Fig. 1. Schematic view of septation in wild-type (A-D) and in chs2 or myo1 mutants (A,E-G). The neck region between the mother and daughter cell is represented, with the cell walls as the grey area and the plasma membrane as the brown line. The red spots indicate the location of Chs2p. Chitin is shown in green. In (B), the membrane invaginates and chitin is laid down in the invagination. Continuation of this process leads to generation of the primary septum disk and to pinching off of the membrane (C). Next, secondary septa (yellow) are built up from both the mother and daughter cell sides. A trilaminar septum results (D). In chs2 and myo1 mutants, invagination of the plasma membrane in a small area and growth of a primary septum do not take place. Instead, inward growth of cell wall material over a large portion of the plasma membrane pushes the membrane toward the center of the channel (E,F), finally closing the latter and generating a thick and uniform septum (G). In our hypothesis, this represents growth of secondary septa, which would occur at 90° to the normal direction. Thus, in the cell cycle, stage E would start between the normal times for (C) and (D). The green spots in (E-G) designate chitin formed by the action of chitin synthase III that is required for the remedial septa but not for the normal secondary septa. In (G), a lacuna resulting from the uneven fusion of the advancing secondary septum is also shown.