15.3: Fungus-Like Organisms

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Myxogastria (formerly Myxomycota): Plasmodial Slime Molds

Myxomycetes (members of the Myxogastria) are fungus-like organisms called slime molds, but they are not members of Kingdom Fungi. In their feeding stage, myxomycetes form one large amoeba with many nuclei and no cell wall. This amoeba moves over damp, decaying material looking for bacteria to engulf and digest. When it dries out or runs out of food, it begins to make fruiting structures called sporangia (sporangium, singular). Inside these sporangia, the myxomycete will undergo meiosis, wall off individual nuclei and make haploid spores for aerial dispersal. Dispersal by spores, heterotrophism, and glycogen as a storage carbohydrate originally classified this group within Kingdom Fungi, but this is the end of the similarities. The spores have cell walls made of cellulose, like plants. When these spores land, they will germinate into haploid cells (called swarm cells) that will fuse together to form a diploid amoeba. This means that, unlike true Fungi, they have a diplontic life cycle.

Observe the slime molds on display. If possible, make a wet mount of a small sample. If there are motile cells (cells that are actively moving around), look for the presence of 2 flagella. This also distinguishes myxomycetes from Kingdom Fungi, where only one flagellum is present. If a mature amoeba is present, can you see cytoplasmic streaming occurring?

Draw what you see and make observations below. Label the name, function, and ploidy of any identifying structures.

Other Slime Molds: Dictyostelia and Protostelia

Quite on theme for fungal classification, there are several different groups of unrelated organisms that are called slime molds. The two mentioned above, Dictyostelia and Protostelia, represent cellular slime molds and are relatively closely related to the plasmodial slime molds. Unlike the plasmodial slime molds, these groups exist as individual amoebae until it is time to leave the area. In the Dictyostelia, the amoebae assemble together to form elaborate fruiting structures where only some of the individuals will become spores and the rest will die. They are studied for this altruistic behavior.

Oomycota -- The Water Molds

Oomycetes are also fungus-like organisms with cell walls made of cellulose. Similar to myxomycetes, they have motile spores with 2 flagella. However, one of these flagella is "normal"-looking (called a whiplash flagellum) and the other is ornamented. This strange characteristic puts organisms into a group called the heterokonts (meaning "different flagella"). Like us, true Fungi are part of the opisthokonts (opisth- meaning rear, -kont meaning flagellum).

Also similar to myxomycetes, oomycetes have a diplontic life cycle. What does this mean?

If supplies are available, place a dead bug into a petri dish with some pond water. Observe the insect under a dissecting scope, then add a cover to your petri dish. Label “Saprolegnia Culture”, your initials, and the date. Set the covered dish aside to incubate until Lab Heterokonts, where you will be learning about heterokonts in more detail.

Contributors and Attributions

Maria Morrow (College of the Redwoods)