Cytokinesis: the initial linear phase crosses over to a multiplicity of non-linear endings

We investigate the final stage of cytokinesis in two types of amoeba, pointing out the existence of biphasic furrow contraction. The first phase is characterized by a constant contraction rate, is better studied, and seems universal to a large extent. The second phase is more diverse. In Dictyostelium discoideum the transition involves a change in the rate of contraction, and occurs when the width of the cleavage furrow is comparable to the height of the cell. In Entamoeba invadens the contractile ring carries the cell through the first phase, but cannot complete the second stage of cytokinesis. As a result, a cooperative mechanism has evolved in that organism, where a neighboring amoeba performs directed motion towards the dividing cell, and physically causes separation by means of extending a pseudopod. We expand here on a previous report of this novel chemotactic signaling mechanism.