Neural development in Onychophora (velvet worms) suggests a step-wise evolution of segmentation in the nervous system of Panarthropoda

Comment on "The lower Cambrian lobopodian Cardiodictyon resolves the origin of euarthropod brains"

Strausfeld et al. (Report, 24 Nov 2022, p. 905) claim that Cambrian fossilized nervous tissue supports the interpretation that the ancestral panarthropod brain was tripartite and unsegmented. We argue that this conclusion is unsupported, and developmental data from living onychophorans contradict it.

A multiscale approach reveals elaborate circulatory system and intermittent heartbeat in velvet worms (Onychophora)

An antagonistic hemolymph-muscular system is essential for soft-bodied invertebrates. Many ecdysozoans (molting animals) possess neither a heart nor a vascular or circulatory system, whereas most arthropods exhibit a well-developed circulatory system. How did this system evolve and how was it subsequently modified in panarthropod lineages? As the closest relatives of arthropods and tardigrades, onychophorans (velvet worms) represent a key group for addressing this question. We therefore analyzed the entire circulatory system of the peripatopsid Euperipatoides rowelli and discovered a surprisingly elaborate organization. Our findings suggest that the last common ancestor of Onychophora and Arthropoda most likely possessed an open vascular system, a posteriorly closed heart with segmental ostia, a pericardial sinus filled with nephrocytes and an impermeable pericardial septum, whereas the evolutionary origin of plical and pericardial channels is unclear. Our study further revealed an intermittent heartbeat-regular breaks of rhythmic, peristaltic contractions of the heart-in velvet worms, which might stimulate similar investigations in arthropods.

Expression of netrin and its receptors uncoordinated-5 and frazzled in arthropods and onychophorans suggests conserved and diverged functions in neuronal pathfinding and synaptogenesis

BACKGROUND

Development of the nervous system and the correct connection of nerve cells require coordinated axonal pathfinding through an extracellular matrix. Outgrowing axons exhibit directional growth toward or away from external guidance cues such as Netrin. Guidance cues can be detected by growth cones that are located at the end of growing axons through membrane-bound receptors such as Uncoordianted-5 and Frazzled. Binding of Netrin causes reformation of the cytoskeleton and growth of the axon toward (or away from) the source of Netrin production.

RESULTS

Here, we investigate the embryonic mRNA expression patterns of netrin genes and their potential receptors, uncoordinated-5 and frazzled in arthropod species that cover all main branches of Arthropoda, that is, Pancrustacea, Myriapoda, and Chelicerata. We also studied the expression patterns in a closely related outgroup species, the onychophoran Euperipatoides kanangrensis, and provide data on expression profiles of these genes in larval tissues of the fly Drosophila melanogaster including the brain and the imaginal disks.

CONCLUSION

Our data reveal conserved and diverged aspects of neuronal guidance in Drosophila with respect to the other investigated species and suggest a conserved function in nervous system patterning of the developing appendages.

Review of extra-embryonic tissues in the closest arthropod relatives, onychophorans and tardigrades

The so-called extra-embryonic tissues are important for embryonic development in many animals, although they are not considered to be part of the germ band or the embryo proper. They can serve a variety of functions, such as nutrient uptake and waste removal, protection of the embryo against mechanical stress, immune response and morphogenesis. In insects, a subgroup of arthropods, extra-embryonic tissues have been studied extensively and there is increasing evidence that they might contribute more to embryonic development than previously thought. In this review, we provide an assessment of the occurrence and possible functions of extra-embryonic tissues in the closest arthropod relatives, onychophorans (velvet worms) and tardigrades (water bears). While there is no evidence for their existence in tardigrades, these tissues show a remarkable diversity across the onychophoran subgroups. A comparison of extra-embryonic tissues of onychophorans to those of arthropods suggests shared functions in embryonic nutrition and morphogenesis. Apparent contribution to the final form of the embryo in onychophorans and at least some arthropods supports the hypothesis that extra-embryonic tissues are involved in organogenesis. In order to account for this role, the commonly used definition of these tissues as 'extra-embryonic' should be reconsidered. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

The lower Cambrian lobopodian Cardiodictyon resolves the origin of euarthropod brains

For more than a century, the origin and evolution of the arthropod head and brain have eluded a unifying rationale reconciling divergent morphologies and phylogenetic relationships. Here, clarification is provided by the fossilized nervous system of the lower Cambrian lobopodian Cardiodictyon catenulum, which reveals an unsegmented head and brain comprising three cephalic domains, distinct from the metameric ventral nervous system serving its appendicular trunk. Each domain aligns with one of three components of the foregut and with a pair of head appendages. Morphological correspondences with stem group arthropods and alignments of homologous gene expression patterns with those of extant panarthropods demonstrate that cephalic domains of C. catenulum predate the evolution of the euarthropod head yet correspond to neuromeres defining brains of living chelicerates and mandibulates.

The velvet worm brain unveils homologies and evolutionary novelties across panarthropods

Background

The evolution of the brain and its major neuropils in Panarthropoda (comprising Arthropoda, Tardigrada and Onychophora) remains enigmatic. As one of the closest relatives of arthropods, onychophorans are regarded as indispensable for a broad understanding of the evolution of panarthropod organ systems, including the brain, whose anatomical and functional organisation is often used to gain insights into evolutionary relations. However, while numerous recent studies have clarified the organisation of many arthropod nervous systems, a detailed investigation of the onychophoran brain with current state-of-the-art approaches is lacking, and further inconsistencies in nomenclature and interpretation hamper its understanding. To clarify the origins and homology of cerebral structures across panarthropods, we analysed the brain architecture in the onychophoran Euperipatoides rowelli by combining X-ray micro-computed tomography, histology, immunohistochemistry, confocal microscopy, and three-dimensional reconstruction.

Results

Here, we use this detailed information to generate a consistent glossary for neuroanatomical studies of Onychophora. In addition, we report novel cerebral structures, provide novel details on previously known brain areas, and characterise further structures and neuropils in order to improve the reproducibility of neuroanatomical observations. Our findings support homology of mushroom bodies and central bodies in onychophorans and arthropods. Their antennal nerve cords and olfactory lobes most likely evolved independently. In contrast to previous reports, we found no evidence for second-order visual neuropils, or a frontal ganglion in the velvet worm brain.

Conclusion

We imaged the velvet worm nervous system at an unprecedented level of detail and compiled a comprehensive glossary of known and previously uncharacterised neuroanatomical structures to provide an in-depth characterisation of the onychophoran brain architecture. We expect that our data will improve the reproducibility and comparability of future neuroanatomical studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01196-w.

Oscillating waves of Fox, Cyclin and CDK gene expression indicate unique spatiotemporal control of cell cycling during nervous system development in onychophorans

Forkhead box (Fox) genes code for a class of transcription factors with many different fundamental functions in animal development including cell cycle control. Other important factors of cell cycle control are Cyclins and Cyclin-dependent kinases (CDKs). Here we report on the oscillating expression of three Fox genes, FoxM, FoxN14 (jumeaux) and FoxN23 (Checkpoint suppressor like-1), Cyclins and CDKs in an onychophoran, a representative of a relatively small group of animals that are closely related to the arthropods. Expression of these genes is in the form of several waves that start as dot-like domains in the center of each segment and then transform into concentric rings that run towards the periphery of the segments. This oscillating gene expression, however, occurs exclusively along the anterior-posterior body axis in the tissue ventral to the base of the appendages, a region where the central nervous system and the enigmatic ventral and preventral organs of the onychophoran develop. We suggest that the oscillating gene expression and the resulting waves of expression we report are likely correlated with cell cycle control during the development of the onychophoran nervous system. This intriguing patterning appears to be unique for onychophorans as it is not found in any of the arthropods we also investigated in this study, and is likely correlated with the slow embryonic development of onychophorans compared to arthropods.

The origin and evolution of the euarthropod labrum

A widely (although not universally) accepted model of arthropod head evolution postulates that the labrum, a structure seen in almost all living euarthropods, evolved from an anterior pair of appendages homologous to the frontal appendages of onychophorans. However, the implications of this model for the interpretation of fossil arthropods have not been fully integrated into reconstructions of the euarthropod stem group, which remains in a state of some disorder. Here I review the evidence for the nature and evolution of the labrum from living taxa, and reconsider how fossils should be interpreted in the light of this. Identification of the segmental identity of head appendage in fossil arthropods remains problematic, and often rests ultimately on unproven assertions. New evidence from the Cambrian stem-group euarthropod Parapeytoia is presented to suggest that an originally protocerebral appendage persisted well up into the upper stem-group of the euarthropods, which prompts a re-evaluation of widely-accepted segmental homologies and the interpretation of fossil central nervous systems. Only a protocerebral brain was implicitly present in a large part of the euarthropod stem group, and the deutocerebrum must have been a relatively late addition.

Expression of NK genes that are not part of the NK cluster in the onychophoran Euperipatoides rowelli (Peripatopsidae)

Background

NK genes are a group of homeobox transcription factors that are involved in various molecular pathways across bilaterians. They are typically divided into two subgroups, the NK cluster (NKC) and NK-linked genes (NKL). While the NKC genes have been studied in various bilaterians, corresponding data of many NKL genes are missing to date. To further investigate the ancestral roles of NK family genes, we analyzed the expression patterns of NKL genes in the onychophoran Euperipatoides rowelli.

Results

The NKL gene complement of E. rowelli comprises eight genes, including BarH, Bari, Emx, Hhex, Nedx, NK2.1, vax and NK2.2, of which only NK2.2 was studied previously. Our data for the remaining seven NKL genes revealed expression in different structures associated with the developing nervous system in embryos of E. rowelli. While NK2.1 and vax are expressed in distinct medial regions of the developing protocerebrum early in development, BarH, Bari, Emx, Hhex and Nedx are expressed in late developmental stages, after all major structures of the nervous system have been established. Furthermore, BarH and Nedx are expressed in distinct mesodermal domains in the developing limbs.

Conclusions

Comparison of our expression data to those of other bilaterians revealed similar patterns of NK2.1, vax, BarH and Emx in various aspects of neural development, such as the formation of anterior neurosecretory cells mediated by a conserved molecular mechanism including NK2.1 and vax, and the development of the central and peripheral nervous system involving BarH and Emx. A conserved role in neural development has also been reported from NK2.2, suggesting that the NKL genes might have been primarily involved in neural development in the last common ancestor of bilaterians or at least nephrozoans (all bilaterians excluding xenacoelomorphs). The lack of comparative data for many of the remaining NKL genes, including Bari, Hhex and Nedx currently hampers further evolutionary conclusions. Hence, future studies should focus on the expression of these genes in other bilaterians, which would provide a basis for comparative studies and might help to better understand the role of NK genes in the diversification of bilaterians.

Electronic supplementary material

The online version of this article (10.1186/s12861-019-0185-9) contains supplementary material, which is available to authorized users.

Evolution of the bilaterian mouth and anus

It is widely held that the bilaterian tubular gut with mouth and anus evolved from a simple gut with one major gastric opening. However, there is no consensus on how this happened. Did the single gastric opening evolve into a mouth, with the anus forming elsewhere in the body (protostomy), or did it evolve into an anus, with the mouth forming elsewhere (deuterostomy), or did it evolve into both mouth and anus (amphistomy)? These questions are addressed by the comparison of developmental fates of the blastopore, the opening of the embryonic gut, in diverse animals that live today. Here we review comparative data on the identity and fate of blastoporal tissue, investigate how the formation of the through-gut relates to the major body axes, and discuss to what extent evolutionary scenarios are consistent with these data. Available evidence indicates that stem bilaterians had a slit-like gastric opening that was partially closed in subsequent evolution, leaving open the anus and most likely also the mouth, which would favour amphistomy. We discuss remaining difficulties, and outline directions for future research.

Analyses of nervous system patterning genes in the tardigrade Hypsibius exemplaris illuminate the evolution of panarthropod brains

BACKGROUND

Both euarthropods and vertebrates have tripartite brains. Several orthologous genes are expressed in similar regionalized patterns during brain development in both vertebrates and euarthropods. These similarities have been used to support direct homology of the tripartite brains of vertebrates and euarthropods. If the tripartite brains of vertebrates and euarthropods are homologous, then one would expect other taxa to share this structure. More generally, examination of other taxa can help in tracing the evolutionary history of brain structures. Tardigrades are an interesting lineage on which to test this hypothesis because they are closely related to euarthropods, and whether they have a tripartite brain or unipartite brain has recently been a focus of debate.

RESULTS

We tested this hypothesis by analyzing the expression patterns of six3, orthodenticle, pax6, unplugged, and pax2/5/8 during brain development in the tardigrade Hypsibius exemplaris-formerly misidentified as Hypsibius dujardini. These genes were expressed in a staggered anteroposterior order in H. exemplaris, similar to what has been reported for mice and flies. However, only six3, orthodenticle, and pax6 were expressed in the developing brain. Unplugged was expressed broadly throughout the trunk and posterior head, before the appearance of the nervous system. Pax2/5/8 was expressed in the developing central and peripheral nervous system in the trunk.

CONCLUSION

Our results buttress the conclusion of our previous study of Hox genes-that the brain of tardigrades is only homologous to the protocerebrum of euarthropods. They support a model based on fossil evidence that the last common ancestor of tardigrades and euarthropods possessed a unipartite brain. Our results are inconsistent with the hypothesis that the tripartite brain of euarthropods is directly homologous to the tripartite brain of vertebrates.

Expression of NK cluster genes in the onychophoran Euperipatoides rowelli: implications for the evolution of NK family genes in nephrozoans

Background

Understanding the evolution and development of morphological traits of the last common bilaterian ancestor is a major goal of the evo-devo discipline. The reconstruction of this "urbilaterian" is mainly based on comparative studies of common molecular patterning mechanisms in recent model organisms. The NK homeobox genes are key players in many of these molecular pathways, including processes regulating mesoderm, heart and neural development. Shared features seen in the expression patterns of NK genes have been used to determine the ancestral bilaterian characters. However, the commonly used model organisms provide only a limited view on the evolution of these molecular pathways. To further investigate the ancestral roles of NK cluster genes, we analyzed their expression patterns in the onychophoran Euperipatoides rowelli.

Results

We identified nine transcripts of NK cluster genes in E. rowelli, including single copies of NK1, NK3, NK4, NK5, Msx, Lbx and Tlx, and two copies of NK6. All of these genes except for NK6.1 and NK6.2 are expressed in different mesodermal organs and tissues in embryos of E. rowelli, including the anlagen of somatic musculature and the heart. Furthermore, we found distinct expression patterns of NK3, NK5, NK6, Lbx and Msx in the developing nervous system. The same holds true for the NKL gene NK2.2, which does not belong to the NK cluster but is a related gene playing a role in neural patterning. Surprisingly, NK1, Msx and Lbx are additionally expressed in a segment polarity-like pattern early in development-a feature that has been otherwise reported only from annelids.

Conclusion

Our results indicate that the NK cluster genes were involved in mesoderm and neural development in the last common ancestor of bilaterians or at least nephrozoans (i.e., bilaterians to the exclusion of xenacoelomorphs). By comparing our data from an onychophoran to those from other bilaterians, we critically review the hypothesis of a complex "urbilaterian" with a segmented body, a pulsatile organ or heart, and a condensed mediolaterally patterned nerve cord.

Comparison of ventral organ development across Pycnogonida (Arthropoda, Chelicerata) provides evidence for a plesiomorphic mode of late neurogenesis in sea spiders and myriapods

BACKGROUND

Comparative studies of neuroanatomy and neurodevelopment provide valuable information for phylogenetic inference. Beyond that, they reveal transformations of neuroanatomical structures during animal evolution and modifications in the developmental processes that have shaped these structures. In the extremely diverse Arthropoda, such comparative studies contribute with ever-increasing structural resolution and taxon coverage to our understanding of nervous system evolution. However, at the neurodevelopmental level, in-depth data remain still largely confined to comparably few laboratory model organisms. Therefore, we studied postembryonic neurogenesis in six species of the bizarre Pycnogonida (sea spiders), which - as the likely sister group of all remaining chelicerates - promise to illuminate neurodevelopmental changes in the chelicerate lineage.

RESULTS

We performed in vivo cell proliferation experiments with the thymidine analogs 5-bromo-2'-deoxyuridine and 5-ethynl-2'-deoxyuridine coupled to fluorescent histochemical staining and immunolabeling, in order to compare ventral nerve cord anatomy and to localize and characterize centers of postembryonic neurogenesis. We report interspecific differences in the architecture of the subesophageal ganglion (SEG) and show the presence of segmental "ventral organs" (VOs) that act as centers of neural cell production during gangliogenesis. These VOs are either incorporated into the ganglionic soma cortex or found on the external ganglion surface. Despite this difference, several shared features support homology of the two VO types, including (1) a specific arrangement of the cells around a small central cavity, (2) the presence of asymmetrically dividing neural stem cell-like precursors, (3) the migration of newborn cells along corresponding pathways into the cortex, and (4) the same VO origin and formation earlier in development.

CONCLUSIONS

Evaluation of our findings relative to current hypotheses on pycnogonid phylogeny resolves a bipartite SEG and internal VOs as plesiomorphic conditions in pycnogonids. Although chelicerate taxa other than Pycnogonida lack comparable VOs, they are a characteristic feature of myriapod gangliogenesis. Accordingly, we propose internal VOs with neurogenic function to be part of the ground pattern of Arthropoda. Further, our findings illustrate the importance of dense sampling in old arthropod lineages - even if as gross-anatomically uniform as Pycnogonida - in order to reliably differentiate plesiomorphic from apomorphic neurodevelopmental characteristics prior to outgroup comparison.

Evaluation of contrasting techniques for X-ray imaging of velvet worms (Onychophora)

Non-invasive imaging techniques like X-ray computed tomography have become very popular in zoology, as they allow for simultaneous imaging of the internal and external morphology of organisms. Nevertheless, the effect of different staining approaches required for this method on samples lacking mineralized tissues, such as soft-bodied invertebrates, remains understudied. Herein, we used synchrotron radiation-based X-ray micro-computed tomography to compare the effects of commonly used contrasting approaches on onychophorans - soft-bodied invertebrates important for studying animal evolution. Representatives of Euperipatoides rowelli were stained with osmium tetroxide (vapour or solution), ruthenium red, phosphotungstic acid, or iodine. Unstained specimens were imaged using both standard attenuation-based and differential phase-contrast setups to simulate analyses with museum material. Our comparative qualitative analyses of several tissue types demonstrate that osmium tetroxide provides the best overall tissue contrast in onychophorans, whereas the remaining staining agents rather favour the visualisation of specific tissues and/or structures. Quantitative analyses using signal-to-noise ratio measurements show that the level of image noise may vary according to the staining agent and scanning medium selected. Furthermore, box-and-whisker plots revealed substantial overlap in grey values among structures in all datasets, suggesting that a combination of semiautomatic and manual segmentation of structures is required for comprehensive 3D reconstructions of Onychophora, irrespective of the approach selected. Our results show that X-ray micro-computed tomography is a promising technique for studying onychophorans and, despite the benefits and disadvantages of different staining agents for specific tissues/structures, this method retrieves informative data that may eventually help address evolutionary questions long associated with Onychophora.

Myoanatomy of the velvet worm leg revealed by laboratory-based nanofocus X-ray source tomography

X-ray computed tomography (CT) is a powerful noninvasive technique for investigating the inner structure of objects and organisms. However, the resolution of laboratory CT systems is typically limited to the micrometer range. In this paper, we present a table-top nanoCT system in conjunction with standard processing tools that is able to routinely reach resolutions down to 100 nm without using X-ray optics. We demonstrate its potential for biological investigations by imaging a walking appendage of Euperipatoides rowelli, a representative of Onychophora-an invertebrate group pivotal for understanding animal evolution. Comparative analyses proved that the nanoCT can depict the external morphology of the limb with an image quality similar to scanning electron microscopy, while simultaneously visualizing internal muscular structures at higher resolutions than confocal laser scanning microscopy. The obtained nanoCT data revealed hitherto unknown aspects of the onychophoran limb musculature, enabling the 3D reconstruction of individual muscle fibers, which was previously impossible using any laboratory-based imaging technique.

Segmentation in Tardigrada and diversification of segmental patterns in Panarthropoda

The origin and diversification of segmented metazoan body plans has fascinated biologists for over a century. The superphylum Panarthropoda includes three phyla of segmented animals-Euarthropoda, Onychophora, and Tardigrada. This superphylum includes representatives with relatively simple and representatives with relatively complex segmented body plans. At one extreme of this continuum, euarthropods exhibit an incredible diversity of serially homologous segments. Furthermore, distinct tagmosis patterns are exhibited by different classes of euarthropods. At the other extreme, all tardigrades share a simple segmented body plan that consists of a head and four leg-bearing segments. The modular body plans of panarthropods make them a tractable model for understanding diversification of animal body plans more generally. Here we review results of recent morphological and developmental studies of tardigrade segmentation. These results complement investigations of segmentation processes in other panarthropods and paleontological studies to illuminate the earliest steps in the evolution of panarthropod body plans.

Conserved versus derived patterns of controlled cell death during the embryonic development of two species of Onychophora (velvet worms)

BACKGROUND

Apoptosis is involved in various developmental processes, including cell migration and tissue and organ formation. Some of these processes are conserved across metazoans, while others are specific to particular taxa. Although the patterns of apoptosis have been investigated in arthropods, no corresponding data are available from one of their closest relatives, the Onychophora (velvet worms).

RESULTS

We analyzed the patterns of apoptosis in embryos of two onychophoran species: the lecithotrophic/matrotrophic viviparous peripatopsid Euperipatoides rowelli, and the placentotrophic viviparous peripatid Principapillatus hitoyensis. Our data show that apoptosis occurs early in development and might be responsible for the degeneration of extra-embryonic tissues. Moreover, apoptosis might be involved in the morphogenesis of the ventral and preventral organs in both species and occurs additionally in the placental stalk of P. hitoyensis.

CONCLUSIONS

Despite the different developmental modes in these onychophoran species, our data suggest that patterns of apoptosis are conserved among onychophorans. While apoptosis in the dorsal extra-embryonic tissue might contribute to dorsal closure-a process also known from arthropods-the involvement of apoptosis in ventral closure might be unique to onychophorans. Apoptosis in the placental stalk of P. hitoyensis is most likely a derived feature of the placentotrophic onychophorans. Developmental Dynamics 246:403-416, 2017. © 2017 Wiley Periodicals, Inc.

Assessing segmental versus non-segmental features in the ventral nervous system of onychophorans (velvet worms)

Background

Due to their phylogenetic position as one of the closest arthropod relatives, studies of the organisation of the nervous system in onychophorans play a key role for understanding the evolution of body segmentation in arthropods. Previous studies revealed that, in contrast to the arthropods, segmentally repeated ganglia are not present within the onychophoran ventral nerve cords, suggesting that segmentation is either reduced or might be incomplete in the onychophoran ventral nervous system.

Results

To assess segmental versus non-segmental features in the ventral nervous system of onychophorans, we screened the nerve cords for various markers, including synapsin, serotonin, gamma-aminobutyric acid, RFamide, dopamine, tyramine and octopamine. In addition, we performed retrograde fills of serially repeated commissures and leg nerves to localise the position of neuronal somata supplying those. Our data revealed a mixture of segmental and non-segmental elements within the onychophoran nervous system.

Conclusions

We suggest that the segmental ganglia of arthropods evolved by a gradual condensation of subsets of neurons either in the arthropod or the arthropod-tardigrade lineage. These findings are in line with the hypothesis of gradual evolution of segmentation in panarthropods and thus contradict a loss of ancestral segmentation within the onychophoran lineage.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0853-3) contains supplementary material, which is available to authorized users.

The larval nervous system of the penis worm Priapulus caudatus (Ecdysozoa)

The origin and extreme diversification of the animal nervous system is a central question in biology. While most of the attention has traditionally been paid to those lineages with highly elaborated nervous systems (e.g. arthropods, vertebrates, annelids), only the study of the vast animal diversity can deliver a comprehensive view of the evolutionary history of this organ system. In this regard, the phylogenetic position and apparently conservative molecular, morphological and embryological features of priapulid worms (Priapulida) place this animal lineage as a key to understanding the evolution of the Ecdysozoa (i.e. arthropods and nematodes). In this study, we characterize the nervous system of the hatching larva and first lorica larva of the priapulid worm Priapulus caudatus by immunolabelling against acetylated and tyrosinated tubulin, pCaMKII, serotonin and FMRFamide. Our results show that a circumoral brain and an unpaired ventral nerve with a caudal ganglion characterize the central nervous system of hatching embryos. After the first moult, the larva attains some adult features: a neck ganglion, an introvert plexus, and conspicuous secondary longitudinal neurites. Our study delivers a neuroanatomical framework for future embryological studies in priapulid worms, and helps illuminate the course of nervous system evolution in the Ecdysozoa.

Gene expression analysis reveals that Delta/Notch signalling is not involved in onychophoran segmentation

Delta/Notch (Dl/N) signalling is involved in the gene regulatory network underlying the segmentation process in vertebrates and possibly also in annelids and arthropods, leading to the hypothesis that segmentation may have evolved in the last common ancestor of bilaterian animals. Because of seemingly contradicting results within the well-studied arthropods, however, the role and origin of Dl/N signalling in segmentation generally is still unclear. In this study, we investigate core components of Dl/N signalling by means of gene expression analysis in the onychophoran Euperipatoides kanangrensis, a close relative to the arthropods. We find that neither Delta or Notch nor any other investigated components of its signalling pathway are likely to be involved in segment addition in onychophorans. We instead suggest that Dl/N signalling may be involved in posterior elongation, another conserved function of these genes. We suggest further that the posterior elongation network, rather than classic Dl/N signalling, may be in the control of the highly conserved segment polarity gene network and the lower-level pair-rule gene network in onychophorans. Consequently, we believe that the pair-rule gene network and its interaction with Dl/N signalling may have evolved within the arthropod lineage and that Dl/N signalling has thus likely been recruited independently for segment addition in different phyla.

Insights into the segmental identity of post-oral commissures and pharyngeal nerves in Onychophora based on retrograde fills

BACKGROUND

While the tripartite brain of arthropods is believed to have evolved by a fusion of initially separate ganglia, the evolutionary origin of the bipartite brain of onychophorans-one of the closest arthropod relatives-remains obscure. Clarifying the segmental identity of post-oral commissures and pharyngeal nerves might provide useful insights into the evolution of the onychophoran brain. We therefore performed retrograde fills of these commissures and nerves in the onychophoran Euperipatoides rowelli.

RESULTS

Our fills of the anterior and posterior pharyngeal nerves revealed groups of somata that are mainly associated with the deutocerebrum. This resembles the innervation pattern of other feeding structures in Onychophora, including the jaws and several lip papillae surrounding the mouth. Our fills of post-oral commissures in E. rowelli revealed a graded arrangement of anteriorly shifted somata associated with post-oral commissures #1 to #5. The number of deutocerebral somata associated with each commissure decreases posteriorly, i.e., commissure #1 shows the highest and commissure #5 the lowest numbers of associated somata, whereas none of the subsequent median commissures, beginning with commissure #6, shows somata located in the deutocerebrum.

CONCLUSIONS

Based on the graded and shifted arrangement of somata associated with the anteriormost post-oral commissures, we suggest that the onychophoran brain, which is a bipartite syncerebrum, might have evolved by a successive anterior/anterodorsal migration of neurons towards the protocerebrum in the last onychophoran ancestor. This implies that the composite brain of onychophorans and the compound brain of arthropods might have independent evolutionary origins, as in contrast to arthropods the onychophoran syncerebrum is unlikely to have evolved by a fusion of initially separate ganglia.

Phylogenetic analysis and expression patterns of Pax genes in the onychophoran Euperipatoides rowelli reveal a novel bilaterian Pax subfamily

Pax family genes encode a class of transcription factors that regulate various developmental processes. To shed light on the evolutionary history of these genes in Panarthropoda (Onychophora + Tardigrada + Arthropoda), we analyzed the Pax repertoire in the embryonic and adult transcriptomes of the onychophoran Euperipatoides rowelli. Our data revealed homologs of all five major bilaterian Pax subfamilies in this species, including Pax2/5/8, Pax4/6, Pox-neuro, Pax1/9/Pox-meso, and Pax3/7. In addition, we identified a new Pax member, pax-α, which does not fall into any other known Pax subfamily but instead clusters in the heterogenic Pax-α/β clade containing deuterostome, ecdysozoan, and lophotrochozoan gene sequences. These findings suggest that the last common bilaterian ancestor possessed six rather than five Pax genes, which have been retained in the panarthropod lineage. The expression data of Pax orthologs in the onychophoran embryo revealed distinctive patterns, some of which might be related to their ancestral roles in the last common panarthropod ancestor, whereas others might be specific to the onychophoran lineage. The derived roles include, for example, an involvement of pax2/5/8, pox-neuro, and pax3/7 in onychophoran nephridiogenesis, and an additional function of pax2/5/8 in the formation of the ventral and preventral organs. Furthermore, our transcriptomic analyses suggest that at least some Pax genes, including pax6 and pax-α, are expressed in the adult onychophoran head, although the corresponding functions remain to be clarified. The remarkable diversity of the Pax expression patterns highlights the functional and evolutionary plasticity of these genes in panarthropods.

Neural development in the tardigrade Hypsibius dujardini based on anti-acetylated α-tubulin immunolabeling

Background

The tardigrades (water bears) are a cosmopolitan group of microscopic ecdysozoans found in a variety of aquatic and temporarily wet environments. They are members of the Panarthropoda (Tardigrada + Onychophora + Arthropoda), although their exact position within this group remains contested. Studies of embryonic development in tardigrades have been scarce and have yielded contradictory data. Therefore, we investigated the development of the nervous system in embryos of the tardigrade Hypsibius dujardini using immunohistochemical techniques in conjunction with confocal laser scanning microscopy in an effort to gain insight into the evolution of the nervous system in panarthropods.

Results

An antiserum against acetylated α-tubulin was used to visualize the axonal processes and general neuroanatomy in whole-mount embryos of the eutardigrade H. dujardini. Our data reveal that the tardigrade nervous system develops in an anterior-to-posterior gradient, beginning with the neural structures of the head. The brain develops as a dorsal, bilaterally symmetric structure and contains a single developing central neuropil. The stomodeal nervous system develops separately and includes at least four separate, ring-like commissures. A circumbuccal nerve ring arises late in development and innervates the circumoral sensory field. The segmental trunk ganglia likewise arise from anterior to posterior and establish links with each other via individual pioneering axons. Each hemiganglion is associated with a number of peripheral nerves, including a pair of leg nerves and a branched, dorsolateral nerve.

Conclusions

The revealed pattern of brain development supports a single-segmented brain in tardigrades and challenges previous assignments of homology between tardigrade brain lobes and arthropod brain segments. Likewise, the tardigrade circumbuccal nerve ring cannot be homologized with the arthropod ‘circumoral’ nerve ring, suggesting that this structure is unique to tardigrades. Finally, we propose that the segmental ganglia of tardigrades and arthropods are homologous and, based on these data, favor a hypothesis that supports tardigrades as the sister group of arthropods.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-015-0008-4) contains supplementary material, which is available to authorized users.

Oscillation of the velvet worm slime jet by passive hydrodynamic instability

The rapid squirt of a proteinaceous slime jet endows velvet worms (Onychophora) with a unique mechanism for defence from predators and for capturing prey by entangling them in a disordered web that immobilizes their target. However, to date, neither qualitative nor quantitative descriptions have been provided for this unique adaptation. Here we investigate the fast oscillatory motion of the oral papillae and the exiting liquid jet that oscillates with frequencies f~30–60 Hz. Using anatomical images, high-speed videography, theoretical analysis and a physical simulacrum, we show that this fast oscillatory motion is the result of an elastohydrodynamic instability driven by the interplay between the elasticity of oral papillae and the fast unsteady flow during squirting. Our results demonstrate how passive strategies can be cleverly harnessed by organisms, while suggesting future oscillating microfluidic devices, as well as novel ways for micro and nanofibre production using bioinspired strategies.

The velvet worm emits a rapidly oscillating jet of proteinaceous slime to capture prey. Here, Concha et al. combine high-speed videography and a physical simulacrum to establish that this passive mechanism is the result of elastohydrodynamic instability during high-speed flow through the oral papillae.

The evolution of early neurogenesis

The foundation of the diverse metazoan nervous systems is laid by embryonic patterning mechanisms, involving the generation and movement of neural progenitors and their progeny. Here we divide early neurogenesis into discrete elements, including origin, pattern, proliferation, and movement of neuronal progenitors, which are controlled by conserved gene cassettes. We review these neurogenetic mechanisms in representatives of the different metazoan clades, with the goal to build a conceptual framework in which one can ask specific questions, such as which of these mechanisms potentially formed part of the developmental "toolkit" of the bilaterian ancestor and which evolved later.

Making sense of 'lower' and 'upper' stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848

The ever-increasing number of studies that address the origin and evolution of Euarthropoda - whose extant representatives include chelicerates, myriapods, crustaceans and hexapods - are gradually reaching a consensus with regard to the overall phylogenetic relationships of some of the earliest representatives of this phylum. The stem-lineage of Euarthropoda includes numerous forms that reflect the major morphological transition from a lobopodian-type to a completely arthrodized body organization. Several methods of classification that aim to reflect such a complex evolutionary history have been proposed as a consequence of this taxonomic diversity. Unfortunately, this has also led to a saturation of nomenclatural schemes, often in conflict with each other, some of which are incompatible with cladistic-based methodologies. Here, I review the convoluted terminology associated with the classification of stem-group Euarthropoda, and propose a synapomorphy-based distinction that allows 'lower stem-Euarthropoda' (e.g. lobopodians, radiodontans) to be separated from 'upper stem-Euarthropoda' (e.g. fuxianhuiids, Cambrian bivalved forms) in terms of the structural organization of the head region and other aspects of overall body architecture. The step-wise acquisition of morphological features associated with the origins of the crown-group indicate that the node defining upper stem-Euarthropoda is phylogenetically stable, and supported by numerous synapomorphic characters; these include the presence of a deutocerebral first appendage pair, multisegmented head region with one or more pairs of post-ocular differentiated limbs, complete body arthrodization, posterior-facing mouth associated with the hypostome/labrum complex, and post-oral biramous arthropodized appendages. The name 'Deuteropoda' nov. is proposed for the scion (monophyletic group including the crown-group and an extension of the stem-group) that comprises upper stem-Euarthropoda and Euarthropoda. A brief account of common terminological inaccuracies in recent palaeontological studies evinces the utility of Deuteropoda nov. as a reference point for discussing aspects of early euarthropod phylogeny.

Controversies surrounding segments and parasegments in onychophora: insights from the expression patterns of four "segment polarity genes" in the peripatopsid Euperipatoides rowelli

Arthropods typically show two types of segmentation: the embryonic parasegments and the adult segments that lie out of register with each other. Such a dual nature of body segmentation has not been described from Onychophora, one of the closest arthropod relatives. Hence, it is unclear whether onychophorans have segments, parasegments, or both, and which of these features was present in the last common ancestor of Onychophora and Arthropoda. To address this issue, we analysed the expression patterns of the "segment polarity genes" engrailed, cubitus interruptus, wingless and hedgehog in embryos of the onychophoran Euperipatoides rowelli. Our data revealed that these genes are expressed in repeated sets with a specific anterior-to-posterior order along the body in embryos of E. rowelli. In contrast to arthropods, the expression occurs after the segmental boundaries have formed. Moreover, the initial segmental furrow retains its position within the engrailed domain throughout development, whereas no new furrow is formed posterior to this domain. This suggests that no re-segmentation of the embryo occurs in E. rowelli. Irrespective of whether or not there is a morphological or genetic manifestation of parasegments in Onychophora, our data clearly show that parasegments, even if present, cannot be regarded as the initial metameric units of the onychophoran embryo, because the expression of key genes that define the parasegmental boundaries in arthropods occurs after the segmental boundaries have formed. This is in contrast to arthropods, in which parasegments rather than segments are the initial metameric units of the embryo. Our data further revealed that the expression patterns of "segment polarity genes" correspond to organogenesis rather than segment formation. This is in line with the concept of segmentation as a result of concerted evolution of individual periodic structures rather than with the interpretation of 'segments' as holistic units.

Brain structure resolves the segmental affinity of anomalocaridid appendages

Despite being among the most celebrated taxa from Cambrian biotas, anomalocaridids (order Radiodonta) have provoked intense debate about their affinities within the moulting-animal clade that includes Arthropoda. Current alternatives identify anomalocaridids as either stem-group euarthropods, crown-group euarthropods near the ancestry of chelicerates, or a segmented ecdysozoan lineage with convergent similarity to arthropods in appendage construction. Determining unambiguous affinities has been impeded by uncertainties about the segmental affiliation of anomalocaridid frontal appendages. These structures are variably homologized with jointed appendages of the second (deutocerebral) head segment, including antennae and 'great appendages' of Cambrian arthropods, or with the paired antenniform frontal appendages of living Onychophora and some Cambrian lobopodians. Here we describe Lyrarapax unguispinus, a new anomalocaridid from the early Cambrian Chengjiang biota, southwest China, nearly complete specimens of which preserve traces of muscles, digestive tract and brain. The traces of brain provide the first direct evidence for the segmental composition of the anomalocaridid head and its appendicular organization. Carbon-rich areas in the head resolve paired pre-protocerebral ganglia at the origin of paired frontal appendages. The ganglia connect to areas indicative of a bilateral pre-oral brain that receives projections from the eyestalk neuropils and compound retina. The dorsal, segmented brain of L. unguispinus reinforces an alliance between anomalocaridids and arthropods rather than cycloneuralians. Correspondences in brain organization between anomalocaridids and Onychophora resolve pre-protocerebral ganglia, associated with pre-ocular frontal appendages, as characters of the last common ancestor of euarthropods and onychophorans. A position of Radiodonta on the euarthropod stem-lineage implies the transformation of frontal appendages to another structure in crown-group euarthropods, with gene expression and neuroanatomy providing strong evidence that the paired, pre-oral labrum is the remnant of paired frontal appendages.

Neuronal tracing of oral nerves in a velvet worm-Implications for the evolution of the ecdysozoan brain

As one of the closest relatives of arthropods, Onychophora plays an important role in understanding the evolution of arthropod body plans. Currently there is controversy surrounding the evolution of the brain among the ecdysozoan clades, which shows a collar-shaped, circumoral organization in cycloneuralians but a ganglionic architecture in panarthropods. Based on the innervation pattern of lip papillae surrounding the mouth, the onychophoran brain has been interpreted as a circumoral ring, suggesting that this organization is an ancestral feature of Ecdysozoa. However, this interpretation is inconsistent with other published data. To explore the evolutionary origin of the onychophoran mouth and to shed light on the evolution of the ecdysozoan brains, we analyzed the innervation pattern and morphogenesis of the oral lip papillae in the onychophoran Euperipatoides rowelli using DNA labeling, immunocytochemistry, and neuronal tracing techniques. Our morphogenetic data revealed that the seven paired and one unpaired oral lip papillae arise from three anterior-most body segments. Retrograde fills show that only the first and the third nerves supplying the lip papillae are associated with cell bodies within the brain, whereas the second nerve exclusively receives fibers from somata of peripheral neurons located in the lip papillae. According to our anterograde fills and immunocytochemical data, the first nerve supplies the anterior-most pair of lip papillae, whereas the second and the third nerves are associated with the second to fifth and second to eighth lip papillae, respectively. These data suggest that the lip papillae of E. rowelli are mainly innervated by the proto- and deutocerebrum, whereas there are only a few additional cell bodies situated posterior to the brain. According to these findings, the overall innervation pattern of the oral lip papillae in E. rowelli is incompatible with the interpretation of the onychophoran brain as a modified circumoral ring.

The role of ventral and preventral organs as attachment sites for segmental limb muscles in Onychophora

BACKGROUND

The so-called ventral organs are amongst the most enigmatic structures in Onychophora (velvet worms). They were described as segmental, ectodermal thickenings in the onychophoran embryo, but the same term has also been applied to mid-ventral, cuticular structures in adults, although the relationship between the embryonic and adult ventral organs is controversial. In the embryo, these structures have been regarded as anlagen of segmental ganglia, but recent studies suggest that they are not associated with neural development. Hence, their function remains obscure. Moreover, their relationship to the anteriorly located preventral organs, described from several onychophoran species, is also unclear. To clarify these issues, we studied the anatomy and development of the ventral and preventral organs in several species of Onychophora.

RESULTS

Our anatomical data, based on histology, and light, confocal and scanning electron microscopy in five species of Peripatidae and three species of Peripatopsidae, revealed that the ventral and preventral organs are present in all species studied. These structures are covered externally with cuticle that forms an internal, longitudinal, apodeme-like ridge. Moreover, phalloidin-rhodamine labelling for f-actin revealed that the anterior and posterior limb depressor muscles in each trunk and the slime papilla segment attach to the preventral and ventral organs, respectively. During embryonic development, the ventral and preventral organs arise as large segmental, paired ectodermal thickenings that decrease in size and are subdivided into the smaller, anterior anlagen of the preventral organs and the larger, posterior anlagen of the ventral organs, both of which persist as paired, medially-fused structures in adults. Our expression data of the genes Delta and Notch from embryos of Euperipatoides rowelli revealed that these genes are expressed in two, paired domains in each body segment, corresponding in number, position and size with the anlagen of the ventral and preventral organs.

CONCLUSIONS

Our findings suggest that the ventral and preventral organs are a common feature of onychophorans that serve as attachment sites for segmental limb depressor muscles. The origin of these structures can be traced back in the embryo as latero-ventral segmental, ectodermal thickenings, previously suggested to be associated with the development of the nervous system.

Embryonic neurogenesis in Pseudopallene sp. (Arthropoda, Pycnogonida) includes two subsequent phases with similarities to different arthropod groups

BACKGROUND

Studies on early neurogenesis have had considerable impact on the discussion of the phylogenetic relationships of arthropods, having revealed striking similarities and differences between the major lineages. In Hexapoda and crustaceans, neurogenesis involves the neuroblast, a type of neural stem cell. In each hemi-segment, a set of neuroblasts produces neural cells by repeated asymmetrical and interiorly directed divisions. In Euchelicerata and Myriapoda, neurogenesis lacks neural stem cells, featuring instead direct immigration of neural cell groups from fixed sites in the neuroectoderm. Accordingly, neural stem cells were hitherto assumed to be an evolutionary novelty of the Tetraconata (Hexapoda + crustaceans). To further test this hypothesis, we investigated neurogenesis in Pycnogonida, or sea spiders, a group of marine arthropods with close affinities to euchelicerates.

RESULTS

We studied neurogenesis during embryonic development of Pseudopallene sp. (Callipallenidae), using fluorescent histochemical staining and immunolabelling. Embryonic neurogenesis has two phases. The first phase shows notable similarities to euchelicerates and myriapods. These include i) the lack of morphologically different cell types in the neuroectoderm; ii) the formation of transiently identifiable, stereotypically arranged cell internalization sites; iii) immigration of predominantly post-mitotic ganglion cells; and iv) restriction of tangentially oriented cell proliferation to the apical cell layer. However, in the second phase, the formation of a central invagination in each hemi-neuromere is accompanied by the differentiation of apical neural stem cells. The latter grow in size, show high mitotic activity and an asymmetrical division mode. A marked increase of ganglion cell numbers follows their differentiation. Directly basal to the neural stem cells, an additional type of intermediate neural precursor is found.

CONCLUSIONS

Embryonic neurogenesis of Pseudopallene sp. combines features of central nervous system development that have been hitherto described separately in different arthropod taxa. The two-phase character of pycnogonid neurogenesis calls for a thorough reinvestigation of other non-model arthropods over the entire course of neurogenesis. With the currently available data, a common origin of pycnogonid neural stem cells and tetraconate neuroblasts remains unresolved. To acknowledge this, we present two possible scenarios on the evolution of arthropod neurogenesis, whereby Myriapoda play a key role in the resolution of this issue.

Selective neuronal staining in tardigrades and onychophorans provides insights into the evolution of segmental ganglia in panarthropods

BACKGROUND

Although molecular analyses have contributed to a better resolution of the animal tree of life, the phylogenetic position of tardigrades (water bears) is still controversial, as they have been united alternatively with nematodes, arthropods, onychophorans (velvet worms), or onychophorans plus arthropods. Depending on the hypothesis favoured, segmental ganglia in tardigrades and arthropods might either have evolved independently, or they might well be homologous, suggesting that they were either lost in onychophorans or are a synapomorphy of tardigrades and arthropods. To evaluate these alternatives, we analysed the organisation of the nervous system in three tardigrade species using antisera directed against tyrosinated and acetylated tubulin, the amine transmitter serotonin, and the invertebrate neuropeptides FMRFamide, allatostatin and perisulfakinin. In addition, we performed retrograde staining of nerves in the onychophoran Euperipatoides rowelli in order to compare the serial locations of motor neurons within the nervous system relative to the appendages they serve in arthropods, tardigrades and onychophorans.

RESULTS

Contrary to a previous report from a Macrobiotus species, our immunocytochemical and electron microscopic data revealed contralateral fibres and bundles of neurites in each trunk ganglion of three tardigrade species, including Macrobiotus cf. harmsworthi, Paramacrobiotus richtersi and Hypsibius dujardini. Moreover, we identified additional, extra-ganglionic commissures in the interpedal regions bridging the paired longitudinal connectives. Within the ganglia we found serially repeated sets of serotonin- and RFamid-like immunoreactive neurons. Furthermore, our data show that the trunk ganglia of tardigrades, which include the somata of motor neurons, are shifted anteriorly with respect to each corresponding leg pair, whereas no such shift is evident in the arrangement of motor neurons in the onychophoran nerve cords.

CONCLUSIONS

Taken together, these data reveal three major correspondences between the segmental ganglia of tardigrades and arthropods, including (i) contralateral projections and commissures in each ganglion, (ii) segmentally repeated sets of immunoreactive neurons, and (iii) an anteriorly shifted (parasegmental) position of ganglia. These correspondences support the homology of segmental ganglia in tardigrades and arthropods, suggesting that these structures were either lost in Onychophora or, alternatively, evolved in the tardigrade/arthropod lineage.

Apodemes associated with limbs support serial homology of claws and jaws in Onychophora (velvet worms)

Although the onychophoran jaw blades are believed to be derivatives of foot claws, serial homology of these structures has not been demonstrated. To shed light on the evolutionary origin of the onychophoran jaws, we searched for morphological landmarks and compared the internal and external anatomy of jaws and distal leg portions in representatives of the two major onychophoran subgroups, the Peripatidae and Peripatopsidae. Our data revealed hitherto unknown structures associated with the onychophoran limbs, such as a soft diastemal membrane separating the anterior and posterior portions of the inner jaw blade (present only in Peripatidae), apodemes associated with feet, an eversible dorsal sac at the basis of each foot claw, and a specific arrangement of musculature associated with the sclerotised claws, jaws and their apodemes. Specific correspondences in structure and position of apodemes support serial homology of claws and jaws, suggesting that the onychophoran jaw evolved from the distal portion rather than the entire limb in the last common ancestor of Onychophora.

Cambrian lobopodians and extant onychophorans provide new insights into early cephalization in Panarthropoda

Cambrian lobopodians are important for understanding the evolution of arthropods, but despite their soft-bodied preservation, the organization of the cephalic region remains obscure. Here we describe new material of the early Cambrian lobopodian Onychodictyon ferox from southern China, which reveals hitherto unknown head structures. These include a proboscis with a terminal mouth, an anterior arcuate sclerite, a pair of ocellus-like eyes and branched, antenniform appendages associated with this ocular segment. These findings, combined with a comparison with other lobopodians, suggest that the head of the last common ancestor of fossil lobopodians and extant panarthropods comprized a single ocular segment with a proboscis and terminal mouth. The lack of specialized mouthparts in O. ferox and the involvement of non-homologous mouthparts in onychophorans, tardigrades and arthropods argue against a common origin of definitive mouth openings among panarthropods, whereas the embryonic stomodaeum might well be homologous at least in Onychophora and Arthropoda.

Lobopodians include stem-group arthropods and panarthropods, and date back to the early Cambrian. Ou et al. describe specimens of the early Cambrian lobopodian Onychodictyon ferox, revealing new head structures such as modified appendages, eyes, a terminal mouth and a sucking pharynx.

Neural markers reveal a one-segmented head in tardigrades (water bears)

BACKGROUND

While recent neuroanatomical and gene expression studies have clarified the alignment of cephalic segments in arthropods and onychophorans, the identity of head segments in tardigrades remains controversial. In particular, it is unclear whether the tardigrade head and its enclosed brain comprises one, or several segments, or a non-segmental structure. To clarify this, we applied a variety of histochemical and immunocytochemical markers to specimens of the tardigrade Macrobiotus cf. harmsworthi and the onychophoran Euperipatoides rowelli.

METHODOLOGY/PRINCIPAL FINDINGS

Our immunolabelling against serotonin, FMRFamide and α-tubulin reveals that the tardigrade brain is a dorsal, bilaterally symmetric structure that resembles the brain of onychophorans and arthropods rather than a circumoesophageal ring typical of cycloneuralians (nematodes and allies). A suboesophageal ganglion is clearly lacking. Our data further reveal a hitherto unknown, unpaired stomatogastric ganglion in Macrobiotus cf. harmsworthi, which innervates the ectodermal oesophagus and the endodermal midgut and is associated with the second leg-bearing segment. In contrast, the oesophagus of the onychophoran E. rowelli possesses no immunoreactive neurons, whereas scattered bipolar, serotonin-like immunoreactive cell bodies are found in the midgut wall. Furthermore, our results show that the onychophoran pharynx is innervated by a medullary loop nerve accompanied by monopolar, serotonin-like immunoreactive cell bodies.

CONCLUSIONS/SIGNIFICANCE

A comparison of the nervous system innervating the foregut and midgut structures in tardigrades and onychophorans to that of arthropods indicates that the stomatogastric ganglion is a potential synapomorphy of Tardigrada and Arthropoda. Its association with the second leg-bearing segment in tardigrades suggests that the second trunk ganglion is a homologue of the arthropod tritocerebrum, whereas the first ganglion corresponds to the deutocerebrum. We therefore conclude that the tardigrade brain consists of a single segmental region corresponding to the arthropod protocerebrum and, accordingly, that the tardigrade head is a non-composite, one-segmented structure.

Genome size and chromosome number in velvet worms (Onychophora)

The Onychophora (velvet worms) represents a small group of invertebrates (~180 valid species), which is commonly united with Tardigrada and Arthropoda in a clade called Panarthropoda. As with the majority of invertebrate taxa, genome size data are very limited for the Onychophora, with only one previously published estimate. Here we use both flow cytometry and Feulgen image analysis densitometry to provide genome size estimates for seven species of velvet worms from both major subgroups, Peripatidae and Peripatopsidae, along with karyotype data for each species. Genome sizes in these species range from roughly 5-19 pg, with densitometric estimates being slightly larger than those obtained by flow cytometry for all species. Chromosome numbers range from 2n = 8 to 2n = 54. No relationship is evident between genome size, chromosome number, or reproductive mode. Various avenues for future genomic research are presented based on these results.

Unexplored character diversity in onychophora (velvet worms): A comparative study of three peripatid species

Low character variation among onychophoran species has been an obstacle for taxonomic and phylogenetic studies in the past, however we have identified a number of new and informative characters using morphological, molecular, and chromosomal techniques. Our analyses involved a detailed examination of Epiperipatus biolleyi from Costa Rica, Eoperipatus sp. from Thailand, and a new onychophoran species and genus from Costa Rica, Principapillatus hitoyensisgen. et sp. nov.. Scanning electron microscopy on embryos and specimens of varying age revealed novel morphological characters and character states, including the distribution of different receptor types along the antennae, the arrangement and form of papillae on the head, body and legs, the presence and shape of interpedal structures and fields of modified scales on the ventral body surface, the arrangement of lips around the mouth, the number, position and structure of crural tubercles and anal gland openings, and the presence and shape of embryonic foot projections. Karyotypic analyses revealed differences in the number and size of chromosomes among the species studied. The results of our phylogenetic analyses using mitochondrial COI and 12S rRNA gene sequences are in line with morphological and karyotype data. However, our data show a large number of unexplored, albeit informative, characters in the Peripatidae. We suggest that analysing these characters in additional species would help unravel species diversity and phylogeny in the Onychophora, and that inconsistencies among most diagnostic features used for the peripatid genera in the literature could be addressed by identifying a suite of characters common to all peripatids.

Slit/Robo-mediated axon guidance in Tribolium and Drosophila: divergent genetic programs build insect nervous systems

As the complexity of animal nervous systems has increased during evolution, developmental control of neuronal connectivity has become increasingly refined. How has functional diversification within related axon guidance molecules contributed to the evolution of nervous systems? To address this question, we explore the evolution of functional diversity within the Roundabout (Robo) family of axon guidance receptors. In Drosophila, Robo and Robo2 promote midline repulsion, while Robo2 and Robo3 specify the position of longitudinal axon pathways. The Robo family has expanded by gene duplication in insects; robo2 and robo3 exist as distinct genes only within dipterans, while other insects, like the flour beetle Tribolium castaneum, retain an ancestral robo2/3 gene. Both Robos from Tribolium can mediate midline repulsion in Drosophila, but unlike the fly Robos cannot be down-regulated by Commissureless. The overall architecture and arrangement of longitudinal pathways are remarkably conserved in Tribolium, despite it having only two Robos. Loss of TcSlit causes midline collapse of axons in the beetle, a phenotype recapitulated by simultaneous knockdown of both Robos. Single gene knockdowns reveal that beetle Robos have specialized axon guidance functions: TcRobo is dedicated to midline repulsion, while TcRobo2/3 also regulates longitudinal pathway formation. TcRobo2/3 knockdown reproduces aspects of both Drosophila robo2 and robo3 mutants, suggesting that TcRobo2/3 has two functions that in Drosophila are divided between Robo2 and Robo3. The ability of Tribolium to organize longitudinal axons into three discrete medial-lateral zones with only two Robo receptors demonstrates that beetle and fly achieve equivalent developmental outcomes using divergent genetic programs.

Autofluorescence imaging, an excellent tool for comparative morphology

Here we present a set of methods for documenting (exo-)morphology by applying autofluorescence imaging. For arthropods, but also for other taxa, autofluorescence imaging combined with composite imaging is a fast documentation method with high-resolution capacities. Compared to conventional micro- and macrophotography, the illumination is much more homogenous, and structures are often better contrasted. Applying different wavelengths to the same object can additionally be used to enhance distinct structures. Autofluorescence imaging can be applied to dried and embedded specimens, but also directly on specimens within their storage liquid. This has an enormous potential for the documentation of rare specimens and especially type specimens without the need of preparation. Also for various fossils, autofluorescence can be used to enhance the contrast between the fossil and the matrix significantly, making even smallest details visible. 'Life-colour' fluorescence especially is identified as a technique with great potential. It provides additional information for which otherwise more complex methods would have to be applied. The complete range of differences and variations between fluorescence macrophotography and different types of fluorescence microscopy techniques are here explored and evaluated in detail. Also future improvements are suggested. In summary, autofluorescence imaging is a powerful, easy and fast-to-apply tool for morphological studies.

The origins of the arthropod nervous system: insights from the Onychophora

A revision of evolutionary relationships of the Arthropoda has provided fresh impetus to tracing the origins of the nervous system of this group of animals: other members of the Ecdysozoa possess a markedly different type of nervous system from both the arthropods and the annelid worms, with which they were previously grouped. Given their status as favoured sister taxon of the arthropods, Onychophora (velvet worms) are a key group for understanding the evolutionary changes that have taken place in the panarthropod (Arthropoda + Onychophora + Tardigrada) lineage. This article reviews our current knowledge of the structure and development of the onychophoran nervous system. The picture that emerges from these studies is that the nervous system of the panarthropod ancestor was substantially different from that of modern arthropods: this animal probably possessed a bipartite, rather than a tripartite brain; its nerve cord displayed only a limited degree of segmentation; and neurons were more numerous but more uniform in morphology than in living arthropods. These observations suggest an evolutionary scenario, by which the arthropod nervous system evolved from a system of orthogonally crossing nerve tracts present in both a presumed protostome ancestor and many extant worm-like invertebrates, including the onychophorans.

Expression of collier in the premandibular segment of myriapods: support for the traditional Atelocerata concept or a case of convergence?

BACKGROUND

A recent study on expression and function of the ortholog of the Drosophila collier (col) gene in various arthropods including insects, crustaceans and chelicerates suggested a de novo function of col in the development of the appendage-less intercalary segment of insects. However, this assumption was made on the background of the now widely-accepted Pancrustacea hypothesis that hexapods represent an in-group of the crustaceans. It was therefore assumed that the expression of col in myriapods would reflect the ancestral state like in crustaceans and chelicerates, i.e. absence from the premandibular/intercalary segment and hence no function in its formation.

RESULTS

We find that col in myriapods is expressed at early developmental stages in the same anterior domain in the head, the parasegment 0, as in insects. Comparable early expression of col is not present in the anterior head of an onychophoran that serves as an out-group species closely related to the arthropods.

CONCLUSIONS

Our findings suggest either that i) the function of col in head development has been conserved between insects and myriapods, and that these two classes of arthropods may be closely related supporting the traditional Atelocerata (or Tracheata) hypothesis; or ii) alternatively col function could have been lost in early head development in crustaceans, or may indeed have evolved convergently in insects and myriapods.

A revision of brain composition in Onychophora (velvet worms) suggests that the tritocerebrum evolved in arthropods

Background

The composition of the arthropod head is one of the most contentious issues in animal evolution. In particular, controversy surrounds the homology and innervation of segmental cephalic appendages by the brain. Onychophora (velvet worms) play a crucial role in understanding the evolution of the arthropod brain, because they are close relatives of arthropods and have apparently changed little since the Early Cambrian. However, the segmental origins of their brain neuropils and the number of cephalic appendages innervated by the brain - key issues in clarifying brain composition in the last common ancestor of Onychophora and Arthropoda - remain unclear.

Results

Using immunolabelling and neuronal tracing techniques in the developing and adult onychophoran brain, we found that the major brain neuropils arise from only the anterior-most body segment, and that two pairs of segmental appendages are innervated by the brain. The region of the central nervous system corresponding to the arthropod tritocerebrum is not differentiated as part of the onychophoran brain but instead belongs to the ventral nerve cords.

Conclusions

Our results contradict the assumptions of a tripartite (three-segmented) brain in Onychophora and instead confirm the hypothesis of bipartite (two-segmented) brain composition. They suggest that the last common ancestor of Onychophora and Arthropoda possessed a brain consisting of protocerebrum and deutocerebrum whereas the tritocerebrum evolved in arthropods.

Ecdysozoan mitogenomics: evidence for a common origin of the legged invertebrates, the Panarthropoda

Ecdysozoa is the recently recognized clade of molting animals that comprises the vast majority of extant animal species and the most important invertebrate model organisms--the fruit fly and the nematode worm. Evolutionary relationships within the ecdysozoans remain, however, unresolved, impairing the correct interpretation of comparative genomic studies. In particular, the affinities of the three Panarthropoda phyla (Arthropoda, Onychophora, and Tardigrada) and the position of Myriapoda within Arthropoda (Mandibulata vs. Myriochelata hypothesis) are among the most contentious issues in animal phylogenetics. To elucidate these relationships, we have determined and analyzed complete or nearly complete mitochondrial genome sequences of two Tardigrada, Hypsibius dujardini and Thulinia sp. (the first genomes to date for this phylum); one Priapulida, Halicryptus spinulosus; and two Onychophora, Peripatoides sp. and Epiperipatus biolleyi; and a partial mitochondrial genome sequence of the Onychophora Euperipatoides kanagrensis. Tardigrada mitochondrial genomes resemble those of the arthropods in term of the gene order and strand asymmetry, whereas Onychophora genomes are characterized by numerous gene order rearrangements and strand asymmetry variations. In addition, Onychophora genomes are extremely enriched in A and T nucleotides, whereas Priapulida and Tardigrada are more balanced. Phylogenetic analyses based on concatenated amino acid coding sequences support a monophyletic origin of the Ecdysozoa and the position of Priapulida as the sister group of a monophyletic Panarthropoda (Tardigrada plus Onychophora plus Arthropoda). The position of Tardigrada is more problematic, most likely because of long branch attraction (LBA). However, experiments designed to reduce LBA suggest that the most likely placement of Tardigrada is as a sister group of Onychophora. The same analyses also recover monophyly of traditionally recognized arthropod lineages such as Arachnida and of the highly debated clade Mandibulata.