Abstract
Multinucleate cells of Dictyostelium discoideum divide usually by unilateral cleavage furrows that ingress from the cell border. Along their path into the cell, they follow regions that are rich in myosin II and cortexillin and leave out the areas around the spindle poles that are populated with microtubule asters. In cells of a D. discoideum mutant that remain spread during mitosis we observed, as a rare event, cleavage by the expansion of a hole that is initiated in the middle of the cell area and has no connection with the cell's periphery. Here we show that these ring-shaped furrows develop in two phases, the first being reversible. During the first phase, the dorsal and ventral cell cortices come in close apposition and the cell membrane detaches locally from the substrate surface. The second phase comprises formation of the hole by membrane fusion and expansion of the opening toward the border of the cell, eventually cutting the multinucleate cell into pieces. We address the three-dimensional organization of ring-shaped furrows, their interaction with lateral furrows, and their association with filamentous myosin II and cortexillin. Thus, despite their geometrical divergence, similar molecular mechanisms might link the expanding hole to the standard contractile ring.