Final answer:
The signaling molecule for aggregation in Dictyostelium discoideum is cyclic adenosine monophosphate (cAMP), prompting the formation of a multicellular slug.
Step-by-step explanation:
In the slime mold species Dictyostelium discoideum, the signaling molecule associated with the aggregation of single-celled individuals into the migratory slug stage is cyclic adenosine monophosphate (cAMP). When food sources are depleted, the amoeboid cells of Dictyostelium discoideum release cAMP, which acts as a chemical signal to nearby cells. This signaling prompts the cells to initiate a coordinated movement towards the source of cAMP, resulting in the formation of a multicellular slug that can move as a single unit. This aggregation allows the slime mold to function as a complex organism, leading to the development of a fruiting body that releases spores, thus completing its life cycle.