Control of cell type proportions by a secreted factor in Dictyostelium discoideum

It has been shown that, in Dictyostelium discoideum, conversion of prestalk cells to prespore cells in suspension cultures is inhibited by coexisting prespore cells. To examine whether the inhibition of conversion requires direct cell contact or is mediated by substances secreted by the cells, prestalk cells and prespore cells were incubated in shaken suspension, separated from each other by a dialysis membrane, and conversion of the prestalk cells to prespore cells scored after 24 h. Prestalk-to-prespore conversion was significantly inhibited if the density of the prespore cells was sufficiently high. In contrast, prestalk cells had little influence on prestalk-to-prespore conversion. Media conditioned by prespore cells, but not by prestalk cells, also inhibited the conversion of prestalk cells. Adenosine, propionate, diethylstilboestrol and differentiation inducing factor (DIF), all of which are known to influence the prestalk/prespore differentiation, were examined for their effects on prestalk-to-prespore conversion. Among these, all except adenosine significantly inhibited the conversion. Based on these results, possible mechanisms for maintenance of the constant cell-type ratio in D. discoideum slugs were discussed.