Fig. 7. Premature apical constriction of amnioserosa end cells is induced by overexpression of Crb or by expression of Drac1V12 in a crb^S010409 mutant background. Dashed yellow lines demarcate clusters of apically constricted amnioserosa cells. Embryos were stained with anti-nonmuscle myosin heavy chain antibodies (A), antiphosphotyrosine antibodies (B-E) or phalloidin (F). (A,C) A doublestained stage 13 GAL4^332.3/UAS-crb^wt embryo showing clusters of apically constricted cells with strong myosin staining at the anterior and posterior ends of the amnioserosa. (B) A stage 14 GAL4^332.3/UAS-crb^wt embryo showing dumbbell-shaped amnioserosa. (D) A stage 13 GAL4^332.3/UAS-crb^wt;UAS-Drac1N17 embryo showing that impairment of Drac1 signaling does not block induction of premature cell constriction by overexpressed Crb. (E) A stage 13 crb^S010409, UAS-Drac1V12/crb^S010409, Hs-GAL4^M-4 embryo showing premature apical cell constriction at the anterior end of the amnioserosa. The posterior end of the amnioserosa in this embryo is obscured by the unretracted germband. (F) A stage 13 Hs-GAL4^M-4/UAS-Drac1V12 embryo is included for comparison, showing contraction of amnioserosa into the posterior end of dorsal hole.