Programmed cell death (PCD) of Dictyostelium discoideum cells was triggered precisely and studied quantitatively in an in vitro system involving differentiation without morphogenesis. In temporal succession after the triggering of differentiation, PCD included first an irreversible step leading to the inability to regrow at 8 hours. At 12 hours, massive vacuolisation was best evidenced by confocal microscopy, and prominent cytoplasmic condensation and focal chromatin condensation could be observed by electron microscopy. Membrane permeabilization occurred only very late (at 40–60 hours) as judged by propidium iodide staining. No early DNA fragmentation could be detected by standard or pulsed field gel electrophoresis. These traits exhibit some similarity to those of previously described non-apoptotic and apoptotic PCD, suggesting the hypothesis of a single core molecular mechanism of PCD emerging in evolution before the postulated multiple emergences of multicellularity. A single core mechanism would underly phenotypic variations of PCD resulting in various cells from differences in enzymatic equipment and mechanical constraints. A prediction is that some of the molecules involved in the core PCD mechanism of even phylogenetically very distant organisms, e.g. Dictyostelium and vertebrates, should be related.
The weak base ammonia inhibits aggregation and culmination of wild-type amoebae of Dictyostelium discoideum. Here we have examined its effect on a series of ‘slugger’ mutants previously assigned to 10 complementation groups, and so-called because they remain as slugs for extended periods. We show that the mutants accumulate normal levels of ammonia and hence may be abnormally susceptible to the ammonia they produce. In agreement with this we find that representatives of the slugger complementation groups are hypersensitive to ammonia inhibition at three clearly recognisable morphological stages of development: aggregation, tip formation and culmination. This finding suggests that a common ammonia-sensitive process underlies each of these developmental events.
Waves of chemotactic movement during the early phase of aggregation in Dictyostelium discoideum are of 2 kinds, concentric waves produced by cells that emit cyclic AMP signals spontaneously, and spirals generated by excitations relayed continuously around loops of excitable cells. The period of a spiral wave is the time taken for the excitation to make one complete circuit of the pacemaker loop. We have compared signal emission from the 2 types of source in time-lapse films made at a variety of temperatures. Our results show that spiral waves have a characteristic period length throughout most if not all of the early phase of aggregation, and that the period of concentric waves is generally longer and more variable. Temperature has a pronounced effect on period length and a lesser effect on propagation velocity. We find that each individual wave is propagated at constant velocity over distances of 1–2 cm but that the velocity of successive waves declines. This decline probably reflects some cumulative effect of the chemotactic excitations on the excitable properties of the aggregating cells.