Ontogeny of the electric organ discharge and of the papillae of the electrocytes in the weakly electric fish Campylomormyrus rhynchophorus (Teleostei: Mormyridae).

The electric organ of the mormyrid weakly electric fish, Campylomormyrus rhynchophorus (Boulenger, 1898), undergoes changes in both the electric organ discharge (EOD) and the light and electron microscopic morphology as the fish mature from the juvenile to the adult form. Of particular interest was the appearance of papillae, surface specializations of the uninnervated anterior face of the electrocyte, which have been hypothesized to increase the duration of the EOD. In a 24.5 mm long juvenile the adult electric organ (EO) was not yet functional, and the electrocytes lacked papillae. A 40 mm long juvenile, which produced a short biphasic EOD of 1.3 ms duration, shows small papillae (average area 136 µm2 ). In contrast, the EOD of a 79 mm long juvenile was triphasic. The large increase in duration of the EOD to 23.2 ms was accompanied by a small change in size of the papillae (average area 159 µm2 ). Similarly, a 150 mm long adult produced a triphasic EOD of comparable duration to the younger stage (24.7 ms) but featured a prominent increase in size of the papillae (average area 402 µm2 ). Thus, there was no linear correlation between EOD duration and papillary size. The most prominent ultrastructural change was at the level of the myofilaments, which regularly extended into the papillae, only in the oldest specimen-probably serving a supporting function. Physiological mechanisms, like gene expression levels, as demonstrated in some Campylomormyrus species, might be more important concerning the duration of the EOD.

Ontogeny of electric organ and electric organ discharge in Campylomormyrus rhynchophorus (Teleostei: Mormyridae).

The aim of this study was a longitudinal description of the ontogeny of the adult electric organ of Campylomormyrus rhynchophorus which produces as adult an electric organ discharge of very long duration (ca. 25 ms). We could indeed show (for the first time in a mormyrid fish) that the electric organ discharge which is first produced early during ontogeny in 33-mm-long juveniles is much shorter in duration and has a different shape than the electric organ discharge in 15-cm-long adults. The change from this juvenile electric organ discharges into the adult electric organ discharge takes at least a year. The increase in electric organ discharge duration could be causally linked to the development of surface evaginations, papillae, at the rostral face of the electrocyte which are recognizable for the first time in 65-mm-long juveniles and are most prominent at the periphery of the electrocyte.

Comparative histology of the adult electric organ among four species of the genus Campylomormyrus (Teleostei: Mormyridae).

The electric organ (EO) of weakly electric mormyrids consists of flat, disk-shaped electrocytes with distinct anterior and posterior faces. There are multiple species-characteristic patterns in the geometry of the electrocytes and their innervation. To further correlate electric organ discharge (EOD) with EO anatomy, we examined four species of the mormyrid genus Campylomormyrus possessing clearly distinct EODs. In C. compressirostris, C. numenius, and C. tshokwe, all of which display biphasic EODs, the posterior face of the electrocytes forms evaginations merging to a stalk system receiving the innervation. In C. tamandua that emits a triphasic EOD, the small stalks of the electrocyte penetrate the electrocyte anteriorly before merging on the anterior side to receive the innervation. Additional differences in electrocyte anatomy among the former three species with the same EO geometry could be associated with further characteristics of their EODs. Furthermore, in C. numenius, ontogenetic changes in EO anatomy correlate with profound changes in the EOD. In the juvenile the anterior face of the electrocyte is smooth, whereas in the adult it exhibits pronounced surface foldings. This anatomical difference, together with disparities in the degree of stalk furcation, probably contributes to the about 12 times longer EOD in the adult.

Reproductive character displacement and signal ontogeny in a sympatric assemblage of electric fish.

The reproductive signals of two or more taxa may diverge in areas of sympatry, due to selection against costly reproductive interference. This divergence, termed reproductive character displacement (RCD), is expected in species-rich assemblages, where interspecific signal partitioning among closely related species is common. However, RCD is usually documented from simple two-taxon cases, via geographical tests for greater divergence of reproductive traits in sympatry than in allopatry. We propose a novel approach to recognizing and understanding RCD in multi-species communities--one that traces the displacement of signals within multivariate signal space during the ontogeny of individual animals. We argue that a case for RCD can be made if the amount of signal displacement between a pair of species after maturation is negatively correlated to distance in signal space before maturation. Our application of this approach, using a dataset of communication signals from a sympatric Amazonian assemblage of the electric fish genus Gymnotus, provides strong evidence for RCD among multiple species. We argue that RCD arose from the costs of heterospecific mismating, but interacted with sexual selection--favoring the evolution of conspicuous male signals that not only serve for mate-choice, but which simultaneously facilitate species recognition.

Post-natal development of the electromotor system in a pulse gymnotid electric fish.

Some fish emit electric fields generated by the coordinated activation of electric organs. Such discharges are used for exploring the environment and for communication. This article deals with the development of the electric organ and its discharge in Gymnotus, a pulse genus in which brief discharges are separated by regular silent intervals. It is focused on the anatomo-functional study of fish sized between 10 and 300 mm from the species of Gymnotus, in which electrogenic mechanisms are best known. It was shown that: (1) electroreception and electromotor control is present from early larval stages; (2) there is a single electric organ from larval to adult stages; (3) pacemaker rhythmicity becomes similar to that of the adult when the body length becomes greater than 45 mm and (4) there is a consistent developmental profile of the electric organ discharge in which waveform components are added according to a programmed sequence. The analysis of these data allowed us to identify three main periods in post-natal development of electrogenesis: (1) before fish reach 55 mm in length, when maturation of neural structures is the main factor determining a characteristic sequence of changes observed in the discharge timing and waveform; (2) between 55 and 100 mm in length, when peripheral maturation of the effector cells and changes in post-effector mechanisms due to the fish's growth determine minor changes in waveform and the increase in amplitude of the discharge and (3) beyond 100 mm in length, when homothetic growth of the fish body explains the continuous increase in electric power of the discharge.

Lateral line receptors: where do they come from developmentally and where is our research going?

The lateral line system is composed of both mechanoreceptors, which exhibit little variation in structure between taxonomic groups, and electroreceptors, which exhibit considerably more variation. Cathodally sensitive ampullary electroreceptors are the primitive condition and are found in agnathans, chondrichthyans, and most osteichthyans. Aquatic amphibians also have ampullary electroreceptors for at least part of their life cycle. The more recently evolved anodally sensitive ampullary electroreceptors and tuberous electroreceptors are only found in four groups of teleost fishes. The basic ontogenetic unit of lateral line development is the dorsolateral placode. Primitively, there are six pairs of placodes, which pass through sequential stages of development into lateral line receptors. There is no question about the origin of primitive mechanoreceptors or electroreceptors, however, we do not have a good understanding of the origin of teleost mechanoreceptors and their ampullary or tuberous electroreceptors; do they come exclusively from dorsolateral placodes or from neural crest or even general ectoderm? A second intriguing lateral line question is how certain teleost fish groups evolved tuberous electroreceptors. Electroreception appears to have re-evolved at least twice in teleosts after being lost during the neopterygian radiation. It has been suggested that the development of tuberous electroreceptors might be due to changes in placodal patterning or a change in the general ectoderm that placodes arise from. Unfortunately, our understanding of lateral line origins in fishes is very sketchy, and, if we are to answer such an evolutionary question, we first need more complete information about lateral line development in a variety of fishes, which can then be combined with gene expression data to better interpret lateral line receptor development.

Anatomy and motor pathways of the electric organ of skates.

The electric organ of skates is a paired structure within the tail consisting of two longitudinal columns of electrocytes contained within the lateral musculature on each side of the vertebral column. The electrocytes develop from hypaxial skeletal muscle fibers, and, depending upon the species, are generally classified as either cup-shaped or disc-shaped. The disc-shaped electrocytes are considered to be the more derived type. Regardless of the morphology of the electrocyte, the electric organ discharge of all skates is characterized as a weak asynchronous and long-lasting signal. Although recent behavioral investigations have revealed a communicative function for the electric organ, details as to which specific behaviors utilize this motor system remain uncertain. The electric organ is innervated by segmental motor nerves that branch from the ventral root of caudal spinal nerves at all levels of the electric organ. The cells of origin of the electromotor nerves, or electromotoneurons (EMNs), are large multipolar neurons with extensive dendrites located within the ventral gray matter of the spinal cord. The EMNs are uniformly distributed among the somatic motoneurons at levels corresponding to the rostrocaudal extent of the electric organ, and therefore do not form a discrete nucleus. The medullary command nucleus is comprised of neurons located within the nucleus raphe magnus, and forms a descending spinal pathway to the EMNs.

Development and regeneration of the electric organ.

The electric organ has evolved independently from muscle in at least six lineages of fish. How does a differentiated muscle cell change its fate to become an electrocyte? Is the process by which this occurs similar in different lineages? We have begun to answer these questions by studying the formation and maintenance of electrocytes in the genus Sternopygus, a weakly electric teleost. Electrocytes arise from the fusion of fully differentiated muscle fibers, mainly those expressing fast isoforms of myosin. Electrocytes briefly co-express sarcomeric proteins, such as myosin and tropomyosin, and keratin, a protein not found in mature muscle. The sarcomeric proteins are subsequently down-regulated, but keratin expression persists. We investigated whether the maintenance of the electrocyte phenotype depends on innervation. We found that, after spinal cord transection, which silences the electromotor neurons that innervate the electrocytes, or destruction of the spinal cord, which denervates the electrocytes, mature electrocytes re-express sarcomeric myosin and tropomyosin, although keratin expression persists. Ultrastructural examination of denervated electrocytes revealed nascent sarcomeres. Thus, the maintenance of the electrocyte phenotype depends on neural activity.

Morphology of the brain and sense organs in the snailfish Paraliparis devriesi: neural convergence and sensory compensation on the Antarctic shelf.

The Antarctic snailfish Paraliparis devriesi (Liparidae) is an epibenthic species, inhabiting depths of 500-650 m in McMurdo Sound. Liparids are the most speciose fish family in the Antarctic Region. We examine the gross morphology and histology of the sense organs and brain of P. devriesi and provide a phyletic perspective by comparing this morphology to that of four scorpaeniforms and of sympatric perciform notothenioids. The brain has numerous derived features, including well-developed olfactory lamellae with thick epithelia, large olfactory nerves and bulbs, and large telencephalic lobes. The retina contains only rods and exhibits a high convergence ratio (82:1). Optic nerves are small and nonpleated. The tectum is small. The corpus of the cerebellum is large, whereas the valvula is vestigial. The rhombencephalon and bulbospinal junction are extended and feature expanded vagal and spinal sensory lobes as well as hypertrophied dorsal horns and funiculi in the rostral spinal cord. The lower lobes of the pectoral fins have taste buds and expanded somatosensory innervation. Although the cephalic lateral line and anterior lateral line nerve are well developed, the trunk lateral line and posterior lateral line nerve are reduced. Near-field mechanoreception by trunk neuromasts may have been compromised by the watery, gelatinous subdermal extracellular matrix employed as a buoyancy mechanism. The expanded somatosensory input to the pectoral fin may compensate for the reduction in the trunk lateral line. The brains of P. devriesi and sympatric notothenioids share well-developed olfactory systems, an enlarged preoptic-hypophyseal axis, and subependymal expansions. Although the functional significance is unknown, the latter two features are correlated with habitation of the deep subzero waters of the Antarctic shelf.

Ontogeny of the electric organ discharge and the electric organ in the weakly electric pulse fish Brachyhypopomus pinnicaudatus (Hypopomidae, Gymnotiformes).

I recorded the electric organ discharges (EODs) of 331 immature Brachyhypopomus pinnicaudatus 6-88 mm long. Larvae produced head-positive pulses 1.3 ms long at 7 mm (6 days) and added a second, small head-negative phase at 12 mm. Both phases shortened duration and increased amplitude during growth. Relative to the whole EOD, the negative phase increased duration until 22 mm and amplitude until 37 mm. Fish above 37 mm produced a "symmetric" EOD like that of adult females. I stained cleared fish with Sudan black, or fluorescently labeled serial sections with anti-desmin (electric organ) or anti-myosin (muscle). From day 6 onward, a single electric organ was found at the ventral margin of the hypaxial muscle. Electrocytes were initially cylindrical, overlapping, and stalk-less, but later shortened along the rostrocaudal axis, separated into rows, and formed caudal stalks. This differentiation started in the posterior electric organ in 12-mm fish and was complete in the anterior region of fish with "symmetric" EODs. The lack of a distinct "larval" electric organ in this pulse-type species weakens the hypothesis that all gymnotiforms develop both a temporary (larval) and a permanent (adult) electric organ.

Development of the jamming avoidance response and its morphological correlates in the gymnotiform electric fish, Eigenmannia.

The electric fish, Eigenmannia, will smoothly shift the frequency of its electric organ discharge away from an interfering electric signal. This shift in frequency is called the jamming avoidance response (JAR). In this article, we analyze the behavioral development of the JAR and the anatomical development of structures critical for the performance of the JAR. The JAR first appears when juvenile Eigenmannia are approximately 1 month old, at a total length of 13-18 mm. We have found that the establishment of much of the sensory periphery and of central connections precedes the onset of the JAR. We describe three aspects of the behavioral development of the JAR: (a) the onset and development of the behavior is closely correlated with size, not age; (b) the magnitude (in Hz) of the JAR increases with size until the juveniles display values within the adult range (10-20 Hz) at a total length of 25-30 mm; and (3) the JAR does not require prior experience or exposure to electrical signals. Raised in total electrical isolation from the egg stage, animals tested at a total length of 25 mm performed a correct JAR when first exposed to the stimulus. We examine the development of anatomical areas important for the performance of the JAR: the peripheral electrosensory system (mechano- and electroreceptors and peripheral nerves); and central electrosensory pathways and nuclei [the electrosensory lateral line lobe (ELL), the lateral lemniscus, the torus semicircularis, and the pace-maker nucleus]. The first recognizable structures in the developing electrosensory system are the peripheral neurites of the anterior lateral line nerve. The afferent nerves are established by day 2, which is prior to the formation of receptors in the epidermis. Thus, the neurites wait for their targets. This sequence of events suggests that receptor formation may be induced by innervation of primordial cells within the epidermis. Mechanoreceptors are first formed between day 3 and 4, while electroreceptors are first formed on day 7. Electroreceptor multiplication is observed for the first time at an age of 25 days and correlates with the onset of the JAR. The somata of the anterior lateral line nerve ganglion project afferents out to peripheral electroreceptors and also send axons centrally into the ELL. The first electroreceptive axons invade the ELL by day 6, and presumably a rough somatotopic organization and segmentation within the ELL may arise as early as day 7. Axonal projections from the ELL to the torus develop after day 18.(ABSTRACT TRUNCATED AT 400 WORDS)

Zebrin II distinguishes the ampullary organ receptive map from the tuberous organ receptive maps during development in the teleost electrosensory lateral line lobe.

In weakly electric gymnotiform teleosts, monoclonal antibody anti-zebrin II recognizes developing pyramidal cells in the ampullary organ-receptive medial segment of the medullary electrosensory lateral line lobe (ELL) and in the mechanoreceptive nucleus medialis. Developing pyramidal cells in the remaining three tuberous organ-receptive lateral ELL segments are unreactive. These results suggest that certain biochemical features of the ELL ampullary organ-receptive medial segment are more similar to the nucleus medialis than to the tuberous organ-receptive ELL segments, and support the hypothesis that the ampullary system evolved from mechanosensory precursors.

[Development, during ontogenetic development of gymnotids, of substance p expression in tuberous organs (electroreceptors)]. / Evolution, durant le développement ontogénétique des gymnotides, de l'expression de la substance P dans les organes tubéreux (électrorécepteurs).

The evolution of the neuropeptidic expression of Substance P has been investigated with immunohistochemistry in the cutaneous electroreceptors, tuberous organs, during ontogenetic development of Apteronotus leptorhynchus. In the present data, antiSP antiserum has been applied to serial sections of Apteronotus leptorhynchus larvae obtained from several egg layings. Larvae were taken during development from hatching up to one hundred days old. SP immunoreactivity appeared just after hatching, in the epidermal zones which give rise to cutaneous sense organs. Four days after hatching, the tuberous organs are differentiated and immunoreactivity was observed in these organs, in both sensory cells and accessory cells. From day 30 after hatching, there was a regular decrease in the number of tuberous organs showing labelled accessory cells, and one hundred days later only 8% of tuberous organs had immunoreactive accessory cells. The adult accessory cells were no longer labelled with anti-SP antiserum. The results showed that in Apteronotus leptorhynchus, the epidermal structures which give rise to the cutaneous sensory organs were immunoreactive at a very early stage of development; this suggests that SP could have an effect upon their differentiation.

The evolution of metamorphosis in amphibians.

A survey is provided of the external transformations that coincide with metamorphosis or a water-to-land transition, and of transformations during water-to-land transition in the retinal projection, the brain stem, the lateral-line system, and the inner ear of amphibians. Among the three orders of amphibians, the frogs are characterized by more pronounced transformations during the water-to-land transition than are the other two orders. Some of the progressive and regressive changes in the sensory and nervous system are presented and a scenario is suggested for the evolution of these transformations among amphibians. Suggestions that metamorphosis in frogs can recapitulate the water-to-land transition of ancestral amniotic vertebrates are refuted.

Developmental expression of the 43K and 58K postsynaptic membrane proteins and nicotinic acetylcholine receptors in Torpedo electrocytes.

The expression of the postsynaptic 43-kDa and 58-kDa proteins and actin during development of the Torpedo marmorata electric organ was compared to that of nicotinic acetylcholine receptors (AChRs). Western blot analysis demonstrates that AChRs and proteins of 43 kDa (43K protein) and 58 kDa (58K protein) are all present prior to synaptogenesis. Subsequently, levels of all 3 synaptic proteins increase dramatically during differentiation and innervation of electrocytes. In contrast, actin is present in relatively high concentrations at early times and decreases thereafter. The equimolar ratio of AChRs and the 43K protein found in the adult electric organ is established early in development. Furthermore, the AChR and 43K protein share a common postsynaptic localization in electrocytes following synapse formation. Aggregates of the AChR that form at the ventral pole of the oval-shaped electrocytes prior to innervation, however, show no detectable immunofluorescence staining with anti-43K monoclonal antibodies. Therefore, in some cases, aggregation of AChRs occurs without the 43K protein.

Asymmetric distribution of dystrophin in developing and adult Torpedo marmorata electrocyte: evidence for its association with the acetylcholine receptor-rich membrane.

Dystrophin has been shown to occur in Torpedo electrocyte [Chang, H. W., Bock, E. & Bonilla, E. (1989) J. Biol. Chem. 264, 20831-20834], a highly polarized syncytium that is embryologically derived from skeletal muscle and displays functionally distinct plasma membrane domains on its innervated and noninnervated faces. In the present study, we investigated the subcellular distribution of dystrophin in the adult electrocyte from Torpedo marmorata and the evolution of its distribution during embryogenesis. Immunofluorescence experiments performed on adult electrocytes with a polyclonal antibody directed against chicken dystrophin revealed that dystrophin immunoreactivity codistributed exclusively with the acetylcholine receptor along the innervated membrane. At the ultrastructural level, dystrophin immunoreactivity appears confined to the face of the subsynaptic membrane exposed to the cytoplasm. In developing electrocytes (45-mm embryo), dystrophin is already detectable at the acetylcholine receptor-rich ventral pole of the cells before the entry of the electromotor axons. Furthermore, we show that dystrophin represents a major component of purified membrane fractions rich in acetylcholine receptor. A putative role of dystrophin in the organization and stabilization of the subsynaptic membrane domain of the electrocyte is discussed.

Hormone-induced and maturational changes in electric organ discharges and electroreceptor tuning in the weakly electric fish Apteronotus.

Plasticity in the frequency of the electric organ discharge (EOD) and electroreceptor tuning of weakly electric fish was studied in the genus Apteronotus. Both hormone-induced and maturational changes in EOD frequency and electroreceptor tuning were examined. Apteronotus is different from all other steroid-responsive weakly electric fish in that estradiol-17 beta, rather than androgens, induces discharge frequency decreases. This result can account for the 'reversed' discharge frequency dimorphism found in Apteronotus in which, counter to all other known sexually dimorphic electric fish, females have lower discharge frequencies than males. Studies of electroreceptor tuning in Apteronotus indicate that electroreceptors are closely tuned to the frequency of the EOD. This finding was noted not only in adult animals, but also in juvenile animals shortly after the onset of their EODs. Tuning plasticity in Apteronotus, as in other species studied, is associated with altered EOD frequencies and was noted in both maturational EOD changes and in estrogen-induced changes. Thus, tuning plasticity appears to be a general phenomenon which occurs concurrent with a variety of EOD changes.

Torpedo electromotor system development: neuronal cell death and electric organ development in the fourth branchial arch.

The fourth branchial arch of Torpedo marmorata has been examined at the light and electron microscopic level during development. Of interest was the determination of the extent of electric organ tissue reported to be present in this arch and its possible relationship to electromotoneuron cell death in the electric lobes. The main electric organ of the torpedo is derived from the hyoid and first three branchial arches and is innervated by four major electromotor nerves. Extensive electromotoneuron cell death occurs in the electric lobes and most notably in the posterior poles. This feature could be due to a tendency for these neurons to innervate the fourth branchial arch where little or no electric tissue is formed. Our findings support this conclusion but are not entirely consistent with the idea that a population mismatch has occurred. This is because cell death precedes the genesis of the target cells. The presence of innervated differentiated electric tissue in this arch is also reported, leading to the conclusion that Torpedo marmorata possesses an accessory electric organ.

Torpedo electromotor system development: a quantitative analysis of synaptogenesis.

Synaptogenesis in the electric organ of Torpedo marmorato has been studied quantitatively at the ultrastructural level of observation. In addition to establishing the normal developmental time course for this event we were interested in determining whether a gradient of synaptogenesis might be present because the electric organ produces several morphologically recognizable spatiotemporal gradients during its early ontogeny. These gradients genesis of electrocyte columns, both gradients of which are operative for periods of weeks. No gradient of synaptogenesis was found, indicating this to be a synchronous process. The idea is advanced that synaptogenesis in the electric organ is modulated by extrinsic influences, many of which may originate from the target electrocytes which, by this time, have become synchronized in their development.