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Slime molds have complex life cycles that may be divided into an animallike motile phase, in which growth and feeding occur, and a plantlike, immotile, reproductive phase. The motile phase is commonly found under rotting logs and damp leaves, where cellulose is abundant. It consists in the cellular slime molds of solitary, amebalike cells, and in the Myxomycota of a coenocytic (multinucleate) mass of protoplasm called a plasmodium, which creeps about by ameboid movement. Plasmodia often grow to a diameter of several inches and are frequently brightly colored. Both types ingest solid food particles using a process called phagocytosis (see endocytosis). They feed on living microorganisms, such as bacteria and yeasts, as well as decaying vegetation. Before entering the reproductive stage, a plasmodium moves to a drier, better-lit place, such as the top of a log. In the amebalike, or cellular, slime molds, up to 125,000 individual cells aggregate and flow together, forming a multicellular mass called a pseudoplasmodium that resembles a slug and crawls about before settling in a location with acceptable warmth and brightness.
In the reproductive stage the plasmodium or pseudoplasmodium is transformed into one or more reproductive structures called fruiting bodies, each consisting of a stalk topped by a spore-producing capsule that resembles the reproductive structures of many fungi. Eventually the cellulose-walled spores are released and dispersed; they germinate in wet places, releasing naked cells. In a typical plasmodial slime mold the germinated spores go through an ameboid or flagellated swimming stage, followed by sexual fusions and cell divisions. The diploid ameboid cell (i.e., the zygote) grows and its nucleus divides repeatedly, resulting in the formation of a new plasmodium. Under adverse conditions a plasmodium may be transformed into a hard, dry, inactive mass called a sclerotium. Resistant to desiccation, it becomes a plasmodium again when favorable conditions return.
In the case of the cellular slime molds, each spore released becomes a single ameba, which feeds individually until starving cells release a chemical signal that causes them to aggregate into a new pseudoplasmodium, and the process is repeated. In sexual reproduction two haploid amebas fuse, then engulf surrounding amebas, forming a single organism called a macrocyst. The macrocyst then undergoes meiosis and mitosis and releases haploid individuals.
There are about 65 cellular and 500 known plasmodial slime mold species, found in forests and sometimes lawns throughout the world. In a few species the plasmodium, under favorable conditions, may cover an area of several square feet. A slime mold is the cause of clubroot, a disease of cabbage and related plants.
Bibliography
See J. T. Bonner, The Cellular Slime Molds (2d ed. 1985).
| WordNet: cellular slime mold |
The noun has one meaning:
Meaning #1:
differing from true slime molds in being cellular and nucleate throughout the life cycle
| Wikipedia: Slime mold |
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This article may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can. (March 2008) |
Slime mold (or slime mould, see spelling differences) is a broad term describing fungus-like organisms that use spores to reproduce.[1] They were formerly classified as fungi, but are no longer considered part of this group.[2]
Their common name refers to part of some of these organism's lifecycles where they can appear gelatinous (hence the name slime). However, this feature is mostly seen with the myxomycetes, which are the only macroscopic slime molds.
Slime molds have been found all over the world and feed on microorganisms that live in any type of dead plant material. For this reason, these organisms are usually found in soil, lawns, and on the forest floor, commonly on deciduous logs. However, in tropical areas they are also common on inflorescences, fruits and in aerial situations (e.g., in the canopy of trees). In urban areas, they are found on mulch or even in the leaf mold in gutters. One of the most commonly encountered slime molds, both in nature in forests in the temperate zones of the earth as well as in classrooms and laboratories is the yellow Physarum polycephalum.
Most slime mold are smaller than a few centimetres, but the largest recorded reached an area of up to thirty square metres,[3] making them the largest undivided cells known (although one could argue that slime molds are made up from individual cells). Many have striking colours such as yellow, brown and white.
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Slime molds can generally be divided into two main groups.
Slime molds as a group, are polyphyletic. They were originally represented by the subkingdom Gymnomycota in the Fungi kingdom and included the defunct phyla Myxomycota, Acrasiomycota and Labyrinthulomycota. Today, slime molds have been divided between several supergroups and not one of them is included in the kingdom Fungi.
In more strict terms, slime molds comprise the group of the mycetozoans (myxomycetes, dictyostelids and protostelids). However, even at this level there are conflicts to be resolved. Recent molecular evidence shows that the first two groups are likely to be monophyletic; however the protostelids seem to be polyphyletic, too. For this reason, scientists are trying to elucidate the relations between these three groups.
The Acrasidae have a life style similar to Dictyostelids, but their amoebae behave differently and are of uncertain taxonomic position.
The Plasmodiophorids also form coenocytes but are internal parasites of plants (e.g., club root disease of cabbages).
Finally, the Labyrinthulomycetes are marine and form labyrinthine networks of tubes in which amoebae without pseudopods can travel.
A common slime mold 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 feet in size. Fuligo forms yellow crusts in mulch.
The Protostelids life cycle is very similar to the above descriptions, but they are much smaller, the fruiting bodies only forming one to a few spores.
The Dictyosteliida, cellular slime molds, are distantly related to the plasmodial slime molds and have a very different life style. Their amoebae do not form huge coenocytes, and remain individual. They live in similar habitats and 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 swarms. These amoeba then join up into a tiny multicellular slug-like coordinated creature, which crawls to an open lit place and grows into a fruiting body. Some of the amoebae become spores to begin the next generation, but some of the amoebae sacrifice themselves to become a dead stalk, lifting the spores up into the air.
They begin life as amoeba-like cells. These unicellular amoebae are commonly haploid and multiply if they encounter their favorite food, bacteria. These amoebae can mate if they encounter the correct mating type and form zygotes which then grow into plasmodia. These contain many nuclei without cell membranes between them, which can grow to be meters in size. One variety 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.[4]
Within each protoplasmic strand the cytoplasmic contents rapidly stream. If one strand is carefully watched for about 50 seconds the cytoplasm can be seen to slow, stop, and then reverse direction. The streaming protoplasm within a plasmodial strand can reach speeds of up to 1.35 mm per second which is the fastest rate recorded for any micro-organism.[5] Migration of the plasmodium is accomplished when more protoplasm streams to advancing areas and protoplasm is withdrawn from rear areas. When the food supply wanes, the plasmodium will migrate to the surface of its substrate and transform into rigid fruiting bodies. The fruiting bodies or sporangia are what we commonly see, they superficially look like fungi or molds but are not related to the true fungi. These sporangia will then release spores which hatch into amoebae to begin the life cycle again.[4]
In Myxomycetes, the plasmoidal portion of the life cycle only occurs after syngamy, which is the fusion of cytoplasm and nuclei of myxoamoebae or swarm cells. Therefore, all of the nuclei are diploid at this stage and mitosis occurs simultaneously throughout the organism. Myxomycete plasmodia are multinucleate masses of protoplasm that move by cytoplasmic streaming. In order for the plasmodium to move, cytoplasm must be diverted towards the leading edge from the lagging end. This process results in the plasmodium advancing in fan-like fronts. As it moves, plasmodium also gains nutrients through the phagocytosis of bacteria and small pieces of organic matter.
The Myxomycete plasmodium also has the ability to subdivide and establish separate plasmodia. Conversely, separate plasmodia that are genetically similar and compatible can fuse together to create a larger plasmodium. In the event that conditions become dry, the plasmodium will form a sclerotium, essentially a dry and dormant state. In the event that conditions become moist again the sclerotium absorbs water and an active plasmodium is restored. When the food supply wanes, the Myxomycete plasmodium will enter the next stage of its life cycle forming haploid spores, often in a well-defined sporangium or other spore-bearing structure.
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved. Read more |
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| Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/. Read more |
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| WordNet. WordNet 1.7.1 Copyright © 2001 by Princeton University. All rights reserved. Read more |
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