Morphogenesis and ultrastructure in Dictyostelium

Abstract

Summary The ultrastructure of the slime moldDictyostelium discoideum during development was investigated. It was revealed that a spore vesicle (SV) appeared which is specific for prespore and mature spore cells and was never found in prestalk cells. The SV disappear upon germination of the spores and reappear in the aggregate during prespore differentiation. Consequently, SV and a prespore vacuole (PV) discovered by Hohl and Hamamoto, (1969) served as specific cell markers which provided an opportunity to study the factors affecting the differentiation or redifferentiation of the cells. It was determined that during normal differentiation of prespore cells in the late aggregate the PV are synthesized first followed by the SV. This sequential appearance of PV and SV also occurred during the redifferentiation of isolated prestalk cells into prespore cells, a finding that suggests the genome is transcribed sequentially during morphogenesis. Prespore cell isolates redifferentiated prestalk cells which no longer contained PV or SV. Experiments with actinomycin D on normal aggregates and on prestalk cell isolates revealed that both PV and SV formation was inhibited. Thus, the synthesis of these components appear to depend upon DNA-dependent RNA synthesis. Additional experiments revealed that prestalk isolates, although mechanically disorganized, could resynthesize prespore cells with SV and PV, but could not differentiate mature spores. The research emphasizes the existence of a relationship between cell differentiation and morphogenesis and suggests that mature spore differentiation depends upon an interaction with differentiating prestalk cells during culmination.