2.3.5: Macrofungi

Ascomycota

Learning Objectives

• Use life history traits and morphological features to distinguish between Ascomycota and Basidiomycota.
• Identify structures in the Ascomycota life cycle and know their ploidy.
• Differentiate between different types of ascocarps; locate fertile surfaces within those structures.

The majority of described fungal species belong to the Phylum Ascomycota. Fungi in this group have simple septate hyphae and most produce fruiting bodies called ascocarps (Figure \(\PageIndex{1}\)) for sexual reproduction. Some genera of Ascomycota either only reproduce asexually via conidia or the sexual phases have not been discovered or described. These were referred to as the Fungi Imperfecti or Deuteromycota. When ascomycetes reproduce sexually, they produce haploid ascospores (usually 8) within a sac-like structure called an ascus (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{1}\): Ascomycetes are called the cup fungi after the cup-shaped fruiting body formed by many of the larger organisms in this group, the apothecium. A few apothecia are shown on the left side of the diagram. The right side shows a long section through an apothecium. Septate hyphae comprise most of the structure. On the interior surface of the cup is the hymenium, which is composed of asci and paraphyses. Ascospores are produced by meiosis within each ascus. Diagram by Nikki Harris, CC-BY-NC.

Figure \(\PageIndex{2}\): In this photo, light and dark colored ascospores are produced inside the asci of the fungus Sordaria macrospora. This demonstrates the linear division of the spores, as pairs of genetically identical light or dark spores are adjacent, produced after mitosis occurs within the ascus. Photo by Aurora Storlazzi, CC BY 4.0, via Wikimedia Commons.

The Ascomycota include some ectomycorrhizal fungi (e.g. truffles and morels), fungi that are used as food, others that are common causes of food spoilage (bread molds and plant pathogens), and still others that are human pathogens. Some notable examples of ascomycetes include:

• Saccharomyces cerevisiae one of the budding yeasts. It ferments sugar to ethanol and carbon dioxide and thus is used to make alcoholic beverages like beer and wine, to make ethanol for industrial use and in baking (it is often called baker's yeast). Here, it is the carbon dioxide that is wanted (to make bread and cakes "rise" and have a spongy texture). Yeast is also used in the commercial production of some vitamins and in the production - using recombinant DNA technology - of some human therapeutic proteins.
• Neurospora crassa, another favorite "model" organism in the laboratory.
• The fungal partner in most lichens is an ascomycete.
• Powdery mildews that attack ornamental plants.
• The chestnut blight, which in a few decades killed almost all of the mature American chestnut trees in the Appalachians of North America.
• The Dutch elm disease, which has killed many of the American elms in the United States.
• Pneumocystis jirovecii, which is a major cause of illness in immunosuppressed people, e.g., patients with AIDS.
• The truffle and the morel, both highly-prized food delicacies. Truffles establish a symbiotic relationship with the roots of such trees as oaks.

Figure \(\PageIndex{3}\): A truffle. It might not look like much, but truffles are some of the most highly prized fungal fruiting bodies! (Public Domain).

Ascomycete Life Cycle

Asexual reproduction is frequent and involves the production of conidiophores that release haploid conidia. Sexual reproduction starts with the development of special hyphae from either one of two types of mating strains. One strain produces an antheridium and a strain of a complementary mating type develops an ascogonium. At fertilization, the antheridium and the ascogonium combine in plasmogamy without nuclear fusion. Dikaryotic ascogenous hyphae arise, in which pairs of nuclei migrate: one from each parent strain. In each ascus mother cell, two nuclei fuse (karyogamy). During sexual reproduction, thousands of asci may fill a fruiting body called the ascocarp. The diploid nucleus gives rise to four haploid nuclei by meiosis. In most ascomycetes, this is followed by a round of mitosis, producing 8 ascospores. The ascospores are then released, germinate, and form haploid hyphae that are disseminated in the environment and start new mycelia (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{4}\): The life cycle 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. In asexual reproduction, haploid (1n) mycelia produce conidiophores which use mitosis to asexually produce spores. These spores then germinate into new haploid mycelia. Sexual reproduction begins when one mycelium produces an ascogonium and another produces an antheridium. In plasmogamy, the ascogonium and antheridium fuse. Mitosis and cell division result in the formation of many dikaryotic (n+n) hyphae, which form a fruiting body called the ascocarp. Asci form at the tips of these hyphae. In karyogamy, the nuclei in the asci fuse to form a diploid (2n) zygote. Then meiosis produces four haploid (1n) nuclei within the ascus. Then mitosis and cell division results in eight haploid ascospores in the ascus. These haploid (1n) ascospores then disperse and germinate into new mycelia. Original diagram altered by Maria Morrow.

Types of Ascocarps

Apothecium

Apothecia are cup-shaped with the asci fully exposed, lining the interior of the cup. Normally, these asci are microscopic. However, in the fungus Ascobolus, the large asci with dark ascospores can be seen with the naked eye or a handlens (Figure \(\PageIndex{5}\)). The typical cup shape can be inverted and take on strange morphologies (see Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{5}\): In the first photo, apothecia of Ascobolus furfuraceus have asci appear black due to their dark spores. In the second photo, a magnified view of Ascobolus immersus shows the dark ascospores. First photo by Salvatore Bacciu and Paola Mereu, (CC-BY-NC). Second photo by Jerry Cooper, some rights reserved (CC-BY).

Figure \(\PageIndex{6}\): Two apothecia of Helvella in the image on the left. This ascomycete, much like a morel, has an apothecium at the end of a stalk. The apothecium at the top of the stalk is not cup-shaped because the cup has been inverted. The spores are produced on the surface of the apothecium that you can see in this image. The image on the right shows another ascoymcete, Heyderia abietis, with a similar strategy. The inverted cup shape makes a smooth tongue-like structure on the top of the stalk. These are only about 1 cm tall. Photos by Maria Morrow, CC-BY-NC.

Perithecium

A perithecium is a flask-shaped fruiting structure (Figure \(\PageIndex{7}\)), often microscopic and embedded within either the substrate it is fruiting in or a fungal structure called a stroma (Figure \(\PageIndex{8}\)). The asci are almost entirely closed off from the external environment, excepting a small hole in the top of the perithecium called an ostiole. Many plant parasites, such as the causal agents of Dutch elm disease, American chestnut blight, and ergot, are perithecial ascomycetes.

Figure \(\PageIndex{7}\): A diagram of a perithecium. m=the mycelium, s=asci with black ascospores inside, a=paraphyses (sterile cells surrounding the asci), e=the ostiole (opening). From Strasburger’s Lehrbuch der Botanik. After v. Tavel., Public domain, via Wikimedia Commons.

Figure \(\PageIndex{8}\): Xylaria species produce perithecia embedded in sterile fungal tissue called a stroma. In Xylaria tucumanensis, the perithecia are large and are not surrounded by much stroma. In the first photo (left), each lumpy projection has a distinct dot in its center; this is the ostiole (opening) of the perithecium. The second photo (right) shows a long section through a fruiting structure. The white tissue is the stroma. The black, concave structures are the perithecia. Photos by Danny Newman, CC-BY-NC-SA.

Cleistothecium

A cleistothecium is a fully-enclosed fruiting structure. These typically have bag-like asci (Figure \(\PageIndex{9}\)). Some (chasmothecia) split open to release their spores. This type of fruiting body can be found in the powdery mildews, a group of plant pathogenic fungi in the order Erysiphales (Figure \(\PageIndex{10}\)).

Figure \(\PageIndex{9}\): A section through a cleistothecium with an ascus circled. A cleistothecium is a fully enclosed fruiting body, generally microscopic, that encloses many bag-like asci. Photo by Melissa Ha, CC-BY-NC with labels added.

Figure \(\PageIndex{10}\): Powdery mildews (Erysiphales) are plant parasites that form cleistothecia. They get their name from the powdery white look they give to the surface of plants they are parasitizing (first). Sometimes within this powdery white fluff (mycelium), cleistothecia can be seen with some magnification (second). Photos by ðejay (Orkney), CC-BY-NC.

Lichenized Fungi

Learning Objectives

• Explain the lichen symbiosis.
• Identify structures in the lichen thallus.

Lichens are fungi that live in a symbiotic association with a green alga or cyanobacterium (the "photobiont"), or both. The primary fungal partner (the "mycobiont") in most lichens (98% of them) is an ascomycete. Basidiomycetes make up the remainder. The relationship is often characterized as mutualistic; that is, both partners benefit. However, evidence (see "The British Soldier" below) suggests that while the fungus is dependent on its autotrophic partner, the photobiont is often perfectly content to live alone. Recently many lichens have been found to harbor a second fungal partner, a basidiomycete yeast. Its function remains to be discovered, though the presence of the basidiomycete can change the outward appearance of the lichen (e.g. color).

The British Solder

The below image is of the colorful lichen called British soldier. The fungus is Cladonia cristatella, an ascomycete. Its name is the name given to the lichen. The photobiont is Trebouxia erici, a green alga. It is found in many other lichens as well, and also can be found growing independently. The algal cells eventually are killed by the fungus, but are continuously replaced by new ones. So, the relationship in this lichen is one of controlled parasitism rather than mutualism.

Figure \(\PageIndex{11}\): British soldier lichen, Cladonia cristatella.

The red cap produces the spores of the fungus, but these alone cannot form new lichens. Other structures (e.g., soredia), containing both partners, are needed to disperse the lichen to new locations. Some lichens release only fungal spores. These mycobionts depend for their continued survival on finding an acceptable photobiont released from other lichens. Phylogenetic trees, based on both ribosomal RNA genes and many protein-coding genes, as well as fossils indicate that lichens have been present on the earth for at least 600 million years.

Today about 14,000 species of fungi are known to form lichens. Lichens display a range of colors and textures and can survive in the most unusual and hostile habitats, though they are extremely sensitive to air pollution.. They cover rocks, gravestones, tree bark, and the ground in the tundra where plant roots cannot penetrate. Lichens can survive extended periods of drought, become completely desiccated, and then rapidly become active once water is available again. In part because of this ability, as well as the pigments in some lichens that protect from UV radiation, lichens are some of the only living things to survive exposure to conditions in space.

Explore the world of lichens using this site from Oregon State University.

Figure \(\PageIndex{12}\): Lichens have many forms. They may be (a) crust-like, (b) hair-like, or (c) leaf-like. Descriptive text: Different lichens are shown. Part A shows a lichen that appears like brown flecks on gray rock. Part B shows a moss-like lichen hanging from a tree. Part C shows lichen that have a wide, flat, convoluted shape. (credit a: modification of work by Jo Naylor; credit b: modification of work by "djpmapleferryman"/Flickr; credit c: modification of work by Cory Zanker)

The body of a lichen, referred to as a thallus (Figure \(\PageIndex{13}\)), is formed of hyphae wrapped around the photosynthetic partner. The photosynthetic organism provides carbon and energy in the form of carbohydrates. If cyanobacteria are involved, they fix nitrogen from the atmosphere, contributing nitrogenous compounds to the association. In return, the fungus supplies minerals and protection from dryness and excessive light by encasing the algae in its mycelium. The fungus also attaches the symbiotic organism to the substrate.

Figure \(\PageIndex{13}\): This cross-section of a lichen thallus shows the (a) upper cortex of fungal hyphae, which provides protection; the (b) algal zone where photosynthesis occurs, the (c) medulla of fungal hyphae, and the (d) lower cortex, which also provides protection and may have (e) rhizines to anchor the thallus to the substrate. Descriptive text: The lichen has multiple layers. The top layer, or cortex, is made up of irregularly shaped cells. Beneath this layer, the cells in the algal zone hyphae wrap around the cyanobacteria. Beneath the algal zone, long, thread-like mycelia occur. Beneath the mycelia is the lower cortex, which is similar in appearance to the upper cortex, but with larger cells. Projections beneath the lower cortex anchor the lichen to its substrate.

The thallus of lichens grows very slowly, expanding its diameter a few millimeters per year. Both the fungus and the alga participate in the formation of dispersal units for reproduction. Some lichens produce soredia (Figure \(\PageIndex{14}\)), clusters of algal cells surrounded by mycelia, for asexual reproduction. Soredia are dispersed by wind and water and form new lichens. To reproduce sexually, the fungus makes spores that must find a new photosynthetic partner shortly after germinating.

Figure \(\PageIndex{14}\): A foliose lichen with marginal soralia. The powdery regions on the edges of this lichen are called soralia. In these soralia, the lichen is producing many soredia--loose bundles of fungal hyphae and algal cells that allow the lichen to reproduce asexually. Three of these soralia have been indicated by white arrows. Photo by Maria Morrow, CC-BY-NC.

Basidiomycota

Learning Objectives

• Use life history traits and morphological features to distinguish between Ascomycota and Basidiomycota
• Identify the parts of a mushroom
• Identify structures in the Basidiomycota life cycle and know their ploidy

The Basidiomycota (basidiomycetes) are fungi that produce haploid basidiospores (spores produced through budding) from club-shaped cells called basidia (Figure \(\PageIndex{13}\)). These are typically formed within fruiting bodies called basidiocarps. They are important as decomposers (particularly of wood), plant pathogens, mutualists, and food sources for many animals. Basidiomycetes include the group of fungi that forms mushrooms, the rusts, and the smuts. Mushrooms are basidiocarps formed from masses of interwoven hyphae growing up from the mycelium. The basidia develop on fertile surfaces of the mushroom and release their spores (typically four from each basidium) into the air.

They hyphae of basidiomycetes are septate with clamp connections where the septa form (Figure \(\PageIndex{14}\)). The septal structure is more complex than in ascomycetes. The septum is swollen around the pore (a dolipore septum) and is flanked by structures called parenthesomes (Figure \(\PageIndex{15}\)).

Fungi with the following structures can be placed in the Basidiomycota*:

*It is important to note that these features may look different or not be present at all in some groups of Basidiomycota, such as the rusts (Pucciniomycotina) and smuts (Ustilagomycotina) -- see Chapter 3.6.4: Rusts & Smuts in the Photographic Atlas. However, there are many physiological and genetic similarities that support grouping these organisms together in the Basidiomycota.

Basidia and Basidiospores

Both karyogamy and meiosis occur within a cell called the basidium (Figure \(\PageIndex{15}\)). Haploid basidiospores from atop projections on the basidum called sterigmata (sing. sterigma). There are generally four spores, as shown in the image below, though the number of spores produced can vary by species. For example, the mushroom you are likely most familiar with, Agaricus bisporus (though you probably know it as a crimini or button mushroom at its immature stage and portobello at maturity), only produces two spores on each basidium (bi- meaning two).

Figure \(\PageIndex{15}\): A section through the gills of a mushroom. On the left, a basidium has four basidiospores sitting atop its four sterigmata. On the right, the spores have been released and the sterigmata are easier to distinguish. Photo by Maria Morrow, CC-BY-NC.

Clamp Connections

Basidiomycetes maintain their dikaryotic (n+n) state in each hyphal compartment by making structures called clamp connections (Figure \(\PageIndex{16}\)). These are not always present, but provide a helpful identification feature when they are!

Figure \(\PageIndex{16}\): These two images show hyphae from a mushroom. In the image on the left, a clamp connection is visible at the septum. In the image on the right, four clamp connections are indicated by arrows. Photos by Maria Morrow, CC-BY-NC.

Complex Septations

Complex septations are shown in Figure \(\PageIndex{17}\).

Figure \(\PageIndex{17}\): The complex dolipore septum found in basidiomycetes. This is not a feature that can be seen with the naked eye or in a standard microscope. "In hyphae of basidiomycete fungi, parenthesomes (1) "cap" a dolipore septum (2). The cell wall (3) swells around the septal pore to form a barrel-shaped ring. Perforations in the parenthesome allow cytoplasm to flow between (4) and (5)." Miguelferig, CC0, via Wikimedia Commons.

General Mushroom Anatomy

Though only a subset of basidiocarps look this way, they are the model for how we describe "mushrooms". In mycology, this type of basidiocarp is called "agaricoid" or "agaric" because it is the general form we see in the genus Agaricus. A more complex version of the agaric mushroom is seen in the genus Amanita (Figure \(\PageIndex{18}\)).

Figure \(\PageIndex{18}\): The anatomy of an Amanita muscaria basidiocarp. The cap (also called the pileus) protects the spore producing region, the hymenophore. The diagram shows a hymenophore composed of gills. However, you might see mushrooms with pores, teeth, or other types of spore-producing surfaces. The partial veil covers the hymenophore while the mushroom develops. When the cap extends as the mushroom grows, the partial veil is pulled and can either end up as an annulus or attached to the edges of the cap. Not all mushrooms have a partial veil and those that do often look quite different from this one! The universal veil (only present in a few mushrooms) covers the entire mushroom when it is young. As the mushroom expands, the universal veil is also pulled apart. Here, the cap has tiny fragments of the universal veil (warts) and the rest is at the base of the stipe (forming a volva). The stipe is the "stem" or "stalk" of the mushroom. Artwork by Nikki Harris, CC-BY-NC.

Basidiomycete Life Cycle

The life cycle of basidiomycetes involves an extension of the dikaryotic phase (Figure \(\PageIndex{19}\)). Mycelia of different mating strains combine (plasmogamy) soon after germination and produce a secondary, dikaryotic mycelium that contains haploid nuclei of two different mating strains (a dikaryon). This is the dikaryotic stage of the basidiomycete life cycle, and it is the dominant stage. Eventually, the secondary mycelium generates a basidiocarp. The basidiocarp can vary greatly in morphology, but in the sense of a standard mushroom (Figure \(\PageIndex{15}\)), the developing basidia are produced on the surface of gills located under cap.Within the club-shaped basidium, meiosis occurs and a diploid zygote is formed (karyogamy). The zygote divides by meiosis to produce four haploid nuclei. The haploid nuclei migrate into basidiospores, which germinate and generate monokaryotic hyphae. The mycelium that results is called a primary mycelium.

Figure \(\PageIndex{19}\): The life cycle of a basidiomycete alternates a haploid generation with a prolonged stage in which two nuclei (dikaryon) are present in the hyphae. Haploid (1n) basidiospores germinate to form haploid mycelia. In this diagram, there are two mating types (+ and -). In plasmogamy, fusion between + and – mating type mycelia results in formation of a dikaryotic (n+n) mycelium. Within the dikaryotic mycelium, one nucleus of each type (+ and -) is maintained between each septation. Under the right environmental conditions, a basidiocarp forms via mitosis. Gills of the basidiocarp contain cells called basidia. A photo of a mushroom labels the mushroom as basidiocap and basidia within the gills. Basidia form diploid nuclei via karyogamy; this produces a diploid zygote. Four haploid nuclei are formed in the basidium via meiosis. Cell walls are formed to produce four haploid (1n) basidiospores as they are pushed out the sterigmata. These spores then disperse and germinate into new haploid mycelia. Original diagram altered by Maria Morrow.

Summary

Macrofungi are represented by two major groups that can form macroscopic fruiting structures. However, many lineages within these groups might only reproduce asexually, form yeasts, or form microscopic fruiting structures (such as the rusts and smuts).

Ascomycetes typically form 8 ascospores within a structure called an ascus. In most lineages, these are produced within an ascocarp, which can be an apothecium (cup), perithecium (flask), or cleistothecium (ball). Ascomycetes have hyphae with simple septations. Their life cycle involves an extended dikaryotic phase that takes place within the ascocarp, forming dikaryotic ascogenous hyphae. These hyphae will eventually form asci, where karyogamy will take place, followed shortly after by meiosis, and usually mitosis.

Basidiomycetes typically form 4 basidiospores externally on a basidium. In the Agaricomycotina, these are produced on or in a basidiocarp, what we call mushrooms, specialized for spore dispersal. The basidiomycete life cycle is almost entirely dikaryotic. Haploid spores germinate, form a monokaryon, then must fuse with another monokaryon shortly afterward. These forms a dikaryotic mycelium with dolipore septations and clamp connections. Karyogamy only takes place within the basidia, follow promptly by meisosis to produce basidiospores.

Both ascomycetes and basidiomycetes for relationships with photosynthetic partners, such as algae or cyanobacteria, to form lichens. In this mutualism, the photobiont provides sugars from photosynthesis, while the mycobiont forms a protective thallus. In the case of cyanobacteria, these may also provide nitrogen fixation.

Attributions

Curated and authored by Maria Morrow, CC-BY-NC, using the following sources:

• 19.1.7 Fungi from Biology by John. W. Kimball (licensed CC-BY)
• 24.2 Classifications of Fungi and 24.3 Ecology of Fungi from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
• 5.3 Fungi from Microbiology by OpenStax (licensed CC-BY). Access for free at openstax.org.