Acrasiomycota: the cellular slime molds
Myxomycota: true or plasmodial slime molds
Anton de Bary (1858, 1859, 1887) - first detailed life history studies that characterize slime molds; distinguished the true slime molds from the cellular slime molds on the basis of plasmodium formation by myxomycetes.
- Dictyostelid and acrasid slime molds are distinguished from the true slime molds by the aggregation of their somatic amoebae to form a pseudoplasmodium also designated as a slug or a grex. The component amoebae do not fuse, but retain their individuality while cooperating as members of a well-organized community until formation of the sorocarp
- stalks consists of walled cells
Acrasin, now known to be cyclic AMP, a pheromone responsible for aggregation in some species of Dictyostelium
Life cycle of Dictyostelium discoideum
Myxomycota: The true slime molds, or plasmodial slime molds
- About 500 described species
- These organisms exhibit phagotrophic nutrition and produce the following life-cycle stages:
- Three types of uninucleate single cells, one of which is flagellate,
- A multinucleate somatic phase known as a plasmodium that moves and exhibits a reversible shuttle streaming of its protoplasm,
- A resistant stage consisting of a sclerotium, and
- A reproductive phase that culminates in the production of stationary sporophores containing walled spores that may be dispersed by wind and water or by arthropods that live and feed on the sporophores.
Myxomycetes exhibit two types of mitotic divisions, one that is centric with a nonpersistent nuclear envelope that breaks down in prophase and another that is acentric with a nuclear envelope remaining more or less intact until late anaphase or early telophase.
Fuligo septica, actually has several common names, "flowers of tan" and, more recently, "the blob."
- Bright yellow plasmodium of this species may spread over areas several feet in diameter before it eventually becomes concentrated and forms the largest sporophores of any slime mold; sporophores often may exceed a foot or more in diameter and more or less interconnected sporophores of this species may spread over several feet.
- Usually take the form of flattened, crust-like masses about one half inch or so thick. The surface of the mass is gray to yellow-brown in color.
- May develop directly on the soil or on grass, they commonly can be found on sawdust piles, bark and wood chips that have been used as mulch, dead tree branches, and tree stumps.
- Substrates upon which slime molds grow are only secondarily important; it is the minute organisms such as fungi, bacteria, and protozoans on these substrates that are the requirement for the presence of the phagotrophic, assimilative stages of slime molds.
- Moisture and temperature also are important abiotic factors and contribute to the periodic appearance and seasonal nature of some species. During a rainy season myxomycetes begin, as a group to appear in May in northern temperate regions, and continue to sporulate through October.
- 1 class Myxomycetes
- 2 - 3 subclasses containing a total of 5 - 6 orders
- Consider 3 orders Physarales, Stemonitales, and Ceratiomyxales.
Orders are distinguished on the basis of
- Sporophore development,
- Type of sporophore produced,
- Method of spore production,
- Spore color,
- Presence or absence of special thread-like structures collectively known as capillitium (pl. capilitia; L. capillus = hair),
- Calcium (often called "lime") content of the sporophore, and
- Plasmodium type.
Endosporous Myxomycetes- Physarales, Stemonitales
- With the exception of the three known species comprising the genus Ceratiomyxa of the Ceratiomyxales, all other myxomycetes produce their spores inside a sporophore of some type that is encased by a peridium
- Referred to as endosporous myxomycetes
- Liberated from their sporophores by wind, water and the activities of animals, including arthropods
- Generally globose with a definite, rather thick wall, the surface of which may be smooth, punctate, spiny, warty, reticulate or areolate; relatively little information is available on the composition of the myxomycete spore wall.
- Mature myxomycete spore typically contains a single haploid nucleus.
- Color of the spores in mass may be pallid, yellow, rosy, purple, olivaceous, gray, deep violet, brown, or black. The pigments are dilute so that individual spores, when viewed under strong transmitted light, will not show the same color as the mass of spores; spore characteristics are extremely important in the taxonomy of myxomycetes at several taxonomic levels; size, shape and color, spore ornamentation is also important.
- Spores of some myxomycetes appear to be exceptionally resistant to unfavorable conditions, especially prolonged periods of desiccation which few other organisms are able to withstand; some spores are capable of germinating after 75 years of storage in a herbarium
- Spores of myxomycetes are small (4-20 µm) and are easily picked up by air currents, arthropods and other animals
Spore Germination, Myxamoeba, and Swarm Cells.
- In nature, myxomycete spores probably germinate in rain water.
- when a spore germinates, one or more myxamoebae or flagellate cells known as swarm cells emerge. In most species the release of the spore protoplasm is accomplished by the splitting of the spore wall to form what appears to be a v-shaped opening; spores of some species germinate through a minute pore dissolved in the spore wall.
- Whether myxamoeba or swarm cells emerge from germinating spores depends to some extent on environmental conditions. If spores are suspended in water, the emerging cells are often flagellate from the very beginning. Sometimes, however, a myxamoeba will issue from the spore, remain quiescent for a few minutes, and then either develop flagella and become a swarm cell or begin to divide repeatedly, resulting in a large population of myxamoeba that may or may not be converted to swarm cells. Swarm cells themselves are not capable of division. However, they readily convert to myxamoebae following retraction of their flagella.
- Swarm cells may possess from 1 to 4 anteriorly attached flagella, all of which are of the whiplash type and anchored in the cell by a complex apparatus. In most cases swarm cells possess one long prominent flagellum and a second shorter, inconspicuous flagellum that is directed backward and appressed to the cell surface
- Swarm cells obtain nourishment by absorbing dissolved food material from the surrounding medium and by ingesting, at their posterior ends, bacteria, yeast cells, some particles of organic matter, and, in some cases, even certain types of fungal spores. The posterior end of the cell appears to be very sticky and may be important in catching food.
- After a period of motility, the swarm cell eventually withdraws its flagella, thus changing into a myxamoeba. Myxamoebae, like swarm cells, are capable of ingesting food particles
- When food is abundant and environmental conditions are favorable, myxamoebae divide repeatedly, giving rise to a large population of cells. The nuclear divisions are centric and open (the nuclear membrane breaks down during prophase and is reconstituted after telophase.) Under unfavorable conditions myxamoebae round up and secrete a galactosamine wall to form microcysts (sclerotia). When favorable conditions return, the microcysts germinate and either a myxamoeba or swarm cell emerges from each wall.
Formation of Zygotes and Plasmodia
"Ever changing, ever flowing, the plasmodium creeps over the surface of the substratum, engulfing particles of food in its path."
- Swarm cells and myxamoeba may function as gametes and in heterothallic strains they eventually fuse in pairs (two swarm cells, two myxamoebae or possibly one of each) to form a zygote.
- Mating is bipolar or unifactoral with multiple alleles occur
- There is evidence to suggest that a "critical mass" of myxamoeba or swarm cells is necessary before fusions occur. If two compatible cells are isolated and placed near each other on a favorable medium, they will not fuse, but start dividing instead, each giving rise to a clone consisting of many cells. Only after a large number of cells are formed in a culture will fusions take place. Also, two compatible cells that come in contact do not fuse immediately. It appears that contact for several hours is necessary before fusion actually takes place.
- Not all myxomycetes exhibit heterothallism and variation has been shown to exist even in a single species; homothallic, apogamic
- Zygote of a heterothallic myxomycete is formed by the union of two gametes (swarm cells/myxamoeba). In the case of swarm cells contact takes place at their posterior ends.
- Resulting zygote is initially flagellate and swims for a time before retracting its flagella and changing into an amoeba-like cell.
- As the zygote grows, its nucleus undergoes successive synchronous mitotic divisions without cytokinesis and the cell becomes transformed into a multinucleate, amoeboid structure, the plasmodium.
- The mitotic divisions that occur in this stage of the life cycle are intranuclear (closed) and do not involve centrioles. The nuclear envelope remains intact until late anaphase or early telophase, with the spindle microtubules confined within the nucleus by the nuclear envelope. This is in contrast to the open divisions with centrioles that occur in myxamoebae.
- lasmodium is a mass of protoplasm, delimited only by a thin plasma membrane and a gelatinous sheath; does not have a definite size or shape
- Plasmodium eventually matures and develops into the sporophore(s) typical of the species
- Plasmodium often is described as being a naked mass of protoplasm; no cell wall around it
- However, in most species, the plasmodium is enveloped by a gelatinous slime sheath that contains microfibrils and is shed as the plasmodium creeps over the substratum leaving traces behind it that easily are visible on dead leaves and on agar over which it has migrated
- Just inside the slime sheath is the plasma membrane that surrounds and confines the cytoplasm
- Plasmodia are of various colors, ranging from colorless to white, gray, black, violet, blue, green, yellow, orange, and red, with the color depending largely though not exclusively on the species.
- Young plasmodia unite readily with other plasmodia or with zygotes of the same strain and thus increase in size. These unions are probably somatic and are not followed by nuclear fusions.
- In nature plasmodia probably feed on bacteria, spores of fungi and plants, and possibly on protozoa and even on bits of nonliving organic matter
- Growth is accompanied by successive mitotic divisions of the nuclei embedded in the cytoplasm. In growing plasmodia of P. polycephalum, nuclear division occurs almost simultaneously every 8-10 hours throughout the plasmodium and requires 20 to 40 minutes for completion. This synchrony is truly remarkable, particularly in view of the fact that the thin, sheet-like plasmodia of this slime mold may be several feet in diameter and contain millions of nuclei.
Three basic types of plasmodia
protoplasmodium - microscopic throughout its existence; gives rise to only a single sporangium when it fruits.
aphanoplasmodium - resembles a protoplasmodium in its initial stages, but soon elongates, branches, and becomes a network of very fine, transparent strands; Stemonitales.
phaneroplasmodium - characteristic of Physarales, also resembles a protoplasmodium at first; it grows larger and becomes more massive. Its protoplasm is very granular, and the plasmodium is visible even at an early stage of development. The gelified and fluid portions of the veins are easily distinguishable and the rhythmic, reversible streaming is very conspicuous.
In the normal course of events, the plasmodium gives rise to sporophores. Under certain conditions, however, a phaneroplasmodium becomes converted into an irregular, hardened mass, the sclerotium, that can remain dormant for a long time, but grows out into a plasmodium again when conditions favorable for growth return.
Sporulation and Sporophores
- Entire plasmodium of a myxomycete usually is converted into one or more sporophores so that the somatic and reproductive phases seldom coexist in the same individual. Whatever happens in a plasmodium that causes it to sporulate appears to be irreversible, for once a plasmodium reaches the fruiting stage it cannot be induced to resume growth.
- The endosporous myxomycetes produce four general types of sporophores
- Plasmodium forms numerous individual stalked or sessile sporangia in close proximity on the portion of the substratum previously occupied by the plasmodium. Each sporangium has a peridium of its own. There also may be a thin, cellophane-like base, the hypothallus, from which the sporangia arise; each sporangium is independent of all the others in the group. Most slime molds produce sporangia; most species are no more than a few millimeters tall.
- Second type of sporophore is the aethalium; fairly large, sometimes massive, generally cushion-shaped sporophore that is derived from an entire plasmodium that has not differentiated into individual sporangial units. In some aethalia the walls of the individual sporangia are quite evident; in others they are difficult to see, and in others the aethalium shows no trace of internal sporangial walls. In all cases, the entire body is enclosed in a more or less continuous peridium. e.g.Fuligo septica , noted earlier in this chapter, produces the largest sporophore of any myxomycete.
- Third type of sporophore is the pseudoaethalium; In this structure, a group of sporangia are crowded together to form what appears to be a single sporophore. The individual sporangia are clearly distinguishable, however, and are by no means fused.
- Fourth type of sporophore, the plasmodiocarp, is similar to a stalkless sporangium, but differs in that it retains, to a certain extent, the morphology of the plasmodium. In the formation of the plasmodiocarp, the protoplasm accumulates in some of the main veins of the plasmodium and develops into a sporophore that more or less retains the shape of the plasmodial venation at the time of fruiting. It is very difficult to draw the line between the sessile types of sporangia and short plasmodiocarps. These two forms actually merge into one another and may be found side by side in the same group of fructifications, developed from a single plasmodium; e.g. Hemitrichia
- Most common type of sporophore produced by myxomycetes is the sporangium.
- Sporangium consists of the following parts:
- +/- hypothallus - a secretion of the plasmodium on the substratum at the time of sporulation
- +/- stalk,
- peridium (persistent or evanescent),
- +/- columella - part of the stalk that continues into the spore-bearing sac
- +/- capillitium, attached to the columella; a system of secreted threads inside the sporophores, intermingled with the spores.