(A,B) In situ hybridization at stage 46 for Kcna5, which encodes the pore-forming alpha subunit of the voltage-gated potassium channel Kv1.5, reveals expression only in the developing ampullary organ fields. Dotted lines indicate approximate boundaries of neuromast canal lines. (C,D) Even at the successively earlier stages shown (stage 41 and 39), Kcna5 is still restricted to the developing ampullary organ fields. (E–H) Expression of Kcnab3, encoding the auxiliary beta subunit Kvβ3, is similarly confined to the developing ampullary organ fields. Dotted lines indicate approximate boundaries of neuromast canal lines. The arrow in H, and the higher-power view of this region shown in the inset, indicate the area where the first Kcnab3 expression is noted, at stage 39. Scale bars: A,C-E,G,H, 0.5 mm; B,F, 50 µm. Abbreviations: ao, ampullary organ; e, eye; olf, olfactory pit. (I) Schematic, linear structure of the pore-forming alpha subunit of a Kv channel, with the positions noted of amino acid substitutions in paddlefish Kv1.5. (J) Amino acid sequences across the S3/4 linker region, the voltage-sensing segment S4, plus S5 and the pore, from paddlefish Kv1.5 (top) and other Shaker-related Kv channels for comparison, indicating the deep conservation of some of these amino acid positions across metazoans. The first set of sequences are from Kv1.5 across the jawed vertebrates, including three ray-finned bony fishes: Polyodon spathula (Mississippi paddlefish, a non-teleost chondrostean fish), Lepisosteus oculatus (spotted gar, a non-teleost neopterygian fish), Danio rerio (zebrafish, a teleost neopterygian fish); four lobe-finned fishes/tetrapods: Latimeria chalumnae (coelacanth), Xenopus tropicalis (tropical clawed frog), Gallus gallus (chicken), Homo sapiens (human); and a cartilaginous fish (Callorhinchus milii, a holocephalan). The second set of sequences are from two other human Shaker-related channels (Kv1.1 and Kv1.4), and three invertebrate Shaker orthologs, from Drosophila melanogaster (an insect, i.e., an ecdysozoan), Aplysia californica (a mollusc, i.e., a spiralian) and Nematostella vectensis (a sea anemone, i.e., a cnidarian).