Fig. 1. Notch signalling is involved in founder cell determination in the visceral mesoderm. The visceral mesoderm is visualised by Fas3 (red in A,B,C,M), ß-galactosidase expression from bap-lacZ (red in H,J) and founder cells are marked by rP298-lacZ expression (green in A,C,M brown in K,L). (A) Visceral mesoderm (vm) of a stage 11 wild-type embryo. (B) In Fas3 staining of Notch^55e11 embryos, it seems as if most cells of the vm are converted into founder cells as indicated by the stronger Fas3 expression and the more rectangular shape. (C) In stage 11 Delta^B2 mutants, the number of founder cells also seems to be increased in the visceral mesoderm and fewer fcms are visible (compare C with A). The clusters of rP298-lacZ-positive cells ventral to the visceral mesoderm belong to the somatic mesoderm (sm). (D-F) sns in situ hybridisation comparing the wild type (D) where sns is expressed in two bands with the sns expression in N^55e11 (E) and Dl^B2 (F) embryos where it is reduced. This reduction is more severe in the ventral band of the fcms of the somatic mesoderm but also visible in the dorsal band of the fcms of the visceral mesoderm. (G,H) Notch expression in the visceral mesoderm in stage 11 embryos. (G) Notch is expressed ubiquitously at the membrane of all cells of the visceral mesoderm. (H) Some cells seem to have a lower expression level of Notch (arrowhead in G). These cells also express bap-lacZ, which at this stage is mainly restricted to the fcms (H). (I,J) In contrast to the N expression Delta is expressed in the cells adjacent to the bap-lacZ positive cell clusters of the visceral mesoderm. (K) rP298-lacZ expression in the gut of a stage 14 wild-type embryo. (L) In gut preparations of stage 14 embryos with ectopic expression of UAS-N^intra in the visceral mesoderm, the number of rP298-lacZ positive founder cells of the circular visceral musculature is decreased. Founder cells of the longitudinal visceral muscles are not affected and migrate normally in anterior direction (arrow in L). (M) In stage 11 embryos overexpressing a dominant-negative form of Notch (UAS-dnN) with a bap-GAL4 driverline, the number of rP298-lacZ positive founder cells is increased compared with the wild type (A) and also some cells which are not marked by lacZ expression exhibit a stronger Fas3 expression, which is characteristic for the founder cells in the visceral mesoderm.