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Fig. 6. Model for heart specification in Ciona. Stages are shown on the top line. (A) At the 16- to 64-cell stages maternal Macho 1 directs the expression of Tbx6b and Tbx6c, whereas maternal ß-catenin directs the expression of an additional activator. At the 110-cell stage, Tbx6 and `Activator X' drive the expression of Mesp in the B7.5 cells. Mesp expression initiates conditional heart specification (represented by the red and yellow pattern). During gastrulation, the B7.5 cells have divided to form a cluster of four cells. An inductive `Signal X' then synergizes with Mesp to permit further heart differentiation in the anterior daughters, whereas the posterior daughters revert to an anterior tail muscle fate. (B) Targeted expression of the constitutively active MespVP16 bypasses the requirement for the inductive signal, leading to heart differentiation in the entire B7.5 lineage.





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