CHAPTER 31

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Defining Fungi

• Mycologists believe there may be as many as 1.5 million fungal species • Single-celled or multicellular • Specialized to extract and absorb nutrients from surroundings 2

• 7 monophyletic phyla – Microsporidia – Blastocladiomycota – Neocallismastigamycota – Chytridiomycota – Glomeromycota – Basidiomycota – Ascomycota 3

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Microsporidia Blastocladiomycota Zygomycota Neocallimastigomycota Chytridiomycota Glomeromycota Basidiomycota Ascomycota 10 µm 520 µm 333 µm 312 µm 160 µm 300 µm Fungi

a: © Dr. Ronny Larsson; b: Contributed by Don Barr, Mycological Society of America; c: © Carolina Biological Supply Company/Phototake; d: Contributed by Don Barr, Mycological Society of America; e: © Dr. Yuuji Tsukii; f: © Yolande Dalpe, Agriculture and Agri-Food Canada; g:© inga spence/Alamy; h: © Michael&Patricia Fogden

Dikarya

4

5

General Biology of the Fungi

• Multicellular fungi consist of long, slender filaments called hyphae – Some hyphae are continuous – Others are divided by septa • Cytoplasm flows throughout hyphae – Allows rapid growth under good conditions 6

• Mycelium – mass of connected hyphae – Grows through and digests its substrate • Fungal cell walls include chitin – Also found in the hard shells (exoskeletons) of arthropods 7

• Spores are the most common means of reproduction among fungi – May form from sexual or asexual processes – Most are dispersed by wind 8

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Fungal loop

Nutrition

Fungus

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Nematode a.

© Carolina Biological Supply Company/Phototake

370 µm b.

© L. West/ Photo Researchers, Inc.

• Obtain food by secreting digestive enzymes into surroundings • Then absorb the organic molecules produced by this external digestion – Great surface area-to-volume ratio • Fungi can break down cellulose and lignin – Decompose wood – Some fungi are carnivorous 9

Microsporidia

• Obligate, intracellular, animal parasites • Long thought to be protists • Lack mitochondria – Ancestors lost them Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fungi

10

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Polar tube Spore 0.5 µm (left): © Daniel P. Fedorko

•

Encephalitozoon cuniculi

– Commonly cause disease in immunosuppressed patients – Infect hosts with their spores, which contain a polar tube – Infects intestinal and neuronal cells, leading to diarrhea and neurodegenerative disease 11

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Chytridiomycota

Fungi

• • Chytridiomycetes or chytrids • Aquatic, flagellated fungi • Closely related to ancestral fungi • Have motile zoospores

Batrachochytrium dendrobatidis

implicated in amphibian die-offs has been 12

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Contributed by Daniel Wubah, Mycological Society of America Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chytrid Oedogonium filament

Contributed by Don Barr, Mycological Society of America 13

Blastocladiomycetes

• Uniflagellated zoospores • Allomyces example – Water mold – Haplodiplontic life cycle – Female gametes secrete pheromone to attract male gametes – Giant mitochondria in its zoospores 14

a.

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100 µm b.

Gametophyte (n) Haploid zoospore (n) Young gametophyte (n) Female gametangium Male gametangium Haploid gametes (n) Mature gametophyte (n) n 2n Sporangium (2n) FERTILIZATION Sporophyte(2n) Sporophyte Young sporophyte (2n) Diploid zoospores (2n) Asexual reproduction Asexual sporangium Mature sporophyte (2n) Young sporophyte (2n)

a: © Carolina Biological Supply Company/Phototake 15

Neocallimastigamycota

• Digest plant biomass in mammalian herbivore rumens – Mammal depends on fungi for sufficient calories • Greatly reduced mitochondria lack cristae • Zoospores have multiple flagella • Horizontal gene transfer brought cellulase gene from bacteria into

Neocallimastix

genome 16

Zygomycota

• Zygomycetes are incredibly diverse • Not monophyletic – still under research • Include the common bread molds • A few human pathogens Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fungi

17

18

Glomeromycota

• Glomeromycetes are a tiny group of fungi • Form intracellular associations with plant roots called arbuscular mycorrhizae – Cannot survive in absence of host plant • No evidence of sexual reproduction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

19

Fungi

Basidiomycota

• Basidiomycetes are some of the most familiar fungi • Mushrooms, toadstools, puffballs, shelf fungi, etc.

• Also important plant pathogens like rusts and smuts Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

20

Fungi

21

Ascomycota

• Contain about 75% of the known fungi • Includes bread yeasts, common molds, cup fungi, truffles, and morels • Serious plant pathogens – cause of chestnut blight and Dutch elm disease • Penicillin-producing fungi are in the genus

Penicillium

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Fungi

22

23

Yeast

• Unicellular ascomycetes • Most reproduce asexually by budding • Yeasts can ferment carbohydrates – Break down glucose into ethanol and CO 2 – Used to make bread, beer, and wine –

Saccharomyces cerevisiae

24

• Yeast is a long-standing model system for genetic research – First eukaryotes to be manipulated extensively –

Saccharomyces cerevisiae

have genome sequenced first eukaryote to 25

Ecology of Fungi

• Fungi, together with bacteria, are the principal decomposers in the biosphere • Break down cellulose and lignin from wood – Release carbon, nitrogen, and phosphorus 26

• Endophytic fungi – Live in the intercellular spaces inside plants – Some parasitic, some commensalistic – Some fungi protect their hosts from herbivores by producing toxins 27

• Italian rye grass is more resistant to aphid feeding in the presence of endophytes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

SCIENTIFIC THINKING Hypothesis: Endophytic fungi can protect their host from herbivory.

Prediction: There will be fewer aphids (Rhopalosiphum padi, an herbivore) on perennial ryegrass (Lolium perenne) infected with endophytic fungi than on uninfected ryegrass.

Test: Place five adult aphids on each pot of 2-week-old grass plants with and without endophytic fungi. Place pots in perforated bags and grow for 36 days. Count the number of aphids in each pot.

5 aphids 5 aphids Fungal endophyte No endophyte Result: Significantly more aphids were found on the uninfectedgrass plants.

140 120 100 80 60 40 20 0 Fungal Endophyte No Endophyte Conclusion: Endophytic fungi protect host plants from herbivory.

Further Experiments: How do you think the fungi protect the plants from herbivory? If they secrete chemical toxins, could you use this basic experimental design to test specific fungal compounds?

(left): © Nigel Cattlin/Alamy; (right): © B. Borrell Casal/Frank Lane Picture Agency/Corbis 28

• Lichens – Symbiotic associations between a fungus and a photosynthetic partner • Cyanobacteria, green algae, or sometimes both – Most are mutualistic – Ascomycetes are found in all but about 20 of the 15,000 lichen species 29

Fungal loop Fungus Nematode

• Fungi in lichens are unable to grow normally without their photosynthetic partners • Fungi protect their partners from strong light and desiccation • Lichens have invaded the harshest habitats • Striking colors play a role in protecting photosynthetic partner • Sensitive to pollutants 30

• Mycorrhizae – Mutualistic relationships between fungi and plants – Found on the roots of about 90% of all known vascular plant species – Function as extensions of root system • Increase soil contact and absorption – Two principal types • Arbuscular mycorrhizae • Ectomycorrhizae 31

• Arbuscular mycorrhizae – By far the most common – Fungal partners are glomeromycetes – No aboveground fruiting structures – Potentially capable of increasing crop yields with lower phosphate and energy inputs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Arbuscular Mycorrhizae Root 3.7 µm a.

© Eye of Science/Photo Researchers, Inc.

Hyphae penetrate the root cell wall but not plant membranes 32

• Ectomycorrhizae – Most hosts are forest trees (pines, oaks) – Fungal partners are mostly basidiomycetes – At least 5000 species of fungi are involved in ectomycorrhizal relationships Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Ectomycorrhizae 50 µm b.

© Dr. Gerald Van Dyke/Visuals Unlimited Hyphae surround but do not penetrate the root cells 33

• Animal mutual symbioses – Ruminant animals host neocallimastigamycete fungi in their gut – Leaf-cutter ants have domesticated fungi which they keep in underground gardens • Ants provide fungi with leaves • Fungi are food for the ants 34

Fungal Parasites and Pathogens

• Fungal species cause many diseases in plants – Among most harmful pests of living plants – Can also spoil harvested or stored food products 35

36

• Fungi may secrete substances making food unpalatable, carcinogenic, or poisonous • •

Fusarium

– vomitoxin

Aspergillus flavus

– aflatoxin 37

• Fungi also cause human and animal diseases –

Candida

– thrush; vaginal infections –

Pneumocystis jiroveci

– pneumonia in AIDS – Athlete’s foot, ringworm, and nail fungus • Fungal diseases are difficult to treat because of the close phylogenetic relationship between fungi and animals 38

Batrachochytrium dendrobatidis

causes chytridiomycosis – Responsible for the worldwide decline in amphibian populations 39