Figure 7

Vrp1 mutant cells of the VM are capable of migrating and fusing. (A) The VM of a stage 12 wild type embryo stained with antibodies against Vrp (red) and Alk (green). At this stage FCs and FCMs have just fused. (A') Vrp protein expression is detected in VM cells (arrow) and SM cells (arrowhead). (A'') Alk protein expression is detected in VM only (arrow). (B-D) Examination of Alk mutant cells migrating from the VM to the SM in stage 12 embryos. (B) Alk¹⁰mutant embryo, where unfused FCMs of the VM, which express Alk and Vrp, migrate towards the SM to fuse with the somatic FCs. SM cells are identified by Vrp protein expression and lack of Alk expression (arrow indicates unfused, detached VM cells and arrowhead indicates VM cells that have migrated into the SM). (B') Close up of B, showing an Alk and Vrp positive cell from the VM stretching out toward a Vrp1 positive cell of the SM (arrowhead indicates Vrp1 expression at the protrusion of the stretched VM cell). (C) Alk-Vrp1^f06715double mutant embryo displays a similar phenotype to that observed in the Alk single mutant where Alk expressing VM cells are detected among Mef2 expressing SM cells (arrowhead). (C') Close up of C shows Alk positive VM cells which have migrated into the SM cell population (arrow), hence Vrp1 disruption does not appear to affect the migration process of VM cells in vivo. (D) The VM of a rp298;Alk-Vrp1 embryo confirms that Alk-Vrp double mutant cells from the VM can fuse with FCs of the SM. Only FCs of the SM (and longitudinal muscles, arrowhead) express lacZ under the Duf/Kirre promoter in an Alk mutant embryo, hence cells that expresses both lacZ and Alk are FCMs originating from the VM which have fused with a FC of the SM (arrow).