Characteristics of Fungi - multicellular, eukaryotic, heterotrophic, obtain nutrients by absorption. Fungi digest matter outside of their bodies and then absorb the nutrients.
Symbiosis - Can be mutualistic, saprobes and parasitic.
Habitat - Most are terrestrial; however, there are some species that live in marine and freshwater habitats. There are species in almost every habitat from pole to pole.
Structure -
Hyphae - the vegetative body of fungi. Can be long and thin or bundled and shaped like what we see as mushrooms. The hyphae that span out into a network are the myciliam and the hyphae that make the above substrate mushroom structure are the reproductive structure.
Septa - crosswalls that section of the cells of the hyphae. These walls are mostly made of chitin (a polysaccharide). Some fungi are aseptate in which their hyphae do not have cells. In aseptate hyphae, the nuclei divide without cytokinesis. Hyphae like this are called coenocytic.
Coenocytic hyphae - Nuclei divide similarly to cellular mitosis. The chromosomes line up during metaphase and are pulled apart by spindle fibers in the nucleus. During this, the nuclear envelope is retained. After the chromosomes are pulled toward the poles, the nuclear envelope pinches in two.
Growth & Reproduction -
Growth of fungi is by hyphal extension. The cells grow in search of or ramify throughout a food source. In Coenocytic fungi, the proteins and nutrients are brought to the ends of the hyphae by cytoplasmic streaming. Growth is fast because it is by length and not girth. It is better for fungi not to be too large in diameter because its nutrition source is by absorption.
Reproduction - fungi reproduce by spores. These spores are produced in or on special areas of hyphae. Spores can be produced sexually or asexually. Sexual reproduction s often in response to a changing environmental conditions to increase the chance of offspring surviving this change. Usually spores and hyphal nuclei are haploid and spores can be produced by mitosis. When spores are produced by sexual reproduction, hyphae of different strains (+ & - instead of male & female) connect and although the cytoplasm may join, the nuclei do not. In fungi syngamy is in two stages, plasmogamy (fusion of plasma) and karyogamy (fusion of nuclei). After plasmogamy, the nuclei may not immediately fuse, in which the cell will be dikaryon (2 nuclei). When the nuclei fuse, they immediately undergo meiosis and produce newly recombined haploid spores.
Divisions of Fungi - the three divisions of fungi are zygomycota, ascomycota & basidiomycota. These divisions are usually based on their reproductive structures.
Zygomycetes - it is estimated that there are about 600 species. These are usually terrestrial saprobes that live in soil.
Reproduction - Zygomycete hyphae are usually coenocytic except the reproductive structures, zygosporangia. Hyphae can produce sporangia with haploid spores. These spores can produce hyphae that can go on and produce sporangia. If hyphae of different mating strains fuse, they can undergo plasmogamy and produce a zygosporangium. Karyogamy can then form diploid nuclei that then undergo meiosis and produce spores. The zygosporangia produces a sporangium that releases these haploid spores. Spores can be carried by the wind or be shot out by squirting plasma. Ex.: Rhizopus (black bread mold), Pilobus (dung decomposing fungi).
One important form of Zygomycete is mycorrhiza. Mycorrhizal fungi live mutualistically with most (~90%) of plants. They grow around roots (ectomycorrhiza) are between the cells and into the cell walls (endomycorrhiza), but not into the cytoplasm. These fungi help plants absorb nutrients and water and the plants supply the fungi with carbohydrates. Although all divisions of fungi make mycorrhizal associations many of them are zygomycetes.
Ascomycetes - sac fungi, includes unicellular yeast, make mutualistic association with algae (lichens). Can also form association with plants in which the fungus covers the leaf and releases substances that will repel insects.
Reproduction - the asexual reproduction is the same as zygomycetes, but spores are produced in a club-like asci instead of zygosporangia. The sexual reproductive stage is very similar to zygomyetes also, but two hyphae produce archegonium and antheridium which fuse to produce the ascogonium. The ascogonium grow ascocarp which have several asci (the fruiting bodies). Inside each fruiting body there are 2 nuclei (dikaryotic) which under go karyogamy and then immediately split into four haploid spores by meiotic division. Then the spores undergo mitosis to produce 8 haploid spores. Ex.: Cup fungi.
Basidiomycetes - these are some of the most commonly seen fungi. Most of the mushrooms that are consumed by people are basidiomycetes. Reproduction in basidiomycetes are very similar to that in ascomycetes. The difference is in the structure in which the spores develop and from where they are dispersed. The mating hyphae experience plasmogamy where they become dikaryotic. Basidiomycetes spend most of their lives as dikaryotic organisms. Many of the mushrooms that we see are n+n. On the underside of many mushrooms are gills that contain basidia. The basidia is where the spores will be produced. When environmental conditions warrant it, the nuclei will fuse and immediately undergo meiosis producing 4 spores. (Pg. 590 Amarillo bulbosa).
