Figure 4. Dynamic behavior of cortical NMY-2–GFP aggregates during centration. Time-lapse sequence of cortical NMY-2–GFP in a wild-type zygote, with corresponding DIC images showing centrosomes (white marks). Time in seconds before NEB is indicated. Vertical lines indicate 50% EL. Bar, 10 μm. shows an intrinsic anterior bias throughout much of the cortex that is independent of aster movement. Interestingly, astral MTs appear to restrict large movements of actomyosin in the equato- rial region of the cortex, suggesting that astral MTs and cortical actomyosin are indeed linked. We also examined NMY-2 aggregate displacements along the transverse axis. In wild type, large displacements (>1 μm/12 s) occurred much more frequently along the transverse axis than along the AP axis (Fig. 7). In every octant, these large displace- ments were biased in the upward direction, which is coincident with the trajectory of the forward- and upward-moving anterior centrosome. Such biased transverse displacement was strongly reduced in the zyg-9(−) mutant, suggesting that transverse aggregate movements are normally stimulated by NCC centration/ rotation or by some other function of astral MTs. Distinct requirements for LET-99 and GOA-1/GPA-16 in NMY-2 aggregate movement In Gα-depleted and let-99(−) mutant zygotes, NMY-2 was simi- larly localized during prophase to a large, dense anterior cap and a small, sparse posterior cap (Fig. S1, Videos 9 and 10, available Figure 5. Using automated particle tracking to measure NMY-2–GFP ag- gregate displacements. (A) Cortical NMY-2–GFP imaged for particle track- ing in a wild-type zygote 30 s after pronuclear meeting. (B) Features identiﬁed from the image in A. (C) Plot of tracks derived from a 60-s 1-Hz video of the same zygote, beginning with the features shown in B. Track ends are marked with diamonds. Every frame is represented in this plot (time scale = 1 s). (D) The same set of tracks, with feature positions plotted every 12th frame (time scale = 12 s). (E) Wild-type centrosome movement in relation to eight regions (octants) deﬁned for regional analysis of NMY-2– GFP aggregate dynamics. Net displacements of wild-type centrosomes dur- ing the particle-tracking period are shown by gray lines, summed for 25 zygotes. Mean centrosome positions at the beginning and end of this period are indicated by open squares; a solid line connects the two mean centro- some positions at each time point. Note that the presumptive anterior spin- dle pole centrosome moves across the full width of octant g. Bar, 10 μm. NUCLEAR MOVEMENT IN C. ELEGANS ZYGOTES • GOULDING ET AL. 1183