Fig. 4. Rescues of mesodermal cell migration defects by hybrid receptors. The embryos were generated in the crosses described in the Materials and Methods and collected at 25°C. The tracheal system was visualized with the 2A12 antibody. Mesoderm migration and pericardial cell formation was analyzed with an anti-Eve antibody (see also Materials and Methods). Evenskipped is specifically expressed in the pericardial cells and was used as a marker for these mesodermal cells. Arrows point to the pericardial cells. (A) Pericardial cells arranged in two rows at the dorsal side of a wild-type embryo. (B) Mesodermal cells were not able to migrate properly in htl mutant embryos. Consequently, pericardial cell fate determination did not occur. (C) Mesodermal cells did not migrate properly in dof mutant embryos and pericardial cells failed to form. (D) Mesodermal cell migration and pericardial cell differentiation were rescued in dof mutant embryos expressing the dof transgene. The arrangement of the cells and cell number were identical to wild-type embryos. (E) Pericardial cell formation was fully rescued by the Htl wild-type construct expressed in htl mutant embryos. (F) By contrast, the Htl wild-type construct was not able to rescue mesodermal cell migration and pericardial cell formation in the absence of Dof. (G) The chimeric receptor construct Htl-Btl was not able to fully replace the endogenous Htl receptor when expressed in htl mutant embryos. Only 12-16 pericardial cells were formed on each side of the embryo (instead of 20). (H) In dof mutant embryos, Htl-Btl failed to rescue pericardial cell formation. (I) Expression of the Htl-Tor fusion protein in htl mutant embryos led to excess cardiac cell formation. Nineteen to 30 scattered cells were observed along the midline in the dorsal part of the embryo. (J) The rescue of pericardial cells in dof mutant embryos expressing Htl-Tor was almost complete. 18-20 pericardial cells were arranged in a slightly disordered manner at the dorsal side of the embryo. (K) 17-20 pericardial cells were formed in htl mutant embryos expressing the Htl-EGFR receptor construct. (L) In dof mutant embryos, the Htl-EGFR construct was able to rescue 12-18 pericardial cells on each side of the embryo but the cells failed to arrange in an anterior-posterior row.
