PMC

Development of the lateral line system. (A) Schematic showing position of lateral line placodes (black) in a 17-day skate (elasmobranch) embryo (modified after ). Epibranchial placodes (light grey ovals) are shown dorsocaudal to each pharyngeal (branchial) cleft. (B) Schematic showing distribution of lateral line neuromasts (black ovals), both at the surface and buried in subepidermal lateral line canals, on the head of a juvenile channel catfish (teleost). Modified after . (C) Schematic section through a mechanosensory neuromast: the kinocilia and stereocilia of each hair cell project into a gelatinous cupula, secreted by support cells. Redrawn from . (D) Schematic showing a mechanosensory lateral line hair cell bearing a kinocilium and stereocilia, innervated by lateral line neurites, with an adjacent support cell. Redrawn from . (E) Basic structure of an elasmobranch electroreceptive ampullary organ, with electroreceptor cells and support cells recessed in a pore canal filled with a mucous jelly. Redrawn from . (F) Scanning electron micrograph of a lateral line neuromast on the trunk of a stage 49 (12-day) Xenopus tadpole, showing the bundle of long kinocilia with smaller stereocilia at its base. Scale bar: 1μm. Xenopus staging after . (G) Higher-power view of the base of the kinocilia from panel F. Scale bar: 1μm. Abbreviations: ad, anterodorsal lateral line placode; av, anteroventral lateral line placode; io, infraorbital lateral line canal; m, middle lateral line placode; o, otic lateral line placode; ot, otic lateral line canal; ov, otic vesicle; po, posterior lateral line placode; prm, preoperculomandibular lateral line canal; so, supraorbital lateral line canal; sp, spiracle (first pharyngeal cleft); st, supratemporal lateral line placode; t, temporal lateral line canal.

Tail beat synchrony decreases when the posterior lateral line system is ablated. (A) Swimming speed increases when the anterior or posterior lateral line is ablated. (B) Tail beat frequency increases when the lateral line is ablated, especially for fish with the anterior (ant) and posterior (pos) region ablated. (C) Rayleigh R calculation for a fish’s tail excursion relative to its nearest neighbor. R decreases when the lateral line system is ablated. The effects remain when the lateral line is regenerated. Asterisks (*) indicates P < 0.05.

Notch signaling pathway genes are expressed in the developing lateral line system in paddlefish. Whole-mount in situ hybridization for paddlefish Notch1, Jag1 and Hes-5-like at different stages, with schematic representations of lateral line development, modified from , showing lateral line placode/organ development at different stages in shades of blue, elongating placodes in light blue and emerging neuromast canal lines/ampullary organs in darker blue. Notch pathway gene expression is indicated within those tissues or organs, depending on stage, in black. (A-D) At stages 30–32, Notch1, Jag1 and Hes5-like all seem to be expressed in the developing otic neuromast line between the eye and otic vesicle; Notch1 and Hes5-like are strongly expressed in the brain, eye and otic vesicle, and in the region of the pre-otic lateral line placodes. Jag1 expression is restricted to the developing otic neuromast line and prospective opercular line, plus the otic vesicle. (E-H) At stages 36–37, the lateral line expression of these Notch pathway transcripts includes the other pre-otic neuromast lines and post-otic lateral line primordia. Weak, patchy expression of Jag1 ventral to the pre-otic neuromast lines likely represents developing ampullary organ fields (F). Similarly, Hes5-like expression is observed in the developing ventral infraorbital ampullary organ field (arrowhead in G). (I-L) At stages 39–41, although lateral line expression of Notch1 (I) is difficult to observe, expression of Jag1 (J) and Hes5-like (K) is present in all neuromast lines, including the posterior lateral line, while expression of Jag1 and Hes5-like is also seen in the flanking ampullary organ fields (J,K: arrowheads indicate the ventral infraorbital ampullary organ field). (M-P) At stages 44–45, expression of Notch1 (J), Jag1 (K) and Hes5-like (L) is seen in both neuromasts and ampullary organs. Abbreviations: adp, anterodorsal lateral line placode; app, anterior preopercular ampullary field; avp, anteroventral lateral line placode; dot, dorsal otic ampullary field; di, dorsal infraorbital ampullary field; ds, dorsal supraorbital ampullary field; e, eye; epi, epibranchial placode region; io, infraorbital lateral line; LL, lateral line; m, middle lateral line; mlp, middle lateral line placode; ol, otic lateral line; olf, olfactory; otp, otic lateral line placode; ov, otic vesicle; pll, posterior lateral line; plp, posterior lateral line placode; pop, preopercular lateral line; ppp, posterior preopercular ampullary field; S, stage; so, supraorbital lateral line; st, supratemporal lateral line; stp, supratemporal lateral line placode; vi, ventral infraorbital ampullary field; vot, ventral otic ampullary field; vs, ventral supraorbital ampullary field. Scale bars: 200 µm.

(a) Simplified vertebrate phylogeny illustrating the distribution of electroreception within the major vertebrate clades and the position of chondrosteans, which includes paddlefishes and sturgeons (blue box). Red bars: loss of electroreception; green bars: teleost groups that independently evolved electroreception. *Hagfishes are not electroreceptive. (b, c) SEM and schematic, respectively, of a juvenile paddlefish showing the distribution of the mechanoreceptive neuromast canals and electroreceptive ampullary organ fields. In b, ampullary organ fields are grouped by different colours based on their innervation patterns (see panel j). (d) Magnification of boxed area in b. Red arrowhead: surface pore of an ampullary organ; arrow: a neuromast recessed in a canal. (e) Magnification of neuromast (arrow) within the preopercular canal and (f) of the neuromast itself showing kinocilia on the apical surface. (g) Lateral view of posterior lateral line trunk neuromasts (arrows). (h) Transverse section through a neuromast. Black arrowhead: kinocilia; asterisks: central sensory cells. (i) Transverse section through an ampullary organ cluster containing two organs (round nuclei: sensory cells in epithelium). (j) Schematic reconstruction of head and anterior trunk lateral line nerves based on Sudan Black-stained specimens. Diagonal hatching: neuromast sensory canal lines. The three pre-otic ganglia and associated nerves innervate distinct ampullary organ fields: GAD innervates the ds, vs, di, vi fields (yellow); GAV innervates the app and ppp fields (turquoise); GOL innervates the dot and vot fields (teal). (k) Schematic of the brain and lateral line ganglia (dotted areas) and associated nerves based on Sudan Black-stained specimens, sectioned material and an osmicated dissection of juvenile cranial nerves. Circles highlight the roots of pre-otic (green) and post-otic (blue) afferents. Scale bars: 1 mm (b, j, k), 200 μm (d), 100 μm (e, g), 10 μm (f), 25 μm (h, i). Abbreviations: AM, anastomosis of otic ramus of anterodorsal lateral line nerve with middle lateral line nerve; an, anterior naris; ao, ampullary organ; app, anterior preopercular ampullary field; AS, anastomosis of otic ramus of anterodorsal lateral line nerve with middle lateral line nerve and supratemporal ramus of posterior lateral line nerve; ba, barbel; BR, buccal ramus of anterodorsal lateral line nerve; dot, dorsal otic ampullary field; di, dorsal infraorbital ampullary field; DR, dorsal root of anterior lateral line nerves; ds, dorsal supraorbital ampullary field; e, eye; GAD, ganglion of anterodorsal lateral line nerve; GAV, ganglion of anteroventral lateral line nerve; GML, ganglion of middle lateral line nerve; GOL, ganglion of otic lateral line nerve; GPL, ganglion of posterior lateral line nerve; GST, ganglion of supratemporal lateral line nerve; HM, hyomandibular trunk (mixed trunk with large component of anteroventral lateral line fibres); HR, hyoid ramus of hyomandibular trunk; io, infraorbital lateral line; m, middle lateral line; MD, external mandibular ramus of hyomandibular trunk; ML, middle lateral line nerve; N, first neuromast ramule of posterior lateral line nerve; nm, neuromast; ol, otic lateral line; OL, otic lateral line nerve; OP, opercular ramus of hyomandibular trunk; PL posterior lateral line nerve; pll, posterior lateral line; pn, posterior naris; pop, preopercular lateral line; ppp, posterior preopercular field; R, ramule to spiracular organ; RAPP, ramule to ampullary organs of anterior preopercular field; RD, ramule to dorsal infraorbital field posterior to eye; RDIR, ramule to dorsal infraorbital field on rostrum; RDOT, ramule to dorsal otic field; RDS, ramule to dorsal supraorbital field; RPL, root of post-otic lateral line nerves; RPPP, ramule to posterior preopercular field; RV, ramule to ventral otic field; RVIB, ramule to ventral infraorbital field on upper jaw; RVIR, ramule to ventral infraorbital field on rostrum; RVS, ramule to ventral supraorbital field; s, spiracle; sc, spiracular chamber; so, supraorbital lateral line; SR, superficial ophthalmic ramus of anterodorsal lateral line nerve; st, supratemporal lateral line; ST, supratemporal lateral line nerve; vi, ventral infraorbital field; vot, ventral otic field; VR, ventral root of anterior lateral line nerves; vs, ventral supraorbital field.