Summary: The cellular slime mould exhibits clear dimorphism in development which, depending upon environmental conditions, leads to (1) macrocyst formation, a sexual process or (2) sorocarp formation, an asexual process. The present work was undertaken to determine the role of protons in the determination of the developmental mode. Intracellular pH (pH) was measured at the single-cell level microfluorometrically by use of the pH-sensitive fluorescent dye carboxyfluorescein. The proton-pump inhibitor diethylstilboestrol, the protonophore 2,4-dinitrophenol and a weak acid, propionate, were found to convert the developmental mode from macrocyst to sorocarp formation under conditions otherwise favouring macrocyst formation. All of these drugs lowered pH. Although no significant difference in pH was detected between dissociated cells shaken in buffers of pH 5·0 and pH 7·0, a marked difference in pH arose between the two if 5 mM-NHCl was included to simulate the expected microenvironment in the cell mass during development. Comparison of the pH of cells developing toward sorocarps with that of cells developing toward macrocysts revealed a few cells with extremely high pH (>7·30) in the latter population. The results obtained suggested that pH or something related to pH, influences the developmental choice.


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