Fish Lineages Research Articles

MiR-206 is required for changes in cell adhesion that drive muscle cell morphogenesis in Xenopus laevis

MicroRNAs (miRNAs) are highly conserved small non-coding RNA molecules that post-transcriptionally regulate gene expression in multicellular organisms. Within the set of muscle-specific miRNAs, miR-206 expression is largely restricted to skeletal muscle and is found exclusively within the bony fish lineage. Although many studies have implicated miR-206 in muscle maintenance and disease, its role in skeletal muscle development remains largely unknown. Here, we examine the role of miR-206 during Xenopus laevis somitogenesis. In Xenopus laevis, miR-206 expression coincides with the onset of somitogenesis. We show that both knockdown and over-expression of miR-206 result in abnormal somite formation affecting muscle cell rotation, attachment, and elongation. In particular, our data suggests that miR-206 regulates changes in cell adhesion that affect the ability of newly formed somites to adhere to the notochord as well as to the intersomitic boundaries. Additionally, we show that β-dystroglycan and F-actin expression levels are significantly reduced, suggesting that knockdown of miR-206 levels affects cellular mechanics necessary for cell shape changes and attachments that are required for proper muscle formation.

Body size evolution and habitat colonization across 100 million years (Late Jurassic–Paleocene) of the actinopterygian evolutionary history

AbstractFishes are characterized by their capacity to occupy all aquatic environments and by their amazing range of size and morphology. While it is known that habitat influenced the diversity dynamics of fish clades, studies on environmental colonization events through the evolutionary history of ray‐finned fishes have yielded conflicting results as to the origin of modern clades and preferential directions of shifts. The effects of habitat over morphological evolution such as body size remain poorly known in vertebrates. However, body size evolution is more frequently addressed in terms of variation through time and numerous studies have demonstrated that successive taxa within a clade tend to increase in size through time (Cope's or Depéret's rule). We use phylogenetic comparative methods on a genus‐level actinopterygian super‐tree based on extant and fossil data covering the Late Jurassic‐Paleogene interval. Results indicate marine ancestry for freshwater lineages and a dominance of colonizations from marine clades towards other habitats. Similar trends in environment occupancy among different ray‐finned clades are explored. Three main trends affecting non‐closely‐related clades are recognized: (i) the freshwater invaders, (ii) the predominantly marine dwellers and (iii) the environmentally labile fishes. Habitat effects on body size evolution are not statistically supported, but most actinopterygian subclades originate from small‐sized ancestors and tend to increase in size in the course of their evolutionary history. This trend is clear for lineages restricted for long periods of time in the same environments, either marine or freshwater, but it is not observed in environmentally labile fish lineages.

Multiple origins and strong phenotypic convergence in fish-cleaning palaemonid shrimp lineages

Several species of palaemonid shrimps are known to act as fish-cleaning symbionts, with cleaning interactions ranging from dedicated (obligate) to facultative. We confirmed five evolutionarily independent origins of fish cleaning symbioses within the family Palaemonidae based on a phylogenetic analysis and the ancestral state reconstruction of 68 species, including 13 fish-cleaners from the genera Ancylomenes, Brachycarpus, Palaemon, Periclimenes, and Urocaridella. We focus in particular on two distantly related lineages of fish cleaning shrimps with allopatric distributions, the Indo-West Pacific Ancylomenes and the western Atlantic monophyletic Ancylomenes/Periclimenes group, which exhibit striking similarities in morphology, colouration and complex behaviour. Specifically, representatives of both lineages are similar in: (1) the general body shape and colour pattern; (2) the utilization of sea anemones as conspicuous cleaning stations; and (3) the use of sideways body swaying to visually promote their bright colour spots in order to attract fish clients. Such morphological, ecological and ethological convergences are apparently due to adaptations to fish cleaning linked to the establishment of similar modes of communication with fish clients in these species.

Vertebrate Genome Evolution in the Light of Fish Cytogenomics and rDNAomics.

To understand the cytogenomic evolution of vertebrates, we must first unravel the complex genomes of fishes, which were the first vertebrates to evolve and were ancestors to all other vertebrates. We must not forget the immense time span during which the fish genomes had to evolve. Fish cytogenomics is endowed with unique features which offer irreplaceable insights into the evolution of the vertebrate genome. Due to the general DNA base compositional homogeneity of fish genomes, fish cytogenomics is largely based on mapping DNA repeats that still represent serious obstacles in genome sequencing and assembling, even in model species. Localization of repeats on chromosomes of hundreds of fish species and populations originating from diversified environments have revealed the biological importance of this genomic fraction. Ribosomal genes (rDNA) belong to the most informative repeats and in fish, they are subject to a more relaxed regulation than in higher vertebrates. This can result in formation of a literal ‘rDNAome’ consisting of more than 20,000 copies with their high proportion employed in extra-coding functions. Because rDNA has high rates of transcription and recombination, it contributes to genome diversification and can form reproductive barrier. Our overall knowledge of fish cytogenomics grows rapidly by a continuously increasing number of fish genomes sequenced and by use of novel sequencing methods improving genome assembly. The recently revealed exceptional compositional heterogeneity in an ancient fish lineage (gars) sheds new light on the compositional genome evolution in vertebrates generally. We highlight the power of synergy of cytogenetics and genomics in fish cytogenomics, its potential to understand the complexity of genome evolution in vertebrates, which is also linked to clinical applications and the chromosomal backgrounds of speciation. We also summarize the current knowledge on fish cytogenomics and outline its main future avenues.

Sex-Chromosome Recombination in Common Frogs Brings Water to the Fountain-of-Youth.

According to the canonical model of sex-chromosome evolution, the degeneration of Y or W chromosomes (as observed in mammals and birds, respectively) results from an arrest of recombination in the heterogametic sex, driven by the fixation of sexually antagonistic mutations. However, sex chromosomes have remained homomorphic in many lineages of fishes, amphibians, and nonavian reptiles. According to the "fountain-of-youth" model, this homomorphy results from occasional events of sex reversal. If recombination arrest in males is controlled by maleness per se (and not by genotype), then Y chromosomes are expected to recombine in XY females, preventing their long-term degeneration. Here, we provide field support for the fountain-of-youth, by showing that sex-chromosome recombination in Rana temporaria only depends on phenotypic sex: naturally occurring XX males show the same restriction of recombination as XY males (average map length ∼2 cM), while XY females recombine as much as XX females (average map length ∼150 cM). Our results challenge several common assumptions regarding the evolution of sex chromosomes, including the role of sexually antagonistic genes as drivers of recombination arrest, and that of chromosomal inversions as underlying mechanisms.

Sex-antagonistic genes, XY recombination and feminized Y chromosomes.

The canonical model of sex-chromosome evolution predicts that sex-antagonistic (SA) genes play an instrumental role in the arrest of XY recombination and ensuing Y chromosome degeneration. Although this model might account for the highly differentiated sex chromosomes of birds and mammals, it does not fit the situation of many lineages of fish, amphibians or nonavian reptiles, where sex chromosomes are maintained homomorphic through occasional XY recombination and/or high turnover rates. Such situations call for alternative explanatory frameworks. A crucial issue at stake is the effect of XY recombination on the dynamics of SA genes and deleterious mutations. Using individual-based simulations, we show that a complete arrest of XY recombination actually benefits females, not males. Male fitness is maximized at different XY recombination rates depending on SA selection, but never at zero XY recombination. This should consistently favour some level of XY recombination, which in turn generates a recombination load at sex-linked SA genes. Hill-Robertson interferences with deleterious mutations also impede the differentiation of sex-linked SA genes, to the point that males may actually fix feminized phenotypes when SA selection and XY recombination are low. We argue that sex chromosomes might not be a good localization for SA genes, and sex conflicts seem better solved through the differential expression of autosomal genes.

Specialized specialists and the narrow niche fallacy: a tale of scale-feeding fishes.

Although rare within the context of 30 000 species of extant fishes, scale-feeding as an ecological strategy has evolved repeatedly across the teleost tree of life. Scale-feeding (lepidophagous) fishes are diverse in terms of their ecology, behaviour, and specialized morphologies for grazing on scales and mucus of sympatric species. Despite this diversity, the underlying ontogenetic changes in functional and biomechanical properties of associated feeding morphologies in lepidophagous fishes are less understood. We examined the ontogeny of feeding mechanics in two evolutionary lineages of scale-feeding fishes: Roeboides, a characin, and Catoprion, a piranha. We compare these two scale-feeding taxa with their nearest, non-lepidophagous taxa to identify traits held in common among scale-feeding fishes. We use a combination of micro-computed tomography scanning and iodine staining to measure biomechanical predictors of feeding behaviour such as tooth shape, jaw lever mechanics and jaw musculature. We recover a stark contrast between the feeding morphology of scale-feeding and non-scale-feeding taxa, with lepidophagous fishes displaying some paedomorphic characters through to adulthood. Few traits are shared between lepidophagous characins and piranhas, except for their highly-modified, stout dentition. Given such variability in development, morphology and behaviour, ecological diversity within lepidophagous fishes has been underestimated.

Adaptive evolution of interferon regulatory factors is not correlated with body scale reduction or loss in schizothoracine fish

Body scales in teleost fish are the first line of defense in protecting the fish from invading pathogens. However, the relationship between the adaptive evolution of immune-related genes and changes in the body scale-covering of fish has not been previously studied. Schizothoracine fish, characterized by progressive reduction of body scales from the primitive to the highly specialized species, are a good group to investigate this relationship. We obtained 11 IRF genes (IRF1-11) from 14 of schizothoracine fish representing primitive, specialized, and highly specialized species, of which seven IRF genes (IRF2, IRF3, IRF5, IRF6, IRF7, IRF8 and IRF9) contained the complete CDS. Sequence analysis demonstrated the deletion or insertion of 4–7 amino acids in IRF2, IRF3, IRF6 and IRF9, which seems to be a common phenomenon in the schizothoracine fish. Selection pressure analysis supported the hypothesis that positive selection has been driving the rapid evolution of IRFs in specific lineages of the schizothoracine fish. This, however, is not correlated with body scale reduction or loss in the evolution of these IRFs. Remarkably, the deletion or insertion found in IRF protein sequences presented a regular pattern corresponding to the scale-covering changes in schizothoracine fish. Our study provides evidence for positive selection in the IRF family, contributing to a better understanding of the adaptive evolution of immune-related genes in schizothoracine fish in response to environmental changes of the Qinghai-Tibetan Plateau.

Molecular evolution and expression of oxygen transport genes in livebearing fishes (Poeciliidae) from hydrogen sulfide rich springs.

Hydrogen sulfide (H2S) is a natural toxicant in some aquatic environments that has diverse molecular targets. It binds to oxygen transport proteins, rendering them non-functional by reducing oxygen-binding affinity. Hence, organisms permanently inhabiting H2S-rich environments are predicted to exhibit adaptive modifications to compensate for the reduced capacity to transport oxygen. We investigated 10 lineages of fish of the family Poeciliidae that have colonized freshwater springs rich in H2S-along with related lineages from non-sulfidic environments-to test hypotheses about the expression and evolution of oxygen transport genes in a phylogenetic context. We predicted shifts in the expression of and signatures of positive selection on oxygen transport genes upon colonization of H2S-rich habitats. Our analyses indicated significant shifts in gene expression for multiple hemoglobin genes in lineages that have colonized H2S-rich environments, and three hemoglobin genes exhibited relaxed selection in sulfidic compared to non-sulfidic lineages. However, neither changes in gene expression nor signatures of selection were consistent among all lineages in H2S-rich environments. Oxygen transport genes may consequently be predictable targets of selection during adaptation to sulfidic environments, but changes in gene expression and molecular evolution of oxygen transport genes in H2S-rich environments are not necessarily repeatable across replicated lineages.

Genomic signature of highland adaptation in fish: a case study in Tibetan Schizothoracinae species

BackgroundGenome-wide studies on highland adaptation mechanism in terrestrial animal have been widely reported with few available for aquatic animals. Tibetan Schizothoracinae species are ideal model systems to study speciation and adaptation of fish. The Schizothoracine fish, Gymnocypris przewalskii ganzihonensis had underwent the ecological niche shift from salt water to freshwater, and also experienced a recent split from Gymnocypris przewalskii przewalskii. In addition, G. p. ganzihonensis inhabited harsh aquatic environment including low temperature and hypoxia as well as other Schizothoracinae species, its genetic mechanism of highland adaptation have yet to be determined.ResultsOur study used comparative genomic analysis based on the transcriptomic data of G. p. ganzihonensis and other four fish genome datasets to investigate the genetic basis of highland adaptation in Schizothoracine fish. We found that Schizothoracine fish lineage on the terminal branch had an elevated dN/dS ratio than its ancestral branch. A total of 202 gene ontology (GO) categories involved into transport, energy metabolism and immune response had accelerated evolutionary rates than zebrafish. Interestingly, we also identified 162 genes showing signature of positive selection (PSG) involved into energy metabolism, transport and immune response in G. p. ganzihonesis. While, we failed to find any PSG related to hypoxia response as previous studies.ConclusionsComparative genomic analysis based on G. p. ganzihonensis transcriptome data revealed significant genomic signature of accelerated evolution ongoing within Tibetan Schizothoracinae species lineage. Molecular evolution analysis suggested that genes involved in energy metabolism, transport and immune response functions in Schizothoracine fish underwent positive selection, especially in innate immunity including toll-like receptor signaling pathway genes. Taken together, our result as a case study in Schizothoracinae species provides novel insights in understanding the aquatic animal adaptation to extreme environment on the Tibetan Plateau, and also provides valuable genomic resource for further functional verification studies.

Molecular investigation of genetic assimilation during the rapid adaptive radiations of East African cichlid fishes.

Adaptive radiations are characterized by adaptive diversification intertwined with rapid speciation within a lineage resulting in many ecologically specialized, phenotypically diverse species. It has been proposed that adaptive radiations can originate from ancestral lineages with pronounced phenotypic plasticity in adaptive traits, facilitating ecologically driven phenotypic diversification that is ultimately fixed through genetic assimilation of gene regulatory regions. This study aimed to investigate how phenotypic plasticity is reflected in gene expression patterns in the trophic apparatus of several lineages of East African cichlid fishes, and whether the observed patterns support genetic assimilation. This investigation used a split brood experimental design to compare adaptive plasticity in species from within and outside of adaptive radiations. The plastic response was induced in the crushing pharyngeal jaws through feeding individuals either a hard or soft diet. We find that nonradiating, basal lineages show higher levels of adaptive morphological plasticity than the derived, radiated lineages, suggesting that these differences have become partially genetically fixed during the formation of the adaptive radiations. Two candidate genes that may have undergone genetic assimilation, gif and alas1, were identified, in addition to alterations in the wiring of LPJ patterning networks. Taken together, our results suggest that genetic assimilation may have dampened the inducibility of plasticity related genes during the adaptive radiations of East African cichlids, flattening the reaction norms and canalizing their feeding phenotypes, driving adaptation to progressively more narrow ecological niches.

Usefulness and limitations of sample pooling for environmental DNA metabarcoding of freshwater fish communities

Environmental DNA (eDNA) metabarcoding has been used increasingly to assess biodiversity of aquatic vertebrates. However, there still remains to be developed a sampling design of eDNA metabarcoding that can ensure high detection rates of species with minimum total survey effort, especially for large-scale surveys of aquatic organisms. We here tested whether pooling of eDNA samples can be used to evaluate biodiversity of freshwater fishes in four satellite lakes of Lake Biwa, Japan. Fish communities detected by eDNA metabarcoding of the mitochondrial 12S region were compared between the individual and pooled samples. In the individual samples, 31, 22, 33, and 31 fish lineages (proxies for species) were observed at the respective sites, within which moderate spatial autocorrelation existed. In the pooled samples, 30, 20, 29, and 27, lineages were detected, respectively, even after 15 PCR replicates. Lineages accounting for < 0.05% of the total read count of each site’s individual samples were mostly undetectable in the pooled samples. Moreover, fish communities detected were similar among PCR replicates in the pooled samples. Because of the decreased detection rates, the pooling strategy is unsuitable for estimating fish species richness. However, this procedure is useful potentially for among-site comparison of representative fish communities.

Aggression supersedes individual oxygen demand to drive group air-breathing in a social catfish.

Group‐living is widespread among animals and comes with numerous costs and benefits. To date, research examining group‐living has focused on trade‐offs surrounding foraging, while other forms of resource acquisition have been largely overlooked.Air‐breathing has evolved in many fish lineages, allowing animals to obtain oxygen in hypoxic aquatic environments. Breathing air increases the threat of predation, so some species perform group air‐breathing, to reduce individual risk. Within species, individual air‐breathing can be influenced by metabolic rate as well as personality, but the mechanisms of group air‐breathing remain unexplored. It is conceivable that keystone individuals with high metabolic demand or intrinsic tendency to breathe air may drive social breathing, especially in hypoxia.We examined social air‐breathing in African sharptooth catfish Clarias gariepinus, to determine whether individual physiological traits and spontaneous tendency to breathe air influence the behaviour of entire groups, and whether such influences vary in relation to aquatic oxygen availability.We studied 11 groups of four catfish in a laboratory arena and recorded air‐breathing behaviour, activity and agonistic interactions at varying levels of hypoxia. Bimodal respirometry was used to estimate individual standard metabolic rate (SMR) and the tendency to utilize aerial oxygen when alone.Fish took more air breaths in groups as compared to when they were alone, regardless of water oxygen content, and displayed temporally clustered air‐breathing behaviour, consistent with existing definitions of synchronous air‐breathing. However, groups displayed tremendous variability in surfacing behaviour. Aggression by dominant individuals within groups was the main factor influencing air‐breathing of the entire group. There was no association between individual SMR, or the tendency to obtain oxygen from air when in isolation, and group air‐breathing.For C. gariepinus, synchronous air‐breathing is strongly influenced by agonistic interactions, which may expose subordinate individuals to risk of predation. Influential individuals exerted an overriding effect on risk‐taking by the entire group, for reasons independent of their physiological oxygen requirements. Overall, this illustrates that social context can obscure interactions between an individual's physiological and behavioural traits and their tendency to take risks to obtain resources.

The non‐visual opsins: eighteen in the ancestor of vertebrates, astonishing increase in ray‐finned fish, and loss in amniotes

Non-visual opsins were discovered in the early 1990s. These genes play roles in circadian rhythm in mammals, seasonal reproduction in birds, light avoidance in amphibian larvae, and neural development in fish. However, the interpretation of such studies and the success of future work are compromised by the fact that non-visual opsin repertoires have not been properly characterized in any of these lineages. Here, we show that non-visual opsins from tetrapods and ray-finned fish are distributed among 18 monophyletic subfamilies. An amphibian sequence occurs in every subfamily, whereas mammalian orthologs occur in only seven. Species in the major ray-finned fish lineages, Holostei, Osteoglossomorpha, Otomorpha, Protacanthopterygii, and Neoteleostei, have large numbers of non-visual opsins (22-32 genes) as a result of gene duplication events including, but not limited to, the teleost genome duplication (TGD). In contrast to visual opsins, where lineage-specific duplication is common, the ray-finned fish non-visual opsin repertoire appears to have stabilized shortly after the TGD event and consequently even distantly related species have repertoires of similar size and composition. Most non-visual opsins have been named without the benefit of a phylogenetic perspective and, accordingly, major revisions are proposed.

Fatty acid-binding protein genes of the ancient, air-breathing, ray-finned fish, spotted gar (Lepisosteus oculatus)

With the advent of high-throughput DNA sequencing technology, the genomic sequence of many disparate species has led to the relatively new discipline of genomics, the study of genome structure, function and evolution. Much work has been focused on the role of whole genome duplications (WGD) in the architecture of extant vertebrate genomes, particularly those of teleost fishes which underwent a WGD early in the teleost radiation >230 million years ago (mya). Our past work has focused on the fate of duplicated copies of a multigene family coding for the intracellular lipid-binding protein (iLBP) genes in the teleost fishes. To define the evolutionary processes that determined the fate of duplicated genes and generated the structure of extant fish genomes, however, requires comparative genomic analysis with a fish lineage that diverged before the teleost WGD, such as the spotted gar (Lepisosteus oculatus), an ancient, air-breathing, ray-finned fish. Here, we describe the genomic organization, chromosomal location and tissue-specific expression of a subfamily of the iLBP genes that code for fatty acid-binding proteins (Fabps) in spotted gar. Based on this work, we have defined the minimum suite of fabp genes prior to their duplication in the teleost lineages ~230–400 mya. Spotted gar, therefore, serves as an appropriate outgroup, or ancestral/ancient fish, that did not undergo the teleost-specific WGD. As such, analyses of the spatio-temporal regulation of spotted gar genes provides a foundation to determine whether the duplicated fabp genes have been retained in teleost genomes owing to either sub- or neofunctionalization.

Carassius praecipuus, a dwarf new species of goldfish from the Mekong drainage in central Laos (Teleostei: Cyprinidae)

Carassius praecipuus, new species, is described from a hill stream in the upper Nam Ngum watershed in Mekong drainage in central Laos. It is distinguished from all congeners by the short dorsal fin, a small number of branched dorsal-fin rays, lateral line scales and gill rakers. It reaches sexual maturity around 70 mm SL. This is the first species of the genus discovered outside East and North Asia and Europe. The upper Nam Ngum watershed includes a part of the Plain of Jars, which had been hypothesised to have allowed the movement of a number of fish lineages between the rivers flowing to the Gulf of Tonkin and the Mekong drainage.

Analysis of the spotted gar genome suggests absence of causative link between ancestral genome duplication and transposable element diversification in teleost fish.

Teleost fish have been shown to contain many superfamilies of transposable elements (TEs) that are absent from most tetrapod genomes. Since theories predict an increase in TE activity following polyploidization, such diversity might be linked to the 3R whole-genome duplication that occurred approximately 300 million years ago before the teleost radiation. To test this hypothesis, we have analyzed the genome of the spotted gar Lepisosteus oculatus, which diverged from the teleost lineage before the 3R duplication. Our results indicate that TE diversity and copy numbers are similar in gar and teleost genomes, suggesting that TE diversity was ancestral and not linked to the 3R whole-genome duplication. We propose that about 25 distinct superfamilies of TEs were present in the last ancestor of gars and teleost fish about 300 million years ago in the ray-finned fish lineage.

Early-branching euteleost relationships: areas of congruence between concatenation and coalescent model inferences.

Phylogenetic inference based on evidence from DNA sequences has led to significant strides in the development of a stable and robustly supported framework for the vertebrate tree of life. To date, the bulk of those advances have relied on sequence data from a small number of genome regions that have proven unable to produce satisfactory answers to consistently recalcitrant phylogenetic questions. Here, we re-examine phylogenetic relationships among early-branching euteleostean fish lineages classically grouped in the Protacanthopterygii using DNA sequence data surrounding ultraconserved elements. We report and examine a dataset of thirty-four OTUs with 17,957 aligned characters from fifty-three nuclear loci. Phylogenetic analysis is conducted in concatenated, joint gene trees and species tree estimation and summary coalescent frameworks. All analytical frameworks yield supporting evidence for existing hypotheses of relationship for the placement of Lepidogalaxias salamandroides, monophyly of the Stomiatii and the presence of an esociform + salmonid clade. Lepidogalaxias salamandroides and the Esociformes + Salmoniformes are successive sister lineages to all other euteleosts in the majority of analyses. The concatenated and joint gene trees and species tree analysis types produce high support values for this arrangement. However, inter-relationships of Argentiniformes, Stomiatii and Neoteleostei remain uncertain as they varied by analysis type while receiving strong and contradictory indices of support. Topological differences between analysis types are also apparent within the otomorph and the percomorph taxa in the data set. Our results identify concordant areas with strong support for relationships within and between early-branching euteleost lineages but they also reveal limitations in the ability of larger datasets to conclusively resolve other aspects of that phylogeny.

Laterality is Universal Among Fishes but Increasingly Cryptic Among Derived Groups.

Laterality has been studied in several vertebrates, mainly in terms of brain lateralization and behavioral laterality, but morphological asymmetry has not been extensively investigated. Asymmetry in fishes was first described in scale-eating cichlids from Lake Tanganyika, in the form of bilateral dimorphism in which some individuals, when opening their mouths, twist them to the right and others to the left. This asymmetry has a genetic basis, and is correlated with lateralized attack behaviors. This has subsequently been found in fishes from numerous taxa with various feeding habits. The generality of such morphological laterality should thus be investigated in as wide a range of fishes as possible. Using specific indicators of lateral differences in mandibles and head inclination, we find that representative species from all 60 orders of extant gnathostome fishes (both bony and cartilaginous) possess morphological laterality. Furthermore, we identify the same laterality in agnathans (hagfish and lamprey), suggesting that this trait appeared early in fish evolution and has been maintained across fish lineages. However, a comparison of asymmetry among groups of bony fishes reveals, unexpectedly, that phylogenetically more recent-groups possess less asymmetry in body structures. The universality of laterality in fishes indicates a monophyletic origin, and may have been present in the ancestors of vertebrates. Ecological factors, predator-prey interactions in particular, may be key drivers in the evolution and maintenance of dimorphism, and may also be responsible for the cryptic trend of asymmetry in derived groups. Because lungfish and coelacanths share this trait, it is likely that tetrapods also inherited it. We believe that study of this morphological laterality will provide insights into the behavioral and sensory lateralization of vertebrates.

A unique Cretaceous\u2013Paleogene lineage of piranha-jawed pycnodont fishes

The extinct group of the Pycnodontiformes is one of the most characteristic components of the Mesozoic and early Cenozoic fish faunas. These ray-finned fishes, which underwent an explosive morphological diversification during the Late Cretaceous, are generally regarded as typical shell-crushers. Here we report unusual cutting-type dentitions from the Paleogene of Morocco which are assigned to a new genus of highly specialized pycnodont fish. This peculiar taxon represents the last member of a new, previously undetected 40-million-year lineage (Serrasalmimidae fam. nov., including two other new genera and Polygyrodus White, 1927) ranging back to the early Late Cretaceous and leading to exclusively carnivorous predatory forms, unique and unexpected among pycnodonts. Our discovery indicates that latest Cretaceous–earliest Paleogene pycnodonts occupied more diverse trophic niches than previously thought, taking advantage of the apparition of new prey types in the changing marine ecosystems of this time interval. The evolutionary sequence of trophic specialization characterizing this new group of pycnodontiforms is strikingly similar to that observed within serrasalmid characiforms, from seed- and fruit-eating pacus to flesh-eating piranhas.