INVITRO STALK CELL-DIFFERENTIATION IN WILD-TYPE AND SLUGGER MUTANTS OF DICTYOSTELIUM-DISCOIDEUM

In 'slugger' mutants of Dictyostelium discoideum, aggregates of cells remain for an abnormally long time in the migratory phase under conditions where wild-type aggregates form fruiting bodies. In the present work, we have examined the relationship between the defect in fruiting body formation in these mutants and their ability to form mature stalk cells. We dissociated anterior cells from slugs of the mutants and their parents and tested their ability to form stalk cells when incubated at low density in the presence of (1) the stalk cell morphogen Differentiation Inducing Factor-1 (DIF-1) together with cyclic AMP, or (2) 8-Br-cAMP, which is believed to penetrate cell membrane and activate cAMP-dependent protein kinase (PKA). Most of the mutants were markedly defective in forming stalk cells in response to DIF-1 plus cAMP, confirming a close relationship between fruiting body formation and stalk cell maturation. On the other hand, many of these same mutants formed stalk cells efficiently in response to 8-Br-cAMP. This supports evidence for an essential role of PKA in stalk cell maturation and fruiting body formation. It also indicates that many of the mutants owe their slugger phenotype to defects in functions required for optimal adenylyl cyclase activity.