Active Rho occupies a wider-than-normal zone in asterless cells. (A) GFP-rGBD at similar stages of furrowing in control (left) and TSA-treated (right) purple urchin embryos; single confocal sections. Rho zones are broader and fainter in TSA-treated cells compared with untreated siblings, but zones are centered equatorially and match furrows. Plots show representative fits between intensity data (red) and a fit curve (blue) that measures zone width along the cell outline (see Materials and methods). (B) Measured zone widths normalized by pole-to-pole cell length, plotted against integrated intensity, minus baseline, within the curve fit as shown in A, expressed as a fraction of baseline. (C) Normal Rho zones, eight-cell purple urchin embryo (red, GFP-rGBD; cyan, 3C-EMTB). (D) Eight-cell purple urchin embryo (same probes as C) treated with 20 µM TSA at time 00:00. Uniform cortical Rho activity during metaphase (00:00) disappears as cells enter anaphase, as in normal cells (Bement et al., 2005). Rho zones (brackets) are barely detectable above background, yet furrows develop and complete with minor delay (compare times in C), which implies that cells normally express more equatorial Rho activity than they require. Time is indicated in minutes:seconds.