Abstract
To find mutants of Dictyostelium discoideum that are unable to respond to exogenous cAMP signals (frigid mutants), amoebae of 218 independent aggregationโdeficient mutants were treated in suspension with artificial pulses of cAMP and screened for the capacity to form EDTAโresistant cohesion sites. Eleven frigid mutants were identified and further characterized. Using parasexual genetic techniques, these strains were assigned to five complementation groups (fgdAโE) and the fgd loci were mapped in three linkage groups: fgdA and D in group II, fgdC in group III, and fgdB and E in group VII. Biochemical and physiological experiments with these strains indicated that fgd mutants are of two general types. When starved, strains in groups fgdB, D, and E failed to produce detectable levels of membraneโassociated cAMP phosphodiesterase, surface cAMP receptors, or extracellular phosphodiesterase inhibitor, and the cells continued to respond chemotactically to folate. Thus, these strains are probably arrested in the vegetative phase or very early in development. In contrast, strains in groups fgdA and C produced low levels of cAMP receptors and secreted phosphodiesterase inhibitor. Moreover, after starvation, some of these mutants elicited a weak chemotactic response to cAMP. Therefore, unlike the former group of mutants, these strains appear to initiate development when starved, but the process is blocked at an early stage.