toward the anterior than in wild type. In three of these four central octants, however, the largest displacements (>1 μm/12 s) were strongly biased toward the posterior (to a more subtle degree, this bias was also present in the two anterior-most oc- tants, a and e). An abnormally polarized trend in large displace- ments occurred in octants b and f: large displacements occurred more frequently toward the posterior and more rarely toward the anterior compared with wild type. The resulting degree of asymmetry was beyond any found in wild-type or Gα-depleted zygotes, although similar profiles of anteriorly biased small and posteriorly biased large displacements were found in zyg-9(−). Surprisingly, the inactivation of LET-99 caused a striking reduction in both the magnitude and directional bias of trans- verse displacements (Fig. 7). Similarly narrow and symmetrical displacement profiles were measured in most regions of Gα- depleted zygotes, with the two posterior octants again showing distinctively nonrandom trends. Thus, both GOA-1/GPA-16 and LET-99 seem to play essential roles in generating the direc- tionally biased movement of NMY-2 aggregates along the trans- verse axis. Time-dependent mobility of NMY-2 aggregates is distinctly altered by inactivating LET-99 or GOA-1/GPA-16 To better understand the mechanics of NMY-2 aggregate move- ment and its control by Gα and LET-99, we next measured the total mobility of NMY-2 aggregates as a function of time scale. In general, the tendency of a particle to move from its initial position over time reflects the forces it experiences in its local environment, including both active transport and constraining forces. Such forces may act at distinct time scales, leading to time-dependent variation in particle mobility. For a population of particles, this can be measured as the time-dependent mean- squared displacement (MSD; see Materials and methods sec- tion Particle tracking). Changes in the character of the movement are reflected by changes in the slope of the MSD as a function of time scale; this slope is proportional to the diffusion coeffi- cient (here termed mobility) of the aggregates and reflects the cumulative contributions of random thermal forces, active trans- port, and constraining forces acting over a given range of time scale. Active transport, which is driven by molecular motors or cytoskeletal assembly dynamics, is expected to generate a posi- tive change in MSD (increased aggregate mobility) over the range of time scale in which it produces directed motion. On the other hand, opposition to such transport will negatively affect aggregate mobility over an independent range of time scales. For each population of aggregates, we measured the pro- jection of the MSD onto the AP and transverse axes. In all wild- type and mutant/RNAi conditions, mobility in some octants increased and/or decreased over time scales ranging from 4 to 16 s (Fig. 8), suggesting that diverse factors or conditions locally modify NMY-2 aggregate mobility. Interestingly, throughout Figure 7. Distribution of NMY-2–GFP aggregate displacements along the transverse axis. Histograms of aggregate displacements projected on the transverse axis at a time scale of 12 s, with >1-μm displacements shaded and labeled as in Fig. 6. NUCLEAR MOVEMENT IN C. ELEGANS ZYGOTES • GOULDING ET AL. 1185