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

Abstract

Abstract The effects of several presumptive morphogens on the expression of three developmentally regulated genes (Dll, D19 and pDd63) has been examined during the in vitro differentiation of strain V12-M2 in cell suspensions and in cell monolayers. Adenosine had no effect on either prestalk or prespore gene expression in strain V12-M2, either in monolayers or in suspensions, casting some doubt as to its importance as a morphogen. In addition, although it had been shown previously that elevated pH stimulated spore cell formation and inhibited stalk cell formation, there was a reduction in the amounts of mRNA corresponding to the prespore-cell-specific gene D19 and the prestalk-cell-specific gene pDd63 when monolayers of strain V12-M2 were exposed to an elevated extracellular pH. Furthermore, although ammonia had been shown previously to have opposite effects on stalk and spore cell formation in monolayers, it had differential effects on D19 and pDd63 gene expression only at low concentrations. At higher concentrations, the expression of both genes was stimulated. Methylamine also affects stalk and spore cell formation in monolayers, but had only slight effects on the expression of the three genes. The expression of the prestalk-cell-enriched gene, Dll, was only slightly affected by elevated pH, ammonia or methylamine. These results illustrate the fact that conditions that affect terminal cell differentiation do not necessarily influence cell-type-specific gene expression and indicate hitherto unexpected complexities in the progression of gene expression during differentiation. During monolayer differentiation, the accumulation of the prespore-cell-specific D19 mRNA preceded the accumulation of the prestalk-cell-specific pDd63 mRNA by 2–4 h. When cyclic AMP was removed from monolayers that had already accumulated D19 mRNA, the subsequent accumulation of the prestalk specific pDd63 mRNA did not occur, indicating that the D19 and pDd63 genes require cyclic AMP for their expression at different times during development.