The Fungi
"myco" = fungus; Mycology = study of fungi
Your text does not have a section devoted to fungi, so it is important to jump around to
various contextual references. As you do, make sure you are aware of the context of the section
you are reading. I've put together some information from my own vast knowledge, and
referenced page numbers in your text to supplement my own information.
The fungi are mostly multicellular (exception = yeasts) life forms that have cell walls
made of chitin. Chitin is also found in the exoskeletons of arthropods. It is generally considered
to be a polysaccharide carbohydrate, except that the monomer is glucosamine (there is an amine
group (NH2) on the #2 carbon of an otherwise normal-looking glucose unit).
The Kingdom is most commonly divided into 4 Phyla, but this is changing. Three of the
Phyla are locked in. They are grouped based on their sexual reproductive (spore-bearing)
structures (p 478, 486). The three major phyla are:
Zygomycota (Zygomycetes): Common Molds. The spore bearing structure is a
zygosporanium. The common black mold Rhizopus nigricans is found in soil all over the
world.
Ascomycota (Ascomycetes): The Sac Fungi. Ascus = sac. Examples include yeasts (the only
unicellular fungi), cup fungi, morels, and truffles (the latter 2 considered delicacies among
connoisseurs).
Basidiomycota (Basidiomycetes): The Club Fungi. Basidium = club. Include mushrooms,
bracket or shelf fungi, and puffballs, among many other less-conspicuous species.
Traditionally, a fourth group of fungi - The Deuteromycetes (Imperfect Fungi) were
classified as such because they have evolved beyond sex (no sexual structures for placement in
other groups), utilizing other means to exchange genetic material. This group is dwindling to
nothing as scientists using genetic technology assign them to (mostly) Ascomycetes or (lesscommonly) Basidiomycetes. Penicillin is a very common member of this group, but has now
been reassigned to the Ascomycetes. Same for Tenia pedis, the pathogen of athletes foot.
The Chytridomycota are a strange group of fungi that have been in and out of the
kingdom, but currently they are back in. The Chytrids are gaining attention these days with the
discovery that they are important pathogens of amphibians and are involved with the global
decline in frogs. Chytrids live in water (often), and have flagellated spores.
Feeding: All fungi are heterotrophic and feed in a manner very similar to bacteria (p 68). They
produce and secrete enzymes onto the food upon which they live (their substrate). They are
important decomposers (p 637). Some are parasites of plants and animals. Those that feed on
dead stuff are called saprobes. There are obligate saprobes, obligate parasites, and others can go
either way. A few fungi parasitize humans, but their virulence is low, skin infections caused by
fungi are more common in tropical areas.
Ecology: The fungi play many vital roles in ecosystems. In addition to being significant
decomposers, most fungi have co-evolved with land plants, forming mutualistic symbiotic
relationships. The most important of these relationships involve the Mycorrhizal fungi.
Mycorrhizae "fungus root" take their nourishment from secretions from plant roots, and in
exchange they provide the plant with water and minerals using their unsurpassed powers of
absorption (p 188). Lichens are intimate relationships between a fungus and a photosynthetic
alga or cyanobacterium (p 479). The partners in these lichen relationships are incapable of living
apart from one another and many scientists have suggested that they be given their own place in
the tree of life and a single species name, instead of the species of both members of the
partnership.
Morphology: Most fungi grow in long filaments called hyphae. The hyphae of Ascomycetes
and Basidiomycetes is divided into distinct cells by crosswalls called septa (septate hyphae).
The Zygomycetes lack these septa, and their hyphae is therefore coenocytic (without septa),
resulting in multinucleate hypha. Mycelium is the name given to the body of most fungi. It
consists of lots and lots of hyphae. A hypha is to mycelium as a tree is to a forest. The
extremely important yeasts do not produce hyphae or mycelium, but they do have sexy time
occasionally, and the resulting spore structure is an ascus. Yeast are unicellular, and reproduce
asexually by the process of budding. They are also the only organisms capable of alcoholic
fermentation which produces ethyl alcohol and CO2, important throughout the history of our
species.
Reproduction: All fungi reproduce asexually, and all but the Deuteromycetes can reproduce
sexually. Sex in the fungi is beneficial for maintaining genetic diversity, but from a reproductive
standpoint it is very inefficient in terms of energy consumption. The life cycles of the three
groups include the sexual stage that defines them, as well as their very unique sexual activities.
Fungi are prodigious producers of reproductive spores, which can be of either sexual or
asexual origin.
Genetics: Fungal cells are almost always haploid. When haploid hyphae of compatible strains
meet at the right place and the right time and under the right conditions, they will fuse their cells
in a process called plasmogamy. Resulting cells may have two separate haploid nuclei, at which
point the cells are functionally diploid, but technically dikayrotic. The cells in a mushroom are
in this stage. Think of this as the courtship phase. The hyphae grow into a dense mass, and
when the time is right the haploid nuclei in the cells fuse en masse in a process called
karyogamy. The resulting zygotes immediately undergo meiosis to produce haploid spores. In
mushrooms, this occurs in the cells of the gills, underneath the cap. Most fungi also produce
asexual spores via mitosis. The life cycles of some plant parasitic fungi are amazingly complex
with multiple hosts and up to 5 separate spore stages.