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

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.

Convergent gene losses and pseudogenizations in multiple lineages of stomachless fishes

The regressive evolution of independent lineages often results in convergent phenotypes. Several teleost groups display secondary loss of the stomach, and four gastric genes, atp4a, atp4b, pgc, and pga2 have been co-deleted in agastric (stomachless) fish. Analyses of genotypic convergence among agastric fishes showed that four genes, slc26a9, kcne2, cldn18a, and vsig1, were co-deleted or pseudogenized in most agastric fishes of the four major groups. kcne2 and vsig1 were also deleted or pseudogenized in the agastric monotreme echidna and platypus, respectively. In the stomachs of sticklebacks, these genes are expressed in gastric gland cells or surface epithelial cells. An ohnolog of cldn18 was retained in some agastric teleosts but exhibited an increased non-synonymous substitution when compared with gastric species. These results revealed novel convergent gene losses at multiple loci among the four major groups of agastric fish, as well as a single gene loss in the echidna and platypus.

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.

A new genome assembly of an African weakly electric fish (Campylomormyrus compressirostris, Mormyridae) indicates rapid gene family evolution in Osteoglossomorpha

BACKGROUND

Teleost fishes comprise more than half of the vertebrate species. Within teleosts, most phylogenies consider the split between Osteoglossomorpha and Euteleosteomorpha/Otomorpha as basal, preceded only by the derivation of the most primitive group of teleosts, the Elopomorpha. While Osteoglossomorpha are generally species poor, the taxon contains the African weakly electric fish (Mormyroidei), which have radiated into numerous species. Within the mormyrids, the genus Campylomormyrus is mostly endemic to the Congo Basin. Campylomormyrus serves as a model to understand mechanisms of adaptive radiation and ecological speciation, especially with regard to its highly diverse species-specific electric organ discharges (EOD). Currently, there are few well-annotated genomes available for electric fish in general and mormyrids in particular. Our study aims at producing a high-quality genome assembly and to use this to examine genome evolution in relation to other teleosts. This will facilitate further understanding of the evolution of the osteoglossomorpha fish in general and of electric fish in particular.

RESULTS

A high-quality weakly electric fish (C. compressirostris) genome was produced from a single individual with a genome size of 862 Mb, consisting of 1,497 contigs with an N50 of 1,399 kb and a GC-content of 43.69%. Gene predictions identified 34,492 protein-coding genes, which is a higher number than in the two other available Osteoglossomorpha genomes of Paramormyrops kingsleyae and Scleropages formosus. A Computational Analysis of gene Family Evolution (CAFE5) comparing 33 teleost fish genomes suggests an overall faster gene family turnover rate in Osteoglossomorpha than in Otomorpha and Euteleosteomorpha. Moreover, the ratios of expanded/contracted gene family numbers in Osteoglossomorpha are significantly higher than in the other two taxa, except for species that had undergone an additional genome duplication (Cyprinus carpio and Oncorhynchus mykiss). As potassium channel proteins are hypothesized to play a key role in EOD diversity among species, we put a special focus on them, and manually curated 16 Kv1 genes. We identified a tandem duplication in the KCNA7a gene in the genome of C. compressirostris.

CONCLUSIONS

We present the fourth genome of an electric fish and the third well-annotated genome for Osteoglossomorpha, enabling us to compare gene family evolution among major teleost lineages. Osteoglossomorpha appear to exhibit rapid gene family evolution, with more gene family expansions than contractions. The curated Kv1 gene family showed seven gene clusters, which is more than in other analyzed fish genomes outside Osteoglossomorpha. The KCNA7a, encoding for a potassium channel central for EOD production and modulation, is tandemly duplicated which may related to the diverse EOD observed among Campylomormyrus species.

Molecular evolution of the ependymin-related gene epdl2 in African weakly electric fish

Gene duplication and subsequent molecular evolution can give rise to taxon-specific gene specializations. In previous work, we found evidence that African weakly electric fish (Mormyridae) may have as many as three copies of the epdl2 gene, and the expression of two epdl2 genes is correlated with electric signal divergence. Epdl2 belongs to the ependymin-related family (EPDR), a functionally diverse family of secretory glycoproteins. In this study, we first describe vertebrate EPDR evolution and then present a detailed evolutionary history of epdl2 in Mormyridae with emphasis on the speciose genus Paramormyrops. Using Sanger sequencing, we confirm three apparently functional epdl2 genes in Paramormyrops kingsleyae. Next, we developed a nanopore-based amplicon sequencing strategy and bioinformatics pipeline to obtain and classify full-length epdl2 gene sequences (N = 34) across Mormyridae. Our phylogenetic analysis proposes three or four epdl2 paralogs dating from early Paramormyrops evolution. Finally, we conducted selection tests which detected positive selection around the duplication events and identified ten sites likely targeted by selection in the resulting paralogs. These sites' locations in our modeled 3D protein structure involve four sites in ligand binding and six sites in homodimer formation. Together, these findings strongly imply an evolutionary mechanism whereby epdl2 genes underwent selection-driven functional specialization after tandem duplications in the rapidly speciating Paramormyrops. Considering previous evidence, we propose that epdl2 may contribute to electric signal diversification in mormyrids, an important aspect of species recognition during mating.

Integrated biochemical, transcriptomic and metabolomic analyses provide insight into heat stress response in Yangtze sturgeon (Acipenser dabryanus)

Temperature fluctuations caused by climate change and global warming pose a great threat to various species. Most fish are particularly vulnerable to elevated temperatures. Understanding the mechanism of high-temperature tolerance in fish can be beneficial for proposing effective strategies to help fish cope with global warming. In this study, we systematically studied the effects of high temperature on Acipenser dabryanus, an ancient living fossil and flagship species of the Yangtze River, at the histological, biochemical, transcriptomic and metabolomic levels. Intestinal and liver tissues from the control groups (18 °C) and acute heat stress groups (30 °C) of A. dabryanus were sampled for histological observation and liver tissues were assessed for transcriptomic and metabolomic profiling. Histopathological analysis showed that the intestine and liver tissues were damaged after heat stress. The plasma cortisol content and the levels of oxidative stress markers (catalase/glutathione reductase) and two aminotransferases (aspartate aminotransferase/alanine aminotransferase) increased significantly in response to acute heat stress. Transcriptomic and metabolomic methods showed 6707 upregulated and 4189 downregulated genes and 64 upregulated and 78 downregulated metabolites in the heat stress group. Heat shock protein (HSP) genes showed striking changes in expression under heat stress, with 21 genes belonging to the HSP30, HSP40, HSP60, HSP70 and HSP90 families significantly upregulated by short-term heat stress. The majority of genes associated with ubiquitin and various immune-related pathways were also markedly upregulated in the heat stress group. In addition, the combined analysis of metabolites and gene profiles suggested an enhancement of amino acid metabolism and glycometabolism and the suppression of fatty acid metabolism during heat stress, which could be a potential energy conservation strategy for A. dabryanus. To the best of our knowledge, the present study represents the first attempt to reveal the mechanisms of heat stress responses in A. dabryanus, which can provide insights into improved cultivation of fish in response to global warming.

Comparative study of the evolution of cancer gene duplications across fish

Comparative studies of cancer‐related genes not only provide novel information about their evolution and function but also an understanding of cancer as a driving force in biological systems and species’ life histories. So far, these studies have focused on mammals. Here, we provide the first comparative study of cancer‐related gene copy number variation in fish. Fishes are a paraphyletic group whose last common ancestor is also an ancestor of the tetrapods, and accordingly, their tumour suppression mechanisms should include most of the mammalian mechanisms and also reveal novel (but potentially phylogenetically older) previously undetected mechanisms. We have matched the sequenced genomes of 65 fish species from the Ensemble database with the cancer gene information from the COSMIC database. By calculating the number of gene copies across species using the Ensembl CAFE data (providing species trees for gene copy number counts), we used a less resource‐demanding method for homolog identification. Our analysis demonstrates a masked relationship between cancer‐related gene copy number variation (CNV) and maximum lifespan in fish species, suggesting that a higher number of copies of tumour suppressor genes lengthens and the number of copies of oncogenes shortens lifespan. Based on the positive correlation between the number of copies of tumour suppressors and oncogenes, we show which species have more tumour suppressors in relation to oncogenes. It could be suggested that these species have stronger genetic defences against oncogenic processes. Fish studies could be a largely unexplored treasure trove for understanding the evolution and ecology of cancer, providing novel insights into the study of cancer and tumour suppression, in addition to fish evolution, life‐history trade‐offs, and ecology.

Evolution of the nitric oxide synthase family in vertebrates and novel insights in gill development

Nitric oxide (NO) is an ancestral key signalling molecule essential for life and has enormous versatility in biological systems, including cardiovascular homeostasis, neurotransmission and immunity. Although our knowledge of NO synthases (Nos), the enzymes that synthesize NO in vivo, is substantial, the origin of a large and diversified repertoire of nos gene orthologues in fishes with respect to tetrapods remains a puzzle. The recent identification of nos3 in the ray-finned fish spotted gar, which was considered lost in this lineage, changed this perspective. This finding prompted us to explore nos gene evolution, surveying vertebrate species representing key evolutionary nodes. This study provides noteworthy findings: first, nos2 experienced several lineage-specific gene duplications and losses. Second, nos3 was found to be lost independently in two different teleost lineages, Elopomorpha and Clupeocephala. Third, the expression of at least one nos paralogue in the gills of developing shark, bichir, sturgeon, and gar, but not in lamprey, suggests that nos expression in this organ may have arisen in the last common ancestor of gnathostomes. These results provide a framework for continuing research on nos genes' roles, highlighting subfunctionalization and reciprocal loss of function that occurred in different lineages during vertebrate genome duplications.

Phylogenomics of bonytongue fishes (Osteoglossomorpha) shed light on the craniofacial evolution and biogeography of the weakly electric clade Mormyridae

Bonytongues (Osteoglossomorpha) constitute an ancient clade of teleost fishes distributed in freshwater habitats throughout the world. The group includes well-known species such as arowanas, featherbacks, pirarucus, and the weakly electric fishes in the family Mormyridae. Their disjunct distribution, extreme morphologies, and electrolocating capabilities (Gymnarchidae and Mormyridae) have attracted much scientific interest, but a comprehensive phylogenetic framework for comparative analysis is missing, especially for the species-rich family Mormyridae. Of particular interest are disparate craniofacial morphologies among mormyrids which might constitute an exceptional model system to study convergent evolution. We present a phylogenomic analysis based on 546 exons of 179 species (out of 260), 28 out of 29 genera, and all six families of extant bonytongues. Based on a recent reassessment of the fossil record of osteoglossomorphs, we inferred dates of divergence among trans-continental clades and the major groups. The estimated ages of divergence among extant taxa (e.g., Osteoglossomorpha, Osteoglossiformes, Mormyroidea) are older than previous reports, but most of the divergence dates obtained for clades on separate continents are too young to be explained by simple vicariance hypotheses. Biogeographic analysis of mormyrids indicates that their high species diversity in the Congo Basin is a consequence of range reductions of previously widespread ancestors and that the highest diversity of craniofacial morphologies among mormyrids originated in this basin. Special emphasis on a taxon-rich representation for mormyrids revealed pervasive misalignment between our phylogenomic results and mormyrid taxonomy due to repeated instances of convergence for extreme craniofacial morphologies. Estimation of ancestral phenotypes revealed contingent evolution of snout elongation and unique projections from the lower jaw to form the distinctive Schnauzenorgan. Synthesis of comparative analyses suggests that the remarkable craniofacial morphologies of mormyrids evolved convergently due to niche partitioning, likely enabled by interactions between their exclusive morphological and electrosensory adaptations.

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.

Sperm competition, sexual selection and the diverse reproductive biology of Osteoglossiformes

Osteoglossiformes are an order of "bony tongue" fish considered the most primitive living order of teleosts. This review seeks to consolidate known hypotheses and identify gaps in the literature regarding the adaptive significance of diverse reproductive traits and behaviour of osteoglossiforms within the context of sperm competition and the wider lens of sexual selection. Many of the unusual traits observed in osteoglossiforms indicate low levels of sperm competition; most species have unpaired gonads, and mormyroids are the only known vertebrate species with aflagellate sperm. Several osteoglossiform families have reproductive anatomy associated with internal fertilization but perform external fertilization, which may be representative of the evolutionary transition from external to internal fertilization and putative trade-offs between sperm competition and the environment. They also employ every type of parental care seen in vertebrates. Geographically widespread and basally situated within teleosts, osteoglossiforms present an effective study system for understanding how sperm competition and sexual selection have shaped the evolution of teleost reproductive behaviour, sperm and gonad morphology, fertilization strategies, courtship and paternal care, and sexual conflict. The authors suggest that the patterns seen in osteoglossiform reproduction are a microcosm of teleost reproductive diversity, potentially signifying the genetic plasticity that contributed to the adaptive radiation of teleost fishes.

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.

African Arowana Genome Provides Insights on Ancient Teleost Evolution

Osteoglossiformes is a basal clade of teleost, evolving since the Jurassic period. The genomes of Osteoglossiformes species would shed light on the evolution and adaptation of teleost. Here, we established a chromosome-level genome of African arowana. Together with the genomes of pirarucu and Asian arowana, we found that they diverged at ∼106.1 million years ago (MYA) and ∼59.2 MYA, respectively, which are coincident with continental separation. Interestingly, we identified a dynamic genome evolution characterized by a fast evolutionary rate and a high pseudogenization rate in African arowana and pirarucu. Additionally, more transposable elements were found in Asian arowana which confer more gene duplications. Moreover, we found the contraction of olfactory receptor and the expansion of UGT in African arowana might be related to its transformation from carnivore to be omnivore. Taken together, we provided valuable genomic resource of Osteoglossidae and revealed the correlation of biogeography and teleost evolution.

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An evolutionary model of Osteoglossidae along the continental drift is provided

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A faster evolving rate of African arowana than Asian arowana is revealed

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The gene duplications of Asian arowana are related to more class I TE insertions

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A mechanism of African arowana’s feeding habits transition is proposed.

Brain transcriptomics of agonistic behaviour in the weakly electric fish Gymnotus omarorum, a wild teleost model of non-breeding aggression

Differences in social status are often mediated by agonistic encounters between competitors. Robust literature has examined social status-dependent brain gene expression profiles across vertebrates, yet social status and reproductive state are often confounded. It has therefore been challenging to identify the neuromolecular mechanisms underlying social status independent of reproductive state. Weakly electric fish, Gymnotus omarorum, display territorial aggression and social dominance independent of reproductive state. We use wild-derived G. omarorum males to conduct a transcriptomic analysis of non-breeding social dominance relationships. After allowing paired rivals to establish a dominance hierarchy, we profiled the transcriptomes of brain sections containing the preoptic area (region involved in regulating aggressive behaviour) in dominant and subordinate individuals. We identified 16 differentially expressed genes (FDR < 0.05) and numerous genes that co-varied with behavioural traits. We also compared our results with previous reports of differential gene expression in other teleost species. Overall, our study establishes G. omarorum as a powerful model system for understanding the neuromolecular bases of social status independent of reproductive state.

The Polycomb Orthologues in Teleost Fishes and Their Expression in the Zebrafish Model

The Polycomb Repressive Complex 1 (PRC1) is a chromatin-associated protein complex involved in transcriptional repression of hundreds of genes controlling development and differentiation processes, but also involved in cancer and stem cell biology. Within the canonical PRC1, members of Pc/CBX protein family are responsible for the targeting of the complex to specific gene loci. In mammals, the Pc/CBX protein family is composed of five members generating, through mutual exclusion, different PRC1 complexes with potentially distinct cellular functions. Here, we performed a global analysis of the cbx gene family in 68 teleost species and traced the distribution of the cbx genes through teleost evolution in six fish super-orders. We showed that after the teleost-specific whole genome duplication, cbx4, cbx7 and cbx8 are retained as pairs of ohnologues. In contrast, cbx2 and cbx6 are present as pairs of ohnologues in the genome of several teleost clades but as singletons in others. Furthermore, since zebrafish is a widely used vertebrate model for studying development, we report on the expression of the cbx family members during zebrafish development and in adult tissues. We showed that all cbx genes are ubiquitously expressed with some variations during early development.

Studying convergent evolution to relate genotype to behavioral phenotype

Neuroscience has a long, rich history in embracing unusual animals for research. Over the past several decades, there has been a technology-driven bottleneck in the species used for neuroscience research. However, an oncoming wave of technologies applicable to many animals hold promise for enabling researchers to address challenging scientific questions that cannot be solved using traditional laboratory animals. Here, we discuss how leveraging the convergent evolution of physiological or behavioral phenotypes can empower research mapping genotype to phenotype interactions. We present two case studies using electric fish and poison frogs and discuss how comparative work can teach us about evolutionary constraint and flexibility at various levels of biological organization. We also offer advice on the potential and pitfalls of establishing novel model systems in neuroscience research. Finally, we end with a discussion on the use of charismatic animals in neuroscience research and their utility in public outreach. Overall, we argue that convergent evolution frameworks can help identify generalizable principles of neuroscience.

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.

Integrating natural history collections and comparative genomics to study the genetic architecture of convergent evolution

Evolutionary convergence has been long considered primary evidence of adaptation driven by natural selection and provides opportunities to explore evolutionary repeatability and predictability. In recent years, there has been increased interest in exploring the genetic mechanisms underlying convergent evolution, in part, owing to the advent of genomic techniques. However, the current 'genomics gold rush' in studies of convergence has overshadowed the reality that most trait classifications are quite broadly defined, resulting in incomplete or potentially biased interpretations of results. Genomic studies of convergence would be greatly improved by integrating deep 'vertical', natural history knowledge with 'horizontal' knowledge focusing on the breadth of taxonomic diversity. Natural history collections have and continue to be best positioned for increasing our comprehensive understanding of phenotypic diversity, with modern practices of digitization and databasing of morphological traits providing exciting improvements in our ability to evaluate the degree of morphological convergence. Combining more detailed phenotypic data with the well-established field of genomics will enable scientists to make progress on an important goal in biology: to understand the degree to which genetic or molecular convergence is associated with phenotypic convergence. Although the fields of comparative biology or comparative genomics alone can separately reveal important insights into convergent evolution, here we suggest that the synergistic and complementary roles of natural history collection-derived phenomic data and comparative genomics methods can be particularly powerful in together elucidating the genomic basis of convergent evolution among higher taxa. This article is part of the theme issue 'Convergent evolution in the genomics era: new insights and directions'.

The Cone Opsin Repertoire of Osteoglossomorph Fishes: Gene Loss in Mormyrid Electric Fish and a Long Wavelength-Sensitive Cone Opsin That Survived 3R

Vertebrates have four classes of cone opsin genes derived from two rounds of genome duplication. These are short wavelength sensitive 1(SWS1), short wavelength sensitive 2(SWS2), medium wavelength sensitive (RH2), and long wavelength sensitive (LWS). Teleosts had another genome duplication at their origin and it is believed that only one of each cone opsin survived the ancestral teleost duplication event. We tested this by examining the retinal cones of a basal teleost group, the osteoglossomorphs. Surprisingly, this lineage has lost the typical vertebrate green-sensitive RH2 opsin gene and, instead, has a duplicate of the LWS opsin that is green sensitive. This parallels the situation in mammalian evolution in which the RH2 opsin gene was lost in basal mammals and a green-sensitive opsin re-evolved in Old World, and independently in some New World, primates from an LWS opsin gene. Another group of fish, the characins, possess green-sensitive LWS cones. Phylogenetic analysis shows that the evolution of green-sensitive LWS opsins in these two teleost groups derives from a common ancestral LWS opsin that acquired green sensitivity. Additionally, the nocturnally active African weakly electric fish (Mormyroideae), which are osteoglossomorphs, show a loss of the SWS1 opsin gene. In comparison with the independently evolved nocturnally active South American weakly electric fish (Gymnotiformes) with a functionally monochromatic LWS opsin cone retina, the presence of SWS2, LWS, and LWS2 cone opsins in mormyrids suggests the possibility of color vision.

A review of the systematic biology of fossil and living bony-tongue fishes, Osteoglossomorpha (Actinopterygii: Teleostei)

ABSTRACT The bony-tongue fishes, Osteoglossomorpha, have been the focus of a great deal of morphological, systematic, and evolutionary study, due in part to their basal position among extant teleostean fishes. This group includes the mooneyes (Hiodontidae), knifefishes (Notopteridae), the abu (Gymnarchidae), elephantfishes (Mormyridae), arawanas and pirarucu (Osteoglossidae), and the African butterfly fish (Pantodontidae). This morphologically heterogeneous group also has a long and diverse fossil record, including taxa from all continents and both freshwater and marine deposits. The phylogenetic relationships among most extant osteoglossomorph families are widely agreed upon. However, there is still much to discover about the systematic biology of these fishes, particularly with regard to the phylogenetic affinities of several fossil taxa, within Mormyridae, and the position of Pantodon. In this paper we review the state of knowledge for osteoglossomorph fishes. We first provide an overview of the diversity of Osteoglossomorpha, and then discuss studies of the phylogeny of Osteoglossomorpha from both morphological and molecular perspectives, as well as biogeographic analyses of the group. Finally, we offer our perspectives on future needs for research on the systematic biology of Osteoglossomorpha.