Fungi Chapter 31

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

7 monophyletic phyla Microsporidia Blastocladiomycota Neocallismastigamycota Chytridiomycota Glomeromycota Basidiomycota Ascomycota

Neocallimastigomycota Chytridiomycota Glomeromycota Basidiomycota Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Microsporidia Blastocladiomycota Zygomycota Neocallimastigomycota Chytridiomycota Glomeromycota Basidiomycota Ascomycota 10 µm 520 µm 333 µm 312 µm 160 µm 300 µm Dikarya 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

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

(right): © Garry T. Cole/ Biological Photo Service Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dikaryotic cell Septa with pores Nuclei Nuclei Hypha Pore Septum 0.2 µm (right): © Garry T. Cole/ Biological Photo Service

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

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

Fungi have an unusual mitosis Cell is not relevant unit of reproduction Nuclear envelope does not break down and re-form Instead, the spindle apparatus is formed within it Fungi lack centrioles Spindle plaques regulate microtubule formation during mitosis

Reproduction Capable of both sexual and asexual reproduction Fusion of two haploid hyphae of compatible mating types In some fungi, fusion immediately results in a diploid (2n) cell Others, have a dikaryon stage (1n + 1n) before parental nuclei form diploid nucleus May form mushrooms or puffballs

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

a: © Carolina Biological Supply Company/Phototake Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Young gametophyte (n) Female gametangium Male gametangium MITOSIS Haploid gametes (n) Gametophyte (n) Mature gametophyte (n) n Haploid zoospore (n) FERTILIZATION 2n Sporangium (2n) Sporophyte(2n) MEIOSIS Young sporophyte (2n) Diploid zoospores (2n) Asexual sporangium Asexual reproduction Sporophyte Mature sporophyte (2n) Young sporophyte (2n) a. b. 100 µm a: © Carolina Biological Supply Company/Phototake

Sexual Reproduction Plasmogamy- Union of cytoplasm of two hyphae Karyogamy- Fusion of 2 nuclei to form a diploid zygote

Asexual reproduction more common 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

Spore 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 mycelium, or secondary mycelium Heterokaryotic mycelium Basidiocarps (mushrooms) are formed entirely of secondary mycelium

Asexual Reproduction Budding Fragmentation Binary fission Sporulation

Nutrition Obtain food by secreting digestive enzymes into surroundings Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nutrition Fungal loop Fungus Nematode a. 370 µm © Carolina Biological Supply Company/Phototake 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

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. Neocallimastigomycota Blastocladiomycota Chytridiomycota Glomeromycota Microsporidia Basidiomycota Zygomycota Ascomycota 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. Neocallimastigomycota Blastocladiomycota Chytridiomycota Microsporidia Glomeromycota Zygomycota Basidiomycota Ascomycota Fungi

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Neocallimastigomycota Blastocladiomycota Chytridiomycota Microsporidia Glomeromycota Basidiomycota Zygomycota Ascomycota Fungi

Yeast Unicellular ascomycetes Most reproduce asexually by budding Yeasts can ferment carbohydrates Break down glucose into ethanol and CO2 Used to make bread, beer, and wine Saccharomyces cerevisiae

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

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

Fungi symbioses Interactions 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

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

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

Fungi protect their partners from strong light and desiccation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fungal loop Fungus Nematode a. b. 370 µm a: © Carolina Biological Supply Company/Phototake; b: © L. West/ Photo Researchers, Inc. 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

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

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

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

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

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

Fungi may secrete substances making food unpalatable, carcinogenic, or poisonous Fusarium – vomitoxin Aspergillus flavus – aflatoxin

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

Batrachochytrium dendrobatidis causes chytridiomycosis – Responsible for the worldwide decline in amphibian populations