Cell shape in the algaPediastrum (Hydrodictyaceae; Chlorophyta)

Summary Species ofPediastrum, a genus in which the colonies assemble from aggregating zoospores, differ in the number and form of prongs on peripheral cells and the amount of space between cells of the colony; cell shape appears to be genetically based. Peripheral cells of theP. boryanum colony, for example, have two prongs per cell;P. simplex has one prong per cell. Prong extension is suppressed in the interior cells ofP. boryanum, but prong sites have been reported in scanning electron micrographs of the cell walls. A mutant unicellular strain in which cells of the colony separate after attaining typical form reveals several prong sites (6 or more) in each cell. Multiple suppressed prong sites are evident inP. simplex cells as well. Polyeders, 4- and 5-pronged unicells, occur in the life cycle ofP. simplex. Based on these observations and a recent report byMarchant (1979) of a microtubule organizing center associated with the prongs, it is suggested that several microtubule organizing centers are to be found in zoospores ofPediastrum species and may be related to species differences in cell shape.Research supported in part by Argonne Center for Educational Affairs, U.S. Department of Energy, under contract No. W-31-109-ENG-38.