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Fig. 4. zyg-9 and tac-1 function during mitosis to ensure correct spindle positioning. (A-D) Images from time-lapse DIC microscopy sequences of wild-type (A), zyg-8(or484) (B), zyg-9(or634) (C) or tac-1(or455) (D) embryos shifted to 25°C during centration/rotation; row 1, metaphase; row 2, anaphase; row 3, cytokinesis. Arrowheads point to spindle poles. Last row of panels represent the trajectories of the anterior (red) and posterior (blue) spindle poles during mitosis; spindle-pole position was plotted every 5 seconds for the entire duration of the movies; arrowheads, positions of spindle poles at the onset (black arrowheads correspond to row 1) and the end (white arrowhead correspond to row 3). Elapsed time is indicated in minutes and seconds. Embryos are 50 µm long. Note that in the wild-type, the anterior spindle pole shifts to a sligthly posterior position before moving to a more anterior one. In 9/18 zyg-9(or634) and 5/23 tac-1(or455) embryos, spindle positioning was as depicted here, with the phenotype typically being somewhat less pronounced in tac-1(or455) embryos; 4/18 zyg-9(or634) and 13/23 tac-1(or455) embryos exhibited analogous but weaker spindle positioning defects, like those observed following partial RNAi-mediated inactivation of zyg-8; the spindle snapped in two in 3/18 zyg-9(or634) and 3/23 tac-1(or455) embryos, as is also observed in occasional zyg-8-mutant embryos (Gönczy et al., 2001); in 2/18 zyg-9(or634) and 2/23 tac-1(or455) embryos, spindle positioning was akin to wild type, perhaps because of incomplete or delayed inactivation. In some zyg-9(or634) and tac-1(or455) embryos, centration/rotation was incomplete (see panel D1). Anterior is left, posterior is right.

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