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The development of paired appendages was a key innovation during evolution and facilitated the aquatic to terrestrial transition of vertebrates. Largely derived from the lateral plate mesoderm (LPM), one hypothesis for the evolution of paired fins invokes derivation from unpaired median fins via a pair of lateral fin folds located between pectoral and pelvic fin territories1. Whilst unpaired and paired fins exhibit similar structural and molecular characteristics, no definitive evidence exists for paired lateral fin folds in larvae or adults of any extant or extinct species. As unpaired fin core components are regarded as exclusively derived from paraxial mesoderm, any transition presumes both co-option of a fin developmental programme to the LPM and bilateral duplication2. Here, we identify that the larval zebrafish unpaired pre-anal fin fold (PAFF) is derived from the LPM and thus may represent a developmental intermediate between median and paired fins. We trace the contribution of LPM to the PAFF in both cyclostomes and gnathostomes, supporting the notion that this is an ancient trait of vertebrates. Finally, we observe that the PAFF can be bifurcated by increasing bone morphogenetic protein signalling, generating LPM-derived paired fin folds. Our work provides evidence that lateral fin folds may have existed as embryonic anlage for elaboration to paired fins. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Published version This work was funded by the Industry Aligned Fund (IAF) Agency for Science, Technology and Research (grant to T.J.C. and K.-W.T.); Ministry of Education (MoE) Tier 3 (grant 2016-T3-1-005 to T.J.C., C.W. and H.M.); Ministry of Education (MoE) Tier 1 (grant 2016-T1-001-055 to T.J.C. and C.Z.); Ministry of Education (MoE) Tier 2 (grant MOE-T2EP30221-0008 to C.W.); the Company of Biologists (travelling fellowship to M.J.T.); the National Science Foundation (grants IOS-1853949 to M.C.D. and 2203311 to C.M.); the Swiss National Science Foundation Sinergia (grant CRSII5_180345 to C.M.); the Swiss Bridge Foundation (C.M.); Additional Ventures Single Ventricle Research Fund (SVRF) (grant 1048003 to C.M.); the University of Colorado School of Medicine Anschutz Medical Campus and the Children’s Hospital Colorado Foundation (C.M.); the National Institutes of Health (NIH), National Institute of General Medical Sciences (grants 1T32GM141742-01 to H.R.M. and 3T32GM121742-02S1 to H.R.M.); Australian Research Council (discovery grant DP200103219 to F.J.T. and P.D.C); National Health and Medical Research Council (senior principal research fellow APP1136567 to P.D.C.); and the NIH (grant R35NS111564 to J.S. and M.E.B.).

BACKGROUND Sturgeons belong to an early-branching lineage often used as a proxy of ancestor-like traits of ray-finned fishes. However, many features of this lineage, such as the transitory presence and the eventual loss of dentition, exemplify specializations that, in fact, provide important information on lineage-specific evolutionary dynamics. RESULTS Here, we introduce a detailed overview of the dentition during the development of the sterlet sturgeon. The dentition is composed of tooth fields at oral, palatal, and anterior pharyngeal regions. Oral fields are single-rowed, non-renewed and are shed early. Palatal and pharyngeal fields are multi-rowed and renewed from the adjacent superficial epithelium without the presence of the successional dental lamina. The early loss of oral fields and subsequent establishment of palatal and pharyngeal fields leads to a translocation of the functional dentition from the front to the rear of the oropharyngeal cavity until the eventual loss of all teeth. CONCLUSION Our survey shows the sterlet dentition as a dynamic organ system displaying differential composition at different time points in the lifetime of this fish. These dynamics represent a conspicuous feature of sturgeons, unparalleled among extant vertebrates, and appropriate to scrutinize developmental and evolutionary underpinnings of vertebrate odontogenesis. This article is protected by copyright. All rights reserved.

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.

The family Buthidae represents an early-diverging and most species-rich lineage of extant scorpions, but its internal phylogenetic relationships are still poorly understood. The family is traditionally divided into six morpho-groups; however, the monophyly of some of them remains unclear. We combined multilocus sequence data with extensive taxon sampling to reconstruct the phylogenetic relationships among Buthidae and assess the validity of the morphology-based groupings. We recovered a monophyletic Buthus group as a sister clade to all the remaining Buthidae. We also found support for the monophyly of the Tityus group, but the remaining morpho-groups were recovered as para-/polyphyletic. Our results also suggest that some genera are in need of a taxonomic revision.

Nitric oxide (NO) is an ancestral key signaling molecule essential for life and has enormous versatility in biological systems, including cardiovascular homeostasis, neurotransmission, and immunity. Although our knowledge of nitric oxide synthases (Nos), the enzymes that synthesize NO in vivo, is substantial, the origin of a large and diversified repertoire of nos gene orthologs in fish with respect to tetrapods remains a puzzle. The recent identification of nos3 in the ray-finned fish spotted gar, which was considered lost in the ray-finned fish lineage, changed this perspective. This prompted us to explore nos gene evolution and expression in depth, 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 paralog in the gills of developing shark, bichir, sturgeon, and gar but not in arctic lamprey, suggest that nos expression in this organ likely arose 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.

During the course of evolution, animals have become increasingly complex by the addition of novel cell types and regulatory mechanisms. A prime example is represented by the lateral neural border, known as the neural plate border in vertebrates, a region of the developing ectoderm where presumptive neural and non-neural tissue meet. This region has been intensively studied as the source of two important embryonic cell types unique to vertebrates-the neural crest and the ectodermal placodes-which contribute to diverse differentiated cell types including the peripheral nervous system, pigment cells, bone, and cartilage. How did these multipotent progenitors originate in animal evolution? What triggered the elaboration of the border during the course of chordate evolution? How is the lateral neural border patterned in various bilaterians and what is its fate? Here, we review and compare the development and fate of the lateral neural border in vertebrates and invertebrates and we speculate about its evolutionary origin. Taken together, the data suggest that the lateral neural border existed in bilaterian ancestors prior to the origin of vertebrates and became a developmental source of exquisite evolutionary change that frequently enabled the acquisition of new cell types.

During the course of evolution, animals have become increasingly complex by the addition of novel cell types and regulatory mechanisms. A prime example is represented by the lateral neural border, known as the neural plate border in vertebrates, a region of the developing ectoderm where presumptive neural and non-neural tissue meet. This region has been intensively studied as the source of two important embryonic cell types unique to vertebrates—the neural crest and the ectodermal placodes—which contribute to diverse differentiated cell types including the peripheral nervous system, pigment cells, bone, and cartilage. How did these multipotent progenitors originate in animal evolution? What triggered the elaboration of the border during the course of chordate evolution? How is the lateral neural border patterned in various bilaterians and what is its fate? Here, we review and compare the development and fate of the lateral neural border in vertebrates and invertebrates and we speculate about its evolutionary origin. Taken together, the data suggest that the lateral neural border existed in bilaterian ancestors prior to the origin of vertebrates and became a developmental source of exquisite evolutionary change that frequently enabled the acquisition of new cell types.

Cercosaurine lizards (subfamily Cercosaurinae of the family Gymnophthalmidae) represent a substantial component of the reptile fauna in the Neotropics. Several attempts have been made to reconstruct the phylogenetic relationships within this group, but most studies focused on particular genera or regions and did not cover the subfamily as a whole. In this study, material from the montane forests of Peru was newly sequenced. In combination with all cercosaurine sequences available on GenBank, an updated phylogeny of Cercosaurinae is provided. Monophyly was not supported for three of the currently recognised genera (Echinosaura,Oreosaurus, andProctoporus). The genusProctoporusis formed by five monophyletic groups, which should be used in future taxonomic revisions as feasible entities. Recognition of two previously identified undescribed clades (Unnamed clades 2 and 3) was supported and yet another undescribed clade (termed here Unnamed clade 4), which deserves recognition as an independent genus, was identified herein.Selvasaurabrava, a new genus and new species of arboreal gymnophthalmid lizard is described from the montane forests of the Pui Pui Protected Forest, Provincia de Chanchamayo, Región Junín, Peru. The new species is characterised by its small size (SVL 42.1–45.9 mm), slender body, smooth head shields, presence of paired prefrontal shields, fused anteriormost supraocular and anteriormost superciliary shields, transparent not divided lower palpebral disc, slightly rugose subimbricate rectangular dorsal scales in adults (slightly keeled in juveniles), distinctly smaller but non-granular lateral scales, smooth squared to rectangular ventral scales, and hemipenial lobes large, distinct from the hemipenial body. Phylogenetic affinities of the new genus to the other cercosaurine genera, as well as basal phylogenetic relationships between the other cercosaurine genera remain unresolved.

Mesalina are small lacertid lizards occurring in the Saharo-Sindian deserts from North Africa to the east of the Iranian plateau. Earlier phylogenetic studies indicated that there are several species complexes within the genus and that thorough taxonomic revisions are needed. In this study, we aim at resolving the phylogeny and taxonomy of the M. brevirostris species complex distributed from the Middle East to the Arabian/Persian Gulf region and Pakistan. We sequenced three mitochondrial and three nuclear gene fragments, and in combination with species delimitation and species-tree estimation, we infer a time-calibrated phylogeny of the complex. The results of the genetic analyses support the presence of four clearly delimited species in the complex that diverged approximately between the middle Pliocene and the Pliocene/Pleistocene boundary. Species distribution models of the four species show that the areas of suitable habitat are geographically well delineated and nearly allopatric, and that most of the species have rather divergent environmental niches. Morphological characters also confirm the differences between the species, although sometimes minute. As a result of all these lines of evidence, we revise the taxonomy of the Mesalina brevirostris species complex. We designate a lectotype for Mesalina brevirostris Blanford, 1874; resurrect the available name Eremias bernoullii Schenkel, 1901 from the synonymy of M. brevirostris; elevate M. brevirostris microlepis (Angel, 1936) to species status; and describe Mesalina saudiarabica, a new species from Saudi Arabia.

Despite the wide variety of adaptive modifications in the oral and facial regions of vertebrates, their early oropharyngeal development is considered strictly uniform. It involves sequential formation of the mouth and pharyngeal pouches, with ectoderm outlining the outer surface and endoderm the inner surface, as a rule. At the extreme anterior domain of vertebrate embryos, the ectoderm and endoderm directly juxtapose and initial development of this earliest ecto-endoderm interface, the primary mouth, typically involves ectodermal stomodeal invagination that limits the anterior expansion of the foregut endoderm. Here we present evidence that in embryos of extant non-teleost fishes, oral (stomodeal) formation is preceded by the development of prominent pre-oral gut diverticula (POGD) between the forebrain and roof of the forming mouth. Micro-computed tomography (micro-CT) imaging of bichir, sturgeon and gar embryos revealed that foregut outpocketing at the pre-oral domain begins even before the sequential formation of pharyngeal pouches. The presence of foregut-derived cells in the front of the mouth was further confirmed by in vivo experiments that allowed specific tracing of the early endodermal lining. We show that POGD in sturgeons contribute to the orofacial surface of their larvae, comprising oral teeth, lips, and sensory barbels. To our knowledge, this is the first thorough evidence for endodermal origin of external craniofacial structures in any vertebrate. In bichir and gar embryos, POGD form prominent cranial adhesive organs that are characteristic of the ancient bauplan of free-living chordate larvae. POGD hence seem arguably to be ancestral for all ray-finned fishes, and their topology, pharyngeal-like morphogenesis and gene expression suggest that they are evolutionarily related to the foregut-derived diverticula of early chordate and hemichordate embryos. The formation of POGD might thus represent an ancestral developmental module with deep deuterostome origins.

The northern pike, Esox lucius, is one of the largest temperate freshwater apex predators with a characteristic morphology: an elongated body with pelvic, dorsal, and anal fins located at the rear as a functional feature to sprint predation. However, the typical pike character is its head, which is characterized by a long, flattened snout, a well-armed mouth with numerous teeth, and large eyes characteristic of shallow water visual predators. Although the northern pike is becoming increasingly popular as a model system for ecology and evolutionary research, a detailed staging table has not yet been reported. In this study, we report the first comprehensive staging table for the northern pike, spanning from the one-cell stage to the freely-swimming juvenile stage. In addition to classical embryological descriptions, we use a DAPI staining to distinguish individual cells and embryonic structures during the early development. This dataset, in combination with the genomic and transcriptomic resources already available, serves as a foundation for in-depth mechanistic studies dealing with development using this species.

In most vertebrates, pharyngeal arches form in a stereotypic anterior-to-posterior progression. To gain insight into the mechanisms underlying evolutionary changes in pharyngeal arch development, here we investigate embryos and larvae of bichirs. Bichirs represent the earliest diverged living group of ray-finned fishes, and possess intriguing traits otherwise typical for lobe-finned fishes such as ventral paired lungs and larval external gills. In bichir embryos, we find that the anteroposterior way of formation of cranial segments is modified by the unique acceleration of the entire hyoid arch segment, with earlier and orchestrated development of the endodermal, mesodermal, and neural crest tissues. This major heterochronic shift in the anteroposterior developmental sequence enables early appearance of the external gills that represent key breathing organs of bichir free-living embryos and early larvae. Bichirs thus stay as unique models for understanding developmental mechanisms facilitating increased breathing capacity.