Size regulation in Dictyostelium.

The division of large aggregate centres into separate slugs was examined in two strains of Dictyostelium discoideum which differ in size. The evidence is consistent with the hypothesis that the size of the slugs is determined by two factors; one is the ability of a tip to inhibit surrounding cells from forming an independent, rival tip; the other is the ability of the surrounding cells to resist being subjected to the inhibition of the newly arisen tip. If the surrounding cells are easily inhibited, then the slugs produced will be large; if they are resistant to inhibition the resulting slugs will be correspondingly small. An assay for tip inhibition was developed which was used to estimate the volume and distance over which inhibition occurs, the time over which it acts and the effect of tip size and cell mass shape on size regulation. The measurements and the results of experiments which showed inhibition across a thin agar layer are consistent with the hypothesis that an inhibitor spreads out from the tip by simple diffusion. In further studies it was found that although inhibition strength varies with the size of the tip, the ability to inhibit was the same in both strains whereas the resistance to inhibition was greater in the smaller strain.