Fungi

Outline

A. Introduction to the Fungi

1. Absorptive nutrition enables fungi to live as decomposers and symbionts
2. Extensive surface area and rapid growth adapt fungi for absorptive nutrition
3. Fungi disperse and reproduce by releasing spores that are produced either sexually or asexually
4. Many fungi have a heterokaryotic stage

B. Diversity of Fungi

1. Phylum Chytridiomycota: Chytrids may provide clues about fungal origins
2. Phylum Zygomycota: Zygote fungi form resistant structures during sexual reproduction
3. Phylum Ascomycota: Sac fungi produce sexual spores in saclike asci
4. Phylum Basidiomycota: Club fungi have long-lived dikaryotic mycelia
5. Molds, yeasts, lichens, and mycorrhizae are specialized lifestyles that evolved independently in diverse fungal phyla

C. Ecological Impacts of Fungi

1. Ecosystems depend on fungi as decomposers and symbionts
2. Some fungi are pathogens
3. Fungi are commercially important

D. Evolution of Fungi

1. Fungi colonized land with plants
2. Fungi and animals evolved from a common protistan ancestor

Introduction

Ecosystems would be in trouble without fungi to decompose 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 that help their roots absorb minerals and water from the soil.

Human have cultivated fungi for centuries for food, to produce antibiotics and other drugs, to make bread rise, and to ferment beer and wine.

Fungi are eukaryotes and most are multicellular.

While once grouped with plants, fungi generally differ from other eukaryotes in nutritional mode, structural organization, growth, and reproduction.

Molecular studies indicate that animals, not plants, are the closest relatives of fungi.

1. 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, digest food outside its body to 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, or 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.
    
• Mutualistic fungi also absorb nutrients from a host organism, but they reciprocate with functions that benefit their partner in some way.
  

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2. Extensive surface area and rapid growth adapt fungi for absorptive nutrition

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.

• One giant individual of Armillaria ostoyae in Oregon is 3.4 miles in diameter and covers 2,200 acres of forest,
  
• It is at least 2,400 years old, and weighs hundreds of tons.
  

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.

• These 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.

• Ten cubic centimeters of rich organic soil may have fungal hyphae with a surface area of over 300 cm².
  

The fungal mycelium grows rapidly, adding as much as a kilometer of hyphae each day.

• Proteins and other materials synthesized by the entire 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.
  

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3. Fungi disperse and reproduce by releasing spores that are produced sexually or asexually

Fungi reproduce by releasing spores that are produced either sexually or asexually.

• The output of spores from one reproductive structure is enormous, with the number reaching into the trillions.
  

Dispersed widely by wind or water, spores germinate to produce mycelia if they land in a moist place where there is food.

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4. Many fungi have a heterokaryotic stage

The nuclei of fungal hyphae and spores of most species are haploid, except for transient diploid stages that form during sexual life cycles.

However, some mycelia become genetically heterogeneous through the fusion of two hyphae that have genetically different nuclei.

In this heterokaryotic mycelium, the nuclei may remain in separate parts of the same mycelium or mingle and even exchange chromosomes and genes.

• One haploid genome may be able to compensate for harmful mutations in the other nucleus.
  

In many fungi with sexual life cycles, karyogamy, fusion of haploid nuclei contributed by two parents, occurs well after plasmogamy, cytoplasmic fusion by the two parents.

• The delay may be hours, days, or even years.
  

In some heterokaryotic mycelium, the haploid nuclei pair off, two to a cell, one from each parent.

• This mycelium is said to be dikaryotic.
  

The two nuclei in each cell divide in tandem.

• 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.
  

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B. Diversity of Fungi

More than 100,000 species of fungi are known and mycologists estimate that there are actually about 1.5 million species worldwide.

Molecular analyses supports the division of the fungi into four phyla.

1. Phylum Chytridiomycota: Chytrids may provide clues about fungal origins

The chytrids are mainly aquatic.

Some are saprobes, while others parasitize protists, plants, and animals.

The presence of flagellated zoospores had been used as evidence for excluding chytrids from kingdom Fungi which lack flagellated cells.

However, recent molecular evidence supports the hypothesis that chytrids are the most primitive fungi.

Like other fungi, chytrids use an absorptive mode of nutrition and have chitinous cell walls.

While there are a few unicellular chytrids, most form coenocytic hyphae.

Some key enzymes and metabolic pathways found in chytrids are shared with other fungal groups, but not with the so-called funguslike protists.

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2. Phylum Zygomycota: Zygote fungi form resistant structures during sexual reproduction

Most of the 600 zygomycete, or zygote fungi, are terrestrial, living in soil or on decaying plant and animal material.

One zygomycete group form mycorrhizae, mutualistic associations with the roots of plants.

Zygomycete hyphae are coenocytic, with septa found only in reproductive structures.

The life cycle and biology of Rhizopus stolonifer, black bread mold, is typical of zygomycetes.

• 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.
  
• If environmental conditions deteriorate, this species of Rhizopus reproduces sexually.
  Plasmogamy of opposite mating types produces a zygosporangium.
  
• Inside this multinucleate structure, the heterokaryotic nuclei fuse to form diploid nuclei that undergo meiosis.
  

The zygomycete Rhizopus can reproduce either asexually or sexually.

The zygosporangia are resistant to freezing and drying.

When conditions improve, the zygosporangia release haploid spores that colonize new substrates.

• Some zygomycetes, such as Pilobolus, can actually aim their spores.
  

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3. Phylum Ascomycota: Sac fungi produce sexual spores in saclike asci

Mycologists have described over 60,000 species of ascomycetes, or sac fungi.

They range in size and complexity from unicellular yeasts to elaborate cup fungi and morels.

Ascomycetes live in a variety of marine, freshwater, and terrestrial habitats.

• Some are devastating plant pathogens.
  
• Many are important saprobes, particularly of plant material.
  
• About half the ascomycete species live with algae 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.
  

The defining feature of the Ascomycota is the production of sexual spores in saclike asci.

• In many species, the spore-forming asci are collected into macroscopic fruiting bodies, the ascocarp.
  
  • Examples of ascocarps include the edible parts of truffles and morels.
    

Ascomycetes reproduce asexually by producing enormous numbers of asexual spores, which are usually dispersed by the wind.

• These naked spores, or conidia, develop in long chains or clusters at the tips of specialized hyphae called conidiophores.
  

Ascomycetes are characterized by an extensive heterokaryotic stage during the formation of ascocarps.

1. The sexual phase of the ascomycete lifestyle begins when haploid mycelia of opposite mating types become intertwined and form an antheridium and ascogonium.
   
2. Plasmogamy occurs via a cytoplasmic bridge and haploid nuclei migrate from the antheridium to the ascogonium, creating a heterokaryon.
   
3. The ascogonium produces dikaryotic hyphae that develop into an ascocarp.
   
4. The tips of the ascocarp hyphae are partitioned into asci.
   
5. Karyogamy occurs within these asci and the diploid nuclei divide by meiosis, yielding four haploid nuclei.
   
6. Each haploid nuclei divides once by mitosis to produce eight nuclei, often in a row, and cell walls develop around each nucleus to form ascospores.
   
7. When mature, all the ascospores in an ascus are dispersed at once, often leading to a chain reaction of release, from other asci.
   
8. Germinating ascospores give rise to new haploid mycelia.
   
9. Asexual reproduction occurs via conidia.
   

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4. Phylum Basidiomycota: Club fungi have long-lived dikaryotic mycelia

Approximately 25,000 fungi, including mushrooms, shelf fungi, puffballs, and rusts, are classified in the phylum Basidiomycota.

The name of the phylum is derived from the basidium, a transient diploid stage.

• The clublike shape of the basidium is responsible for the common name club fungus.
  

Basidiomycetes are important decomposers of wood and other plant materials.

• Of all fungi, these are the best at decomposing the complex polymer lignin, abundant in wood.
  

Two groups of basidiomycetes, the rusts and smuts, include particularly destructive plant parasites.

The life cycle of a club fungus usually includes a long-lived dikaryotic mycelium.

1. Two haploid mycelia of opposite mating type undergo plasmogamy, creating a dikaryotic mycelium that ultimately crowds out the haploid parents.
   
2. Environmental cues, such as rain or temperature change, induce the dikaryotic mycelium to form compact masses that develop into basidiocarps.
   
   • Cytoplasmic streaming from the mycelium swells the hyphae, rapidly expanding them into an elaborate fruiting body, the basidiocarp (mushrooms in many species).
     
   • The dikaryotic mycelia are long-lived, generally producing a new crop of basidiocarp each year.
     
3. The surface of the basidiocarp's gills are lined with terminal dikaryotic cells called basidia.
   
4. Karyogamy produces diploid nuclei which then undergo meiosis, each yielding four haploid nuclei.
   
   • Each basidium grows four appendages, and one haploid nucleus enters each and develops into a basidiospore.
     
5. When mature, the basidiospores are propelled slightly by electrostatic forces into the spaces between the gills and then dispersed by the wind.
   
6. The basidiospores germinate in a suitable habitat and grow into a short-lived haploid mycelia.
   

Asexual reproduction in basidiomycetes is much less common than in ascomycetes.

A billion sexually-produced basidiospores may be produced by a single, store-bought mushroom.

• The cap of the mushrooms support a huge surface area of basidia on gills.
  
• These spores drop beneath the cap and are blown away.
  

By concentration growth in the hyphae of mushrooms, a basidiomycete mycelium can erect basidiocarps in just a few hours.

• A ring of mushrooms may appear overnight.
  
• At the center of the ring are areas where the mycelium has already consumed all the available nutrients.
  
• As the mycelium radiates out, it decomposes the organic matter in the soil and mushrooms form just behind this advancing edge.
  

The four fungal phyla can be distinguished by their reproductive features.

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5. Molds, yeasts, lichens, and mycorrhizae are specialized lifestyles that evolved independently in diverse fungal taxa

Four fungal forms: molds, yeasts, lichens, and mycorrhizae, have evolved morphological and ecological adaptations for specialized ways of life.

• These have occurred independently among the zygote fungi, sac fungi, and club fungi.
  

A mold is a rapidly growing, asexually reproducing fungus.

• The mycelia of these fungi grow as saprobes or parasites on a variety of substrates.
  
• Early in life, a mold, a term that applies properly only to the asexual stage, produces asexual spores.
  
• Later, the same fungus may reproduce sexually, producing zygosporangia, ascocarps, or basidiocarps.
  

Some molds cannot be classified as zygomycetes, ascomycetes, or basidiomycetes because they have no known sexual stages.

Collectively called deuteromycetes, or imperfect fungi, these fungi reproduce asexually by producing haploid spores.

• This is an informal grouping without phylogenetic basis.
  

Whenever a sexual stage for one of these fungi is discovered, it is moved to the phylum that matches its type of sexual structures.

Yeasts are unicellular fungi that inhabit liquid or moist habitats, including plant sap and animal tissues.

• Yeasts reproduce asexually by simple cell division or budding off a parent cell.
  
• Some yeast reproduce sexually, forming asci (Ascomycota) or basidia (Basidiomycota), but others have no known sexual stage (imperfect fungi).
  

Humans have used yeasts to raise bread or ferment alcoholic beverages for thousands of years.

Various strains of the yeast Saccharomyces cerevisiae, an ascomycete, have been developed as baker's yeast and brewer's yeast.

• Baker's yeast releases small bubbles of CO₂ that leaven dough.
  
• Brewer's yeast ferment sugars into alcohol.
  

Researchers have used Saccharomyces to investigate the molecular genetics of eukaryotes because they are easy to culture and manipulate.

Some yeasts cause problems for humans.

• A pink yeast, Rhodotorula, grows on shower curtains and other moist surfaces in our homes.
  
• Another yeast, Candida, is a normal inhabitant of moist human epithelial surfaces, such as the vaginal lining.
  
  • An environmental change, such as a change in pH or compromise to the human immune system, can cause Candida to become pathogenic by growing too rapidly and releasing harmful substances.
    

While often mistaken for mosses or other simple plants when viewed at a distance, lichens are actually a symbiotic association of millions of photosynthetic microorganisms held in a mesh of fungal hyphae.

The fungal component is commonly an ascomycete, but several basidiomycete lichens are known.

The photosynthetic partners are usually unicellular or filamentous green algae or cyanobacteria.

The merger of fungus and algae is so complete that they are actually given genus and species names, as though they were single organisms.

• Over 25,000 species have been described.
  

The fungal hyphae provides most of the lichen's mass and gives it its overall shape and structure.

The algal component usually occupies an inner layer below the lichen surface.

In most cases, each partner provides things the other could not obtain on its own.

• For example, the alga provides the fungus with food by "leaking" carbohydrate from their cells.
  
• The cyanobacteria provide organic nitrogen through nitrogen fixation.
  
• The fungus provides a suitable physical environment for growth, retaining water and minerals, allowing for gas exchange, protecting the algae from intense sunlight with pigments, and deterring consumers with toxic compounds.
  
  • The fungi also secrete acids, which aid in the uptake of minerals.
    

The fungi of many lichens reproduce sexually by forming ascocarps or basidiocarps.

Lichen algae reproduce independently by asexual cell division.

Asexual reproduction of symbiotic units occurs either by fragmentation of the parental lichen or by the formation of structures, called soredia, small clusters of hyphae with embedded algae.

The nature of lichen symbiosis is probably best described as mutual exploitation instead of mutual benefit.

• Lichens live in environments where neither fungi nor algae could live alone.
  
• While the fungi do not not grow alone in the wild, some lichen algae occur as free-living organisms.
  
• If cultured separately, the fungi do not produce lichen compounds and the algae do not "leak" carbohydrate from their cells.
  
• In some lichens, the fungus invades algal cells with haustoria and kills some of them, but not as fast as the algae replenish its numbers by reproduction.
  

Lichens are important pioneers on newly cleared rock and soil surfaces, such as burned forests and volcanic flows.

• The lichen acids penetrate the outer crystals of rocks and help break down the rock.
  
• This allows soil-trapping lichens to establish and starts the process of succession.
  
• Nitrogen-fixing lichens also add organic nitrogen to some ecosystems.
  

Some lichens survive severe cold or desiccation.

• In the arctic tundra, herds of caribou and reindeer graze on carpets of reindeer lichens under the snow in winter.
  
• In dry habitats, lichens absorb water quickly from fog or rain, gaining more than ten times their mass in water.
  
• In dry air, lichens rapidly dehydrate and stop photosynthesis.
  
  • In arid climates, lichens grow very slowly, often less than a millimeter per year.
    

Lichens are particularly sensitive to air pollution and their deaths can serve as an early warning of deteriorating air quality.

Mycorrhizae are mutualistic associations of plant roots and fungi.

• The anatomy of this symbiosis depends on the type of fungus.
  

The extensions of the fungal mycelium from the mycorrhizae greatly increases the absorptive surface of the plant roots.

The fungus provides minerals from the soil for the plant, and the plant provides organic nutrients.

Mycorrhizae are enormously important in natural ecosystems and in agriculture.

• Almost all vascular plants have mycorrhizae and the Basidiomycota, Ascomycota, and Zygomycota all have members that form mycorrhizae.
  
• The fungi in these permanent associations periodically form fruiting bodies for sexual reproduction.
  
• Plant growth without mycorrhizae is often stunted.
  

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C. Ecological Impacts of Fungi

1. Ecosystems depend on fungi as decomposers and symbionts

Fungi and bacteria are the principle decomposers that keep ecosystems stocked with the inorganic nutrients essential for plant growth.

• Without decomposers, carbon, nitrogen, and other elements would become tied up in organic matter.
  

In their role as decomposers, fungal hyphae invade the tissues and cells of dead organic matter.

• Exoenzymes hydrolyze polymers.
  

A succession of fungi, bacteria, and even some invertebrates break down plant litter or corpses.

On the other hand, the aggressive decomposition by fungi can be a problem.

• Between 10% and 50% of the world's fruit harvest is lost each year to fungal attack.
  
• Ethylene, a plant hormone that causes fruit to ripen, also stimulates fungal spores on the fruit surface to germinate.
  
• Fungi do not distinguish between wood debris and human structures built of wood.
  

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2. Some fungi are pathogens

About 30% of the 100,000 known species of fungi are parasites, mostly on or in plants.

• Invasive ascomycetes have had drastic effects on forest trees, such as American elms and American chestnut, in the northeastern United States.
  
• Other fungi, such as rusts and ergots, infect grain crops, causing tremendous economic losses each year.
  

Some fungi that attack food crops produce compounds that are harmful to humans.

• For example, the mold Aspergillus can contaminate improperly stored grains and peanuts with aflatoxins, which are carcinogenic.
  
• Poisons produced by the ascomycete Claviceps purpurea can cause gangrene, nervous spasms, burning sensations, hallucinations, and temporary insanity when infected rye is milled into flour and consumed.
  
• On the other hand, some toxin extracted from fungi have medicinal uses when administered at weak doses.
  

Animals are much less susceptible to parasitic fungi than are plants.

• Only about 50 fungal species are known to parasitize humans and other animals, but their damage can be disproportionate to their taxonomic diversity.
  

The general term for a fungal infection is mycosis.

• Infections of ascomycetes produce the disease ringworm, known as athlete's foot when they grow on the feet.
  
• Inhaled infections of other species can cause tuberculosis-like symptoms.
  
• Candida albicans is a normal inhabitant of the human body, but it can become an opportunistic pathogen.
  

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3. Fungi are commercially important

In addition to the benefits that we receive from fungi in their roles as decomposers and recyclers of organic matter, we use fungi in a number of ways.

• Most people have eaten mushrooms, the fruiting bodies (basidiocarps) of subterranean fungi.
  
• The fruiting bodies of certain mycorrhizal ascomycetes, truffles, are prized by gourmets for their complex flavors.
  
• The distinctive flavors of certain cheeses come from the fungi used to ripen them.
  
• The ascomycete mold Aspergillus is used to produce citric acid for colas.
  

Yeast are even more important in food production.

• Yeasts are used in baking, brewing, and winemaking.
  

Contributing to medicine, some fungi produce antibiotics used to treat bacterial diseases.

• In fact, the first antibiotic discovered was penicillin, made by the common mold Penicillium.
  

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D. Evolution of Fungi

1. Fungi colonized land with plants

The fossil record indicates that terrestrial communities have always been dependent on fungi as decomposers and symbionts.

The oldest undisputed fossil fungi date back 460 million years, about the time plants began to colonize land.

Fossils of the first vascular plants from the late Silurian period have petrified mycorrhizae.

Plants probably moved onto land in the company of fungi.

Molecular evidence supports the widely held view that the four fungal divisions are monophyletic.

• The occurrence of flagella in chytrids, the oldest fungal lineage, indicates that fungal ancestors were aquatic flagellated organisms.
  
• Flagellated cells were lost as ancestral fungi became increasingly adapted to life on land.
  
• Many of the differences among the Zygomycota, Ascomycota, and Basidiomycota are different solutions to the problem of reproducing and dispersing on land.
  

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2. Fungi and animals evolved from a common protistan ancestor

Animals probably evolved from aquatic flagellated organisms too.

Molecular evidence from comparisons of several proteins and ribosomal RNA indicates that fungi are more closely related to animals than to plants.

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