The only sentences Railsback requires his students to read and understand are the two sentences in red. The rest is for background or general interest only.
Slime molds were once considered one monophyletic group (i.e., all relatively closely related) within the fungi or perhaps within the protozoa (hence the confusing name Mycetozoa or "fungus animals'). The modern view increasingly favors the idea that there are mutliple kinds of slime molds that are not closely related, and in fact some workers would put those different kinds in different phyla. Depending on your viewpoint, there are at least three types or perhaps as many as six types:
Plasmodial (or "true") slime molds (the Myxomycota or Myxogastria or myxomycetes, of which Physarum is an example). These are single-celled amoeboflagellates that combine to make "plasmodia" by merging cells into one huge cell (a bag of cytoplasm at this point) with thousands of nuclei. Different kinds produce different fruiting bodies, the most common of which is some sort of sporangium from which spores are realeased for the next generation. There are about 450 species, which include mobile slime molds. See also the Plasmodiophoromycota (below).
Cellular slime molds (the Dictyostelids or Dictyosteliomycota, of which Dictyostelium is an example). These are amoeboid single-celled organisms that combine to make larger-scale single organisms in which individual cells remain distinct within the "pseudoplasmodium" they have formed. That's why they are "cellular" rather than "plasmodial". There about 50 species, or 65 if one includes the Acrasids (below).
The Protostelia, which are microscopic slime molds. Nematostelium is an example. These were first described in the 1960s and are less well-known than the plasmodial and cellular slime molds. Like plasmodial or "true" slime molds, they form a multinucleate plasmodium.
The Acrasids or Acrasiomycota, which include Fonticula alba. These organisms function like cellular slime molds, but molecular and ultrastructural evidence indicate they are evolutionarily unrelated to the other mycetozoa. They have often been classified with the Dictyostelids within the cellular slime molds, but at least one classification now considers Dictyostelids and Acrasids as separate phyla of cellular slime molds. Dictyostelids are typically soil organisms, whereas Acrasids are found on dead plants, tree bark, dung, and soil. There are 15 species known.
The Labyrinthulomycota or slime nets, which include Labyrinthuloides and Aplanochytrium. These are sometime called "slime molds", but they don't seem to be closely related to other kinds of slime molds, and they form a separate phylum in some classifications. They form a "net-plasmodium" and are mostly if not entirely marine.
The Plasmodiophoromycota. These are "true" or plasmodial slime molds, but differ from the Myxomycota in that the Myxomycota are free-living, whereas the Plasmodiophoromycota are parasitic on plants and fungi. They are considered by some to be an entirely different phylum.
The Myxobacteria, which include Chondromyces apiculatus, Myxococcus fulvus, Myxococcus stipitatus, and Stigmatella aurantiaca. All of the above are eukaryotes, but this group is prokaryotic or bacterial and part of the proteobacteria. They nonetheless go through a life cycle much like that shown above.
For the purposes of GEOL 1122, the significance of slime molds, and particularly of the cellular slime molds, is that they provide an example of how single-celled organisms can become organisms with mutliple cells and with distinct form and structure (like "higher" organisms). For a more scholarly treatment of this idea, see Richard H. Kessin's Dictyostelium - Evolution, Cell Biology, and the Development of Multicellularity (Cambridge University Press, 2001, 308 p.).
Sources used to construct this page include
Olive, L.S., 1975, The Mycetozoans: New York, Academic Press.
Stephenson, S.L., and Stempen, H., Myxomycetes - A handbook of slime molds: Portland, Timber Press.
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