Fig. 2. Expression of casanova requires Nodal signalling. (A-D) Expression of casanova requires zygotic contribution of oep. (A,B) Animal pole views, dorsal to the right. (C,D) Dorsal views. Compared with wild-type controls (A,C), casanova endodermal expression is not initiated or maintained in blastula (B, 40% epiboly) nor during gastrulation (D, 70-80% epiboly) in Zoep homozygous mutants. Expression in the forerunner cells and YSL is not affected. (E,F) Induction of cas upon Nodal signalling activation is cell autonomous. In late blastula, a few wild-type cells expressing nls-lacZ alone or combined with tar* were transplanted to the animal pole of a host wild-type embryo. During gastrulation (60% epiboly) tar* expressing cells transplanted to the animal pole (brown nuclear staining) autonomously express casanova (F; 96%, n=93) whereas control cells do not (E; 100%, n=12).