Phototaxis during the slug stage of Dictyostelium discoideum: a model study

During the slug stage, the cellular slime mould Dictylosielium discoideum m oves towards light sources. We have modelled this phototactic behaviour usi ng a hybrid cellular automata/partial differential equation model. In our m odel, individual amoebae are not able to measure the direction from which t he light comes, and differences in light intensity do not lead to different iation in motion velocity among the amoebae. Nevertheless, the whole slug o rientates itself towards the light. This behaviour is mediated by a modific ation of the cyclic AMP (cAMP) waves. As an explanation for phototaxis we p ropose the following mechanism, which is basically characterized by four pr ocesses: (i) light is focused on the distal side of the slug as a result of the so-called 'lens-effect'; (ii) differences in luminous intensity cause differences in NH3 concentration; (iii) NH3 alters the excitability of the cell, and thereby the shape of the cAMP wave; and (iv) chemotaxis towards c AMP causes the slug to turn. We show that this mechanism can account for a number of other behaviours that have been observed in experiments, such as bidirectional phototaxis and the cancellation of bidirectionality by a decr ease in the light intensity or the addition of charcoal to the medium.