Aspects of Aggregation in Cellular Slime Moulds 1. Orientation and Chemotaxis

The movement of slime-mould amoebae towards certain collecting centers, which have been started by a few of the amoebae, depends basically on their orientation to the maximum gradient of a chemical, acrasin. Because the gradient is produced by diffusion, the direct range of a center must be limited by the size and maximum sensitivity of the responding amoebae, the amount of chemical that can practicably be produced, and the time needed for orientation to occur after secretion has begun. In fact, its range is rapidly and economically extended by a relay system, as acrasin not only orients an amoeba but induces it to become sticky and secrete acrasin. But as a result, there may be a hundred-thousand sources within an aggregation; and the organism meets the problem of maintaining detectable gradients by inactivating secreted acrasin with an extracellular protein. However, this further reduces the importance of any influence the chemical secreted by the centre could have at the periphery. The amoebae move towards the center not because of a sustained gradient of secretion outwards from it, but because of the time sequence in which the relay operates. In one type of aggregation, the delay before an oriented amoeba begins to secrete is sufficient for it first to reach the amoebae that attracted it; and as it can then depend on contact guidance, it is not disturbed by the acrasin production of the amoebae on its flanks and at its rear. The result is that the amoebae, stuck together in radial streams, converge on the point where acrasin secretion began. In another type of aggregation, the delay is less, and the amoebae start to make acrasin before they reach their primary attractors; nevertheless, they too condense into radial streams, because, once the field has been polarized, there is a greater probability that an amoeba will continue to be attracted in the direction in which it has been oriented than in any other. The center owes its position not to constantly secreting more than the peripheral amoebae but to secreting before them; and this is primarily due to the ability of its initiator to release acrasin into the medium without their being any there already. If the time relationships of the relay are seriously disturbed, aggregation becomes chaotic. The secretion of a center may fluctuate, and this may lead to the periodic re-excitation of the relay system. The resultant pulses spreading out over an aggregation may reinforce flow guidance in its streams and are especially useful in repetitively reasserting the authority of the center in an aggregation in which the oriented amoebae are not in contact. Also, acrasin is used more economically if an adequate gradient is maintained for only a fraction of the total time. Various reactions with neighboring relay systems are possible: and peculiarities in the spread of a relay may lead to an aggregation's being circularly polarized. Specificity in aggregation may depend on differences in cellular adhesiveness, the acrasin, or the pulse pattern. There are many resemblances between the orientation of aggregating amoebae and of growing nerve fibers.