Cell sorting within the prespore zone of Dictyostelium discoideum

Dictyostelium discoideum forms elongate cell aggregates called "slugs" which migrate over the substrate before completing their conversion into fruiting bodies. Prespore cells are found in a zone which occupies the rear four-fifths of the slug. Both front- and rear-prespore cells, marked by a bacterial beta-galactosidase gene, sort out to their original positions in experiments in which slugs are reconstituted from disaggregated tissue. When cells from the rear of the prespore zone are transplanted to the middle or front, sorting is also observed: the transplanted cells return rapidly to the rear. Cells from the front of the prespore zone, however, were not observed to "home" to the front after transplantation to the rear. Since front-prespore cells sort out in disaggregation/reaggregation experiments, but fail to do so after transplantation to the rear, it is possible that the transplanted cells are converted to rear-prespore cells by extracellular signals present in the rear of the slug. In an experiment designed to test this hypothesis, front cells were transplanted to the rear and the host and transplant together then subjected to disaggregation/reaggregation. The results showed that front-prespore cells had not been converted to rear-prespore cells. Instead, there was an unanticipated effect: cells placed in the rear of the prespore zone underwent an anterior shift in positional preference, while cells placed in the front of the prespore zone showed a posterior shift. The specific sorting properties of front- and rear-prespore cells thus do not appear to result from the action of positional signals; positional signals destabilize rather than reinforce sorting preferences. Our observations are consistent with a model in which innate differences among cells bias them to differentiate as front-prespore or rear-prespore types, but the proportions of these types are also modulated by a negative-feedback mechanism.