Figure 1. G∝ and myosin II contribute to the rate of centration in wild-type zygotes. (A) Stereotyped move- ments of pronuclei and centrosomes in the C. elegans zygote. Male and female signs indicate the sperm and egg-derived pronuclei. Centrosomes are marked with arrowheads. The pronuclei initially appear at opposite poles of the cell and migrate toward each other (top). Because the female pronucleus migrates relatively fast from the anterior, pronuclear meeting occurs in the posterior cytoplasm (middle). The joined pronuclei and sperm- derived centrosome pair then move as a unit to the cell center (bottom). To measure rates of centration, we tracked the centroid of the NCC, which is marked here with aster- isks. In all ﬁgures, zygotes are oriented with the anterior pole to the left. (B) NCC velocity along the AP axis plot- ted over time in wild-type (WT; n = 10 zygotes) and G∝- depleted zygotes (n = 9 zygotes). Negative velocities indicate movement from posterior to anterior. (C) NCC velocity along the AP axis plotted over time from pro- nuclear meeting to NEB (n = 3 for each genotype/RNAi treatment) for wild-type, nmy-2(RNAi), and par-3(it71) zygotes fertilized near the site of polar body formation (reversed fertilization; see Results for details). Note the longer time axis compared with A. Each point represents a 30-s rolling average derived from measurements made at 10-s intervals; error bars show one SD from the mean of three successive measurements in each zygote. See Table I for peak and mean velocities during centration. Bar, 10 μm. past the center into the anterior cytoplasm (unpublished data). Finally, the rate of NCC movement in par-3(−) zygotes was reduced more than twofold by NMY-2 depletion to levels slightly below those measured in NMY-2–depleted wild-type zygotes (Table I), indicating that NMY-2 does not act merely by re- stricting PAR-3 localization. We conclude that although PAR-3 makes a minor contribution to the rate of centration, the require- ment for NMY-2 is at least partially independent of PAR-3 and cell polarity. We next examined the rate of NCC rotation to determine whether actomyosin and Gα signaling also contribute to this movement. In wild-type zygotes (n = 30), the NCC rotated through an angle of 24 ± 11° during the 1-min period preceding nuclear envelope breakdown (NEB; see Materials and methods section DIC video microscopy and analysis…). Similar to the effects on centration, NCC rotation during this time interval was reduced by a factor of approximately two in both GOA-1/ GPA-16–depleted and NMY-2–depleted zygotes (12 ± 6°/min for NUCLEAR MOVEMENT IN C. ELEGANS ZYGOTES • GOULDING ET AL. 1179