Lecture III.3. Fungi.

Fungi, choanoflagellates and animals form a clade.

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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 func-tions 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 im-portant decomposers .

• Multicellular fungi form branching, thread-like hy-phae that absorb nutrients. 1. Long, thin growth form

maximizes surface area to volume ratio and therefore absorption.

2. Hyphae organized into mycelia .

Fungal mycelia are com-posed of hyphae.

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 func-tion?

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 sensi-

tive 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 to ward 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 carbohy-

drates from the plant.

2. Plant obtains water and soil nutrients – principally N and P, from the fungus

3. Important: N and P are often limiting .

Root tip, root hairs (arrows) and mycorrhizae.

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Experiment using labeled CO2, P and N to establish the mutualistic na-ture 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 phospho-rus, 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 cyanobacte-ria or photosynthetic eu-karyotes (algae ).

2. “Pioneer” species; col-

onize bare rock, e.g., af-ter 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 produc-tion 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 con-nections ensure that each compartment contains one nu-cleus from each parent hypha.

Top. Hyphal fusion. Bot-tom. Clamp formation assures that each com-partment receives one nucleus of each type.

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4. If no septa , the structure is called a coenocyte .

• In most organisms , nuclear

fusion (karyogamy) immedi-ately 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 dif-ferent individuals (right ) and a persistent dikaryon (here “heterokary-otic mycelium”). Eventual nuclear fusion results in a diploid zygote and the production of haploid spores from which new mycelia develop mi-totically. 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 absorp-tion of water and nutri-ents also increases vul-nerability .

6. As the mycelium grows, its

constituent hyphae a. Hunt for food by dispersing until they find it;

b. Exploit it by then proliferating.

Armillaria solidipes. Most of the fungus is underground. The enormous mycelia can be more than a thousand years old.

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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 environ-

ments 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 re-sulted 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 off-

spring. 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 (mito-sis).

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 nema-

tode eggs – spores swim and / or hyphae grow to its prey.

• Conventional alternation of generations – no dikaryon.

Chytrid spores will emerge from this nematode egg.

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• Parasitic Batrachochytrium dendrobatidis (BD) the appar-ent 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 preda-ceous.

2. Examples: Bread mold; Pilobolus.

• Sexual reproduction entails 1. Formation of coenocytic

hyphae.

2. Fusion of hyphae of differ-ent (+/-) mating types.

3. Zoosporangium (n + n) develops at sites of fusion –

within it, diploid zygote, then haploid spores.

Asexual and sexual reproduction in Zygomycota. Sexual reproduction involves hyphal and nuclear fusion, zygospore formation and the pro-duction of haploid spores by meio-sis.

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Ascomycota (Cup fungi). • Cup-like ascocarp bears as-

ci, each of which typically contains eight spores.

• Include lichens – symbiotic

with cyanobacteria . • Many plant parasites. • Molds and mildews . • Commercial Uses.

1. Brewer’s / baker’s yeast; 2. Cheese / soya sauce manufacture. 3. Source of penicillin . 4. Food – truffles, morels.

Ascocarp. Haploid asci contain eight spores each.

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Basidiomycota (Club Fungi). • Includes “mushrooms ”. • Reproductive structure is

the diploid basidium (grow from the ventral “gills”) wherein haploid basidio-spores (four per basidium) formed by meiosis.

• Multiple indistinguishable

hyphal mating types . • Clamp connections (not all species) maintain dikaryon in-

tegrity – two nuclei (one from each of two different hyphae) per compartment.

Pileus (“mushroom cap”) and spore-bearing basidia in Basidio-mycota.

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Reproductive Structures in Multicellular Fungi.

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Yeasts. • Polyphyletic group with As-

comycota (most) and Basidi-omycota representatives.

• Examples: Saccharomyces

cerevisiae (Ascomycota). 1. Bread and beer. 2. Anaerobe . Ferments glu-

cose 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.

4. Airborne contaminant of skin, lungs, urine, feces.

Top. Life cycle of S. cervisiae. Each of the two haploid mating types, a and α, produces a pher-omone to which the other re-sponds. Both haploid and diploid phases reproduce by budding. Diploid yeasts also produce hap-loid spores. Bottom. Rhodoto-rula mucilaginosa.