Fungi

Characteristics of Fungi

Fungi, latin for mushroom, are eukaryotes which are responsible for decomposition and nutrient cycling through the environment.

Learning Objectives

Describe the role of fungi in the ecosystem

Key Takeaways

Key Points

  • Fungi are more closely related to animals than plants.
  • Fungi are heterotrophic: they use complex organic compounds as sources of energy and carbon, not photosynthesis.
  • Fungi multiply either asexually, sexually, or both.
  • The majority of fungi produce spores, which are defined as haploid cells that can undergo mitosis to form multicellular, haploid individuals.
  • Fungi interact with other organisms by either forming beneficial or mutualistic associations (mycorrhizae and lichens ) or by causing serious infections.

Key Terms

  • mycorrhiza: a symbiotic association between a fungus and the roots of a vascular plant
  • spore: a reproductive particle, usually a single cell, released by a fungus, alga, or plant that may germinate into another
  • lichen: any of many symbiotic organisms, being associations of fungi and algae; often found as white or yellow patches on old walls, etc.
  • Ascomycota: a taxonomic division within the kingdom Fungi; those fungi that produce spores in a microscopic sporangium called an ascus
  • heterotrophic: organisms that use complex organic compounds as sources of energy and carbon

Introduction to Fungi

The word fungus comes from the Latin word for mushrooms. Indeed, the familiar mushroom is a reproductive structure used by many types of fungi. However, there are also many fungi species that don’t produce mushrooms at all. Being eukaryotes, a typical fungal cell contains a true nucleus and many membrane-bound organelles. The kingdom Fungi includes an enormous variety of living organisms collectively referred to as Ascomycota, or true Fungi. While scientists have identified about 100,000 species of fungi, this is only a fraction of the 1.5 million species of fungus probably present on earth. Edible mushrooms, yeasts, black mold, and the producer of the antibiotic penicillin, Penicillium notatum, are all members of the kingdom Fungi, which belongs to the domain Eukarya.

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Examples of fungi: Many species of fungus produce the familiar mushroom (a) which is a reproductive structure. This (b) coral fungus displays brightly-colored fruiting bodies. This electron micrograph shows (c) the spore-bearing structures of Aspergillus, a type of toxic fungi found mostly in soil and plants.

Fungi, once considered plant-like organisms, are more closely related to animals than plants. Fungi are not capable of photosynthesis: they are heterotrophic because they use complex organic compounds as sources of energy and carbon. Some fungal organisms multiply only asexually, whereas others undergo both asexual reproduction and sexual reproduction with alternation of generations. Most fungi produce a large number of spores, which are haploid cells that can undergo mitosis to form multicellular, haploid individuals. Like bacteria, fungi play an essential role in ecosystems because they are decomposers and participate in the cycling of nutrients by breaking down organic and inorganic materials to simple molecules.

Fungi often interact with other organisms, forming beneficial or mutualistic associations. For example most terrestrial plants form symbiotic relationships with fungi. The roots of the plant connect with the underground parts of the fungus forming mycorrhizae. Through mycorrhizae, the fungus and plant exchange nutrients and water, greatly aiding the survival of both species Alternatively, lichens are an association between a fungus and its photosynthetic partner (usually an alga). Fungi also cause serious infections in plants and animals. For example, Dutch elm disease, which is caused by the fungus Ophiostoma ulmi, is a particularly devastating type of fungal infestation that destroys many native species of elm (Ulmus sp.) by infecting the tree’s vascular system. The elm bark beetle acts as a vector, transmitting the disease from tree to tree. Accidentally introduced in the 1900s, the fungus decimated elm trees across the continent. Many European and Asiatic elms are less susceptible to Dutch elm disease than American elms.

In humans, fungal infections are generally considered challenging to treat. Unlike bacteria, fungi do not respond to traditional antibiotic therapy because they are eukaryotes. Fungal infections may prove deadly for individuals with compromised immune systems.

Fungi have many commercial applications. The food industry uses yeasts in baking, brewing, and cheese and wine making. Many industrial compounds are byproducts of fungal fermentation. Fungi are the source of many commercial enzymes and antibiotics.

Fungi as Plant, Animal, and Human Pathogens

From crop and food spoilage to severe infections in animal species, fungal parasites and pathogens are wide spread and difficult to treat.

Learning Objectives

Give examples of fungi that are plant and animal parasites and pathogens

Key Takeaways

Key Points

  • In plants, fungi can destroy plant tissue directly or through the production of potent toxins, which usually ends in host death and can even lead to ergotism in animals like humans.
  • During mycosis, fungi, like dermatophytes, successfully attack hosts directly by colonizing and destroying their tissues.
  • Examples of fungal parasites and pathogens in animals that cause mycoses include Batrachochytrium dendrobatidis, Geomyces destructans, and Histoplasma capsulatum.
  • Systemic mycoses, such as valley fever, Histoplasmosis, or pulmonary disease, are fungal diseases that spread to internal organs and commonly enter the body through the respiratory system.
  • Opportunistic mycoses, fungal infections that are common in all environments, mainly take advantage of individuals who have a compromised immune system, such as AIDS patients.
  • Fungi can also cause mycetismus, a disease caused by the ingestion of toxic mushrooms that leads to poisoning.

Key Terms

  • mycosis: a fungal disease caused by infection and direct damage
  • dermatophyte: a parasitic fungus that secretes extracellular enzymes that break down keratin, causing infections the skin, such as jock itch and athlete’s foot
  • aflatoxin: toxic, carcinogenic compounds released by fungi of the genus Aspergillus; contaminate nut and grain harvests
  • ergot: any fungus in the genus Claviceps which are parasitic on grasses

Fungal Parasites and Pathogens

The production of sufficient good-quality crops is essential to human existence. Plant diseases have ruined crops, bringing widespread famine. Many plant pathogens are fungi that cause tissue decay and eventual death of the host. In addition to destroying plant tissue directly, some plant pathogens spoil crops by producing potent toxins. Fungi are also responsible for food spoilage and the rotting of stored crops. For example, the fungus Claviceps purpurea causes ergot, a disease of cereal crops (especially of rye). Although the fungus reduces the yield of cereals, the effects of the ergot’s alkaloid toxins on humans and animals are of much greater significance. In animals, the disease is referred to as ergotism. The most common signs and symptoms are convulsions, hallucinations, gangrene, and loss of milk in cattle. The active ingredient of ergot is lysergic acid, which is a precursor of the drug LSD. Smuts, rusts, and powdery or downy mildew are other examples of common fungal pathogens that affect crops.

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Fungal pathogens: Some fungal pathogens include (a) green mold on grapefruit, (b) powdery mildew on a zinnia, (c) stem rust on a sheaf of barley, and (d) grey rot on grapes.

Aflatoxins are toxic, carcinogenic compounds released by fungi of the genus Aspergillus. Periodically, harvests of nuts and grains are tainted by aflatoxins, leading to massive recall of produce. This sometimes ruins producers and causes food shortages in developing countries.

Animal and Human Parasites and Pathogens

Fungi can affect animals, including humans, in several ways. A mycosis is a fungal disease that results from infection and direct damage. Fungi attack animals directly by colonizing and destroying tissues. Mycotoxicosis is the poisoning of humans (and other animals) by foods contaminated by fungal toxins (mycotoxins). Mycetismus describes the ingestion of preformed toxins in poisonous mushrooms. In addition, individuals who display hypersensitivity to molds and spores develop strong and dangerous allergic reactions. Fungal infections are generally very difficult to treat because, unlike bacteria, fungi are eukaryotes. Antibiotics only target prokaryotic cells, whereas compounds that kill fungi also harm the eukaryotic animal host.

Many fungal infections are superficial; that is, they occur on the animal’s skin. Termed cutaneous (“skin”) mycoses, they can have devastating effects. For example, the decline of the world’s frog population in recent years may be caused by the fungus Batrachochytrium dendrobatidis, which infects the skin of frogs and presumably interferes with gaseous exchange. Similarly, more than a million bats in the United States have been killed by white-nose syndrome, which appears as a white ring around the mouth of the bat. It is caused by the cold-loving fungus Geomyces destructans, which disseminates its deadly spores in caves where bats hibernate. Mycologists are researching the transmission, mechanism, and control of G. destructans to stop its spread.

Fungi that cause the superficial mycoses of the epidermis, hair, and nails rarely spread to the underlying tissue. These fungi are often misnamed “dermatophytes”, from the Greek words dermis meaning skin and phyte meaning plant, although they are not plants. Dermatophytes are also called “ringworms” because of the red ring they cause on skin. They secrete extracellular enzymes that break down keratin (a protein found in hair, skin, and nails), causing conditions such as athlete’s foot and jock itch. These conditions are usually treated with over-the-counter topical creams and powders; they are easily cleared. More persistent superficial mycoses may require prescription oral medications.

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Mycosis infection: (a) Ringworm presents as a red ring on skin; (b) Trichophyton violaceum, shown in this bright field light micrograph, causes superficial mycoses on the scalp; (c) Histoplasma capsulatum is an ascomycete that infects airways and causes symptoms similar to influenza.

Systemic mycoses spread to internal organs, most commonly entering the body through the respiratory system. For example, coccidioidomycosis (valley fever) is commonly found in the southwestern United States where the fungus resides in the dust. Once inhaled, the spores develop in the lungs and cause symptoms similar to those of tuberculosis. Histoplasmosis is caused by the dimorphic fungus Histoplasma capsulatum. It also causes pulmonary infections. In rarer cases, it causes swelling of the membranes of the brain and spinal cord. Treatment of these and many other fungal diseases requires the use of antifungal medications that have serious side effects.

Opportunistic mycoses are fungal infections that are either common in all environments or are part of the normal biota. They mainly affect individuals who have a compromised immune system. Patients in the late stages of AIDS suffer from opportunistic mycoses that can be life threatening. The yeast Candida sp., a common member of the natural biota, can grow unchecked and infect the vagina or mouth (oral thrush) if the pH of the surrounding environment, the person’s immune defenses, or the normal population of bacteria are altered.

Mycetismus can occur when poisonous mushrooms are eaten. It causes a number of human fatalities during mushroom-picking season. Many edible fruiting bodies of fungi resemble highly-poisonous relatives. Amateur mushroom hunters are cautioned to carefully inspect their harvest and avoid eating mushrooms of doubtful origin.

Fungi Habitat, Decomposition, and Recycling

Fungi are the major decomposers of nature; they break down organic matter which would otherwise not be recycled.

Learning Objectives

Explain the roles played by fungi in decomposition and recycling

Key Takeaways

Key Points

  • Aiding the survival of species from other kingdoms through the supply of nutrients, fungi play a major role as decomposers and recyclers in the wide variety of habitats in which they exist.
  • Fungi provide a vital role in releasing scarce, yet biologically-essential elements, such as nitrogen and phosphorus, from decaying matter.
  • Their mode of nutrition, which involves digestion before ingestion, allows fungi to degrade many large and insoluble molecules that would otherwise remain trapped in a habitat.

Key Terms

  • decomposer: any organism that feeds off decomposing organic material, especially bacterium or fungi
  • exoenzyme: any enzyme, generated by a cell, that functions outside of that cell
  • saprobe: an organism that lives off of dead or decaying organic material

Fungi & Their Roles as Decomposers and Recyclers

Fungi play a crucial role in the balance of ecosystems. They colonize most habitats on earth, preferring dark, moist conditions. They can thrive in seemingly-hostile environments, such as the tundra. However, most members of the Kingdom Fungi grow on the forest floor where the dark and damp environment is rich in decaying debris from plants and animals. In these environments, fungi play a major role as decomposers and recyclers, making it possible for members of the other kingdoms to be supplied with nutrients and to live.

The food web would be incomplete without organisms that decompose organic matter. Some elements, such as nitrogen and phosphorus, are required in large quantities by biological systems; yet, they are not abundant in the environment. The action of fungi releases these elements from decaying matter, making them available to other living organisms. Trace elements present in low amounts in many habitats are essential for growth, but would remain tied up in rotting organic matter if fungi and bacteria did not return them to the environment via their metabolic activity.

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Fungi as decomposers: Fungi are an important part of ecosystem nutrient cycles. These bracket fungi growing on the side of a tree are the fruiting structures of a basidiomycete. They receive their nutrients through their hyphae, which invade and decay the tree trunk.

The ability of fungi to degrade many large and insoluble molecules is due to their mode of nutrition. As seen earlier, digestion precedes ingestion. Fungi produce a variety of exoenzymes to digest nutrients. These enzymes are either released into the substrate or remain bound to the outside of the fungal cell wall. Large molecules are broken down into small molecules, which are transported into the cell by a system of protein carriers embedded in the cell membrane. Because the movement of small molecules and enzymes is dependent on the presence of water, active growth depends on a relatively-high percentage of moisture in the environment.

As saprobes, fungi help maintain a sustainable ecosystem for the animals and plants that share the same habitat. In addition to replenishing the environment with nutrients, fungi interact directly with other organisms in beneficial, but sometimes damaging, ways.

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Fungi: beneficial & pathogenic: Shelf fungi, so called because they grow on trees in a stack, attack and digest the trunk or branches of a tree. While some shelf fungi are found only on dead trees, others can parasitize living trees, causing eventual death. They are considered serious tree pathogens.

Chytridiomycota: The Chytrids

Chytrids are the most primitive group of fungi and the only group that possess gametes with flagella.

Learning Objectives

Describe the ecology and reproduction of chytrids

Key Takeaways

Key Points

  • The first recognizable chytrids appeared more than 500 million years ago during the late pre-Cambrian period.
  • Like protists, chytrids usually live in aquatic environments, but some species live on land.
  • Some chytrids are saprobes while others are parasites that may be harmful to amphibians and other animals.
  • Chytrids reproduce both sexually and asexually, which leads to the production of zoospores.
  • Chytrids have chitin in their cell walls; one unique group also has cellulose along with chitin.
  • Chytrids are mostly unicellular, but multicellular organisms do exist.

Key Terms

  • chytridiomycete: an organism of the phylum Chytridiomycota
  • zoospore: an asexual spore of some algae and fungi
  • flagellum: a flagellum is a lash-like appendage that protrudes from the cell body of certain prokaryotic and eukaryotic cells
  • coenocytic: a multinucleate cell that can result from multiple nuclear divisions without their accompanying cytokinesis

Chytridiomycota: The Chytrids

The kingdom Fungi contains five major phyla, which were established according to their mode of sexual reproduction or use of molecular data. The Phylum Chytridiomycota (chytrids) is one of the five true phyla of fungi. There is only one class in the Phylum Chytridiomycota, the Chytridiomycetes. The chytrids are the simplest and most primitive Eumycota, or true fungi. The evolutionary record shows that the first, recognizable chytrids appeared during the late pre-Cambrian period, more than 500 million years ago. Like all fungi, chytrids have chitin in their cell walls, but one group of chytrids has both cellulose and chitin in the cell wall. Most chytrids are unicellular; a few form multicellular organisms and hyphae, which have no septa between cells (coenocytic). They reproduce both sexually and asexually; the asexual spores are called diploid zoospores. Their gametes are the only fungal cells known to have a flagellum.

The ecological habitat and cell structure of chytrids have much in common with protists. Chytrids usually live in aquatic environments, although some species live on land. Some species thrive as parasites on plants, insects, or amphibians, while others are saprobes. Some chytrids cause diseases in many species of amphibians, resulting in species decline and extinction. An example of a harmful parasitic chytrid is Batrachochytrium dendrobatidis, which is known to cause skin disease. Another chytrid species, Allomyces, is well characterized as an experimental organism. Its reproductive cycle includes both asexual and sexual phases. Allomyces produces diploid or haploid flagellated zoospores in a sporangium.

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Parasitic chytrids: The chytrid Batrachochytrium dendrobatidis is seen in these light micrographs as transparent spheres growing on (a) a freshwater arthropod and (b) algae. This chytrid causes skin diseases in many species of amphibians, resulting in species decline and extinction.

Zygomycota: The Conjugated Fungi

Zygomycota, a small group in the fungi kingdom, can reproduce asexually or sexually, in a process called conjugation.

Learning Objectives

Describe the ecology and reproduction of Zygomycetes

Key Takeaways

Key Points

  • Most zygomycota are saprobes, while a few species are parasites.
  • Zygomycota usually reproduce asexually by producing sporangiospores.
  • Zygomycota reproduce sexually when environmental conditions become unfavorable.
  • To reproduce sexually, two opposing mating strains must fuse or conjugate, thereby, sharing genetic content and creating zygospores.
  • The resulting diploid zygospores remain dormant and protected by thick coats until environmental conditions have improved.
  • When conditions become favorable, zygospores undergo meiosis to produce haploid spores, which will eventually grow into a new organism.

Key Terms

  • zygomycete: an organism of the phylum Zygomycota
  • karyogamy: the fusion of two nuclei within a cell
  • zygospore: a spore formed by the union of several zoospores
  • conjugation: the temporary fusion of organisms, especially as part of sexual reproduction

Zygomycota: The Conjugated Fungi

The zygomycetes are a relatively small group in the fungi kingdom and belong to the Phylum Zygomycota. They include the familiar bread mold, Rhizopus stolonifer, which rapidly propagates on the surfaces of breads, fruits, and vegetables. They are mostly terrestrial in habitat, living in soil or on plants and animals. Most species are saprobes meaning they live off decaying organic material. Some are parasites of plants, insects, and small animals, while others form symbiotic relationships with plants. Zygomycetes play a considerable commercial role. The metabolic products of other species of Rhizopus are intermediates in the synthesis of semi-synthetic steroid hormones.

Zygomycetes have a thallus of coenocytic hyphae in which the nuclei are haploid when the organism is in the vegetative stage. The fungi usually reproduce asexually by producing sporangiospores. The black tips of bread mold, Rhizopus stolonifer, are the swollen sporangia packed with black spores. When spores land on a suitable substrate, they germinate and produce a new mycelium.

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Sporangia of bread mold: Sporangia grow at the end of stalks, which appear as (a) white fuzz seen on this bread mold, Rhizopus stolonifer. The (b) tips of bread mold are the spore-containing sporangia.

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Zygomycete life cycle: Zygomycetes have asexual and sexual life cycles. In the sexual life cycle, plus and minus mating types conjugate to form a zygosporangium.

Sexual reproduction starts when conditions become unfavorable. Two opposing mating strains (type + and type –) must be in close proximity for gametangia (singular: gametangium) from the hyphae to be produced and fuse, leading to karyogamy. The developing diploid zygospores have thick coats that protect them from desiccation and other hazards. They may remain dormant until environmental conditions become favorable. When the zygospore germinates, it undergoes meiosis and produces haploid spores, which will, in turn, grow into a new organism. This form of sexual reproduction in fungi is called conjugation (although it differs markedly from conjugation in bacteria and protists), giving rise to the name “conjugated fungi”.

Glomeromycota

Glomeromycetes are an important group of fungi that live in close symbiotic association with the roots of trees and plants.

Learning Objectives

Describe the ecology and reproduction of Glomeromycetes

Key Takeaways

Key Points

  • Most glomeromycetes form arbuscular mycorrhizae, a type of symbiotic relationship between a fungus and plant roots; the plants supply a source of energy to the fungus while the fungus supplies essential minerals to the plant.
  • Glomeromycota that have arbuscular mycorrhizal are mostly terrestrial, but can also be found in wetlands.
  • The glomeromycetes reproduce asexually by producing glomerospores and cannot survive without the presence of plant roots.
  • DNA analysis shows that all glomeromycetes probably descended from a common ancestor 462 and 353 million years ago.
  • The classification of fungi as Glomeromycota has been redefined with adoption of molecular techniques.

Key Terms

  • biotrophic: describing a parasite that needs its host to stay alive
  • arbuscular mycorrhizae: a type of symbiotic relationship between a fungus and the roots of a plant where the plants supply a source of energy to the fungus while the fungus supplies essential minerals to the plant
  • glomeromycete: an organism of the phylum Glomeromycota

Glomeromycota

In the kingdom Fungi, the Glomeromycota is a newly-established phylum comprised of about 230 species that live in close association with the roots of trees and plants. Fossil records indicate that trees and their root symbionts share a long evolutionary history. It appears that most members of this family form arbuscular mycorrhizae: the hyphae interact with the root cells forming a mutually-beneficial association where the plants supply the carbon source and energy in the form of carbohydrates to the fungus while the fungus supplies essential minerals from the soil to the plant. This association is termed biotrophic. The Glomeromycota species that have arbuscular mycorrhizal are terrestrial and widely distributed in soils worldwide where they form symbioses with the roots of the majority of plant species. They can also be found in wetlands, including salt-marshes, and are associated with epiphytic plants.

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Glyomeromycetes and tree roots: This image illustrates the bitrophic relationship between a glomeromycota (Gigaspora margarita) and the roots of a plant (Lotus corniculatus).

The glomeromycetes do not reproduce sexually and cannot survive without the presence of plant roots. They have coenocytic hyphae and reproduce asexually, producing glomerospores. The biochemical and genetic characterization of the Glomeromycota has been hindered by their biotrophic nature, which impedes laboratory culturing. This obstacle was eventually surpassed with the use of root cultures. With the advent of molecular techniques, such as gene sequencing, the phylogenetic classification of Glomeromycota has become clearer. The first mycorrhizal gene to be sequenced was the small-subunit ribosomal RNA (SSU rRNA). This gene is highly conserved and commonly used in phylogenetic studies so it was isolated from spores of each taxonomic group. Using a molecular clock approach based on the substitution rates of SSU sequences, scientists were able to estimate the time of divergence of the fungi. This analysis shows that all glomeromycetes probably descended from a common ancestor 462 and 353 million years ago, making them a monophyletic lineage. A long-held theory is that Glomeromycota were instrumental in the colonization of land by plants.

Ascomycota: The Sac Fungi

Most fungi belong to the Phylum Ascomycota, which uniquely forms of an ascus, a sac-like structure that contains haploid ascospores.

Learning Objectives

Describe the ecology and the reproduction of Ascomycetes

Key Takeaways

Key Points

  • Ascomycota fungi are the yeasts used in baking, brewing, and wine fermentation, plus delicacies such as truffles and morels.
  • Ascomycetes are filamentous and produce hyphae divided by perforated septa.
  • Ascomycetes frequently reproduce asexually which leads to the production of conidiophores that release haploid conidiospores.
  • Two types of mating strains, a “male” strain which produces an antheridium and a “female” strain which develops an ascogonium, are required for sexual reproduction.
  • The antheridium and the ascogonium combine in plasmogamy at the time of fertilization, followed by nuclei fusion in the asci.
  • In the ascocarp, a fruiting body, thousands of asci undergo meiosis to generate haploid ascospores ready to be released to the world.

Key Terms

  • plasmogamy: stage of sexual reproduction joining the cytoplasm of two parent mycelia without the fusion of nuclei
  • Ascomycota: a taxonomic division within the kingdom Fungi; those fungi that produce spores in a microscopic sporangium called an ascus
  • ascus: a sac-shaped cell present in ascomycete fungi; it is a reproductive cell in which meiosis and an additional cell division produce eight spores
  • ascospore: a sexually-produced spore from the ascus of an Ascomycetes fungus
  • conidia: asexual, non-motile spores of a fungus, named after the Greek word for dust, conia; also known as conidiospores and mitospores
  • antheridia: a haploid structure or organ producing and containing male gametes (called antherozoids or sperm) present in lower plants like mosses and ferns, primitive vascular psilotophytes, and fungi
  • ascogonium: a haploid structure or organ producing and containing female gametes in certain Ascomycota fungi
  • ascocarp: the sporocarp of an ascomycete, typically bowl-shaped
  • ascomycete: any fungus of the phylum Ascomycota, characterized by the production of a sac, or ascus, which contains non-motile spores

Ascomycota: The Sac Fungi

The majority of known fungi belong to the Phylum Ascomycota, which is characterized by the formation of an ascus (plural, asci), a sac-like structure that contains haploid ascospores. Many ascomycetes are of commercial importance. Some play a beneficial role, such as the yeasts used in baking, brewing, and wine fermentation, plus truffles and morels, which are held as gourmet delicacies. Aspergillus oryzae is used in the fermentation of rice to produce sake. Other ascomycetes parasitize plants and animals, including humans. For example, fungal pneumonia poses a significant threat to AIDS patients who have a compromised immune system. Ascomycetes not only infest and destroy crops directly, they also produce poisonous secondary metabolites that make crops unfit for consumption. Filamentous ascomycetes produce hyphae divided by perforated septa, allowing streaming of cytoplasm from one cell to the other. Conidia and asci, which are used respectively for asexual and sexual reproductions, are usually separated from the vegetative hyphae by blocked (non-perforated) septa.

Asexual reproduction is frequent and involves the production of conidiophores that release haploid conidiospores. Sexual reproduction starts with the development of special hyphae from either one of two types of mating strains. The “male” strain produces an antheridium (plural: antheridia) and the “female” strain develops an ascogonium (plural: ascogonia). At fertilization, the antheridium and the ascogonium combine in plasmogamy without nuclear fusion. Special ascogenous hyphae arise, in which pairs of nuclei migrate: one from the “male” strain and one from the “female” strain. In each ascus, two or more haploid ascospores fuse their nuclei in karyogamy. During sexual reproduction, thousands of asci fill a fruiting body called the ascocarp. The diploid nucleus gives rise to haploid nuclei by meiosis. The ascospores are then released, germinate, and form hyphae that are disseminated in the environment and start new mycelia.

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Release of ascospores: The bright field light micrograph shows ascospores being released from asci in the fungus Talaromyces flavus var. flavus.

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Lifecycle of an ascomycete: The lifecycle of an ascomycete is characterized by the production of asci during the sexual phase. The haploid phase is the predominant phase of the life cycle.

Basidiomycota: The Club Fungi

The basidiomycota are mushroom-producing fungi with developing, club-shaped fruiting bodies called basidia on the gills under its cap.

Learning Objectives

Describe the ecology and reproduction of the Basidiomycota

Key Takeaways

Key Points

  • The majority of edible fungi belong to the Phylum Basidiomycota.
  • The basidiomycota includes shelf fungus, toadstools, and smuts and rusts.
  • Unlike most fungi, basidiomycota reproduce sexually as opposed to asexually.
  • Two different mating strains are required for the fusion of genetic material in the basidium which is followed by meiosis producing haploid basidiospores.
  • Mycelia of different mating strains combine to produce a secondary mycelium that contains haploid basidiospores in what is called the dikaryotic stage, where the fungi remains until a basidiocarp (mushroom) is generated with the developing basidia on the gills under its cap.

Key Terms

  • basidiocarp: a fruiting body that protrudes from the ground, known as a mushroom, which has a developing basidia on the gills under its cap
  • basidiomycete: a fungus of the phylum Basidiomycota, which produces sexual spores on a basidium
  • Basidiomycota: a taxonomic division within the kingdom Fungi: 30,000 species of fungi that produce spores from a basidium
  • basidium: a small structure, shaped like a club, found in the Basidiomycota phylum of fungi, that bears four spores at the tips of small projections
  • basidiospore: a sexually-reproductive spore produced by fungi of the phylum Basidiomycota

Basidiomycota: The Club Fungi

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Fruiting bodies of a basidiomycete: The fruiting bodies of a basidiomycete form a ring in a meadow, commonly called “fairy ring.” The best-known fairy ring fungus has the scientific name Marasmius oreades. The body of this fungus, its mycelium, is underground and grows outward in a circle. As it grows, the mycelium depletes the soil of nitrogen, causing the mycelia to grow away from the center, leading to the “fairy ring” of fruiting bodies where there is adequate soil nitrogen.

The fungi in the Phylum Basidiomycota are easily recognizable under a light microscope by their club-shaped fruiting bodies called basidia (singular, basidium), which are the swollen terminal cell of a hypha. The basidia, which are the reproductive organs of these fungi, are often contained within the familiar mushroom, commonly seen in fields after rain, on the supermarket shelves, and growing on your lawn. These mushroom-producing basidiomyces are sometimes referred to as “gill fungi” because of the presence of gill-like structures on the underside of the cap. The “gills” are actually compacted hyphae on which the basidia are borne. This group also includes shelf fungus, which cling to the bark of trees like small shelves. In addition, the basidiomycota includes smuts and rusts, which are important plant pathogens, and toadstools. Most edible fungi belong to the Phylum Basidiomycota; however, some basidiomycetes produce deadly toxins. For example, Cryptococcus neoformans causes severe respiratory illness.

The lifecycle of basidiomycetes includes alternation of generations. Spores are generally produced through sexual reproduction, rather than asexual reproduction. The club-shaped basidium carries spores called basidiospores. In the basidium, nuclei of two different mating strains fuse (karyogamy), giving rise to a diploid zygote that then undergoes meiosis. The haploid nuclei migrate into basidiospores, which germinate and generate monokaryotic hyphae. The mycelium that results is called a primary mycelium. Mycelia of different mating strains can combine and produce a secondary mycelium that contains haploid nuclei of two different mating strains. This is the dikaryotic stage of the basidiomyces lifecyle and it is the dominant stage. Eventually, the secondary mycelium generates a basidiocarp, which is a fruiting body that protrudes from the ground; this is what we think of as a mushroom. The basidiocarp bears the developing basidia on the gills under its cap.

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Lifecycle of a basidiomycete: The lifecycle of a basidiomycete alternates generation with a prolonged stage in which two nuclei (dikaryon) are present in the hyphae.