Chapter 32 Fungi

Defining Fungi Mycologists believe there may be as many as 1.5 million fungal species Single-celled or multicellular Sexual or asexual Specialized to extract and absorb nutrients from surroundings Animal and fungi last shared a common ancestor 460 MYA –Some debate on timing –May have 670 MYA ancestor 2

7 monophyletic phyla Microsporidia Blastocladiomycota Neocallismastigamycota Chytridiomycota Glomeromycota Basidiomycota Ascomycota 1 polyphyletic Zygomycota 3

4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. c. MicrosporidiaBlastocladiomycotaZygomycotaNeocallimastigomycotaChytridiomycotaGlomeromycotaBasidiomycotaAscomycota Dikarya Fungi a. 10 μm b.h.g. 300 μm200 μm300 μm e.f.d. 300 μm500 μm 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

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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 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 6

Hyphae may have more than one nucleus Monokaryotic – 1 nucleus Dikaryotic – 2 nuclei Both genomes transcribed Sometimes many nuclei intermingle in the common cytoplasm of the fungal mycelium –Heterokaryotic – nuclei from genetically distinct individuals –Homokaryotic – nuclei are genetically similar to one another 8

Fungi have an unusual mitosis Nuclear envelope does not break down and re- form Instead, the spindle apparatus is formed within it Fungi lack centrioles 9

Reproduction Capable of both sexual and asexual reproduction Sexual reproduction –Fusion of two haploid hyphae of compatible mating types You will see it displayed on figures as “mating strain +” and “mating strain –” –No “male” or “female” In some fungi, fusion immediately results in a diploid (2n) cell Others, have a dikaryon stage (1n + 1n) before parental nuclei form diploid nucleus 10

Spores are the most common means of reproduction among fungi –May form from sexual or asexual processes –Most are dispersed by wind 11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10 μm © Eye of Science/Photo Researchers, Inc.

Nutrition 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 12 b. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Now we will talk about the different groups of fungi 13

Microsporidia Obligate, intracellular, animal parasites –Long thought to be protists –Lack mitochondria Ancestors lost them 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 14

Chytridiomycota Chytridiomycetes or chytrids Aquatic, flagellated fungi Closely related to ancestral fungi Have motile spores: zoospores Batrachochytrium dendrobatidis has been implicated in amphibian die-offs 15 NeocallimastigomycotaZygomycotaChytridiomycota GlomeromycotaBasidiomycota Ascomycota Blastocladiomycota Microsporidia Fungi Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Neocallimastigomycota Digest plant biomass in mammalian herbivore rumens Mammal depends on fungi for sufficient calories Greatly reduced mitochondria lack cristae Zoospores have multiple flagella 16

Zygomycota Zygomycetes are incredibly diverse Not monophyletic – still under research Include the common bread molds A few human pathogens 17 Neocallimastigomycota ZygomycotaChytridiomycota GlomeromycotaBasidiomycota Ascomycota BlastocladiomycotaMicrosporidia Fungi

Sexual reproduction Fusion of gametangia Haploid nuclei fuse to form diploid zygote nuclei – karyogamy Develops into zygosporangium in which zygospore develops Meiosis occurs during germination of zygospore –Releases haploid spores Asexual reproduction more common –Sporangiophores have sporangia that release spores 18

Sexual reproduction in Zygomycota

Asexual reproduction in zygomycota 20

Glomeromycota Glomeromycetes are a tiny group of fungi Form intracellular associations with plant roots called arbuscular mycorrhizae Provide plant with minerals Cannot survive in absence of host plant No evidence of sexual reproduction 21 Neocallimastigomycota Zygomycota ChytridiomycotaGlomeromycotaBasidiomycota Ascomycota Blastocladiomycota Microsporidia Fungi

Basidiomycota Basidiomycetes are some of the most familiar fungi Mushrooms, toadstools, puffballs, shelf fungi, etc. Also important plant pathogens like rusts and smuts 22 Neocallimastigomycota Zygomycota ChytridiomycotaGlomeromycotaBasidiomycota Ascomycota Blastocladiomycota Microsporidia Fungi

Named for basidium club-shaped sexual reproductive structure Karyogamy (fusion of 2 nuclei from different mating types) occurs within basidia –Only diploid cell in life cycle Meiosis follows –The four haploid (1n) products are incorporated into basidiospores –Basidiospores are released Basidiospores land and germination leads to the production of monokaryotic hyphae Results in a monokaryotic mycelium, or primary mycelium Different mating types of monokaryotic hyphae may fuse –Results in a dikaryotic (2 nuclei) mycelium, or secondary mycelium –Heterokaryotic (2 nuclei from different mating types) mycelium –The basidiocarps (mushrooms) are then formed entirely of secondary mycelium 23

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 25 Neocallimastigomycota Zygomycota ChytridiomycotaGlomeromycota Basidiomycota Ascomycota Blastocladiomycota Microsporidia Fungi

Named for ascus – microscopic, saclike reproductive structure Karyogamy occurs within asci –Only diploid nucleus of life cycle Asci differentiate in ascocarp –Meiosis and mitosis follow, producing 8 haploid nuclei that become walled ascospores –Ascospores released and land, germinate –2 mating strands come together to form dikaryotic hyphae that grows into ascus 26

Asexual reproduction is very common –Conidia formed at the ends of modified hyphae called conidiophores –Allow for the rapid colonization of a new food source –Many conidia are multinucleate 28

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 29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © David Scharf/Photo Researchers, Inc. 5 µm

Yeast is a long-standing model system for genetic research First eukaryotes to be manipulated extensively Saccharomyces cerevisiae first eukaryote to have genome sequenced Yeast two-hybrid system has been an important component of research on protein interactions 30

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 31

Fungi symbioses Obligate symbiosis – essential for fungus survival Facultative symbiosis – nonessential Interactions Pathogen – pathogens harm host by causing disease Parasites cause harm to host (do not cause disease) Commensal relationships benefit one partner but does not harm the other Mutualistic relationships benefit both partners 32

Endophytic fungi –Live in the intercellular spaces inside plants –Some parasitic, some commensalistic –Some fungi protect their hosts from herbivores by producing toxins 33

Italian rye grass is more resistant to aphid feeding in the presence of endophytes 34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fungal endophyteNo endophyte Fungal EndophyteNo Endophyte Aphids after 36 days aphids 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. Result: Significantly more aphids were found on the uninfected grass plants. 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

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 35 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Algal cells Fungal hyphae 40 μm © Ed Reschke

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 from sun’s rays Sensitive to pollutants 36 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a.b.c. Fruticose LichenCrustose LichenFoliose Lichen

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 37

Arbuscular mycorrhizae –By far the most common –Fungal partners are glomeromycetes –No above ground fruiting structures –Potentially capable of increasing crop yields with lower phosphate and energy inputs 38 Hyphae penetrate the root cell wall but not plant membranes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Eye of Science/Photo Researchers, Inc. a. Root 5 µm Arbuscular Mycorrhizae

Ectomycorrhizae –Most hosts are forest trees (pines, oaks) –Fungal partners are mostly basidiomycetes –At least 5000 species of fungi are involved in ectomycorrhizal relationships 39 Hyphae surround but do not penetrate the root cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Dr. Gerald Van Dyke/Visuals Unlimited b. 50 µm Ectomycorrhizae

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 40

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 41 c. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. b: © agefotostock/SuperStock; c: USDA Forest Service Archive, USDA Forest Service, Bugwood.org b.

Fungi may secrete substances making food unpalatable, carcinogenic, or poisonous Fusarium – vomitoxin Aspergillus flavus – aflatoxin 42 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a.b. 5 μm

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 43

44 Batrachochytrium dendrobatidis causes chytridiomycosis – Responsible for the worldwide decline in amphibian populations Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chytrid 10 μm (left): © School of Biological Sciences, University of Canterbury, New Zealand; (inset): Courtesy of Dr. Peter Daszak