The Major Groups of Fungi Help (page 2)
Introduction to The Major Groups of Fungi
More than 100,000 different species of fungi have been identified, yet mycologists (my- KAHL -uh- jists ) – “those who specialize in the study of fungi” ( myc ) – usually group all of these species into just four different phyla. The names of these phyla are very tongue-twisting! Nevertheless, a helpful hint is that each phylum name ends with the suffix, - mycetes (my- SEE -teez), which comes from the Greek for “fungus” ( mycet ). So, look for and recognize -mycetes, and you know a fungus is somehow involved!
Phylum Basidiomycetes (the Club Fungi)
A good fungal phylum to start with is the Phylum Basidiomycetes (bah- sid -e-oh-my- SEE -teez). The Phylum Basidiomycetes includes mushrooms, puffballs, and the shelf fungi often seen growing on the barks of trees. This particular fungal phylum, then, is quite familiar to those of us who like to walk through woods and fields (or even stroll through our own backyard).
The mushrooms get the first part of their phylum name from their basidia (bah- SID -e-uh) – the “little bases” ( basidi ) growing out from the gills under their caps (see Figure 8.3). This group is therefore called the Basidiomycetes (“little base fungi”) or club fungi because of the tiny, “club”-shaped basidia present along the edges of their gills. The basidia are important because they produce the mushroom’s spores, which can be blown off from under the cap and distributed long distances by the wind.
Phylum Ascomycetes (the Sac Fungi)
You might also encounter various members of the Phylum Ascomycetes ( as -koh-my- SEE -teez) or “leather bag” (asco-) “fungi” as you walk through a forest. This group is commonly called the sac fungi, because they produce spores within their sac-like or bag-like caps. Consider, for instance, the very popular morel mushroom, often nicknamed spongy-cap due to the wrinkled, sponge-like appearance of its cap.
Much less dramatic members of Ascomycetes, however, are the unicellular yeasts. Yeast cells reproduce by smaller cells budding off and eventually separating from larger ones. Yeast belonging to the genus Saccharomyces ( sak -ah-roh- MY -seez) are the main “sugar” (sacchar) “fungi” (myc). This genus of yeasts has been used to help make bread, beer, and wine since ancient times. The yeast cells are added to raw bread dough or a vat of grape juice, and they soon begin to consume lots of sugar molecules in the dough or juice. Initially, these Saccharomyces organisms operate aerobically, utilizing oxygen. The yeast cells give off CO 2 gas as a waste product, and these gas bubbles are what make bread dough rise! Later, when the bread is finally baked, the yeast cells are killed by the high temperature of the oven.
During winemaking, in contrast, the sealed wine vats become anaerobic, after the active yeast cells consume all available O 2 . The yeast cells then switch their metabolism from aerobic to anaerobic, and carry out the process of alcoholic fermentation ( fer -men- TAY -shun). By this process, yeast cells produce both carbon dioxide and ethyl alcohol (common drinking alcohol) as waste products. When the fermenting wine reaches an alcohol content of about 12–16%, the yeast cells become poisoned and die from too much alcohol!
Some common types of molds are also considered members of the Phylum Ascomycetes (sac fungi). In general, a mold is a fuzzy coating of fungus growing on the surface of some food or animal or plant substances, when they are decaying or left for too long in a moist, warm place. A familiar example of such a fuzzy mold is the genus Penicillium ( pen -ih- SIL -e-um), named from the Latin for “brush.” The Penicillium fungi are bluish-colored molds growing on bread, fruits, and cheeses. They have a somewhat brushlike appearance under the light microscope, and several Penicillium species produce penicillin , the powerful, bacteria-killing, antibiotic drug.
Phylum Zygomycetes (the Zygote Fungi)
A third group of fungi are those of the Phylum Zygomycetes ( zeye -go-my- SEE -teez), literally the “yoked together” ( zygo -) “fungi.” This phylum of fungi derives its name from the inclusion of a zygote ( ZEYE -goat) within its life cycle. A zygote consists of two sex cells, called gametes ( GAM -eats), that are literally fused, yoked, or “married” ( gamet ) together during fertilization.
The so-called zygote fungi in this group utilize sexual reproduction – the fusion of two gametes together to create a zygote. Once formed, the zygote then divides repeatedly by the process of mitosis (Chapter 5), thereby creating a new adult organism. The zygotes are enclosed in thick-walled zygospores ( ZEYE -go-spoors), which are shed and sent wafting out through the air to new locations.
A typical zygomycetic ( zeye -go-my- SEE -tick) fungus is the black bread mold, genus Rhizopus ( RYE -zuh-pus) – “root” ( rhiz ) “feet” ( pus ). Interestingly enough, like many fungi, the Rhizopus group can engage in both sexual reproduction (gametes uniting to form zygotes), as well as asexual ( AY - sex -you-al) reproduction that occurs “without” ( a -) gametes. For simplicity, just the asexual reproduction of the black bread mold is displayed in Figure 8.4.
A spore lands on a piece of white bread, then begins to germinate ( JER -muh- nayt ). The landed spore “sprouts” a number of slender, thread-like hyphae, which soon merge to form a white, extensively branching mycelium, deep inside the bread slice. Soon, a large number of round-topped sporangia (spoh- RAN -jee-ah) or “seed” (spor) “vessels” (angi), appear like tiny black puffballs. The round, black sporangia are attached to the bread surface by long, narrow stalks. The mold is called Rhizopoda, because the sporangia and their stalks are firmly anchored into the bread surface by means of rhizoids ( RYE -zoyds) – blunt, “root-resembling” or “foot-resembling” projections. When the wind blows, masses of black spores are scattered from the rounded sporangia, which hold thousands of them. A spore may land on another piece of bread, and the life cycle of the mold begins anew.
Phylum Chytridiomycetes and Fungi Summary
Phylum Chytridiomycetes (water-dwelling Fungi)
A fourth phylum of fungi is the Phylum Chytridiomycetes ( KIH -trid-e-oh-my- SEE -teez) or “water-dwelling” (chytrid) “fungi.” The members of this group with an extremely tongue-twisting name are more easily called chytrids ( KIT -rids), because they are mainly “water-dwellers.”
Recent evidence suggests that the chytrids are the most primitive fungi, and that they were the first type to evolve from protists having flagella. Dwelling in an aqueous (watery) environment, each chytrid has a small, globe-shaped body. It produces a highly active spore which has a flagellum attached. In their adult stage, the chytrids make a large mycelium (network) consisting of an extensive tangle of slender hyphae. These highly branched hyphae create a large area for the easy absorption of nutrients dissolved in the surrounding water.
Summary Diagram of the Fungi
Figure 8.5 provides summary pictures and brief descriptions of each of the four major phyla of fungi we have been studying.
Practice problems for these concepts can be found at: The Fungi: Not Just Mushrooms Test