The effects of presumptive morphogens on prestalk and prespore cell gene expression in monolayers of Dictyostelium discoideum

Abstract

Abstract The expression of three prestalk cell-specific genes ( ecmA, ecmB and pDd26) was examined during in vitro differentiation in cell monolayers, in an attempt to explain the spatial heterogeneity of the prestalk region of migrating Dictyostelium pseudoplasmodia. Under these conditions ecmA, ecmB and pDd26 mRNAs were expressed sequentially in response to the addition of differentiation inducing factor-1 (DIF)-1, a temporal sequence similar to that observed during normal development. ecmA and ecmB mRNAs reached a maximum level 2–4 h after DIF-1 supplementation and then declined, whereas pDd26 mRNA levels increased more slowly but remained high 24 h after DIF addition. The increases in expression in response to increasing concentrations of either DIF-1 or DIF-2 were identical for the three genes, suggesting that neither alteration in DIF concentration nor species was an important determinant of spatial heterogeneity. Ammonia had the same inhibitory effect on the expression of all three prestalk cell-specific genes and stimulated the expression of the prespore cell-specific gene, D19. These results indicate that ammonia is also not responsible for the spatial heterogeneity of the prestalk cell region. In contrast, cyclic AMP had a differential effect on the expression of the prestalk cell specific genes: ecmA expression was variably stimulated, pDd26 expression was inhibited and ecmB expression was sometimes stimulated and sometimes inhibited. These results are difficult to explain in terms of a gradient of cyclic AMP in the prestalk region. We postulate that temporal responses are more important than spatial responses to cyclic AMP in regulating stalk cell differentiation.