Chapter 31 Fungi Eukaryotes most multicellular Differ in nutritional mode structural organization growth reproduction Once grouped with plants molecular studies indicate that animals are closest relatives Heterotrophs acquire nutrients by absorption Digest food outside body by secreting powerful hydrolytic enzymes exoenzymes Fungi function as 1 decomposers saprobes absorb nutrients from non living organic material 2 parasites absorb nutrients from cells of living hosts 3 Mutualistic symbionts absorb nutrients from living host but reciprocate Extensive surface area and rapid growth adapt fungi for absorptive nutrition Vegetative bodies diffusely organized around and within food source Except for yeasts microsporidia fungi constructed of tiny filaments called hyphae Composed of tubular walls that surround plasma membranes cytoplasm Hyphae form interwoven mat mycelium Mycelia can be huge maximizes surface area Armillaria ostoyae 3 4 miles in diameter 2200 acres of forest 1900 years old and weighs hundreds of tons Most fungi multicellular with hyphae divided into cells by septa Large enough holes in septa to allow cellular organelles to flow from cell to cell Cell walls of fungi built mainly of chitin Some fungi are aseptate hyphae not divided called coenocytic fungi continuous cytoplasmic mass hundereds or thousnads of nuclei Parasitic fungi hyphae modified as haustoria absorbing hyphal tips that penetrate host tissue Filamentous structure of mycelium extensive surface area for rapid growth kilometer of hyphae each day Fungi disperse and reproduce by releasing spores that are produced sexually or asexually Output is enormous trillions of spores Distributed by air spores have been found 100 miles above Earth Nuclei of fungal hyphae spores of most species are haploid except for transient diploid stages that form during sexual life cycle Some mycelia have become genetically heterogeneous through the fusion of 2 hyphae with genetically different nuclei heterokaryon In many fungi with sexual life cycles union occurs in two distinct stages called 1 plasmogamy fusion of 2 parents cytoplasm 2 karyogamy fusion of haploid nuclei May be separated by hours to centuries Meiosis restores haploid condition before spores are produced During interim hybrid mycelium exists as heterokaryon Haploid n Heterokaryotic Diploid 2n Another mycelium PLASMOGAMY fusion of cytoplasm Heterokaryotic stage Sporeproducing structures Spores ASEXUAL Mycelium REPRODUCTION GERMINATION SEXUAL REPRODUCTION GERMINATION KARYOGAMY fusion of nuclei Zygote MEIOSIS Spores Origin of Fungi Fungi evolved from an aquatic single celled flagellated protist as did animals DNA sequence data indicate that fungi animals and their protistan relatives form a clade opisthokonts More closely related to nuclearids than to the choanoflagellates the protistan group to which animals are more closely related Multicellularity must have evolved in animals and fungi independently Animals and their close protistan relatives Nucleariids Chytrids Fungi Other fungi Opisthokonts UNICELLULAR FLAGELLATED ANCESTOR Fungi classified into five major phyla Chytrids 1 000 species Hyphae 25 m Zygomycetes 1 000 species Glomeromycetes 160 species Fungal hypha 100 000 species of fungi are known 1 5x106 possible Ascomycetes 65 000 species 25 m Basidiomycetes 30 000 species Phylum Chytridomycota Chytrids may provide clues about fungal origins Chytrids mainly aquatic some saprobes other parasites protists plants animals Form uniflagellated spores called zoospores Chytrids are the most primitive fungi Fungi evolved from protists that had flagella now only found in chytrids Phylum Zygomycota Zygote fungi form resistant structures during sexual reproduction 1000 species mostly terrestrial One group forms mycorrhizae mutualistic association with roots of plants Common zygomycete is black bread mold Rhizopus stolonifer hyphae spread out over food penetrate absorb nutrients Asexual phase bulbous black sporangia develop at tips of upright hyphae Can reproduce sexually if food supply used up Plasmogamy produces a resistant structure called a zygosporangium in which karyogamy meiosis occur Resistant to freeze and drying are metabolically inactive When conditions improve release genetically diverse haploid spores Haploid n Heterokaryotic n n Diploid 2n PLASMOGAMY Mating type Rhizopus growing on bread New mycelia ASEXUAL REPRODUCTION Mycelium 100 m Young zygosporangium heterokaryotic SEXUAL REPRODUCTION Dispersal and germination of spores Sporangia 50 m Mating type Gametangia with haploid nuclei KARYOGAMY Sporangium MEIOSIS Diploid nuclei Zygosporangium Phylum Glomeromycota Form a distinct type of mycorrhizae arbuscular mycorrhizae Push into plant root cells form arbuscules tiny treelike structures 160 species Found in about 90 of plants Phylum Ascomycota Sac fungi produce sexual spores in saclike asci 65 000 species of ascomycetes or sac fungi marine freshwater terrestrial Range in size from unicellular yeasts to elaborate cap fungi morels Include some of most devastating plant pathogens Many are important saprobes Defining feature is production of sexual spores in saclike asci Sexual stages occur in macroscopic fruiting bodies ascocarps Produce asexually by producing enormous of spores at tips of specialized hyphae conidiophores spores called conidia Conidia mating type Dispersal Haploid n Dikaryotic n n Diploid 2n Mating type Germination ASEXUAL REPROHypha DUCTION PLASMOGAMY Ascus dikaryotic Conidiophore Dikaryotic hyphae Mycelia Mycelium Germination Dispersal Asci Ascocarp SEXUAL REPRODUCTION KARYOGAMY Diploid nucleus zygote Eight ascospores Four haploid nuclei MEIOSIS Phylum Basidiomycota Club fungi 30 000 species including mushrooms shelf fungi puffballs rusts and smuts Basidium L little pedestal transient diploid stage in organisms life cycle Important decomposers of wood other plant material best at decomposing lignin Life cycle includes long lived dikaryotic mycelium Periodically mycelium reproduces sexually by producing elaborate fruiting bodies called basidiocarps A mushroom is a basidiocarp Basidia of a basidiocarp are sources of sexual spores Cap supports protects large surface area of basidia on gills Mycelium can erect fruiting structure in just a few hours Fairy ring may appear overnight Increases about 30 cm y Shelf fungi Puffballs Maiden veil fungus PLASMOGAMY Dikaryotic mycelium Mating type Mating type Gills lined with basidia SEXUAL REPRODUCTION Haploid