Figure 5. Decreased P3 cell size in small embryos destabilizes polarity and induces premature loss of division asymmetry.

(a) Histogram of GFP::PAR-2 fluorescence values (yellow and blue bars) taken from the surface of the two cell halves bisected by the plane that maximizes asymmetry of the cell shown. Histogram overlap (o_H) is highlighted. (b) Same as (a), but for a wild-type P4 cell that divides symmetrically. (c) Plots of PAR-2 asymmetry (1 - o_H) by cell type or condition as a function of time before cytokinesis onset. Note loss of asymmetry in small ima-3 P3 cells as they approach division. Mean ± SEM shown. (d) Plot of asymmetry vs. cell size for P lineage cells taken from wild-type or genetically-induced large or small embryos. Vertical dashed line indicates predicted CPSS calculated from experimental parameters, with grey region denoting 95% CI estimate from parameter measurement variance. Measurements are taken 1 min before onset of cytokinesis. Sample sizes: P4 C27D09.1 n= 3, P4 wt n=4, P3 ima-3 n=13, P3 C27D9.1 n=5, P3 wt n=7, P2 wt n=6, P1 wt n=3, P0 wt n=5. (e) Z projections of GFP::PAR-2 in P3 cells 1 min prior to cytokinesis (-1) and the resulting daughter cells 2 min. after (+2’). Solid and outlined arrowheads denote P4 and its sister D. Note PAR-2 is inherited symmetrically between the presumptive D and P4 cells in ima-3 embryos. See Supplementary Figure S4, Movie S3 and Table S1. Scale bar, 5 µm. (f-g) ima-3 embryos exhibit reduced asymmetry in size (f) and GFP::PAR-2 fluorescence (g) between P3 daughter cells. Same samples as in (d), except one ima-3 cell could not be followed for sufficient time after division. Two sample t-test, two-tailed. Mean ± STD indicated.