For over 30 years, the cellular slime molds, and in particular the species Dictyostelium discoideum, have been viewed as a eukaryotic model system for the study of a variety of regulatory and developmental processes [l]. In particular, the transition from the free-living amoeboid stage to the multicellular slug stage has been the focus of a great deal of attention by those investigators studying the regulation of development [ 1-31. The first morphological evidence that differentiation of a starved population of cells is proceeding on schedule is the appearance of ripples or streams in a lawn of cells resting on a moist surface [l-31. This prelude to the readily identified tight aggregate stage indicates the functioning of the cells' endogenous chemosensory system. In 1969, Bonner and coworkers [4] identified the endogenously produced chemoattractant or acrasin of these cells as cyclic AMP (CAMP). While cAMP is widely encountered as an intracellular regulatory agent or second messenger, the Dictyostelia provide one of the few well documented examples of cAMP as an extracellular messenger molecule.The Polysphondylia such as P violaceum and P pallidum appear to employ a nonnucleotide molecule as their acrasin during starvation induced chemotaxis. This compound has been studied by Bonner and coworkers [S] and has been tentatively identified as a peptide with a blocked amino terminus, although further characterization is required for unambiguous identification. Bonner's group has also been responsible for the identification of folic acid and pterin compounds as chemoattractants of vegetative Dictyostelium amoebae [6-81. This identification of a nonacrasin chemoattractant raises the possibility of studying the transition from a vegetative to a developmental mode of chemotaxis.
DEVELOPMENTAL CHEMOTAXIS IN D discoideumThe phenomenology of the CAMP chemotactic system in D discoideum has been well established [9,10]. It is convenient for descriptive purposes to imagine