Early events in myofibrillogenesis and innervation of skeletal, sound-generating muscle in a teleost fish

Ontogeny of the swim bladder of the Plainfin Midshipman, Porichthys notatus (Percomorphacea: Batrachoidiformes)

The batracoidid Plainfin Midshipmen Porichthys notatus Girard has been extensively studied due to the sound production abilities and specializations of its swim bladder. The present study describes three-dimensional variations of the morphology of the swim bladder and sonic muscles of P. notatus during its post-hatch larval development, with the use of three-dimensional computed tomography. This study also includes descriptions of the relative position of the swim bladder to other visceral organs. The swim bladder, digestive tract, and liver were already present in the smallest examined specimens (5.9 mm; newly hatched larvae) along with the yolk sac. In the smallest specimens, the digestive tract is straight, but from 7.1 mm TL, the digestive tract forms the first intestinal loops, and at 25.5 mm TL, a second intestinal loop. In smallest specimens, the swim bladder is oval, but at 7.1 mm TL, the anterior margin starts invaginating, forming a pair of anterior lobes. The first appearance of the intrinsic sonic muscles in swim bladder occurs at 13.1 mm TL. Additionally, we provide comparisons between the shape of the swim bladder of P. notatus and other species. The shape of the swim bladder of P. notatus and other members of Porichthyinae have an ovoid posterior region with two anterior lobes and differs from the cordiform or semiconected/bilobed the swim bladders observed in the other Batrachoididae.

Development of the ultrastructure of sonic muscles: a kind of neoteny?

Background

Drumming muscles of some sound-producing fish are ‘champions’ of contraction speed, their rate setting the fundamental frequency. In the piranha, contraction of these muscles at 150 Hz drives a sound at the same frequency. Drumming muscles of different not closely related species show evolutionary convergences. Interestingly, some characters of sonic muscles can also be found in the trunk muscles of newly hatched larvae that are able to maintain tail beat frequencies up to 100 Hz. The aim of this work was to study the development of sound production and sonic and epaxial muscles simultaneously in the red bellied piranhas (Pygocentrus nattereri) to seek for possible common characteristics.

Results

Call, pulse and period durations increased significantly with the fish size, but the call dominant frequencies decreased, and the number of pulses and the call amplitude formed a bell curve. In epaxial muscles, the fibre diameters of younger fish are first positioned in the graphical slope corresponding to sonic muscles, before diverging. The fibre diameter of older fish trunk muscles was bigger, and the area of the myofibrils was larger than in sonic muscles. Moreover, in two of the biggest fish, the sonic muscles were invaded by fat cells and the sonic muscle ultrastructure was similar to the epaxial one. These two fish were also unable to produce any sound, meaning they lost their ability to contract quickly.

Conclusions

The volume occupied by myofibrils determines the force of contraction, the volume of sarcoplasmic reticulum sets the contraction frequency, and the volume of mitochondria sets the level of sustained performance. The functional outcomes in muscles are all attributable to shifts in the proportions of those structures. A single delay in the development restricts the quantity of myofibrils, maintains a high proportion of space in the sarcoplasm and develops sarcoplasmic reticulum. High-speed sonic muscles could thus be skeletal muscles with delayed development. This hypothesis has the advantage that it could easily explain why high-speed sonic muscles have evolved so many times in different lineages.

Novel vocal repertoire and paired swimbladders of the three-spined toadfish, Batrachomoeus trispinosus: insights into the diversity of the Batrachoididae

Toadfishes (Teleostei: Batrachoididae) are one of the best-studied groups for understanding vocal communication in fishes. However, sounds have only been recorded from a low proportion of taxa within the family. Here, we used quantitative bioacoustic, morphological and phylogenetic methods to characterize vocal behavior and mechanisms in the three-spined toadfish, Batrachomoeus trispinosus. B. trispinosus produced two types of sound: long-duration 'hoots' and short-duration 'grunts' that were multiharmonic, amplitude and frequency modulated, with a dominant frequency below 1 kHz. Grunts and hoots formed four major classes of calls. Hoots were typically produced in succession as trains, while grunts occurred either singly or as grunt trains. Aside from hoot trains, grunts and grunt trains, a fourth class of calls consisted of single grunts with acoustic beats, apparently not previously reported for individuals from any teleost taxon. Beats typically had a predominant frequency around 2 kHz with a beat frequency around 300 Hz. Vocalizations also exhibited diel and lunar periodicities. Spectrographic cross-correlation and principal coordinates analysis of hoots from five other toadfish species revealed that B. trispinosus hoots were distinct. Unlike any other reported fish, B. trispinosus had a bilaterally divided swimbladder, forming two separate swimbladders. Phylogenetic analysis suggested B. trispinosus was a relatively basal batrachoidid, and the swimbladder and acoustic beats were independently derived. The swimbladder in B. trispinosus demonstrates that toadfishes have undergone a diversification of peripheral sonic mechanisms, which may be responsible for the concomitant innovations in vocal communication, namely the individual production of acoustic beats as reported in some tetrapods.

Myosin flares and actin leptomeres as myofibril assembly/disassembly intermediates in sonic muscle fibers

The sonic muscle of type 1 male midshipman fish produces loud and enduring mating calls. Each sonic muscle fiber contains a tubular contractile apparatus with radially arranged myofibrillar plates encased in a desmin-rich cytoskeleton that is anchored to broad Z bands (approximately 1.2 micro m wide). Immunomicroscopy has revealed patches of myosin-rich "flares" emanating from the contractile tubes into the peripheral sarcoplasm along the length of the fibers. These flares contain swirls of thick filaments devoid of associated thin filaments. In other regions of the sarcoplasm at the inner surface of the sarcolemma and near Z bands, abundant ladder-like leptomeres occur with rungs every 160 nm. Leptomeres consist of dense arrays of filaments (approximately 4 nm) with a structure that resembles myofibrillar Z band structure. We propose that flares and leptomeres are distinct filamentous arrays representing site-specific processing of myofibrillar components during the assembly and disassembly of the sarcomere. Recent reports that myosin assembles into filamentous aggregates before incorporating into the A band in the skeletal muscles of vertebrates and Caenorhabditis elegans suggest that sonic fibers utilize a similar pathway. Thus, sonic muscle fibers, with their tubular design and abundant sarcoplasmic space, may provide an attractive muscle model to identify myofibrillar intermediates by structural and molecular techniques.

Sonic Motor Pathways in Piranhas with a Reassessment of Phylogenetic Patterns of Sonic Mechanisms among Teleosts

Sound production has evolved independently a number of times among teleost fishes. In most cases, sound is generated by fast contracting muscles that vibrate the swim bladder by way of their direct attachment (intrinsic muscles) or indirectly by way of other skeletal elements (extrinsic muscles). This study focuses on the red and black piranha, Pygocentrus nattereri and Serrasalmus rhombeus (superorder Ostariophysi, Order Characiformes), that have extrinsic swim bladder sonic muscles innervated by the third and fourth spinal nerves. This innervation pattern diverges from that found in most teleosts, including the closely related catfishes (Ostariophysi, Siluriformes), where sonic muscles are innervated by ventral occipital nerve roots that arise just caudal to the vagus nerve. Here, we tested the hypothesis that piranhas would also differ from most other teleosts in the location of their sonic motor neurons. Following biotin labeling of branches of the third and fourth spinal nerves that innervate the sonic muscles in the red and black piranha, sonic motor neurons were identified amongst other non-sonic motor neurons in the central part of the spinal cord, slightly ventrolateral to the central canal. To our knowledge, this is the first example of sonic motor neurons positioned entirely within the spinal cord. In the other species so far studied, sonic motor neurons form well-defined nuclei that extend from far caudal levels of the medulla into the rostral spinal cord and are located either within the ventral motor column or near the midline, close to or just ventrolateral to the fourth ventricle and central canal. A piranha-like pattern may be more widespread among characiforms and is likely present in other teleost orders, e.g., Sciaenidae (drumfishes), that also have sonic muscles innervated by spinal nerves.

Ontogenetic changes in the response properties of individual, primary auditory afferents in the vocal plainfin midshipman fish Porichthys notatus Girard

The auditory system of adult midshipman fish Porichthys notatusGirard is an important sensory receiver system used during intraspecific social communication to encode conspecific vocalizations, but the response properties and function of this system in the pre-adult stages are unknown. Midshipman fish, like other teleosts, use the saccule as the main acoustic end organ of the inner ear. In this study, we examined the discharge properties and the frequency response dynamics of auditory saccular afferent neurons in pre-adult midshipman (∼4–12 months of age) to determine whether encoding of auditory information, inclusive of conspecific vocalizations,changes across life history stages. Extracellular single unit recordings were made from saccular afferents while sound was presented via an underwater speaker. Comparisons with adult data show that the resting discharge rate and auditory threshold sensitivity increased with age/size,while temporal encoding of frequency did not show any significant shifts. The results indicate that the saccular afferents of juveniles, like those of non-reproductive adults, are best adapted to temporally encode the low frequency components (≤100 Hz) of midshipman vocalizations. This report represents the first in vivo investigation of age-related changes in the encoding properties of individual auditory neurons for any fish species.

Power output of sound-producing muscles in the tree frogs Hyla versicolor and Hyla chrysoscelis

Sound-producing muscles provide the opportunity of studying the limits of power production at high contractile frequencies. We used the work loop technique to determine the power available from the external oblique muscles in two related species of North American gray tree frog, Hyla chrysoscelis and Hyla versicolor. These trunk muscles contract cyclically, powering high-intensity sound production in anuran amphibians. The external oblique muscles in H. chrysoscelis have an in vivo operating frequency of 40–55 Hz at 20–25 degrees C, whereas in H. versicolor these muscles contract with a frequency of 20–25 Hz at these temperatures. In vivo investigations have shown that these muscles use an asymmetrical sawtooth length trajectory (with a longer shortening phase compared with the lengthening phase) during natural cycles. To study the influence of this particular length trajectory on power output, we subjected the muscles to both sinusoidal and sawtooth length trajectories. In both species, the sawtooth trajectory yielded a significantly higher power output than the sinusoidal length pattern. The maximum power output during sawtooth cycles was similar in both species (54 W kg(−)(1) in H. chrysoscelis and 58 W kg(−)(1) in H. versicolor). These values are impressive, particularly at the operating frequencies and temperatures of the muscle. The sinusoidal length trajectory yielded only 60 % of the total power output compared with the sawtooth trajectory (34 W kg(−)(1) for H. chrysoscelis and 36 W kg(−)(1) for H. versicolor). The optimum cycle frequencies maximizing the power output using a sawtooth length pattern were approximately 44 Hz for H. chrysoscelis and 21 Hz for H. versicolor. These frequencies are close to those used by the two species during calling. Operating at higher frequencies, H. chrysoscelis maximized power at a strain amplitude of only 8 % compared with a value of 12 % in H. versicolor. These strains match those used in vivo during calling. The stimulus timing observed in vivo during calling was also similar to that yielding maximum power at optimal frequency in both species (6 ms and 8 ms before the start of shortening in H. chrysoscelis and H. versicolor, respectively). As expected, twitch duration in H. chrysoscelis is much shorter than that in H. versicolor (23 ms and 37 ms, respectively). There was a less remarkable difference between their maximum shortening velocities (V(max)) of 13.6 L(0)s(−)(1) in H. chrysoscelis and 11.1 L(0)s(−)(1) in H. versicolor, where L(0) is muscle length. The force-velocity curves are very flat, which increases power output. At the myofibrillar level, the flat force-velocity curves more than compensate for the lower peak isometric force found in these muscles. The data presented here emphasize the importance of incorporating in vivo variables in designing in vitro studies.

Early development of the motor and premotor circuitry of a sexually dimorphic vocal pathway in a teleost fish

The plainfin midshipman fish (Porichthys notatus) has a caudal hindbrain vocal motor circuit that has been proposed to share a common embryonic origin with the hindbrain vocal networks of other vertebrates. In midshipman, this vocal circuit includes three groups of neurons: sonic motor, pacemaker, and ventral medullary. Here, transneuronal transport of biocytin or neurobiotin was used to delineate the early ontogeny of the three hindbrain vocal nuclei and their pattern of connectivity. The organization of the vocal nuclei was studied in animals beginning soon after hatching until the nuclei have the adult phenotype at the time fish become free-swimming. There is a clear sequence of events whereby motoneurons establish their connections with the sonic muscle prior to establishing connections with premotor neurons; developmental milestones of the vocal pathway parallel those of the sonic muscle. The results also indicate that sexual differentiation of the vocal motor system in midshipman begins early in development, well before any evidence of sexual maturation. Embryonic males and females differ in the relationship between soma size and body length for the three hindbrain nuclei. Males are also more variable than females in body mass, volume of the sonic motor nucleus, and motoneuron cell size.

Embryonic, juvenile, and adult development of the toadfish sonic muscle

BACKGROUND

Sonic muscle fibers intrinsic to the swim bladder of the oyster toadfish Opsanus tau proliferate throughout adult life and have an unusual radial morphology: alternating ribbons of sarcoplasmic reticulum (SR) and myofibrils surround a central core of sarcoplasm. Large fibers in adults form multiple cores, fragment, and appear to divide into smaller, more energy efficient units.

METHODS

We examined embryonic to adult development of sonic muscle using electron and light microscopy and focused on the incidence of satellite cells (SC).

RESULTS

Muscle fibers form late in the larval period from myoblasts, which do not appear to fuse into myotubes, but enlarge and differentiate myofibrils in a single patch. The SR differentiates from the outside inward, separating the myofibrils into bundles of varying thickness, which often exceed the thickness seen in adults. SCs in juveniles and adults have a sparse cytoplasm and a heterochromatic nucleus. The % SC nuclei (SC nuclei/total nuclei) decreases from a high of 88% in larvae to a low of 1% in adults although the adult average is 10%. No embryonic type fibers in the process of differentiating myofibrils were seen in adults. Small immature fibers, which had not yet formed the central core, have a complete radially organized contractile cylinder.

CONCLUSIONS

Immature muscle fibers formed embryonically in the larval period have a different morphology from immature fibers in adults, suggesting that splitting rather than SCs is a major source of new fibers in adults.

Phenotypic specification of hindbrain rhombomeres and the origins of rhythmic circuits in vertebrates

This essay considers the ontogeny and phylogeny of the cranial neural circuitry producing rhythmic behaviors in vertebrates. These behaviors are characterized by predictable temporal patterns established by a neuronal network variously referred to as either a pacemaker, neural oscillator or central pattern generator. Comparative vertebrate studies have demonstrated that the embryonic hindbrain is divided into segmented compartments called rhombomeres, each of which gives rise to a distinct complement of cranial motoneurons and, as yet, unidentified populations of interneurons. We now propose that novel rhythmic circuits were innovations associated with the adoption of cardiac and respiratory pumps during the protochordate-vertebrate transition. We further suggest that the pattern-generating circuits of more recent innovations, such as the vocal, electromotor and extraocular systems, have originated from the same Hox gene-specified compartments of the embryonic hindbrain (rhombomeres 7-8) that gave rise to rhythmically active cardiac and respiratory circuits. Lastly, we propose that the capability for pattern generation by neurons originating from rhombomeres 7 and 8 is due to their electroresponsive properties producing pacemaker oscillations, as best typified by the inferior olive which also has origins from these same hindbrain compartments and has been suggested to establish rhythmic oscillations coupled to sensorimotor function throughout the neuraxis of vertebrates.

Sonic/vocal-acousticolateralis pathways in teleost fishes: a transneuronal biocytin study in mochokid catfish

Mochokid catfish have two sound producing (sonic) organs--a pectoral spine stridulatory apparatus and a swimbladder whose vibration is established by nearby "drumming" muscles. Dextran-biotin or biocytin application to sonic nerves or muscles identified topographically separated motoneuron pools. Pectoral spine-related motoneurons are located within the ventral motor column whereas swimbladder motoneurons lie just ventral to the central canal or fourth ventricle. Axons of both groups of motoneurons exit the brain and spinal cord via ventral roots of occipital (swimbladder and pectoral) and spinal (swimbladder only) nerves. Transneuronal biocytin transport identified an extensive premotor network only for the swimbladder motor nuclei. Premotoneuron somata are located ipsilaterally in 1) a dorsolateral region of the sonic motor nucleus (SMN); motoneurons were clustered in the ventromedial region of the SMN and 2) the ventromedial medulla at the rostral pole of the SMN. Biocytin-filled fibers and less frequently premotoneuron somata were also found in the contralateral SMN. Biocytin-labeled fibers were continuous farther rostrally with 1) a commissural bundle that terminated bilaterally in the medial reticular formation near the caudal pole of the descending octaval nucleus and 2) a lateral brainstem bundle that terminated ipsilaterally in regions of the medulla and cerebellum considered to subserve acoustic and lateral line functions. Together with other data in distantly related teleost fishes, the results support the hypotheses that 1) central pathways linking sound-generating (sonic or vocal) and acoustic regions of the brain are traits common to both teleosts fishes and tetrapods that actively generate sounds, and 2) sonic/vocal pathways in teleosts have a conserved pattern of organization suggestive of common developmental origins.

Androgen effects on vocal muscle structure in a teleost fish with inter- and intra-sexual dimorphism

The plainfin midshipman fish Porichthys notatus has both inter- and intra-sexual dimorphism in the sound-producing (vocal or sonic) muscles attached to the swimbladder wall. The "Type I" and "Type II" male morphs differ in that dramatic structural changes related to sexual maturity occur in the mass, the area of mitochondria-filled sarcoplasm, and the myofiber number of the sonic muscles of Type I males, but not in those of Type II males (nor of females). Androgen implantation for 9 weeks markedly increased the relative sonic muscle size in juvenile males, juvenile females, and Type II males, whereas estradiol or cholesterol treatment did not. The principal androgen effect on myofiber structure was an increase in the area of mitochondria-filled sarcoplasm. The ratio of sarcoplasm area to myofibril area (Sr/Mf) increased by 1.4- to 2-fold in myofibers of all androgen-treated groups, with the greatest structural change occurring in juvenile males. When androgen implants were removed from juvenile males, the muscle mass and Sr/Mf ratio reverted toward the unimplanted juvenile phenotype. Total fiber number in sonic muscle increased significantly in juvenile males following androgen implantation but did not detectably change in juvenile females or Type II males. These results suggest: 1) sonic muscle in Porichthys notatus is an androgen target tissue, 2) fiber structure and fiber number are androgen-sensitive features, and 3) there exist sex- and morph-specific patterns of sonic muscle responsiveness to androgen implants.