24.1: Introduction
There are many groups of unrelated organisms that are referred to as ‘fungus’ or ‘mold’. Formerly, these groups were all believed to have been related, sharing the characteristics of being heterotrophic eukaryotes, generally with cell walls, usually decomposers or parasites, and often with the storage carbohydrate glycogen. These organisms were placed into groups ending with “-mycota”, meaning fungus. On closer inspection, we find that these groups differ on many important life history traits and very few of them actually belong to the true Fungi. However, if you take a mycology course, you will most likely study these unrelated organisms, as well. Similarly, fungi are still studied under the umbrella of botany as a relic of past classification, but they are more closely related to animals than they are to plants.
Fungus-Like Organisms
Myxogastria (formerly Myxomycota): Plasmodial Slime Molds
Myxomycetes (members of the Myxogastria) are fungus-like organisms called slime molds, but they are not members of Kingdom Fungi. In their feeding stage, myxomycetes form one large amoeba with many nuclei and no cell wall. This amoeba moves over damp, decaying material looking for bacteria to engulf and digest. When it dries out or runs out of food, it begins to make fruiting structures called sporangia (sporangium, singular). Inside these sporangia, the myxomycete will undergo meiosis, wall off individual nuclei and make haploid spores for aerial dispersal. Dispersal by spores, heterotrophism, and glycogen as a storage carbohydrate originally classified this group within Kingdom Fungi, but this is the end of the similarities. The spores have cell walls made of cellulose , like plants. When these spores land, they will germinate into haploid cells (called swarm cells) that will fuse together to form a diploid amoeba. This means that, unlike true Fungi, they have a diplontic life cycle.
Observe the slime molds on display. If possible, make a wet mount of a small sample. If there are motile cells (cells that are actively moving around), look for the presence of 2 flagella. This also distinguishes myxomycetes from Kingdom Fungi, where only one flagellum is present. If a mature amoeba is present, can you see cytoplasmic streaming occurring?
Draw what you see and make observations below. Label the name, function, and ploidy of any identifying structures.
Other Slime Molds: Dictyostelia and Protostelia
Quite on theme for fungal classification, there are several different groups of unrelated organisms that are called slime molds. The two mentioned above, Dictyostelia and Protostelia, represent cellular slime molds and are relatively closely related to the plasmodial slime molds. Unlike the plasmodial slime molds, these groups exist as individual amoebae until it is time to leave the area. In the Dictyostelia, the amoebae assemble together to form elaborate fruiting structures where only some of the individuals will become spores and the rest will die. They are studied for this altruistic behavior.
Oomycota -- The Water Molds
Oomycetes are also fungus-like organisms with cell walls made of cellulose . Similar to myxomycetes, they have motile spores with 2 flagella. However, one of these flagella is "normal"-looking (called a whiplash flagellum) and the other is ornamented. This strange characteristic puts organisms into a group called the heterokonts (meaning "different flagella"). Like us, true Fungi are part of the opisthokonts (opisth- meaning rear, -kont meaning flagellum).
Also similar to myxomycetes, oomycetes have a diplontic life cycle . What does this mean?
Kingdom Fungi - The True Fungi
Though many heterotrophic decomposers and parasites are fungus-like, an organism must be in Kingdom Fungi to be considered a true fungus. In addition to genetic relationships, organisms are classified into Kingdom Fungi based on the following traits:
- Eukaryotic
- Heterotrophic by absorption
- Morphology: Unicellular or a thallus composed of hyphae . If present, motile cells have only a single, whiplash flagellum (like us).
- Cell walls: Chitin
- Storage carbohydrate: Glycogen
- Life cycle: Haplontic (though this one gets a bit weird)
- Ecology: Many fungi are decomposers, primarily in terrestrial ecosystems. Other groups of fungi live in symbiosis with other organisms as parasites, mutualists, or commensalists.
Determining the ancestry and relatedness of groups of fungi is surprisingly difficult. DNA sequencing has led to continual rearrangements of the fungal tree of life and, at the time of writing, there is no one agreed upon picture of the history of fungi that the author is aware of. However, there are a few major groups that Kingdom Fungi is commonly broken into, and these will be discussed in the following section.
The image below is from an open-access publication (doi.org/10.1128/ mBio.01739-16) and presents one possible hypothesis for the relationships between groups of fungi. This hypothesis is called a phylogeny and is based on genetics, as well as physiological and morphological features. No single phylogeny is currently accepted by all mycologists.
Chytrids: includes Chytridiomycota and Blastocladiomycota from the above phylogeny
Chytrids comprise the oldest lineages of fungi. Unlike any other group within this kingdom, they are aquatic and have swimming spores (zoospores) with a single flagellum. Though many in this group are harmless decomposers, the most famous of the chytrids is Batrachochytrium dendrobatidis , a fungus that infects the skin of amphibians. This chytrid is contributing to a worldwide decline in many amphibian species (though there are numerous other contributing factors), particularly frogs.
If available view specimens of chytrids under the dissecting and compound microscopes. What features can you find?
Zygomycetes: includes all of the non-flagellated, early diverging fungi above, except Glomeromycotina
The zygomycetes are composed of at least two distinct lineages of fungi that all share a common structure during sexual reproduction, the zygosporangium . This is a large, ornamented, orange-to-brown structure where both fertilization and meiosis occur. Zygomycetes have no septations in their hyphae, which is referred to as being coenocytic . These fungi are commonly found on high-sugar substrates, such as rotting fruits or molding bread, or as insect parasites. Zygomycetes can reproduce asexually by producing mitosporangia (shown below), making haploid spores by mitosis.
Above are three mature zygosporangia produced during sexual reproduction of Rhizopus stolonifer . In the center of the image, two compatible mycelia (+ and -) have connected together and are currently forming gametangia. They will flood these gametangia with haploid nuclei, which will fuse within the zygosporangium to create diploid zygotes. Each of these will immediately undergo meiosis to produce haploid spores.
View specimens of zygomycetes available in lab and record your observations below. Which features would help you identify this group?
Glomeromycota: listed as Glomeromycotina in the fungal phylogeny
This single lineage within Kingdom Fungi forms relationships with the roots of almost all land plant species and thalli of the earliest plant lineages, who evolved before roots. This mycorrhizal (myco- meaning fungus, rhiza meaning root) relationship has existed for 400 million years and was likely involved in the movement of plants onto land. Glomeromycetes are called endomycorrhizal because the fungal hyphae enter inside of plant cells. The fungus enters the plant tissue, usually through the roots, and penetrates the cell walls of the cortex cells in the root. However, they hyphae do not go through the plasma membrane. Instead, they form highly branched, tree-like structures called arbuscles . This provides a large amount of surface area for the plant and fungus to interact with each other. How are they interacting?
These images are from an open access paper (doi: 10.1038/srep29733) studying fungal colonization of plant cells. In the image on the right, the fungal tissue was stained with a fluorescent dye. In the lower two (B and C), fungal hyphae were stained dark and are forming arbuscules within the plant cell walls.
Because it is a heterotroph, the fungus takes sugars from the plant. The fungal hyphae extend beyond the plant roots into the soil, where it can absorb water and nutrients that are transferred to the plant. Each partner gains a benefit from this relationship, so this is called a mutualistic symbiosis, or mutualism .
A symbiotic relationship refers to a shared relationship between at least two organisms of different species. This relationship can benefit both parties, as above, only benefit one, potentially causing harm to the other. What would you call this last type of symbiosis?
View a mycorrhizal root tip under the compound microscope, either as a prepared slide or from a fresh sample. If from a fresh sample, use 5% KOH + Phloxine B or Cotton Blue to stain the fungal tissue. Draw what you see below and label any distinguishing features of both the plant and fungus.
The Dikarya: Ascomycota and Basidiomycota
These two groups of fungi are referred to as the dikarya because some portion of their life cycle is dikaryotic . When two compatible mating types meet, they fuse together and begin the process of fertilization with plasmogamy (fusion of the cytoplasm). However, the nuclei do not fuse. Instead, a new mycelium is formed with two different haploid nuclei in each cell, making the ploidy n + n (as opposed to diploid, 2n). This state is called dikaryotic. Karyogamy (fusion of the nuclei) does not occur until the fungus is about to make spores. The two nuclei fuse and the zygote immediately undergoes meiosis, forming haploid spores.
Which type of life cycle would you classify this as and why?
Though there are microscopic species and life stages in each group (including the yeast you saw earlier), members of the Ascomycota and Basidiomycota both form macroscopic fruiting bodies.
Fungi make a diversity of fruiting structures, structures in which meiosis occurs to form spores. In the microfungi, you saw zygomycetes making their fruiting structures: zygosporangia. In the macrofungi, you will learn three different types of ascocarps and the anatomy of a basidiocarp, otherwise known as a mushroom. These structures are analogous to fruits produced on a tree. The rest of the fungal body (or, the rest of the tree, in this analogy) is the mycelium, buried within the substrate and busily acquiring food. Note: The mycelium is not the equivalent of the roots of the tree, but of the entire tree, including the roots, stems, and branches. Mushrooms are the ephemeral fruits of this structure, emerging for sexual reproduction.
Ascomycota -- The Sac Fungi
This group of fungi is when the life cycle starts to get particularly strange and the fungus forms something called a dikaryon (di- meaning two, karyo- meaning nucleus). When two different hyphal bodies fuse, they do not fuse their nuclei, only the cytoplasm. The result is a single fungal body with two separate types of nuclei floating around inside it. Because it is neither haploid (n), nor truly diploid (2n), this condition is called being dikaryotic (n+n).
Ascomycota are distinguished from other groups of fungi by the following characteristics:
- Production of ascospores (usually 8) within an ascus , a normally elongate, sac-like structure
- Hyphae with simple septations
- The mycelium is primarily haploid. When two haploid (n) mycelia of the correct mating types meet, they can form a dikaryotic (n+n) fruiting body called an ascocarp .
There are three different types of ascocarps:
- Apothecium - a (usually) cup-shaped fruiting body with asci lining the interior of the cup. Two exceptions are Helvella and Morchella (morels), where the cup has been inverted, no longer looking like a cup, and the asci cover the now-exterior surface.
- Perithecium - a round fruiting body with a long neck, much like a bottle, with the asci inside the bottom of the bottle. Microscopic and quite similar to a Fucus conceptacle.
- Cleistothecium - a spherical, enclosed fruiting body packed with rounded asci. These are also microscopic and can be beautifully ornamented.
Above is the spore producing surface of an apothecium. The long asci each contain 8 ascospores.
Basidiomycota -- The Club Fungi
Basidiomycota are the other group of dikaryotic fungi. This group includes the mushroom-forming species of fungi, as well as two groups of (mostly) plant parasites , delightfully referred to as the rusts and the smuts.
Basidiomycota are distinguished from other groups of fungi by the following characteristics:
- Basidiospores (usually 4) produced from a basidium --a normally squat, roundish structure with prongs called sterigmata that spores are produced on.
- Hyphae with complex septations. These specialized septations look like they are surrounded by parentheses and help the mycelium maintain its dikaryotic state.
- Clamp connections can sometimes be seen, though are only obvious in certain genera.
- The mycelium is primarily dikaryotic. The haploid spores germinate and must find another germinating spore of the correct mating type to form a dikaryotic mycelium. This can then produce fruiting bodies called basidiocarps , or as we commonly call them, mushrooms.
The image above shows the spore producing surface of a basidiomycete, Coprinus . The dark football-shaped structures are the haploid basidiospores . They sit atop small projections called sterigmata ( sterigma , singular) that emerge from the top of the basidium . In most mushrooms, each basidium produces four basidiospores.