12.13–12.16: Fungi and plants are partners but not close relatives.

Cup fungi (Cookeina tricholoma) from Borneo.

Think about your toes for a minute. Have you ever had an itchy burning sensation between them? Almost all of us have athlete’s foot at some time during our life. One study found that 15% of all people in the United Kingdom currently had this fungal infection.

Fungi make up their own monophyletic kingdom (see Chapter 10) within the eukarya domain. Most fungi are multicellular, sessile decomposers (FIGURE 12-28). Although they were originally thought to be plants lacking chlorophyll, it turns out that they have little in common with plants. In fact, DNA sequence comparisons reveal that fungi are more closely related to animals than they are to plants (FIGURE 12-29). As eukaryotes, fungi have all the basic cellular components you would expect to find: nuclei, mitochondria, an endomembrane system, and a cytoskeleton. They also have cell walls, but the cell walls, instead of including cellulose, as in plants, are made of the carbohydrate chitin, a chemical also important in producing the exoskeleton of insects.

Figure 12.28: The defining characteristics of fungi.

Figure 12.29: Where the fungi fit in.

Fungi most likely arose from a unicellular, flagellated, aquatic protist more than 500 million years ago (and possibly as long as 1.3 billion years ago). Close to 100,000 species of fungi have been described, but the total number of species is estimated to be about 1.5 million. These species are divided into about seven phyla, but the overall phylogenetic classification is still very much in flux, with many of the relationships unresolved. Molecular evidence is, increasingly, guiding the process. About 98% of the described species belong to two monophyletic phyla, Ascomycota (about 64,000 species) and Basidiomycota (about 31,000 species). Besides these phyla, the two phyla with the largest numbers of described species are Microsporidia, with about 1,300 species, and Chytridiomycota, with about 700 species. These two groups, however, are not monophyletic.

The most commonly encountered types of fungi are (1) yeasts (the only single-celled fungi), truffles, and morels of the phylum Ascomycota; (2) mushrooms, of the phylum Basidiomycota; and (3) molds (which are not a phylogenetic grouping), such as Penicillium—the source of the antibiotic penicillin—of the phylum Ascomycota, and black bread mold, of the monophyletic phylum Zygomycota (FIGURE 12-30).

Figure 12.30: The fungi that humans know best. There are more than 1.5 million fungal species, usually divided among seven phyla. Those most recognizable to humans are mushrooms of the Basidiomycota (basidiomycetes), yeasts of the Ascomycota (ascomycetes), and molds (a term that does not refer to a monophyletic group), which occur in multiple phyla.

The fungus that causes athlete’s foot is multicellular and consists of thread-like structures made up of long strings of cells called hyphae (pronounced high-fee). Because of their thinness and length, hyphae have an enormous surface area. This means they are very good at taking up nutrients from your skin, but it also makes them very vulnerable to drying out. The fungus responsible for athlete’s foot grows best in moist places—like on the skin between your toes.

Unless you have an active athlete’s foot infection right now, however, your most recent encounter with a fungus was most likely when you last ate bread. Bread rises because yeast cells are mixed into the dough, which is then kept in a warm place while the yeast consumes sugar and produces carbon dioxide through fermentation. The carbon dioxide released by the yeast makes the dough swell into a loaf.