Effect of temperature on the mutual adhesion of preaggregation cells of the slime mould, Dictyostelium discoideum

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Abstract
The mutual adhesion of slime-mould cells was investigated at 24 and 1 °C in stirred and unstirred medium. Adhesion was induced by adding sodium chloride and followed by recording the diminution in optical density of the cell suspension associated with the formation of cell aggregates. It was found that: (1) when cells, previously stored at 1 °C, were stirred in distilled water at 24 °C the optical density of the suspension increased; (2) when sodium chloride was added after this increase, the optical density fell as cells adhered to each other; (3) both the increase in optical density and subsequent adhesion were reversibly inhibited at 1 °C; and (4) the rise in optical density was a prerequisite for adhesion. Comparison of cell shape and volume at the 2 temperatures suggested that the rise in optical density was due to (a) extension of pseudopodia and a consequent change from a spherical to an irregular shape, and (b) an increase in mean cell volume of about 17%. Observations on the formation of adhesions in still medium at 24 °C showed that initial contacts were formed either by microspikes or by rounded parts of the cell surface. The cell surfaces then flattened against each other, thereby increasing the area of contact. No flattening occurred at 1 °C. Microspikes were present on the surfaces of both cold and warm cells and contained longitudinal fibrils. It is suggested that adhesion of these cells is inhibited in the cold in stirred medium because the cells are unable to expand areas of mutual contact. Cold adhesions are, therefore, weak and easily disrupted by shearing forces. The inability to expand adhesive contacts may be due to the inhibition of cell motility at low temperature.