3D Standard Brain of the Red Flour Beetle Tribolium Castaneum: A Tool to Study Metamorphic Development and Adult Plasticity

Anatomical changes of Tenebrio molitor and Tribolium castaneum during complete metamorphosis

In holometabolous insects, extensive reorganisation of tissues and cells occurs at the pupal stage. The remodelling of the external exoskeleton and internal organs that intervenes during metamorphosis has been traditionally studied in many insect species based on histological or ultrastructural methods. This study demonstrates the use of synchrotron X-ray phase-contrast micro-computed tomography as a powerful, non-destructive tool for in situ morphological observation of anatomical structures at the pupal stage in two Tenebrionid beetles, i.e. Tribolium castaneum and Tenebrio molitor, known as important pests, as well as emerging and promising models in experimental biology. Virtual sections and three-dimensional reconstructions were performed on both males and females at early, intermediate, and late pupal stage. The dataset allowed us to observe the remodelling of the gut and nervous system as well as the shaping of the female and male reproductive system at different pupal ages in both mealworm and red flour beetles. Moreover, we observed that the timing and duration pattern of organ development varied between the species analysed, likely related to the species-specific adaptations of the pre-imaginal stages to environmental conditions, which ultimately affect their life cycle. This research provides new knowledge on the morphological modifications that occur during the pupal stage of holometabolous insects and provides a baseline set of information on beetle metamorphosis that may support future research in forensics, physiology, and ecology as well as an image atlas for educational purposes.

Anatomical study of the red flour beetle using synchrotron radiation X-ray phase-contrast micro-tomography

Synchrotron X-ray phase-contrast microtomography (SR-PhC micro-CT) is well established, fast and non-destructive imaging technique for data acquisition that is currently being used to obtain new insights into insect anatomy and function in physiological, morphological and phylogenetic studies. In this study, we described in situ the internal organs of the red flour beetle Tribolium castaneum Herbst 1797, a widespread pest of cereals and stored food causing serious damage to the human economy. Two-dimensional virtual sections and volumetric reconstructions of the nervous, alimentary and reproductive systems were carried out in both sexes. The results provided a comprehensive overview of the morphological characteristics of this species, such as the different maturation stages of ovarioles and the realistic location, size and shape of internal organs. Given the great interest in this model species in experimental biology and forensic entomology, complete knowledge of the general anatomy is required for future functional applications in pest control and experimental studies. In addition, this study confirms SR-PhC micro-CT as a powerful and innovative tool in entomology, particularly suitable for small species and chitinized structures that are difficult to analyse using conventional dissection and histological methods.

Anatomic and neurochemical analysis of the palpal olfactory system in the red flour beetle Tribolium castaneum, HERBST

The paired antennal lobes were long considered the sole primary processing centers of the olfactory pathway in holometabolous insects receiving input from the olfactory sensory neurons of the antennae and mouthparts. In hemimetabolous insects, however, olfactory cues of the antennae and palps are processed separately. For the holometabolous red flour beetle Tribolium castaneum, we could show that primary processing of the palpal and antennal olfactory input also occurs separately and at distinct neuronal centers. While the antennal olfactory sensory neurons project into the antennal lobes, those of the palps project into the paired glomerular lobes and the unpaired gnathal olfactory center. Here we provide an extended analysis of the palpal olfactory pathway by combining scanning electron micrographs with confocal imaging of immunohistochemical staining and reporter expression identifying chemosensory and odorant receptor-expressing neurons in the palpal sensilla. In addition, we extended the anatomical characterization of the gnathal olfactory center by 3D reconstructions and investigated the distribution of several neuromediators. The similarities in the neuromediator repertoire between antennal lobes, glomerular lobes, and gnathal olfactory center underline the role of the latter two as additional primary olfactory processing centers.

The red flour beetle T. castaneum: elaborate genetic toolkit and unbiased large scale RNAi screening to study insect biology and evolution

The red flour beetle Tribolium castaneum has emerged as an important insect model system for a variety of topics. With respect to studying gene function, it is second only to the vinegar fly D. melanogaster. The RNAi response in T. castaneum is exceptionally strong and systemic, and it appears to target all cell types and processes. Uniquely for emerging model organisms, T. castaneum offers the opportunity of performing time- and cost-efficient large-scale RNAi screening, based on commercially available dsRNAs targeting all genes, which are simply injected into the body cavity. Well established transgenic and genome editing approaches are met by ease of husbandry and a relatively short generation time. Consequently, a number of transgenic tools like UAS/Gal4, Cre/Lox, imaging lines and enhancer trap lines are already available. T. castaneum has been a genetic experimental system for decades and now has become a workhorse for molecular and reverse genetics as well as in vivo imaging. Many aspects of development and general biology are more insect-typical in this beetle compared to D. melanogaster. Thus, studying beetle orthologs of well-described fly genes has allowed macro-evolutionary comparisons in developmental processes such as axis formation, body segmentation, and appendage, head and brain development. Transgenic approaches have opened new ways for in vivo imaging. Moreover, this emerging model system is the first choice for research on processes that are not represented in the fly, or are difficult to study there, e.g. extraembryonic tissues, cryptonephridial organs, stink gland function, or dsRNA-based pesticides.

An atlas of the developing Tribolium castaneum brain reveals conservation in anatomy and divergence in timing to Drosophila melanogaster

Insect brains are formed by conserved sets of neural lineages whose fibers form cohesive bundles with characteristic projection patterns. Within the brain neuropil, these bundles establish a system of fascicles constituting the macrocircuitry of the brain. The overall architecture of the neuropils and the macrocircuitry appear to be conserved. However, variation is observed, for example, in size, shape, and timing of development. Unfortunately, the developmental and genetic basis of this variation is poorly understood, although the rise of new genetically tractable model organisms such as the red flour beetle Tribolium castaneum allows the possibility to gain mechanistic insights. To facilitate such work, we present an atlas of the developing brain of T. castaneum, covering the first larval instar, the prepupal stage, and the adult, by combining wholemount immunohistochemical labeling of fiber bundles (acetylated tubulin) and neuropils (synapsin) with digital 3D reconstruction using the TrakEM2 software package. Upon comparing this anatomical dataset with the published work in Drosophila melanogaster, we confirm an overall high degree of conservation. Fiber tracts and neuropil fascicles, which can be visualized by global neuronal antibodies like antiacetylated tubulin in all invertebrate brains, create a rich anatomical framework to which individual neurons or other regions of interest can be referred to. The framework of a largely conserved pattern allowed us to describe differences between the two species with respect to parameters such as timing of neuron proliferation and maturation. These features likely reflect adaptive changes in developmental timing that govern the change from larval to adult brain.

Shaking hands is a homeodomain transcription factor that controls axon outgrowth of central complex neurons in the insect model Tribolium

Gene regulatory mechanisms that specify subtype identity of central complex (CX) neurons are the subject of intense investigation. The CX is a compartment within the brain common to all insect species and functions as a ‘command center’ that directs motor actions. It is made up of several thousand neurons, with more than 60 morphologically distinct identities. Accordingly, transcriptional programs must effect the specification of at least as many neuronal subtypes. We demonstrate a role for the transcription factor Shaking hands (Skh) in the specification of embryonic CX neurons in Tribolium. The developmental dynamics of skh expression are characteristic of terminal selectors of subtype identity. In the embryonic brain, skh expression is restricted to a subset of neurons, many of which survive to adulthood and contribute to the mature CX. skh expression is maintained throughout the lifetime in at least some CX neurons. skh knockdown results in axon outgrowth defects, thus preventing the formation of an embryonic CX primordium. The previously unstudied Drosophila skh shows a similar embryonic expression pattern, suggesting that subtype specification of CX neurons may be conserved.

Summary: A detailed examination of the developmental expression of the homeodomain transcription factor Shaking hands in Tribolium reveals a role in the formation of the central complex primordium.

A unified platform to manage, share, and archive morphological and functional data in insect neuroscience

Insect neuroscience generates vast amounts of highly diverse data, of which only a small fraction are findable, accessible and reusable. To promote an open data culture, we have therefore developed the InsectBrainDatabase (IBdb), a free online platform for insect neuroanatomical and functional data. The IBdb facilitates biological insight by enabling effective cross-species comparisons, by linking neural structure with function, and by serving as general information hub for insect neuroscience. The IBdb allows users to not only effectively locate and visualize data, but to make them widely available for easy, automated reuse via an application programming interface. A unique private mode of the database expands the IBdb functionality beyond public data deposition, additionally providing the means for managing, visualizing, and sharing of unpublished data. This dual function creates an incentive for data contribution early in data management workflows and eliminates the additional effort normally associated with publicly depositing research data.

Insect neuroscience, like any field in the natural sciences, generates vast amounts of data. Currently, only a fraction are publicly available, and even less are reusable. This is because insect neuroscience data come in many formats and from many species. Some experiments focus on what insect brains look like (morphology), while others focus on how insect brains work (function). Some data come in the form of high-speed video, while other data contain voltage traces from individual neurons. Sharing is not as simple as uploading the raw files to the internet.

To get a clear picture of how insect brains work, researchers need a way to cross-reference and connect different experiments. But, as it stands, there is no dedicated place for insect neuroscientists to share and explore such a diverse body of work. The community needs an open data repository that can link different types of data across many species, and can evolve as more data become available. Above all, this repository needs to be easy for researchers to use.

To meet these specifications, Heinze et al. developed the Insect Brain Database. The database organizes data into three categories: species, brain structures, and neuron types. Within these categories, each entry has its own profile page. These pages bring different experiments together under one heading, allowing researchers to combine and compare data of different types. As researchers add more experiments, the profile pages will grow and evolve. To make the data easy to navigate, Heinze et al. developed a visual search tool. A combination of 2D and 3D images allow users to explore the data by anatomical location, without the need for expert knowledge. Researchers also have the option to upload their work in private mode, allowing them to securely share unpublished data.

The Insect Brain Database brings data together in a way that is accessible not only to researchers, but also to students, and non-scientists. It will help researchers to find related work, to reuse existing data, and to build an open data culture. This has the potential to drive new discoveries combining research across the whole of the insect neuroscience field.

Metamorphic development of the olfactory system in the red flour beetle (Tribolium castaneum, HERBST)

BACKGROUND

Insects depend on their olfactory sense as a vital system. Olfactory cues are processed by a rather complex system and translated into various types of behavior. In holometabolous insects like the red flour beetle Tribolium castaneum, the nervous system typically undergoes considerable remodeling during metamorphosis. This process includes the integration of new neurons, as well as remodeling and elimination of larval neurons.

RESULTS

We find that the sensory neurons of the larval antennae are reused in the adult antennae. Further, the larval antennal lobe gets transformed into its adult version. The beetle's larval antennal lobe is already glomerularly structured, but its glomeruli dissolve in the last larval stage. However, the axons of the olfactory sensory neurons remain within the antennal lobe volume. The glomeruli of the adult antennal lobe then form from mid-metamorphosis independently of the presence of a functional OR/Orco complex but mature dependent on the latter during a postmetamorphic phase.

CONCLUSIONS

We provide insights into the metamorphic development of the red flour beetle's olfactory system and compared it to data on Drosophila melanogaster, Manduca sexta, and Apis mellifera. The comparison revealed that some aspects, such as the formation of the antennal lobe's adult glomeruli at mid-metamorphosis, are common, while others like the development of sensory appendages or the role of Orco seemingly differ.

A micro-CT-based standard brain atlas of the bumblebee

In recent years, bumblebees have become a prominent insect model organism for a variety of biological disciplines, particularly to investigate learning behaviors as well as visual performance. Understanding these behaviors and their underlying neurobiological principles requires a clear understanding of brain anatomy. Furthermore, to be able to compare neuronal branching patterns across individuals, a common framework is required, which has led to the development of 3D standard brain atlases in most of the neurobiological insect model species. Yet, no bumblebee 3D standard brain atlas has been generated. Here we present a brain atlas for the buff-tailed bumblebee Bombus terrestris using micro-computed tomography (micro-CT) scans as a source for the raw data sets, rather than traditional confocal microscopy, to produce the first ever micro-CT-based insect brain atlas. We illustrate the advantages of the micro-CT technique, namely, identical native resolution in the three cardinal planes and 3D structure being better preserved. Our Bombus terrestris brain atlas consists of 30 neuropils reconstructed from ten individual worker bees, with micro-CT allowing us to segment neuropils completely intact, including the lamina, which is a tissue structure often damaged when dissecting for immunolabeling. Our brain atlas can serve as a platform to facilitate future neuroscience studies in bumblebees and illustrates the advantages of micro-CT for specific applications in insect neuroanatomy.

A digital 3D reference atlas reveals cellular growth patterns shaping the Arabidopsis ovule

A fundamental question in biology is how morphogenesis integrates the multitude of processes that act at different scales, ranging from the molecular control of gene expression to cellular coordination in a tissue. Using machine-learning-based digital image analysis, we generated a three-dimensional atlas of ovule development in Arabidopsis thaliana, enabling the quantitative spatio-temporal analysis of cellular and gene expression patterns with cell and tissue resolution. We discovered novel morphological manifestations of ovule polarity, a new mode of cell layer formation, and previously unrecognized subepidermal cell populations that initiate ovule curvature. The data suggest an irregular cellular build-up of WUSCHEL expression in the primordium and new functions for INNER NO OUTER in restricting nucellar cell proliferation and the organization of the interior chalaza. Our work demonstrates the analytical power of a three-dimensional digital representation when studying the morphogenesis of an organ of complex architecture that eventually consists of 1900 cells.

Coding of odour and space in the hemimetabolous insect Periplaneta americana

The general architecture of the olfactory system is highly conserved from insects to humans, but neuroanatomical and physiological differences can be observed across species. The American cockroach, inhabiting dark shelters with a rather stable olfactory landscape, is equipped with long antennae used for sampling the surrounding air-space for orientation and navigation. The antennae's exceptional length provides a wide spatial working range for odour detection; however, it is still largely unknown whether and how this is also used for mapping the structure of the olfactory environment. By selectively labelling antennal lobe projection neurons with a calcium-sensitive dye, we investigated the logic of olfactory coding in this hemimetabolous insect. We show that odour responses are stimulus specific and concentration dependent, and that structurally related odorants evoke physiologically similar responses. By using spatially confined stimuli, we show that proximal stimulations induce stronger and faster responses than distal ones. Spatially confined stimuli of the female pheromone periplanone B activate a subregion of the male macroglomerulus. Thus, we report that the combinatorial logic of odour coding deduced from holometabolous insects applies also to this hemimetabolous species. Furthermore, a fast decrease in sensitivity along the antenna, not supported by a proportionate decrease in sensillar density, suggests a neural architecture that strongly emphasizes neuronal inputs from the proximal portion of the antenna.

Brain Organization of Apolygus lucorum: A Hemipteran Species With Prominent Antennal Lobes

The anatomical organization of distinct regions in the insect brain often reflects their functions. In the present study, the brain structure of Apolygus lucorum was examined by using immunolabeling and three-dimensional reconstruction. The results revealed the location and volume of prominent neuropils, such as the antennal lobes (AL), optic lobes (OL), anterior optic tubercles (AOTU), central body (CB), lateral accessory lobes (LAL), mushroom lobes, and distinct tritocerebral neuropils. As expected, this brain is similar to that of other insects. One exception, however, is that the antennal lobes were found to be the most prominent neuropils. Their size relative to the entire brain is the largest among all insect species studied so far. In contrast, the calyx, a region getting direct input from the antennal lobe, has a smaller size relative to the brain than that of other species. These findings may suggest that olfaction plays an essential role for A. lucorum.

Odorant receptors and antennal lobe morphology offer a new approach to understanding olfaction in the Asian longhorned beetle

The Asian longhorned beetle Anoplophora glabripennis (Motchulsky) is an exotic forest pest that has repeatedly invaded North America and Europe from Asia, and has the potential to kill millions of trees and cause billions of dollars in damage. Traps baited with an attractive mixture of volatile organic compounds from hosts have been of limited success in monitoring invasion sites. We propose that lures might be improved through studying the olfactory system of adult beetles, especially the gene family of odorant receptors (ORs) and the structure of the antennal lobes of the brain. Here, we report identification of 132 ORs in the genome of A. glabripennis (inclusive of one Orco gene and 11 pseudogenes), some of which are orthologous to known pheromone receptors of other cerambycid beetles. We also identified three ORs that are strongly biased toward expression in the female transcriptome, and a single OR strongly biased toward males. Three-dimensional reconstruction of the antennal lobes of adults suggested a male-specific macroglomerulus and several enlarged glomeruli in females. We predict that functional characterization of ORs and glomeruli will lead to identification of key odorants in the life history of A. glabripennis that may aid in monitoring and controlling future invasions.

Variations on a Theme: Antennal Lobe Architecture across Coleoptera

Beetles comprise about 400,000 described species, nearly one third of all known animal species. The enormous success of the order Coleoptera is reflected by a rich diversity of lifestyles, behaviors, morphological, and physiological adaptions. All these evolutionary adaptions that have been driven by a variety of parameters over the last about 300 million years, make the Coleoptera an ideal field to study the evolution of the brain on the interface between the basic bauplan of the insect brain and the adaptions that occurred. In the current study we concentrated on the paired antennal lobes (AL), the part of the brain that is typically responsible for the first processing of olfactory information collected from olfactory sensilla on antenna and mouthparts. We analyzed 63 beetle species from 22 different families and thus provide an extensive comparison of principal neuroarchitecture of the AL. On the examined anatomical level, we found a broad diversity including AL containing a wide range of glomeruli numbers reaching from 50 to 150 glomeruli and several species with numerous small glomeruli, resembling the microglomerular design described in acridid grasshoppers and diving beetles, and substructures within the glomeruli that have to date only been described for the small hive beetle, Aethina tumida. A first comparison of the various anatomical features of the AL with available descriptions of lifestyle and behaviors did so far not reveal useful correlations. In summary, the current study provides a solid basis for further studies to unravel mechanisms that are basic to evolutionary adaptions of the insect olfactory system.

Morphological and Transcriptomic Analysis of a Beetle Chemosensory System Reveals a Gnathal Olfactory Center

BACKGROUND

The red flour beetle Tribolium castaneum is an emerging insect model organism representing the largest insect order, Coleoptera, which encompasses several serious agricultural and forest pests. Despite the ecological and economic importance of beetles, most insect olfaction studies have so far focused on dipteran, lepidopteran, or hymenopteran systems.

RESULTS

Here, we present the first detailed morphological description of a coleopteran olfactory pathway in combination with genome-wide expression analysis of the relevant gene families involved in chemoreception. Our study revealed that besides the antennae, also the mouthparts are highly involved in olfaction and that their respective contribution is processed separately. In this beetle, olfactory sensory neurons from the mouthparts project to the lobus glomerulatus, a structure so far only characterized in hemimetabolous insects, as well as to a so far non-described unpaired glomerularly organized olfactory neuropil in the gnathal ganglion, which we term the gnathal olfactory center. The high number of functional odorant receptor genes expressed in the mouthparts also supports the importance of the maxillary and labial palps in olfaction of this beetle. Moreover, gustatory perception seems equally distributed between antenna and mouthparts, since the number of expressed gustatory receptors is similar for both organs.

CONCLUSIONS

Our analysis of the T. castaneum chemosensory system confirms that olfactory and gustatory perception are not organotopically separated to the antennae and mouthparts, respectively. The identification of additional olfactory processing centers, the lobus glomerulatus and the gnathal olfactory center, is in contrast to the current picture that in holometabolous insects all olfactory inputs allegedly converge in the antennal lobe. These findings indicate that Holometabola have evolved a wider variety of solutions to chemoreception than previously assumed.

The insect central complex as model for heterochronic brain development-background, concepts, and tools

The adult insect brain is composed of neuropils present in most taxa. However, the relative size, shape, and developmental timing differ between species. This diversity of adult insect brain morphology has been extensively described while the genetic mechanisms of brain development are studied predominantly in Drosophila melanogaster. However, it has remained enigmatic what cellular and genetic mechanisms underlie the evolution of neuropil diversity or heterochronic development. In this perspective paper, we propose a novel approach to study these questions. We suggest using genome editing to mark homologous neural cells in the fly D. melanogaster, the beetle Tribolium castaneum, and the Mediterranean field cricket Gryllus bimaculatus to investigate developmental differences leading to brain diversification. One interesting aspect is the heterochrony observed in central complex development. Ancestrally, the central complex is formed during embryogenesis (as in Gryllus) but in Drosophila, it arises during late larval and metamorphic stages. In Tribolium, it forms partially during embryogenesis. Finally, we present tools for brain research in Tribolium including 3D reconstruction and immunohistochemistry data of first instar brains and the generation of transgenic brain imaging lines. Further, we characterize reporter lines labeling the mushroom bodies and reflecting the expression of the neuroblast marker gene Tc-asense, respectively.

Morphological characterization of the antennal lobes in the Mediterranean fruit fly Ceratitis capitata

The medfly Ceratitis capitata is one of the most important pests for horticulture worldwide. The knowledge about anatomy and function of the medfly olfactory system is still limited. The first brain structure to process olfactory information in insects is the antennal lobe (AL), which is composed of its functional and morphological units, the olfactory glomeruli. Here, we present a morphological three-dimensional reconstruction of AL glomeruli in adult brains. We used unilateral antennal backfills of olfactory receptor neurons (ORNs) with neural tracers, revealing the AL structure. We recorded confocal stacks acquired from whole-mount specimens, and analyzed them with the software AMIRA. The ALs in C. capitata are organized in glomeruli which are more tightly packed in the anterior part than the posterior one. Axons of ORNs bilaterally connect the ALs through a commissure between the two ALs. This commissure is formed by several distinct fascicles. Contralateral dye transfer suggests the presence of gap junctions connecting ORNs from both antennae. There was no statistical difference between the average volumes of female ALs (204,166 ± 12,554 μm(3)) and of male ALs (190,287 ± 11,823 μm(3)). In most specimens, we counted 53 glomeruli in each AL, seven of which were sexually dimorphic in size.

Functional brain regeneration in the acoel worm Symsagittifera roscoffensis

The ability of some animals to regrow their head and brain after decapitation provides a striking example of the regenerative capacity within the animal kingdom. The acoel worm Symsagittifera roscoffensis can regrow its head, brain and sensory head organs within only a few weeks after decapitation. How rapidly and to what degree it also reacquires its functionality to control behavior however remains unknown. We provide here a neuroanatomical map of the brain neuropils of the adult S. roscoffensis and show that after decapitation a normal neuroanatomical organization of the brain is restored in the majority of animals. By testing different behaviors we further show that functionality of both sensory perception and the underlying brain architecture are restored within weeks after decapitation. Interestingly not all behaviors are restored at the same speed and to the same extent. While we find that phototaxis recovered rapidly, geotaxis is not restored within 7 weeks. Our findings show that regeneration of the head, sensory organs and brain result in the restoration of directed navigation behavior, suggesting a tight coordination in the regeneration of certain sensory organs with that of their underlying neural circuits. Thus, at least in S. roscoffensis, the regenerative capacity of different sensory modalities follows distinct paths.

Summary: Brain and head regeneration in the acoel Symsagittifera roscoffensis is coordinated with restoration of directed navigation behavior, suggesting that the regenerative capacity of different sensory modalities follows distinct paths. 

A clearer view of the insect brain-combining bleaching with standard whole-mount immunocytochemistry allows confocal imaging of pigment-covered brain areas for 3D reconstruction

In the study of insect neuroanatomy, three-dimensional (3D) reconstructions of neurons and neuropils have become a standard technique. As images have to be obtained from whole-mount brain preparations, pigmentation on the brain surface poses a serious challenge to imaging. In insects, this is a major problematic in the first visual neuropil of the optic lobe, the lamina, which is obstructed by the pigment of the retina as well as by the pigmented fenestration layer. This has prevented inclusion of this major processing center of the insect visual system into most neuroanatomical brain atlases and hinders imaging of neurons within the lamina by confocal microscopy. It has recently been shown that hydrogen peroxide bleaching is compatible with immunohistochemical labeling in insect brains, and we therefore developed a simple technique for removal of pigments on the surface of insect brains by chemical bleaching. We show that our technique enables imaging of the pigment-obstructed regions of insect brains when combined with standard protocols for both anti-synapsin-labeled as well as neurobiotin-injected samples. This method can be combined with different fixation procedures, as well as different fluorophore excitation wavelengths without negative effects on staining quality. It can therefore serve as an effective addition to most standard histology protocols used in insect neuroanatomy.

Object-based representation and analysis of light and electron microscopic volume data using Blender

Background

Rapid improvements in light and electron microscopy imaging techniques and the development of 3D anatomical atlases necessitate new approaches for the visualization and analysis of image data. Pixel-based representations of raw light microscopy data suffer from limitations in the number of channels that can be visualized simultaneously. Complex electron microscopic reconstructions from large tissue volumes are also challenging to visualize and analyze.

Results

Here we exploit the advanced visualization capabilities and flexibility of the open-source platform Blender to visualize and analyze anatomical atlases. We use light-microscopy-based gene expression atlases and electron microscopy connectome volume data from larval stages of the marine annelid Platynereis dumerilii. We build object-based larval gene expression atlases in Blender and develop tools for annotation and coexpression analysis. We also represent and analyze connectome data including neuronal reconstructions and underlying synaptic connectivity.

Conclusions

We demonstrate the power and flexibility of Blender for visualizing and exploring complex anatomical atlases. The resources we have developed for Platynereis will facilitate data sharing and the standardization of anatomical atlases for this species. The flexibility of Blender, particularly its embedded Python application programming interface, means that our methods can be easily extended to other organisms.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-015-0652-7) contains supplementary material, which is available to authorized users.

Brain composition in Godyris zavaleta, a diurnal butterfly, Reflects an increased reliance on olfactory information

Interspecific comparisons of brain structure can inform our functional understanding of brain regions, identify adaptations to species-specific ecologies, and explore what constrains adaptive changes in brain structure, and coevolution between functionally related structures. The value of such comparisons is enhanced when the species considered have known ecological differences. The Lepidoptera have long been a favored model in evolutionary biology, but to date descriptions of brain anatomy have largely focused on a few commonly used neurobiological model species. We describe the brain of Godyris zavaleta (Ithomiinae), a member of a subfamily of Neotropical butterflies with enhanced reliance on olfactory information. We demonstrate for the first time the presence of sexually dimorphic glomeruli within a distinct macroglomerular complex (MGC) in the antennal lobe of a diurnal butterfly. This presents a striking convergence with the well-known moth MGC, prompting a discussion of the potential mechanisms behind the independent evolution of specialized glomeruli. Interspecific analyses across four Lepidoptera further show that the relative size of sensory neuropils closely mirror interspecific variation in sensory ecology, with G. zavaleta displaying levels of sensory investment intermediate between the diurnal monarch butterfly (Danaus plexippus), which invests heavily in visual neuropil, and night-flying moths, which invest more in olfactory neuropil. We identify several traits that distinguish butterflies from moths, and several that distinguish D. plexippus and G. zavaleta. Our results illustrate that ecological selection pressures mold the structure of invertebrate brains, and exemplify how comparative analyses across ecologically divergent species can illuminate the functional significance of variation in brain structure.

Potential and limitations of X-Ray micro-computed tomography in arthropod neuroanatomy: a methodological and comparative survey

Classical histology or immunohistochemistry combined with fluorescence or confocal laser scanning microscopy are common techniques in arthropod neuroanatomy, and these methods often require time-consuming and difficult dissections and sample preparations. Moreover, these methods are prone to artifacts due to compression and distortion of tissues, which often result in information loss and especially affect the spatial relationships of the examined parts of the nervous system in their natural anatomical context. Noninvasive approaches such as X-ray micro-computed tomography (micro-CT) can overcome such limitations and have been shown to be a valuable tool for understanding and visualizing internal anatomy and structural complexity. Nevertheless, knowledge about the potential of this method for analyzing the anatomy and organization of nervous systems, especially of taxa with smaller body size (e.g., many arthropods), is limited. This study set out to analyze the brains of selected arthropods with micro-CT, and to compare these results with available histological and immunohistochemical data. Specifically, we explored the influence of different sample preparation procedures. Our study shows that micro-CT is highly suitable for analyzing arthropod neuroarchitecture in situ and allows specific neuropils to be distinguished within the brain to extract quantitative data such as neuropil volumes. Moreover, data acquisition is considerably faster compared with many classical histological techniques. Thus, we conclude that micro-CT is highly suitable for targeting neuroanatomy, as it reduces the risk of artifacts and is faster than classical techniques.

A map of brain neuropils and fiber systems in the ant Cardiocondyla obscurior

A wide spectrum of occupied ecological niches and spectacular morphological adaptations make social insects a prime object for comparative neuroanatomical studies. Eusocial insects have evolved complex societies based on caste polyphenism. A diverse behavioral repertoire of morphologically distinct castes of the same species requires a high degree of plasticity in the central nervous system. We have analyzed the central brain neuropils and fiber tract systems of the worker of the ant Cardiocondylaobscurior, a model for the study of social traits. Our analysis is based on whole mount preparations of adult brains labeled with an antibody against Drosophila-Synapsin, which cross-reacts strongly with synapses in Cardiocondyla. Neuropil compartments stand out as domains with a certain texture and intensity of the anti-Synapsin signal. By contrast, fiber tracts, which are composed of bundles of axons accompanied by glia and are devoid of synapses, appear as channels or sheaths with low anti-Synapsin signal. We have generated a digital 3D atlas of the Cardiocondyla brain neuropil. The atlas provides a reference for future studies of brain polymorphisms in distinct castes, brain development or localization of neurotransmitter systems.

Tissue-specific transcriptomics, chromosomal localization, and phylogeny of chemosensory and odorant binding proteins from the red flour beetle Tribolium castaneum reveal subgroup specificities for olfaction or more general functions

Background

Chemoreception is based on the senses of smell and taste that are crucial for animals to find new food sources, shelter, and mates. The initial step in olfaction involves the translocation of odorants from the periphery through the aqueous lymph of the olfactory sensilla to the odorant receptors most likely by chemosensory proteins (CSPs) or odorant binding proteins (OBPs).

Results

To better understand the roles of CSPs and OBPs in a coleopteran pest species, the red flour beetle Tribolium castaneum (Coleoptera, Tenebrionidae), we performed transcriptome analyses of male and female antennae, heads, mouthparts, legs, and bodies, which revealed that all 20 CSPs and 49 of the 50 previously annotated OBPs are transcribed. Only six of the 20 CSP are significantly transcriptionally enriched in the main chemosensory tissues (antenna and/or mouthparts), whereas of the OBPs all eight members of the antenna binding proteins II (ABPII) subgroup, 18 of the 20 classic OBP subgroup, the C + OBP, and only five of the 21 C-OBPs show increased chemosensory tissue expression. By MALDI-TOF-TOF MS protein fingerprinting, we confirmed three CSPs, four ABPIIs, three classic OBPs, and four C-OBPs in the antennae.

Conclusions

Most of the classic OBPs and all ABPIIs are likely involved in chemoreception. A few are also present in other tissues such as odoriferous glands and testes and may be involved in release or transfer of chemical signals. The majority of the CSPs as well as the C-OBPs are not enriched in antennae or mouthparts, suggesting a more general role in the transport of hydrophobic molecules.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1141) contains supplementary material, which is available to authorized users.

Controlling feeding behavior by chemical or gene-directed targeting in the brain: what's so spatial about our methods?

Intracranial chemical injection (ICI) methods have been used to identify the locations in the brain where feeding behavior can be controlled acutely. Scientists conducting ICI studies often document their injection site locations, thereby leaving kernels of valuable location data for others to use to further characterize feeding control circuits. Unfortunately, this rich dataset has not yet been formally contextualized with other published neuroanatomical data. In particular, axonal tracing studies have delineated several neural circuits originating in the same areas where ICI injection feeding-control sites have been documented, but it remains unclear whether these circuits participate in feeding control. Comparing injection sites with other types of location data would require careful anatomical registration between the datasets. Here, a conceptual framework is presented for how such anatomical registration efforts can be performed. For example, by using a simple atlas alignment tool, a hypothalamic locus sensitive to the orexigenic effects of neuropeptide Y (NPY) can be aligned accurately with the locations of neurons labeled by anterograde tracers or those known to express NPY receptors or feeding-related peptides. This approach can also be applied to those intracranial "gene-directed" injection (IGI) methods (e.g., site-specific recombinase methods, RNA expression or interference, optogenetics, and pharmacosynthetics) that involve viral injections to targeted neuronal populations. Spatial alignment efforts can be accelerated if location data from ICI/IGI methods are mapped to stereotaxic brain atlases to allow powerful neuroinformatics tools to overlay different types of data in the same reference space. Atlas-based mapping will be critical for community-based sharing of location data for feeding control circuits, and will accelerate our understanding of structure-function relationships in the brain for mammalian models of obesity and metabolic disorders.

TrOn: an anatomical ontology for the beetle Tribolium castaneum

In a morphological ontology the expert's knowledge is represented in terms, which describe morphological structures and how these structures relate to each other. With the assistance of ontologies this expert knowledge is made processable by machines, through a formal and standardized representation of terms and their relations to each other. The red flour beetle Tribolium castaneum, a representative of the most species rich animal taxon on earth (the Coleoptera), is an emerging model organism for development, evolution, physiology, and pest control. In order to foster Tribolium research, we have initiated the Tribolium Ontology (TrOn), which describes the morphology of the red flour beetle. The content of this ontology comprises so far most external morphological structures as well as some internal ones. All modeled structures are consistently annotated for the developmental stages larva, pupa and adult. In TrOn all terms are grouped into three categories: Generic terms represent morphological structures, which are independent of a developmental stage. In contrast, downstream of such terms are concrete terms which stand for a dissectible structure of a beetle at a specific life stage. Finally, there are mixed terms describing structures that are only found at one developmental stage. These terms combine the characteristics of generic and concrete terms with features of both. These annotation principles take into account the changing morphology of the beetle during development and provide generic terms to be used in applications or for cross linking with other ontologies and data resources. We use the ontology for implementing an intuitive search function at the electronic iBeetle-Base, which stores morphological defects found in a genome wide RNA interference (RNAi) screen. The ontology is available for download at http://ibeetle-base.uni-goettingen.de.

Postembryonic lineages of the Drosophila brain: I. Development of the lineage-associated fiber tracts

Neurons of the Drosophila central brain fall into approximately 100 paired groups, termed lineages. Each lineage is derived from a single asymmetrically-dividing neuroblast. Embryonic neuroblasts produce 1,500 primary neurons (per hemisphere) that make up the larval CNS followed by a second mitotic period in the larva that generates approximately 10,000 secondary, adult-specific neurons. Clonal analyses based on previous works using lineage-specific Gal4 drivers have established that such lineages form highly invariant morphological units. All neurons of a lineage project as one or a few axon tracts (secondary axon tracts, SATs) with characteristic trajectories, thereby representing unique hallmarks. In the neuropil, SATs assemble into larger fiber bundles (fascicles) which interconnect different neuropil compartments. We have analyzed the SATs and fascicles formed by lineages during larval, pupal, and adult stages using antibodies against membrane molecules (Neurotactin/Neuroglian) and synaptic proteins (Bruchpilot/N-Cadherin). The use of these markers allows one to identify fiber bundles of the adult brain and associate them with SATs and fascicles of the larval brain. This work lays the foundation for assigning the lineage identity of GFP-labeled MARCM clones on the basis of their close association with specific SATs and neuropil fascicles, as described in the accompanying paper (Wong et al., 2013. Postembryonic lineages of the Drosophila brain: II. Identification of lineage projection patterns based on MARCM clones. Submitted.).

Whole-body gene expression pattern registration in Platynereis larvae

Background

Digital anatomical atlases are increasingly used in order to depict different gene expression patterns and neuronal morphologies within a standardized reference template. In evo-devo, a discipline in which the comparison of gene expression patterns is a widely used approach, such standardized anatomical atlases would allow a more rigorous assessment of the conservation of and changes in gene expression patterns during micro- and macroevolutionary time scales. Due to its small size and invariant early development, the annelid Platynereis dumerilii is particularly well suited for such studies. Recently a reference template with registered gene expression patterns has been generated for the anterior part (episphere) of the Platynereis trochophore larva and used for the detailed study of neuronal development.

Results

Here we introduce and evaluate a method for whole-body gene expression pattern registration for Platynereis trochophore and nectochaete larvae based on whole-mount in situ hybridization, confocal microscopy, and image registration. We achieved high-resolution whole-body scanning using the mounting medium 2,2’-thiodiethanol (TDE), which allows the matching of the refractive index of the sample to that of glass and immersion oil thereby reducing spherical aberration and improving depth penetration. This approach allowed us to scan entire whole-mount larvae stained with nitroblue tetrazolium/5-bromo-4-chloro-3-indolyl phosphate (NBT/BCIP) in situ hybridization and counterstained fluorescently with an acetylated-tubulin antibody and the nuclear stain 4’6-diamidino-2-phenylindole (DAPI). Due to the submicron isotropic voxel size whole-mount larvae could be scanned in any orientation. Based on the whole-body scans, we generated four different reference templates by the iterative registration and averaging of 40 individual image stacks using either the acetylated-tubulin or the nuclear-stain signal for each developmental stage. We then registered to these templates the expression patterns of cell-type specific genes. In order to evaluate the gene expression pattern registration, we analyzed the absolute deviation of cell-center positions. Both the acetylated-tubulin- and the nuclear-stain-based templates allowed near-cellular-resolution gene expression registration. Nuclear-stain-based templates often performed significantly better than acetylated-tubulin-based templates. We provide detailed guidelines and scripts for the use and further expansion of the Platynereis gene expression atlas.

Conclusions

We established whole-body reference templates for the generation of gene expression atlases for Platynereis trochophore and nectochaete larvae. We anticipate that nuclear-staining-based image registration will be applicable for whole-body alignment of the embryonic and larval stages of other organisms in a similar size range.

Cockchafer larvae smell host root scents in soil

In many insect species olfaction is a key sensory modality. However, examination of the chemical ecology of insects has focussed up to now on insects living above ground. Evidence for behavioral responses to chemical cues in the soil other than CO(2) is scarce and the role played by olfaction in the process of finding host roots below ground is not yet understood. The question of whether soil-dwelling beetle larvae can smell their host plant roots has been under debate, but proof is as yet lacking that olfactory perception of volatile compounds released by damaged host plants, as is known for insects living above ground, occurs. Here we show that soil-dwelling larvae of Melolontha hippocastani are well equipped for olfactory perception and respond electrophysiologically and behaviorally to volatiles released by damaged host-plant roots. An olfactory apparatus consisting of pore plates at the antennae and about 70 glomeruli as primary olfactory processing units indicates a highly developed olfactory system. Damage induced host plant volatiles released by oak roots such as eucalyptol and anisol are detected by larval antennae down to 5 ppbv in soil air and elicit directed movement of the larvae in natural soil towards the odor source. Our results demonstrate that plant-root volatiles are likely to be perceived by the larval olfactory system and to guide soil-dwelling white grubs through the dark below ground to their host plants. Thus, to find below-ground host plants cockchafer larvae employ mechanisms that are similar to those employed by the adult beetles flying above ground, despite strikingly different physicochemical conditions in the soil.

Retention of features on a mapped Drosophila brain surface using a Bézier-tube-based surface model averaging technique

Model averaging is a widely used technique in biomedical applications. Two established model averaging methods, iterative shape averaging (ISA) method and virtual insect brain (VIB) method, have been applied to several organisms to generate average representations of their brain surfaces. However, without sufficient samples, some features of the average Drosophila brain surface obtained using the above methods may disappear or become distorted. To overcome this problem, we propose a Bézier-tube-based surface model averaging strategy. The proposed method first compensates for disparities in position, orientation, and dimension of input surfaces, and then evaluates the average surface by performing shape-based interpolation. Structural features with larger individual disparities are simplified with half-ellipse-shaped Bézier tubes, and are unified according to these tubes to avoid distortion during the averaging process. Experimental results show that the average model yielded by our method could preserve fine features and avoid structural distortions even if only a limit amount of input samples are used. Finally, we qualitatively compare our results with those obtained by ISA and VIB methods by measuring the surface-to-surface distances between input surfaces and the averaged ones. The comparisons show that the proposed method could generate a more representative average surface than both ISA and VIB methods.

Report on the 12th symposium on invertebrate neurobiology held 31 August-4 September 2011 at the Balaton Limnological Research Institute of the Hungarian Academy of Sciences, Tihany, Hungary

In August 2011, the 12th international symposium of ISIN was held by Lake Balaton in Tihany, Hungary. This convivial and stimulating meeting provided a forum for discussion of a range of invertebrate organisms in neuroscience research. Here the main topics covered at the meeting are reviewed.

Comparative analysis of deutocerebral neuropils in Chilopoda (Myriapoda): implications for the evolution of the arthropod olfactory system and support for the Mandibulata concept

Background

Originating from a marine ancestor, the myriapods most likely invaded land independently of the hexapods. As these two evolutionary lineages conquered land in parallel but separately, we are interested in comparing the myriapod chemosensory system to that of hexapods to gain insights into possible adaptations for olfaction in air. Our study connects to a previous analysis of the brain and behavior of the chilopod (centipede) Scutigera coleoptrata in which we demonstrated that these animals do respond to volatile substances and analyzed the structure of their central olfactory pathway.

Results

Here, we examined the architecture of the deutocerebral brain areas (which process input from the antennae) in seven additional representatives of the Chilopoda, covering all major subtaxa, by histology, confocal laser-scan microscopy, and 3D reconstruction. We found that in all species that we studied the majority of antennal afferents target two separate neuropils, the olfactory lobe (chemosensory, composed of glomerular neuropil compartments) and the corpus lamellosum (mechanosensory). The numbers of olfactory glomeruli in the different chilopod taxa ranged from ca. 35 up to ca. 90 and the shape of the glomeruli ranged from spheroid across ovoid or drop-shape to elongate.

Conclusion

A split of the afferents from the (first) pair of antennae into separate chemosensory and mechanosensory components is also typical for Crustacea and Hexapoda, but this set of characters is absent in Chelicerata. We suggest that this character set strongly supports the Mandibulata hypothesis (Myriapoda + (Crustacea + Hexapoda)) as opposed to the Myriochelata concept (Myriapoda + Chelicerata). The evolutionary implications of our findings, particularly the plasticity of glomerular shape, are discussed.

Technical and organizational considerations for the long-term maintenance and development of digital brain atlases and web-based databases

Digital brain atlas is a kind of image database that specifically provide information about neurons and glial cells in the brain. It has various advantages that are unmatched by conventional paper-based atlases. Such advantages, however, may become disadvantages if appropriate cares are not taken. Because digital atlases can provide unlimited amount of data, they should be designed to minimize redundancy and keep consistency of the records that may be added incrementally by different staffs. The fact that digital atlases can easily be revised necessitates a system to assure that users can access previous versions that might have been cited in papers at a particular period. To inherit our knowledge to our descendants, such databases should be maintained for a very long period, well over 100 years, like printed books and papers. Technical and organizational measures to enable long-term archive should be considered seriously. Compared to the initial development of the database, subsequent efforts to increase the quality and quantity of its contents are not regarded highly, because such tasks do not materialize in the form of publications. This fact strongly discourages continuous expansion of, and external contributions to, the digital atlases after its initial launch. To solve these problems, the role of the biocurators is vital. Appreciation of the scientific achievements of the people who do not write papers, and establishment of the secure academic career path for them, are indispensable for recruiting talents for this very important job.

3D-Reconstructions and Virtual 4D-Visualization to Study Metamorphic Brain Development in the Sphinx Moth Manduca Sexta

DURING METAMORPHOSIS, THE TRANSITION FROM THE LARVA TO THE ADULT, THE INSECT BRAIN UNDERGOES CONSIDERABLE REMODELING: new neurons are integrated while larval neurons are remodeled or eliminated. One well acknowledged model to study metamorphic brain development is the sphinx moth Manduca sexta. To further understand mechanisms involved in the metamorphic transition of the brain we generated a 3D standard brain based on selected brain areas of adult females and 3D reconstructed the same areas during defined stages of pupal development. Selected brain areas include for example mushroom bodies, central complex, antennal- and optic lobes. With this approach we eventually want to quantify developmental changes in neuropilar architecture, but also quantify changes in the neuronal complement and monitor the development of selected neuronal populations. Furthermore, we used a modeling software (Cinema 4D) to create a virtual 4D brain, morphing through its developmental stages. Thus the didactical advantages of 3D visualization are expanded to better comprehend complex processes of neuropil formation and remodeling during development. To obtain datasets of the M. sexta brain areas, we stained whole brains with an antiserum against the synaptic vesicle protein synapsin. Such labeled brains were then scanned with a confocal laser scanning microscope and selected neuropils were reconstructed with the 3D software AMIRA 4.1.