What are FUNGI?

© Else Vellinga
Lab
Original publication: Mycena News, September 2013

Fig. 1. Yeast cells (Saccharomyces cerevisiae). The two cells on the left are budding, and making new yeast cells.

It is surprisingly difficult to come up with a definition that characterizes fungi.

For plants it is relatively easy to list the most important characteristics: they are planted in the ground and make their own sugars in special little cell-organelles called chloroplasts with the help of sunlight. Plant cells contain a nucleus in a membrane. The cell’s walls contain cellulose and lignin.

Animals are also not too hard to define: organisms that are able to move around—at least during one stage of their life. They lack the capacity to make their own sugars and must depend on other organisms for this essential source of energy. They are able to ingest food and digest it. They’re also made up of cells with a nucleus in a membrane, but distinct cell walls are absent.

Fig. 2. Fungal hyphae forming a mantel around a tree root. (photo by Hugues Massicotte)

Fungi have characteristics in common with both these groups. Most fungi, and certainly the mushrooms we all know so well, are sessile just like plants—they sit there and do not move around. And like animals, they have to break down organic material and absorb it for nutrition. Their cell walls do not contain cellulose, but chitin; a substance also present in many animal groups (think beetles and crabs for instance). Fungal cell walls also have glucans, just like plants.

Fungi are a diverse group and their body shapes vary enormously. The oldest branches of the fungal family tree are unicellular, in other words, each individual is a solitary, single cell. These cells live in wet environments and their spores have a flagellum—a whip-like hair that propels the cell around. The more advanced species of fungi lost their flagellum and changed from single-cell individuals to complexes of thread-like hyphae that form fruiting bodies like boletes or morels. They moved onto the land and conquered that new environment. Hyphae perform the same function as roots do for plants. Every day I gaze at the roots of the orchid above my computer, roots that are slowly but surely growing along the shelf, and I contemplate the similarities of roots and hyphae.

Fig. 3. Asci with 8 ascospores. Every spore has a refractive drop. The long cells between the asci are called paraphyses, and protect the growing asci. (photo by the author)

Fungi digest food—but not inside the cells. Enzymes are made inside the cells and are exuded into the environment (wood or soil) where they do their work of breaking down complex molecules into smaller molecules. The cells are then able to absorb these smaller molecules and put them to work.

Here, hyphae are at their best; they have a lot of surface in relation to the contents.

There are two large and species-rich fungal groups that we are very familiar with: the basidiomycetes and the ascomycetes. They differ mainly by the structure on which, or in which, their spores are formed. Basidiomycetes have basidia: club-shaped cells with four prongs on which spores are formed. The hallmark of ascomycetes are their asci: tube-shaped cells in which 8 ascospores are formed.

Fig. 4. Basidium seen from above with four asidiospores on it. (photo by the author)

These basidia and asci are on or in the fruitbodies, whereas the rest of the fungal organism is out of sight––too small to be seen with the naked eye.

Both these groups conquered the world by growing everywhere with their hyphae. Hyphae lengthen just behind each tip and they have rather rigid walls.

What I have written so far is all very general and that means there are plenty of exceptions. Here are a few:

Some plants have lost their chlorophyll and parasitize other plants (either directly or indirectly via a fungus) for their carbohydrates and sugars.
Some groups of filamentous fungi have gone back to a cellular form or stage; some Leucocoprinus species (parasol mushroom relatives), for instance, form yeast-like colonies when grown and cultivated by ants. Yeasts form colonies, but every individual is still just a single cell that behaves more like bacteria than like fungi. One example is Saccharomyces cerevisiae or baker’s yeast, which makes our bread rise, ferments grapes into wine and is instrumental in brewing beer.
Some basidiomycetes have basidia with six spores, or five, or two, or one and some ascomycetes have asci with four spores or some multiple of eight, like, 1032.

Fig. 5. Lichens growing on a wall in Avignon
(France). The flat round objects in the middle are the fruitbodies of the fungus, but the rest of the organism is visible as well (in non-lichenized fungi, that part is invisible to the naked eye and is hidden in soil, etc.). (photo by the author)

Lichens show off their whole organism; nothing is hidden from sight.
Quite recently, a group of widespread and diverse aquatic organisms was discovered that does not fit the fungal bill completely. These unicellular fungi lack chitin in their cell walls. Are they still fungi or do they belong to other groups? The morphology of the cells does not help much; it is just a balloon-shaped cell and a spore with a flagellum. All these were found in British pond water, river sediments, harbors, even the Berkeley Marina, etc., and they form one basal branch of the big fungal tree. This branch has a lot of leaves on it; many different organisms fit in here. One of these organisms was already known: Rozella, a genus that can be grown in the lab. Rozella also lacks chitin in the cell wall, but only in some stages of its life, namely when it is actively feeding. Some authors describe this as “no dinner jacket for Rozella”. Chitin is present in the walls of its resting sporangia (a kind of over-wintering spore container) and that can easily be revealed with a chitin-specific marker.

This is what makes fungi such intriguing organisms: completely new groups keep being discovered. Exceptions on generalities are the rule and we still know surprisingly little about the vast majority of the individual species.

Fig. 6. Rozella allomycis – the resting sporangia (the brown balls) in groups within the cells of the fungus Allomyces. The cell walls of these sporangia contain chitin, but the active stages of the fungi do not have chitin. (photo from Wikimedia commons)

Further reading

Jens H. Petersen. 2013. The Kingdom of Fungi. Princeton University Press. Princeton, NJ. 265 p. (At present the best introduction to fungi in all their beauty and weirdness)
Jones, M.D.M., I. Forn, C. Gadelha, M.J. Egan, D. Bass, R. Massana & T.A. Richards. 2011. Discovery of novel intermediate forms redefines the fungal tree of life. Nature 474: 200-203. doi:10.1038/nature09984 (Abstract) (PDF)
James, T.Y., M.L. Berbee. 2011. No jacket required—new fungal lineage defies dress code. Bioessays 34: 94-102. DOI 10.1002/bies.201100110 (PDF)

Else Vellinga, Ph.D., is interested in mushroom taxonomy and has been studying mushrooms in California and beyond for years. A frequent contributor to Mycena News, she is also fascinated by interactions between fungi and other organisms. In her free time she knits, and knits, and knits!