They are essential for the well-being of most terrestrial ecosystems because they break down organic material and recycle vital nutrients.
Fungi are heterotrophs and absorb nutrients from outside of their body.
Fungi use enzymes to break down a large variety of complex molecules into smaller organic compounds.
The versatility of these enzymes contributes to fungi’s ecological success.
Fungal Body Structures and Lifestyles
The most common body structures are multicellular filaments and single cells (yeasts).
Some species grow as either filaments or yeasts; others grow as both.
Fungi exhibit diverse lifestyles:
Decomposers / saprophytes
Parasites (+ -)
Mutualists (+ +)
Fungal Morphology: Hyphae
The morphology of multicellular fungi enhances their ability to absorb nutrients.
Fungi consist of mycelia, networks of branched hyphae adapted for absorption.
Most fungi have cell walls made of chitin.
Septate and Coenocytic Fungi
Septate fungi: Some fungi have hyphae divided into cells by septa, with pores allowing cell-to-cell movement of organelles.
Coenocytic fungi lack septa.
Some unique fungi have specialized hyphae called haustoria that allow them to penetrate the tissues of their host.
Specialized Hyphae in Mycorrhizal Fungi
Mycorrhizae are mutually beneficial symbiotic relationships between fungi and plant roots (+ +).
Ectomycorrhizal fungi form sheaths of hyphae over a root and also grow into the extracellular spaces of the root cortex.
Arbuscular mycorrhizal fungi extend hyphae through the cell walls of root cells and into tubes formed by invagination of the root cell membrane.
Specialized Hyphae
Fungi have specialized hyphae.
Haustoria penetrate cell walls of plants (+ +) or (+ -).
Some hyphae are adapted for trapping and killing prey (e.g., nematodes).
Fungal Reproduction
Fungi produce spores through sexual or asexual life cycles.
Fungi propagate themselves by producing vast numbers of spores, either sexually or asexually.
Fungal nuclei are normally haploid.
Sexual reproduction requires the fusion of hyphae from different mating types.
Fungi use sexual signaling molecules called pheromones to communicate their mating type.
Fungal Life Cycle
Spores germinate into mycelia.
Asexual reproduction occurs through spore-producing structures.
Sexual reproduction involves plasmogamy (fusion of cytoplasm) and karyogamy (fusion of nuclei).
The heterokaryotic stage contains unfused nuclei from different parents.
Meiosis occurs in the zygote to produce haploid spores.
Plasmogamy and Karyogamy
Plasmogamy is the union of two parent hyphae, mycelia.
In most fungi, the haploid nuclei from each parent do not fuse right away; they coexist in the hyphae; this mycelium is called a heterokaryon.
In some fungi, the haploid nuclei pair off two to a cell; such a mycelium is dikaryotic.
During karyogamy, the haploid nuclei fuse, producing diploid cells: n+n=2n.
Timing of Karyogamy
Hours, days, or even centuries may pass before the occurrence of karyogamy, nuclear fusion.
The diploid phase is short-lived and undergoes meiosis, producing haploid spores.
Asexual Reproduction
In addition to sexual reproduction, many fungi can reproduce asexually.
Molds produce haploid spores by mitosis and form visible mycelia.
Other fungi that can reproduce asexually are yeasts, which inhabit moist environments.
Instead of producing spores, yeasts reproduce asexually by budding: simple cell division and pinching of “bud cells” from a parent cell.
Fungal Ancestry
The ancestor of fungi was an aquatic, single-celled, flagellated protist.
Fungi and animals are more closely related to each other than they are to plants or other eukaryotes.
DNA evidence suggests that fungi are most closely related to unicellular nucleariids, while animals are most closely related to unicellular choanoflagellates.
This suggests that fungi and animals evolved from a common flagellated unicellular ancestor, and multicellularity arose separately in the two groups.
The oldest undisputed fossils of fungi are only about 460 million years old.
Move to Land
Fungi were among the earliest colonizers of land and probably formed mutualistic relationships with early land plants (+ +).
Chytrids (phylum Chytridiomycota) are found in freshwater and terrestrial habitats.
They can be decomposers, parasites, or mutualists.
Chytrids are unique among fungi in having flagellated spores, called zoospores.
Fungal Diversity
Chytrids (1,000 species)
Zygomycetes (1,000 species)
Glomeromycetes (160 species)
Ascomycetes (65,000 species)
Basidiomycetes (30,000 species)
Zygomycetes
The zygomycetes (phylum Zygomycota) exhibit great diversity of life histories.
They include fast-growing molds, parasites, and commensal symbionts.
The zygomycetes are named for their sexually produced zygosporangia.
Zygosporangia, which are resistant to freezing and drying, can survive unfavorable conditions.
Rhizopus Life Cycle
Rhizopus (black bread mold) reproduces sexually and asexually.
Plasmogamy leads to the formation of a young zygosporangium (heterokaryotic).
Karyogamy and meiosis occur within the zygosporangium.
Sporangia release spores through asexual reproduction.
Ascomycetes
Ascomycetes (phylum Ascomycota) live in marine, freshwater, and terrestrial habitats.
The phylum is defined by the production of sexual spores in saclike asci, usually contained in fruiting bodies called ascocarps.
Ascomycetes are commonly called sac fungi.
Ascomycetes vary in size and complexity from unicellular yeasts to elaborate cup fungi and morels.
Ascomycete Characteristics
Ascomycetes include plant pathogens, decomposers, and symbionts.
Ascomycetes reproduce asexually by enormous numbers of asexual spores called conidia.
Conidia are not formed inside sporangia; they are produced asexually at the tips of specialized hyphae called conidiophores.
Neurospora is a model organism with a well-studied genome.
Neurospora Life Cycle
Asexual reproduction occurs through conidia.
Sexual reproduction involves plasmogamy, karyogamy, and meiosis within the ascus.
Ascospores are formed and dispersed from the ascocarp.
Basidiomycetes
Basidiomycetes (phylum Basidiomycota) include mushrooms, puffballs, and shelf fungi, mutualists, and plant parasites.
The phylum is defined by a clublike structure called a basidium, a transient diploid stage in the life cycle.
The basidiomycetes are also called club fungi.
Ecological Roles of Fungi
Fungi play key roles in nutrient cycling, ecological interactions, and human welfare.
Fungi interact with other organisms in many ways.
Fungi are efficient decomposers. They perform essential recycling of chemical elements between the living and nonliving world.
Fungi form mutualistic relationships with plants, algae, cyanobacteria, and animals.
All of these relationships have profound ecological effects.
Fungus-Plant Mutualisms
Mycorrhizae live symbiotically in plant roots (+ +) and are enormously important in natural ecosystems and agriculture.
Plants harbor harmless symbiotic endophytes that live inside leaves or other plant parts (+ +).
Endophytes make toxins that deter herbivores and defend against pathogens.
Fungus-Animal Symbioses
Some fungi share their digestive services with animals.
These fungi help break down plant material in the guts of cows and other grazing mammals.
Many species of ants and termites use the digestive power of fungi by raising them in “farms.”
Lichens
A lichen is a symbiotic association between a photosynthetic microorganism and a fungus (+ +).
Millions of photosynthetic cells are held in a mass of fungal hyphae.
The fungal component of a lichen is most often an ascomycete.
Algae or cyanobacteria occupy an inner layer below the lichen surface.
Lichen Structure and Reproduction
The algae provide carbon compounds, cyanobacteria provide organic nitrogen, and fungi provide the environment for growth.
The fungi of lichens can reproduce sexually and asexually. Asexual reproduction is by fragmentation or the formation of soredia, small clusters of hyphae with embedded algae.
Lichens are important pioneers on new rock and soil surfaces.
Lichens are sensitive to pollution, and their death can be a warning that air quality is deteriorating. Lichen (+ +).
Harmful Fungi
About 30% of known fungal species are parasites or pathogens (+ -), mostly on or in plants.
Some fungi that attack food crops are toxic to humans.
Animals are much less susceptible to parasitic fungi than are plants.
The general term for a fungal infection in animals is mycosis.
Practical Uses of Fungi
Food: Humans eat many fungi and use others to make cheeses, alcoholic beverages, and bread.
Some fungi are used to produce antibiotics for the treatment of bacterial infections, for example, the ascomycete Penicillium.
Genetic research on fungi is leading to applications in biotechnology:
For example, insulin-like growth factor can be produced in the fungus Saccharomyces cerevisiae.
Review of Fungal Phylums
Chytridiomycota (chytrids): Flagellated spores
Zygomycota (zygote fungi): Resistant zygosporangium as a sexual stage
Glomeromycota (arbuscular mycorrhizal fungi): Form arbuscular mycorrhizae with plants
Ascomycota (sac fungi): Sexual spores (ascospores) borne internally in sacs called asci; ascomycetes also produce vast numbers of asexual spores (conidia)
Basidiomycota (club fungi): Elaborate fruiting body (basidiocarp) containing many basidia that produce sexual spores (basidiospores)