Shared developmental and evolutionary origins for neural basis of vocal-acoustic and pectoral-gestural signaling

Abstract

Acoustic signaling behaviors are widespread among bony vertebrates, which include the majority of living fishes and tetrapods. Developmental studies in sound-producing fishes and tetrapods indicate that central pattern generating networks dedicated to vocalization originate from the same caudal hindbrain rhombomere (rh) 8-spinal compartment. Together, the evidence suggests that vocalization and its morphophysiological basis, including mechanisms of vocal-respiratory coupling that are widespread among tetrapods, are ancestral characters for bony vertebrates. Premotor-motor circuitry for pectoral appendages that function in locomotion and acoustic signaling develops in the same rh8-spinal compartment. Hence, vocal and pectoral phenotypes in fishes share both developmental origins and roles in acoustic communication. These findings lead to the proposal that the coupling of more highly derived vocal and pectoral mechanisms among tetrapods, including those adapted for nonvocal acoustic and gestural signaling, originated in fishes. Comparative studies further show that rh8 premotor populations have distinct neurophysiological properties coding for equally distinct behavioral attributes such as call duration. We conclude that neural network innovations in the spatiotemporal patterning of vocal and pectoral mechanisms of social communication, including forelimb gestural signaling, have their evolutionary origins in the caudal hindbrain of fishes.

Fig. 1.

Evolution of vocal–pectoral motor systems in fishes and tetrapods. (A) Waveforms of representative social vocalizations of bullfrog (time base 1 s), zebra finch (250 ms), squirrel monkey (200 ms), midshipman fish (500 ms), catfish (250 ms), and club-winged manakin (100 ms). Vocal (v) and nonvocal pectoral (p) basis is indicated. (B) Cladogram of vertebrates, including jawless (agnatha) and jawed (gnathostome) radiations (Osteostracans represent an extinct agnathan group with pectoral fins). (C) Summary of location of vocal and sonic motoneurons. Among fishes, the occipitospinal motor column (black) gives rise to motoneurons innervating muscles of vocal organs dedicated to sonic functions (e.g., swim bladder) and pectoral fins that can also serve a sonic function. This same column gives rise to vocal motoneurons in tetrapods. Among tetrapods, forelimb motoneurons (orange) that function in both sonic and gestural signaling are located in the spinal cord. (A adapted from ref. ; B and C adapted from ref. .)

Fig. 2.

Vocal behavior and neural network of plainfin midshipman fish. (A) Oscillogram record of repetitive series of natural calls (“grunt train”) recorded with hydrophone; lower trace shows one call. (B) Spontaneous vocal motor volley recorded from vocal occipital nerve (VOC) with temporal properties like those of natural vocalization; lower trace shows one VOC. VOC duration is time between first and last pulses; frequency is pulse repetition rate. (C) Vocal motor nuclei superimposed on lateral view of intact brain. Indicated are VPN, VPP, and VMN nuclei and vocal nerve (VN). Vocal midbrain (VMB) and forebrain preoptic area (POA) are vocally active sites. (D) Premotor compartmentalization of neurons code for distinct acoustic attributes. Representative intracellular records from vocal nuclei and vocal nerve superimposed on background sagittal image of caudal hindbrain. Descending input from vocal midbrain/forebrain neurons activates vocal hindbrain. Vocal prepacemaker nucleus is source of known corollary discharge informing auditory nuclei about a vocalization’s temporal properties. (Adapted from ref. .)

Fig. 3.

Map of developing vocal pattern generator in rh8-spinal compartment. (A) Fluorescently labeled neurons in plainfin midshipman fish larvae visualized with laser scanning confocal microscopy (horizontal plane). Simultaneous visualization of reticulospinal neurons labeled via retrograde transport from the spinal cord (Alexa 546 dextran-amine, red) and VMN (Alexa 488 dextran-amine, green) labeled via the developing vocal muscle. Yellow is composite overlap and does not indicate double labeling. Inset: Clusters of reticulospinal neurons (Alexa biocytin 488, green) in each rh, from 1 to 8. (Scale bars: 0.2 mm.) (B and C) Mapping in horizontal plane of VPP, VPN, and VMN neurons (black) in Gulf toadfish larvae; labeling via transneuronal transport of neurobiotin from developing vocal muscle. Cresyl violet counterstain reveals segmental, reticulospinal clusters. (Scale bar: 0.2 mm.) (D) Transverse section in caudal hindbrain of toadfish showing transneuronal neurobiotin labeling (brown) of paired midline VMN and adjacent VPNs; VMNs and VPNs have extensive dendritic and axonal branching. VMN axons exit via occipital vocal nerve root (OVN; cresyl violet counterstain). (Scale bar: 100 μm.) (E) Sagittal view summarizing relative positions of hindbrain vocal premotor-motor networks in rh8-spinal compartment of fish, birds, frogs, and mammals including primates, based on early-stage and adult phenotypes (see ref. for details). Most laryngeal motor neurons that shape the temporal envelope of mammalian calls originate from caudal nucleus ambiguus (Amb). Drt, dorsal reticular nucleus; PAm, nucleus parambigualis; RAb, nucleus retroambiguus; RAm, nucleus retroambigualis; Ri, inferior reticular formation; XIIts, tracheosyringeal division of hypoglossal motor nucleus; XMNc, caudal XMN. (Adapted from ref. .)

Fig. 4.

Map of developing pectoral motor nucleus in rh8-spinal compartment of basal and derived groups of actinopterygian fish (A and B, dorsal views). (A) Craniovertebral junction (asterisk) in postlarval, juvenile midshipman fish cleared and stained with alcian blue and alizarin red. (B) Demarcation of rh8-spinal boundary (yellow hatching) in zebrafish hoxb4a enhancer trap line. (C) Alignment of myotomes (“M”), occipital (Oc1, Oc2), and spinal (Sp1, Sp2) nerves and pectoral (red) and occipital (gray) motoneurons. Phylogeny of study species is also shown (Right). (Adapted from ref. .)

Fig. 5.

Spatiotemporal coding of behavioral attributes by rh8 premotor nuclei. Shown are single traces of neuronal activity (black) of midshipman fish vocal pacemaker (A) and VPP (B), goldfish oculomotor (C) and guinea pig inferior olive (D), and corresponding behavioral readout (red). (A) Inset: Dependency of membrane oscillations (i.e., cycles) and pulse repetition rate/frequency of vocal output. (B) Inset: Dependency of duration of membrane-sustained depolarization and call duration (i.e., length). (D) Inset: Correlation in rats between tongue licking behavior (red) and cerebellar complex spike activity (black) that directly reflects levels of inferior olive activity. [A and B adapted from ref. ; C reprinted by permission from Macmillan Publishers Ltd: Nature Neuroscience (ref. 93), copyright 2001; D reproduced with permission from John Wiley & Sons (ref. 95); D (Inset) reprinted by permission from Macmillan Publishers Ltd: Nature (ref. 96), copyright 1995.]