Dictyostelium Discoideum (D.D.) is a widely used system for studying a variety of basic processes in development including cell-cell signaling, signal transduction, pattern formation and cell motility. Cells of D.D typically exist as single cells (amoebae) that upon exhaustion of food supply aggregate to form a multicellular structure. A mound of cells is first formed, which then elongates vertically like a finger and finally crawls in a manner resembling the movement of a garden slug (thus called a "migrating slug"). Both aggregation and probably slug migration are mediated by chemotactic waves of extracellular cyclic AMP signal. However, the mechanisms by which mechanical forces are exerted, their magnitude and their location, are unknown.
We present here the first measurements of the distribution of forces exerted by slugs using the elastic substrate method. Deformation field is measured with a confocal microscope from the displacement of fluorescent beads embedded in soft polyacrylamide substrate. We show that force calculations are simple and robust when the noise level on bead displacements is low. We are able to identify clearly separate friction areas in the tip and in the trail, and traction in the prespore area. Surprisingly, the magnitude of friction and traction forces is decreasing with slug velocity indicating that these quantities are probably related to the dynamics of cell/substrate adhesion. Contrary to what is assumed in models and simulations, friction is not a viscous drag but rather close to solid friction. We also measure large perpendicular forces around slug boundary suggesting a large role of the slime sheath surrounding the slug in the transmission of forces to the substrate.