Differential expression of genes and proteins between electric organ and skeletal muscle in the mormyrid electric fish Brienomyrus brachyistius

A review of the reproductive biology of mormyroid fishes: An emerging model for biomedical research

Mormyroidea is a superfamily of weakly electric African fishes with great potential as a model in a variety of biomedical research areas including systems neuroscience, muscle cell and craniofacial development, ion channel biophysics, and flagellar/ciliary biology. However, they are currently difficult to breed in the laboratory setting, which is essential for any tractable model organism. As such, there is a need to better understand the reproductive biology of mormyroids to breed them more reliably in the laboratory to effectively use them as a biomedical research model. This review seeks to (1) briefly highlight the biomedically relevant phenotypes of mormyroids and (2) compile information about mormyroid reproduction including sex differences, breeding season, sexual maturity, gonads, gametes, and courtship/spawning behaviors. We also highlight areas of mormyroid reproductive biology that are currently unexplored and/or have the potential for further investigation that may provide insights into more successful mormyroid laboratory breeding methods.

Gene and Allele-Specific Expression Underlying the Electric Signal Divergence in African Weakly Electric Fish

In the African weakly electric fish genus Campylomormyrus, electric organ discharge signals are strikingly different in shape and duration among closely related species, contribute to prezygotic isolation, and may have triggered an adaptive radiation. We performed mRNA sequencing on electric organs and skeletal muscles (from which the electric organs derive) from 3 species with short (0.4 ms), medium (5 ms), and long (40 ms) electric organ discharges and 2 different cross-species hybrids. We identified 1,444 upregulated genes in electric organ shared by all 5 species/hybrid cohorts, rendering them candidate genes for electric organ-specific properties in Campylomormyrus. We further identified several candidate genes, including KCNJ2 and KLF5, and their upregulation may contribute to increased electric organ discharge duration. Hybrids between a short (Campylomormyrus compressirostris) and a long (Campylomormyrus rhynchophorus) discharging species exhibit electric organ discharges of intermediate duration and showed imbalanced expression of KCNJ2 alleles, pointing toward a cis-regulatory difference at this locus, relative to electric organ discharge duration. KLF5 is a transcription factor potentially balancing potassium channel gene expression, a crucial process for the formation of an electric organ discharge. Unraveling the genetic basis of the species-specific modulation of the electric organ discharge in Campylomormyrus is crucial for understanding the adaptive radiation of this emerging model taxon of ecological (perhaps even sympatric) speciation.

Thirty Years with ERH: An mRNA Splicing and Mitosis Factor Only or Rather a Novel Genome Integrity Protector?

ERH is a 100 to about 110 aa nuclear protein with unique primary and three-dimensional structures that are very conserved from simple eukaryotes to humans, albeit some species have lost its gene, with most higher fungi being a noteworthy example. Initially, studies on Drosophila melanogaster implied its function in pyrimidine metabolism. Subsequently, research on Xenopus laevis suggested that it acts as a transcriptional repressor. Finally, studies in humans pointed to a role in pre-mRNA splicing and in mitosis but further research, also in Caenorhabditis elegans and Schizosaccharomyces pombe, demonstrated its much broader activity, namely involvement in the biogenesis of mRNA, and miRNA, piRNA and some other ncRNAs, and in repressive heterochromatin formation. ERH interacts with numerous, mostly taxon-specific proteins, like Mmi1 and Mei2 in S. pombe, PID-3/PICS-1, TOST-1 and PID-1 in C. elegans, and DGCR8, CIZ1, PDIP46/SKAR and SAFB1/2 in humans. There are, however, some common themes in this wide range of processes and partners, such as: (a) ERH homodimerizes to form a scaffold for several complexes involved in the metabolism of nucleic acids, (b) all these RNAs are RNA polymerase II transcripts, (c) pre-mRNAs, whose splicing depends on ERH, are enriched in transcripts of DNA damage response and DNA metabolism genes, and (d) heterochromatin is formed to silence unwanted transcription, e.g., from repetitive elements. Thus, it seems that ERH has been adopted for various pathways that serve to maintain genome integrity.

ERH Interacts With EIF2α and Regulates the EIF2α/ATF4/CHOP Pathway in Bladder Cancer Cells

Background

There is a lack of research on the molecular interaction of the enhancers of rudimentary homolog (ERH) in bladder cancer (BC) cells. This study aimed to determine the interacting proteins of ERH in human T24 cells.

Methods

First, the ERH gene was overexpressed in human T24 cells. Coimmunoprecipitation (co-IP) and shotgun mass spectrometry (MS) analyses were performed to obtain a list of proteins that interact with ERH. Subsequently, bioinformatic analyses with Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and protein–protein interaction (PPI) studies were performed to analyze the ERH-interactive protein list (ERH-IPL). Then, we selected one of the interacting proteins, EIF2α for verification. An immunofluorescence colocalization assay was performed to validate the co-expression of the selected protein, and the binding sites of the two proteins were predicted by ZDOCK technology. Finally, PCR analysis on the downstream molecules of the interacting protein was performed for verification.

Results

ERH protein was successfully overexpressed in human T24 cells. We obtained a list of 205 proteins that might directly or indirectly interact with the ERH protein by mass spectrometric analysis. The bioinformatic analysis showed that ERH-interacting proteins were related to “ribonucleoprotein complex”, “ATPase activity”, “nuclear speck”, and “translation factor activity, RNA binding”. We further identified one of the key genes, EIF2S1, and confirmed that the corresponding protein EIF2α is co-expressed and may bind with ERH in human T24 cells. The mRNA levels of molecules ATF4 and CHOP were found to be upregulated by ERH.

Conclusion

ERH protein affects “ribonucleoprotein complex”, “ATPase activity”, “nuclear speck”, and “translation factor activity, RNA binding”. The ERH protein can interact with EIF2α and regulate the EIF2α-ATF4/CHOP signaling pathway in human T24 cells.

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.

Genomic Evidence for Convergent Molecular Adaptation in Electric Fishes

Fishes have independently evolved electric organs (EOs) at least six times, and the electric fields are used for communication, defense, and predation. However, the genetic basis of convergent evolution of EOs remains unclear. In this study, we conducted comparative genomic analyses to detect genes showing signatures of positive selection and convergent substitutions in electric fishes from three independent lineages (Mormyroidea, Siluriformes, and Gymnotiformes). Analysis of 4,657 orthologs between electric fishes and their corresponding control groups identified consistent evidence for accelerated evolution in electric fish lineages. A total of 702 positively selected genes (PSGs) were identified in electric fishes, and many of these genes corresponded to cell membrane structure, ion channels, and transmembrane transporter activity. Comparative genomic analyses revealed that widespread convergent amino acid substitutions occurred along the electric fish lineages. The overlap of convergent genes and PSGs was identified as adaptive convergence, and a subset of genes was putatively associated with electrical and muscular activities, especially scn4aa (a voltage-gated sodium channel gene). Our results provide hints to the genetic basis for the independent evolution of EOs during millions of years of evolution.

Transcriptome-wide single nucleotide polymorphisms related to electric organ discharge differentiation among African weakly electric fish species

African weakly electric fish of the mormyrid genus Campylomormyrus generate pulse-type electric organ discharges (EODs) for orientation and communication. Their pulse durations are species-specific and elongated EODs are a derived trait. So far, differential gene expression among tissue-specific transcriptomes across species with different pulses and point mutations in single ion channel genes indicate a relation of pulse duration and electrocyte geometry/excitability. However, a comprehensive assessment of expressed Single Nucleotide Polymorphisms (SNPs) throughout the entire transcriptome of African weakly electric fish, with the potential to identify further genes influencing EOD duration, is still lacking. This is of particular value, as discharge duration is likely based on multiple cellular mechanisms and various genes. Here we provide the first transcriptome-wide SNP analysis of African weakly electric fish species (genus Campylomormyrus) differing by EOD duration to identify candidate genes and cellular mechanisms potentially involved in the determination of an elongated discharge of C. tshokwe. Non-synonymous substitutions specific to C. tshokwe were found in 27 candidate genes with inferred positive selection among Campylomormyrus species. These candidate genes had mainly functions linked to transcriptional regulation, cell proliferation and cell differentiation. Further, by comparing gene annotations between C. compressirostris (ancestral short EOD) and C. tshokwe (derived elongated EOD), we identified 27 GO terms and 2 KEGG pathway categories for which C. tshokwe significantly more frequently exhibited a species-specific expressed substitution than C. compressirostris. The results indicate that transcriptional regulation as well cell proliferation and differentiation take part in the determination of elongated pulse durations in C. tshokwe. Those cellular processes are pivotal for tissue morphogenesis and might determine the shape of electric organs supporting the observed correlation between electrocyte geometry/tissue structure and discharge duration. The inferred expressed SNPs and their functional implications are a valuable resource for future investigations on EOD durations.

The transcriptional correlates of divergent electric organ discharges in Paramormyrops electric fish

Background

Understanding the genomic basis of phenotypic diversity can be greatly facilitated by examining adaptive radiations with hypervariable traits. In this study, we focus on a rapidly diverged species group of mormyrid electric fish in the genus Paramormyrops, which are characterized by extensive phenotypic variation in electric organ discharges (EODs). The main components of EOD diversity are waveform duration, complexity and polarity. Using an RNA-sequencing based approach, we sought to identify gene expression correlates for each of these EOD waveform features by comparing 11 specimens of Paramormyrops that exhibit variation in these features.

Results

Patterns of gene expression among Paramormyrops are highly correlated, and 3274 genes (16%) were differentially expressed. Using our most restrictive criteria, we detected 145–183 differentially expressed genes correlated with each EOD feature, with little overlap between them. The predicted functions of several of these genes are related to extracellular matrix, cation homeostasis, lipid metabolism, and cytoskeletal and sarcomeric proteins. These genes are of significant interest given the known morphological differences between electric organs that underlie differences in the EOD waveform features studied.

Conclusions

In this study, we identified plausible candidate genes that may contribute to phenotypic differences in EOD waveforms among a rapidly diverged group of mormyrid electric fish. These genes may be important targets of selection in the evolution of species-specific differences in mate-recognition signals.

Neurogranin is expressed in mammalian skeletal muscle and inhibits calcineurin signaling and myoblast fusion

Calcineurin is a Ca²⁺/calmodulin (CaM)-dependent phosphatase that plays a critical role in promoting the slow fiber phenotype and myoblast fusion in skeletal muscle, thereby making calcineurin an attractive cellular target for enhancing fatigue resistance, muscle metabolism, and muscle repair. Neurogranin (Ng) is a CaM-binding protein thought to be expressed solely in brain and neurons, where it inhibits calcineurin signaling by sequestering CaM, thus lowering its cellular availability. Here, we demonstrate for the first time the expression of Ng protein and mRNA in mammalian skeletal muscle. Both protein and mRNA levels are greater in slow-oxidative compared with fast-glycolytic muscles. Coimmunoprecipitation of CaM with Ng in homogenates of C2C12 myotubes, mouse soleus, and human vastus lateralis suggests that these proteins physically interact. To determine whether Ng inhibits calcineurin signaling in muscle, we used Ng siRNA with C2C12 myotubes to reduce Ng protein levels by 60%. As a result of reduced Ng expression, C2C12 myotubes had enhanced CaM-calcineurin binding and calcineurin signaling as indicated by reduced phosphorylation of nuclear factor of activated T cells and increased utrophin mRNA. In addition, calcineurin signaling affects the expression of myogenin and stabilin-2, which are involved in myogenic differentiation and myoblast fusion, respectively. Here, we found that both myogenin and stabilin-2 were significantly elevated by Ng siRNA in C2C12 cells, concomitantly with an increased fusion index. Taken together, these results demonstrate the expression of Ng in mammalian skeletal muscle where it appears to be a novel regulator of calcineurin signaling.

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.

The Genome and Adult Somatic Transcriptome of the Mormyrid Electric Fish Paramormyrops kingsleyae

Several studies have begun to elucidate the genetic and developmental processes underlying major vertebrate traits. Few of these traits have evolved repeatedly in vertebrates, preventing the analysis of molecular mechanisms underlying these traits comparatively. Electric organs have evolved multiple times among vertebrates, presenting a unique opportunity to understand the degree of constraint and repeatability of the evolutionary processes underlying novel vertebrate traits. As there is now a completed genome sequence representing South American electric eels, we were motivated to obtain genomic sequence from a linage that independently evolved electric organs to facilitate future comparative analyses of the evolution and development of electric organs. We report here the sequencing and de novo assembly of the genome of the mormyrid Paramormyrops kingsleyae using short-read sequencing. In addition, we have completed a somatic transcriptome from 11 tissues to construct a gene expression atlas of predicted genes from this assembly, enabling us to identify candidate housekeeping genes as well as genes differentially expressed in the major somatic tissues of the mormyrid electric fish. We anticipate that this resource will greatly facilitate comparative studies on the evolution and development of electric organs and electroreceptors.

Electric fish genomics: Progress, prospects, and new tools for neuroethology

Electric fish have served as a model system in biology since the 18th century, providing deep insight into the nature of bioelectrogenesis, the molecular structure of the synapse, and brain circuitry underlying complex behavior. Neuroethologists have collected extensive phenotypic data that span biological levels of analysis from molecules to ecosystems. This phenotypic data, together with genomic resources obtained over the past decades, have motivated new and exciting hypotheses that position the weakly electric fish model to address fundamental 21^st century biological questions. This review article considers the molecular data collected for weakly electric fish over the past three decades, and the insights that data of this nature has motivated. For readers relatively new to molecular genetics techniques, we also provide a table of terminology aimed at clarifying the numerous acronyms and techniques that accompany this field. Next, we pose a research agenda for expanding genomic resources for electric fish research over the next 10years. We conclude by considering some of the exciting research prospects for neuroethology that electric fish genomics may offer over the coming decades, if the electric fish community is successful in these endeavors.

Cross-tissue and cross-species analysis of gene expression in skeletal muscle and electric organ of African weakly-electric fish (Teleostei; Mormyridae)

BACKGROUND

African weakly-electric fishes of the family Mormyridae are able to produce and perceive weak electric signals (typically less than one volt in amplitude) owing to the presence of a specialized, muscle-derived electric organ (EO) in their tail region. Such electric signals, also known as Electric Organ Discharges (EODs), are used for objects/prey localization, for the identification of conspecifics, and in social and reproductive behaviour. This feature might have promoted the adaptive radiation of this family by acting as an effective pre-zygotic isolation mechanism. Despite the physiological and evolutionary importance of this trait, the investigation of the genetic basis of its function and modification has so far remained limited. In this study, we aim at: i) identifying constitutive differences in terms of gene expression between electric organ and skeletal muscle (SM) in two mormyrid species of the genus Campylomormyrus: C. compressirostris and C. tshokwe, and ii) exploring cross-specific patterns of gene expression within the two tissues among C. compressirostris, C. tshokwe, and the outgroup species Gnathonemus petersii.

RESULTS

Twelve paired-end (100 bp) strand-specific RNA-seq Illumina libraries were sequenced, producing circa 330 M quality-filtered short read pairs. The obtained reads were assembled de novo into four reference transcriptomes. In silico cross-tissue DE-analysis allowed us to identify 271 shared differentially expressed genes between EO and SM in C. compressirostris and C.tshokwe. Many of these genes correspond to myogenic factors, ion channels and pumps, and genes involved in several metabolic pathways. Cross-species analysis has revealed that the electric organ transcriptome is more variable in terms of gene expression levels across species than the skeletal muscle transcriptome.

CONCLUSIONS

The data obtained indicate that: i) the loss of contractile activity and the decoupling of the excitation-contraction processes are reflected by the down-regulation of the corresponding genes in the electric organ's transcriptome; ii) the metabolic activity of the EO might be specialized towards the production and turn-over of membrane structures; iii) several ion channels are highly expressed in the EO in order to increase excitability; iv) several myogenic factors might be down-regulated by transcription repressors in the EO.

Unique patterns of transcript and miRNA expression in the South American strong voltage electric eel (Electrophorus electricus)

Background

With its unique ability to produce high-voltage electric discharges in excess of 600 volts, the South American strong voltage electric eel (Electrophorus electricus) has played an important role in the history of science. Remarkably little is understood about the molecular nature of its electric organs.

Results

We present an in-depth analysis of the genome of E. electricus, including the transcriptomes of eight mature tissues: brain, spinal cord, kidney, heart, skeletal muscle, Sachs’ electric organ, main electric organ, and Hunter’s electric organ. A gene set enrichment analysis based on gene ontology reveals enriched functions in all three electric organs related to transmembrane transport, androgen binding, and signaling. This study also represents the first analysis of miRNA in electric fish. It identified a number of miRNAs displaying electric organ-specific expression patterns, including one novel miRNA highly over-expressed in all three electric organs of E. electricus. All three electric organ tissues also express three conserved miRNAs that have been reported to inhibit muscle development in mammals, suggesting that miRNA-dependent regulation of gene expression might play an important role in specifying an electric organ identity from its muscle precursor. These miRNA data were supported using another complete miRNA profile from muscle and electric organ tissues of a second gymnotiform species.

Conclusions

Our work on the E. electricus genome and eight tissue-specific gene expression profiles will greatly facilitate future research on determining the coding and regulatory sequences that specify the function, development, and evolution of electric organs. Moreover, these data and future studies will be informed by the first comprehensive analysis of miRNA expression in an electric fish presented here.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1288-8) contains supplementary material, which is available to authorized users.

Na+/K+-ATPase α-subunit (nkaα) isoforms and their mRNA expression levels, overall Nkaα protein abundance, and kinetic properties of Nka in the skeletal muscle and three electric organs of the electric eel, Electrophorus electricus

This study aimed to obtain the coding cDNA sequences of Na⁺/K⁺-ATPase α (nkaα) isoforms from, and to quantify their mRNA expression in, the skeletal muscle (SM), the main electric organ (EO), the Hunter’s EO and the Sach’s EO of the electric eel, Electrophorus electricus. Four nkaα isoforms (nkaα1c1, nkaα1c2, nkaα2 and nkaα3) were obtained from the SM and the EOs of E. electricus. Based on mRNA expression levels, the major nkaα expressed in the SM and the three EOs of juvenile and adult E. electricus were nkaα1c1 and nkaα2, respectively. Molecular characterization of the deduced Nkaα1c1 and Nkaα2 sequences indicates that they probably have different affinities to Na⁺ and K⁺. Western blotting demonstrated that the protein abundance of Nkaα was barely detectable in the SM, but strongly detected in the main and Hunter’s EOs and weakly in the Sach’s EO of juvenile and adult E. electricus. These results corroborate the fact that the main EO and Hunter’s EO have high densities of Na⁺ channels and produce high voltage discharges while the Sach’s EO produces low voltage discharges. More importantly, there were significant differences in kinetic properties of Nka among the three EOs of juvenile E. electricus. The highest and lowest V_max of Nka were detected in the main EO and the Sach’s EO, respectively, with the Hunter’s EO having a V_max value intermediate between the two, indicating that the metabolic costs of EO discharge could be the highest in the main EO. Furthermore, the Nka from the main EO had the lowest K_m (or highest affinity) for Na⁺ and K⁺ among the three EOs, suggesting that the Nka of the main EO was more effective than those of the other two EOs in maintaining intracellular Na⁺ and K⁺ homeostasis and in clearing extracellular K⁺ after EO discharge.

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.

Nonhuman genetics. Genomic basis for the convergent evolution of electric organs

Little is known about the genetic basis of convergent traits that originate repeatedly over broad taxonomic scales. The myogenic electric organ has evolved six times in fishes to produce electric fields used in communication, navigation, predation, or defense. We have examined the genomic basis of the convergent anatomical and physiological origins of these organs by assembling the genome of the electric eel (Electrophorus electricus) and sequencing electric organ and skeletal muscle transcriptomes from three lineages that have independently evolved electric organs. Our results indicate that, despite millions of years of evolution and large differences in the morphology of electric organ cells, independent lineages have leveraged similar transcription factors and developmental and cellular pathways in the evolution of electric organs.

De novo assembly and characterization of the skeletal muscle and electric organ transcriptomes of the African weakly electric fish Campylomormyrus compressirostris (Mormyridae, Teleostei)

African weakly electric fishes (Mormyridae) underwent an outstanding adaptive radiation (about 200 species), putatively owing to their ability to communicate through species-specific weak electric signals. The electric organ discharge (EOD) is produced by muscle-derived electrocytes organized in piles to form an electric organ. Despite the importance of this trait as a prezygotic isolation mechanism, genomic resources remained limited. We present here a first draft of the skeletal muscle and electric organ transcriptomes from the weakly electric fish species Campylomormyrus compressirostris, obtained using the Illumina HiSeq2000 sequencing technology. Approximately 6.8 Gbp of cDNA sequence data were produced from both tissues, resulting in 57,268,109 raw reads for the skeletal muscle and 46,934,923 for the electric organ, and assembled de novo into 46,143 and 89,270 contigs, respectively. About 50% of both transcriptomes were annotated after protein databases search. The two transcriptomes show similar profiles in terms of Gene Ontology categories composition. We identified several candidate genes which are likely to play a central role in the production and evolution of the electric signal. For most of these genes, and for many other housekeeping genes, we were able to obtain the complete or partial coding DNA sequences (CDS), which can be used for the development of primers to be utilized in qRT-PCR experiments. We present also the complete mitochondrial genome and compare it to those available from other weakly electric fish species. Additionally, we located 1671 SSR-containing regions with their flanking sites and designed the relative primers. This study establishes a first step in the development of genomic tools aimed at understanding the role of electric communication during speciation.

Electrocyte physiology: 50 years later

Weakly electric gymnotiform and mormyrid fish generate and detect weak electric fields to image their worlds and communicate. These multi-purpose electric signals are generated by electrocytes, the specialized electric organ (EO) cells that produce the electric organ discharge (EOD). Just over 50 years ago the first experimental analyses of electrocyte physiology demonstrated that the EOD is produced and shaped by the timing and waveform of electrocyte action potentials (APs). Electrocytes of some species generate a single AP from a distinct region of excitable membrane, and this AP waveform determines EOD waveform. In other species, electrocytes possess two independent regions of excitable membrane that generate asynchronous APs with different waveforms, thereby increasing EOD complexity. Signal complexity is further enhanced in some gymnotiforms by the spatio-temporal activation of distinct EO regions with different electrocyte properties. For many mormyrids, additional EOD waveform components are produced by APs that propagate along stalks that connect postsynaptic regions to the main body of the electrocyte. I review here the history of research on electrocyte physiology in weakly electric fish, as well as recent discoveries of key phenomena not anticipated during early work in this field. Recent areas of investigation include the regulation of electrocyte activity by steroid and peptide hormones, the molecular evolution of electrocyte ion channels, and the evolutionary selection of ion channels expressed in excitable cells. These emerging research areas have generated renewed interest in electrocyte function and clear future directions for research addressing a broad range of new and important questions.

From sequence to spike to spark: evo-devo-neuroethology of electric communication in mormyrid fishes

Mormyrid fishes communicate using pulses of electricity, conveying information about their identity, behavioral state, and location. They have long been used as neuroethological model systems because they are uniquely suited to identifying cellular mechanisms for behavior. They are also remarkably diverse, and they have recently emerged as a model system for studying how communication systems may influence the process of speciation. These two lines of inquiry have now converged, generating insights into the neural basis of evolutionary change in behavior, as well as the influence of sensory and motor systems on behavioral diversification and speciation. Here, we review the mechanisms of electric signal generation, reception, and analysis and relate these to our current understanding of the evolution and development of electromotor and electrosensory systems. We highlight the enormous potential of mormyrids for studying evolutionary developmental mechanisms of behavioral diversification, and make the case for developing genomic and transcriptomic resources. A complete mormyrid genome sequence would enable studies that extend our understanding of mormyrid behavior to the molecular level by linking morphological and physiological mechanisms to their genetic basis. Applied in a comparative framework, genomic resources would facilitate analysis of evolutionary processes underlying mormyrid diversification, reveal the genetic basis of species differences in behavior, and illuminate the origins of a novel vertebrate sensory and motor system. Genomic approaches to studying the evo-devo-neuroethology of mormyrid communication represent a deeply integrative approach to understanding the evolution, function, development, and mechanisms of behavior.