Slime mold orslime mould is an informal name given to apolyphyletic assemblage of unrelatedeukaryotic organisms in theStramenopiles,Rhizaria,Discoba,Amoebozoa andHolomycotaclades. Most are near-microscopic; those in theMyxogastria form largerplasmodial slime molds visible to the naked eye.

The slime moldlife cycle includes a free-livingsingle-celled stage and the formation of spores. Spores are often produced in macroscopic multicellular ormultinucleate fruiting bodies that may be formed through aggregation or fusion; aggregation is driven bychemical signals calledacrasins. Slime molds contribute to thedecomposition of dead vegetation; some areparasitic.

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 andprotostelians are aquatic or semi-aquatic. Thephytomyxea are parasitic, living inside their planthosts. Geographically, slime molds arecosmopolitan in distribution. A small number of species occur in regions as dry as theAtacama Desert and as cold as theArctic; they are abundant in thetropics, especially inrainforests.

Slime molds have a variety of behaviors otherwise seen in animals with brains. Species such asPhysarum polycephalum have been used to simulate traffic networks. Some species have traditionally been eaten in countries such as Ecuador.

The first account of slime molds wasThomas Panckow [de]'s 1654 discussion ofLycogala epidendrum. He called itFungus cito crescentes, "a fast-growing fungus".^[2][1]

German mycologistHeinrich Anton de Bary, in 1860 and 1887, classified theMyxomycetes (plasmodial slime molds) andAcrasieae (cellular slime molds) as Mycetozoa, a new class. He also introduced a "Doubtful Mycetozoa" section forPlasmodiophora (now inPhytomyxea) andLabyrinthula, emphasizing their distinction from plants and fungi.^[3][4] In 1880, the French botanistPhilippe van Tieghem analyzed the two groups further.^[4]

In 1868, the German biologistErnst Haeckel placed the Mycetozoa in a kingdom he namedProtista.^[4] In 1885, the British zoologistRay Lankester grouped the Mycetozoa alongside theProteomyxa as part of the Gymnomyxa in the phylumProtozoa.^[4]Arthur andGulielma Lister published monographs of the group in 1894, 1911, and 1925.^[5][6]

In 1932 and 1960, the American mycologistGeorge Willard Martin argued that the slime molds evolved from fungi.^[7][8] In 1956, the American biologistHerbert 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 thealgae in a new kingdom, Protoctista.^[4][9]

In 1969, the taxonomistR. 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 textbookThe Myxomycetes.^[6]

In 1975, Olive distinguished thedictyostelids and theacrasids as separate groups.^[4] In 1992,David J. Patterson and M. L. Sogin proposed that the dictyostelids diverged before plants, animals, and fungi.^[11]

Slime molds have little or no fossil history, as might be expected given that they are small and soft-bodied.^[12] The grouping ispolyphyletic, consisting of multipleclades (emphasised in thephylogenetic tree) widely scattered across theEukaryotes. Paraphyletic groups are shown in quotation marks:^[13][14]

Various estimates of the number of species of slime molds agree that there are around 1000 species, most beingMyxogastria. Collection ofenvironmental 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 thesporangia, 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]

The Myxogastria orplasmodial slime molds are the onlymacroscopic scale slime molds; they gave the group its informal name, since for part of their life cycle they are slimy to the touch.^[16] A myxogastrian consists of a large cell withthousands of nuclei within a single membrane without walls, forming asyncytium.^[17] Most are smaller than a few centimeters, but some species may reach sizes up to several square meters, and in the case ofBrefeldia maxima, a mass of up to 20 kilograms (44 lb).^[18][19][20]

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  Stemonitis shows stalked sporangia for airborne spore dispersal.
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  Diachea leucopodia
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  Fuligo septica cells aggregate to form a soft mass.
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  Trichia varia
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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 spiralelaters to eject their lids and disperse their spores.^[22]
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  Mucilago crustacea aggregating from a streamingplasmodium (network of filaments) to asporangium (large mass)

TheDictyosteliida or cellular slime molds do not form hugecoenocytes like the Myxogastria; their amoebae remain individual for most of their lives as individual unicellularprotists, feeding on microorganisms. When food is depleted and they are ready to form sporangia, they form swarms. Theamoebae join up into a tiny multicellular slug which crawls to an open lit place and grows into a fruiting body, asorocarp. Some of the amoebae become spores to begin the next generation, but others sacrifice themselves to become a dead stalk, lifting the spores up into the air.^[23][24]

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  Dictyostelium discoideum is a microscopic organism. The cells can aggregate to form agrex or slug, and then to asorocarp or fruiting body (shown) on a delicate stalk.

TheProtosteliida, a polyphyletic group, have characters intermediate between the previous two groups, but they are much smaller, the fruiting bodies only forming one to a fewspores.^[25]

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  Ceratiomyxa is microscopic; each stalk is topped by only one or a very few spores.

Thelobosans, a paraphyletic group of amoebae, include theCopromyxa slime molds.^[26][27]

Among the non-amoebozoan slime molds are theAcrasids, which have sluglike amoebae. In locomotion, the amoebae'spseudopodia are eruptive, meaning that hemispherical bulges appear at the front.^[28] ThePhytomyxea are obligateparasites, with hosts among the plants,diatoms,oomycetes, andbrown algae. They cause plant diseases likecabbage club root andpowdery scab.^[29] TheLabyrinthulomycetes are marine slime nets, forming labyrinthine networks of tubes in which amoeba without pseudopods can travel.^[30] TheFonticulida are cellular slime molds that form a fruiting body in a "volcano" shape.^[31]

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  TheLabyrinthulomyceteAplanochytrium is a marine protist.

Slime molds, with their small size and moist surface, live mostly in damp habitats including shaded forests, rotting wood, fallen or living leaves, and onbryophytes.^[32][18] Most Myxogastria are terrestrial,^[18] though some, likeDidymium aquatilis are aquatic,^[33][34] andD. nigripes is semi-aquatic.^[34] Myxogastria are not limited to wet regions; 34 species are known from Saudi Arabia, living on bark, in plant litter, and rotting wood, even indeserts.^[35] They occur, too, in Arizona'sSonoran Desert (46 species), and in Chile's exceptionally dryAtacama Desert (24 species). In contrast, the semi-dryTehuacán-Cuicatlán Biosphere Reserve has 105 species, and Russia and Kazakhstan'sVolga river basin has 158 species.^[35] Intropical rainforests of Latin America, species such as ofArcyria andDidymium are commonlyepiphyllous, growing on the leaves ofliverworts.^[36]

The dictyostelids are mostly terrestrial.^[37] OnChangbai Mountain in China, six species of dictyostelids were found in forest soils at elevations up to 2,038 m (6,686 ft), the highest recorded species there beingDictyostelium mucoroides.^[38]The protostelids live mainly on dead plant matter, where they consume the spores ofbacteria,yeasts, andfungi.^[37] They include some aquatic species, which live on dead plant parts submerged in ponds.^[33] Cellular slime molds are most numerous in the tropics, decreasing withlatitude, but arecosmopolitan in distribution, occurring in soil even in the Arctic and the Antarctic.^[39] In the Alaskantundra, the only slime molds are the dictyostelidsD. mucoroides andD. sphaerocephalum.^[36]

The species ofCopromyxa arecoprophilous, feeding on dung.^[27]

Some myxogastrians have their spores dispersed by animals. The slime mold flyEpicypta testata lay its eggs within the spore mass ofEnteridium 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]

Plasmodial slime molds begin life asamoeba-likecells. These unicellular amoebae are commonlyhaploid and feed on small prey such asbacteria, yeast cells, and fungal spores byphagocytosis, engulfing them with itscell membrane. These amoebae can mate if they encounter the correctmating type and formzygotes that then grow intoplasmodia. These contain manynuclei withoutcell membranes between them, and can grow to meters in size. The speciesFuligo septica is often seen as a slimy yellow network in and on rotting logs. The amoebae and the plasmodia engulf microorganisms.^[41] The plasmodium grows into an interconnected network of protoplasmic strands.^[42] Within each protoplasmic strand, the cytoplasmic contents rapidly stream, periodically reversing direction. The streaming protoplasm within a plasmodial strand can reach speeds of up to 1.35 mm per second inPhysarum polycephalum, the fastest for any microorganism.^[43]

Slime molds areisogamous, which means that theirgametes (reproductive cells) are all the same size, unlike the eggs and sperms of animals.^[45]Physarum polycephalum has threegenes involved in reproduction:matA andmatB, with thirteen variants each, andmatC with three variants. Each reproductively mature slime mold isdiploid, meaning that it contains two copies of each of the three reproductive genes.^[46] WhenP. polycephalum is ready to make its reproductive cells, it grows a bulbous extension of its body to contain them.^[47] Each cell has a random combination of the genes that the slime mold contains within itsgenome. Therefore, it can create cells of up to eight different gene types. Released cells then independently seek another compatible cell for fusion. Other individuals ofP. polycephalum may contain different combinations of thematA,matB, andmatC genes, allowing over 500 possible variations. It is advantageous for organisms with this type of reproductive cell to have many mating types because the likelihood of the cells finding a partner is greatly increased, and the risk ofinbreeding is drastically reduced.^[46]

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 moldDictyostelium 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]

The chemicals that aggregate cellular slime molds are small molecules calledacrasins; motion towards a chemical signal is calledchemotaxis. The first acrasin to be discovered wascyclic adenosine monophosphate (cyclic AMP), a common cell signaling molecule, inDictyostelium discoideum. During the aggregation phase of their life cycle,Dictyostelium discoideum amoebae communicate with each other using traveling waves of cyclic AMP.^[51][52][53] There is an amplification of cyclic AMP when they aggregate.^[54] Pre-stalk cells move toward cyclic AMP, but pre-spore cells ignore the signal.^[55] Other acrasins exist; the acrasin forPolysphondylium violaceum, purified in 1983, is thedipeptide glorin.^[56]Calcium ions too serve to attract slime mold amoebae, at least at short distances. It has been suggested that acrasins may be taxon-specific, since specificity is required to form an aggregation of genetically similar cells. Many dictyostelid species indeed do not respond to cyclic AMP, but as of 2023 their acrasins remained unknown.^[57]

The practical study of slime molds was facilitated by the introduction of the "moist culture chamber" by H. C. Gilbert andG. W. Martin in 1933.^[58] Slime molds can be used to teachconvergent 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 moldDidymium iridis had two strains (+ and −) of cells, equivalent to gametes, that these could formimmortal cell lines inculture, and that the system was controlled byalleles of a single gene. This made the species amodel organism for exploring incompatibility, asexual reproduction, and mating types.^[59]

Slime molds have been studied for their production of unusual organic compounds, includingpigments,antibiotics, andanti-cancer drugs.^[59] Pigments includenaphthoquinones, physarochrome A, and compounds of tetramic acid.Bisindolylmaleimides produced byArcyria denudata include somephosphorescent compounds.^[60] The sporophores (fruiting bodies) ofArcyria denudata are colored red by arcyriaflavins A–C, which contain an unusual indolo[2,3-a]carbazolealkaloid ring.^[61] By 2022, more than 100 pigments had been isolated from slime molds, mostly from sporophores. It has been suggested that the many yellow-to-red pigments might be useful incosmetics.^[15] Some 42% of patients withseasonal allergic rhinitis reacted to myxogastrian spores, so the spores may contribute significantly as airborneallergens.^[62]

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 onPhysarum 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 moldalgorithm is ameta-heuristic algorithm, based on the behavior of aggregated slime molds as they stream in search of food. It is described as a simple, efficient, and flexible way of solvingoptimization problems, such as finding theshortest path betweennodes in a network. However, it can become trapped in alocal optimum.^[70]

Toshiyuki Nakagaki and colleagues studied slime molds and their abilities to solve mazes by placing nodes at two points separated by a maze of plastic film. The mold explored all possible paths and solved it for the shortest path.^[71]

Atsushi Tero and colleagues grewPhysarum 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. AsPhysarum 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 resembledTokyo'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 ofP. polycephalum forming a network to food sources is similar to the large scalegalaxy filament structure of theuniverse. This observation has led astronomers to use simulations based on the behaviour of slime molds to inform their search fordark matter.^[77][78]

In central Mexico, thefalse puffballEnteridium lycoperdon was traditionally used as food; it was one of the species which mushroom-collectors orhongueros 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 amaize leaf, and baked in the ashes of a campfire; or boiled and eaten with maizetortillas.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]

Oscar Requejo and N. Floro Andres-Rodriguez suggest thatFuligo septica may have inspiredIrvin Yeaworth's 1958 filmThe Blob, in which a giant amoeba from space sets about engulfing people in a small American town.^[79]

• Swarming motility – rapid and coordinated translocation of a bacterial population across solid or semi-solid surfacesPages displaying wikidata descriptions as a fallback
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