Chapter 31
Fungi
Lecture Outline
Overview:
• The honey mushroom Armillaria ostoyae in Malheur National Park in eastern Oregon is enormous.
? Its subterranean mycelium covers 890 hectares, weighs hundreds of tons, and has been growing for 2,600 years.
• Ten thousand species of fungi have been described, but it is estimated that there are actually up to 1.5 million species of fungi.
• Fungal spores have been found 160 km above the ground.
• Fungi play an important role in ecosystems, decomposing dead organisms, fallen leaves, feces, and other organic materials.
? This decomposition recycles vital chemical elements back to the environment in forms other organisms can assimilate.
• Most plants depend on mutualistic fungi to help their roots absorb minerals and water from the soil.
? Humans have cultivated fungi for centuries for food, to produce antibiotics and other drugs, to make bread rise, and to ferment beer and wine.
Concept 31.1 Fungi are heterotrophs that feed by absorption
Absorptive nutrition enables fungi to live as decomposers and symbionts.
• Fungi are heterotrophs that acquire their nutrients by absorption.
? They absorb small organic molecules from the surrounding medium.
? Exoenzymes, powerful hydrolytic enzymes secreted by the fungus, break down food outside its body into simpler compounds that the fungus can absorb and use.
• The absorptive mode of nutrition is associated with the ecological roles of fungi as decomposers (saprobes), parasites, and mutualistic symbionts.
? Saprobic fungi absorb nutrients from nonliving organisms.
? Parasitic fungi absorb nutrients from the cells of living hosts.
? Some parasitic fungi, including some that infect humans and plants, are pathogenic.
? Fungi cause 80% of plant diseases.
? Mutualistic fungi also absorb nutrients from a host organism, but they reciprocate with functions that benefit their partner in some way.
Extensive surface area and rapid growth adapt fungi for absorptive nutrition.
• Yeasts are single-celled fungi. Most other species of fungi are multicellular.
• The vegetative bodies of most fungi are constructed of tiny filaments called hyphae that form an interwoven mat called a mycelium.
? Fungal mycelia can be huge, but they usually escape notice because they are subterranean.
• Fungal hyphae have cell walls.
? These are built mainly of chitin, a strong but flexible nitrogen-containing polysaccharide identical to that found in arthropods.
• Most fungi are multicellular with hyphae divided into cells by cross walls, or septa.
? Septa generally have pores large enough for ribosomes, mitochondria, and even nuclei to flow from cell to cell.
• Fungi that lack septa, coenocytic fungi, consist of a continuous cytoplasmic mass with hundreds or thousands of nuclei.
? This results from repeated nuclear division without cytoplasmic division.
• Parasitic fungi usually have some hyphae modified as haustoria, nutrient-absorbing hyphal tips that penetrate the tissues of their host.
• Some fungi even have hyphae adapted for preying on animals.
• The filamentous structure of the mycelium provides an extensive surface area that suits the absorptive nutrition of fungi.
? One cubic centimeter of rich organic soil may contain 1 km of fungal hyphae with a surface area of more than 300 cm2.
• A fungal mycelium grows rapidly.
? Proteins and other materials made by the mycelium are channeled by cytoplasmic streaming to the tips of the extending hyphae.
• The fungus concentrates its energy and resources on adding hyphal length and absorptive surface area.
? While fungal mycelia are nonmotile, by swiftly extending the tips of its hyphae it can extend into new territory.
Concept 31.2 Fungi produce spores through sexual or asexual life cycles
• Fungi reproduce by producing vast numbers of spores, either sexually or asexually.
? The output of spores from one reproductive structure can be enormous.
? Puffballs may release trillions of spores.
• Dispersed widely by wind or water, spores germinate to produce mycelia if they land in a moist place where there is food.
Many fungi have a heterokaryotic stage.
• The nuclei of fungal hyphae and spores of most species are haploid, except for brief diploid stages that form during sexual life cycles.
• Sexual reproduction in fungi begins when hyphae from two genetically distinct mycelia release sexual signaling molecules called pheromones.
So, 2 genetically different mats of mycelia release sexual signalling molecules called pheromones.
? Pheromones from each partner bind to receptors on the surface of the other.
• The union of the cytoplasm of the two parent mycelia is known as plasmogamy.
? In many fungi, the haploid nuclei do not fuse right away.
• In some species, heterokaryotic mycelia become mosaics, with different nuclei remaining in separate parts of the same mycelium or mingling and even exchanging chromosomes and genes.
• In some fungi, the haploid nuclei pair off two to a cell, one from each parent.
? Such a mycelium is called dikaryotic, meaning “two nuclei.”
• In many fungi with sexual life cycles, karyogamy, fusion of haploid nuclei contributed by two parents, occurs well after plasmogamy, cytoplasmic fusion of cells from the two parents.
? The delay may be hours, days, or even centuries.
• During karyogamy, the haploid nuclei contributed by the two parents fuse, producing diploid cells.
? In most fungi, the zygotes of transient structures formed by karyogamy are the only diploid stage in the life cycle.
? These undergo meiosis to produce haploid cells that develop as spores in specialized reproductive structures.
? These spores disperse to form new haploid mycelia.
• The sexual processes of karyogamy and meiosis generate genetic variation.
• The heterokaryotic condition also offers some of the advantages of diploidy, in that one haploid genome may be able to compensate for harmful mutations in the other.
Many fungi reproduce asexually.
• The processes of asexual reproduction in fungi vary widely.
? Some species reproduce only asexually.
• Some fungi that can reproduce asexually grow as mould.
? Moulds grow rapidly as mycelia and produce spores.
• Yeasts live in liquid or moist habitats.
• Instead of producing spores, yeasts reproduce asexually by simple cell division or by budding of small cells.
• Most moulds and yeasts have no known sexual stage.
? Such fungi are called deuteromycetes, or imperfect fungi.
? Whenever a sexual stage of a deuteromycete is discovered, the species is classified in a particular phylum depending on its sexual structures.
• Fungi can be identified from their sexual stages and by new genetic techniques.
Concept 31.3 Fungi descended from an aquatic, single-celled, flagellated protist
• Data from paleontology and molecular systematics offer insights into the early evolution of fungi.
• Systematists recognize Fungi and Animalia as sister kingdoms.
? Fungi and animals are more closely related to each other than they are to plants or other eukaryotes.
Phylum Chytridiomycota: Chytrids may provide clues about fungal origins.
• Phylogenetic systematics suggests that fungi evolved from a unicellular, flagellated protist.
? The lineages of fungi that diverged earliest (the chytrids) have flagella.
? Members of the clade Opisthokonta, including animals, fungi, and closely related protists, possess flagella.
? This name refers to the posterior (opistho) location of the flagellum.
• Scientists estimate that the ancestors of animals and fungi diverged into separate lineages 1.5 billion years ago.
? However, the oldest undisputed fungal structures are only 460 million years old.
? It is likely that the first fungi were unicellular and did not fossilize.
• Fungi underwent an adaptive radiation when life began to colonize land.
• Fossils of the first vascular plants from the Silurian period contain evidence of mycorrhizae, symbiotic relationships between plants and subterranean fungi.
Concept 31.4 Fungi have radiated into a diverse set of lineages
• Fungi classified in the phylum Chytridiomycota, called the chytrids, are ever-present in lakes, ponds, and soil.
? Some are saprobes, while others parasitize protists, plants, and animals.
• However, recent molecular evidence supports the hypothesis that chytrids diverged earliest in fungal evolution.
• Like other fungi, chytrids use an absorptive mode of nutrition, have chitinous cell walls, and have similar key enzymes and metabolic pathways.
• While there are a few unicellular chytrids, most form coenocytic hyphae.
• Chytrids are unique among fungi in having flagellated spores, called zoospores.
• Until recently, systematists thought that fungi lost flagella only once in their history, after chytrids had diverged from other lineages.
? However, molecular data now indicates that some flagellated fungi are more closely related to another fungal group, the zygomycetes.
• If this is true, flagella were lost on more than one occasion during fungal evolution.
Phylum Zygomycota: Zygote fungi form resistant structures during sexual reproduction.
• The 1,000 zygomycetes exhibit a considerable diversity of life history. He says, 600 different.
• The phylum includes fast-growing moulds, parasites, and commensal symbionts.
• Include some moulds.
• The life cycle and biology of Rhizopus stolonifer, black bread mould, is typical of zygomycetes.
• The hyphae are coenocytic, however, with septa found only where reproductive cells are formed. REVIEW DIAGRAM
? Horizontal hyphae spread out over food, penetrate it, and digest nutrients.
• In the asexual phase, hundreds of haploid spores develop in sporangia at the tips of upright hyphae.
? Some zygomycetes, such as Pilobolus, can actually aim their sporangia toward conditions that would be favorable for their spores. Direct their spores towards the LIGHT. More successful than others.
• If environmental conditions deteriorate, Rhizopus may reproduce sexually.
• Plasmogamy of opposite mating types produces a zygosporangium.
? Inside this multinucleate structure, the heterokaryotic nuclei fuse to form diploid nuclei that undergo meiosis. Will not undergo karyogomy until certain conditions are meet.
• The zygosporangia are resistant to freezing and drying.
• When conditions improve, the zygosporangia undergo meiosis and release haploid spores that colonize new substrates.
Microsporidia are unicellular parasites.
• Microsporidia are unicellular parasites of animals and protists.
• They are often used in biological control of insect pests.
DIDN’T TALK ABOUT THE THINGS BELOW
• Microsporidia lack conventional mitochondria, and represent something of a taxonomic mystery.
? Some researchers suggest that they are an ancient, deep-branching eukaryotic lineage.
? Recent evidence suggests that they are highly derived parasites that may be related to zygomycete fungi.
Glomeromycetes form mycorrhizae ONCE CONSIDERED ZYGOMYCETES..
• Only 160 species of glomeromycetes have been identified.
• Nonetheless, they are an economically significant group.
• All glomeromycetes form symbiotic mycorrhizae with plant roots.
? Mycorrhizal fungi can deliver phosphate ions and other minerals to plants.
? In exchange, the plants supply the fungi with organic nutrients.
• There are several different types of mycorrhizal fungi.
• Ectomycorrhizal fungi form sheaths of hyphae over the surface of the plant root and grow into the extracellular spaces of the root cortex.
• Endomycorrhizal fungi extend their hyphae through the root cell wall and into tubes formed by inward grow of the root cell membrane.
? The tips of the hyphae that push into plant root cells branch into tiny treelike structures known as arbuscles.
• Such symbiotic partnerships with glomeromycetes are present in 90% of all plants.
Phylum Ascomycota: Sac fungi produce sexual spores in saclike asci.
• Mycologists (fungi studiers) have described more than 32,000 (he say’s 60 000) species of ascomycetes, or sac fungi, from a variety of marine, freshwater, and terrestrial habitats. For the most part terrestrial.
• Ascomycetes produce sexual spores in saclike asci and are called sac fungi.
• Most ascomycetes bear their sexual stages in fruiting bodies called ascocarps.
• They range in size and complexity from unicellular yeasts to elaborate cup fungi and morels. Some are devastating plant pathogens. SEE DIAGRAM.
• Many are important saprobes, particularly of plant material.
• About 40% of ascomycete species live with green algae or cyanobacteria in mutualistic associations called lichens.
? Some ascomycetes form mycorrhizae with plants or live between mesophyll cells in leaves where they may help protect the plant tissue from insects by releasing toxins.