SLIME MOLDS

Slime molds are organisms in two taxonomic groups, the cellular slime molds (Phylum Acrasiomycota) and the plasmodial slime molds (Phylum Myxomycota). Organisms in both groups are eukaryotic (meaning that their cells have nuclei) and are fungus-like in appearance during part of their life cycle. For this reason, they were traditionally included in mycology textbooks. However, modern biologists consider both groups to be only distantly related to the fungi. The two groups of slime molds are considered separately below.

Species in the cellular slime mold group are microscopic during most stages of their life cycle, when they exist as haploid (having one copy of each chromosome in the nucleus), single-celled amoebas. The amoebas typically feed on bacteria by engulfing them, in a process known as phagocytosis, and they reproduce by mitosis and fission. Sexual reproduction occurs but is uncommon. Most of what we know about this group is from study of the species Dictyostelium discoideum. When there is a shortage of food, the individual haploid amoebas of a cellular slime mold aggregate into a mass of cells called a pseudoplasmodium. A pseudoplasmodium typically contains many thousands of individual cells. In contrast to the plasmodial slime molds, the individual cells in a pseudoplasmodium maintain their own plasma membranes during aggregation. The migrating amoebas often form beautiful aggregation patterns, which change form over time.

After a pseudoplasmodium has formed, the amoebas continue to aggregate until they form a mound on the ground surface. Then, the mound elongates into a "slug." The slug is typically less than 0.04 in (1 mm) in length and migrates in response to heat, light, and other environmental stimuli.

The slug then develops into a sporocarp, a fruiting body with cells specialized for different functions. A sporocarp typically contains about 100,000 cells. The sporocarp of Dictyostelium is about 0.08 in (2 mm) tall and has cells in a base, stalk, and ball-like cap. The cells in the cap develop into asexual reproductive spores, which germinate to form new amoebas. The different species of cellular slime molds are distinguished by sporocarp morphology.

Dictyostelium discoideum has been favored by many biologists as a model organism for studies of development, biochemistry, and genetics. Aspects of its development are analogous to that of higher organisms, in that a mass of undifferentiated cells develops into a multicellular organism, with different cells specialized for different functions. The development of Dictyostelium is much easier to study in the laboratory than is the development of higher organisms.

A food shortage induces aggregation in Dictyostelium. In aggregation, individual amoebas near the center of a group of amoebas secrete pulses of cAMP (cyclic adenosine-3'5'-monophosphate). The cAMP binds to special receptors on the plasma membranes of nearby amoebas, causing the cells to move toward the cAMP source for about a minute. Then, these amoebas stop moving and in turn secrete cAMP, to induce other more distant amoebas to move toward the developing aggregation. This process continues until a large, undifferentiated mass of cells, the pseudoplasmodium, is formed.

Interestingly, cAMP is also found in higher organisms, including humans. In Dictyostelium and these higher organisms, cAMP activates various biochemical pathways and is synthesized in response to hormones, neurotransmitters, and other stimuli.

The plasmodial slime molds are relatively common in temperate regions and can be found living on decaying plant matter. There are about 400 different species. Depending on the species, the color of the amorphous cell mass, the plasmodium, can be red, yellow, brown, orange, green, or other colors. The color of the plasmodium and the morphology of the reproductive body, the sporocarp, are used to identify the different species.

The plasmodial slime molds are superficially similar to the cellular slime molds. Both have a haploid amoeba phase in when cells feed by phagocytosis, followed by a phase with a large amorphous cell mass, and then a reproductive phase with a stalked fruiting body.

However, the plasmodial slime molds are distinguished from the cellular slime molds by several unique features of their life cycle. First, the germinating spores produce flagellated as well as unflagellated cells. Second, two separate haploid cells fuse to produce a zygote with a diploid nucleus. Third, the zygote develops into a plasmodium, which typically contains many thousands of diploid nuclei, all surrounded by a continuous plasma membrane.

The cytoplasm of the plasmodium moves about within the cell, a process known as cytoplasmic streaming. This is readily visible with a microscope. The function of cytoplasmic streaming is presumably to move nutrients about within the giant cell.

In nature, plasmodial slime molds grow well in wet and humid environments, and under such conditions the plasmodium of some species can be quite large. After a particularly wet spring in Texas in 1973, several residents of a Dallas suburb reported a large, moving, slimy mass, which they termed "the Blob." One reporter in the local press speculated that the Blob was a mutant bacterium, able to take over the earth. Fortunately, a local mycologist soberly identified the Blob as Fuligo septica, a species of plasmodial slime mold.

Another plasmodial slime mold, Physarum polycephalum, is easily grown in the laboratory and is often used by biologists as a model organism for studies of cytoplasmic streaming, biochemistry, and cytology. The plasmodium of this species moves in response to various stimuli, including ultraviolet and blue light. The proteins actin and myosin are involved in this movement. Interestingly, actin and myosin also control the movement of muscles in higher organisms, including humans.