23. Colony Formation in Pediastrum boryanum

23. Colony Formation in Pediastrum boryanum

The preservation of the developmental Biology Film Series was made possible by generous contributions from Publisher of “The Biggest Picture” Producer of the documentary, “Symbiotic Earth” Pediastrum boryanum is a disc-shaped green freshwater alga about 100 microns in diameter. It is a colonial plant made up of a number of cells adhering edge to edge. The number of cells in an individual colony may be two, four, eight, sixteen, or thirty-two. It rarely exceeds sixty-four. All of these cells are produced asexually from identical zoospores derived from a single common mother cell of the parent colony. When reproduction takes place, the nucleus of each mother cell undergoes a series of mitoses. Within an individual mother cell the mitoses are synchronous, and the process may stop at any point between the first and the sixth nuclear division. This establishes the number of cells in the daughter colony and hence its size. When the nuclear divisions have ceased, The cytoplasm is cleaved into uninuclear cells about seven microns in diameter. These cells separate and become active biflagellate zoospores as can be seen if they are experimentally released from the mother cell. Under natural conditions however, the outer wall of the mother cell ruptures and the zoopores are extruded, But they are still confined within a membranous vesicle which previously served as an inner wall to the mother cell. Restrained by this small transparent bag, which swells as development proceeds, the zoospore swarm actively for from five to fifteen minutes before they slow down and begin to aggregate. In these scenes the action has been speeded up by time-lapse cinematography. During aggregation the cells can alter their positions relative to one another before all movement ceases. How the colony comes to achieve the shape of a flattened disc one cell in thickness is something which remains to be explained. Once the disk has become stable, each of the peripheral cells grows a pair of prongs. This takes about thirty minutes. The number of prongs per cell is specific for the organism. Pediastrum boryanum has two. But Pediastrum simplex possesses only one on each of the outer cells. Despite the fact that the cells of a colony were originally identical sister cells derived from a single mother cell, they nevertheless differ in shape when finally incorporated into the colony. Most obviously the inner cells lack prongs. A Stereo Scan Electron Micrograph however, shows that the inner cells actually do have stunted prongs which have failed to develop. This seems to indicate that all the cells are initially capable of growing prongs. But the development of prongs is suppressed in cells which are surrounded. Often all the vesicles containing aggregating zoospores will have been ejected from the parent colony within an hour or so after the first one was released. An empty skeleton Is all that remains of the parent colony. In it we can see the semicircular holes through which the vesicles emerged. These escape hatches can be seen in this electron micrograph of an empty colony. In addition this shows the complex, reticulated pattern of the wall construction. The vesicles disintegrate and dissolve away, freeing the daughter colonies to expand and grow in size. They reach maturity during the next few days. The basic developmental processes shown in the film still remain to be explained. For instance, what regulates the number of nuclear divisions before the formation of a daughter colony? Why do the zoospores separate, swarm, and then immediately reassemble to form a stable daughter colony? How do the zoospores, which are apparently free to assemble into any shape, always organize into a disc? [Music]


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