Lecture III.3. Fungi.
Fungi, choanoflagellates and animals form a clade.
The Good, the Bad and the Ugly.
Top left. Mushroom. Top right. Baker’s yeast. Bottom left. Corn
smut. Bottom right. Oral yeast infection (thrush).
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Phylogenetic Relationships.
• Choanoflagellates, Fungi and Animalia form a clade.
• Shared derived traits include:
1. Chitin
a.
Choanoflagellates: Component of
the surrounding lorica – most /
mostly silicious);
b.
Fungi – cell walls;
c.
Metazoa – exoskeletons.
2. Glycogen – so-called “animal starch”
(storage molecule).
3. Receptor tyrosine kinase (RTK) – a
signal receptor molecule.
Above right. A choanoflagellate and its lorica. Following page.
Cladograms based on different proteins linking choanoflagellates
to metazoa. From King and Carroll (2001). The abbreviations MP,
ME and QP, refer to measures of reconstruction reliability.
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Computer generated images of lorical development. Lorica functions include resisting locomotory forces generated by the flagellum
and directing water flow to the collar. Not all Choanoflagellates
have a lorica. From Leadbeater et al. (2009).
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Extra-Cellular Digestion / Hyphae.
• Fungi secrete enzymes that
break down living or dead
organisms on (in) which they
grow – makes them important decomposers.
• Multicellular
fungi
form
branching, thread-like hyphae that absorb nutrients.
1. Long, thin growth form Fungal mycelia are composed of hyphae.
maximizes surface area
to volume ratio and therefore absorption.
2. Hyphae organized into mycelia.
Left. Both vegetative and reproductive parts of fungi are composed of
hyphae. Right. Hyphal structure showing partial division by septa.
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• Not all fungi multicellular – yeasts are unicells.
Two fungal growth forms.
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Questions.
1. (2 pts) How do the cell walls of fungi differ from those of
plants?
2. (2 pts) The thread-like structure (large surface area to
volume ratio) of hyphae maximizes nutrient absorption.
What structures in the human gut perform the same function?
3. (2 pts) Give other (at least two) examples from vertebrate
anatomy in which increased surface area to volume ratio
facilitates the exchange of materials.
4. (2 pts) What are receptor tyrosine kinases (RTKs)?
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Nutrition.
• Most fungi saprobes – feed on dead matter.
1. Fungi important decomposers.
2. Important decomposers of cellulose (cellulase) and
lignin (lignin peroxidase) – coevolved w. plants.
3. Recycle nutrients in terrestrial ecosystems.
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• Pilobolus – Cleaning Up After the Rhinos.
1. Lives on dung.
2. Has an “eye” – light sensitive organ that points the
fruiting body to the light.
3. Hydrostatic pressure (up
to 7 Atm) propels spores
2-3 meters.
4. Spores accelerated at 20,000 – 180,000 g – contrast w.
maximum force sustainable by WWII pilots of < 10 g .
Pilobolus. Top right. In its native habitat. Bottom left. Sporangial
vesicles, each with a single sporangium, orients toward the
light and launches the spores (Bottom right).
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• Some fungi are predators.
Fungus “capturing” a nematode (round worm).
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Human Health / Economic Impacts.
• Some are parasites; a few are human pathogens. Fungal
diseases include
1. Diaper rash;
2. Meningitis;
3. Pneumonia;
4. Ringworm;
5. Thrush;
6. Vaginitis.
• Fungi are important crop / food parasites.
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Symbioses.
• Mycorrhizae: Plant-fungus mutualisms.
1. Fungus obtains carbohydrates from the plant.
2. Plant obtains water and
soil nutrients – principally
N and P, from the fungus
3. Important: N and P are Root tip, root hairs (arrows) and
mycorrhizae.
often limiting.
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Experiment using labeled CO2, P and N to establish the mutualistic nature of plant-mycorrhizae associations. From Freeman, Ch. 32.
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4. Gardeners often supplement (substitute) conventional
fertilizers with mycorrhizal spores.
Left. Conventional fertilizers contain nitrogen and phosphorus, the availability of which (in the soil) often limits plant
growth. Right. A mycorrhizal fungal additive (spores).
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• Lichens
1. Symbiotic associations of
fungi and cyanobacteria or photosynthetic eukaryotes (algae).
2. “Pioneer” species; colonize bare rock, e.g., after volcanic eruptions.
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Life Cycles.
• Life cycle is haplontic.
1. Mycelial cells haploid.
2. Nuclear fusion (zygote formation)
followed by meiosis and production of haploid spores.
3. Spores divide mitotically to form
mycelia.
• Most fungal life cycles include a
dikaryon (heterokaryon) stage in
which hyphal cells contain two (or
more) haploid nuclei.
1. Results from hyphal fusion.
2. In septate fungi, hyphae partially
divided by septa.
3. In Basidiomycota, clamp connections ensure that each
compartment contains one nucleus from each parent hypha.
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Top. Hyphal fusion. Bottom. Clamp formation
assures that each compartment receives one
nucleus of each type.
4. If no septa, the structure
is called a coenocyte.
• In most organisms, nuclear
fusion (karyogamy) immediately follows cell fusion
(plasmogamy) – e.g., when
a sperm fertilizes an egg.
• In fungi, karyogamy is delayed.
Generalized fungal life cycle includes fusion of hyphal cells from different individuals (right) and a persistent dikaryon (here “heterokaryotic mycelium”). Eventual nuclear fusion results in a diploid zygote and
the production of haploid spores from which new mycelia develop mitotically. Mycelia can also produce spores mitotically (left).
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5. Delayed karyogamy may enhance fungal flexibility in
responding to environmental variation encountered by
different parts of the mycelium as it spreads.
a. Extent of underground
mycelia often enormous
– can exceed a square
mile.
b. The same morphology
that facilitates absorption of water and nutrients also increases vulArmillaria solidipes. Most of the
nerability.
fungus is underground. The
enormous mycelia can be more
6. As the mycelium grows, its than a thousand years old.
constituent hyphae
a. Hunt for food by dispersing until they find it;
b. Exploit it by then proliferating.
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Questions.
5. (2 pts) The large surface area to volume ratio of hyphae
increases the absorption of water and nutrients from the
soil. What’s a potential down side?
6. (2 pts) Name a common fungal disease in arid environments to which all of us have been exposed.
7. (2 pts) a. What are yeasts good for – besides rising
dough? b. Name two fungal plant diseases that have resulted in the decimation / near extinction of important
North American trees.
8. (6 pts) Why should Pilobolus discharge its spores away
from the dung heap on which it’s growing?
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Sex and Reproduction in Fungi.
• Sex: Fusion of two nuclei followed by meiosis.
• Recombination involves gene exchange.
• Reproduction: One individual produces one or more offspring. Can be sexual or asexual.
• Sexual reproduction in fungi.
1. Results from fusion of hyphae of different mating type
(except in Chytrids, which have motile gametes).
2. Except in Chytrids, karyogamy follows plasmogamy.
3. Spores produced by meiosis.
• Asexual reproduction in fungi.
1. Mycelial fragmentation.
2. Spore production by mitosis.
3. Fission (equal sized products) and budding (small cell
buds off from large) in yeasts.
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• Spores.
1. Can be produced sexually (meiosis) or asexually (mitosis).
2. Typically small, tough cells that germinate upon ex-
posure to favorable conditions.
3. Often long-lived, resistant to heat, cold, desicca-
tion, etc.
4. Fungal spores are everywhere – 104 per m3 of air.
Question.
9. (4 pts) Approximately how many fungal spores do
each of us inhale per minute? Show your work.
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Four Major Fungal Groups.
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Chitridiomycota (Little Pots).
• Name refers to shape of
spore-bearing structures.
• Aquatic (most freshwater).
• Only fungal group retaining
flagella
(gametes
and
spores).
• One species attacks nematode eggs – spores swim and
/ or hyphae grow to its prey.
Chytrid spores will emerge from
this nematode egg.
• Conventional alternation of generations – no dikaryon.
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• Parasitic Batrachochytrium dendrobatidis (BD) the apparent cause of world-wide amphibian die-off.
1. 1st attacks the skin – impedes respiration, water balance
maintenance.
2. Progressively affects internal organs, nervous system in
particular.
.
3. Xenopus (pets and research animals) a vector?
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Zygomycota (Zygote Fungi).
• Mostly soil dwellers.
1. Many saprophytes; some
parasitic; a few predaceous.
2. Examples: Bread mold;
Pilobolus.
• Sexual reproduction entails
1. Formation
hyphae.
of
coenocytic
Asexual and sexual reproduction in
Zygomycota. Sexual reproduction
involves hyphal and nuclear fusion,
zygospore formation and the production of haploid spores by meiosis.
2. Fusion of hyphae of different (+/-) mating types.
3. Zoosporangium (n + n) develops at sites of fusion –
within it, diploid zygote, then haploid spores.
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Ascomycota (Cup fungi).
• Cup-like ascocarp bears asci, each of which typically
contains eight spores.
• Include lichens – symbiotic
with cyanobacteria.
• Many plant parasites.
• Molds and mildews.
Ascocarp. Haploid asci contain
eight spores each.
• Commercial Uses.
1. Brewer’s / baker’s yeast;
2. Cheese / soya sauce manufacture.
3. Source of penicillin.
4. Food – truffles, morels.
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Basidiomycota (Club Fungi).
• Includes “mushrooms”.
• Reproductive structure is
the diploid basidium (grow
from the ventral “gills”)
wherein haploid basidiospores (four per basidium)
formed by meiosis.
• Multiple
indistinguishable
hyphal mating types.
Pileus (“mushroom cap”) and
spore-bearing basidia in Basidiomycota.
• Clamp connections (not all species) maintain dikaryon integrity – two nuclei (one from each of two different hyphae)
per compartment.
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Reproductive Structures in Multicellular Fungi.
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Yeasts.
• Polyphyletic group with Ascomycota (most) and Basidiomycota representatives.
• Examples: Saccharomyces
cerevisiae (Ascomycota).
1. Bread and beer.
2. Anaerobe. Ferments glucose to ethanol.
3. Reproduces sexually by
spores and asexually by
fission and budding.
• Rhodotorula (Basidiomycota)
1. Orange-red color result of
UV-protective pigments.
2. Lives in soil, water air.
3. Nitrogen scavenger.
Top. Life cycle of S. cervisiae.
Each of the two haploid mating
types, a and α, produces a pheromone to which the other responds. Both haploid and diploid
phases reproduce by budding.
Diploid yeasts also produce haploid spores. Bottom. Rhodotorula mucilaginosa.
4. Airborne contaminant of skin, lungs, urine, feces.
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