Brian McCauley 12/12/07 Page 1 of 5 08 Fungi Bio 6A/Sanhita Datta Winter 2008  Reading: Chapter 31 in Campbell. By the time you complete this lab, you should: • Understand fungal body plans, including mycelia, hyphae, and yeasts. • Understand the general characteristics of fungal life cycles, including the heterokaryotic stage. • Be able to recognize fungi in a wide variety of forms. • Compare and contrast the fungi with plants and animals in terms of body plan and life cycle. FUNGAL BASICS CELL FEATURES: • Eukaryotes. • Often more than one nucleus per cell. • Nuclei are haploid for most of the life cycle; when diploid nuclei are formed, they immediately undergo meiosis to make more haploid nuclei. • Cell wall present, made of chitin. • Long, thin hyphae or roundish cells (yeasts). BODY PLAN: • Usually multicellular (except for the unicellular yeasts). Little or no differentiation other than reproductive cells. MODE OF NUTRITION: • Heterotrophs; often use extracellular digestion; may consume dead material or be parasites or predators. Fungi are among our closest relatives outside the animal kingdom. Like animals, they are heterotrophs. Their bodies are specialized for absorbing their food, and, like animals, they do much of their digestion outside their bodies. They secrete enzymes outside their cells to break down food so it can be absorbed. However, unlike animals, they don’t swim, walk, or fly to find their food. Fungi move into new food sources by growing or by the passive dispersal of nonmotile spores. The basic body design of fungi is simple, but highly functional: it’s all about absorbing food molecules from the environment. Like plants, fungi have cell walls. However, the cell wall material of fungi is completely different from that of plants. Fungi have cell walls made of chitin, the same tough polysaccharide that makes up insect exoskeletons. Plants have cell walls made of cellulose, a different polysaccharide. Multicellular fungi are typically composed of long, thin filaments called hyphae; the whole body of many hyphae is called a mycelium. Multicellular fungi are always composed of hyphae; even mushrooms, which are the reproductive structures of underground fungi, are composed of these threadlike filaments of cells. Fungal mycelia have enormous surface area for absorption. Also, the hyphae can rapidly grow through soil as they move toward new food sources.Brian McCauley 12/12/07 Page 2 of 6 Some fungi are unicellular; these are called yeasts. Yeasts are described in more detail below. SYSTEMATICS OF FUNGI There are four main phyla of fungi: • Chytridiomycota. These unusual aquatic (and sometimes parasitic) fungi are the only fungi with flagellated reproductive cells. We don’t have any examples in lab. • Zygomycota. Common molds such as Rhizopus, the black bread mold. • Ascomycota. Sac or cup fungi, so called for the saclike form of their reproductive structures. This group includes some edible fungi such as truffles and morels. • Basidiomycota. Club fungi, including most common mushrooms and some others such as the woody shelf fungi that form on trees. In practice, it’s often hard to distinguish the different groups if you’re not looking at their reproductive structures. In fact, even experts have been unable to classify certain fungi in which reproductive structures have not been observed. For that reason, you don’t need to remember the names of the phyla of fungi for the lab test! THE LIFE CYCLES OF FUNGI Fungal life cycles are similar to the life cycles of other eukaryotes: a haploid stage is created by meiosis, and two different haploid individuals join together to make a diploid individual. However, most fungi have one key life cycle stage that is unusual: they form a heterokaryotic stage, which has two genetically different haploid nuclei living in the same cell. An example of a fungal life cycle is shown by the common bread mold Rhizopus: 1) The haploid cells that fuse together are not gametes, they’re just part of the regular mycelium. 2) Hyphae of different mating types (there are no males and females in most fungi) join together and form gametangia, each with several haploid nuclei. This fusion is called plasmogamy. 3) The gametangia fuse, creating a heterokaryotic stage: a cell that has two different types of haploid nuclei (one from each parent fungal mycelium). 4) This heterokaryotic stage forms a tough outer wall and can remain dormant for a long time. 5) When conditions are favorable, pairs of haploid nuclei fuse, forming diploid nuclei. The fusion of these nuclei is called karyogamy. 6) These diploid nuclei immediately undergo meiosis, creating haploid spores.Brian McCauley 12/12/07 Page 3 of 6 7) The spores can then grow into new haploid mycelia. 8) Some of these mycelia undergo sexual reproduction. 9) Some of the mycelia undergo asexual reproduction, making new spores by mitosis. Notice the distinctive features of this life cycle: • There’s no fertilization, in which an egg and a sperm join together. Instead, two haploid hyphae join together. They join in two stages. First, there is plasmogamy, in which the membranes of the two cells fuse. This creates a heterokaryotic cell, with two different kinds of haploid nuclei – one from each parent. The heterokaryotic stage may have multiple nuclei, or just two. The heterokaryotic stage may be short-lived and small, as in Rhizopus, or it may be larger and longer-lived, as in mushrooms. Eventually, two different haploid nuclei in the heterokaryotic stage fuse, creating a diploid nucleus. The fusion of the nuclei is called karyogamy. (In animals, fertilization consists of fusion of an egg cell and a sperm cell, followed by fusion of their nuclei. The term heterokaryotic is not used for animal life cycles, but there is a brief moment, just before egg and sperm nuclei fuse, in which there is a cell with two genetically different haploid nuclei.) • The diploid stage is only a single cell (the zygote), which then undergoes meiosis to return to the haploid state. LIFESTYLES OF THE FUNGI It’s not easy to recognize taxonomic group a fungus belongs to