Fig. 6. (A) Theoretical prediction of the effect of a gPtdIns(4,5)P₂ increase in the vegetal hemisphere of the ascidian egg. A high amplitude increase in gPtdIns(4,5)P₂ initiated in the vegetal hemisphere of the egg (indicated by a black square) evokes two successive Ca²⁺ waves, the first one emanating from the locus of stimulation and the second one emanating from the region of higher ER density in the animal pole region (PM3). Simulations have been performed as in Fig. 4 with a gPtdIns(4,5)P₂ increase I_{gPtdIns(4,5)P2}=12 µM second^-1 for 4 seconds, at the mesh points (45 to 55) along the X axis and (80 to 90) along the Y axis, as indicated by the black box in the first panel. The successive panels show the spatial distribution of Ca²⁺ at the times indicated by the points on the curve on the left. (B) Effect of local photo-release of gPtdIns(4,5)P₂ in an unfertilized ascidian egg. First row: confocal image of [Ca²⁺]_c (the concentration of cytosolic Ca²⁺) taken 2 seconds (2'') after local UV uncaging, the area of UV uncaging is indicated by a black square. Second row: a Ca²⁺ wave is initiated at 1 minute 45 seconds (t=1'45'') in the animal pole and traverses the whole egg.