lost once
new phylogeny indicates
flagellae were lost often
Campbell & Reece 2002
Chytrids
May be single-celled or form
multi-cellular hyphae
Only fungi w/ flagellated
spores, called zoospores
Relationship to Zygomycetes
is still controversial
Aquatic
Chytrids
zoospore
Branching hyphae increase surface area for uptake of nutrients
from surrounding aquatic medium
Zygomycetes life cycle
Haploid
1
1) hyphae of opposite
mating type fuse to
form heterokaryotic
(N+N) zoosporangium
- contains many haploid
nuclei from each
parent
- resists bad conditions
Zygomycetes life cycle
Haploid
1
2) when conditions get
better, karyogamy
occurs: haploid nuclei
fuse into diploid nuclei
- zygote then undergoes
meiosis, producing
genetically diverse spores
- regular sporangia form
Zygomycetes life cycle
Haploid
1
3) Regular sporangia may
form and produce spores
by mitosis (asexual
reproduction)
3
Phylum Zygomycota (Zygomycetes)
Pilobolus sp. – dung fungus
Microsporidia - highly modified
parasites
(Encephalitozoon intestinalis)
Phylum Glomeromycota - Glomeromycetes
Formerly put in zygomycetes; now their own phylum
Only 160 known species, but ecologically critical
Form endomycorrhizae, mutualistic associations
inside of plant roots
>90% of plants have endomycorrhizae associated with roots
Fungal partner takes up minerals like phosphate from soil,
transfers them to root tissue of host plant

Phylum Glomeromycota - Glomeromycetes
Formerly put in zygomycetes; now their own phylum
Only 160 known species, but ecologically critical
Endomycorrhizae
Hyphae penetrate cell walls,
but do not puncture plasma
membrane of plant cells
Instead, push inside host cell
surrounded by plant membrane
like fingers in a glove
Taiz & Zeiger 2002
Phylum Ascomycota (ascomycetes)
“Sac fungi” produce sexual
spores in saclike asci
Hyphae fuse into N+N
heterokaryotic stage
Septa form cells with 2
haploid nuclei each
- one of these grows
into an ascus
Developing asci are
housed in the ascocarp,
the fruiting body that will
later eject the spores
Hyphae fuse into N+N
heterokaryotic stage
Septa form cells with 2
haploid nuclei each
- one of these grows
into an ascus
Karyogamy in the ascus
combines both parental
genomes  meiosis
produces 4 haploid cells
Each undergoes mitosis
 8 haploid ascospores
Penicillium sp. – an ascoymycete (formerly called a deuteromycete)
- source of antibiotic penicillin
Campbell & Reece 2005
Saccharomyces sp. -- “yeast”
Common disease organisms
Used by humans to ferment sugar
in dough or grains, for baking
or production of adult beverages
- under anaerobic conditions,
metabolize sugar to ethanol
and CO2 (makes dough rise)
S. cerevisiae is model eukaryotic cell
for molecular biologists to study
- 1st fully sequenced eukaryotic
genome
Saccharomyces sp. - “yeast”
Many yeasts have no known
sexual stage
Yeast cells budding
(asexual reproduction)
Phylum Basidiomycota - basidiomycetes
includes common
mushrooms,
toadstools, shelf
fungi
important decomposers of wood
long-lived heterokaryotic stage,
giving rise to basidiocarp
(“mushroom”) in bad conditions
A single mushroom cap produces
a billion basidiospores
Campbell & Reece 2005
Phylum Basidiomycota – sexual stages
heterokaryotic
N+N
Basidiocarps of a basidiomycete form a “fairy ring” overnight
Heterokaryotic mycelium connects mushrooms underground
- expands outward, digesting organic matter in soil
Ectomycorrhizae
Freeman 2005
Many basidiomycetes form ectomycorrhizae with plant roots
- roots are completely covered in a layer of hyphae
Exoenzymes release nitrogen from decaying matter  transferred
to hyphae extending in between outer-most root cells
Self-quiz: Know the 4 different types of reproductive structures
that are characteristic of the different fungal phyla
Fungal Symbioses
Lichens – fungus & alga mutualism
- Endosymbiont is usually a chlorophyte (green alga)
- Fungus is the host
Mycorrhizae – fungus & plant root mutualism
Endomycorrhizae - glomeromycete symbiont
(inner)
Ectomycorrhizae - basidiomycete symbiont
(outer)
Parasitic fungi cause many plants & animal diseases
Lichens – mutualism between fungus (Ascomycete) & green alga
3 growth forms of lichens:
- Foliose (leaf-like)
- Crustose
- Fruticose (shrub-like)
This relationship evolved
3 separate times
Lichens – mutualism between fungus (Ascomycete) & green alga
This relationship evolved 3 separate times
Enable plants to eventually grow on what was bare rock by eroding
the rock surface, trapping soil
Are very sensitive to air pollution, acid rain
Ecto-mycorrhizae
Endo-mycorrhizae
Taiz & Zeiger 2002
Fungi are responsible for many plant diseases;
destroy 10-50% of crops worldwide
Eating fungus-infected grains is
a severe human health issue in
much of the world
- contributes to high rate of liver
cancer in areas that consume
peanut meal, which supports
fungi that produce liver toxins
Historically, eating ergot-infected grain caused outbreaks
of madness and death, once thought to be demon possession
- ergot fungus produces lysergic acid, similar to LSD