Physarium polycephalum is one of the most widely recognized and cultivated plasmodia. P. polycephalum undergoes distinctive transformations from spore, to amoeboid, to plasmoid and sporangium. To the naked eye, the spores appear as a reddish or purplish brown mass; amoebas and myxoflagellates can only be seen under microscope; the plasmodium will be a bright yellow, highly reticulate network and have a much finer, denser, distinctive “fan” shape in the direction of movement. Sporangia are dark orange to brown and consist of gregarious lobes connected to a shriveled, thin stipe.
Spores are uninucleate, globose, minutely spinose, 8 to 11 microns in diameter, brownish yellow individually, appear reddish to purplish brown in mass, and require 15 to 48 hours for incubation, defined as the time when seeds are sowed to the splitting of the spore walls. Incubation duration is constant whether a spore is fresh or 18 months old. Cultures are readily started on water drops on the surface of slides. Once germination begins, the spore walls split open and the protoplast emerges.
The amoeboid protoplast will divide mitotically one time right before, during, or after it exits the spore case. The resulting hyaline non-flagellate protoplasmic bodies typically become quiescent temporarily once they exit the spore case (Howard 1931). If the amoeboid are grown in liquid media, they will become uniflagellate, occasionally monopolar biflagellate with a very short secondary flagellum, will assume a comma shape, temporarily move about through posterior pseudopodia, and then begin their characteristic rotating motion. The transformation into myxoflagellates (“swarmcells”) is rapid and reversible, does not require gene activation or protein synthesis (Adelman 2003). If subjected to dry conditions, myxoflagellates will form cysts and can be stored in this state for several months. Swarmcell behavior lasts about 24 hours, but gradually disappears from cultures (Howard 1931). During this time stained preparations will show a characteristically beaked nucleus connected by deeply stained threads to the blepharoplast and flagellum (Howard 1931). It is typical for two swarmcells to touch their non-flagellated ends and begin to fuse, acting as isogametes. The resulting zygotes will them merge or grow into the plasmoid phase and retract the flagella (Howard 1931).
The plasmodium tends to be yellow in color, and is in actuality a large syncytium (multinucleate organism with a common cytoplasm) that can spread over many square centimeters, reportedly up to two square feet. During this stage P. polycephalum can move by forming a network of tubes through which it streams its protoplasm and can do so at a rate of up to 3 centimeters per hour (Howard 1931). The reticulate is quite distinctive and assumes a general, broad, fan shape as P. polycephalum searches for food. Once food has been found, the plasmodium will then concentrate around this source and has the ability to recognize the shortest distance between possible nutrient sources.
When P. polycephalum encounters low atmospheric humidity, the plasmodium will begin to desiccate, forming a bright orange or dark brown sclerotium. The sclerotium can be reactivated many months later in suitably aerobic and humid conditions (Howard 1931).
Sporangia are formed at night through a process spanning approximately 12 hours. In anticipation of fruiting, P. polycephalum will assume a very dense, opaque yellow color, will search for a dryer location, and then form a thick, continuous, verrucose layer that will segment within an hour. Over the following hour, the flat segments round up into knobs, which then lengthen out into vertical, finger-like pillars, eventually the stipe and sporangium become distinct although no membrane separates the two (Howard 1931). The protoplasm moves up in apical swelling, and the sporangial head begins a series of dichotomous division into lobes although each lobe remains connected to the center stipe. At this point, one can observe nuclei measuring 3.2 to 4.8 microns in diameter, as well as degenerating, deeply staining nuclei 2.5 to 3.5 microns in diameter. Mitosis then begins from the periphery and advances to the center of the lobe. Cytokinesis and mitosis then occur over the next 30 to 60 minutes, sometimes concurrently (Howard 1931). The segments then differentiate into spores, which exit once the outer sporangium wall ruptures in moist conditions.
Created: 2012-06-10 01:03:21 BST (+0100) by Eduardo A. Esquivel Rios (Eduardo27)
Last modified: 2013-01-21 14:44:21 GMT (+0000) by Erlon (Herbert Baker)
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