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Fig. 6. Control of Wg expression in ostial cardioblasts and summary of regulatory interactions in the heart region. Detection of Wg (green), Doc2+3 (red) and Tin (blue) in the dorsal vessel of stage 16 embryos. (A) In the wild type, Wg marks the three posterior pairs of Doc⁺/Tin^- cardioblasts as ostia (green arrows). (B) Wg and Doc proteins are not detectable in the dorsal vessel of homozygous svp^AE127 mutants, whereas Tin is expanded. (C) Embryo expressing svp1 ectopically throughout the dorsal vessel. All cardioblasts of the `heart' region express Doc and Wg (bracket). (D) Misexpression of Doc2 causes less efficient ectopic activation of Wg when compared with svp (arrow). (E) tin³⁴⁶ mutant and (F) tin³⁴⁶, svp^AE127 double mutant embryos carrying tin-ABD. Wg is expressed in all Doc-labeled cardioblasts of the posterior dorsal vessel (brackets). (G) Top: schematic representation of the dorsal vessel with corresponding epidermal segment numbers and expression domains of homeotic selector proteins. Middle: in the wild type, tin is activated in cardioblasts downstream of the Tbx20-homolog mid. This activation is blocked by the COUP-TF-homolog Svp (which itself depends on Hh inputs during stages 11-12) in presumptive ostial cells. Doc is expressed by default in these cells and contributes to the repression of tin. Myocardial Tin represses Doc and prevents wg expression. Wg is expressed only in Doc-positive ostial cells that also express Abd-A. These cells, which feature an elongated shape, differentiate into inflow valves. Bottom: in embryos that lack cardiac tin expression (either owing to the absence of the required cis-regulatory element tinC or because of the missing tin trans-activator Mid), all cardioblasts express Doc independently of Svp and wg is activated in all cardioblasts of the Abd-A domain. (H) Misexpression of tin, which causes repression of Doc in Svp⁺ cardioblasts, leads to loss of Wg in those cells (red arrows; green arrows are as in A). (I) Forced expression of mouse Nkx2.5 in the dorsal vessel using S59-Mef2-HtD-Gal4 and UAS-Nkx2.5 leads to repression of Doc and wg in the heart, similar to UAS-tin.

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