13 Reading: Fungi
Zygomycota
Fungi in the phylum Zygomycota are called zygomycetes. The zygomycetes are terrestrial. They are usually saprotrophs but there are some parasites. The hyphae are coenocytic (they lack septa). Septa are found only in the reproductive structures.
Reproduction in Zygomycota
Fusion of two hyphae leads to the formation of a zygosporangium, a thick-walled structure that is capable of surviving environmental extremes. Before karyogamy, the zygosporangium contains many haploid nuclei. after karyogamy, it contains many diploid nuclei.
Rhizopus (Bread Mold)
Asexual reproduction involves mycelia producing sporangia that produce haploid spores by mitosis. The spores produce new mycelia.
When environmental conditions deteriorate, sexual reproduction may occur. Hyphae from opposite mating types produce structures that contain several haploid nuclei. Fusion of two of these structures from opposite mating types results in a heterokaryotic zygosporangium. A thick wall develops that functions to protect the zygospore until environmental conditions become favorable. When conditions are favorable, nuclear fusion (karyogamy) occurs within the zygosporangium producing diploid nuclei. This is followed by meiosis. The zygosporangium then germinates to produce a sporangium which releases haploid spores.
Observe Rhizopus (bread mold) growing on a culture dish. Use a dissecting microscope to see details of the hyphae and sporangia. Is there any evidence of sexual reproduction?
Phylum: Ascomycota (Sac Fungi)
Examples: Yeasts, molds, morels, truffles
Ascomycetes are important in digesting resistant materials such as cellulose (found in plant cell walls), lignin (found in wood), and collagen (a connective tissue found in animals). This group also includes many important plant pathogens.
Many, perhaps half of the species of ascomycota form lichens—a symbiotic relationship between a fungus and a photosynthetic cell such as green algae or cyanobacteria. The fungal component of most lichens is an Ascomycete.
Reproduction in Sac Fungi
Sexual
Hyphae from opposite mating types fuse, forming a heterokaryotic structure which then produces dikaryotic hyphae.
The fruiting body is called an ascocarp. It is composed of dikaryotic hyphae and haploid hyphae.
Dikaryotic hyphae within the ascocarp produce asci (singular: ascus), sacs that are walled off from the rest of the hyphae. Nuclear fusion within an ascus will produce a diploid zygote. The zygote will undergo meiosis, followed by mitosis to produce 8 haploid ascospores.
Asci with ascospores can be seen in figure 5.
Asexual
Most reproduction is by asexual spores called conidia. Unlike the zygomycetes, which produce asexual spores within sporangia, conidia are produced on the ends of specialized hyphae called conidiophores.
Examples of Sac Fungi
Morels and truffles are gourmet delicacies. This group includes many important plant parasites such as Dutch elm disease, chestnut blight, leaf curl fungi, and Clavic