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. Because of this, they will be covered in more detail in Lab Macrofungi and Lichens.