Bimodal distribution of motility and cell fate in Dictyostelium discoideum

Pre-starvation amoebae of Dictyostelium discoideum exhibit random movements.Starved cells aggregate by directed movements (chemotaxis) towards cyclic AMP and differentiateinto live spores or dead stalk cells. Many differences between presumptive spore and stalk cellsprecede differentiation. We have examined whether cell motility-related factors are also amongthem. Cell speeds and localisation of motility-related signalling molecules were monitored by livecell imaging and immunostaining (a) in nutrient medium during growth, (b) immediately follow-ing transfer to starvation medium and (c) in nutrient medium that was re-introduced after a briefperiod of starvation. Cells moved randomly under all three conditions but mean speeds increasedfollowing transfer from nutrient medium to starvation medium; the transition occurred within 15min. The distribution of speeds in starvation medium was bimodal: about 20% of the cells movedsignificantly faster than the remaining 80%. The motility-related molecules F-actin, PTEN and PI3kinase were distributed differently in slow and fast cells. Among starved cells, the calcium contentof slower cells was lower than that of the faster cells. All differences reverted within 15 min afterrestoration of the nutrient medium. The slow/fast distinction was missing in Polysphondyliumpallidum, a cellular slime mould that lacks the presumptive stalk and spore cell classes, and in thetrishanku (triA-) mutant of D. discoideum, in which the classes exist but are unstable. The transitionfrom growth to starvation triggers a spontaneous and reversible switch in the distribution of D.discoideum cell speeds. Cells whose calcium content is relatively low (known to be presumptivespore cells) move slower than those whose calcium levels are higher (known to be presumptivestalk cells). Slow and fast cells show different distributions of motility-related proteins. The switchis indicative of a bistable mechanism underlying cell motility