Chapter 17 – The Evolution of Plant and Fungal Diversity
The Fungi
17.14 Fungi absorb food after digesting it outside their bodies
· Fungus body structures and mode of reproduction are different from any other organism. They are heterotrophs, but they do not ingest their food. They acquire nutrients by absorption. Fungi secrete enzymes that digest the food outside of the body and then absorb the nutrients into their cells.
· Fungi are found almost everywhere (both in soil and in water) and serve as decomposers in the ecosystems. Most fungi are beneficial, but a few parasitic species exist. Fungi cannot make their own food, but depend on organic molecules from other sources. In some instances, fungi make associations with plant roots; this is called mycorrhizae.
· Most fungi consist of threadlike filaments, hyphae that branch to form the feeding network called the mycelium. Fungal hyphae are surrounded by a cell wall usually made of chitin, a nitrogen containing polysaccharide identical to that found in insect exoskeletons. The hyphae consist of chains separated by cross-walls that have pores large enough to allow ribosomes, mitochondria, and sometimes nuclei to move from cell to cell. Those that lack cross walls have many nuclei within one mass of cytoplasm.
· Mushrooms are tightly packed hyphae that serve as an above ground reproductive structure. Fungi are not mobile so cannot search for food, but the mycelium grows very fast and makes up for any lack in mobility.
17.15 Fungi produce spores in both asexual and sexual life cycles
· Many fungal species can produce both sexually and asexually. Reproduction usually involves the release of haploid spores that can travel by wind or water. Spores land in a moist area with food, germinate, and produce new fungus
· In sexual reproduction, two haploid mycelia of different mating types release sexual signaling molecules, grow toward each other, and fuse. Cytoplasmic fusion is not immediately followed by nuclei fusion and the fungi may enter a heterokaryotic stage in which cells contain two genetically distinct haploid nuclei. Much time may pass before fusion occurs.
· Many fungi, including yeasts and molds, reproduce asexually.
o Mold – any rapidly growing fungus that reproduces asexually by producing spores. They are often furry looking and appear on fruit, bread, or other food.
o Yeasts – any single-celled fungus that reproduces asexually by cell division or budding, pinching off small “buds” from the parent cell. Yeasts inhabit liquid or moist habitats, such as sap or animal tissue.
17.16 Fungi are classified into five groups
· Over 100,000 species of fungus have been described. Sexual reproductive structures are usually used to classify species. Fungi that have no known sexual stage are informally called imperfect fungi. All but one group of fungi lack flagella, which was initially a criterion for being placed in Kingdom Fungi
· Chytrids: The only fungi with flagellated spores. Common in lakes, ponds, and soil. Some are decomposers, others parasitize protists, plants, or animals
· Zygomycetes (zygote fungi) Characterized by resistant zygosporangium, where haploid spores form by meiosis. Includes fast-growing molds, such as black bread mold, and molds that rot produce like peaches, strawberries, and sweet potatoes. Include some species that parasitize animals.
· Glomeromycetes (arbuscular mycorrhizal fungi) Form a distinct mycorrhiza in which hyphae that invade plant roots branch into tiny treelike structures called arbuscules. This is a symbiotic relationship formed by most plants, where the fungus delivers phosphates and other minerals in return for organic nutrients from the plant.
· Ascomycetes (sac fungi) Named for saclike structures called asci that produce spores for sexual reproduction. They live in marine, freshwater and terrestrial habitats. Range in size from unicellular yeasts to multicellular morels and cap fungi. Include some plant pathogens, as well as symbiotes like lichen
· Basidiomycetes (club fungi) Includes mushrooms, puffballs, and shelf fungi. Named for their club shaped, spore producing structure called a basidium. Many species are able to break down lignin in wood and serve as decomposers. Some shelf fungi break down the wood of weak or dying trees, and continue to decompose the wood after the tree dies. Also include some parasitic species called rusts and smuts.
17.17 Fungal groups differ in their life cycles and reproductive structures
· Zygomycetes – as hyphae expand through the food, the fungus reproduces asexually, forming spores at the tips of upright hyphae.
· When food is depleted, the fungus reproduces asexually
· Ascomycetes – reproduce sexually when conditions are suitable and asexually when they are not. Hyphae of two mating types fuse forming a heterokaryotic mycelium that will grow to produce a mushroom.
· In the mushroom are basidia cells that line the gills of the mushroom. Within the gills, haploid nuclei form to make diploid nuclei. Each diploid nucleus undergoes meiosis to form haploid spores, which are released. If the spores land on moist matter they germinate to form haploid mycelia.
17.18 Parasitic fungi harm plants and animals
· Only about 30% of fungi are parasitic, and most of that is in or on plants. Fungi are serious agricultural pests, causing 10%-50% of the world’s fruit to be lost. Species of club fungi, the smuts and rusts, are common on grain crops.
· Some fungi that attack food crops are toxic to humans.
· Ergots infect many seed heads of grains, including rye, wheat and oats. Consumption of flour from infected grain can lead to gangrene, nervous spasms, burning sensations, hallucinations, temporary insanity, or death. In small doses, these ergots can be used medicinally to treat high blood pressure or to stop bleeding after childbirth.
· Only about 50 species of fungus are parasitic to humans. These include athlete’s foot, ringworm, and some deadly lung diseases. Mycosis is the term used for fungal infection. Most fungal infections appear on the skin.
· Systemic mycoses are fungal infections that spread throughout the body, usually from the inhalation of spores. Coccidioidomycosis is the fungus that affects the lungs and is so toxic that it is considered a biological weapon.
17.19 Fungi have enormous ecological benefits
Fungi are essential decomposers, breaking down organic matter and returning it to the environment. If fungi did not decompose, then plants would soon die off from lack of nutrients. Animals would then starve. Other fungi have the ability to break down toxic pollutants, including DDT and some chemicals that cause cancer. It is being investigated to see if fungus can be used to clean up after oil spills.
17.20 Fungi have many practical uses
Fungi have practical uses for humans. We eat the mushrooms, use fungus to ripen cheeses and give them a distinct taste, use yeast to make alcoholic beverages and cause bread to rise, and we use them as medicine. Penicillium is a common mold that is converted into the antibiotic penicillin.
17.21 Lichens are symbiotic associations of fungi and photosynthetic organisms
· Lichens are associations of green algae or cyanobacteria held in a mass of fungal hyphae. In most of these associations, each partner provides something that the other could not obtain on its own. The fungus receives food, while the algae receives habitat, helping it to absorb and retain water and minerals.
· Lichen can live where there is little or no soil, making it ideal for inhabiting new land. Lichens can tolerate extreme cold, and are found in the arctic where they serve as food for caribou.
· Lichens can withstand drought. They are opportunists that will grow in spurts when conditions are favorable.
· When it rains, they absorb water and photosynthesize. When the air is dry, they dehydrate and wait for conditions to improve.
· They cannot, however, withstand air pollution. Since most of their nutrition is in the air, dust or other compounds can kill them.