Teaching the fungal tree of life

Introduction to fungal biology

I. More than Mushrooms and Mold

click here for a sample lesson plan for introducing the fungi to your students

or continue on for an overview of Fungal biology

Fungi are a difficult group to define because they are a group whose members appear so different when looking only at their shapes and structures. Just look at the logo in the upper left of this webpage. The five types of fungi represented there have very different morphology.

Features shared by all fungi:

Like Plants and Animals, Fungi have a Eukaryotic cell structure (but some appear to have lost their mitochondria—to be discussed later)

Fungi have a Heterotrophic nutritional mode—meaning that they must ingest organic compounds for their carbon nutrition (but some live in close symbioses with photosynthetic algae—these are lichens)

Fungi survive by absorptive nutrition—meaning that they secrete enzymes to digest organic compounds outside their cells, and then absorb the small molecules such as sugars that are left..

Fungi have cell walls that are made of chitin—, which is similar to the cellulose of wood. However, the sugars that are combined to form chitin contain Nitrogen. Chitin is also found in the exoskeletons of insects and other arthropods.

Variable features of fungi:

Unicellular or multicellular—multicellular forms are composed of filaments called hyphae.
With or without complex, multicellular fruiting bodies (reproductive structures)
Sexual or asexual reproduction
With or without flagella—if they have flagella, then these are the same as all other eukaryotic flagellae (i.e., with the“9+2” arrangement of microtubules, ensheathed by the plasma membrane)
Typically reproduce and disperse via spores
Occur on land (including deserts) or in aquatic habitats (including deep-sea thermal vent communities)
Function as decomposers of dead organic matter or as symbionts of other living organisms—the latter include mutualists, pathogens, parasites, and commensals (examples to be given later)

Familiar examples of fungi include mushrooms, molds, yeasts, lichens, puffballs, bracket fungi, and others.

There are about 70,000 described species of fungi. The actual number of species is a matter of conjecture (translation: we haven’t a clue!). One popular estimate is that there are 1.5 million species of fungi.

A phylogenetic definition of fungi (more details on this topic will be provided in Lecture 4):

Fungi form a monophyletic group (a single branch of the tree of life, including an ancestor and all of its descendants—also called a clade).

The closest relatives of the fungi include animals and slime molds (Mycetozoa). Fungi have traditionally been studied by“botanists” but they are not closely related to plants.

Some organisms that have traditionally been considered fungi are now known not to be in the fungus clade, including slime molds and Oomycetes. The latter includes the organism that caused the Irish potato famine, Phytophthora infestans. Phytophthora is a member of the Stramenopiles, a group that also includes kelps (including the largest seaweeds), diatoms (unicellular phytoplankton with glass cell walls) and others.

The major groups of fungi:

Chytridiomycota*—flagellate, mostly aquatic, unicellular or filamentous. ca 1000 described spp.

Zygomycota*—non-flagellate, mostly terrestrial, filamentous, no multicellular fruiting body, zygospores or asexual, aseptate hyphae. ca 1000 described spp.

Ascomycota—non-flagellate, mostly terrestrial, filamentous or unicellular (yeasts), ascospores or asexual spores, septate hyphae, many with multicellular fruiting bodies. ca 45,000 described spp.

Basidiomycota—non-flagellate, mostly terrestrial, filamentous or unicellular, basidiospores or asexual spores, septate hyphae—often with“clamp connections”, multicellular fruiting bodies. ca 22,000 described spp.

*non-monophyletic groups