Re: Do acellular slime molds have sexes or breeding types?

Hello there,

I hope the following adequately answers your question on the slime molds. As is the case with many other acellular slime molds, Physarum polycephalum grows on bacteria and fungi as well as bits of decaying organic material (i.e. it is holozoic). The most commonly-observed form is the plasmodium.

If you wish to view the diagram that goes with the following description go to
http://bic.usuf1.usuhs.mil/Mark/PhysarumPlus.html

1. The plasmodium is the main vegetative phase of the life cycle. Usually diploid, it is a large syncytium (multiple nuclei in a common cytoplasm) that can grow to very large sizes (under laboratory conditions it can be many centimeters in extent). The plasmodium diagrammed here is in the actively migrating stage, "searching" for additional food. Such plasmodia either cease to migrate when they encounter a fresh source or enter one of two other stages.

2. Under certain conditions of starvation and dessication, plasmodia assume a dormant stage called sclerotia. Properly-prepared and -stored sclerotia can be stored for many years and then reactivated by placing small fragments on a moist food source; a favorite such food (for biologists who study plasmodia) is oatmeal flakes.

3. Under conditions of starvation and more moderate environmental stress (such as flooding, high or low pH, etc.) the plasmodium undergoes the process of sporulation. This involves condensation of the plasmodial mass into several focal thickenings, from which aricytoplasmic projectiona that rise several millimeters above the substratum and come to be "topped" by "fruiting bodies". These sporangia divide up into smaller clumps, within which meiotic divisions occur, producing haploid nuclei that become packed as spores.

4. Sporulation involves the rupture of the sporangial mass and the release of spores into the surround. Mechanisms for dispersing such spores are not yet well-studied.

5. Spores are induced to open in environments that have "proper" levels of moisture and nutrients, releasing haploid amoebae.

6. The amoebae that are released from the spore coat are, in most cases, haploid cells that form the gametes of the system. Amoebae can be cultured on solid substrates, with bacteria (live or formalin-killed) as a food source or in suspension culture, with a semi-defined nutrient medium. Amoebae can undergo at least four distinctive stage conversions.

7. Under unfavorable circumstances, such as limited nutrients, dessication, too many neighboring amoebae, etc. the amoebae can form cysts, each of which is a dormant form that is resistant to adverse conditions but can excyst when conditions become more favorable. Encysted amoebae can be stored, at low temperatures, for extended periods of time.

8. When amoebae growing (in the laboratory) on "lawns" of bacteria are immersed in any of a variety of aqueous solutions, they transform into flagellate swimming cells called myxoflagellates or "swarm cells". This amoeboflagellate transformation is rapid and reversible, does not require gene activation or protein synthesis, and involves extensive rearrangement of cytoskeletal elements such as actin filaments and microtubules.

9. Amoebae can also mate (fuse with) other amoebae with complementary mating alleles (6a), thus forming a diploid cell from which a new plasmodium grows up. Certain strains of amoebae have the ability to "self" and create haploid plasmodia.

10. The diploid (or haploid) cells thus formed can be considered uninucleate plasmodia that, upon being cultured, become multinucleate (syncytial) plasmodia.

11. Small plasmodia can be grown on solid substrata with a suitable food source to yield the large plasmodia discussed above (1).

12. Small plasmodia growing on filter paper wet with a semi-defined liquid growth medium can be vigorously shaken and fragmented into microplasmodia, which can be subcultured repeatedly to yield large quantities of microplasmodia grown in suspension.

13. If cultured in liquid medium that is depleted of nutrients (starvation), microplasmodia form another dormant phase, spherules which can be dried by streaking on dry filter paper, stored indefinitely and used to start new shaker cultures of microplasmodia. Microplasmodia can also be fused to form macroplasmodia and then cultured on solid substrata.

http://www.ucmp.berkeley.edu/protista/slimemolds.html

Plasmodial slime molds, like Physarum, are basically enormous single cells with thousands of nuclei. They are formed when individual flagellated cells swarm together and fuse. The result is one large bag of cytoplasm with many diploid nuclei. These "giant cells" have been extremely useful in studies of cytoplasmic streaming (the movement of cell contents) because it is possible to see this happening even under relatively low magnification. In addition, the large size of the slime mold "cell" makes them easier to manipulate than most cells.

A second group, the cellular slime molds, spend most of their lives as separate single-celled amoeboid protists, but upon the release of a chemical signal, the individual cells aggregate into a great swarm. Cellular slime molds are thus of great interest to cell and developmental biologists, because they provide a comparatively simple and easily manipulated system for understanding how cells interact to generate a multicellular organism. There are two groups of cellular slime molds, the Dictyostelida and the Acrasida, which may not be closely related to each other.

A third group, the Labyrinthulomycota or slime nets, are also called "slime molds", but appear to be more closely related to the Chromista, and not relatives of the other "slime mold" groups.

What these three groups have in common is a life cycle that superficially resembles that of the fungi. When conditions become unfavorable, these slime molds form sporangia - clusters of spores, often on the tips of stalks such as in the sporangium of a Physarum shown at right. Spores from the sporangia are dispersed to new habitats, "germinate" into small amoebae, and the life cycle begins again. Similarities in the life cycle do not, however, imply close relationship, especially when one considers that certain bacteria (the myxobacteria) and even an unusual ciliate have very similar life cycles, aggregating to form spores on a sporangium.

Slime molds have almost no fossil record, which is not surprising. Not only do slime molds produce few resistant structures (except for spores, which are often overlooked or unidentifiable), but they live in moist terrestrial habitats, such as on decaying wood or fresh cow dung, where their potential for preservation is low. A few fossil slime molds have been found in amber (Poinar and Waggoner, 1992).

For more information:
Information and links on cellular slime molds is avilable from the Dictyostelium WWW Server at Northwestern University. Available resources include the Cellular Slime Mold Newsletter on gopher.
http://dicty.cmb.nwu.edu/dicty/dicty.html

More information and movies are available at Rice University from Eugenio de Hostos' Dictyostelium Cytoskeleton page.
http://www-bioc.rice.edu/~hostos/science.html

An additional movie showing cytoskeleton dynamics in Dictyostelium is available at the Cells Alive! website.
http://www.cellsalive.com/

For lots of info on the plasmodial slime molds, try the Myxomycetes Home Page or PhysarumPlus. You can find many more images on the Japanese Myxomycetes page, with text in both English and Japanese.
Myxo Web: http://www.wvonline.com/myxo/index.htm
PhysarumPlus: http://bic.usuf1.usuhs.mil/Mark/PhysarumPlus.html

The picture of Physarum is from Tom Volk's plasmodial slime mold image collection at the University of Wisconsin-La Crosse, where you can also find a Quicktime Movie of cytoplasmic streaming in Physarum (but be warned, it's a big file!). The picture of a Physarum sporangium was taken by Henry Aldrich, and kindly provided by PhysarumPlus.
http://www.wisc.edu/botany/fungi/volkmyco.html

June Wingert  RM(ASM)
Center For Comparative Medicine
Comparative Pathology Laboratory
Baylor College of Medicine
One Baylor Plaza
Houston, TX  77030
(713) 798-6591
Fax: (713) 798-8395