Signal Propagation during Aggregation in the Slime Mould Dictyostelium discoideum

Summary: We have analysed the pattern of concentric waves visible at the onset of aggregation of amoebae of Dictyostelium discoideum and have shown that each wave consists of a light band of elongated, moving cells, and a darker interband zone of rounded cells. Our analysis supports earlier suggestions that aggregation occurs in response to chemotactic signals emanating from a centre and which are propagated outwards through the field by a relay mechanism. The width of each band of moving cells corresponds to the distance the signal is propagated during the time that the cells remain elongated after stimulation, and does not vary with signal frequency. The width of the interbands of non-moving cells depends upon the distance the signal is propagated between signalling events, and varies with the signal frequency which increases during aggregation. The velocity of signal propagation decreases slightly with increase in the density of the monolayer of aggregating cells. We have shown by time-lapse films taken at high magnification that the signal is relayed radially outwards in steps of approximately 57 μm (relay zones) and that the response of each successive zone occurs approximately 12 s after the previous one (relay time). We have attempted to demonstrate the existence of a refractory period for chemotactic responsiveness. Our results indicate that such a refractory period, if it exists, cannot be more than 12 s.