Slime mold
Slime molds or slime moulds are a variety of small or microscopic organisms in different groups. They have both single-celled and multicellular forms during their life cycle, the individual cells coming together to form fruiting bodies that produce spores. Most live in damp places such as rotting wood.
More formally, the slime molds are a polyphyletic assemblage of distantly related eukaryotic organisms in the Stramenopiles, Rhizaria, Discoba, Amoebozoa and Holomycota clades. Most are near-microscopic; those in the Myxogastria form larger plasmodial slime molds visible to the naked eye. Spores are often produced in macroscopic multicellular or multinucleate fruiting bodies formed through aggregation or fusion; aggregation is driven by chemical signals called acrasins. Slime molds contribute to the decomposition of dead vegetation; some are parasitic.
Most slime molds are terrestrial and free-living, typically in damp shady habitats. 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 by humans in countries such as Ecuador.
Evolution
Taxonomic history
The first account of slime molds was 's 1654 discussion of Lycogala epidendrum. He called it Fungus cito crescentes, "a fast-growing fungus".German mycologist Heinrich Anton de Bary, in 1860 and 1887, classified the Myxomycetes and Acrasieae as Mycetozoa, a new class. He also introduced a "Doubtful Mycetozoa" section for Plasmodiophora and Labyrinthula, emphasizing their distinction from plants and fungi. In 1880, the French botanist Philippe van Tieghem analyzed the two groups further.
In 1868, the German biologist Ernst Haeckel placed the Mycetozoa in a kingdom he named Protista. In 1885, the British zoologist Ray Lankester grouped the Mycetozoa alongside the Proteomyxa as part of the Gymnomyxa in the phylum Protozoa. Arthur and Gulielma Lister published monographs of the group in 1894, 1911, and 1925.
In 1932 and 1960, the American mycologist George Willard Martin argued that the slime molds evolved from fungi. In 1956, the American biologist Herbert Copeland placed the Mycetozoa and the Sarkodina in a phylum called Protoplasta, which he placed alongside the fungi and the algae in a new kingdom, Protoctista.
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. Whittaker placed three phyla, namely the Myxomycota, Acrasiomycota, and Labyrinthulomycota in a subkingdom Gymnomycota within the Fungi. The same year, Martin and Alexopoulos published their influential textbook The Myxomycetes.
In 1975, Olive distinguished the dictyostelids and the acrasids as separate groups. In 1992, David J. Patterson and M. L. Sogin proposed that the dictyostelids diverged before plants, animals, and fungi.
Phylogeny
Slime molds have little or no fossil history, as might be expected given that they are small and soft-bodied. The grouping is polyphyletic, consisting of multiple clades widely scattered across the Eukaryotes. Paraphyletic groups are shown in quotation marks: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. 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.Macroscopic, plasmodial slime molds: Myxogastria
The Myxogastria or plasmodial slime molds are the only macroscopic scale slime molds; they gave the group its informal name, since for part of their life cycle they are slimy to the touch. A myxogastrian consists of a large cell with thousands of nuclei within a single membrane without walls, forming a syncytium. Most are smaller than a few centimeters, but some species may reach sizes up to several square meters, and in the case of Brefeldia maxima, a mass of up to.Cellular slime molds: Dictyosteliida
The Dictyosteliida or cellular slime molds do not form huge coenocytes like the Myxogastria; their amoebae remain individual for most of their lives as individual unicellular protists, feeding on microorganisms. When food is depleted and they are ready to form sporangia, they form swarms. The amoebae join up into a tiny multicellular slug which crawls to an open lit place and grows into a fruiting body, a sorocarp. 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.Protosteliida
The Protosteliida, a polyphyletic group, have characters intermediate between the previous two groups, but they are much smaller, the fruiting bodies only forming one to a few spores.''Copromyxa''
The lobosans, a paraphyletic group of amoebae, include the Copromyxa slime molds.Non-amoebozoan slime molds
Among the non-amoebozoan slime molds are the Acrasids, which have sluglike amoebae. In locomotion, the amoebae's pseudopodia are eruptive, meaning that hemispherical bulges appear at the front. The Phytomyxea are obligate parasites, with hosts among the plants, diatoms, oomycetes, and brown algae. They cause plant diseases like cabbage club root and powdery scab. The Labyrinthulomycetes are marine slime nets, forming labyrinthine networks of tubes in which amoeba without pseudopods can travel. The Fonticulida are cellular slime molds that form a fruiting body in a "volcano" shape.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 bryophytes. Most Myxogastria are terrestrial, though some, such as Didymium aquatilis, are aquatic, and D. nigripes is semi-aquatic. Myxogastria are not limited to wet regions; 34 species are known from Saudi Arabia, living on bark, in plant litter and rotting wood, and even in deserts. They also occur in Arizona's Sonoran Desert, and in Chile's exceptionally dry Atacama Desert. In contrast, the semi-dry Tehuacán-Cuicatlán Biosphere Reserve has 105 species, and Russia and Kazakhstan's Volga river basin has 158 species. In tropical rainforests of Latin America, species such as of Arcyria and Didymium are commonly epiphyllous, growing on the leaves of liverworts.The dictyostelids are mostly terrestrial. On Changbai Mountain in China, six species of dictyostelids were found in forest soils at elevations up to, the highest recorded species there being Dictyostelium mucoroides.
The protostelids live mainly on dead plant matter, where they consume the spores of bacteria, yeasts, and fungi. They include some aquatic species, which live on dead plant parts submerged in ponds. Cellular slime molds are most numerous in the tropics, decreasing with latitude, but are cosmopolitan in distribution, occurring in soil even in the Arctic and the Antarctic. In the Alaskan tundra, the only slime molds are the dictyostelids D. mucoroides and D. sphaerocephalum.
The species of Copromyxa are coprophilous, feeding on dung.
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. While various insects consume slime molds, Sphindidae slime mold beetles, both larvae and adults, feed exclusively on them.
Life cycle
Plasmodial slime molds
Plasmodial slime molds begin life as amoeba-like cells. These unicellular amoebae are commonly haploid and feed on small prey such as bacteria, yeast cells, and fungal spores by phagocytosis, engulfing them with its cell membrane. These amoebae can mate if they encounter the correct mating type and form zygotes that then grow into plasmodia. These contain many nuclei without cell membranes between them, and can grow to meters in size. The species Fuligo septica is often seen as a slimy yellow network in and on rotting logs. The amoebae and the plasmodia engulf microorganisms. The plasmodium grows into an interconnected network of protoplasmic strands. 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 in Physarum polycephalum, the fastest for any microorganism.File:Plasmodial slime mold life cycle English.png|thumb|upright=1.75|Life cycle of a plasmodial slime mold. Haploid gametes undergo sexual fusion to form a diploid cell. Its nucleus divides to form a multinucleate plasmodium. Meiosis halves the number of chromosomes to form haploid cells with just one nucleus.
Slime molds are isogamous, which means that their gametes are all the same size, unlike the eggs and sperms of animals. Physarum polycephalum has three genes involved in reproduction: matA and matB, with thirteen variants each, and matC with three variants. Each reproductively mature slime mold is diploid, meaning that it contains two copies of each of the three reproductive genes. When P. polycephalum is ready to make its reproductive cells, it grows a bulbous extension of its body to contain them. Each cell has a random combination of the genes that the slime mold contains within its genome. Therefore, it can create cells of up to eight different gene types. Released cells then independently seek another compatible cell for fusion. Other individuals of P. polycephalum may contain different combinations of the matA, matB, and matC 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 of inbreeding is drastically reduced.