Supplementary Figure S1: Confocal images (DIC optics) of Dictyostelium cells taken in the

substrate plane showing pseudopods extended off the substrate (surrounded by a dotted ellipse).

(a)-(b) Vegetative cells; (c)-(d) pre-aggregating cells respectively in a buffer and in a spatial

cAMP gradient; (e)-(f) aggregating cells. Bars, 5 μm.

Supplementary Figure S2: (a)-(d) Changes in the distribution of the forces exerted by a

vegetative cell (cell v4 in Table 1) on a soft polyacrylamide elastomer substrate (E ~ 1150Pa)

presenting a cycle of movement with a sudden retraction of the rear. The frame # is indicated on

the bottom right of each panel. The time interval is 6 sec between two recorded frames; small

arrows indicate the direction of the cell motion. The cycle starts with an already partially

stretched cell (a) and an increase in F0, A and M while V slightly decreases. The shape and the

force distribution at the peak of force (F0 =15 nN) are symmetric (b). The higher traction forces

are located at the two opposite edges of the cell and are oriented toward the cell body giving the

cell a stretched appearance. The cell begins to retract its upper edge and move rapidly in the

opposite direction (c). It develops a front-rear asymmetry in the traction force oriented in the

direction of the movement with higher forces in the uropod that gradually decreases in size (see

large arrow) then disappears as the rear detaches abruptly from the substratum. Retraction

comes with a decrease in the cell area, the cell becomes more rounded in shape, forces have

decreased in amplitude (F0 =4 nN) and are mostly concentrated at the cell periphery (d). Here, a

new cycle immediately starts as a new protrusion is extended. (e) Plots as a function of time of

the overall absolute value of the force (bullets), cell area (open squares), cell major axis (black

diamonds) and cell speed (circles). Bar, 5 μm.

Supplementary Figure S3: Bar histograms of length weighted means obtained for the force,

stress, and cell speed at the different stages of the Dictyostelium development analysed in this

study (Vegetative, Pre-aggrating in buffer, Pre-aggregating stimulated with a cAMP gradient,

Aggregating). Error bars represent standard errors of the mean. The statistical differences

between groups are indicated by stars (*: p<0.05, **: p<0.01).

Supplementary Figure S4: Co-localization of myosin II and forces using AX-2 cells

expressing GFP- myosin II. The optical sections showing the myosin II distribution or the cell

contours were recorded 1 μm above the substratum with a confocal microscope. The optical

sections showing fluorescent beads in the elastomer for the traction microscopy distribution

were recorded 1 μm below the substratum. (a)-(d) Vegetative cell presenting both a rounded

shape with a peripheral force distribution (a)-(b) and an asymmetric shape and force distribution

(c)-(d). (e)-(f) Starved cell with an asymmetric force distribution. (g)-(h) Long starved cell with

a retracting pseudopod (large arrows in (g)). In each case, note the good co-localization of the

forward directed retraction forces with the myosin II but not of the propulsive forces (compare

arrowheads in (e) and (f)). Scale bar, 5 μm.