UNIVERSAL SIGNALS CONTROL SLIME-MOLD STALK FORMATION

The primitive slime mold Dictyostelium minutum does not display oscill ations during aggregation, cannot form migrating dugs, and does not fo rm a prestalk/prespore pattern, all of which are characteristic for de velopment of its advanced relative Dictyostelium discoideum. We used D . minutum to investigate whether slime molds share common mechanisms c ontrolling development. In D. discoideum, the morphogen differentiatio n inducing factor (DIF) can induce stalk-cell differentiation in vitro . However, stalk formation in vivo is supposedly triggered by local de pletion of DIF antagonists such as ammonia or cAMP. A homologue of the D. discoideum stalk gene ecmB was cloned in D. minutum that encodes a 3.4-kb mRNA, and its deduced amino acid sequence shows repeats of 24 amino acids that are characteristic for the D. discoideum ecmB gene. R emarkably, DIF effectively induces expression of the D. minutum ecmB g ene and ammonia inhibits its expression. D. discoideum cells were tran sformed with a construct of the D. minutum ecmB promoter fused to the lacZ reporter gene and showed expression in the stalk, but not in the upper and lower cup of the fruiting body, which also express the D. di scoideum ecmB gene. In D. discoideum, the D. minutum ecmB and the ecmB promoter are similarly activated by DIF and repressed by both cAMP an d ammonia, suggesting that additional signaling is required for ecmB e xpression in upper and lower cup cells. Our data indicate that the ext racellular signals controlling stalk formation and their intracellular signaling cascades including gene regulatory proteins remained highly conserved during slime mold evolution.