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Fungus-like protists Fungi and animals share a common ancestor with amoebozoans (amoebae + slime molds) Fungus-like protists 1) Plasmodial slime molds (Physarum, below) 2) Cellular slime molds Dictyostelium… Plasmodial slime mold life cycle spores hatch into 2 haploid forms that switch back + forth, fuse to form a zygote Cellular slime mold life cycle 2 cells may fuse into a “giant cell” (2N) that eats other amoebae as it grows thousands of unrelated haploid cells aggregate to form a single “slug” fruiting body forms; some cells become nonreproductive stalk Fungi Heterotrophic decomposers - feed by absorbing nutrients Single-celled (yeasts) or multicellular May be free-living, parasitic, or mutualists (= symbionts) Bodies composed of threadlike structures called hyphae Cell walls contain chitin Key Terms: Symbiosis – two organisms (or species) living in close association (e.g. mycorrhizae) mutualism – both benefit parasitism – parasite benefits, host suffers commensalism – one benefits, the other is unaffected Endosymbiosis – one organism living inside another (host) Examples: Plastids, mitochondria evolved through endosymbiosis Rhizobium bacteria living in root nodules – fix nitrogen Lichens – algal symbionts within a fungal host Heterotrophs – ingest or absorb an external source of organic carbon (molecules with CH’s) - Fungi are heterotrophs that absorb organic carbon from their surroundings Decomposers (saprobes) – break down complex organic molecules into simpler organic molecules - Fungi and bacteria are important decomposers, recycling carbon and other nutrients Biogeochemistry – the (re-)cycling of key elements O, C, N, P, S Basic fungal structure: hyphae and mycelia (singular: mycelium) Hyphae release digestive exo-enzymes into their surrounding Hyphae form a web called the mycelium that greatly increases surface area, maximizing uptake of dissolved nutrients from the substrate (= whatever the fungus is growing on) Hyphae release digestive exoenzymes into their surrounding - break down organic matter into small molecules that can be absorbed Produce enzyme that can break down.. 1- lignin, a complex polymer that makes wood tough 2- cellulose, a polymer of glucose (sugar) that animals can’t break down (hence why termites + cows need gut symbionts) Fungi grow by extending the tips of their hyphae through cytoplasmic streaming Hyphae can be divided into individual cells by partitions called septa Some fungi grow by repeated mitotic divisions of nuclei without cell division coenocytic condition - giant multi-nucleated cells, similar to slime molds Some fungi form mutualistic or parasitic associations with plants Use special hyphae called haustoria to penetrate cell wall of plants - push into cell surrounded by plant plasma membrane Fungal Life Cycle Fungi spread by producing huge #’s of spores - structures that resist harsh environmental conditions - can disperse long distances by wind Produced during both sexual and asexual phases of life cycle Stage 1: Plasmogamy Most hyphae contain haploid nuclei N+N 2 hyphae of different mating types can grow together + fuse = plasmogamy 2N N Stage 2: Karyogamy Eventually, 2 haploid nuclei from different parents fuse into diploid nuclei N+N Zygote quickly undergoes meiosis, producing haploid spores 2N N Heterokaryotic stage = separate haploid nuclei from different parents, in the same hyphae N+N Karyogamy = the 2 haploid nuclei fuse into 1 diploid nucleus 2N (like syngamy) Fungal Evolution Fungi evolved from an ancestor that was an aquatic protist with a flagellum (like sperm cells of animals) Molecular evidence indicates this ancestor was also single-celled - thus, animals & fungi independently evolved multicellularity - only primitive fungi have flagellated spores phylogeny assuming flagellae were lost once new phylogeny indicates flagellae were lost often Campbell & Reece 2002 Chytrids May be single-celled or form multi-cellular hyphae Only fungi w/ flagellated spores, called zoospores Relationship to Zygomycetes is still controversial Aquatic Chytrids zoospore Branching hyphae increase surface area for uptake of nutrients from surrounding aquatic medium Zygomycetes life cycle Haploid 1 1) hyphae of opposite mating type fuse to form heterokaryotic (N+N) zoosporangium - contains many haploid nuclei from each parent - resists bad conditions Zygomycetes life cycle Haploid 1 2) when conditions get better, karyogamy occurs: haploid nuclei fuse into diploid nuclei - zygote then undergoes meiosis, producing genetically diverse spores - regular sporangia form Zygomycetes life cycle Haploid 1 3) Regular sporangia may form and produce spores by mitosis (asexual reproduction) 3 Phylum Zygomycota (Zygomycetes) Pilobolus sp. – dung fungus Microsporidia - highly modified parasites (Encephalitozoon intestinalis) Phylum Glomeromycota - Glomeromycetes Formerly put in zygomycetes; now their own phylum Only 160 known species, but ecologically critical Form endomycorrhizae, mutualistic associations inside of plant roots >90% of plants have endomycorrhizae associated with roots Fungal partner takes up minerals like phosphate from soil, transfers them to root tissue of host plant Phylum Glomeromycota - Glomeromycetes Formerly put in zygomycetes; now their own phylum Only 160 known species, but ecologically critical Endomycorrhizae Hyphae penetrate cell walls, but do not puncture plasma membrane of plant cells Instead, push inside host cell surrounded by plant membrane like fingers in a glove Taiz & Zeiger 2002 Phylum Ascomycota (ascomycetes) “Sac fungi” produce sexual spores in saclike asci Hyphae fuse into N+N heterokaryotic stage Septa form cells with 2 haploid nuclei each - one of these grows into an ascus Developing asci are housed in the ascocarp, the fruiting body that will later eject the spores Hyphae fuse into N+N heterokaryotic stage Septa form cells with 2 haploid nuclei each - one of these grows into an ascus Karyogamy in the ascus combines both parental genomes meiosis produces 4 haploid cells Each undergoes mitosis 8 haploid ascospores Penicillium sp. – an ascoymycete (formerly called a deuteromycete) - source of antibiotic penicillin Campbell & Reece 2005 Saccharomyces sp. -- “yeast” Common disease organisms Used by humans to ferment sugar in dough or grains, for baking or production of adult beverages - under anaerobic conditions, metabolize sugar to ethanol and CO2 (makes dough rise) S. cerevisiae is model eukaryotic cell for molecular biologists to study - 1st fully sequenced eukaryotic genome Saccharomyces sp. - “yeast” Many yeasts have no known sexual stage Yeast cells budding (asexual reproduction) Phylum Basidiomycota - basidiomycetes includes common mushrooms, toadstools, shelf fungi important decomposers of wood long-lived heterokaryotic stage, giving rise to basidiocarp (“mushroom”) in bad conditions A single mushroom cap produces a billion basidiospores Campbell & Reece 2005 Phylum Basidiomycota – sexual stages heterokaryotic N+N Basidiocarps of a basidiomycete form a “fairy ring” overnight Heterokaryotic mycelium connects mushrooms underground - expands outward, digesting organic matter in soil Ectomycorrhizae Freeman 2005 Many basidiomycetes form ectomycorrhizae with plant roots - roots are completely covered in a layer of hyphae Exoenzymes release nitrogen from decaying matter transferred to hyphae extending in between outer-most root cells Self-quiz: Know the 4 different types of reproductive structures that are characteristic of the different fungal phyla Fungal Symbioses Lichens – fungus & alga mutualism - Endosymbiont is usually a chlorophyte (green alga) - Fungus is the host Mycorrhizae – fungus & plant root mutualism Endomycorrhizae - glomeromycete symbiont (inner) Ectomycorrhizae - basidiomycete symbiont (outer) Parasitic fungi cause many plants & animal diseases Lichens – mutualism between fungus (Ascomycete) & green alga 3 growth forms of lichens: - Foliose (leaf-like) - Crustose - Fruticose (shrub-like) This relationship evolved 3 separate times Lichens – mutualism between fungus (Ascomycete) & green alga This relationship evolved 3 separate times Enable plants to eventually grow on what was bare rock by eroding the rock surface, trapping soil Are very sensitive to air pollution, acid rain Ecto-mycorrhizae Endo-mycorrhizae Taiz & Zeiger 2002 Fungi are responsible for many plant diseases; destroy 10-50% of crops worldwide Eating fungus-infected grains is a severe human health issue in much of the world - contributes to high rate of liver cancer in areas that consume peanut meal, which supports fungi that produce liver toxins Historically, eating ergot-infected grain caused outbreaks of madness and death, once thought to be demon possession - ergot fungus produces lysergic acid, similar to LSD