Fungi
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Chapter 28 – Fungi
I. Overview
- heterotrophs
- obtain nutrition by absorption
- secrete enzymes to degrade complex molecules
- saprobic fungi – absorb from nonliving material
- parasitic fungi-absorb from living hosts.
- Mutualistic fungi absorb from a living organism but reciprocate.
II. Structure
- vegetative structures are hidden
- hyphae – minute threads of tubular walls surrounding plasma membranes and cytoplasm. Cytoplasm has eukaryotic organelles.
- Mycelium – mat of hyphae that act as a feeding network and are usually subterranean.
- Septa – divide the cell of hyphae. They have pores to allow ribosomes, mitochondria and nuclei to flow.
- Chitin – makes up the cell walls of fungi. Made of a strong flexible nitrogen containing polysaccharide.
- Some hyphae are aseptate – not divided
- Coenocytic fungi results from repeated division of the nuclei without cytoplasmic division.
- Mycelium provides a larger absorption area
- Haustoria are modified hyphal tips used to penetrate the host’s cells
III. Growth and Repair
- grow rapidly because nutrients are able to move between cells.
- Nonmotile
- Chromosomes and nuclei are small.
- Go through mitosis but from prophase to anaphase, the nuclear envelop remains intact.
- After anaphase, the envelope itself pinches in half.
- Produce spores sexually or asexually.
i. Most are unicellular.
ii. Produced in specialized hyphal compartments.
iii. Good conditions means lots of spores
iv. Spread by wind or water
v. Sexual reproduction takes place during harsh times to give genetic variation
- nuclei and spores are haploid
- Sexual reproduction
i. Occurs in two stages
1. Plasmogamy is the fusion of the cytoplasm between the two hyphae
2. karyogamy is the fusion of the nuclei
ii. After plasmogamy, the two nuclei pair forming a dikaryon
iii. This can be maintained for long periods of time
iv. After the nuclei fuse, the immediately go through meiosis
IV. Zygomycota
- Zygote fungi
- Terrestrial-soil or decaying organisms
- Some form mycorrhizae which are mutualistic associations with the roots of plants.
- Coencytic septa are found only where reproductive structures are found.
- Zygosporangia are resistant structure formed during sexual reproduction.
- Example is Rhizopus stolonifer (bread mold)
- See p. 576
V. Ascomycota
- 60,000 species
- sac fungi
- include some main plant pathogens
- examples are yeast, cup fungi, truffles
- some live with algae to form lichens.
- Some form mycorrhizae
- Produce spores in asci
- Sexual stages are seen in ascocarps
- Reproduce by forming multiple, asexual spores at the ends of hyphae
- Naked spores are called conidia
- Have a more extensive dikaryotic stage with the formation of ascocarps
- Plasmogamy-> dikaryotic hyphae -> asci -> karyogamy -> meiosis
- See p. 578
VI. Basidiomycota
- 25,000 species
- mushrooms, shelf- fungi, puff balls, and rusts
- forms a basidium which is a transient diploid stage.
- Decompose wood and other plant material like lignin
- Long lived dikaryotic mycelium
- May reproduce sexually by forming basidiocarps
- Asexual reproduction is less common but forms conidia
- See p. 580
VII. Mold
- -rapidly growing asexually reproducing fungus
- mycelia grow as saprobes or parasites
- early in life – produce asexual spores, later, it may reproduce sexually
- other molds that cannot be classified are called Deuteromycota or imperfect fungi
i. they have no known sexual stages and reproduce asexually by producing spores.
ii. More unusual imperfect fungi are some predatory fungi in soil that trap, kill and consume protists and small animals
- some are sources of antibiotics like penicillin which are ascomycetes.
- Others are used to ferment cheese
VIII. Yeasts
- Unicellular fungi that inhabit liquid or moist habitats
- Reproduce asexually by simple cell division or by pinching off from a parent cell.
- A pink yeast grows on shower curtains and other moist surfaces in the home.
- Others inhabit the moist human epithelial tissue
IX. Lichens
- Symbiotic association of photosynthetic microorganisms held in a mesh of fungal hyphae.
- Usually an ascomycete
- Photosynthetic part made of Cyanobacteria or green algae
- Over 25,000 species
- Fungus gives the lichen its shape and algae are usually found in the inner layer
- Photosynthetic Cyanobacteria fix nitrogen for the fungus and the fungus provides a protected place to live.
- Absorb minerals from dust or rain
- Retain water
- May reproduce sexually
- Soredia may be formed which are small clusters of hyphae with embedded algae.
- Used as the initial growth in both primary and secondary succession
- Tolerate severe cold
- Do not stand up well to air pollution
X. Mycorrhizae
- Mutualistic association of plant roots and fungi
- Increase the absorptive surface of plant roots
- Exchange minerals accumulated from the soil
- Important in natural ecosystem
- 95% of all vascular plants have mycorrhizae
XI. Ecological impacts
- Dominant group of organisms during the Permian extinction
- Principle decomposers
- 10-50% of the world’s fruit harvest is lost to fungal attacks.
- Ethylene, a plant hormone that causes fruit to ripen also stimulates fungi growth
XII. Pathogenic Fungi
- Plants are susceptible to fungal diseases.
i. Dutch elm disease
ii. Some are toxic like Aspergillus, ergot
