The urbilaterian brain revisited: novel insights into old questions from new flatworm clades

Brain size scaling through development in the whitelined sphinx moth (Hyles lineata) shows mass and cell number comparable to flies, bees, and wasps

Factors regulating larval growth and determinants of adult body size are described for several holometabolous insects, but less is known about brain size scaling through development. Here we use the isotropic fractionation ("brain soup") method to estimate the number of brain cells and cell density for the whitelined sphinx moth (Lepidoptera: Hyles lineata) from the first instar through the adult stage. We measure mass and brain cell number and find that, during the larval stages, body mass shows an exponential relationship with head width, while the total number of brain cells increases asymptotically. Larval brain cell number increases by a factor of ten from nearly 8000 in the first instar to over 80,000 in the fifth instar. Brain cell number increases by another factor of 10 during metamorphosis, with the adult brain containing more than 900,000 cells. This is similar to increases during development in the vinegar fly (Drosophila melanogaster) and the black soldier fly (Hermetia illucens). The adult brain falls slightly below the brain-to-body allometry for wasps and bees but is comparable in the number of cells per unit brain mass, indicating a general conservation of brain cell density across these divergent lineages.

Circular and unified analysis in network neuroscience

Genuinely new discovery transcends existing knowledge. Despite this, many analyses in systems neuroscience neglect to test new speculative hypotheses against benchmark empirical facts. Some of these analyses inadvertently use circular reasoning to present existing knowledge as new discovery. Here, I discuss that this problem can confound key results and estimate that it has affected more than three thousand studies in network neuroscience over the last decade. I suggest that future studies can reduce this problem by limiting the use of speculative evidence, integrating existing knowledge into benchmark models, and rigorously testing proposed discoveries against these models. I conclude with a summary of practical challenges and recommendations.

Cadherin preserves cohesion across involuting tissues during C. elegans neurulation

The internalization of the central nervous system, termed neurulation in vertebrates, is a critical step in embryogenesis. Open questions remain regarding how force propels coordinated tissue movement during the process, and little is known as to how internalization happens in invertebrates. We show that in C. elegans morphogenesis, apical constriction in the retracting pharynx drives involution of the adjacent neuroectoderm. HMR-1/cadherin mediates this process via inter-tissue attachment, as well as cohesion within the neuroectoderm. Our results demonstrate that HMR-1 is capable of mediating embryo-wide reorganization driven by a centrally located force generator, and indicate a non-canonical use of cadherin on the basal side of an epithelium that may apply to vertebrate neurulation. Additionally, we highlight shared morphology and gene expression in tissues driving involution, which suggests that neuroectoderm involution in C. elegans is potentially homologous with vertebrate neurulation and thus may help elucidate the evolutionary origin of the brain.

Fine structure of the nervous system of Cercaria parvicaudata Stunkard & Shaw, 1931 (Digenea, Renicolidae)

The biology of free-living and parasitic Platyhelminthes is diverse. Taking into account the widespread prevalence of parasitic flatworms, Digenea is the least studied group regarding the fine structure of nervous system especially of the cercarial life stage. Here, we present a description of the fine structure of central nervous system (CNS) and two types of uniciliate sensory papillae of xiphidiocercaria Cercaria parvicaudata (Microphalloidea, Renicolidae). The present study documents that C. parvicaudata has a complex nervous system that includes a well-developed ganglion with a cortex of perikarya and glia-like sheaths, myelin-like structures within one of the dorsal nerve cords and four types of polarized synapses between neurites. Different types of neurons in the CNS could not be distinguished on ultrastructural level due to high similarity in their fine structure. Shared polarized synapses with high electron density of presynaptic components are numerous in the neuropile and nerve cords of this larva. Within the larval body, we detected specialized "support" processes that relate to different tissues. Some "support" processes are also closely related to the nervous system of C. parvicaudata, where they are considered as glia-like structures. In this case, the fine structure of glia-like "support" cells of C. parvicaudata differs from those described as glia-like cells in adult flatworms. We suggest a wide prevalence of glia-like cells among cercariae, as well as the fact that glia-like structures in digenean nervous systems can develop from various nonneuronal tissues.

Drosophila menthol sensitivity and the Precambrian origins of transient receptor potential-dependent chemosensation

Transient receptor potential (TRP) cation channels are highly conserved, polymodal sensors which respond to a wide variety of stimuli. Perhaps most notably, TRP channels serve critical functions in nociception and pain. A growing body of evidence suggests that transient receptor potential melastatin (TRPM) and transient receptor potential ankyrin (TRPA) thermal and electrophile sensitivities predate the protostome-deuterostome split (greater than 550 Ma). However, TRPM and TRPA channels are also thought to detect modified terpenes (e.g. menthol). Although terpenoids like menthol are thought to be aversive and/or harmful to insects, mechanistic sensitivity studies have been largely restricted to chordates. Furthermore, it is unknown if TRP-menthol sensing is as ancient as thermal and/or electrophile sensitivity. Combining genetic, optical, electrophysiological, behavioural and phylogenetic approaches, we tested the hypothesis that insect TRP channels play a conserved role in menthol sensing. We found that topical application of menthol to Drosophila melanogaster larvae elicits a Trpm- and TrpA1-dependent nocifensive rolling behaviour, which requires activation of Class IV nociceptor neurons. Further, in characterizing the evolution of TRP channels, we put forth the hypotheses that three previously undescribed TRPM channel clades (basal, αTRPM and βTRPM), as well as TRPs with residues critical for menthol sensing, were present in ancestral bilaterians. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

The evolution of wetness perception: A comparison of arachnid, insect and human models

Hygroreceptors are a type of humidity sensor that have been identified in several invertebrate classes including Insecta and Arachnida. While their structure has been well researched, the nature of the mechanisms behind their function is debated as being either mechanical, evaporative, or psychrometric in insects and potentially also olfactory in arachnids. There is evidence that can be used to support or oppose each of these concepts, which also invites the possibility of multiple unified mechanisms occurring together. The integration of multiple sensory modalities has also formed the foundation of wetness perception in humans, led by thermal and tactile cues with supplementary information from vision and sound. These inputs are integrated by a vast neural network in the brain, which also occurs on a smaller scale in insects and arachnids. It is possible that as cerebral capacity increased throughout human evolution, this facilitated a preferable system of wetness perception via multisensory integration and rendered hygroreceptors obsolete. While this cerebral development hypothesis is only speculative, it gives a framework for further investigation. Additional research needs to be conducted to correctly classify hygroreceptor types in invertebrates and their relative prevalence before evolutionary associations can be made with vertebrate species. This integratory premise also applies to the human system, as knowing the relative contribution and compounding effects of each sensory modality on wetness perception will aid the overall understanding of the system and help to uncover the evolutionary development pathways underpinning each sense.

The neuromuscular system of Pycnophyes kielensis (Kinorhyncha: Allomalorhagida) investigated by confocal laser scanning microscopy

Background

Kinorhynchs are ecdysozoan animals with a phylogenetic position close to priapulids and loriciferans. To understand the nature of segmentation within Kinorhyncha and to infer a probable ancestry of segmentation within the last common ancestor of Ecdysozoa, the musculature and the nervous system of the allomalorhagid kinorhynch Pycnophyes kielensis were investigated by use of immunohistochemistry, confocal laser scanning microscopy, and 3D reconstruction software.

Results

The kinorhynch body plan comprises 11 trunk segments. Trunk musculature consists of paired ventral and dorsal longitudinal muscles in segments 1–10 as well as dorsoventral muscles in segments 1–11. Dorsal and ventral longitudinal muscles insert on apodemes of the cuticle inside the animal within each segment. Strands of longitudinal musculature extend over segment borders in segments 1–6. In segments 7–10, the trunk musculature is confined to the segments. Musculature of the digestive system comprises a strong pharyngeal bulb with attached mouth cone muscles as well as pharyngeal bulb protractors and retractors. The musculature of the digestive system shows no sign of segmentation. Judged by the size of the pharyngeal bulb protractors and retractors, the pharyngeal bulb, as well as the introvert, is moved passively by internal pressure caused by concerted action of the dorsoventral muscles. The nervous system comprises a neuropil ring anterior to the pharyngeal bulb. Associated with the neuropil ring are flask-shaped serotonergic somata extending anteriorly and posteriorly. A ventral nerve cord is connected to the neuropil ring and runs toward the anterior until an attachment point in segment 1, and from there toward the posterior with one ganglion in segment 6.

Conclusions

Segmentation within Kinorhyncha likely evolved from an unsegmented ancestor. This conclusion is supported by continuous trunk musculature in the anterior segments 1–6, continuous pharyngeal bulb protractors and retractors throughout the anterior segments, no sign of segmentation within the digestive system, and the absence of ganglia in most segments. The musculature shows evidence of segmentation that fit the definition of an anteroposteriorly repeated body unit only in segments 7–10.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0062-6) contains supplementary material, which is available to authorized users.

Genealogical correspondence of a forebrain centre implies an executive brain in the protostome-deuterostome bilaterian ancestor

Orthologous genes involved in the formation of proteins associated with memory acquisition are similarly expressed in forebrain centres that exhibit similar cognitive properties. These proteins include cAMP-dependent protein kinase A catalytic subunit (PKA-Cα) and phosphorylated Ca(2+)/calmodulin-dependent protein kinase II (pCaMKII), both required for long-term memory formation which is enriched in rodent hippocampus and insect mushroom bodies, both implicated in allocentric memory and both possessing corresponding neuronal architectures. Antibodies against these proteins resolve forebrain centres, or their equivalents, having the same ground pattern of neuronal organization in species across five phyla. The ground pattern is defined by olfactory or chemosensory afferents supplying systems of parallel fibres of intrinsic neurons intersected by orthogonal domains of afferent and efferent arborizations with local interneurons providing feedback loops. The totality of shared characters implies a deep origin in the protostome-deuterostome bilaterian ancestor of elements of a learning and memory circuit. Proxies for such an ancestral taxon are simple extant bilaterians, particularly acoels that express PKA-Cα and pCaMKII in discrete anterior domains that can be properly referred to as brains.

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.

Of Fighting Flies, Mice, and Men: Are Some of the Molecular and Neuronal Mechanisms of Aggression Universal in the Animal Kingdom?

Aggressive behavior is widespread in the animal kingdom, but the degree of molecular conservation between distantly related species is still unclear. Recent reports suggest that at least some of the molecular mechanisms underlying this complex behavior in flies show remarkable similarities with such mechanisms in mice and even humans. Surprisingly, some aspects of neuronal control of aggression also show remarkable similarity between these distantly related species. We will review these recent findings, address the evolutionary implications, and discuss the potential impact for our understanding of human diseases characterized by excessive aggression.

Evolution of flatworm central nervous systems: Insights from polyclads

The nervous systems of flatworms have diversified extensively as a consequence of the broad range of adaptations in the group. Here we examined the central nervous system (CNS) of 12 species of polyclad flatworms belonging to 11 different families by morphological and histological studies. These comparisons revealed that the overall organization and architecture of polyclad central nervous systems can be classified into three categories (I, II, and III) based on the presence of globuli cell masses -ganglion cells of granular appearance-, the cross-sectional shape of the main nerve cords, and the tissue type surrounding the nerve cords. In addition, four different cell types were identified in polyclad brains based on location and size. We also characterize the serotonergic and FMRFamidergic nervous systems in the cotylean Boninia divae by immunocytochemistry. Although both neurotransmitters were broadly expressed, expression of serotonin was particularly strong in the sucker, whereas FMRFamide was particularly strong in the pharynx. Finally, we test some of the major hypothesized trends during the evolution of the CNS in the phylum by a character state reconstruction based on current understanding of the nervous system across different species of Platyhelminthes and on up-to-date molecular phylogenies.

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.

Development and organization of the larval nervous system in Phoronopsis harmeri: new insights into phoronid phylogeny

BACKGROUND

The organization and development of the nervous system has traditionally been used as an important character for establishing the relationships among large groups of animals. According to this criterion, phoronids were initially regarded as deuterostomian but have more recently been regarded as protostomian. The resolving of this conflict requires detailed information from poorly investigated members of phoronids, such as Phoronopsis harmeri.

RESULTS

The serotonin-like immunoreactive part of the P. harmeri nervous system changes during larval development. These changes mostly concern the nervous system of the hood and correlate with the appearance of the median and two marginal neurite bundles, the frontal organ, and the sensory field. The apical organ has bilateral symmetry. The tentacular neurite bundle passes under the tentacles, contains several types of perikarya, and gives rise to intertentacular bundles, which branch in the tentacle base and penetrate into adjacent tentacles by two lateroabfrontal bundles. There are two groups of dorsolateral perikarya, which exhibit serotonin-like immunoreactivity, contact the tentacular neurite bundle, and are located near the youngest tentacles. Larvae have a minor nerve ring, which originates from the posterior marginal neurite bundle of the hood, passes above the tentacle base, and gives rise to the mediofrontal neurite bundle in each tentacle. Paired laterofrontal neurite bundles of tentacles form a continuous nerve tract that conducts to the postoral ciliated band.

DISCUSSION

The organization of the nervous system differs among the planktotrophic larvae of phoronid species. These differences may correlate with differences in phoronid biology. Data concerning the innervation of tentacles in different phoronid larvae are conflicting and require careful reinvestigation. The overall organization of the nervous system in phoronid larvae has more in common with the deuterostomian than with the protostomian nervous system. Phoronid larvae demonstrate some "deuterostome-like" features, which are, in fact, have to be ancestral bilaterian characters. Our new results and previous data indicate that phoronids have retained some plesiomorphic features, which were inherited from the last common ancestor of all Bilateria. It follows that phoronids should be extracted from the Trochozoan (=Spiralia) clade and placed at the base of the Lophotrochozoan stem.

Evolution of bilaterian central nervous systems: a single origin?

The question of whether the ancestral bilaterian had a central nervous system (CNS) or a diffuse ectodermal nervous system has been hotly debated. Considerable evidence supports the theory that a CNS evolved just once. However, an alternative view proposes that the chordate CNS evolved from the ectodermal nerve net of a hemichordate-like ancestral deuterostome, implying independent evolution of the CNS in chordates and protostomes. To specify morphological divisions along the anterior/posterior axis, this ancestor used gene networks homologous to those patterning three organizing centers in the vertebrate brain: the anterior neural ridge, the zona limitans intrathalamica and the isthmic organizer, and subsequent evolution of the vertebrate brain involved elaboration of these ancestral signaling centers; however, all or part of these signaling centers were lost from the CNS of invertebrate chordates. The present review analyzes the evidence for and against these theories. The bulk of the evidence indicates that a CNS evolved just once - in the ancestral bilaterian. Importantly, in both protostomes and deuterostomes, the CNS represents a portion of a generally neurogenic ectoderm that is internalized and receives and integrates inputs from sensory cells in the remainder of the ectoderm. The expression patterns of genes involved in medio/lateral (dorso/ventral) patterning of the CNS are similar in protostomes and chordates; however, these genes are not similarly expressed in the ectoderm outside the CNS. Thus, their expression is a better criterion for CNS homologs than the expression of anterior/posterior patterning genes, many of which (for example, Hox genes) are similarly expressed both in the CNS and in the remainder of the ectoderm in many bilaterians. The evidence leaves hemichordates in an ambiguous position - either CNS centralization was lost to some extent at the base of the hemichordates, or even earlier, at the base of the hemichordates + echinoderms, or one of the two hemichordate nerve cords is homologous to the CNS of protostomes and chordates. In any event, the presence of part of the genetic machinery for the anterior neural ridge, the zona limitans intrathalamica and the isthmic organizer in invertebrate chordates together with similar morphology indicates that these organizers were present, at least in part, at the base of the chordates and were probably elaborated upon in the vertebrate lineage.