Fungi Flashcards
Characteristics shared by most fungal taxa
- Hyphae and mycelium (thread-like filaments and the structure they form)
- Chitinous cell wall (provides sturctural support and protection)
- Absorptive nutrition (absorbs nutrients directly from the environment)
- Spore production (may be generated through asexual or sexual reproduction)
Fungal absorptive nutrition
Fungi are chemoheterotrophic eukaryotes that obtain nutrients by absorbing dissolved organic compounds directly from the external environment (absorptive heterotrophs).
- i.e. utilize organic compounds for both carbon and energy
Fungi secrete enzymes (exoenzymes) to break down a large variety of complex molecules into smaller organic compounds (external digestion)
- Once broken down into simpler organic molecules, such as sugars and amino acids, these nutrients are absorbed by the fungal body.
- The versatility of these enzymes contributes to the ecological success of fungi.
- Fungi can digest cellulose and lignin from plant tissues, and chitin and keratin from animal tissues.
Hyphae
Fungi
Cylindrical, branched, multicellular filaments that absorb nutrients
- Most fungi have these
Mycelium
Fungi
When fungi encounter a food source, their hyphae form a branching filamentous network of hyphae (mycelium), adapted for nutrient absorption.
True or False
All fungi produce hyphae
False
Not all fungi produce hyphae.
- Early-diverging fungal lineages lack hyphae.
- Single-celled fungi (yeasts) live in moist, nutrient-rich environments.
- Yeasts descended from hyphae-forming ancestors; yeasts evolved independently several times (convergent evolution).
Multicellular hyphal morphology
Fungi
The multicellular hyphal morphology enhances fungi’s ability to absorb nutrients.
- Hyphae are thin, tubular cells (singlecell diameter, 2-10 µm Ø) filled with cytoplasm and organelles.
- Hyphae grow at their tips, elongating in length, not width.
- Some fungi grow rapidly, adding up to 1 km of hyphae per day in some species.
- The filamentous structure of a mycelium maximizes its surface areato volume ratio, facilitating more efficient enzyme secretion and nutrient absorption.
- Thin hyphae are protected by strong, flexible cell walls composed of chitin, a glucosamine polymer.
- Chitin is a nitrogenous polysaccharide, a structural component that has evolved independently in some invertebrates.
Coenocytic fungi
The earliest fungal lineages exhibit a
coenocytic (aseptate) structure, lacking septa or dividing walls within their hyphae.
- The hyphae of coenocytic fungi form a continuous compartment containing numerous nuclei but no dividing cell walls.
- Continuous cytoplasmic mass with thousands of nuclei dispersed throughout.
Septate fungi
In later evolving groups, nuclear divisions are accompanied by the formation of septa (cross-walls) that divide the cytoplasm into separate cells.
- Most fungal species are septate.
- Pores allow cell-to-cell movement of water/solutes, cytoplasm, and small organelles.
- Septate fungi have single nuclei per cell.
True or False
Fungal mycelia are diploid (2n)
False
Fungal mycelia are haploid (1n) that produce haploid (1n) spores that grow to produce hyphae.
- Spores are the dispersal stage, as they are resistant to desiccation and can be carried by wind, water, or animals
Generalized life cycle of fungi
The fungal life cycle is similar to haploid-dominant life cycles observed in many eukaryotic organisms.
Except fungi have a unique separation of plasmogamy (cell fusion) and karyogamy (nuclear fusion).
- Between plasmogamy and karyogamy is a heterokaryotic stage where genetically distinct haploid nuclei coexist within a single cell.
Monokaryotic vs homokaryotic hypae
Fungi
Septate hyphae are monokaryotic
(single, identical nuclei).
Coenocytic hyphae are homokaryotic
(multiple, identical nuclei).
Plasmogamy
The merging of cytoplasm from two parental mycelia, initiates the process.
- In most fungi, the haploid nuclei of each parent coexist in the resulting mycelium, termed a heterokaryon, where multiple genetically distinct haploid nuclei cohabit.
Dikaryotic mycelium
Fungi
In septate fungi, haploid nuclei pair off during mitosis to form a dikaryotic mycelium, containing two nuclei per cell.
How long is the interval between plasmogamy
and karyogamy
Fungi Reproduction
The interval between plasmogamy and karyogamy can vary widely, ranging from hours to centuries.
- During karyogamy, haploid nuclei fuse to form a diploid cell (zygote).
- The formation of dikaryotic mycelia enables multiple karyogamy events, yielding numerous diploid zygotes.
How long doe the diploid zygote live?
Fungal Reproduction
The diploid zygote is short-lived, immediately undergoing meiosis to yield haploid spores.
- This separation of plasmogamy and karyogamy ensures mating and spore production occur at optimal times.
- Fungi lack diploid multicellular stages.
- i.e. fungi do not exhibit alternation of generations.
- Karyogamy and meiosis generate genetic diversity within fungal populations.
- Fungi do not produce gametes; the zygote arises from the fusion of regular haploid nuclei
Methods of Asexual Reproduction
Fungi
Fragmentation: fragments of hyphae grow into new mycelia.
Budding: unicellular yeasts reproduce asexually by budding.
- Asymmetric mitosis is where a small bud cell emerges from the parent cell.
- Some fungi can grow both as budding yeasts and filamentous mycelia.
Asexual spores: moulds (or molds) form visible mycelia and generate haploid asexual spores via mitosis during dispersal.
Many moulds and yeasts have no
known sexual stage.
What clade are fungi and animals, (alongside ameobozians) from?
the opisthokonts clade
What is the closest relative to fungi?
Unicellular nucleariids
Nucleariids are characterized as nonflagellated, spherical or flat amoebae, with radiating filopodia.
- Nucleariids are aquatic phagotrophs that feed on bacteria and unicellular algae.
What are the 7 main clades of the fungi?
- Cryptomycetes (Unicellular)
- Microsporidians (Unicellular)
- Chytrids (most unicellular)
- Zoopagomycetes (Coenocytic)
- Mucoromycetes (Coenocytic)
- Ascomycetes (Septate)
- Basidiomycetes (Septate)
Cryptomycetes
Cryptomycetes (phylum Cryptomycota) are found in marine, freshwater, and soil environments.
- ~30 identified species, but environmental DNA sequencing suggests cryptomycetes are likely diverse.
- Cryptomycetes are unicellular, have flagellated spores, and lack chitinous cell walls.
- Chitin deposition has only been observed in the spore walls of some cryptomycete species.
- Many identified cryptomycetes are parasites of protists and other fungi.
Microsporidians
Microsporidians (phylum Microsporidia) are spore-forming, unicellular parasites.
- Microsporidia parasitize animal hosts; most infect insects.
- e.g. Nosema ceranae is a parasite of honeybees and may contribute to Colony Collapse Disorder of honeybee colonies
- Microsporidians lack flagellated spores; instead, they produce unique spores that infect host cells via an infection organelle, the polar tube.
- Microsporidia can form resistant spores with chitin-rich cell walls, capable of surviving outside their host for several years.
Chytrids
Fungi
Chytrids (phylum Chytridiomycota) are low diversity (~1k species), ubiquitous inhabitants of aquatic ecosystems or moist environments.
- Some species are found in soil, estuaries, or within the digestive tracts of animals.
- Most chytrids are unicellular, while others form colonies with hyphae.
- Nearly all chytrids have flagellated asexual spores, known as zoospores.
- Like other fungi, chytrids have chitin cell walls and utilize external digestion
Chytrids have diverse ecological roles:
- Many are free-living decomposers.
- Certain species are cellulose-digesting mutualists within the digestive systems of ruminants like sheep and cattle.
- Some chytrids are parasitic, targeting hosts such as plants, animals, or other fungi
Zoopagomycetes
Zoopagomycetes (phylum Zoopagomycota) are a low diversity (~900 species) group with:
- Chitinous cell walls.
- Coenocytic (nonseptate) filamentous hyphae that form simple, branched or unbranched bodies.
- Reproduce asexually by producing non-flagellated spores.
- Basal fungal lineages have flagellated spores that disperse through water.
- Zoopagomycetes, along with mucoromycetes, ascomycetes, and basidiomycetes, are terrestrial fungi that have non-flagellated spores, which are dispersed by wind.
- During karyogamy (sexual reproduction), zoopagomycetes form diploid zygotes enclosed in a protective covering, the zygosporangium, which can survive unfavourable conditions.
- Zygosporangia are the site of karyogamy and then meiosis.
- Zoopagomycetes are typically parasites of other fungi, soil microbes, or animals.
- Some are commensal (neutral) symbionts of animals.
Mucoromycetes
Mucoromycetes (phylum Mucoromycota; ~750 species).
- Mucoromycetes include fast-growing decomposers (moulds) of plant tissues, parasites or pathogens of plants, a mutualists (including some mycorrhizae).
- The life cycle of black bread mould (Rhizopus stolonifer) is typical of a decomposer mucoromycete.
- Asexual sporangia produce haploid spores that are dispersed through the air.
- Some mucoromycetes, such as Pilobolus, “aim” and shoot their sporangia toward bright light sources.
How do mucoromycetes reproduce?
Mucoromycetes reproduce sexually
through the formation of zygosporangia, which form after the fusion of specialized hyphae
- Zygosporangia are resistant to harsh conditions such as freezing and drying, enabling them to survive unfavourable environments.
- Karyogamy (the fusion of nuclei) and meiosis occur within zygosporangia.
Glomeromycetes
Mucoromycota includes glomeromycetes, a clade of fungi that form mycorrhizae, mutually beneficial symbiotic relationships between fungi and plant roots.
- Glomeromycetes play a significant ecological role by establishing arbuscular mycorrhizae (endomycorrhizae) with ~70–80% of plant species.
- Mycorrhizae facilitate the exchange of minerals and nutrients, enhancing the overall health and growth of plants.
- Specialized hyphae that push into plant root cells branch into tiny treelike arbuscules.
- Glomeromycetes reproduce via asexual soil spores; no evidence of sexual reproduction.
Dikarya fungi
Most fungi are these (~98% of fungal diversity)
- Includes: edible mushrooms, yeast strains utilized in bread, beer, and cheese production, significant wooddecaying fungi, as well as various pathogens affecting both crops and humans.
Characterized by:
- formation of septate hyphae
- the development of dikaryotic hyphae after plasmogamy
Two Clades:
- Ascomycetes and Basidiomycetes
Dikaryotic stage of Ascomycetes vs Basidiomycetes
Ascomycetes typically exhibit a brief dikaryotic stage, with plasmogamy and karyogamy occurring in relatively rapid succession.
Basidiomycetes often maintain a
prolonged dikaryotic phase, where dikaryotic hyphae frequently dominate the mycelium structure.
Heterokaryotic Stage of Dikarya
During the heterokaryotic stage, these fungi produce dikaryotic cells (n + n) containing two haploid nuclei, each contributed by a different parental strain.
Ascomycetes
Ascomycetes (phylum Ascomycota, “sac-fungi”) are the most diverse fungi (~90k species) and are found in a variety of marine, freshwater, and terrestrial habitats]
- Can be very small (yeast) to very large (sup fungi, truffles)
- Includes wood-decyaing fungi, symbitic organsms, pathogens, fungal lichens, yeasts
Produce sexual spores within sac-like structures called asci, which are contained in specialized fruiting bodies known as ascocarps (carp = fruit).
Ascomycetes reproduce asexually via the production of vast quantities of asexual spores termed conidia.
- Conidia are not formed within sporangia; instead, conidia are produced at the tips of specialized hyphae called conidiophores.
Basidiomycetes
Include: a range of familiar fungi, including toadstools, puffballs, shelf fungi, ectomycorrhizae, and the plant pathogens smuts and rusts.
- They play crucial roles as decomposers, particularly in breaking down lignin-rich wood.
Typically, the life cycle of basidiomycetes involves a prolonged phase of dikaryotic mycelium, where two different nuclei coexist within the same hyphal compartment.
A defining feature of Basidiomycetes is their club-shaped structures called basidia, transient diploid stages in their life cycle (“club fungi”).
In response to environmental cues, Basidiomycete mycelia undergo sexual reproduction by generating elaborate, multicellular fruiting bodies known as basidiocarps.
- Basidiocarps include classic mushrooms and toadstools.
Reproduction of Basidiomycetes
In response to environmental cues, Basidiomycete mycelia undergo sexual reproduction by generating elaborate, multicellular fruiting bodies known as basidiocarps.
- Basidiocarps include classic
mushrooms and toadstools.
Within a short-lived basidiocarp (cap), thin vertical sheets of mycelia called gills are lined with millions of basidia.
- Young basidia start as dikaryotic cells (n+n), which undergo karyogamy to become diploid (2n) nuclei.
- At maturity, diploid basidia undergo meiosis to produce four basidiospores, each containing a single haploid nucleus.
- A single mushroom cap can release up to a billion basidiospores.
Ecological roles of fungi
− Decomposers
− Mutualists
− Pathogens
Where does more dead organic matter come from?
Most dead organic matter in soils is from plant tissues: cellulose and lignin.
- While some saprotrophic bacteria and protists can decompose these materials, fungi are the primary decomposers of most plant matter.
Saprotrophs
Saprotrophs (sapro = detritus) are organisms that obtain their nutrition by decomposing organic matter from dead organisms or waste material.
- Saprotrophic fungi play a crucial role in ecosystems by breaking down complex organic molecules into simpler, inorganic forms, which are then recycled back into the environment
What are the types of fungi mutual relationships
- Endophytes (fungus-plant mutualisms)
- Mycorrhizae (The roots one)
- Lichens
- Animal-fungus mutualisms
Endophytes (fungus-plant mutualisms)
Plants host many harmless symbiotic fungal endophytes, which reside in the intercellular spaces of leaves or other plant tissues.
- This relationship is often mutualistic, as endophytes enhance the health and survival of host plants by producing toxins that deter herbivores and pathogens.
- Most fungal endophytes are Ascomycetes
Mycorrhizae
Mycorrhizae (fungus-plant mutualisms) are mutually beneficial symbiotic relationships between fungi and the roots of many plant species
Mycorrhizal fungi supply plant roots with water and essential nutrients, such as phosphorus and nitrogen, extracted from the soil.
- In return, the fungi receive carbohydrates from their plant hosts.
- Mycorrhizae play a crucial role in natural ecosystems and agriculture.
- Plants grown with mycorrhizae typically exhibit better growth compared to those without, especially in nutrient-poor soils.
Around 80–90% of plant species form some type of mycorrhizal associations, primarily of two types:
- Ectomycorrhiza (limited to ~6,000 mostly woody species, e.g. conifers).
- Endomycorrhiza (all other mycorrhizal associations).
Evidence from the fossil record indicates that mycorrhizal associations likely existed with the earliest vascular plants, suggesting a co-evolutionary history between plants and fungi on land.
Ectomycorrhizal fungi
Ectomycorrhizal fungi envelop the roots of plants without penetrating the root cells.
- The mycelium forms a dense sheath of hyphae (mantle) over the root, while hyphae extend into the surrounding soil.
- Hyphae also penetrate the spaces between the cell walls of the root.
- Ectomycorrhizae are found across multiple fungal groups (mucoro-, asco- and basidiomycetes) and are believed to have evolved independently on multiple occasions.
- Ectomycorrhizal associations are limited to about 5–10% of plant species, primarily in certain tree families, including pines, oaks, and birches.
- Play a crucial role in nutrient cycling in temperate and boreal forest ecosystems
Endomycorrhizal fungi
The most common endomycorrhizae are arbuscular mycorrhiza.
- Hyphae of arbuscular mycorrhizae penetrate the cell walls of root cells, forming intricate invaginations of the root cell membrane.
- Highly branched hyphal structures (arbuscules) greatly expand the surface area available for nutrient exchange.
Arbuscular mycorrhizae are a low diversity (~300 species) monophyletic group (clade Glomeromycetes) within Mucoromycota.
- Arbuscular mycorrhizal associations are found in ~70–80% of plant species and are most common among herbaceous plants, grasses, and crop species.
Lichens
Lichens are mutually beneficial (symbiotic) associations between photosynthetic microorganisms and fungi.
- A lichen appears, functions, and reproduces as a single organism.
- The photosynthetic component can come from ~100 species of green algae (Chlorophyta) or cyanobacteria (prokaryote).
- ~20,000 lichen species of lichen that are named for the fungal partner.
- Most (>90%) of lichen fungi are Ascomycetes.
Fungi provide an environment for photosymbiont growth and reproduction (protection, free from competition, fungal mycelia retains water and minerals).
The algae or cyanobacteria provide organic carbon compounds and, in the case of nitrogen-fixing cyanobacteria, organic nitrogen.
Lichens are pioneers of mineral surfaces.
- Some lichens grow on rocks without soil, breaking down the rocks into soil for plants.
- Lichen secretes weak acids that dissolve rock.
- The swelling of hyphae in cracks promotes the fracturing of rock.
- Lichens trap windblown particles.
Lichens may have helped colonization of land by plants 550–600 mya by promoting soil formation.
Lichen reproduction
The fungi of lichens can reproduce sexually or asexually.
- Asexual reproduction is through fragmentation or the formation of soredia, small clusters of hyphae containing embedded algae
Animal-fungus mutualisms
Some fungi share their digestive services with animals, such as cellulose-digesting chytrid mutualists inhabiting the guts of sheep and cattle.
Ex. Many species of leafcutter ants utilize the digestive power of fungi by cultivating them in “farms” to feed their larvae, while adult ants feed on leaf sap
Types of parasitic fungi
- Plant-parasitic fungi
- Invertebate-parasitic fungi
- Vertebrate-parasitic fungi
Plant-parasitic fungi
Fungi are the 2nd most important pest of plants after insects
Some fungal pathogens targeting food crops can produce toxins hazardous to human health.
- Ex. ergot
- Causes ergotism in mammals
The globalization of trade has facilitated the spread of plant-parasitic fungi
- Ex. Dutch elm disease, Mountian pine beetle (blue-stain fungus)
Ergot
A plant parasitc fungi that can lead to ergotism in humans and other mammals when contaminated grains are consumed
- Ergot replaces seed with its mycelium mass, which produces alkaloid toxins, including lysergic acid derivatives, which are similar to psychedelic drugs (LSD), that affect circulation and neurotransmission.
- Ergotism is characterized by gangrene, nervous spasms, burning sensations, hallucinations, and temporary insanity.
- More than 40,000 people died from ergotism during the Middle Ages (“St. Anthony’s Fire”).
Dutch Elm Disease
Caused by plant parasitic fungi
- Dutch elm disease was accidentally introduced into North America in the 1930s in imported elm wood.
- Ascomycete fungi (two Ophiostoma species) are spread by elm bark beetles.
- The tree blocks its xylem to prevent fungal infection, causing the tree to wilt, eventually leading to death.
Invertebrate-parasitic fungi
Fungi can parasitize arthropods, such as insects and arachnids.
- Arthropods have chitin-reinforced exoskeletons.
- Fungi have chitin cell walls and can digest chitin to reabsorb old mycelia.
Invertebrate-parasitic fungi often act as parasitoids that slowly kill their hosts.
- Examples: Entomophthora (Zoopagomycota), which produces spores inside hosts; Ophiocordyceps unilateralis (Ascomycota), known as the zombie-ant fungus, which produces fruiting structures that burst through the host’s body
Vertebrate-parasitic fungi
Fungi can infect vertebrates causing mycosis, fungal infections of animals or humans.
- A cutaneous mycosis occurs on the outer layers of the skin, e.g. fungi digest keratin (structural proteins of the skin).
- e.g. human skin disease called ringworm (ring-shaped growth of hyphae).
- e.g. the global decline of amphibians has been linked to chytrid infections.
- Systemic mycosis occurs when a fungal pathogen spreads widely inside the host’s body to many organs.
- Immuno-compromised people are susceptible to systemic fungal infections.
Candida albicans yeast infections
Candida albicans is commonly found in the gut microbiome and on moist surfaces where it is usually harmless.
Candida albicans infections are opportunistic:
- Cause oral or genital infections under humid conditions, e.g. diaper rash.
- Colonizes mucous membranes if the normal microbiota is absent, or if chemical conditions change, e.g. following antibiotic treatment.
Histoplasmosis
Histoplasmosis is a systemic mycosis commonly found in North America.
- Histoplasma capsulatum (ascomycete) fungus grows in soil contaminated with bird or bat droppings.
- Disturbing the substrate leads to the inhalation of asexual spores.
- Histoplasmosis primarily affects the lungs where the fungus morphs into yeast at body temperature.
- Can spread from the lungs to other organs, becoming systemic and fatal if left untreated.
- This disease poses a significant risk to immunocompromised individuals.
Fungi as human food
Humans consume a variety of fungi:
- The common cultivated mushroom (Agaricus bisporus), a basidiomycete.
- Originally domesticated in France during the 17th century, its annual worldwide production is ~1 million tons.
- Varieties such as white, crimini, and portobello mushrooms are widely cultivated and consumed.
- Other mass-produced mushrooms species, include oyster and shiitake.
- Certain high-value wild fungi, such as truffles and morels, are prized for their unique flavours and are often used in gourmet dishes.
- Some fungi possess psychoactive properties, as seen in “magic” mushrooms, which have been used for ceremonial and recreational purposes in various cultures.
Fungi in food production:
- Penicillium camemberti, an ascomycete, is utilized in cheese-making processes, contributing to the distinctive flavours and textures of cheeses like Camembert.
- In fermentation processes, Saccharomyces cerevisiae (ascomycete), commonly known as brewer’s or baker’s yeast, is integral to the production of alcoholic beverages and leavened bread.
Systemic vs cutaneous mycosis
A cutaneous mycosis occurs on the outer layers of the skin, e.g. fungi digest keratin (structural proteins of the skin).
- e.g. human skin disease called ringworm (ring-shaped growth of hyphae).
- e.g. the global decline of amphibians has been linked to chytrid infections.
Systemic mycosis occurs when a fungal pathogen spreads widely inside the host’s body to many organs.
* Immuno-compromised people are susceptible to systemic fungal infections
