Rapid evolution of a voltage-gated sodium channel gene in a lineage of electric fish leads to a persistent sodium current

Ventral-to-dorsal electrocyte development in electric organs of electric eel (Electrophorus)

Electric eels (Electrophorus) are renowned for their ability to generate electric discharge, which is used for prey capture and defense. Their electric organs (EOs) are located along the lateral-ventral region of the tail and contain electrocytes, which are multinucleated syncytium cells. The developmental origin of the electrocyte is mesodermal lineage cells observed in the ventral part of the myotome. However, it is unclear whether these precursor cells are also maintained in later stages and contribute to electric organ growth in adulthood. In this study, we report regional differences in cell morphology within the main EO (mEO) and identify a candidate cell population for electrocyte progenitors at the juvenile stages of Electrophorus. The cell morphology and distribution from the ventral terminal to the dorsal region of the mEO suggest the segregation of progenitors from the ventral cluster and their gradual transformation into mature multinucleated electrocytes via cell fusion and layering along the dorsal axis. Immunohistochemistry revealed strong expression of sodium-potassium adenosine triphosphatase (Na+/K+-ATPase), a key component in generating electric discharge in the mEO, across most mEO regions, except in the ventral cluster cells. Based on these observations, we propose that electrocyte progenitors develop from ventral cluster cells in the mEO and differentiate into mature multinucleated cells as they migrate dorsally. This is the first report to approach the developmental process of Electrophorus electrocytes from cell morphology and genetic profiles. Our findings represent a breakthrough in understanding the differentiation of electrocytes during the growth stages of Electrophorus.

Normal locomotion in zebrafish lacking the sodium channel NaV1.4 suggests that the need for muscle action potentials is not universal

Extensive studies over decades have firmly established the concept that action potentials (APs) in muscles are indispensable for muscle contraction. To re-examine the significance of APs, we generated zebrafish lacking APs by editing the scn4aa and scn4ab genes, which together encode NaV1.4 (NaVDKO), using the CRISPR-Cas9 system. Surprisingly, the escape response of NaVDKOs to tactile stimuli, both in the embryonic and adult stages, was indistinguishable from that of wild-type (WT) fish. Ca2+ imaging using the calcium indicator protein GCaMP revealed that myofibers isolated from WT fish could be excited by the application of acetylcholine (ACh), even in the presence of tetrodotoxin (TTX) indicating that NaVs are dispensable for skeletal muscle contraction in zebrafish. Mathematical simulations showed that the end-plate potential was able to elicit a change in membrane potential large enough to activate the dihydropyridine receptors of the entire muscle fiber owing to the small fiber size and the disseminated distribution of neuromuscular synapses in both adults and embryos. Our data demonstrate that NaVs are not essential for muscle contraction in zebrafish and that the physiological significance of NaV1.4 in muscle is not uniform across vertebrates.

Differential expression of steroid-related genes across electrosensory brain regions in two sexually dimorphic species of electric knifefish

The production of communication signals can be modulated by hormones acting on the brain regions that regulate these signals. However, less is known about how signal perception is regulated by hormones. The electrocommunication signals of weakly electric fishes are sexually dimorphic, sensitive to hormones, and vary across species. The neural circuits that regulate the production and perception of these signals are also well-characterized, and electric fishes are thus an excellent model to examine the neuroendocrine regulation of sensorimotor mechanisms of communication. We investigated (1) whether steroid-related genes are expressed in sensory brain regions that process communication signals; and (2) whether this expression differs across sexes and species that have different patterns of sexual dimorphism in their signals. Apteronotus leptorhynchus and Apteronotus albifrons produce continuous electric organ discharges (EODs) that are used for communication. Two brain regions, the electrosensory lateral line lobe (ELL) and the dorsal torus semicircularis (TSd), process inputs from electroreceptors to allow fish to detect and discriminate electrocommunication signals. We used qPCR to quantify the expression of genes for two androgen receptors (ar1, ar2), two estrogen receptors (esr1, esr2b), and aromatase (cyp19a1b). Four out of five steroid-related genes were expressed in both sensory brain regions, and their expression often varied between sexes and species. These results suggest that expression of steroid-related genes in the brain may differentially influence how EOD signals are encoded across species and sexes, and that gonadal steroids may coordinately regulate central circuits that control both the production and perception of EODs.

Neuromodulation and the toolkit for behavioural evolution: can ecdysis shed light on an old problem?

The geneticist Thomas Dobzhansky famously declared: 'Nothing in biology makes sense except in the light of evolution'. A key evolutionary adaptation of Metazoa is directed movement, which has been elaborated into a spectacularly varied number of behaviours in animal clades. The mechanisms by which animal behaviours have evolved, however, remain unresolved. This is due, in part, to the indirect control of behaviour by the genome, which provides the components for both building and operating the brain circuits that generate behaviour. These brain circuits are adapted to respond flexibly to environmental contingencies and physiological needs and can change as a function of experience. The resulting plasticity of behavioural expression makes it difficult to characterize homologous elements of behaviour and to track their evolution. Here, we evaluate progress in identifying the genetic substrates of behavioural evolution and suggest that examining adaptive changes in neuromodulatory signalling may be a particularly productive focus for future studies. We propose that the behavioural sequences used by ecdysozoans to moult are an attractive model for studying the role of neuromodulation in behavioural evolution.

Ecologically mediated differences in electric organ discharge drive evolution in a sodium channel gene in South American electric fishes

Active electroreception-the ability to detect objects and communicate with conspecifics via the detection and generation of electric organ discharges (EODs)-has evolved convergently in several fish lineages. South American electric fishes (Gymnotiformes) are a highly species-rich group, possibly in part due to evolution of an electric organ (EO) that can produce diverse EODs. Neofunctionalization of a voltage-gated sodium channel gene accompanied the evolution of electrogenic tissue from muscle and resulted in a novel gene (scn4aa) uniquely expressed in the EO. Here, we investigate the link between variation in scn4aa and differences in EOD waveform. We combine gymnotiform scn4aa sequences encoding the C-terminus of the Nav1.4a protein, with biogeographic data and EOD recordings to test whether physiological transitions among EOD types accompany differential selection pressures on scn4aa. We found positive selection on scn4aa coincided with shifts in EOD types. Species that evolved in the absence of predators, which likely selected for reduced EOD complexity, exhibited increased scn4aa evolutionary rates. We model mutations in the protein that may underlie changes in protein function and discuss our findings in the context of gymnotiform signalling ecology. Together, this work sheds light on the selective forces underpinning major evolutionary transitions in electric signal production.

Living life with an electric touch

The electric organ discharges (EODs) produced by weakly electric fish have long been a source of scientific intrigue and inspiration. The study of these species has contributed to our understanding of the organization of fixed action patterns, as well as enriching general imaging theory by unveiling the dual impact of an agent's actions on the environment and its own sensory system during the imaging process. This Centenary Review firstly compares how weakly electric fish generate species- and sex-specific stereotyped electric fields by considering: (1) peripheral mechanisms, including the geometry, channel repertoire and innervation of the electrogenic units; (2) the organization of the electric organs (EOs); and (3) neural coordination mechanisms. Secondly, the Review discusses the threefold function of the fish-centered electric fields: (1) to generate electric signals that encode the material, geometry and distance of nearby objects, serving as a short-range sensory modality or 'electric touch'; (2) to mark emitter identity and location; and (3) to convey social messages encoded in stereotypical modulations of the electric field that might be considered as species-specific communication symbols. Finally, this Review considers a range of potential research directions that are likely to be productive in the future.

Neuroendocrine mechanisms contributing to the coevolution of sociality and communication

Communication is inherently social, so signaling systems should evolve with social systems. The 'social complexity hypothesis' posits that social complexity necessitates communicative complexity and is generally supported in vocalizing mammals. This hypothesis, however, has seldom been tested outside the acoustic modality, and comparisons across studies are confounded by varying definitions of complexity. Moreover, proximate mechanisms underlying coevolution of sociality and communication remain largely unexamined. In this review, we argue that to uncover how sociality and communication coevolve, we need to examine variation in the neuroendocrine mechanisms that coregulate social behavior and signal production and perception. Specifically, we focus on steroid hormones, monoamines, and nonapeptides, which modulate both social behavior and sensorimotor circuits and are likely targets of selection during social evolution. Lastly, we highlight weakly electric fishes as an ideal system in which to comparatively address the proximate mechanisms underlying relationships between social and signal diversity in a novel modality.

Transcriptome mining extends the host range of the Flaviviridae to non-bilaterians

The flavivirids (family Flaviviridae) are a group of positive-sense RNA viruses that include well-documented agents of human disease. Despite their importance and ubiquity, the timescale of flavivirid evolution is uncertain. An ancient origin, spanning millions of years, is supported by their presence in both vertebrates and invertebrates and by the identification of a flavivirus-derived endogenous viral element in the peach blossom jellyfish genome (Craspedacusta sowerbii, phylum Cnidaria), implying that the flaviviruses arose early in the evolution of the Metazoa. To date, however, no exogenous flavivirid sequences have been identified in these hosts. To help resolve the antiquity of the Flaviviridae, we mined publicly available transcriptome data across the Metazoa. From this, we expanded the diversity within the family through the identification of 32 novel viral sequences and extended the host range of the pestiviruses to include amphibians, reptiles, and ray-finned fish. Through co-phylogenetic analysis we found cross-species transmission to be the predominate macroevolutionary event across the non-vectored flavivirid genera (median, 68 per cent), including a cross-species transmission event between bats and rodents, although long-term virus-host co-divergence was still a regular occurrence (median, 23 per cent). Notably, we discovered flavivirus-like sequences in basal metazoan species, including the first associated with Cnidaria. This sequence formed a basal lineage to the genus Flavivirus and was closer to arthropod and crustacean flaviviruses than those in the tamanavirus group, which includes a variety of invertebrate and vertebrate viruses. Combined, these data attest to an ancient origin of the flaviviruses, likely close to the emergence of the metazoans 750-800 million years ago.

Divergent cis-regulatory evolution underlies the convergent loss of sodium channel expression in electric fish

South American and African weakly electric fish independently evolved electric organs from muscle. In both groups, a voltage-gated sodium channel gene independently lost expression from muscle and gained it in the electric organ, allowing the channel to become specialized for generating electric signals. It is unknown how this voltage-gated sodium channel gene is targeted to muscle in any vertebrate. We describe an enhancer that selectively targets sodium channel expression to muscle. Next, we demonstrate how the loss of this enhancer, but not trans-activating factors, drove the loss of sodium channel gene expression from muscle in South American electric fish. While this enhancer is also altered in African electric fish, key transcription factor binding sites and enhancer activity are retained, suggesting that the convergent loss of sodium channel expression from muscle in these two electric fish lineages occurred via different processes.

Advancing human disease research with fish evolutionary mutant models

Model organism research is essential to understand disease mechanisms. However, laboratory-induced genetic models can lack genetic variation and often fail to mimic the spectrum of disease severity. Evolutionary mutant models (EMMs) are species with evolved phenotypes that mimic human disease. EMMs complement traditional laboratory models by providing unique avenues to study gene-by-environment interactions, modular mutations in noncoding regions, and their evolved compensations. EMMs have improved our understanding of complex diseases, including cancer, diabetes, and aging, and illuminated mechanisms in many organs. Rapid advancements of sequencing and genome-editing technologies have catapulted the utility of EMMs, particularly in fish. Fish are the most diverse group of vertebrates, exhibiting a kaleidoscope of specialized phenotypes, many that would be pathogenic in humans but are adaptive in the species' specialized habitat. Importantly, evolved compensations can suggest avenues for novel disease therapies. This review summarizes current research using fish EMMs to advance our understanding of human disease.

Vocal and Electric Fish: Revisiting a Comparison of Two Teleost Models in the Neuroethology of Social Behavior

The communication behaviors of vocal fish and electric fish are among the vertebrate social behaviors best understood at the level of neural circuits. Both forms of signaling rely on midbrain inputs to hindbrain pattern generators that activate peripheral effectors (sonic muscles and electrocytes) to produce pulsatile signals that are modulated by frequency/repetition rate, amplitude and call duration. To generate signals that vary by sex, male phenotype, and social context, these circuits are responsive to a wide range of hormones and neuromodulators acting on different timescales at multiple loci. Bass and Zakon (2005) reviewed the behavioral neuroendocrinology of these two teleost groups, comparing how the regulation of their communication systems have both converged and diverged during their parallel evolution. Here, we revisit this comparison and review the complementary developments over the past 16 years. We (a) summarize recent work that expands our knowledge of the neural circuits underlying these two communication systems, (b) review parallel studies on the action of neuromodulators (e.g., serotonin, AVT, melatonin), brain steroidogenesis (via aromatase), and social stimuli on the output of these circuits, (c) highlight recent transcriptomic studies that illustrate how contemporary molecular methods have elucidated the genetic regulation of social behavior in these fish, and (d) describe recent studies of mochokid catfish, which use both vocal and electric communication, and that use both vocal and electric communication and consider how these two systems are spliced together in the same species. Finally, we offer avenues for future research to further probe how similarities and differences between these two communication systems emerge over ontogeny and evolution.

Beyond the Kármán gait: knifefish swimming in periodic and irregular vortex streets

Neotropical freshwater fishes such as knifefishes are commonly faced with navigating intense and highly unsteady streams. However, our knowledge on locomotion in apteronotids comes from laminar flows, where the ribbon fin dominates over the pectoral fins or body bending. Here, we studied the 3D kinematics and swimming control of seven black ghost knifefish (Apteronotus albifrons) moving in laminar flows (flow speed U∞≈1-5 BL s-1) and in periodic vortex streets (U∞≈2-4 BL s-1). Two different cylinders (∼2 and ∼3 cm diameter) were used to generate the latter. Additionally, fish were exposed to an irregular wake produced by a free oscillating cylinder (∼2 cm diameter; U∞≈2 BL s-1). In laminar flows, knifefish mainly used their ribbon fin, with wave frequency, speed and acceleration increasing with U∞. In contrast, knifefish swimming behind a fixed cylinder increased the use of pectoral fins, which resulted in changes in body orientation that mimicked steady backward swimming. Meanwhile, individuals behind the oscillating cylinder presented a combination of body bending and ribbon and pectoral fin movements that counteract the out-of-phase yaw oscillations induced by the irregular shedding of vortices. We corroborated passive out-of-phase oscillations by placing a printed knifefish model just downstream of the moving cylinder, but when placed one cylinder diameter downstream, the model oscillated in phase. Thus, the wake left behind an oscillating body is more challenging than a periodic vortex shedding for an animal located downstream, which may have consequences on inter- and intra-specific interactions.

Functional evolution of vertebrate sensory receptors

Sensory receptors enable animals to perceive their external world, and functional properties of receptors evolve to detect the specific cues relevant for an organism's survival. Changes in sensory receptor function or tuning can directly impact an organism's behavior. Functional tests of receptors from multiple species and the generation of chimeric receptors between orthologs with different properties allow for the dissection of the molecular basis of receptor function and identification of the key residues that impart functional changes in different species. Knowledge of these functionally important sites facilitates investigation into questions regarding the role of epistasis and the extent of convergence, as well as the timing of sensory shifts relative to other phenotypic changes. However, as receptors can also play roles in non-sensory tissues, and receptor responses can be modulated by numerous other factors including varying expression levels, alternative splicing, and morphological features of the sensory cell, behavioral validation can be instrumental in confirming that responses observed in heterologous systems play a sensory role. Expression profiling of sensory cells and comparative genomics approaches can shed light on cell-type specific modifications and identify other proteins that may affect receptor function and can provide insight into the correlated evolution of complex suites of traits. Here we review the evolutionary history and diversity of functional responses of the major classes of sensory receptors in vertebrates, including opsins, chemosensory receptors, and ion channels involved in temperature-sensing, mechanosensation and electroreception.

Comparative Transcriptome Analyses Reveal the Role of Conserved Function in Electric Organ Convergence Across Electric Fishes

The independent origins of multiple electric organs (EOs) of fish are fascinating examples of convergent evolution. However, comparative transcriptomics of different electric fish lineages are scarce. In this study, we found that the gene expression of EOs and skeletal muscles from three lineages (Mormyroidea, Siluriformes, and Gymnotiformes) tended to cluster together based on the species of origin, irrespective of the organ from which they are derived. A pairwise comparison of differentially expressed genes (DEGs) revealed that no less than half of shared DEGs exhibited parallel expression differentiation, indicating conserved directionality of differential expression either in or between lineages, but only a few shared DEGs were identified across all focal species. Nevertheless, the functional enrichment analysis of DEGs indicated that there were more parallel gene expression changes at the level of pathways and biological functions. Therefore, we may conclude that there is no parallel evolution of the entire transcriptomes of EOs among different lineages. Further, our results support the hypothesis that it is not different genes but conserved biological functions that play a crucial role in the convergence of complex phenotypes. This study provides insight into the genetic basis underlying the EO convergent evolution; however, more studies in different cases will be needed to demonstrate whether this pattern can be extended to other cases to derive a general rule for convergent evolution.

Neural Evolution of Context-Dependent Fly Song

It is unclear where in the nervous system evolutionary changes tend to occur. To localize the source of neural evolution that has generated divergent behaviors, we developed a new approach to label and functionally manipulate homologous neurons across Drosophila species. We examined homologous descending neurons that drive courtship song in two species that sing divergent song types and localized relevant evolutionary changes in circuit function downstream of the intrinsic physiology of these descending neurons. This evolutionary change causes different species to produce divergent motor patterns in similar social contexts. Artificial stimulation of these descending neurons drives multiple song types, suggesting that multifunctional properties of song circuits may facilitate rapid evolution of song types.

Eukaryotic Voltage-Gated Sodium Channels: On Their Origins, Asymmetries, Losses, Diversification and Adaptations

The appearance of voltage-gated, sodium-selective channels with rapid gating kinetics was a limiting factor in the evolution of nervous systems. Two rounds of domain duplications generated a common 24 transmembrane segment (4 × 6 TM) template that is shared amongst voltage-gated sodium (Na_v1 and Na_v2) and calcium channels (Ca_v1, Ca_v2, and Ca_v3) and leak channel (NALCN) plus homologs from yeast, different single-cell protists (heterokont and unikont) and algae (green and brown). A shared architecture in 4 × 6 TM channels include an asymmetrical arrangement of extended extracellular L5/L6 turrets containing a 4-0-2-2 pattern of cysteines, glycosylated residues, a universally short III-IV cytoplasmic linker and often a recognizable, C-terminal PDZ binding motif. Six intron splice junctions are conserved in the first domain, including a rare U12-type of the minor spliceosome provides support for a shared heritage for sodium and calcium channels, and a separate lineage for NALCN. The asymmetrically arranged pores of 4x6 TM channels allows for a changeable ion selectivity by means of a single lysine residue change in the high field strength site of the ion selectivity filter in Domains II or III. Multicellularity and the appearance of systems was an impetus for Na_v1 channels to adapt to sodium ion selectivity and fast ion gating. A non-selective, and slowly gating Na_v2 channel homolog in single cell eukaryotes, predate the diversification of Na_v1 channels from a basal homolog in a common ancestor to extant cnidarians to the nine vertebrate Na_v1.x channel genes plus Nax. A close kinship between Na_v2 and Na_v1 homologs is evident in the sharing of most (twenty) intron splice junctions. Different metazoan groups have lost their Na_v1 channel genes altogether, while vertebrates rapidly expanded their gene numbers. The expansion in vertebrate Na_v1 channel genes fills unique functional niches and generates overlapping properties contributing to redundancies. Specific nervous system adaptations include cytoplasmic linkers with phosphorylation sites and tethered elements to protein assemblies in First Initial Segments and nodes of Ranvier. Analogous accessory beta subunit appeared alongside Na_v1 channels within different animal sub-phyla. Na_v1 channels contribute to pace-making as persistent or resurgent currents, the former which is widespread across animals, while the latter is a likely vertebrate adaptation.