Membrane formation during sclerotization of Physarum polycephalum plasmodia

Variance of ploidy in mitochondrial nucleus during spherulation in Physarum polycephalum

In Physarum polycephalum, microplasmodia differentiated into spherules when cultures were aged for 8-10 days. Respiration rates of the microplasmodia decreased rapidly with ageing to a 90% decrease in oxygen consumption over 9 days. We studied this phenomena by isolating and characterizing mitochondria from microplasmodia and spherules at different stages of spherulation. Oxygen uptake by the isolated mitochondria decreased with spherulation. Morphological and biochemical analyses showed that mitochondrial differentiation to inactive state was characterized by a decrease not only in dimension but also of content (DNA, RNA and protein). Diminutive mitochondria contained small particle-shaped mitochondrial nuclei. The DNA content, measured by microscopic fluorometry, was about 1.15 and 0.58 X 10(-10) g, which corresponded to about 16 and 8 genome copies, respectively (e.g., 32 genome copies per mitochondrion at mitochondrial G1). Restriction endonuclease analysis showed that the physical structure and methylation pattern of the mtDNA had not changed although the DNA content per mitochondrion had decreased remarkably with spherulation. This showed that changes in the ploidy level of the mitochondrial nucleus during spherulation were due to reduction in the number of whole mitochondrial genomes.

Translational regulation of polysome formation during dormancy of Physarum polycephalum

The translational activity of actively growing microplasmodia and dormant microsclerotia of Physarum polycephalum was investigated by analyzing the distribution of ribosomes in polysomes. Microplasmodial post-mitochondrial fractions contained substantial amounts of polysomes and ribosomal subunits but very few native monosomes. During the starvation period which preceded microsclerotium formation, polysome levels remained constant, whereas the subunit titer began to increase. During encystment ribosomal subunits continued to accumulate as the level of polysomes gradually decreased. Dormant microsclerotia contained a large surplus of stored ribosomal subunits but no detectable polysomes. However, incubation of microsclerotia with concentrations of cycloheximide sufficient to slow polypeptide elongation without affecting initiation caused the gradual reappearance of polysomes at the expense of the subunits. Under these conditions the percentage of subunits driven into polysomes reached values similar to those of actively growing microplasmodia. Microsclerotia returned to nutrient medium contained very low levels of polysomes during the lag period which preceded germination. These were formed with preexisting, stored messenger ribonucleic acid. During the germination period, polysome levels were markedly increased. This elevation was dependent on new ribonucleic acid transcription. It is concluded that dormant microsclerotia contain functional messenger ribonucleic acid and ribosomes which are subject to translational repression at the level of initiation.

Calcium excretion and deposition during sporogenesis in Physarella oblonga

Calcium excretion and deposition were studied during sporogenesis in the myxomycete, Physarella oblonga. Prior to fruiting, the migrating vegetative plasmodium contains numerous, large channels connected to the external environment. During early sporangial development, these channels become smaller and are isolated as channel remnants. Plasmodial microfilament bundles also disappear at this time. Internally, two types of spheres, each about 0.5-1.5 mum in diameter, are formed in the channel remnants. A dense sphere appears to originate from small particles initially secreted from the mitochondria. These particles then enter the channels and channel remnants by reverse pinocytosis and fuse to form the dense spheres. A second, less dense sphere has no detectable cytoplasmic origin but also appears to form in the channels and channel remnants. Both types of spheres are concentrated by the fusion of channel remnants to form enlarged regions (knots and spikes) of the developing capillitium. Some of the channel remnants also fuse with the outer surface of the sporangium, depositing both spheres to the outside. Both spheres have been studied by various light and electron microscopic techniques and with specific histochemical and analytical procedures. The less dense spheres appear to contain a major portion of calcium in the form of calcium carbonate. These results are compared to other recent studies on calcium deposition in the Myxomycetes and a different mechanism for calcium deposition is proposed.