Fungi are eukaryotic, almost entirely multicellular (with yeast, e.g. Saccharomyces cerviseae, being a prominent unicellular fungus), heterotrophic (deriving their energy from another organism, whether alive or dead), and usually having some cells with two nuclei (dikaryon), in contrast with the more common uninucleate cell. Ecologically this kingdom is important (along with many bacteria) as decomposers and recyclers of nutrients. Economically, the Fungi provide us with food (mushrooms; Bleu cheese/Roquefort cheese; baking and brewing), antibiotics (the first of the wonder drugs, penicillin, was isolated from the fungus Penicillium notatum), and crop parasites (doing several million dollars of damage annually).
Evolution of multicellular eukaryotes increased the size and complexity of organisms, allowing them to exploit the terrestrial habitat. Fungi first evolved in water but made the transition to land through the development of specialized structures (e.g. cell walls of chitin and resistant spores) that prevented desiccation. Prolific asexual and sexual spore production enables the Fungi to disperse by wind, water and animals, rapidly colonize a food source and complete their life cycles, sometimes within only a few hours under favorable conditions (e.g. optimal temperature and moisture).
Body Plans and Nutritional Modes
The kingdom Fungi includes unicellular, multinucleate, and multicellular forms.They are classified on the basis of their sexual recombination, formation of spores and the nature of their multinucleate hyphae (elongated cells) and body of multicellular filaments (connected hyphae) known as a mycelium (plural, mycelia). Fungal cells have highly resistant (e.g. to desiccation) cell walls containing the complex polysaccharide, chitin.
Fungi are absorptive heterotrophs. They breakdown food by secreting digestive enzymes onto a substrates and then absorbing the resulting small food molecules. Fungal hyphae have a small volume but large surface area, enhancing the fungal absorptive capacity.
Most fungi are saprotrophic, i.e. obtaining their food from the decaying bodies of plants and animals. Other fungi are parasites causing disease. Many are mutualistic symbionts with the roots of plants, called mycorrhizae, or form commensalistic associations with algae or cyanobacteria, called lichens.
Fungi are important both as a source of food and in the preparation of food. Edible fungi include mushrooms, truffles, and morels. Cheeses such as Gorgonzola, Roquefort, Stilton, and bleu have fungal colonies that give these cheeses their distinctive flavors.
Beer and wine are produced through the action of fungi known as yeasts, such as Saccharomyces cerevisae. Many antibiotics are produced by fungi (e.g. Penicillium notatum). Fungi also are important crop parasites, causing loss of food plants, spoilage of food and some infectious diseases.
Classification of Fungi
The taxonomy and systematics of the Fungi is currently undergoing revision. Over 60,000 species of fungi are known. Fungi have been classified traditionally by their cell structure method of reproduction (both sexual and asexual). Historically they have been divided into five taxonomic divisions: Chytridiomycota, Zygomycota, Ascomycota, Basidiomycota, and Deuteromycota. These taxonomic categories will likely be modified as new molecular evidence of relationships within and among them is discovered.
The phylum Chytridiomycota, the chytrids, represents a group of primitive aquatic fungi. They are characterized by having gametes that are motile by means of flagellum. Chytrids are aquatic, not terrestrial as are almost all other fungi. They thus may provide us with a glimpse of the earliest fungi, both in terms of ecological requirements and morphology. This division also has quite a diverse array of body plans and habitats.
The zygomycetes, in phylum Zygomycota, are characterized by the formation of sexual spores called zygospores. The zygospores are contained within a specialized structure called a zygosporangium. Zygospores form when the haploid nuclei at the ends of two hyphae fuse together in a process of fertilization to form a diploid zygote. The zygote immediately undergoes meiosis in an elevated structure called a sporangium to form haploid cells that develop into zygospores. The zygospores are highly resistant to desiccation and may be dormant for years until a suitable substrate is available for rapid asexual growth and haploid spore dispersal. An example of a zygomycete is the common black bread mold, Rhizopus nigricans.
The ascomycetes are members of the phylum Ascomycota. Large species often form a cup-shaped fruiting body. They are also called the “sac fungi” because their sexual spores (ascospores) are enclosed in tube-like sacs known as asci. The formation of ascospores is similar to that of zygospores, except that the ascospores (usually eight) formed by meiosis are enclosed in the asci. Neurospora crassa is an ascomycete mold that was important in studies of genetic linkage.
Basidiomycetes, also known as the “club fungi,” are classified in phylum Basidiomycota. Their sexual spores, or basidiospores, are formed on complex fruiting body structures called basidia. Basidiomycetes include some of the most complex fungi, including large, mushrooms and puffballs, and microscopic wheat rusts.
The group known as deuteromycetes are also called the “imperfect fungi.” This group contains species for which no sexual stage has been discovered. At one time, many of the parasitic fungi were classified into this group. However, many of these species have recently been reclassified into one of the other phyla when the sexual stage was discovered. An example of a deuteromycete is Candida albicans, a dimorphic fungus responsible for disease in humans.
Draw the sexual life cycles of different groups of fungi (excluding Deuteromycota). You can either use the charts provided in the lab or your textbook (pages 660-661). Make sure you label all the parts.
2. Examine the Rhizopus growing on the bread. Note the white mycelium, numerous black fruiting heads, and the mature sporangium. Draw and labeled all the parts.
3. After preparing a wet-mount from the yeast culture, examine it under high power and record and describe the form, size, color, and content of the yeast cell. Make sure you find the “bud”.
4. Examine a mushroom fruiting body and draw the gills, then from the prepared slide showing cuts through the mushroom gills (Coprinus) find and draw the Basidiospores and Basidium.
Mushroom & gills Coprinus
5. Using the stereomicroscope exam the Lichen types demonstrations. Draw and label the Physcia slides. Finally, write a paragraph explaining what Lichen is.
6. Examine the Penicillium growing on Micrococcus lutenus and the Penicillium slide. From the slide find and draw the spore containing structures.
Post lab questions (answer on a separate sheet and attach to the lab report)
Do the hyphae of zygomycetes contain cross walls (septa)?
How would you describe the hyphae of zygomycetes?
Is zygospore the results of sexual or asexual reproduction?
Zygomycetes are sometimes called the “conjugation fungi.” Why would this be a fitting term for this group?
Describe the reproduction in yeasts.
Describe the hyphae of basidiomycetes.
Are Basidiospores the result of asexual or sexual reproduction?
In general terms, compare asci and basidia with respect to the roles of meiosis and mitosis in their development.
Why are Deuteromycota referred to as “imperfect fungi”?
First Land Plants
OBJECTIVES: Be able to identify gross and microscopic specimens as mosses and liverworts. Be able to identify reproductive structures and explain how all parts connect to the processes and phases of the life cycles. Also be able to compare and contrast the characteristics of mosses and liverworts with those of their ancestors and with those of the vascular plants.
All land plants, including bryophytes, have multicellular tissues, alternation of generation life cycles, cellulose cell walls, and store starch for energy. Except for a very few parasitic angiosperms, all are photosynthetic (autotrophic). They all evolved on land, but a few ferns and angiosperms have secondarily returned to aquatic or marine habitats. Vascular plants (next week’s lab) have true xylem and phloem and other characteristics biologists consider more advanced. The bryophytes are smaller and lacking true roots and stems and leaves, but they are more ancient.
Three separate phyla are included in the bryophyte group. These include the Phylum Hepatophyta (liverworts); Phylum Anthocerophyta (hornworts); and Phylum Bryophyta (mosses). Bryophytes lack vascular tissues, xylem and phloem, that are found in other plants. Therefore, these plants do not grow very large and require damp environments.
Bryophytes (and other plants) demonstrate ‘alternation of generations” (figure 28.4 page 614 in your text) The gametophyte generation is the dominate generation in all three phyla. Gametagia may be female (archegonia) or male (antheridia).
1. Draw the Polytrichum life cycle and label all the parts. Include the scale of your drawing (i.e. the size of your drawing relative to the actual size of the object, e.g. 0.25x).
2. Examine the bryophyte specimens (moss-types, liverworts type) and write their scientific name. You may be required to correctly identify these on the lab exam.
3. Obtain the Mnium slide and draw the antheridial head and archegonial head. Make sure you label all the parts and include the scale of your drawing.
4. Obtain the Marchantia slide and draw the antheridia and archegonia. Make sure you label all the parts and include the scale.