Physarum polycephalum

Physarum polycephalum belongs to the supergroup Amoebozoa, phylum Mycetozoa, and class Myxogastria. P. polycephalum, often referred to as the “many-headed slime,” is a slime mold that inhabits shady, cool, moist areas, such as decaying leaves and logs.

Characteristics

This protist may be seen without a microscope; P. polycephalum is typically yellow in color, and eats fungal spores, bacteria, and other microbes. P. polycephalum is one of the easiest eukaryotic microbes to grow in culture, and has been used as a model organism for many studies involving amoeboid movement and cell motility. Most organisms receive mitochondrial DNA from their mother, but it is not known from where P. polycephalum receives its mitochondrial DNA as it is currently not possible to distinguish between male and female. It is also believed that the P. polycephalum is the first eukaryotic cell to have organelles such as mitochondria and ribosomal features.

Life cycle

The main vegetative phase of P. polycephalum is the plasmodium (the active, streaming form of slime molds). The plasmodium consists of networks of protoplasmic veins, and many nuclei. It is during this stage that the organism searches for food. The plasmodium surrounds its food and secretes enzymes to digest it.

If environmental conditions cause the plasmodium to desiccate during feeding or migration, Physarum will form a sclerotium. The sclerotium is basically hardened multinucleated tissue that serves as a dormant stage, protecting Physarum for long periods of time. Once favorable conditions resume, the plasmodium reappears to continue its quest for food.

As the food supply runs out, the plasmodium stops feeding and begins its reproductive phase. Stalks of sporangia form from the plasmodium; it is within these structures that meiosis occurs and spores are formed. Sporangia are usually formed in the open so that the spores they release will be spread by wind currents.

Spores can remain dormant for years if need be. However, when environmental conditions are favorable for growth, the spores germinate and release either flagellated or amoeboid swarm cells (motile stage); the swarm cells then fuse together to form a new plasmodium.

Streaming behavior

The movement of P. polycephalum is termed shuttle streaming. Shuttle streaming is characterized by the rhythmic back-and-forth flow of the protoplasm; the time interval is approximately two minutes. The forces of the streaming vary for each type of microplasmodium.

The force in amoeboid microplasmodia is generated by contraction and relaxation of a membranous layer probably consisting of actin (type of filament associated with contraction). The filament layer creates a pressure gradient, over which the protoplasm flows within limits of the cell periphery.

The force behind streaming in the dumbbell-shaped microplasmodia is generated by volume changes in both the periphery of the cell and in the invagination system of the cell membrane.

Intelligence

Physarum Polycephalum demonstrates a surprising amount of intelligence for a single-celled creature.

Maze-solving

A team of Japanese and Hungarian researchers, writing in the journal Nature^[1], claimed to have found the slime mold Physarum polycephalum is capable of finding the shortest way through a maze. Pieces of the slime mould were enticed through a 30-square-centimetre (five-square-inch) maze by the prospect of food at the end of the puzzle. The researchers concluded that the creature was exhibiting a kind of primitive intelligence.

Normally, the slime spreads out its network of tube-like legs, or pseudopodia, to fill all the available space. But when two pieces of food were placed at separate exit points in the labyrinth, the organism squeezed its entire body between the two nutrients. It adopted the shortest possible route, effectively solving the puzzle.

Event anticipation

Biophysicist Toshiyuki Nakagaki of Hokkaido University and colleagues manipulated the environment of Physarum slime-mold amoebas. As the cells crawled across an agar plate, the researchers subjected them to cold, dry conditions for the first 10 minutes of every hour. During these cool spells, the cells slowed down their motion. After three cold snaps the scientists stopped changing the temperature and humidity and watched to see whether the amoebas had learned the pattern. Indeed, many of the cells throttled back right on the hour in anticipation of another bout of cold weather. When conditions stayed stable for a while, the slime-mold amoebas gave up on their hourly braking, but when another single jolt of cold was applied, they resumed the behavior and correctly recalled the 60-minute interval. The amoebas were also able to respond to other intervals, ranging from 30 to 90 minutes.^[2]

Care

Physarum Polycephalum can be easily cultivated in the lab for research purposes and there are many resources available online for ordering cultures. The most simple way of cultivating P. polycephalum is to grow it in its plasmodial stage on an agar or filter paper medium. Keep the culture out of direct light and "rinse" it lightly weekly to ensure growth. Once your culture begins to grow you will need to feed it. In the wild, P. polycephalum will usually eat bacteria and dead organic matter. However, in the lab you will need to feed your culture daily with old fashioned oats. It is important to note that instant oats will kill the P. polycephalum and should not be used. As your culture grows you may wish to transfer it to multiple petri dishes. Simply remove a portion of the active plasmodium and place it gently on your agar, or filter paper culture medium. If you wish to induce sporangia formation, expose the plasmodium stage P. polycephalum to small increments of sunlight each day until sporangia form.

References

• Gawlitta,W, KV Wolf, HU Hoffmann, and W. Stockem. 1980. Studies on microplasmodia of Physarum polycephalum. I. Classification and locomotive behavior. Cell Tissue Res; 209(1): 71-86.
• http://www.carolina.com/tips/01pdfs/August_2001_Tips.pdf
• http://www.educationalassistance.org/Physarum/PhysarumPlus.html
• Cellular memory hints at the origins of intelligence - The learning and memory potential of Physarum polycephalum
• slime mold - Slime shown negotiating a maze