Tip regeneration and positional information in the slug of Dictyostelium discoideum.

We show in this paper that in the case of the slug of the cellular slime mould Dictyostelium discoideum the time which it takes for a new tip to regenerate at a given level can be used as a measure of positional information at that level. Our basic experiment consists of amputating slugs at various distances from the existing tip and thereby inducing the regeneration of a fresh tip; the time needed for regeneration is estimated by two independent methods. An identical operation, when performed in the anterior portion of a previously cut slug, tells us how this position-dependent regeneration time adjusts to a sudden change in the size of the slug. The reasons which lead us to conclude that tip regeneration times are in one-to-one correspondence with positional information are as follows. (i) Depending on their positions, the cells in a slug take different times to regenerate a new tip following amputation; (ii) regeneration times are scaled in relation to the total length of the slug and increase monotonically with the length cut off; and (iii) as judged by the regeneration time, cells can assess and remember their positions relative to the length of the slug. Our results highlight the importance of two rate processes. One might think of the slower process as being related to the setting up of a system of positional information in the slug, and the faster process as being a reflection of the kinetics of the positional value changing locally till it reaches the level appropriate to a tip.