Division :- Myxomycota (plasmodial slime molds)
The organisms included in this division are commonly known as the plasmodial or acellular slime molds. The exact systemic position of the Myxomycota is not clear. Olive (1975) classified them in the Kingdom Protista. Ainsworth (1973) classification is most accepted in which Division Myxomycota belong to Kingdom Mycetae (=Fungi).
- Vegetative phase is unicellular, without a cell wall, a multinucleate mass of protoplasm called a plasmodium.
- They engulf bacteria, protozoa, and other food particles by phagocytosis.During this mode of ingestion, the food particles, usually bacteria, beceome surrounded by the pseudopodia of the myxamoeba. Once the food has been engulfed in this matter, it is surrounded by a membrane or food vacuole where hydrolytic enzymes are secreted that will digest the food.
- The vegetative stage in slime molds is morphologically similar to that of an amoeba, because of that known as myxamoeba. Plasmodium produces one or more sporangia where meiosis takes place.
Within the Myxomycota, the class Myxomycetes, known as the acellular slime molds.
Class: Myxomycetes There are approximately 71 genera and 500 species of Myxomycetes.
Habitats (Occurrence) :-There are commonly occur in damp places especially on decaying wood, all of which are terrestrial (found on land).
- Asexual reproduction occurs by binary fission of myxamoeba or fragmentation of the plasmodium.
- Sexual reproduction takes place by the plasmogamy (fusion of compatible gametes) occurs between myxamoebae or swarm cells (some species are heterothallic).Karyogamy (fusion of nucleus) occurs shortly after plasmogamy to form zygote (2n). Zygote forms plasmodium – longest lived vegetative stage.
Life Cycle of Myxomycetes:- Bi-phasic life cycle with a haploid and a diploid phase. The diploid stage is a naked coenocytic protoplast (that is, it consists of a multinucleate mass of cytoplasm that is enclosed only by a plasma membrane and does not have a cell wall).When the sporangia matured and turned grey. Normally globose, with a definite, rather thick cell wall,unicellular, uninucelate and haploid spores are released by rupture of the sporangial coat.The spore surface may range from almost smooth to reticulate. Spores of myxomycetes are small (4-20 µm) and are easily picked up by air currents, arthropods and other animals On germination a spore produces one or four myxamoebae or flagellate cells known as swarm cells- that have one or two anterior flagella, all of which are of the whiplash type. 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. Myxamoebae and swarm cells can be interconverted. 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.
When free water is available myxamoeba can differentiate into flagellated swarm cells Swarm cells and myxamoeba may function as gametes (both in homothallic or heterothallic strains).The compatible gametes fuse in pairs (two swarm cells, two myxamoebae) to form a diploid zygote.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.
Plasmodium:-The plasmodium is a diploid structure.Plasmodium is a naked, multinucleate, motile mass of protoplasm; no cell wall around it. However, in most species, the plasmodium is enveloped by a gelatinous slime sheath that contains microfibrils. Just inside the slime sheath is the plasma membrane that surrounds and confines the cytoplasm Plasmodia are of various colors. Examples- Physarum polycephalum it is a bright yellow, slimy structure and Didymium iridis the plasmodium is colorless 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. There are three 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
Sporulation and Sporophores :-
- Under favorable conditions, the plasmodium will migrate and feed for a period of time before being converted to one or more sporophores.Sporophores posses brightly coloured sporangia.
- 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.
- Sporangium consists of the following parts: -
- Peridium (persistent or evanescent):- The fragile, outer layer of the sporangium is the peridium (pl.=peridia), which may be persistent or degenerate by the time the sporangium is ready to disperse its spores.
- Hypothallus - The hypothallus is a plasmodial remnant forming the base for one or more fruiting bodies. The hypothallus connects the stalk or stipe to the substrate. It may be dull or brightly colored, thin and delicate or coarse.The hypothallus may be composed of calcium carbonate
- Stalk :- Basal portion of sporangium, may or may not be present, may be hollow or filled with material Stalks formed from secretions of plasmodium and are acellular (in contrast to cellular slime molds
- Columella and Pseudocolumella :-The columella appears as an extension of the stalk into the spore mass, although it may not resemble the stalk. In a sessile fruiting body, the columella may be an area on the inside of the peridium where it contacts the substrate or appears as a dome-shaped structure. A pseudocolumella (pseudo=false) is a columella that does not attach to the stalk. The pseudocolumella is found only in the order Physarales, existing as a lime mass within the spore mass. Capillitial elements may be attached to the columella or pseudocolumella.
- Capillitium and Pseudocapillitium :-The capillitium consists of threadlike elements inside the sporophores, intermingled with the spores.Some elements of capillitium may be elastic, allowing for expansion when the peridium opens, while other types are hygroscopic and capable of dispersing spores by a twisting motion. A pseudocapillitium is present in some aethalia and pseudoaethalia producing species. Pseudocapillitial elements are highly variable in size and shape, and may appear as bristles, threads or perforated plates.