The Cells Founding Aggregation Centres in the Slime Mould Polysphondylium Violaceum

Abstract

1. In cloned populations aggregation centres are started by the differentiation of single cells called founders. These attract their neighbours and induce them to attract theirs. 2. Founders vary in their degree and rate of differentiation. Those most highly differentiated are rounded up, stationary, and highly adhesive. They are very different from the ‘I-cells’ and hypothetical ‘initiator’ cells described in the literature. 3. A technique has been developed for reducing a centre to a monolayer of cells and dispersing them to any desired extent. This reveals the founder if not previously apparent. The original founder will repeatedly re-organize a group of cells subjected to successive dispersals, though additional founders may eventually differentiate among them. 4. If a young aggregate is dispersed, the founder killed, and further founder differentiation inhibited, the cells do not re-aggregate. 5. If kept separate a founder invariably dedifferentiates within a few hours. 6. Illuminating a culture produces an outburst of founder activity. If the cells are sandwiched between agar and glass this outburst is delayed. And if sandwiched cells are kept in darkness the differentiation of founders is very severely inhibited, even at extremely high cell densities. In the same conditions aggregation streams once started grow extensively. The developmental pathway of the founder appears to be quite different from that of the other cells in the aggregate. 7. Re-aggregation after dispersal is not inhibited by a density of edible bacteria that would suffice to keep cells in the vegetative state. 8. A separate, elongated cell oriented by an acrasin gradient may directly reverse its direction of movement if the gradient is reversed.