Introduction to Fungi
Our lab today focuses on two very unusual groups of organisms, the fungi and the slime molds. Fungi were originally grouped together with algae (as "thallophytes"), partly because most fungi are composed of long slender threads that superficially resemble certain types of algae. Many mycologists thought that fungi were descended from algae or primitive plants that had lost their ability to photosynthesize. The most recent molecular studies, however, reveal that fungi are actually more closely related to animals than to plants.
By now it should be coming apparent that our classification of organisms is often tentative and arbitrary, a best guess based on current information. As a taxonomic souvenir of our earlier hypothesis, fungi are grouped into divisions, a taxonomic term used for plants (the equivalent term for animals is phylum). Many other common botanical terms are used to describe analogous structures in fungi. These superficial similarities may be the result of convergent evolution between fungi and algae.
There are over 100,000 known species of fungi, but the differences between species are not always readily apparent. Subtle differences in biochemistry set many fungal species apart. Fungi are heterotrophic, and many fungi are parasitic. Fungi and other organisms that feed on dead or decaying matter are called detritivores. Many species of fungi are predators, catching their prey with tiny lassos or miniature missiles or toxic chemicals.
Fungi also participate in two important symbiotic relationships. Fungi can form lichens, an association of a fungus with a green algae or a cyanobacteria. Lichens are usually presented as a classic example of mutualism, where each partner benefits from the relationship, but some authorities believe that this relationship might be a form of controlled parasitism. Certain fungi are also symbiotic with the roots or gametophyte stages of many types of plants. These mycorrhizae can grow inside roots (endomycorrhizae), or on the outside of roots (ectomycorrhizae).
Fungal hyphae only form a complete cross wall at the very tip of a hyphae undergoing sexual reproduction. This lack of cross walls may be the secret behind the evolutionary success of this widespread group of organisms. Fungi can move materials like nutrients and proteins back and forth very quickly by cytoplasmic streaming. Fungal digestion is extracellular, with the hyphae secreting powerful enzymes to digest the host tissue, then absorbing the breakdown products through their cell walls. Organisms that feed in such a fashion are called saprobes.
Fungi reproduce by conjugation, a fusion of nuclei analogous to conjugation in bacteria and certain types of algae. The hyphae of two mating strains of fungi (usually referred to as + or -) lie side by side, and each grows a projection toward the other. These projections, called gametangia, meet and fuse together. The intervening cell walls break down, so that nuclei from each strand can then fuse directly into a diploid zygote. This zygote, the only diploid stage in the life cycle of fungi, undergoes meiosis to form four haploid spores, contained in a small sporangia. A spore is a cell that can develop directly into a complete adult haploid organism. Like most spores, fungal spores are enclosed a special protective wrapper that guards against mechanical or chemical damage.
The fusion of nuclei in conjugation is delayed in both ascomycetes and basidiomycetes. The two nuclei continue to lie side by side, reproducing separately by mitosis, until each cellular compartment in the hyphal strand may contain two nuclei. The hyphae of these fungi are called dikaryotic, to distinguish them from monokaryotic hyphae. Fungi can also reproduce asexually, usually by forming groups of long hyphae called conidiophores, which resemble a tiny brush. The tips of these conidiophores fragment into hundreds of tiny haploid spores called conidia.
Ascomycetes reproduce by forming a mushroom-like fruiting body called an ascocarp. Conjugation, with delayed fusion of nuclei, produces dikaryotic hyphal strands. The tips of these strands form cross walls to isolate a tiny sac or ascus. Ascomycetes are often referred to as sac fungi. The two nuclei in the ascus fuse together into a diploid nucleus, which then undergo meiosis to form four haploid ascospores, which then divide again by mitosis to form eight haploid ascospores. Asexual reproduction is accomplished by conidia bearing conidiospores.
Basidiomycetes produce fruiting bodies called basidiocarps. Club-shaped structures called basidia hang from the underside of the mushroom, lining thin flaps of tissue called gills. Within these basidia, nuclear fusion occurs, followed by meiosis to produce four basidiospores. Because of the shape of the basidium, basidiomycetes are sometimes called club fungi.
By focusing on sexual reproduction in classifying fungi, we fell into a taxonomic trap. Many species of fungi have never been observed to undergo sexual reproduction. We lump these species together in the artificial taxon Deuteromycota, the imperfect fungi or fungi imperfecti. Most of these fungi imperfecti are probably ascomycetes.
Kingdom Fungi
Division Zygomycota - molds, Rhizopus (bread mold)
Division Ascomycota - sac fungi (yeasts, morels, truffles)
Division Basidiomycota - club fungi (mushrooms, puffballs, shelf fungi, rusts, smuts)
Division Deuteromycota - fungi imperfecti - Penicillium, Aspergillus
orphan phyla:
Division Myxomycota - plasmodial slime molds, Physarum
Division Acrasiomycota - cellular slime molds, Dictyostelium
Division Zygomycota - molds, Rhizopus (bread mold)
All members of this group form characteristic sexual structures called zygospores. When the mold reproduces sexually, its’ gametangium looks like two little ice cream cones smashed together. The zygote divides by meiosis to form haploid spores.
Asexual reproduction in zygomycetes, like Rhizopus, produces a growth pattern resembling that of the strawberry. Long hyphae called stolons run along the surface of their food, periodically sinking down root-like projections called rhizoids. Long stalks called sporangiophores arise from the stolons, bearing tiny round sporangia, which break open to release spores.
Division Ascomycota - sac fungi (yeasts, morels, truffles, Dutch elm disease, chestnut blight, ergot)
Ascomycetes, often called sac fungi, have a wide range of body forms, from the single-celled yeasts to mushroom-like morels. The mushroom-like fruiting body is called an ascocarp. They reproduce asexually by means of special hyphae called conidiophores, which fragment to produce thousands of tiny spores called conidia. Yeasts form tiny buds that break off and grow into larger cells. Sexual reproduction in this Division involves conjugation, with the two nuclei fusing together at the tip of a hypha to form a nucleated sac called an ascus. Meiosis creates 4 haploid nuclei, which divide again by mitosis to form the characteristic 8 ascospores.
Ergot fungi of the genus Claviceps infests rye bread and other grains. All by itself, this little fungus has altered the course of human history in areas like Russia where it is widespread. People eating infected rye bread were thought to have been possessed by the devil, because of their wild dancing and uncontrolled behavior. We know now that the chemical causing this behavior is none other than LSD. Mary Matossian wrote a wonderful book about the effects of ergot and other fungi on human affairs, called Poisons of the Past: Molds, Epidemics and History, (Yale UP, 1989). She also argues that ergot poisoning may have been behind the Salem witch trials!
Division Basidiomycota - club fungi (mushrooms, puffballs, shelf fungi, rusts, smuts)
Basidiomycetes, often called club fungi, form a basidiocarp, the fruiting body commonly called a mushroom. The underside of the mushroom cap is filled with thin plates called gills, which they superficially resemble. These gills hold club-shaped sexual reproductive structure called basidia (-ium). The nuclei inside the basidia fuse to form a 2N zygote, which undergoes meiosis to form 4 haploid basidiospores, which appear at the tips of the basidia. Asexual reproduction is very rare in this group.
Most of the body of a basidiomycete is actually growing under the ground. Because the hyphae of basidiomycetes grow at roughly equal rates from the center of growth, when the hyphae emerge from the ground as the fruiting bodies we call mushrooms, the mushrooms often appear in a large circular ring. These rings of mushrooms are called fairy rings, and in simpler times they were thought to be magical places, where the fairies came to dance at night. After a heavy rain, you can see these fairy rings in yards and parks all over the city.
Division Deuteromycota - fungi imperfecti - Penicillium, Aspergillus
This “catch-all category” contains many species of fungi which have never been observed to reproduce sexually, and cannot, therefore, be classed in the other three Divisions, although most of these are probably different species of ascomycetes. They reproduce asexually in a manner similar to ascomycetes, with hyphae called conidiophores fragmenting into numerous conidiospores. Penicillin and Aspergillus are typical fungi imperfecti.
Examine the zygomycete Rhizopus on display. There are two Rhizopus plates to examine. The first shows a normal culture of Rhizopus, such as you might find lurking on a slice of bread or other food in the back of the fridge. Find and identify the stolons, rhizoids, sporangiophore and sporangia. The second plate shows two mating strains of Rhizopus that have produced several zygospores.
Examine slides of Rhizopus to get a closer look at these reproductive structures.
Examine the ascomycetes on display. Note the warty appearance of the ascocarp of the morel, a common edible ascomycete.
Examine ls slides of the ascomycete Peziza, called cup fungi. Look for the ascus and ascospores at the top of the cup.
Examine slides of the yeast Saccharomyces. Yeast is a unicellular fungi, a curious ascomycete that usually reproduces asexually by budding, forming a new cell that develops as a smaller bud attached to the parent cell.
Examine the basidiomycetes on display. Shelf fungi are often found on dead and decaying trees.
Observe the sliced edible mushrooms. Notice the way the gills hang down below the cap.
Examine cs slides of Coprinus, a typical basidiomycete. Observe the basidia and basidiospores located on the sides of the gills.
Examine the deuteromycete Arthrobotrys. This curious fungus catches live nematodes (little worms), by constricting little loops formed by the hyphae, much like cowboys use a lasso to rope an errant steer.
Examine the deuteromycete Penicillium. You often see this fungus growing on stale bread and cheese. It is the source of the antibiotic penicillin.
Examine the deuteromycete Aspergillus. Some species of this deuteromycete are used in the manufacture of soy sauce. This organism typifies the confusion that can arise in fungal taxonomy. Many mycologists classify Aspergillus as an ascomycete.
Examine slides of Penicillium. Note the conidiophores
and conidia.
Examine slides of mycorrhizae. Why is this symbiotic relationship so important?
Examine the lichens on display. Notice that lichens come in three basic forms: crustose (flat crust, often seen on rocks), foliose (leaf like), and fruticose (highly branched).
Examine slides of the lichen thallus (body). Notice the algal cells toward the top, and the tangle of fungal hyphae below.
Examine slides of mycorrhizae. Observe the fungal hyphae winding through the root tissues. The fungus can grow much faster than the roots of the plants it interacts with, and can obtain nutrients from a large area of soil. It funnels these nutrients back to the plant, in exchange for some of the stored food in the root.
Know the general life cycle of fungi. The life cycle of Rhizopus, will serve as typical fungal life cycle. Compare and contrast the special structures unique to sexual reproduction in each division of fungi.
Many fungi are edible, including gourmet fungi like morels and truffles.
Without fungi we would have no
soy
sauce, no fermented tofu (big deal!), no saki, no soy sauce, no beer,
no wine,
no bread, no cheese, and therefore (gasp) no pizza!!
Fungi cause many diseases, such as athlete's foot, yeast infections, ring worm, and histoplasmosis (lung disease). Fungi are also the source of many antibiotics, including penicillin.
Fungal rusts and smuts are major agricultural pests.
Many plants rely on symbiotic root fungi (mycorrhizae) for healthy growth.
Lichens are an important food for tundra animals like reindeer.
Ergot fungi are ascomycetes that produce LSD, a strong hallucinogen. This fungus, a type of ascomycete, changed the history of the Russian empire, and may have been the root cause of the Salem witch trials.
Slime molds as sometimes referred to as "orphan phyla", sometimes treated as a Division of fungi, sometimes classified as a weird type of colonial protist. Given the immense amount of material in the protist lab, we've elected to study them beside the fungi which they superficially resemble. Slime molds can be either plasmodial slime molds, like Physarum, or cellular slime molds like Dictyostelium.
The Acrasiomycota, or cellular slime molds, spend much of their lives as little creeping amoeboid cells, feeding on decaying vegetation. When the local food supply begins to run dry, a chemical signal goes out to reproduce, and they swarm together, climbing up over one another to form a slug-like body. This mass of cells even leaves a trail of slime behind as it moves, like a real slug. It develops a tiny stalk, with a sporangium on top in which spores develop. New amoeba emerge from the dispersed spores.
The Myxomycota, or plasmodial slime molds, are basically similar to the cellular slime molds, but have a far more complex life cycle. The feeding stage, or plasmodium, has many nuclei inside a network of cytoplasm. If you look closely you might see cytoplasmic streaming, the constant back and forth flow of the slime mold's cytoplasm, which is thought to circulate oxygen and food throughout the body. Slime molds are a wonderful example of what happens when you try to pin neat little labels like plant, or animal, or fungi on the incredible diversity of living things!!
Mycorrhizal fungi were vital to the successful invasion of the land surface by primitive plants. Why? (Hint: What was the surface of the land like before the first plants or animals?)
The largest organisms on Earth are not blue whales or dinosaurs, but basidiomycetes. The current record holder is a single basidiomycete, underlying 37 acres of a Montana conifer forest. It is estimated to be 1,500 years old, and weighs about 10,000 kilograms (22,000 pounds!!). How do we know that every mushroom in the woods is a fruiting body of the same fungus? (Hint: what can a study of genetics tell us about these mushrooms?)
You'll find a well-referenced guide to the latest discoveries about these curious creatures at the Tree of Life: