Pattern formation in Dictyostelium discoideum. II. Differentiation and pattern formation in non-polar aggregates.

Cells of the cellular slime mould D. discoideum were allowed to form into spherical aggregates, by shaking vegetative cells as a suspension in phosphate buffer. In such conditions, grex polarity is never established and surface sheath is not formed (Loomis, 1975 a). Despite the absence of such characteristics of normal development, differentiation of prespore cells, as tested for by immunofluorescent staining, and the organization of such cells into a patterned structure still occurred within the aggregates. Differentiation of prespore cells was found to occur within the cultures at times equivalent to those in the normal life cycle; such differentiation could be advanced by pulsation of the cultures with cyclic-AMP. When cell contact and aggregate formation was prevented, differentiation never occurred within the single cells. Our results suggest that the prespore cells develop randomly within the aggregate and that a pattern is subsequently formed as a result of sorting out of cell types within the cell mass. Aggregates shaken for extended periods of time showed development into cyst-like structures. The process of pattern formation that occurred within these aggregates which possess neither polarity nor a grex tip, would be unlikely to involve any mechanism of positional information signalling. The relevance of polar organization in the generation of pattern in the normal life cycle may therefore be questionable. We present a model of pattern formation in the slime mould in which sorting out of predetermined cell types is viewed as the major mechanism in bringing about patterned organization of the grex precursor cells.