GFP::PIE-1 exhibits unrestricted mobility within the zygote. (a and b) At pronuclear meeting, 4.5-µm-diameter circular regions (arrow) were photobleached in the posterior cytoplasm of embryos expressing GFP::PIE-1. Mean fluorescence intensity in the photobleached regions recovered to within 95% of their initial value (n = 7). (c and d) Repeated photobleaching of a 15-µm circle adjacent to the posterior cortex causes anterior fluorescence to decrease to a value of 55 ± 5% compared with controls (P = 0.00011, n = 4). (e and f) Similarly, repeated photobleaching of a 30-µm circle in the anterior cytoplasm caused a decrease of posterior fluorescence to a value of 40 ± 6% compared with controls (P = 0.000032, n = 5). Time 0 represents the time of photobleaching in each experiment, which was chosen to be coincident with pronuclear meeting. The difference in the size of the areas chosen for photobleaching is due to the difference in relative volume of the anterior and posterior cytoplasm. Error bars represent SEM. ***, P < 0.001. Bars, 10 µm.