THE ROLE OF THE INITIATOR CELL IN SLIME MOLD AGGREGATION

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Abstract
Dictyostelium discoideum myxamoebae occur as two distinct morphological types, termed I-cells and R-cells. Data presented in a previous publication demonstrate that I-cells can initiate centers of aggregation and suggest compellingly that they are in fact the initiator cells for normal aggregation. The present communication extends and amplifies these findings. A. Time lapse camera lucida drawings and photomicrographs illustrate the sequence of events during the onset of aggregation. B. Small population samples of myxamoebae, when isolated from their neighbors shortly after deposition on washed agar, showed a distribution of aggregative centers consistent with the distribution of I-cells within the samples. Longer periods of contact with neighboring cells (including other I-cells) that surrounded the samples prior to isolation permitted progressively greater proportions of the samples to aggregate. The possibility arises of an "initiator substance" whose effect may extend over relatively great distances. C. R-cells, incubated for long periods of time on washed agar, were found to have acquired initiative capacity. At best, only a small proportion did so and furthermore could only induce the formation of aggregative centers amongst their developmental juniors (by twelve hours) but not amongst their developmental contemporaries.