Islet1 regulates Islet2 expression. In this and subsequent figures, all photographs show 8-12 segment region of whole-mount embryos with rostral towards the left and dorsal towards the top, unless otherwise noted. (A) Schematic showing expression of islet1 (blue) and islet2 (red) in CaP and MiP between 11 and 24 hpf. Blue and red stripes indicate co-expression of islet1 and islet2; grey indicates downregulation of islet1. (B) At 20 hpf, islet1 RNA is expressed in dorsal Rohon-Beard sensory neurons (RB) and MiPs in control embryos. MN designates the row containing MiPs; individual MiPs are marked by black dots. In the segment farthest to the left, both MiP and RoP are indicated by black dots. RoPs are PMNs that express islet1 later than MiPs (Appel et al., 1995), and thus are absent from most of our figures. (C) islet1 MO-injected embryos express islet1 in RBs and MiPs, but the RNA is nuclear instead of cytoplasmic as it is in controls (47% of PMNs have nuclear islet1 RNA staining in islet1 MO-injected embryos when compared with 1% in controls*; n=7 islet1 MO-injected embryos; n=8 control embryos); black dots indicate MiPs. (D) At 28 hpf, Islet Ab (red) labels RBs and motoneurons (MN) in control embryos. (E) Islet Ab labeling is absent from islet1 MO-injected embryos (92% fewer cells labeled with Islet Ab in the pMN domain*; n=22 islet1 MO-injected embryos; n=26 control embryos). *P<0.001. Scale bar: 20 µm.