Slime mold
Slime mold or slime mould is an informal name given to a
Most slime molds are terrestrial and free-living, typically in damp shady habitats such as in or on the surface of rotting wood. Some myxogastrians and protostelians are aquatic or semi-aquatic. The phytomyxea are parasitic, living inside their plant hosts. Geographically, slime molds are cosmopolitan in distribution. A small number of species occur in regions as dry as the Atacama Desert and as cold as the Arctic; they are abundant in the tropics, especially in rainforests.
Slime molds have a variety of behaviors otherwise seen in animals with brains. Species such as Physarum polycephalum have been used to simulate traffic networks. Some species have traditionally been eaten in countries such as Ecuador.
Evolution
Taxonomic history
The first account of slime molds was Thomas Panckow 's 1654 discussion of Lycogala epidendrum. He called it Fungus cito crescentes, "a fast-growing fungus".[2][1]
German mycologist
In 1932 and 1960, the American mycologist George Willard Martin argued that the slime molds evolved from fungi.[7][8] In 1956, the American biologist Herbert Copeland placed the Mycetozoa (the myxomycetes and plasmodiophorids) and the Sarkodina (the labyrinthulids and the cellular slime molds) in a phylum called Protoplasta, which he placed alongside the fungi and the algae in a new kingdom, Protoctista.[4][9]
In 1969, the taxonomist R. H. Whittaker observed that slime molds were highly conspicuous and distinct within the Fungi, the group to which they were then classified. He concurred with Lindsay S. Olive's proposal to reclassify the Gymnomycota, which includes slime molds, as part of the Protista.[10] Whittaker placed three phyla, namely the Myxomycota, Acrasiomycota, and Labyrinthulomycota in a subkingdom Gymnomycota within the Fungi.[4] The same year, Martin and Alexopoulos published their influential textbook The Myxomycetes.[6]
In 1975, Olive distinguished the dictyostelids and the acrasids as separate groups.[4] In 1992, David J. Patterson and M. L. Sogin proposed that the dictyostelids diverged before plants, animals, and fungi.[11]
Phylogeny
Slime molds have little or no fossil history, as might be expected given that they are small and soft-bodied.
Eukaryotes |
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Diversity
Various estimates of the number of species of slime molds agree that there are around 1000 species, most being Myxogastria. Collection of environmental DNA gives a higher estimate, from 1200 to 1500 species.[6] These are diverse both taxonomically and in appearance, the largest and most familiar species being among the Myxogastria. The growth forms most commonly noticed are the sporangia, the spore-forming bodies, which are often roughly spherical; these may be directly on the surface, such as on rotting wood, or may be on a thin stalk which elevates the spores for release above the surface. Other species have the spores in a large mass, which may be visited by insects for food; they disperse spores when they leave.[15]
Macroscopic, plasmodial slime molds: Myxogastria
The Myxogastria or
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Stemonitis shows stalked sporangia for airborne spore dispersal.
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Fuligo septica cells aggregate to form a soft mass.
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Enteridium lycoperdon sporangium. Spores can disperse in air or water, or by slime mold flies.[21]
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Metatrichia vesparium has small round sporangia that have spiral elaters to eject their lids and disperse their spores.[22]
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Mucilago crustacea aggregating from a streaming plasmodium (network of filaments) to a sporangium(large mass)
Cellular slime molds: Dictyosteliida
The
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Dictyostelium discoideum is a microscopic organism. The cells can aggregate to form a grex or slug, and then to a sorocarp or fruiting body (shown) on a delicate stalk.
Protosteliida
The
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Ceratiomyxa is microscopic; each stalk is topped by only one or a very few spores.
Copromyxa
The lobosans, a paraphyletic group of amoebae, include the Copromyxa slime molds.[26][27]
Non-amoebozoan slime molds
Among the non-amoebozoan slime molds are the
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The Labyrinthulomycete Aplanochytrium is a marine protist.
Distribution, habitats, and ecology
Slime molds, with their small size and moist surface, live mostly in damp habitats including shaded forests, rotting wood, fallen or living leaves, and on
The dictyostelids are mostly terrestrial.
The species of Copromyxa are coprophilous, feeding on dung.[27]
Some myxogastrians have their spores dispersed by animals. The slime mold fly Epicypta testata lay its eggs within the spore mass of Enteridium lycoperdon, which the larvae feed on. These pupate, and the hatching adults carry and disperse spores that have stuck to them.[21] While various insects consume slime molds, Sphindidae slime mold beetles, both larvae and adults, exclusively feed on them.[40]
Life cycle
Plasmodial slime molds
Plasmodial slime molds begin life as
Slime molds are
Cellular slime molds
The cellular slime molds are a group of approximately 150 described species. They occur primarily in the humus layer of forest soils[48] and feed on bacteria but also are found in animal dung and agricultural fields. They exist as single-celled organisms while food is plentiful. When food is in short supply, many of the single-celled amoebae congregate and start moving as a single body, called a 'slug'. The ability of the single celled organisms to aggregate into multicellular forms are why they are also called the social amoebae. In this state they are sensitive to airborne chemicals and can detect food sources. They readily change the shape and function of parts, and may form stalks that produce fruiting bodies, releasing countless spores, light enough to be carried on the wind or on passing animals.[23] The cellular slime mold Dictyostelium discoideum has many different mating types. When this organism has entered the stage of reproduction, it releases a chemical attractant.[49] When it comes time for the cells to fuse, Dictyostelium discoideum has mating types of its own that dictate which cells are compatible with each other. There are at least eleven mating types; macrocysts form after cell contact between compatible mating types.[50]
Chemical signals
The chemicals that aggregate cellular slime molds are small molecules called
Study
Use in research and teaching
The practical study of slime molds was facilitated by the introduction of the "moist culture chamber" by H. C. Gilbert and G. W. Martin in 1933.[58] Slime molds can be used to teach convergent evolution, as the habit of forming a stalk with a sporangium that can release spores into the air, off the ground, has evolved repeatedly, such as in myxogastria (eukaryotes) and in myxobacteria (prokaryotes).[59] Further, both the (macroscopic) dictyostelids and the (microscopic) protostelids have a phase with motile amoebae and a phase with a stalk; in the protostelids, the stalk is tiny, supporting just one spore, but the logic of airborne spore dispersal is the same.[59]
O. R. Collins showed that the slime mold Didymium iridis had two strains (+ and −) of cells, equivalent to gametes, that these could form immortal cell lines in culture, and that the system was controlled by alleles of a single gene. This made the species a model organism for exploring incompatibility, asexual reproduction, and mating types.[59]
Biochemicals
Slime molds have been studied for their production of unusual organic compounds, including
Computation
Slime molds share some similarities with neural systems in animals.[63] The membranes of both slime molds and neural cells contain receptor sites, which alter electrical properties of the membrane when it is bound.[64] Therefore, some studies on the early evolution of animal neural systems are inspired by slime molds.[65][66][67] When a slime mold mass or mound is physically separated, the cells find their way back to re-unite. Studies on Physarum polycephalum have even shown the organism to have an ability to learn and predict periodic unfavorable conditions in laboratory experiments.[68] John Tyler Bonner, a professor of ecology known for his studies of slime molds, argues that they are "no more than a bag of amoebae encased in a thin slime sheath, yet they manage to have various behaviors that are equal to those of animals who possess muscles and nerves with ganglia – that is, simple brains."[69]
The slime mold
Toshiyuki Nakagaki and colleagues studies slime molds and their abilities to solve mazes by placing nodes at two point separated by a maze of plastic film. The mold explored all possible paths and solves it for the shortest path.[71]
Traffic system inspirations
Atsushi Tero and colleagues grew Physarum in a flat wet dish, placing the mold in a central position representing Tokyo, and oat flakes surrounding it corresponding to the locations of other major cities in the Greater Tokyo Area. As Physarum avoids bright light, light was used to simulate mountains, water and other obstacles in the dish. The mold first densely filled the space with plasmodia, and then thinned the network to focus on efficiently connected branches. The network closely resembled Tokyo's rail system.[72][73] P. polycephalum was used in experimental laboratory approximations of motorway networks of 14 geographical areas: Australia, Africa, Belgium, Brazil, Canada, China, Germany, Iberia, Italy, Malaysia, Mexico, the Netherlands, UK and US.[74][75][76] The filamentary structure of P. polycephalum forming a network to food sources is similar to the large scale galaxy filament structure of the universe. This observation has led astronomers to use simulations based on the behaviour of slime molds to inform their search for dark matter.[77][78]
Use as food
In central Mexico, the false puffball Enteridium lycoperdon was traditionally used as food; it was one of the species which mushroom-collectors or hongueros gathered on trips into the forest in the rainy season. One of its local names is "cheese mushroom", so called for its texture and flavor when cooked. It was salted, wrapped in a maize leaf, and baked in the ashes of a campfire; or boiled and eaten with maize tortillas. Fuligo septica was similarly collected in Mexico, cooked with onions and peppers and eaten in a tortilla. In Ecuador, Lycogala epidendrum was called "yakich" and eaten raw as an appetizer.[79]
In popular culture
Oscar Requejo and N. Floro Andres-Rodriguez suggest that Fuligo septica may have inspired Irvin Yeaworth's 1958 film The Blob, in which a giant amoeba from space sets about engulfing people in a small American town.[79]
See also
- Swarming motility – rapid and coordinated translocation of a bacterial population across solid or semi-solid surfaces
- Water mold– Fungus-like eukaryotic microorganism
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External links
- Slime molds are gorgeous (you just never knew it!) | Oregon Field Guide on YouTube
- Slime mold photo series by Barry Webb, 2023