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.