Fig. 3. Ectopic expression of abd-A in the mesoderm resulted in an aorta-to-heart transformation. (A-D) Ectopic expression of abd-A in the ectoderm did not affect heart cell identity. (A) MEF2 staining revealed that the dorsal vessel did not form normally; however, expression of Hand in the dorsal vessel was detectable (B, inset). (B) Closer examination of Hand-expression cells revealed that those at the posterior (Ht, the presumptive heart region) were larger than those more anterior (Ao, the presumptive aorta region), suggesting that heart cell identity had been retained. This was confirmed by staining for MHC (C), which showed stronger staining in the presumptive heart region. Heart identity at the molecular level was also retained, as determined by expression of Tina-1 (D), which was restricted to the posterior of the dorsal vessel. Ectopic expression of abd-A in the mesoderm using the 24B-gal4 driver (E-H) or the twi-gal4 driver (I-L) transformed the aorta into a heart. (E,I) MEF2 staining revealed a gap between the two rows of cardial cells throughout the length of the dorsal vessel suggesting the formation of a larger lumen than wild type. This was confirmed by examination of Hand expression in embryos (F,J), and by accumulation of muscle MHC (G,K). (H,L) Tina-1, the marker of heart cells in wild-type embryos showed an expansion of its expression domain throughout the dorsal vessel. Scale bars: 50 µm for all panels except B (15 µm). All micrographs are dorsal views of stage 16 embryos with anterior towards the left. The location of the heart is bracketed.