by Tristan Wang
Slime molds are some of the world’s ancient mysteries. From the independent unicellular amoeba to the cooperation of many individuals, these globs of ooze share biological functions that few other species or even kingdoms exhibit. Even though they are not seen conspicuously in our day-to-day lives, slime molds may hold the key to understanding fundamental problems in biology, such as spatial memory and altruism, through their social behavior.
Overview of Mycetozoa
Historically, scientists have not had much luck in classifying these mysterious critters. Animal biologists liked to point out the animalistic behavior of these slug-like creatures, while plant enthusiasts noted their stalk-like structures. Mycologists, in the meantime, argued that the root-like arms of feeding slime molds resembled a feeding fungus—an argument that eventually positioned the slime molds at the base of the fungal lineage (1).
Nowadays, scientists have classified these creatures under the infraphylum of Mycetozoa within the kingdom of Amoebozoa — or amoebas — which are commonly known to be tiny slug-like creatures that move via the flow of internal fluids within cells, a process called cytoplasmic flow (1). For easier classification, Amoebozoa is often referred to as a part of the kingdom of protists, a group that excludes fungi, plants, and animals. Within Mycetozoa, slime molds are further categorized into two main groups: cellular (dictyostelic) and plasmodial (myxogastric) slime molds.
Plasmodial slime molds live most of their lives as masses of protoplasm, a form of slime that crawls around before finding a suitable food substrate, like bacteria or other organic matter (2). Often, these masses of slime can be found in cool, moist areas, but, during reproduction, they move to a more open location where their spores can be carried by the wind (2). Cellular slime molds, on the other hand, live mostly as independent amoebas that feed until there is an external stimulus, such as a change in environment or food source, which prompts the individual amoebas to congregate into a slug-like form (2). This mass of cells ultimately forms a stalk with a clump of spores sitting on top, where they can released into the air (2).
Plasmodial Slime Molds and Navigation
One of the most fascinating abilities of slime molds is their ability to solve mazes. There are several well-known videos attesting to the mold’s ability to not only navigate labyrinths but also find the most efficient route from the entrance to the exit (3). From this, it is not too far of a leap to find a use for these critters. Scientists have applied the mold’s ability to organize itself to model the shortest distance between city train stations, using a miniaturized model with oatmeal flakes to represent train stops (4). Simulations like these show that the shape in which the slime mold organizes its protoplasm often resembles actual railway networks (4).
Behind this phenomenon of connecting different parts of slime molds is the process of shuttling nutrients around the protoplasm called cytoplasmic streaming (5). These protoplasmic connections allow efficient transportation between food sources and growing parts of the organism (5). In addition, scientists have explored the processes that occur during movement of slime molds. According to a paper in the Proceedings of the National Academy of Sciences, a slime mold’s plasmodium is made up of individual “oscillating units” that are influenced by external stimuli from neighboring units and the environment (6). When food is detected by receptor molecules, the cell membrane closest to the food allows cytoplasm to flow into that area of the attractant (6). As a result, it looks like the slime mold is growing towards a specific location.
Even more interesting is the ability of slime mold to find its way to food sources, a phenomenon known as “spatial memory,” which is often only associated with organisms with higher mental capacities. When slime molds move, they leave behind a mass of extracellular slime that contains distinctive sugar polymers (6). Slime molds have been shown to be very repulsed by the slime they produce, thereby allowing the individual organisms to mark the territory that they have already traveled through (6). It has been theorized that this rudimentary spatial memory is the predecessor to the more sophisticated memory of other organisms (6).
Cellular Slime Mold and Altruism
Altruism is selfless concern for others over concerns for oneself. In biology, however, this practice is rarely found outside of animals. Currently, it is thought that motives for altruism either involve selection for closely related kin (much like how worker bees work for the queen bee) or the expectation of reciprocation from the benefiting party (7).
When reproduction occurs in cellular slime molds, some individuals of the population must create a stalk that holds a mass of spores for dispersal (2). While the spores on top get the opportunity to reproduce, the individual amoebas that create the stalk die and do not get the chance to pass on their genes (2). However, in biology, there is always a catch.
One study that looked at pure cultures of cellular slime molds and their reproduction found that combinations of less related slime molds created a much smaller stalk relative to the spore capsule than did pure cultures of either variety (8). These results imply that when populations of differing slime molds encounter each other, neither invests heavily in a needed structure (9).
Indeed, some studies have even noted that cheating occurs with coexisting slime molds. “Cheater” cultures may sometimes invest a smaller portion of cells to the formation of the stalk when cooperating with other populations (7). As a result, these unfair cultures are in an evolutionarily more advantageous position in terms of reproductive advantage. Exactly how can a system of altruism still exist when there is an unfair advantage? It has been shown that, when this trend of cheating is allowed to occur, it leads to the development of slime molds with and without stalks (7). When both populations coexist, the stalk-less slime mold takes advantage of the stalked population, but the stalk-less population cannot persist on its own, while the stalked population can (7). Thus, in a sense, nature punishes cheaters.
The study of slime molds has revealed these organisms to be not just a mass of slime on the forest floor but very charismatic organisms that are able to solve mazes, modeling city planning. Not only do slime molds play an integral part of our ecology, but their social behavior also provides important insight into the inner workings of communication, movement, and altruism. At least in biology, slime molds are truly extraordinary.
Tristan Wang ’16 is an Organismic and Evolutionary Biology concentrator in Kirkland House.
- Baldauf, S. L. “Origin and Evolution of the Slime Molds (Mycetozoa).” Proceedings of the National Academy of Sciences 94.22 (1997): 12007-2012.
- Stephenson, Steven L., and Henry Stempen. Myxomycetes a Handbook of Slime Molds. Portland, Or.: Timberland, 2000.
- Nakagaki, Toshiyuki. “Smart Behavior of True Slime Mold in a Labyrinth.” Research in Microbiology 152.9 (2001): 767-70.
- Yong, Ed. “Slime Mould Attacks Simulates Tokyo Rail Network.” Web log post. Scienceblogs.com. ScienceBlogs, 21 Jan. 2010.
- Adamatzky, Andrew, and Jeff Jones. “Road Planning With Slime Mould: If Physarum Built Motorways It Would Route M6/m74 Through Newcastle.” International Journal of Bifurcation and Chaos 20.10 (2010): 3065
- Reid, C. R., T. Latty, A. Dussutour, and M. Beekman. “Slime Mold Uses an Externalized Spatial “memory” to Navigate in Complex Environments.” Proceedings of the National Academy of Sciences 109.43 (2012): 17490-7494.
- Brannstrom, A., and U. Dieckmann. “Evolutionary Dynamics of Altruism and Cheating among Social Amoebas.” Proceedings of the Royal Society B: Biological Sciences 272.1572 (2005): 1609-616.
- Deangelo, M.j., V.m. Kish, and S.a. Kolmes. “Altruism, Selfishness, and Heterocytosis in Cellular Slime Molds.” Ethology Ecology & Evolution 2.4 (1990): 439-43.