Organismal Biology

Key Concepts

Chapter  9 - Fungi

Basic Structure

Fungi - whether the microscopic yeasts, the bigger molds or even larger, with visible fruiting bodies like mushrooms, are an assembly of thread-like structures called hyphae.  Hyphae first grow out from spores (usually sexually-produced, but not always), branching and surrounding the "landing site" before spreading in every direction that nutrition is available, often cross-connecting between neighbors.  Hyphae extend but do not thicken as they grow;  growth is in the overall network.  It is primarily the tips of the hyphae that secrete digestive enzymes, absorb nutrients, and pass these along.  It is not unusual for the eventual overall structure to be a ring, with the nutrient-poor center broken back down and absorbed.  Fungi are often said to be divided into growth zones:  the outer edge extension zone, with the nutrient-capture productive zone a bit farther in, the innermost aging zone, and the spore-producing fruiting zone.

In some species, hyphae are bundled into rhizomorphs, providing structures that can be extended across spaces.  If you have pulled apart rotting wood, the white threadlike stuff in there is probably a rhizomorph fungus.  Other bundling is used to produce fruiting bodies, mentioned above and discussed in more detail below, and if conditions are particularly bad, tight bundles can produce a sclerotium, a drying-resistant dormancy structure used to wait for improved conditions.

Ecology

Fungi are some of the primary decomposers of the world (along with bacteria), breaking down dead and waste materials to the basic molecules used by producers - they are major recyclers.  They may live on or inside other organisms, taking nutrients from them, as parasites.  There are a number of diseases of humans that are produced by fungi.

A critical function performed by fungi (and some bacteria) is nutrient fixation for plants, where the fungi are symbionts, often embedded in the root tissues of land plants.  Fungi are much better of transforming molecular nitrogen into usable nitrates (which plants can then use to make amino acids), and may be involved in transforming phophorus compounds as well.  Plants very likely would not have been successful in colonizing land without these mycorrhizas.

Some symbiotic fungi may function to defend the partner organism against disease organisms - this is an area of ongoing research.  It isn't yet known whether any common fungi on people do this.

Reproduction

Spores are the main dispersal form to spread a fungus beyond its immediate environment.  These are sometimes produced asexually, in non-specialized structures, but to make fruiting bodies, two genetically-different individual hyphae must come in contact.  The cells in hyphae are haploid,  with only one set of chromosomes.  The hyphae fuse and share cell contents, including nuclei.  The nuclei eventually fuse into diploid nuclei, and inside the fruiting bodies, spores are produced by meiosis; depending upon the total number of chromosomes, this generates variety in the fungi that the spores will germinate into.  Because the new fungi are genetically distinct from the "parents," this is a form of sexual reproduction.

If spores are produced inside a special cell, the asci, this is the basis of the major subgroup Ascomycetes;  if spores develop on projections of special basidia cells, it's the subgroup Basidiomycetes.

Spores may just be released into the air, but some fungi have special "launching" processes to get the spores clear of the potentially-sticky surfaces of the fruiting body.  In most mushroom, the spore has a wet coating almost all of the way around;  in a dry area near where it attaches, a small blob of sugar is produced.  Sugar molecules have a attraction for water molecules, and a drop of water forms and grows there, bending the support structure down.  The drop eventually gets big enough to contact the wet surface of the spore, which makes the drop release;  the spring-back of the support tosses the spore free of the "fins" inside the mushroom cap, so it can drop freely into the air below and get carried away.

Yeasts are a special-case fungus:  they are single-celled, so do not form hyphae, and they reproduce by budding, a reproductive process where the offspring are much smaller than the original.  Slime molds are sometimes thought of as fungi but are actually protozoans, a type of ameba with some odd abilities:  in low-nutrient conditions, they can fuse together as a larger (and more mobile) "giant ameba";  in very bad conditions, they assemble into spore-producing fruiting bodies, which is why they are sometimes thought of as fungi, although their genetics say otherwise.

KEY CONCEPTS -
Click on term to go to it in the text.
Terms are in the order they appear.

 Molds
Mushrooms
Hyphae
Growth Zones
Extension Zone
Productive Zone
Aging Zone
Fruiting Zone
Rhizomorphs
Sclerotium
Fungal Niches
Nutrient Fixation
Symbionts
Mycorrhizas
Reproduction
Spore Production
Fruiting Body
Fungal Subgroups
Ascomycetes / Asci
Basidiomycetes / Basidia
Spore Release
Yeasts
Slime molds