Figure 8. Mobility of NMY-2–GFP aggregates varies in a spatial pattern that depends on GOA-1/GPA-16 and LET-99. One-dimensional MSD of NMY-2–GFP aggregates plotted over time for wild-type, let-99(or513ts), and goa-1 (RNAi);gpa-16(RNAi) zygotes. MSD values are shown as open diamonds (red, displace- ments projected on the AP axis; blue, pro- jected on the transverse axis). Solid lines are extrapolated from MSD values at time scales of 1 and 2 s; their slopes (with values shown) indicate short-time aggregate mobilities. most of the regions examined, mobility over time scales of 1–2 s was higher in wild type than in the corresponding octants of Gα-depleted or let-99(−) zygotes, suggesting that wild-type aggregates move relatively fast and/or in a more directed man- ner at these short time scales. At longer time scales as well, wild- type NMY-2 aggregates in most regions of the cortex showed relatively high AP mobility, which is consistent with a normal predominance of directed transport that is lacking or constrained in the absence of Gα or LET-99. Both short-time and long-time mobilities varied substan- tially with position, being highest in wild type near the NCC (octants c and g) and lowest in the anterior octants (a and e), with more than a twofold difference between these regions. In general, mobility along the AP axis was markedly diminished in Gα-depleted zygotes, with the exception of the most posterior region, where asymmetric displacement peaks had been observed (Fig. 6). Together, these data suggest that GOA-1/GPA-16 are required for directed movement in anterior regions, but only for the spatial regulation of directed movement in the posterior. In let-99(−) zygotes, an interesting asymmetry was detected: in anterior regions, mobility did not increase over time but rather decreased over time scales of 10–15 s. This time-dependent decrease was expected as a reflection of oscillatory cortical flows that occur in approximate synchrony with NCC rocking (unpublished data). Strikingly, mobility in posterior regions of let-99(−) zygotes tended to increase over time, showing only a slight decrease at the time scale of NCC oscillation. This re- gional difference in aggregate mobility could account for the abnormal distribution of NMY-2–dependent forces that act on the NCC in let-99(−) mutant zygotes (see Discussion). Transverse mobility increased over time in most regions of the wild-type cortex, but the pattern of regional variation was roughly opposite that of AP mobility, indicating local prefer- ences for aggregates to move along one axis or the other (Fig. 8). Strikingly, time-dependent mobility increases along the trans- verse axis were largely absent in both goa-1(−);gpa-16(−) and let-99(−) zygotes. Together with the symmetrically distributed displacements along the transverse axis in both conditions 1186 JCB • VOLUME 178 • NUMBER 7 • 2007