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5.2.1: Slime Molds

  • Page ID
    37000
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    Learning Objectives
    • Differentiate between the three major groups of slime molds.
    • Differentiate between slime molds, fungi, and plants.
    • Identify structures and phases in the myxomycete life cycle and know their ploidy.

    Slime molds are an unusual group of organisms that have previously been classified as animals, fungi, and plants. Like plants, slime molds have cellulose in the cell walls of their spores. Unlike plants, slime molds are heterotrophs! Though they were formally classified as fungi, slime molds do not have chitin in their cell walls and have a diplontic life cycle (Figure \(\PageIndex{12}\)). These organisms move about as amoebae engulfing bacteria (unlike fungi, who digest food externally). When conditions become unfavorable, whether due to lack of food or lack of moisture, they form spores. They can be found in damp substrates with ample bacteria and are most frequently found on decaying logs and forest duff.

    There are several different lineages of organisms commonly referred to as slime molds. Cellular slime molds (dictyostelids, Figure \(\PageIndex{1}\)(a)) are groups of unicellular amoebae that collaborate to form fruiting structures to disperse spores. Protostelids make small fruiting bodies that have cellular stalks. Plasmodial slime molds (classified as Myxogastria or Myxomycetes, Figure \(\PageIndex{1}\)(b)) form a large, multinucleate amoeba with no cell wall that will eventually wall off individual nuclei to form spores.

    a) A circular dome with long branches emanating outward. B) A yellow structure that looks like foam on a branch.
    Figure \(\PageIndex{1}\): (a) Dictyostelium discoideum is a cellular slime mold can be grown on agar in a Petri dish. In this image, individual amoeboid cells (visible as small spheres) are streaming together to form an aggregation that is beginning to rise in the upper right corner of the image. The primitively multicellular aggregation consists of individual cells that each have their own nucleus. (b) Fuligo septica is a plasmodial slime mold. This brightly colored organism consists of a single large cell with many nuclei.

    Dictyostelids

    The cellular slime molds exist as individual amoeboid cells that periodically aggregate. The individual amoebe can be seen aggregating in Figure \(\PageIndex{1}\)(a). The aggregate then forms a fruiting body (Figure \(\PageIndex{2}\)) that produces haploid spores. One cellular slime mold, Dictyostelium discoideum, has been an important study organism for understanding cell differentiation, because it has both single-celled and multicelled life stages, with the cells showing some degree of differentiation in the multicelled form. Watch Video \(\PageIndex{1}\) to see how these individuals aggregate into a single fruiting body.

    A collage of photos showing the culminating stages of Dictyostelium
    Fruiting bodies with distinguishable stalks and sporangia
    Figure \(\PageIndex{2}\): These images show a species of Dictyostelium developing fruiting bodies. In the first image, the culmination stages are visible. Many of these look like a settled teardrop with a point emerging. Others, in later stages, look more like a worm moving vertically. In the second image, several sporangia are in the process of forming, with individuals still migrating up the stalk to the apex. Only the individuals who end up in the sporangium will survive to "reproduce" as spores. Photos by Jerry Cooper, CC BY 4.0.

    Video \(\PageIndex{1}\): Watch the strange behavior of the cellular slime mold Dictostelium discoideum as individual amoebae respond to an aggregation signal (cAMP), form a mobile slug, and eventually produce a stalked fruiting structure and spores. Sourced from YouTube.

    The organisms in this group have a complex life cycle (Figure \(\PageIndex{3}\)) during the course of which they go through unicellular, multicellular, spore producing, and amoeboid stages. Thousands of individual amoebae aggregate into a slimy mass - each cell retaining its identity (unlike plasmodial slime molds). The aggregating cells are attracted to each other by the cyclic AMP (cAMP) that they release when conditions become stressful, such as a depletion in food. Individual amoebae respond to the chemical signal by moving to areas of higher cAMP concentration (chemotaxis), eventually aggregating into a single slug. The slug can respond to moisture and light gradients, navigating to a good spot for spore production. Some cells in the slug contribute to a 2–3-millimeter stalk, drying up and dying in the process. Cells atop the stalk form an asexual fruiting body that contains haploid spores. The spores are disseminated and can germinate if they land in a moist environment.

    The Dictyostelium discoideum life cycle
    Figure \(\PageIndex{3}\): Dictyostelium life cycle (text from the original figure caption). "(A) During the growth phase of development, amoeboid cells feed on bacteria and replicate by binary fission. The development cycle is initiated upon resource depletion, and aggregation occurs when starving cells secrete cyclic AMP to recruit additional cells (B). The aggregating cells organize to form the mound stage enclosed within an extracellular matrix composed of cellulose and mucopolysaccharide (26) (C) and continue to develop into the standing slug (D). Depending on its environment, the standing slug either falls over to become a migrating slug that moves toward heat and light (e) or proceeds directly to the culmination stages (F) that ultimately produce the fruiting body, which consists of a spore-containing structure, the sorus, held aloft by a stalk of dead cells (g). Spores are released from the sorus and germinate into growing cells (H). Under optimal conditions, the developmental cycle takes around 24 h. If the slug forms underground, it migrates toward the surface to maximize spore dissemination. To protect itself from infection during migration, the slug possesses a rudimentary immune system comprising phagocytic sentinel cells. These cells move throughout the slug, take up bacteria and toxins, and are shed along with extracellular matrix as the slug moves (e). In response to bacteria, sentinel cells release extracellular traps, derived from mitochondrial DNA, via an unknown mechanism involving NADPH oxidase (NOX)-generated reactive oxygen species (ROS) and TirA, a soluble protein containing a toll/interleukin 1 receptor domain (i)." Figure sourced from the publication Eat Prey, Live: Dictyostelium discoideum as a model for cell-autonomous defenses. Dunn et al., (2018). CC BY 4.0 DOI: 10.3389/fimmu.2017.01906

    Protostelids

    Protostelids are a group that has received less attention than either the Dictyostelids or plasmodial slime molds, as each of the latter groups contains a model organism used to study a specific system. Protostelids make simple fruiting bodies, similar to the Dictyostelids, with a stalk and spores at the apex. The slime mold Ceratiomyxa looks more like a plasmodial slime mold, but closer inspection reveals that spores are formed on minute, stalked fruiting bodies covering the external surface of the tentacle-like structures (Figure \(\PageIndex{5}\)). Ceratiomyxa may not actually be a protostelid, but the small, stalked fruiting bodies formed on the external surface are similar to what would be found in a true protostelid.

    A cellular slime mold growing on a log
    Figure \(\PageIndex{4}\): This image shows the slime mold Ceratiomyxa fruticulosa, which looks a bit like an organism you'd find under the sea. This slime mold (likely) belongs to the protostelid group because it makes its spores externally. Each coral-like extension of this slime mold is covered with tiny spores. Photo by Maria Morrow, CC BY-NC.
    Projections of Ceratiomyxa covered in tiny fruiting structures with thin stalks and globose sporangia.
    Figure \(\PageIndex{5}\): A close-up of Ceratiomyxa fruticulosa, showing the fruiting structures covering the outside of the strange, coralloid projections. Photo by Damon Tighe, CC BY-NC. See another great example here.

    Plasmodial Slime Molds (Myxogastria)

    Plasmodial slime molds represent a vast diversity of morphologies. While still a plasmodium (see Figure \(\PageIndex{6}\)), they can be difficult to distinguish. However, once they have formed into a fruiting structure, they can form distinct, varied, and amazing shapes (see Figures \(\PageIndex{7-10}\))!

    The Plasmodium

    In their feeding stage, myxomycetes form one large amoeba called a plasmodium with many nuclei and no cell wall. This plasmodium moves over damp, decaying material looking for bacteria (and sometimes fungi) to engulf and digest. When it dries out or runs out of food, it begins to make fruiting structures called sporangia (sporangium, singular). Inside these sporangia, the diploid nuclei will undergo meiosis and haploid nuclei will be walled off to make spores for aerial dispersal. Dispersal by spores, heterotrophism, and glycogen as a storage carbohydrate originally classified this group within Kingdom Fungi, but this is the end of the similarities. The spores have cell walls made of cellulose, like plants. When these spores land, they will germinate into haploid cells with two flagella (called swarm cells) or amoebae that will fuse together to form a diploid plasmodium. See Figure \(\PageIndex{11}\) for a diagram of this life cycle.

    A bright yellow slime has fanned across the surface of some dead wood. Raised veins are visible traversing the plasmodium.
    Figure \(\PageIndex{6}\): This image shows Physarum polycephalum exploring some decaying wood. This is the feeding plasmodium. During this stage, the giant, multinucleate amoeba moves over the substrate engulfing bacteria. The veins allow for streaming of the cytoplasm and efficient connections between sources of food. Photo by Daniel Folds, CC BY-NC.

    Sporocarp Diversity

    The diversity of sporulating structures, or sporocarps, has led many to fall in love with this group of organisms. In Hemitrichia serpula, the plasmodium forms into a network of veins that then become fruiting structures (a plasmodiocarp, see Figure \(\PageIndex{7}\)). In some slime molds, like Fuligo and Lycogala, the entire plasmodium forms a cushion that dries and produces spores (an aethalium, see Figure \(\PageIndex{8}\)). In other slime molds, individual sporangia are so closely clustered together, they appear to be a single fruiting structure (a pseudoaethalium, see Figure \(\PageIndex{9}\)). The last type of sporocarp is more familiar, forming many distinct stalked sporangia (see Figure \(\PageIndex{10}\)).

    A plasmodiocarp. The veins of the plasmodium have formed into a network of tubes filled with spores..
    Figure \(\PageIndex{7}\): Hemitrichia serpula forms an uncommon fruiting body called a plasmodiocarp. The feeding stage accumulates its protoplasm into the veins of the plasmodium, forming strange linear, intertwining shapes. Photo by Roman Providukhin, CC-BY-NC.
    Round, pink Lycogala fruiting bodies oozing after being poked
    Figure \(\PageIndex{8}\): Fruiting bodies of the plasmodial slime mold Lycogala epidendrum form into cushion-like structures called aethalia. The plasmodium has formed into pink ball-like structures on the surface of a rotten log. One of these structures has been popped and is oozing a pink slime, full of immature spores. This pink slime gives Lycogala its name, wolf's milk. Photo by Maria Morrow, CC BY-NC.
    A pink slime mold with distinct columns of sporangia that have formed into a single (sort of spiky) cushion
    Figure \(\PageIndex{9}\): Another option for a fruiting structure is the pseudoaethalium, where there are distinct sporangia but they still form together like a cushion. This is the type of fruiting structure formed by Tubifera ferruginosa, the red raspberry slime mold. Photo by Hiromi Karagiannis, CC BY-NC.
    Four fruiting structures, each with a pale white stalk and dark sporangium that has an oil-sheen look to it (rainbowed)
    Figure \(\PageIndex{10}\): Fruiting bodies of Diachea leucopodia have a distinct stalk and sporangium. The stalk in this species is white, while the elongate sporangium displays an oil-sheen rainbow of colors. Photo by Sypster, CC BY-NC.

    Life Cycle

    The life cycle of plasmodial slime molds is best classified as diplontic: the "multicellular" (actually just multinucleate) phase is diploid. Haploid cells that germinate from spores (amoebae or biflagellate swarm cells) do not grow until after they have fused with another haploid cell. In some myxomycetes, amoebae or swarm cells produced from the same parent plasmodium can fuse together to form a new plasmodium. This is called homothallism (homo- meaning same, thallus). In other myxomycetes, these gametes must be from different individuals (heterothallism, hetero- meaning other). The discovery of different mating types in myxomycetes, as well as the genes that determine mating type, was made by O'Neil Ray Collins (Figure \(\PageIndex{11}\)).

    O’Neil Ray Collins, wearing a suit with a tie
    Figure \(\PageIndex{11}\): O'Neil Ray Collins (1931-1989) was an American mycologist and botanist. He is well known for his research on slime-mold genetics and was the first African-American biologist to hold a tenured position at University of California, Berkeley. In addition to discovering the existence of mating types and the associated alleles in slime molds, Collins's work to increase diversity and support minority students are important components of his legacy at UC Berkeley. (Credit: Public Domain)
    Life cycle of a plasmodial slime mold
    Figure \(\PageIndex{12}\): Plasmodial slime molds exist as large amoebae filled with many diploid (2n) nuclei. When conditions signal the amoeba to end its vegetative phase (lack of food or moisture), the plasmodium coalesces into a fruiting structure (sporocarp). Inside the sporocarp, the nuclei undergo meiosis to produce haploid spores. Spores are released and germinate when conditions are right. The spore might germinate to produce an amoeba or a flagellated swarm cell. Swarm cells and amoebae are able to transition between these stages. At some point, these haploid (1n) cells will fuse to produce a diploid amoeboid zygote. Plasmogomy is the fusion of cytoplasm of two cells. Karyogamy is the fusion of nuclei and leads to the production of a diploid zygote. The zygote will grow, replicating the nuclei with no cytokinesis, to form the multinucleate plasmodium. A mature plasmodium (multinucleated free-flowing mass of protoplasm) can produce sclerotium (small cells) in a dry habitat.

    Summary

    Slime molds represent several different lineages: the cellular slime molds (Dictyostelids), Protostelids, and plasmodial slime molds (Myxomycetes). These organisms move about as amoebae consuming bacteria until conditions become unfavorable, at which point they form spores. They can be found in damp substrates with ample bacteria and are most frequently found on decaying logs and forest duff.

    Dictyostelids are model organisms for studying altruism. They are unicellular, but collaborate to form multicellular structures where only some of the individuals involved go on to make spores. Protostelids are less well-understood and form a single sporangium at the tip of a cellular stalk. Plasmodial slime molds (the myxomycetes) form a large, multinucleate amoeba during their feeding stage called a plasmodium. They have diplontic life cycles and there is a lot of morphological diversity of sporocarps represented in this group. Some organisms in this group are studied for their ability to solve mazes and spatial puzzles.

    Though these organisms seem primitive, they have complex interactions with each other and their environments.

    Attributions

    Curated and authored by Maria Morrow, CC BY-NC, using the following sources:

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    This page titled 5.2.1: Slime Molds is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers (ASCCC Open Educational Resources Initiative) .