Slime mould

Slime (or slime mold) is a broad term often referring to roughly six groups of Eukaryotes. The taxonomy is still in flux. Originally, they were considered Fungi, but now they have been split into various groups:


 * Myxogastria plasmodial or syncytial slime moulds.
 * Protostelia smaller plasmodial slime moulds.
 * Dictyosteliida cellular slime moulds.
 * Acrasidae similar life style to Dictyostelids, but of uncertain taxonomy.
 * Plasmodiophorids cabbage club root disease.
 * Labyrinthulomycetes slime nets.

The Myxogastria, Protosteli, and Dictyosteliida make up the group Mycetozoa. The mycetozoan groups all fit into the unikont supergroup Amoebozoa, whereas the others fit into various bikont groups. Slime moulds feed on microorganisms in decaying vegetable matter. They can be found in the soil, on lawns, in the forest. They begin life as amoeba like cells. These amoeba then grow into plasmodia which contain many nuclei without cell membranes between them, which can become many centimeters in size. One variety is often seen as a slimy network of yellow fibers in rotting logs. The amoeba and the plasmodia engulf microorganisms. The plasmodium grows in an amorphous fashion with much cytoplasmic streaming and can even travel. When the food supply wanes, the plasmodium will migrate to the surface of its substrate and transform into rigid fruiting bodies. The fruiting bodies are what we commonly see, superficially like fungi or moulds but are not related. These will then release spores which hatch into amoeba to begin the life cycle again.

History
In the early 16th century, the Dutch artist Hieronymus Bosch depicted an estimated 22 species of slime mould in The Garden of Earthly Delights. The Slime mould Lycogala epidendrum, then called Simonus Fungi cito crescentes, was first described in the year 1654. In 1753 Linnaeus described five separate species, laying the foundation for the nomenclature of slime moulds. Slime moulds were originally considered fungi by mycologists and amoebae by zoologists, respectively classified as Myxomycota (slime fungi) or Mycetozoa (fungus animals). Both names are still used among different groups of specialists. The first person to connect the different stages of the creature was Elias Fries; even though he still considered them to be a species of fungus.

Life Cycle
The plasmodial slime moulds begin as amoeboid cells each with a single haploid nucleus. These may begin feeding on bacteria and multiply. Under very moist conditions they may convert into biflagellate swarmers. In either case, pairs of amoebas or swarmers will fuse and become diploid.

Most organisms reproduce by cell fission when their cells reach a certain size, that is, the nucleus divides by mitosis and the cell divides with it to result in two smaller daughter cells each with their own nucleus. This is what slime mould amoeba do at first, but then they transform into a different behavior. Instead of reproducing this way, they just grow bigger. The nucleus divides by mitosis, but the cell does not divide with it, resulting in a new cell with two nuclei. This is called a syncytium (or plasmodium). As the syncytium grows, the nuclei keep dividing. In some species the syncytium will be many centimeters in size with thousands of nuclei.

This syncytium spreads out like an amoeba and can even migrate slowly. It continues to grow and feed on various microorganisms in the rotting vegetable matter the slime moulds live on. When environmental conditions change or food runs out, the slime mould changes behavior. It migrates to the outer surface of a rotting log if that's where it is living or it migrates up grass blades, etc. It then transforms into a fruiting body or sporangia. In different species these take on many forms from ugly blobs to delicate lacy structures.

First, the nuclei undergo meiosis and become haploid. Then, cell membranes finally form around each of these nuclei one to a cell, to form normal cells again. Other structures like stalks and fibers to hold the spores in place may also form. Finally the cells transform into spores, the structure dries out and the spores can be released into the air to travel to new substrates. These spores are haploid like the original amoeba from which the slime mould started.

These spores can then germinate. They will either form new amoeba to begin the cycle over again, or they may form biflagellate swarming cells. These cells (still haploid) serve as gametes and start the cycle over again.

Under dry conditions, the plasmodia can also form resting structures called sclerotia, which then begin growing again when moist conditions come back.

Types of slime mould
Most slime mould are smaller than a few centimetres, but the very largest reach areas of up to two square metres, making them the largest undivided cells known. Many have bright colours such as yellow, brown, and white.

A common slime mould which forms tiny brown tufts on rotting logs is Stemonitis. Another form which lives in rotting logs and is often used in research is Physarum polycephalum. In logs it has the appearance of a slimy webwork of yellow threads, up to a few inches in size. Fuligo forms yellow crusts.

The Protostelids life cycle is very similar to the above descriptions, but these are much smaller, the fruiting bodies only forming one to a few spores.

The Dictyosteliida, cellular slime moulds, are related to the plasmodial slime moulds but have a very different life style. Their amoeba do not form huge syncytiums and remain individual. They live in similar habitats and also feed on microorganisms. When food runs out and they are ready to form sporangia, they do something radically different. They release signal molecules into their environment, by which they find each other and create huge swarms. These amoeba then join up into a tiny slug like coordinated creature which crawls to an open lit place and grows into a fruiting body. Some of the amoeba become spores to begin the next generation, but some of the amoeba sacrifice themselves to become a dead stalk, lifting the spores up into the air.

The Acrasidae, have a similar life style to Dictyostelids, but their amoeba behave differently and are of uncertain taxonomic position.

The Plasmodiophorids also form syncytia but are internal parasites of plants (e.g., club root disease of cabbages).

Finally the Labyrinthulomycetes are marine and form labyrynthine networks of tubes in which amoebas without pseudopods can travel.

Evolutionary origins
It now appears that the protostelids gave rise to both the myxogastrids and dictyostelids.

Utility in research
Dictyostelids are used as examples of cellular communication and differentiation, and may provide insights into how multicellular organisms develop.

Slime moulds like Physarum polycephalum are useful for studying cytoplasmic streaming. It has been observed that they can find their way through mazes by spreading out and choosing the shortest path, an interesting example of information processing without a nervous system.

In 2006, researchers at the University of Southampton and the University of Kobe reported that they had built a six-legged robot whose movement was remotely controlled by a Physarum slime mould. The mould directed the robot into a dark corner most similar to its natural habitat.

Slime molds are sometimes studied in advanced mathematics courses. Slime mold aggregation is a natural process that can be approximated with partial differential equations.

Slime moulds in culture
Although usually overlooked, slime moulds have occasionally found their way into art and literature. Traditional Finnish lore describes how malicious witches used yellow Fuligo (there called "paranvoi," or butter of the familiar) to spoil milk. More recently, in the popular RPG NetHack, "slime mold" is the default name of a sought-after and delicious food item. Whether or not most actual slime moulds are delicious, or even edible, is unclear, and some may be poisonous. However, mycologist Tom Volk reports that the plasmodium of Fuligo is eaten in Mexico. The graphic novel Nausicaä of the Valley of Wind features a highly dangerous mutated slime mould that engulfs entire cities. Philip K. Dick's novel Clans of the Alphane Moon contains a character called Lord Running Clam, that is a "Ganymedean Slime Mold", who talks and is very intelligent and has telepathic powers. In Jeffrey Darlington's comic General Protection Fault, one character's poor hygiene led to the development of a sentient species of slime mould in his apartment that split the rent with him. In the DVD release of "This is Spinal Tap" there is an outtake of an interview with David St. Hubbins where he speaks of slime moulds, saying "They are both plant AND animal...it's like they can't make up their mind...and, you know, they think it's them...who've been running the earth all this time."

Book References

 * Sleigh, Michael. "Protozoa and Other Protists". Routledge, Chapman and Hall inc. 1989
 * Alexopolous, C.J., Charles W. Mims, M. Blackwell  et al., Introductory Mycology, 4th ed. (John Wiley and Sons, Hoboken NJ, 2004)  ISBN 0-471-52229-5
 * Martin, G.W and C. J. Alexopoulos. 1969. "The Myxomycota" Iowa University Press.
 * Raper, K.B. (1984) The Dictyostelids. Princeton University Press.
 * Karling, J.S. (1968) The Plasmodiophorales. Hafner Publishing Co.
 * Bill Bryson's "A Short History of Nearly Everything".
 * Nick Arnold's "Nasty Nature" (a volume in the "Horrible Science" series).