Jim's Fungi Discussion Notes - overview

Fungi Discussion Notes

What is the difference between fungi and plants?

(Pass out mushrooms)

· Sketch whole;

· separate stalk and cap;

· try to break the stalk in both directions;

· crumble a piece of the stalk;

· break the cap in two and examine the thin sheets underneath

What is the scope of the Fungi Kingdom?

· molds, yeasts, morels, mushrroms

· but also rust, mildew and lichens

And how do they reproduce? (spores)

Defining characteristics:

· eukaryotic

· all are multicellular except for yeast

· heterotrophs

· cell walls, made of chitin

chitin: complex carbohydrate also found in insect exoskeletons

heterotrophs: depend on other organisms for food

fungi do not ingest their food - digest it externally then absorb it

some feed on decaying organisms, others are parasites

Structure and Function

· Muticellular yeasts are composed of tiny filaments called hyphae;

· Each hyphae is only one cell thick;

· in some, cross walls (septa - plural of septum) divide the hyphae into cells with one or two nuclei;

· the septa have small pores that allow cytoplasm to flow between cells;

· other hyphae lack cross walls and contain many nuclei.

· A mushroom is the fruiting body of a fungus with a mycelium formed from many tangle hyphae in the substrate (below ground).

· The mycelium has a large surface area and is very efficient in absorbing food.

· Clusters of mushrooms are usually part of the same mycelium, and therefore part of the same organism.

· After several years, nutrients near the center are depleted and the fruiting bosies appear around the edges in "fairy rings" (up to 20-30 meters in diameter!)

Reproduction in Fungi

Most fungi reproduce both sexually and asexually.

Asexually:

· when cells or hyphae break off and grow on their own;

· when spores are produced through mitosis;

Spores are sometimes formed in sporangia, which are found at the tip of specialized hyphae called sporangiophores.

Sexually:

· Usually two different mating types (+ and -, rather than male and female).

· When hyphae of opposite mating types meet, each hypha forms a gametangium.

· The two gametangia join and the haploid nuclei fuse to form a diploid nucleus or zygote.

· Meiosis then takes place forming two haploid nuclei, which dominate most of the life cycle.

· In most fungi, the zygote is the only diploid part of the life cycle.

Dispersal (spreading)

Fungal spores are found in most all environments.

Most spores are weightless;

Chances of an individual spore landing in a suitable environment, germinating and growing into a mature organism are miniscule (1:1B).

Other fungi lure animals, such as insects, and pass through their digestive system unharmed.

(scrape spores from under a mushroom cap and view under a microscope)

Classification of Fungi

over 100,000 known species

Four main groups:

· common molds (Zygomycota)

· sac fungi (Ascomycota)

· club fungi (Basidiomycota)

· imperfect fungi (Deuteromycota)

Common molds (Zygomycota)

Zygomycetes grow on meat, cheese and bread.

life cycles include a zygospore, a 2N resting spore in the sexual phase of the life cycle

hyphae lack cross walls except in their reproductive structure

Structure and function of molds:

· bread without preservatives will develop a black mold, Rhizopus stolonifer.

· under magnification, rhizoids penetrate the surface of the bread

· rhizoids anchor the mold, release enzymes, and absorb nutrients

· stolons are horizontal hyphae

· vertical hyphae are sporangiophores, with sporangia at the tip, each with up to 40K spores

Life cycle of molds:

· sexual as described earlier

· thick walls form around the zygospore, which can be dormant for months

· then forms sporangium and releases spores under good conditions

Sac Fungi (Ascomycota)

Ascomycota have a reproductive ascus, which contains spores

30,000 known species (largest phylum of Fungi)

some macroscopic (cup fungi), some microscopic (yeast)

Life cycle:

Asexual reproduction (production of conidia)

· tiny spores called conida form on conidiophores (Greek konis = dust)

· in a suitable environment, a conidium becomes a haploid mycelium

Sexual reproduction (production of ascospores)

· when haploid (+ and -) hyphae are adjacent, gametangia fuse but the haploid nuclei do not

· a fruiting body forms asci, in which the two nuclei fuse to form a diploid zygote

· the zygote divides by meiosis, producing four haploid cells

· in most, mitosis follows, resulting in eight haploid cells (ascospores)

· each has the potential to form a haploid mycelium.

(add a teaspoon of yeast to a half cup of warm water 20 minutes in advance)

Yeasts are unicellular ascomycetes because they form ascospores.

· the dry particles you use contain ascospores which become active with moisture

· they reproduce asexually by budding

· Yeasts used for baking and brewing are of the genus Saccharomyces (sugar fungi)

· they use very little oxygen, and get energy through alcoholic fermentation (producing alcohol and CO2)

Club fungi (Basidiomycota)

Basidiomycetes have a spore-bearing structure called a basidium

Basidia are found on the gills on the underside of the cap

Life cycle of Basidiomycetes:

· basidiospore germinates to produce a haploid primary mycelium, which grows

· mycelia of different mating types fuse to form a secondary mycelium

· the secondary mycelium can grow to hundreds of meters in diameter, making them perhaps the largest organism in the world

· when conditions are right, the fruiting bodies form (mushrooms),

· sometimes they fully form overnight by cell enlargement (water)

· Each gill is lined with basidia

· two nuclei in each basidium fuse to form a zygote cell (2N)

· after meiosis, there are clusters of haploid basidiospores

· mushrooms produce billions of spores, and some puffballs form trillions

Diversity of club fungi

mushrooms

shelf fungi (surface of dead or decaying trees)

puffballs

earthstars

jelly fungi

plant rusts

Edible mushrooms - many are poisonous

The Imperfect Fungi (Deuteromycota)

a varied phylum of fungi not classified in other phyla, mostly because their sexual cycle has never been observed

Penecillium notatum