Cellular slime molds

Categories: Microorganisms; molds; Protista; taxonomic groups; water-related life

Although once thought to be fungi, the protists of the phylum Dictyosteliomycota actually have more in common with the paramecium or amoeba that can be observed in a drop of pond water when viewed under the microscope than they do with mushrooms and toadstools. Cellular slime molds are essentially microscopic throughout their entire life cycle, and only rarely can they be observed directly in nature, as is the case for the plasmodial slime molds. Cellular slime molds must therefore be grown under controlled laboratory conditions in order to be studied.

Life Cycle

Since their discovery in the late nineteenth century, cellular slime molds have intrigued biologists. Their life cycle exhibits a curious alternative to the way in which most other creatures on earth grow, develop, and become multicellular, with different specialized tissues produced as a result of the process. Most plants and animals begin life as a single cell (called a zygote) that is the product of the fusion of an egg cell and sperm cell. Shortly after the two cells fuse (through a process termed fertilization), the zygote divides into two cells that stick together. These cells soon divide again to produce a cluster of four cells that in turn divide, and so on. Within hours or days (depending upon the particular plant or animal), clusters of dozens to thousands of cells form an embryo. Specialized cells begin to take form, and the basic shape of the body of the organism begins to become apparent.

Cellular slime molds approach reproduction differently. Like fungi and plasmodial slime molds, they produce spores as reproductive structures. When a spore germinates (no fusion of cells is required), it releases a single amoeboid cell that begins to engulf and digest bacteria in soil and decaying plant debris, the usual habitats for cellular slime molds. When the amoeboid cell divides, the two cells produced separate and become completely independent of each other, with each continuing to feed and undergo additional divisions for a number of hours or days. Only after the growing population of amoeboid cells depletes the local supply of bacteria is there any indication that a multicellular structure will be produced.

In response to the production of chemical attractants, thousands of amoeboid cells that have been operating as individual single-celled organisms begin to move, either singly or in streaming masses, to form multicellular clumps, or aggregations. Shortly thereafter, one or more cigar-shaped structures called pseudoplasmodia emerge from each aggregation. A pseudoplasmodium is a unified collection of thousands of what had once been separate, independent amoeboid cells. The cells remain distinct in the pseudoplasmodium but no longer act independently. Instead, they cooperate as parts of a multicellular entity. Remarkably, when amoeboid cells of two or more different species of cellular slime molds are grown together, the amoeboid cells of the different species can recognize each other, so that the cells that form any one aggregation are all of a single species rather than a mixture.

Immediately, or perhaps after the entire structure has migrated a short distance toward a light source, cells of the pseudoplasmodium begin to display different patterns of specialization. Cells that happen to have been positioned near the anterior end of the moving “cigar” begin to secrete a wall consisting of cellulose. These cells bind together to form a slender stalk that grows upward from the surface of the substrate upon which the pseudoplasmodium occurs. Other cells, those that happened to have been nearer the posterior end of the pseudoplasmodium, are lifted off the surface on the end of the extending stalk. These cells begin to become encapsulated and specialized as spores. Only the latter live on and produce another generation of amoeboid cells to feed on soil bacteria. The cells that produced the stalk in order to elevate the spore cluster above the substrate eventually die, dry up, and decay.

Reproduction

It appears that cellular slime molds reproduce asexually most of the time, at least under laboratory conditions. All of the cells that originate from the same spore are basically genetically identical to one another and collectively represent a genetic clone. As is the case for asexual reproduction in other life-forms, finding a “mate” is not necessary to perpetuate the species. If amoeboid cells are equipped with the genetic characteristics necessary to survive long enough to produce spores, the same gene combinations will be passed faithfully to all offspring, thus providing the same qualities for survival.

However, a method of sexual reproduction, with its potential of introducing genetic variability, also seems to exist in cellular slime molds. Occasionally in laboratory cultures, a number of large, thick-walled cells are found that are quite different from spores or encysted amoeboid cells. These giant cells are called macrocysts. Macrocysts appear to form when several amoeboid cells (sometimes described as being of compatible “mating types”) fuse together and rearrange their genetic libraries and those of other amoeboid cells that may be engulfed. When macrocysts germinate, the amoeboid cells that emerge seem to have different combinations of genetic information than the cells that initially formed the macrocysts. This mixing up of genetic information, along with the genetic changes resulting from mutations, provides cellular slime molds with an ability to cope with changing environments.

Distribution and Ecology

Most of what is known about cellular slime molds has been acquired from studying these organisms in laboratory culture. What about the biology of “wild” slime molds in nature? In natural ecosystems, it is quite likely that cellular slime molds play a significant role in controlling the size of bacterial populations in soil and decaying litter. Nutrients that are taken up from decaying plants and animals by bacteria are transferred to cellular slime cells when the latter feed upon these bacteria. The cellular slime molds, in turn, become food for soil protozoans, nematode worms, microscopic arthropods such as mites, and other small invertebrate animals. Because of this, cellular slime molds play an essential role in patterns of energy flow and nutrient cycles within terrestrial ecosystems.

There are about seventy-five described species of cellular slime molds. These have been assigned to one of three genera: Dictyostelium, Polysphondylium, and Acytostelium. Some species of cellular slime molds have been found in almost all parts of the world. Two good examples are Dictyostelium mucoroides and Polysphondylium pallidum. Numbers of species of cellular slime molds appear to be highest in the American tropics, which suggests that this region represents a center of evolutionary diversification of the group. More than thirty-five different species have been found in the small area around the Mayan ruins at Tikal in Guatemala. In general, numbers of species of cellular slime molds decrease with increasing elevation and with increasing latitude.

Some species have restricted habitat associations. One species (Dictyostelium caveatum) has been found only in a single cave system in Arkansas. Another species (Dictyostelium rosarium), known from a number of localities worldwide but rarely aboveground, also seems to have an affinity for the type of conditions found in caves. Of the thirty-five species that occur at Tikal, many appear to be restricted to tropical or subtropical locations. Dictyostelium discoideum is the most intensively studied cellular slime mold and the one most widely used in research on developmental biology and genetics. Any search for information about cellular slime molds would probably turn up numerous references to this particular form.

Dispersal of Spores

Unlike most spore-producing organisms (including plasmodial slime molds), cellular slime molds produce spores that do not seem to be carried appreciable distances by wind. Instead, dispersal of cellular slime mold spores seems to depend more upon their accidental transport on the body surface or within the digestive tract of some animal. Viable spores of cellular slime molds have been recovered from the droppings of a number of animals, including rodents, amphibians, bats, and even migratory birds that travel great distances between winter and summer homes. In tropical forests, many living plants and considerable amounts of organic material are found high above the ground in the forest canopy. Cellular slime molds have been isolated from the mass of organic material (literally a “canopy soil”) found at the bases of epiphytic plants growing on the trunks and branches of trees in these forests. It seems likely that they are introduced to such habitats by being carried up from the ground by birds, insects, or other animals that move between the forest floor and the canopy above it.

Bibliography

Bonner, John T. The Cellular Slime Molds. Princeton, N.J.: Princeton University Press, 1967. A bit dated but a good general reference to the biology of cellular slime molds.

Hagiwara, Hiromitsu. The Taxonomic Study of Japanese Cellular Slime Molds. Tokyo: National Science Museum, 1989. This monograph is available in English. The majority of the species described are found in many parts of the world other than Japan.

Raper, Kenneth B. The Dictyostelids. Princeton, N.J.: Princeton University Press, 1984. A comprehensive monograph on all aspects of the biology, ecology, and taxonomy of cellular slime molds.