BIOL 3007W: 11/15/10

by Gilbert Keith

THis lecture is on the basal fungal groups: Chytrids, zygomycetes, and glomeromycota. None of them make large fruiting bodies.

chytrid: characterized by anchoring structures.
Zygo = yoke. zygospore is formed.
Arbusculus = small tree. (glomeromycota.)

basidio and ascomycota are the higher fungi. Chytrids appear to be polyphyletic, as do zygomycetes.

Chytrids: typically found in aquatic (freshwater) organisms. They occur as parasites on terrestrial organisms. They are the only fugi with motile cells in their life cycle (posterior, whiplash flagella = opisthokonta.) THey are mostly saprobic, and are important biodegraders. Some of them live in guts of herbivores as cellulose degraders (in which case they are obligate anaerobes. SOme are parasites on algae and other fungi (useful in case of feeding on algal blooms.)

Chytrid baits: chitin , cellulose (cellophane, wood), keratin (eg red hair), pollen grains (pine, spruce, oak) , seeds.
Thallus form: Monocentric thallus (one center with rhizoids.) Inside the host  they do not make rhizoids.  They make a sporangium (center of the picutre) anucleate rhizoids that anchor it. Papilla (where the zoospores emerge from.)

Life cycle of allomyces. allo=different. It is a tropical/subtropical fungus.  One of the best studies chytrids. Starting with the sporophyte: Fusion of two different size gametes. Starts growing by dichotomous branching. The branches have two kinds of sporangia on them. Resistant sporangia are brown. Germinate immediately to produce diploid zoospores. The resistant sporangia lie dormant for many months, so they are released under stress. They produce haploid gametophytes. The gametophytes will produce two kinds of gametangia (make and female.) The female gamete produces sirenin, which will cause male gametes to swim to the females. The AoG is isomorphic.

Chitridiomycosis: parasite of amphibians. (frogs) and has caused mass extinction. The cause is not certain. It attacks the skin of the frog, making the skin thick and inhbiting the ability of frogs to respire.

Zygomycetes: not a monophyletic group. Variable life styles: saprobic, arthropod guts, pathogens. fluffy white growth on bread and strawberries is prime example. They have both sexual and sexual reproduction. The mycelium is coenocytic.

Sexual reproduction involves a zygospore, whereas the asexual reproduction involves non-motile sporangiospores. Sporangia sitting on aerial mycelium. Sterile part = columella.

Mature: protogametangia (two cells that grow towards each other and then swell up.) they each cut off a cell at the tip, called suspensores. We see plasmogamy and karyogamy to produce the zygote, and the zygopore is like a yoke between the two cells,

Mucor: Sticky or slimy sporangia. Don’t get blown away. Insects or water disperse the sporangia.
rhizopus: dry spores. The wall breaks and the wind carries them away.
Thamnidium: Sporangium at the tip have sticky spores. The things down at the bottom are smaller sporangia which are dispersed by the wind.
coemansia: Sporangia have one spore. They are desert organisms. A droplet of water develops around it….
Cunninghamella: like thamidium, but also with 1 spore per sporangium.

Spirodactylon aureum. There ar a lot of spiraly things, which prevent the dispersal by air.
Entomophthora. Destroy flies and stuff. They parasitize flies and other insects. They are ballistospores, so they are shot away. They penetrate the exoskeleton
Pilobolus: Hat throw (the sporangia is the cap.) The sporangia can be shot for over 6 feet. They are adapted to be eaten by animals. They move according to the sun, and shoot at about 9 in the morning. Sticky stuff in the ring at the bottom of the spore.  Great video on pilobolus spore dispersal:

Life cycle of rhizopus: Asexual reproduction by sporangiospores. THey have two mating types (+ and -) There are compounds produced which diffiuse through the agar (and then through the air) to form the gametangia. The gametangia fuse to form the zygosporangium. When the zygosporangium germinates, it forms a sporangium (undergoing meiosis.) The mycelium is haploid, unlike in the oomyces.

Mating types: + and -. Takes two different thalli for mating (heterothallic)

Zygomycetes products: tempeh. Rhizopus oligosporus.

Glomeromycota: They make endomycorrhizae. Within a cell of a root, it looksl ike a root (arbuscule.) The fungus collapses without any obvious impact on the plant cell.

AM vs VAM: VAM: vesicle forms between the cells. It is storage structure that contains nutrients.  Herbaceous and woody plants usually make the arbuscular mycorrhizae.

Glomeromycota are obligate symbionts. They are difficult to grow in culture. They push in the plasma membrane, but the membrane remains intact. The arbuscule is short lived. They penetrate the cell wall, though. We don’t understand the life cycle at all.  They make large spores. Extension of the root system, aggregate the soil, glue together fine soil particles. They aid in phosphate and nitrogen uptake. Defense against parasites.