Fungi Chapter 32

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

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Hyphae 10 μm Mycelium (inset): © Micro Discovery/Corbis; (right): © Michael & Patricia Fogden/Corbis 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

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

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

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. 400 µm b. © Carolina Biological Supply Company/Phototake © 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

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. Neocallimastigomycota Blastocladiomycota Chytridiomycota Glomeromycota Microsporidia Basidiomycota Zygomycota Ascomycota Fungi

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

a: © Carolina Biological Supply Company/Phototake Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. GERMINATION Asexual reproduction Spores Spores Sporangium Sporangiophore – Mating strain Germinating zygosporangium Rhizoid GERMINATION Hypha + Mating strain (+) (–) (Meiosis occurs during germination) a. n 700 µm MEIOSIS 2n n + n Gametangia KARYOGAMY FERTILIZATION Zygosporangium b. a: © Carolina Biological Supply Company/Phototake

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

Named for basidium – club-shaped sexual reproductive structure Karyogamy occurs within basidia Only diploid cell in life cycle Meiosis follows The four haploid products are incorporated into basidiospores

a. b. Basidiocarp Gills lined with basidia Basidium n + n Secondary Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Basidiocarp Gills lined with basidia Basidium n + n KARYOGAMY Secondary mycelium (dikaryotic) 2n n a. Zygote FERTILIZATION Basidium MEIOSIS – Mating strain + Mating strain Primary mycelium (monokaryotic) Basidiospores Sterigma b. a: © Alexandra Lowry/The National Audubon Society Collection/Photo Researchers, Inc.

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

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

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

a: © Richard Kolar/Animals Animals; b: © Ed Reschke/Getty Images Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Conidia Asexual reproduction Developing mycelium Ascospore Each haploid nucleus divides once by mitosis Conidia GERMINATION Ascogonium Antheridium – Mating strain a. FERTILIZATION + Mating strain MITOSIS n n + n 2n b. MEIOSIS KARYOGAMY Young ascus Dikaryotic hyphae form From ascogonium (formation of young ascus) Fully developed ascocarp composed of dikaryotic (ascogenic) hyphae and sterile hyphae c. a: © Richard Kolar/Animals Animals; b: © Ed Reschke/Getty Images

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

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

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

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