EXPRESSION OF AN ACTIVATED RASD GENE CHANGES CELL FATE DECISIONS DURING DICTYOSTELIUM DEVELOPMENT

It has been previously demonstrated that the expression of an activate d rasD gene in wild-type Dictyostelium cells results in formation of a ggregates with multitips, instead of the normal single tips, and a blo ck in further development. In an attempt to better understand the role of activated RasD development, we examined cell-type-specific gene ex pression in a strain stably expressing high levels of RasD[G12T]. We f ound that the expression of prestalk cell-specific genes ecmA and tagB was markedly enhanced, whereas the expression of the prespore cell-sp ecific gene cotC was reduced to very low levels. When the fate of cell s in the multitipped aggregate was monitored with an ecmA/lacZ fusion, it appeared that most of the cells eventually adopted prestalk gene e xpression characteristics. When mixtures of the [G12T]rasD cells and A x3 cells were induced to differentiate, chimeric pseudoplasmodia were not formed. Thus, although the [G12T]rasD transformant had a marked pr opensity to form prestalk cells, it could not supply the prestalk cell population when mixed with wild-type cells. Both stalk and spore cell formation occurred in low cell density monolayers of the [G12T]rasD s train, suggesting that at least part of the inhibition of stalk and sp ore formation during multicellular development involved inhibitory cel l interactions within the cell mass. Models for the possible role of r asD in development are discussed.