Survey
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Chapter 31: Fungi 1. General Features of Fungi 2. Survey of the Fungi A. Chytrids B. Zygomycetes C. Glomeromycetes D. Ascomycetes E. Basidiomycetes 3. Ecological Roles of Fungi 1. General Features of Fungi General Characteristics of Fungi Fungi are eukaryotic absorptive heterotrophs. • secrete enzymes externally to digest, absorb nutrients Fungi are unicellular (yeasts) or multicellular. Fungi reproduce asexually OR sexually. Fungi typically consist of haploid cells with cell walls made of chitin. Most fungi are terrestrial. **The study of fungi is known as mycology. ** Reproductive structure Fungal Body Structures Hyphae The thallus (“body”) of a fungus consists largely of filamentous chains of cells called hyphae. Spore-producing structures • vegetative (nonreproductive) or aerial (reproductive) 60 μm Mycelium (a mass of hyphae) • most multicellular fungi have a mass of branched hyphae called a mycelium Fungal Hyphae Fungal hyphae can be septate or ceonocytic: • septate hyphae have porous septa between cell nuclei • coenocytic hyphae lack septa and thus nuclei distributed throughout a continuous cytoplasm Cell wall Cell wall Pore Septum Nuclei Nuclei Septate hypha Coenocytic hypha Specialized Hyphae Nematode Hyphae 25 μm Some unique fungi have specialized hyphae adapted for capturing prey. (a) Hyphae adapted for trapping and killing prey Some unique fungi have specialized hyphae called haustoria that allow them to penetrate the tissues of their plant host. Plant cell wall Fungal hypha Plant cell Haustorium (b) Haustoria Plant cell plasma membrane Mycorrhizal Fungi Mycorrhizae are fungi with a mutually beneficial symbiotic relationship with plants via the roots. • ectomycorrhizal fungi form sheaths of hyphae over a root & grow into extracellular spaces of the root cortex arbuscular mycorrhizal fungi extend hyphae through the cell walls of root cells and into tubes formed by invagination of the root cell membrane Mantle 1.5 mm (fungal sheath) Mantle (fungal sheath) Epidermal cell Endodermis (a) Ectomycorrhizae Epidermis Cortex Fungal hyphae Root hair (b) Arbuscular mycorrhizae Fungal hyphae between cortical cells (LM) 50 μm Cortical cell Endodermis Fungal vesicle Casparian strip Arbuscules Plasma membrane (LM) 10 μm • Cortex (Colorized SEM) Epidermis Fungal Reproduction Haploid (n) Heterokaryotic Another mycelium Diploid (2n) PLASMOGAMY (fusion of cytoplasm) Heterokaryotic stage Sporeproducing structures Spores ASEXUAL REPRODUCTION GERMINATION Mycelium SEXUAL REPRODUCTION GERMINATION KARYOGAMY (fusion of nuclei) Zygote MEIOSIS Spores Fungi reproduce by generating haploid spores which become airborne once released. Haploid spores can be produced sexually or asexually. Sexual Reproduction in Fungi Sexual spores in fungi require 3 phases not seen in the production of asexual spores: 1) Transfer of a haploid nucleus to a cell of the opposite mating type: plasmogamy • opposite mating types recognize each other via pheromones 2) Fusion of the haploid nuclei to form a diploid zygote nucleus: karyogamy • haploid nuclei may reproduce by mitosis before fusing • this is the ONLY occasion when fungal cells are diploid 3) Meiosis to produce genetically unique haploid sexual spores Asexual Spore Production in Fungi Asexual spores are produced by mitosis at the tips of aerial hyphae. • derived from a single parent fungus 1.5 μm • no genetic diversity (i.e., spores are genetic clones of the parent fungus) 2A. Survey of the Fungi: Chytrids Chytrids Zygomycetes Glomeromycetes Ascomycetes Basidiomycetes The Evolutionary History of Fungi Genomic studies have identified chytrids as an early diverging fungal lineage, a basal taxon in the fungal clade. Animals (and their close protistan relatives Nucleariids Chytrids Fungi Other fungi Opisthokonts UNICELLULAR, FLAGELLATED ANCESTOR The Fungal Clade Chytrids (1,000 species) Hyphae 25 μm Zygomycetes (1,000 species) • fungi are most closely related to the unicellular nucleariids Glomeromycetes (160 species) Fungal hypha 25 μm Ascomycetes (65,000 species) Basidiomycetes (30,000 species) • the oldest fossil evidence of fungi dates to 450 million years ago General Characteristics of Chytrids Chytrids (phylum Chytridiomycota) are found in terrestrial, freshwater, and marine habitats including hydrothermal vents. • chytrids can be decomposers, parasites, or mutualists • chytrids are unique among fungi in having flagellated spores called zoospores 4 μm Flagellum • chytrids reproduce asexually California Sixty Lake Basin Chytrid Pathogens 2 N 2 ▲ Yellow-legged frogs killed by B. dendrobatidis infection Key Boundary of chytrid spread Lake status in 2009: Frog population extinct 07 Treatment lake: frogs treated with fungicides and released • a chytrid species is thought to be the cause of the recent decline in amphibians worldwide 2B. Survey of the Fungi: Zygomycetes Chytrids Zygomycetes Glomeromycetes Ascomycetes Basidiomycetes General Characteristics of Zygomycetes Sporangia Zygomycetes (phylum “Zygomycota”) have coenocytic hyphae and produce sporangiospores (“sporangia”) in a sac-like sporangium. 100 μm 50 μm Sexual reproduction in zygomycetes involves the production of a zygosporangium. Zygosporangium Haploid (n) Heterokaryotic (n + n) Diploid (2n) PLASMOGAMY Mating type (−) Mating type (+) Rhizopus growing on bread New mycelia ASEXUAL REPRODUCTION 50 μm Gametangia with haploid nuclei Mycelium 100 μm Young zygosporangium (heterokaryotic) SEXUAL REPRODUCTION Dispersal and germination of spores Sporangia Zygomycete Life Cycle KARYOGAMY Sporangium MEIOSIS Diploid nuclei Zygosporangium Examples of Zygomycetes Some zygomycetes, such as Pilobolus, can actually “aim” and shoot their sporangia toward bright light. Rhizopus, a common bread mold, is a classic example of a zygomycete 0.5 mm Pilobolus Rhizopus growing on bread 2C. Survey of the Fungi: Glomeromycetes Chytrids Zygomycetes Glomeromycetes Ascomycetes Basidiomycetes General Characteristics of Glomeromycetes Glomeromycetes (phylum Glomeromycota) form arbuscular mycorrhizae. • the glomeromycetes (phylum Glomeromycota) were once considered zygomycetes but due to molecular analyses they now belong in their own clade 2.5 μm 2D. Survey of the Fungi: Ascomycetes Chytrids Zygomycetes Glomeromycetes Ascomycetes Basidiomycetes General Characteristics of Ascomycetes Ascomycetes (phylum Ascomycota) have septate hyphae and produce asexual conidiospores (“conidia”) that are not enclosed in any sac. Sexual reproduction in ascomycetes yields 4 or 8 ascospores in a sac-like ascus many of which may be contained in a larger fruiting body – an ascocarp. Conidia; mating type (−) Dispersal Mating type (+) Germination ASEXUAL REPRODUCTION Haploid (n) Dikaryotic (n + n) Diploid (2n) Ascomycete Life Cycle PLASMOGAMY Hypha Ascus (dikaryotic) Conidiophore Dikaryotic hyphae Mycelia Mycelium Germination Dispersal Asci Ascocarp SEXUAL REPRODUCTION KARYOGAMY Diploid nucleus (zygote) Eight ascospores Four haploid nuclei MEIOSIS Examples of Ascomycetes Ascomycetes include a wide variety of fungi such as molds (Neurospora, Penicillium), morels, truffles and the unicellular yeasts. Saccharomyces Parent cell Morchella esculenta Tuber melanosporum Bud 10 μm 2E. Survey of the Fungi: Basidiomycetes Chytrids Zygomycetes Glomeromycetes Ascomycetes Basidiomycetes General Characteristics of Basidiomycetes Basidiomycetes (phylum Basidiomycota) are characterized by having microscopic club-shaped basidia from which basidiospores are produced. Basidium • these structures are produced large fruiting bodies called basiocarps (e.g., a mushroom) that result from sexual reproduction 1 μm Basidiospore Examples of Basidiomycetes Shelf fungi Puffballs Maiden veil fungus Mushroom “fairy ring” PLASMOGAMY Dikaryotic mycelium Mating type (−) Mating type (+) Basidiomycete Life Cycle Gills lined with basidia Haploid mycelia SEXUAL REPRODUCTION Basidiocarp (n + n) Dispersal and germination Basidiospores (n) Basidium Basidium with four basidiospores Basidium containing four haploid nuclei Basidia (n + n) KARYOGAMY MEIOSIS 1 μm Haploid (n) Dikaryotic (n + n) Diploid (2n) Basidiospore Diploid nuclei Fungal Phylum Distinguishing Features Chytridiomycota (chytrids) Flagellated spores Zygomycota (zygomycetes) Resistant zygosporangium as sexual stage Glomeromycota (arbuscular mycorrhizal fungi) Arbuscular mycorrhizae formed with plants Ascomycota (ascomycetes) Sexual spores (ascospores) borne internally in sacs called asci; vast numbers of asexual spores (conidia) produced Basidiomycota (basidiomycetes) Elaborate fruiting body (basidiocarp) containing many basidia that produce sexual spores (basidiospores) Key Features of Each Fungal Phylum 3. Ecological Roles of Fungi Fungi as Mutualists Plants harbor harmless symbiotic endophytes, fungi that live inside leaves or other plant parts, most of which are ascomycetes. Results Endophyte not present; pathogen present (E−P+) Both endophyte and pathogen present (E+P+) Leaf area damaged (%) Leaf mortality (%) 30 20 10 0 E−P+ E+P+ • endophytes make toxins that deter herbivores and defend against pathogens 15 10 5 0 E−P+ E+P+ Some fungi share their digestive services with animals such as those that help break down plant material in the guts of cows and other grazing mammals. Many species of ants use the digestive power of fungi by raising them in “farms”. Lichens Lichens are actually 2 different organisms in a mutualistic symbiosis: • cyanobacteria or green algae living among the hyphae of typically an ascomycete A foliose (leaflike) lichen A fruticose (shrublike) lichen • fungus gets free carbs! • algae or cyanobacteria protected from the elements Crustose (encrusting) lichens Soredia Fungal hyphae Algal layer 50 μm Lichen Structure Ascocarp of fungus Fungal hyphae Algal cell Lichens are also important pioneers that can grow on inorganic surfaces, beginning the process of ecological succession. Fungi as Parasites About 30% of known fungal species are parasites or pathogens, mostly on or in plants. • each year up to 50% of world fruit harvests are lost due to fungi • some fungal plant parasites can be toxic to humans • e.g., ergot which causes a variety of neurological symptoms and contains lysergic acid, the raw material used to make LSD (b) Tar spot fungus on maple leaves (a) Corn smut on corn (c) Ergots on rye Fungal Illnesses in Humans The general term for a fungal infection in animals is mycosis. Examples of humans mycoses include: • ringworm and athlete’s foot • coccidiomycosis or “valley fever” • opportunistic “yeast infections” by Candida albicans