Introduction: Fungi

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Introduction: Fungi

• Fungi comprise a diverse group of (mostly) multicellular eukaryotes.

• Fungi feed by absorbing nutrients from living or dead organisms.

– important decomposers, because of their unique ability to absorb the cellulose and lignin that comprise wood.


Introduction

• Some fungi absorb nutrients from their hosts, lowering the hosts’ fitness and so acting as parasites.

• Most fungi that live in association with other organisms, however, benefit their hosts and are mutualists.

• The roots of virtually every land plant are colonized by mutualistic fungi. In exchange for sugars synthesized by the plant, the fungi provide the plant with water and key nutrients without which the host plants grow more slowly or even starve.


Introduction

• Fungi can be thought of as the master traders and recyclers in terrestrial ecosystems; some fungi release nutrients from dead plants and animals; others transfer nutrients they obtain to living plants.


Why Do Biologists Study Fungi?

• Fungi nourish the plants that nourish us.

• They affect global warming, because they are critical to the carbon cycle on land.

• A handful of species can cause debilitating diseases in humans and crop plants.


Importance: Fungi Provide Nutrients for Land Plants

• Mycorrhizal associations between fungi and plant roots allow faster plant growth.

– Experiments show that plant growth suffers in the absence of these fungi.


Importance: Fungi Speed the Carbon Cycle on Land

• Saprophytes are fungi that make their living by digesting dead plant material.

• The carbon cycle on land has two basic components:

1. The fixation of carbon by land plants.

2. The release of CO2 from plants, animals, and fungi as the result of cellular respiration.

• For most carbon atoms, fungi connect the two components, as they are the organisms that break down the complex molecules in wood into reusable organic compounds.


Fungi Have Important Economic Impacts

• Although parasitic fungi cause athlete’s foot, vaginitis, diaper rash, ringworm, pneumonia, and thrush in humans, the incidence of fungal infections in humans is relatively low.

• Fungi have many beneficial impacts on humans.

– Fungi are the source for many antibiotics, including penicillin.

– Many cultures eat mushrooms.

– We have used yeast for thousands of years to make bread, cheese, soy sauce, tofu, beer, wine, and other foods.


Fungi Have Important Economic Impacts

• The major destructive impact of fungi is on crops.

– Fungi are responsible for fruit and vegetable spoilage.

– Fungi are the cause of several epidemics that have killed millions of trees, such as chestnuts and elms.


How Do Biologists Study Fungi?

• About 80,000 species of fungi have been described and named; about 1000 more are discovered each year.

• The known species are widely regarded as a tiny fraction of the real total, estimated at 1.65 million species or even more.


Analyzing Morphological Traits

• Fungi have very simple bodies. Two growth forms exist:

1. Single-celled forms—yeasts.

2. Multicellular, filamentous forms—mycelia.

• Some fungi adopt both forms.


The Nature of the Fungal Mycelium and Hyphae

• All mycelia are dynamic; they constantly grow in the direction of food sources and die back in areas where food is running out.

• The filaments that make up a mycelium are called hyphae

• Hyphae may be haploid or heterokaryotic, containing several haploid nuclei from different parents.

– Most heterokaryotic hyphae are dikaryotic, with two haploid nuclei, one from each parent.


The Nature of the Fungal Mycelium and Hyphae

• Each filament is separated into cell-like compartments by cross-walls called septa.

– Gaps in septa called pores enable materials to flow between compartments

• Some fungi are coenocytic, meaning they lack septa entirely.

• Hyphae are exceedingly thin, allowing fungal mycelia to penetrate tiny fissures in soil and absorb nutrients that are inaccessible to plant roots.


Mycelia Have a Large Surface Area

• Because mycelia are composed of branching networks of very thin hyphae, the body of a fungus has the highest surface-area-to-volume ratio observed in a multicellular organism.

• This makes absorption extremely efficient but also makes fungi prone to drying out.

• Mycelia are an adaptation to the absorptive lifestyle of fungi. Thus, reproductive organs—not feeding structures—are the only thick, fleshy structures that fungi produce.

– Reproductive spores are resistant to drying out.


4 types of Reproductive Structures

1. Both the sexually produced gametes and the asexually produced spores in the chytrids have flagella, and are the only known motile fungal cells.

2. The zygomycetes have zygosporangia, formed when cells from two yoked-together haploid hyphae fuse.

3. Basidia are specialized spore-producing cells that form at the ends of basidiomycetes hyphae.

4. Asci are sac-like spore-producing cells that form at the end of ascomycetes hyphae.


Evaluating Molecular Phylogenies

• Fungi are more closely related to animals than to land plants.

– Fungal infections in humans are more difficult to treat than bacterial infections. Recent shared ancestry results in similar cellular and molecular structures—drugs that affect fungal structures may similarly affect human structures.

• Evidence for this relationship includes:

– DNA sequence data.

– Both animals and fungi synthesize chitin.

– Chytrid and animal flagella are similar in structure and function.

– Both groups store glucose as glycogen.



• Conclusions:

– Unicellular eukaryotes called microsporidians are fungi.

– Molecules that kill fungi, fungicides, may be able to cure microsporidian infections.

– The Chytridiomycota and Zygomycota are paraphyletic. A single common ancestor did not give rise to all species within each phylum.

– The Glomeromycota, a group of zygomycetes, is monophyletic.



• The Basidiomycota (club fungi) and Ascomycota (sac fungi) are monophyletic, and are the most highly derived groups of fungi.

– The two groups are both dikaryotic and together form a monophyletic group.

• The sister group to fungi consists of protists called choanoflagellates and the animals.

– The earliest fungi, like the choanoflagellates and earliest animals, were most likely aquatic.


Experimental Studies of Mutualism

• Fungi and land plants often have a symbiotic relationship—one of close association. Several types of symbiosis are found.

• Mutualism is a symbiotic relationship that provides benefits to both species involved.

• In a parasitic symbiotic relationship, one species benefits at the expense of the other.

• In a commensal relationship, one species benefits while the other is unaffected.

• Experimental evidence indicates that mycorrhizal fungi and plants are mutualistic.


Themes in the Diversification of Fungi

• The evolution of novel methods for absorbing nutrients from a wide array of food sources drove the diversification of fungi.


Fungi Participate in Several Types of Mutualisms

• Fungi can be involved in both mycorrhizal associations and endophytic (aboveground) associations with plants.

• There are two primary types of plant-mycorrhizal interactions.

1. Ectomycorrhizal fungi (EMF) are usually Basidiomycetes.

2. Arbuscular mycorrhizal fungi (AMF) are all Glomeromycetes.


Ectomycorrhizal Fungi

• Ectomycorrhizal fungi (EMF) form a dense network of hyphae that cover a plant's roots and extend into the soil but do not enter the root cells.

• EMF are found on many tree species in temperate latitudes and in northern coniferous forests.

• EMF release peptidases which cleave proteins, releasing amino acids which the hyphae transport to spaces between the plant root cells.

– EMF also provide nitrogen and phosphate ions to the host plant.

• In return, the EMF receive sugars.


Arbuscular Mycorrhizal Fungi

• Arbuscular mycorrhizal fungi (AMF) grow into the cells of root tissue and directly contact the plasma membrane of the plant cell, thus increasing the surface area available for exchange of molecules between the fungus and its host.

• AMF are extremely ancient, having been found in fossilized root cells that are 400 million years old.

• AMF are found in 80 percent of land plant species, and are particularly common in grasslands and in tropical forests.


What Do AMF Do?

• Unlike EMF, AMF do not provide nitrogen to the plants.

• However, AMF does transfer phosphorus from the soil to the host plant. In return, the plant supplies the AMF with sugar.

• AMF are also extremely important in soil formation. Their hyphae contain large quantities of a glycoprotein called glomalin, which helps bind organic compounds to sand or clay, thus enriching the organic matter in soil.


Are Endophytes Mutualists?

• Endophytes are quite common and highly diverse.

• Recent research has found that endophytes in some grasses produce compounds that deter herbivores in exchange for absorbing sugars from the plants.

– Therefore, these endophytes and their plant hosts are in a mutualistic relationship.

• Some endophytes and plants may be commensals.

– The association does not produce a noticeable effect, either deleterious or beneficial, on either species.


Mutualisms with Other Species

• Lichens are a mutualistic partnership between a species of ascomycete and either a cyanobacterium or an alga.

• Some ant species actively farm fungi inside their colonies. The ants fertilize and “weed” the fungal gardens, then harvest the fungi for food.


Why Are Fungi Effective Decomposers?

• Although bacteria and archaea are also important decomposers in terrestrial environments, fungi and a few bacterial species are the only organisms that can digest wood completely, due to two adaptations:

1. The large surface area on the mycelium enhances absorption.

2. Saprophytic fungi are able to grow toward the dead tissues that supply their food.


Extracellular Digestion

Instead of digesting food inside a stomach or food vacuole, as most animals and some protists do, respectively, fungi synthesize digestive enzymes and then secrete them outside their hyphae, into their food.

– This process is called extracellular digestion.

• Basidiomycetes can completely degrade both lignin and cellulose, components of woody cells.


Lignin Degradation

• Fungi break down lignin with the use of an enzyme called lignin peroxidase.

– This enzyme catalyzes an oxidation step that creates a free radical, leading to a series of uncontrolled reactions that split the polymer into smaller units.

• Fungi break down lignin in order to get to cellulose.


Cellulose Digestion

• Fungi break down cellulose, which they can live on, with enzymes called cellulases.

• Fungi secrete these enzymes, which are highly specific in their action, into the extracellular environment.

• These cellulases together convert cellulose into glucose.


Variation in Reproduction

• Fungi may produce

– swimming gametes and spores,

– yoked hyphae in which nuclei from different individuals fuse to form a zygote inside a protective structure, or

– specialized spore-producing cells called basidia and asci.


Spores as Key Reproductive Cells

• The spore is the most fundamental reproductive cell in fungi.

– They are the dispersal stage in the fungal life cycle and are produced in huge numbers during both asexual and sexual reproduction.

• If a spore falls on a food source and is able to germinate, a mycelium begins to form. As the fungus expands, hyphae grow in the direction in which food is most abundant.


Spores as Key Reproductive Cells

• If food begins to run out, the mycelium responds by making spores, which are dispersed by wind or animals.

• Spore production allows starving mycelia to disperse offspring to new habitats where more food might be available.


Multiple Mating Types

• In many fungal lineages, hyphae come in different mating types rather than morphologically distinct sexes.

– Hyphae of the same mating type will not combine during sexual reproduction.

• In this way, mating types function as sexes. Instead of having just two sexes, a single fungal species may have tens of thousands.

• Having multiple mating types helps generate genetic diversity in offspring.


How Does Fertilization Occur?

Only members of the Chytridiomycota produce gametes, and no fungus produces gametes that are different enough in size to be called sperm and egg.

• In many fungi, fertilization occurs in two steps, which can be separated in both time and space:

1. Fusion of cells.

2. Fusion of nuclei from the fused cells.


How Does Fertilization Occur?

• The process of sexual reproduction begins when hyphae from two individuals fuse to form a hybrid hypha. Plasmogamy occurs when the cytoplasms fuse.

• If the nuclei remain independent, the mycelium becomes heterokaryotic.

• Karyogamy occurs when the nuclei fuse to form a diploid zygote.


Four Major Types of Fungal Life Cycles

• The four major types of life cycles in fungi were the basis for the designation of the four phyla.

• The Chytridiomycota are the only fungi that produce gametes and exhibit alternation of generations.

– Swimming gametes are produced by mitosis in haploid adults.

– Gametes fuse to form a diploid zygote, which grows into a sporophyte. Haploid spores, which disperse by swimming, are produced by meiosis inside the sporophyte’s sporangium.



• Zygomycota form yoked hyphae that produce a spore-forming structure.

• Plasmogamy forms a zygosporangium that develops a tough, resistant coat. Inside the zygosporangium, nuclei from the mating partners fuse—meaning that karyogamy occurs.

• Mycelia can also reproduce asexually by making sporangia, which produce haploid spores by mitosis. The spores are then dispersed by the wind.



• Basidiomycota have reproductive structures with many spore-producing basidia. All basidiomycete reproductive structures originate from the heterokaryotic hyphae of mated individuals.

• Karyogamy occurs within the basidia. The diploid nucleus that results undergoes meiosis, and four haploid spores, which will eventually be ejected and dispersed by the wind, mature.

• A single individual can produce a billion spores.



• Ascomycota have reproductive structures, which are produced by dikaryotic hyphae, with many spore-producing asci.

• After karyogamy occurs inside each ascus, meiosis takes place and haploid spores are produced. When the ascus matures, the spores inside are forcibly ejected and then dispersed by the wind.


Key Lineages of Fungi

• Chytridiomycota and Zygomycota are not monophyletic and so are not true phyla, although the names are still used until more accurate relationships are worked out.


Microsporidia

• All microsporidia are single celled and parasitic, with a polar tube that allows them to enter the interior of the cells they parasitize.

• Microsporidians have a dramatically reduced genome and lack functioning mitochondria.

• Variation in life cycles is extensive.

• Species from eight different genera are parasitic in humans.

– Generally, they only cause serious infections in people who are severely immunocompromised, such as people with AIDS.


Chytridiomycota (Chytrids)

• Chytrids are largely aquatic, and are particularly common in freshwater environments.

• Members of this group are the only fungi that can produce motile cells.

– Most chytrid species produce spores that swim to new habitats via a flagellum.

• Many chytrids can digest cellulose, and have lifestyles ranging from decomposers to parasites to mutualists.

• Some chytrids are the only fungi to exhibit alternation of generations.


Zygomycota

• Zygomycota are primarily soil dwellers.

• Their hyphae yoke together and fuse during sexual reproduction and then form a durable, thick-walled zygosporangium.

• Many are saprophytes, some are parasitic, and some are predatory.

– Species include the common bread mold as well as lineages that cause rot in fruit and vegetables.

• Some species are used in the production of medical steroids, and others in the commercial production of organic acids, pigments, alcohols, and fermented foods.


Glomeromycota

• The AMF are all members of the Glomeromycota.

• Most species form spores underground and are difficult to grow in the lab, so their life cycle is not well known.

• Because grasslands and tropical forests are among the most productive habitats on Earth, the AMF are enormously important to both human and natural economies.


Basidiomycota (Club Fungi)

• The entire array of absorptive lifestyles found in fungi (saprophytic, mutualistic, and parasitic) is found within the Basidiomycota.

– Some species are EMF, which are important in forestry.

• During sexual reproduction, all basidiomycetes produce basidia. The largest subgroup forms basidia in large, aboveground mushrooms, brackets, earthstars, or puffballs. Asexual reproduction is less common than in other fungal lineages.

• Basidiomycota and Ascomycota are the only fungi with dikaryotic mycelia.

• Species are important as food sources, biological research.


Ascomycota (Sac Fungi)

• Over half of all known fungi belong to the Ascomycota. The group is extremely large and diverse, however, and the phylogenetic relationships among subgroups are largely unknown.

• Two major groups of Ascomycota are the lichen-formers and the non-lichen-formers.


Ascomycota (Sac Fungi): Lichen-Formers

• The lichen-forming ascomycetes grow in association with cyanobacteria or green algae to form lichens.

– The fungal hyphae form a dense protective layer that shields the photosynthetic species and reduces water loss. In return, the cyanobacterium or alga provides carbohydrates that the fungus uses as a source of carbon and energy.

• Many lichens reproduce asexually via the production of small “mini-lichen” structures called soredia that contain both symbionts.

• Lichens are the primary food of many animals, initiate soil production in barren areas, and are used in the production of perfume.


Ascomycota: Non-Lichen-Formers

• Non-lichen-forming ascomycetes are found in virtually every terrestrial habitat, as well as some freshwater and marine environments.

• They are sometimes called cup fungi because of their ascus-bearing cup- or saucer-shaped reproductive structure, called an ascocarp.

• Some form EMF associations with tree roots, and they are the most common fungal endophytes. Other species are parasitic, and some are even predatory.

• Some ascomycetes are used to clean up contamination; this group also includes the fungus used to make penicillin, as well as yeast and the valuable foods truffles and morels.

Introduction: Fungi

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