Comparative analysis of Hmx expression and the distribution of neuronal somata in the trigeminal ganglion in lamprey and shark: insights into the homology of the trigeminal nerve branches and the evolutionary origin of the vertebrate jaw.

The evolutionary origin of the jaw remains one of the most enigmatic events in vertebrate evolution. The trigeminal nerve is a key component for understanding jaw evolution, as it plays a crucial role as a sensorimotor interface for the effective manipulation of the jaw. This nerve is also found in the lamprey, an extant jawless vertebrate. The trigeminal nerve has three major branches in both the lamprey and jawed vertebrates. Although each of these branches was classically thought to be homologous between these two taxa, this homology is now in doubt. In the present study, we compared expression patterns of Hmx, a candidate genetic marker of the mandibular nerve (rV3, the third branch of the trigeminal nerve in jawed vertebrates), and the distribution of neuronal somata of trigeminal nerve branches in the trigeminal ganglion in lamprey and shark. We first confirmed the conserved expression pattern of Hmx1 in the shark rV3 neuronal somata, which are distributed in the caudal part of the trigeminal ganglion. By contrast, lamprey Hmx genes showed peculiar expression patterns, with expression in the ventrocaudal part of the trigeminal ganglion similar to Hmx1 expression in jawed vertebrates, which labeled the neuronal somata of the second branch. Based on these results, we propose two alternative hypotheses regarding the homology of the trigeminal nerve branches, providing new insights into the evolutionary origin of the vertebrate jaw.

Four myriapod relatives - but who are sisters? No end to debates on relationships among the four major myriapod subgroups.

BACKGROUND: Phylogenetic relationships among the myriapod subgroups Chilopoda, Diplopoda, Symphyla and Pauropoda are still not robustly resolved. The first phylogenomic study covering all subgroups resolved phylogenetic relationships congruently to morphological evidence but is in conflict with most previously published phylogenetic trees based on diverse molecular data. Outgroup choice and long-branch attraction effects were stated as possible explanations for these incongruencies. In this study, we addressed these issues by extending the myriapod and outgroup taxon sampling using transcriptome data. RESULTS: We generated new transcriptome data of 42 panarthropod species, including all four myriapod subgroups and additional outgroup taxa. Our taxon sampling was complemented by published transcriptome and genome data resulting in a supermatrix covering 59 species. We compiled two data sets, the first with a full coverage of genes per species (292 single-copy protein-coding genes), the second with a less stringent coverage (988 genes). We inferred phylogenetic relationships among myriapods using different data types, tree inference, and quartet computation approaches. Our results unambiguously support monophyletic Mandibulata and Myriapoda. Our analyses clearly showed that there is strong signal for a single unrooted topology, but a sensitivity of the position of the internal root on the choice of outgroups. However, we observe strong evidence for a clade Pauropoda+Symphyla, as well as for a clade Chilopoda+Diplopoda. CONCLUSIONS: Our best quartet topology is incongruent with current morphological phylogenies which were supported in another phylogenomic study. AU tests and quartet mapping reject the quartet topology congruent to trees inferred with morphological characters. Moreover, quartet mapping shows that confounding signal present in the data set is sufficient to explain the weak signal for the quartet topology derived from morphological characters. Although outgroup choice affects results, our study could narrow possible trees to derivatives of a single quartet topology. For highly disputed relationships, we propose to apply a series of tests (AU and quartet mapping), since results of such tests allow to narrow down possible relationships and to rule out confounding signal.

FGF- and SHH-based molecular signals regulate barbel and craniofacial development in catfish.

BACKGROUND: Catfish (Siluriformes) are characterized by unique morphologies, including enlarged jaws with movable barbels and taste buds covering the entire body surface. Evolution of these characteristics was a crucial step in their adaptive radiation to freshwater environments. However, the developmental processes of the catfish craniofacial region and taste buds remain to be elucidated; moreover, little is known about the molecular mechanisms underlying the morphogenesis of these structures. RESULTS: In Amur catfish (Silurus asotus), three pairs of barbel primordia are formed by 2 days post-fertilization (dpf). Innervation of the peripheral nerves and formation of muscle precursors are also established during early development. Taste buds from the oral region to the body trunk are formed by 4 dpf. We then isolated catfish cognates Shh (SaShh) and Fgf8 (SaFgf8), which are expressed in maxillary barbel primordium at 1-2 dpf. Further, SHH signal inhibition induces reduction of mandibular barbels with abnormal morphology of skeletal elements, whereas it causes no apparent abnormality in the trigeminal and facial nerve morphology. We also found that mandibular barbel lengths and number of taste buds are reduced by FGF inhibition, as seen in SHH signal inhibition. However, unlike with SHH inhibition, the abnormal morphology of the trigeminal and facial nerves was observed in FGF signal-inhibited embryos. CONCLUSION: The developmental processes of Amur catfish are consistent with those reported for other catfish species. Thus, developmental aspects of craniofacial structures and taste buds may be conserved in Siluriformes. Our findings also suggest that SHH signaling plays a crucial role in the formation of barbels and taste buds, without affecting nerve projection, while FGF signaling is required for the development of barbels, taste buds, and branchial nerves. Thus, SHH and FGF signaling plays key roles in the ontogenesis and evolution of some catfish-specific characteristics.

Specialized movement and laterality of fin-biting behaviour in Genyochoromis mento in Lake Malawi.

Several vertebrates, including fish, exhibit behavioural laterality and associated morphological asymmetry. Laterality may increase individual fitness as well as foraging strength, accuracy and speed. However, little is known about which behaviours are affected by laterality or what fish species exhibit obvious laterality. Previous research on the predatory behaviour of the scale-eating Lake Tanganyika cichlid Perissodus microlepis indicates behavioural laterality that reflects asymmetric jaw morphology. The Lake Malawi cichlid Genyochromis mento feeds on the fins of other fish, a behaviour that G. mento developed independently from the Tanganyikan Perissodini scale eaters. We investigated stomach contents and behavioural laterality of predation in aquarium to clarify the functional roles and evolution of laterality in cichlids. We also compared the behavioural laterality and mouth asymmetry of G. mento and P. microlepis The diet of G. mento mostly includes fin fragments, but also scales of several fish species. Most individual G. mento specimens showed significant attack bias favouring the skew mouth direction. However, there was no difference in success rate between attacks from the preferred side and those from the non-preferred side, and no lateralized kinetic elements in predation behaviour. Genyochromismento showed weaker laterality than P. microlepis, partly because of their different feeding habits, the phylogenetic constraints from their shorter evolutionary history and their origin from ancestor Haplochromini omnivorous/herbivorous species. Taken together, this study provides new insights into the functional roles of behavioural laterality: predatory fish aiming for prey that show escape behaviours frequently exhibit lateralized behaviour in predation.

Egg structure and outline of embryonic development of the basal mantodean, Metallyticus splendidus Westwood, 1835 (Insecta, Mantodea, Metallyticidae).

The egg structure and outline of the embryonic development of Metallyticus splendidus of one of the basal Mantodea representatives, Metallyticidae, were described in the present study. The results obtained were compared with those from the previous studies, to reconstruct and discuss the groundplan of Mantodea and Dictyoptera. In M. splendidus, the egg is spheroidal, it has a convex ventral side at the center in which numerous micropyles are grouped, and it possesses a conspicuous hatching line in its anterior half. These are the groundplan features of mantodean eggs and the "grouped micropyles in the ventral side of the egg" are regarded as an apomorphic groundplan feature of Dictyoptera. A small circular embryo is formed by a simple concentration of blastoderm cells, which then undergoes embryogenesis of the typical short germ band type. Blastokinesis is of the "non-reversion type" and the embryo keeps its original superficial position and original orientation throughout embryonic development. During the middle stages of development, the embryo undergoes rotation around the egg's anteroposterior axis. These features are a part of the groundplan of Mantodea. It is uncertain whether sharing of the "non-reversion type" of blastokinesis by Mantodea and blaberoidean Blattodea can be regarded as homology or homoplasy.

Expression patterns of Sema3A in developing amniote limbs: With reference to the diversification of peripheral nerve innervation.

Paired limbs were acquired in the ancestor of tetrapods and their morphology has been highly diversified in amniotes in relation to the adaptive radiation to the terrestrial environment. These morphological changes may have been induced by modification of the developmental program of the skeletal or muscular system. To complete limb modification, it is also important to change the neuronal framework, because the functions of the limbs rely on neural circuits that involve coordinated movement. Previous studies have shown that class 3 semaphorins (Sema3 semaphorins), which act as repulsive axonal guidance cues, play a crucial role in the formation of the peripheral nerves in mice. Here, we studied the expression pattern of Sema3A orthologues in embryos of developing amniotes, including mouse, chick, soft-shelled turtle, and ocelot gecko. Sema3A transcripts were expressed in restricted mesenchymal parts of the developing limb primordium in all animals studied, and developing spinal nerves appeared to extend through Sema3A-negative regions. These results suggest that a Sema3A-dependent guidance system plays a key role in neuronal circuit formation in amniote limbs. We also found that Sema3A partially overlapped with the distribution of cartilage precursor cells. Based on these results, we propose a model in which axon guidance and skeletogenesis are linked by Sema3A; such mechanisms may underlie functional neuron rearrangement during limb diversification.

Nervous system disruption and swimming abnormality in early-hatched pufferfish (Takifugu niphobles) larvae caused by pyrene is independent of aryl hydrocarbon receptors.

Pyrene, a member of the polycyclic aromatic hydrocarbons (PAHs), contributes to abnormality in the size of the brain and the swimming behavior of pufferfish (Takifugu niphobles) larvae. We hypothesized that the aryl hydrocarbon receptor (AHR) may mediate pyrene-induced toxic effects because AHR is assumed to be a candidate for the downstream target of PAHs in many cases. To identify the contribution of AHR on developing pufferfish, we performed exposure experiments using ß-naphthoflavone, an agonist of AHR. We found that the toxic effects of pyrene and ß-naphthoflavone in pufferfish larvae are fundamentally different. Pyrene specifically induced problems in the developing midbrain and in swimming behavior, while ß-naphthoflavone affected the heartbeat rate and the size of the yolk. These results suggest that the behavioral and morphological abnormality caused by pyrene exposure is mediated by an AHR-independent pathway. Alternatively, defects caused by pyrene may be attributed to the inhibition of the FGF signal.

Egg structure and ultrastructure of Paterdecolyus yanbarensis (Insecta, Orthoptera, Anostostomatidae, Anabropsinae).

The egg structure of Paterdecolyus yanbarensis was examined using light, scanning electron and transmission electron microscopy. The egg surface shows a distinct honeycomb pattern formed by exochorionic ridges. Several micropyles are clustered on the ventral side of the egg. The egg membrane is composed of an exochorion penetrated with numerous aeropyles, an endochorion, and an extremely thin vitelline membrane. The endochorion is thickened at the posterior egg pole, probably associated with water absorption. A comparison of egg structure among Orthoptera revealed that the micropylar distribution pattern is conserved in Ensifera and Caelifera and might be regarded as a groundplan feature for each group; in Ensifera, multiple micropyles are clustered on the ventral side of the egg, whereas in Caelifera, micropyles are arranged circularly around the posterior pole of the egg.

Involvement of Slit-Robo signaling in the development of the posterior commissure and concomitant swimming behavior in Xenopus laevis.

INTRODUCTION: During vertebrate development, the central nervous system (CNS) has stereotyped neuronal tracts (scaffolds) that include longitudinal and commissural axonal bundles, such as the medial longitudinal fascicle or the posterior commissure (PC). As these early tracts appear to guide later-developing neurons, they are thought to provide the basic framework of vertebrate neuronal circuitry. The proper construction of these neuronal circuits is thought to be a crucial step for eliciting coordinated behaviors, as these circuits transmit sensory information to the integrative center, which produces motor commands for the effective apparatus. However, the developmental plan underlying some commissures and the evolutionary transitions they have undergone remain to be elucidated. Little is known about the role of axon guidance molecules in the elicitation of early-hatched larval behavior as well. RESULTS: Here, we report the developmentally regulated expression pattern of axon-guidance molecules Slit2 ligand and Robo2 receptor in Xenopus laevis and show that treatment of X. laevis larvae with a slit2- or robo2-morpholino resulted in abnormal swimming behavior. We also observed an abnormal morphology of the PC, which is part of the early axonal scaffold. CONCLUSION: Our present findings suggest that expression patterns of Slit2 and Robo2 are conserved in tetrapods, and that their signaling contributes to the construction of the PC in Xenopus. Given that the PC also includes several types of neurons stemming from various parts of the CNS, it may represent a candidate prerequisite neuronal tract in the construction of subsequent complex neuronal circuits that trigger coordinated behavior.

Development of the thalamo-dorsal ventricular ridge tract in the Chinese soft-shelled turtle, Pelodiscus sinensis.

With the exception of that from the olfactory system, the vertebrate sensory information is relayed by the dorsal thalamus (dTh) to be carried to the telencephalon via the thalamo-telencephalic tract. Although the trajectory of the tract from the dTh to the basal telencephalon seems to be highly conserved among amniotes, the axonal terminals vary in each group. In mammals, thalamic axons project onto the neocortex, whereas they project onto the dorsal pallium and the dorsal ventricular ridge (DVR) in reptiles and birds. To ascertain the evolutionary development of the thalamo-telencephalic connection in amniotes, we focused on reptiles. Using the Chinese soft-shelled turtle (Pelodiscus sinensis), we studied the developmental course of the thalamic axons projecting onto the DVR. We found, during the developmental period when the thalamo-DVR connection forms, that transcripts of axon guidance molecules, including EphA4 and Slit2, were expressed in the diencephalon, similar to the mouse embryo. These results suggest that the basic mechanisms responsible for the formation of the thalamo-telencephalic tract are shared across amniote lineages. Conversely, there was a characteristic difference in the expression patterns of Slit2, Netrin1, and EphrinA5 in the telencephalon between synapsid (mammalian) and diapsid (reptilian and avian) lineages. This indicates that changes in the expression domains of axon guidance molecules may modify the thalamic axon projection and lead to the diversity of neuronal circuits in amniotes.

Phylogenomics resolves the timing and pattern of insect evolution.

Insects are the most speciose group of animals, but the phylogenetic relationships of many major lineages remain unresolved. We inferred the phylogeny of insects from 1478 protein-coding genes. Phylogenomic analyses of nucleotide and amino acid sequences, with site-specific nucleotide or domain-specific amino acid substitution models, produced statistically robust and congruent results resolving previously controversial phylogenetic relations hips. We dated the origin of insects to the Early Ordovician [~479 million years ago (Ma)], of insect flight to the Early Devonian (~406 Ma), of major extant lineages to the Mississippian (~345 Ma), and the major diversification of holometabolous insects to the Early Cretaceous. Our phylogenomic study provides a comprehensive reliable scaffold for future comparative analyses of evolutionary innovations among insects.