| Physarum polycephalum | |
|---|---|
| Scientific classification | |
| Kingdom: | Amoebozoa |
| Phylum: | Mycetozoa |
| Class: | Myxogastria |
| Order: | Physarida |
| Family: | Physaridae |
| Genus: | Physarum |
| Species: | P. polycephalum |
| Binomial name | |
| 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. It is sensitive to light; in particular, light can repel the slime mold and be a factor in triggering spore growth.
Contents |
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.
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.
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.
Physarum polycephalum has been shown to exhibit intelligent characteristics similar to those seen in single-celled creatures and eusocial insects.
A team of Japanese and Hungarian researchers claims that a specimen of P. polycephalum was able to navigate a maze made of agar using the shortest route possible when two pieces of food were placed at two separate exits of the maze.[1]
By repeatedly making the test environment of a specimen of P. polycephalum cold and dry for 60-minute intervals, Hokkaido University biophysicists discovered that the slime mould appears to anticipate the pattern by reacting to the conditions when they did not repeat the conditions for the next interval. Upon repeating the conditions, it would react to expect the 60-minute intervals, as well as testing with 30- and 90-minute intervals.[2][3]
P. polycephalum have also been shown to dynamically re-allocate to apparently maintain constant levels of different nutrients simultaneously.[4][5] In particular, specimen placed at the center of a petri dish spatially re-allocated over combinations of food sources that each had different protein–carbohydrate ratios. After 60 hours, the slime mould area over each food source was measured. For each specimen, the results were consistent with the hypothesis that the amoeba would balance total protein and carbohydrate intake to reach particular levels that were invariant to the actual ratios presented to the slime mould.
Similarly, in a preprint that has not yet been scientifically peer reviewed[6], P. polycephalum are again shown to prefer food patches based on the nutrients present within the patches; in particular, the slime moulds are reported to prefer patches with herbs like valerian root that are known to have sedative properties.
With more than two sources, the amoeba also produces efficient networks.[7] In particular, the pattern connecting multiple food sources was shown to form efficient network structures like cycles and Steiner minimum trees.
In a 2010 paper, oatflakes were dispersed to represent Tokyo and 36 surrounding towns.[8][9] P. polycephalum created a network similar to the existing train system, and "with comparable efficiency, fault tolerance, and cost". Similar results have been shown based on road networks in the United Kingdom[10] and the Iberian peninsula (i.e., Spain and Portugal).[11]
The organism's reaction to its environment has also been used in a USB sensor[12] and to control a robot[13].
In a book[14] and several preprints that have not been scientifically peer reviewed,[15][16] it has been claimed that because plasmodia appear to react in a consistent way to the each stimulus, they are the "ideal substrate for future and emerging bio-computing devices".[16] For example,