Fig. 4. Mutants that lack paraxial mesoderm form hybrid PMNs. (A-D) Lateral views of ntl;spt mutant trunks. (E-H) Lateral views of spt mutant trunks. (A,E) islet1 in situ hybridization at 17-18 hpf. In both spt and ntl;spt mutants, islet1-expressing PMNs form continuous rows or clumps. Most of these PMNs also express islet 2 (see B,C,F,G). (B,F) islet2 in situ hybridization at 18-20 hpf. In both spt and ntl;spt mutants, islet2-expressing PMNs form continuous rows or clumps.(C,G) Islet antibody + islet2 in situ hybridization at 18-21 hpf. Islet antibody staining is nuclear and brown; islet2 staining is blue and cytoplasmic. ^*Brown-only cell (MiP). Most PMNs in spt mutants express islet2 (only a couple of brown-only cells in G) and almost all PMNs in ntl;spt mutants express islet2 (no brown-only cells in C). Insets show higher magnification of cells with brown and blue staining. (D,H) Islet antibody + islet1 in situ hybridization at 18-21 hpf. All PMNs in spt and ntl;spt mutants express islet1 (there are no brown-only cells in D or H). A small number of cells only express islet1 RNA (blue-only cells; +); these may be RoPs or SMNs that have just started to express islet1 RNA but not Islet protein. Scale bar: 50 µm.