Chapter 31

Describe three commercial roles played by fungi.

1. Most people eat mushrooms, the fruiting bodies (basidiocarps) of subterranean fungi. 2. Yeasts are very important in food production. Humans have used yeasts to produce alcoholic beverages and make bread rise for thousands of years. 3. Contributing to medicine, some fungi produce antibiotics used to treat bacterial diseases.

Explain the significance of the reduced mitochondria of the microsporidia.

A 2006 analysis of DNA sequence data from six genes in nearly 200 fungal species indicates that microsporidia are members of an early-diverging lineage of fungi.

Describe the role of fungi as agricultural pests.

About 30% of the 100,000 known species of fungi are pathogens or parasites, mostly of plants.

Describe an animal-fungi mutualistic symbiosis.

Anaerobic chytrids that live in the digestive tracts of sheep and cattle help break down plant matter.

Describe the life cycle of the bread mold, Neurospora crassa. Explain how this organism has played an important role in biological research.

Ascomycetes reproduce asexually by producing enormous numbers of asexual spores called conidia. ----Conidia develop in long chains or clusters at the tips of specialized hyphae called conidiophores. Once released, they are dispersed by the wind. In Neurospora, conidia may also be involved in sexual reproduction, fusing with hyphae from a mycelium of a different mating type. In the 1930s, biologists used Neurospora to formulate the one gene-one enzyme hypothesis.

Explain the possible significance of the flagellated spores of chytrid fungi.

Chytrids are unique among fungi in having flagellated spores, called zoospores. Molecular evidence supports the hypothesis that chytrids diverged early in fungal evolution.

Describe the evidence that multicellularity evolved independently in fungi and animals.

DNA sequence data also indicate that fungi are more closely related to several groups of single-celled protists than to animals, suggesting that the ancestor of fungi was unicellular. ----One such single-celled group, the nucleariids, consists of amoebae that feed on algae and bacteria. Animals are more closely related to a different group of protists, the choanoflagellates.

Explain how fungi acquire their nutrients.

Decomposer fungi break down and absorb nutrients from nonliving organisms. Parasitic fungi absorb nutrients from the cells of living hosts. Mutualistic fungi also absorb nutrients from a host organism, but they reciprocate with functions that benefit their partner in some way.

Distinguish between ectomycorrhizal fungi and arbuscular mycorrhizal fungi.

Ectomycorrhizal fungi form sheaths of hyphae over the surface of the plant root and grow into the extracellular spaces of the root cortex. Arbuscular mycorrhizal fungi extend their branching hyphae through the root cell wall and into tubes formed by invagination of the root cell membrane.

Describe some of the roles of fungi in ecosystems.

Fungi are important decomposers of organic material, including the cellulose and lignin of plant cell walls. Fungi form symbiotic relationships with plants, algae, cyanobacteria, and animals.

Define mycosis, and describe some human mycoses.

General term for a fungal infection. 1. Coccidiodomycosis is a systemic mycosis that produces tuberculosis-like symptoms in the lungs. 2. Candida albicans is a normal inhabitant of moist epithelia such as human vaginal lining, but it can become an opportunistic pathogen, leading to so-called "yeast infections."

Explain the significance of heterokaryotic stages in fungal life cycles.

In some species, heterokaryotic mycelia become mosaics, with different nuclei remaining in separate parts of the same mycelium or mingling and even exchanging chromosomes and genes.

Describe the structure of a lichen. Explain the roles of the fungal component of the lichen.

Lichens are symbiotic associations of millions of photosynthetic microorganisms held in a mass of fungal hyphae The fungal component is commonly an ascomycete, but one glomeromycete and 75 basidiomycete lichens are known. The photosynthetic partners are usually unicellular or filamentous green algae or cyanobacteria.

List the characteristics that distinguish fungi from members of other multicellular kingdoms.

Members of the kingdom Fungi are eukaryotes, meaning they have complex cells with a nucleus and organelles. Most are multicellular, with the exception of single-celled yeast.

Explain why glomeromycete fungi are ecologically important.

Nearly all glomeromycetes form arbuscular mycorrhize. ----The tips of the hyphae that push into plant root cells branch into tiny treelike structures known as arbuscles. ----Such mutualistic partnerships with glomeromycetes are present in 90% of all plants.

Describe the evidence that suggests that Fungi and Animalia are sister kingdoms

Phylogenetic systematics suggests that fungi evolved from a unicellular, flagellated protist. DNA sequence data indicate that fungi, animals, and their protist relatives form a clade called opisthokonts, a name that refers to the posterior (opistho-) location of the flagellum.

Describe the processes of plasmogamy and karyogamy in fungi.

Plasamogamy is the union of the cytoplasm of the two parent mycelia. Haryogamy is the the fusion of haploid nuclei contributed by two parents, occurs well after plasmogamy, the cytoplasmic fusion of cells from the two parents.

Describe the life cycle of the black bread mold, Rhizopus stolonifer.

The hyphae are coenocytic, with septa found only where reproductive cells are formed. ----Horizontal hyphae spread out over food, penetrate it, and digest nutrients. In the asexual phase, hundreds of haploid spores develop in sporangia at the tips of upright hyphae. ----Within each sporangium, hundreds of haploid spores develop and are dispersed through the air. ----Spores that happen to land on moist food germinate, growing into new mycelia. Plasmogamy of opposite mating types produces a zygosporangium within which karyogamy and then meiosis occur. Although the zygosporangium represents the zygote (2n) stage in the life cycle, it is not a zygote in the sense of a cell with one diploid nucleus. Zygosporangia are resistant to freezing and drying and are metabolically inactive. ----When conditions improve, a zygosporangium undergoes meiosis, germinates into a sporangium, and releases genetically diverse haploid spores that colonize new substrates.

Explain how lichens may act as pioneers on newly burned soil or volcanic rock.

The lichen acids penetrate the outer crystals of rocks and help break down the rock. ----This breakdown allows the trapping of soil and starts the process of succession.

Describe the basic body plan of a fungus.

The vegetative bodies of multicellular fungi are constructed of tiny filaments called hyphae that form an interwoven mat called a mycelium that infiltrates the material on which the fungus feeds. Fungal hyphae have cell walls built mainly of chitin, a strong but flexible nitrogen-containing polysaccharide identical to that found in arthropods. Most fungi are multicellular with hyphae divided into cells by cross walls, or septa. Coenocytic fungi that lack septa consist of a continuous cytoplasmic mass with hundreds or thousands of nuclei. Some parasitic fungi have specialized hyphae called haustoria, nutrient-absorbing hyphal tips that penetrate the tissues of their host.