Morphogenetic reorganization of the brain during embryogenesis in the grasshopper

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.

Ontogeny and development of the tritocerebral commissure giant (TCG): an identified neuron in the brain of the grasshopper Schistocerca gregaria

The tritocerebral commissure giant (TCG) of the grasshopper Schistocerca gregaria is one of the best anatomically and physiologically described arthropod brain neurons. A member of the so-called Ventral Giant cluster of cells, it integrates sensory information from visual, antennal and hair receptors, and synapses with thoracic motor neurons in order to initiate and regulate flight behavior. Its ontogeny, however, remains unclear. In this study, we use bromodeoxyuridine incorporation and cyclin labeling to reveal proliferative neuroblasts in the region of the embryonic brain where the ventral giant cluster is located. Engrailed labeling confirms the deutocerebral identity of this cluster. Comparison of soma locations and initial neurite projections into tracts of the striate deutocerebrum help identify the cells of the ventral cluster in both the embryonic and adult brain. Reconstructions of embryonic cell lineages suggest deutocerebral NB1 as being the putative neuroblast of origin. Intracellular dye injection coupled with immunolabeling against neuron-specific horseradish peroxidase is used to identify the VG1 (TCG) and VG3 neurons from the ventral cluster in embryonic brain slices. Dye injection and backfilling are used to document axogenesis and the progressive expansion of the dendritic arbor of the TCG from mid-embryogenesis up to hatching. Comparative maps of embryonic neuroblasts from several orthopteroid insects suggest equivalent deutocerebral neuroblasts from which the homologous TCG neurons already identified in the adult brain could originate. Our data offer the prospect of identifying further lineage-related neurons from the cluster and so understand a brain connectome from both a developmental and evolutionary perspective.

Embryonic development of the insect central complex: insights from lineages in the grasshopper and Drosophila

The neurons of the insect brain derive from neuroblasts which delaminate from the neuroectoderm at stereotypic locations during early embryogenesis. In both grasshopper and Drosophila, each developing neuroblast acquires an intrinsic capacity for neuronal proliferation in a cell autonomous manner and generates a specific lineage of neural progeny which is nearly invariant and unique. Maps revealing numbers and distributions of brain neuroblasts now exist for various species, and in both grasshopper and Drosophila four putatively homologous neuroblasts have been identified whose progeny direct axons to the protocerebral bridge and then to the central body via an equivalent set of tracts. Lineage analysis in the grasshopper nervous system reveals that the progeny of a neuroblast maintain their topological position within the lineage throughout embryogenesis. We have taken advantage of this to study the pioneering of the so-called w, x, y, z tracts, to show how fascicle switching generates central body neuroarchitecture, and to evaluate the roles of so-called intermediate progenitors as well as programmed cell death in shaping lineage structure. The novel form of neurogenesis involving intermediate progenitors has been demonstrated in grasshopper, Drosophila and mammalian cortical development and may represent a general strategy for increasing brain size and complexity. An analysis of gap junctional communication involving serotonergic cells reveals an intrinsic cellular organization which may relate to the presence of such transient progenitors in central complex lineages.

Early embryonic development of the head region of Gryllus assimilis Fabricius, 1775 (Orthoptera, Insecta)

We report our investigations on the embryonic development of Gryllus assimilis, with particular attention to the head. Significant findings revealed with scanning electron microscopy (SEM) images include: (1) the pre-antennal lobes represent the anterior-most segment that does not bear any appendages; (2) each of the lobes consists of central and marginal regions; (3) the central region thereof develops into the protocerebrum and the optic lobes, whereas the marginal region thereof becomes the anterior portion of the head capsule; (4) the initial position of the antennal segment is posterior to the mouth region; (5) appendage anlagen are transitorily present in the intercalary segment, and they later vanish together with the segment itself; (6) a bulged sternum appears to develop from the ventral surface of the mandibular, maxillary and labial segments. Embryonic features are then compared across the Insecta and further extended to the embryos of a spider (Araneae, Chelicerata). Striking similarities shared by the anterior-most region of the insect and spider embryos lead the authors to conclude that such comparison should be further undertaken to cover the entire Euarthropoda. This will help us to understand the embryology and evolution of the arthropod head.

Neural lineages of the Drosophila brain: a three-dimensional digital atlas of the pattern of lineage location and projection at the late larval stage

The late larval brain consists of embryonically produced primary neurons forming a deep core cortex, surrounded at the surface by approximately 100 secondary lineages. Each secondary lineage forms a tract (secondary lineage tract) with an invariant and characteristic trajectory. Within the neuropile, tracts of neighboring lineages bundle together to form secondary tract systems. In this paper, we visualized secondary lineages by the global marker BP106 (neurotactin), as well as green fluorescent protein-labeled clones and thereby establish a comprehensive digital atlas of secondary lineages. The information contained in this atlas is the location of the lineage within the cortex, the neuropile compartment contacted by the lineage tract, and the projection pattern of the lineage tract within the neuropile. We have digitally mapped the expression pattern of three genes, sine oculis, period, and engrailed into the lineage atlas. The atlas will enable us and others to analyze the phenotype of mutant clones in the larval brain. Mutant clones can only be interpreted if the corresponding wild-type clone is well characterized, and our lineage atlas, which visualizes all wild-type lineages, will provide this information. Secondly, secondary lineage tracts form a scaffold of connections in the neuropile that foreshadows adult nerve connections. Thus, starting from the larval atlas and proceeding forward through pupal development, one will be able to reconstruct adult brain connectivity at a high level of resolution. Third, the atlas can serve as a repository for genes expressed in lineage-specific patterns.

Comparative analysis of Wingless patterning in the embryonic grasshopper eye

The signaling factor Wingless regulates multiple steps during the postembryonic development of the Drosophila eye. To obtain insight into the molecular regulation of embryonic eye development in primitive insects, we studied the expression of wg and genes projected to interact with wg in the grasshopper Schistocerca americana. We find that the dynamic and complex expression of wg in the early grasshopper procephalon results in three paired expression domains with relevance to eye primordium development. By comparison with Drosophila, these domains are compatible with a conserved function of wg during anteroposterior and dorsoventral axis formation by repression of retinal differentiation and stimulation of tissue proliferation. This is further supported by the expression of grasshopper orthologs of the retina determination genes sine oculis and eyes absent, and by inhibition of retina differentiation in grasshopper eye primordia cultured with LiCl. Surprisingly, the expression of wg and the grasshopper orthologs of pannier, fringe, Delta, and Iroquois complex is inconsistent with induction of midline centered Notch signaling activity, which is essential for Drosophila retina development. Similarly substantial evolutionary divergence is found concerning the control of retina versus head epidermis specification. The transcription factor Extradenticle (Exd), which cooperates with wg in specifying the Drosophila head epidermis, is not detected outside the labral and antennal primordia in the embryonic grasshopper head. Our results, which provide the first insight into the molecular control of eye primordium formation in primitive insects, suggest substantial modification of this process during the evolution of the Drosophila mode of postembryonic eye development.

Development of neural lineages derived from the sine oculis positive eye field of Drosophila

The Anlage of the Drosophila visual system, called eye field, comprises a domain in the dorso-medial neurectoderm of the embryonic head and is defined by the expression of the early eye gene sine oculis (so). Beside the eye and optic lobe, the eye field gives rise to several neuroblasts that contribute their lineages to the central brain. Since so expression is only very short lived, the later development of these neuroblasts has so far been elusive. Using the P-element replacement technique [Genetics, 151 (1999) 1093] we generated a so-Gal4 line driving the reporter gene LacZ that perdures in the eye field derived cells throughout embryogenesis and into the larval period. This allowed us to reconstruct the morphogenetic movements of the eye field derived lineages, as well as the projection pattern of their neurons. The eye field produces a dorsal (Pc1/2) and a ventral (Pp3) group of three to four neuroblasts each. In addition, the target neurons of the larval eye, the optic lobe pioneers (OLPs) are derived from the eye field. The embryonically born (primary) neurons of the Pp3 lineages spread out at the inner surface of the optic lobe. Together with the OLPs, their axons project to the dorsal neuropile of the protocerebrum. Pp3 neuroblasts reassume expression of so-Gal4 in the larval period and produce secondary neurons whose axonal projection coincides with the pattern formed by the primary Pp3 neurons. Several other small clusters of neurons that originate from outside the eye field, but have axonal connections to the dorsal protocerebrum, also express so and are labeled by so-Gal4 driven LacZ. We discuss the dynamic pattern of the so-positive lineages as a tool to reconstruct the morphogenesis of the larval brain.

Early steps in building the insect brain: neuroblast formation and segmental patterning in the developing brain of different insect species

In insects, morphological, molecular and genetic studies have provided a detailed insight into the ontogenetic processes that shape the ventral nerve cord. On the other hand, owing to its complexity and less obvious segmental composition, the knowledge about the development of the brain is still fragmentary. A promising approach towards gaining insight into fundamental processes underlying brain development is the comparison of embryonic brain development among different insect species. However, so far such comparative analyses are scarce. In this review, we summarize and compare data on the early steps in brain formation in different hemi- and holometabolous insects. We show that basic aspects of the spatial and temporal development of the embryonic brain neuroblast pattern are conserved among insects. Furthermore, we compare the number and proliferation patterns of neuroblasts related to major neuropil structures such as mushroom bodies, central complex, and antennal lobe. Finally, comparing the expression patterns of engrailed in different species, and considering new data from Drosophila melanogaster, we discuss the segmental organization of the insect brain.

Spatial and temporal pattern of neuroblasts, proliferation, and Engrailed expression during early brain development in Tenebrio molitor L. (Coleoptera)

In insects, the knowledge of embryonic brain development is still fragmentary, and comparative data are scarce. In this study, we explored aspects of embryonic brain development in the coleopteran Tenebrio molitor. A detailed description is provided of the spatial and temporal pattern of the embryonic brain neuroblasts during 18-60% of embryonic development. Approximately 125 brain NBs have been identified in each hemisphere of the brain at about 40% of embryonic development. A subset of five neuroblasts, among them the two progenitors of the mushroom bodies and two progenitors of the larval antennal lobe, are morphologically identifiable by their larger size. As revealed by incorporation of BrdU, their mitotic behaviour is distinct from that of all other brain NBs, exhibiting an extended period of proliferation into postembryonic stages, and a significantly higher rate of division. To gain insight into the segmental organization of the T. molitor brain, Engrailed expression was examined in the head ectoderm and the deriving components of the CNS (including neuroblasts and their progeny) at different stages of embryonic development.

Early development of the Drosophila brain: IV. Larval neuropile compartments defined by glial septa

In this study, we have analyzed the architecture of the brain neuropile of the Drosophila larva, which is formed by two main structural elements: long axon tracts and terminal axonal/dendritic arborizations carrying synapses. By using several molecular markers expressed in neurons and glial cells, we show that the early larval neuropile is subdivided by glial sheaths into numerous compartments. The three-dimensional layout of these compartments and their relationship to the pattern of long axon tracts described in the accompanying article (Nassif et al. [2003] J. Comp. Neurol 417-434) was modeled by using a three-dimensional illustration computer software. On the basis of their location relative to each other and to long axon tracts, larval brain compartments can be identified with compartments defined by structural and functional criteria for the adult fly brain. We find that small precursors of most of the compartments of the adult central brain can be identified in the early larva. Changes in brain compartmental organization occurring during larval growth are described. Neuropile compartments, representing easily identifiable landmark structures, will assist in future analyses of Drosophila brain development in which the exact location of neurons and their axonal trajectories is of importance.

Early development of the Drosophila brain: V. Pattern of postembryonic neuronal lineages expressing DE-cadherin

The Drosophila E-cadherin homolog, DE-cadherin, is expressed postembryonically by brain neuroblasts and their lineages of neurons ("secondary lineages"). DE-cadherin appears in neuroblasts as soon as they can be identified by their increase in size and then remains expressed uninterruptedly throughout larval life. DE-cadherin remains transiently expressed in the cell bodies and axons of neurons produced by neuroblast proliferation. In general, axons of neurons belonging to one lineage form tight bundles. The trajectories of these bundles are correlated with the location of the neuronal lineages to which they belong. Thus, axon bundles of lineages that are neighbors in the cortex travel parallel to each other and reach the neuropile at similar positions. It is, therefore, possible to assign coherent groups of neuroblasts and their lineages to the individual neuropile compartments and long axon tracts introduced in the accompanying articles (Nassif et al. [2003] J Comp Neurol 455:417-434; Younossi-Hartenstein et al. [2003] J Comp Neurol 455:435-450). In this study, we have reconstructed the pattern of secondary lineages and their projection in relationship to the compartments and Fasciclin II-positive long axon tracts. Based on topology and axonal trajectory, the lineages of the central brain can be subdivided into 11 groups that can be followed throughout successive larval stages. The map of larval lineages and their axonal projection will be important for future studies on postembryonic neurogenesis in Drosophila. It also lays a groundwork for investigating the role of DE-cadherin in larval brain development.

Early development of the Drosophila brain: III. The pattern of neuropile founder tracts during the larval period

The Drosophila N-CAM homolog Fasciclin II (FasII) is expressed during the embryonic period in a subset of central neurons that pioneer the neuropile of the larval brain. Toward the end of embryogenesis, FasII expression in axon tracts diminishes but resumes from the late first larval instar in an increasingly complex pattern of axon tracts that join the tracts laid down in the embryo. We present evidence that FasII is expressed in a major fraction of the long axon tracts that interconnect different domains of the larval brain. For many tracts, FasII expression remains stable throughout larval development and pupal development. Therefore, the FasII pattern of axon tracts, along with the mushroom body and optic lobe, both of which are also FasII-positive, represents a useful set of landmarks that define different regions in the Drosophila brain throughout development. In this study, serial confocal brain sections were used to generate digital three-dimensional models of larval axon tracts at different stages. These models form part of our effort to generate an anatomic framework of Drosophila larval brain structure required for accurate localization of gene expression and gene function in experimental studies of neural development.

Developmental expression of nitric oxide/cyclic GMP signaling pathways in the brain of the embryonic grasshopper

Biochemical, cytochemical, and physiological investigations have demonstrated the presence of the nitric oxide/cyclic GMP signaling system in the brain of the adult locust, Schistocerca gregaria. Here, we characterize nitric oxide (NO) releasing neurons and neurons that synthesize cyclic GMP (cGMP) in response to a NO stimulus in the brain of the embryonic grasshopper. Using NADPH-diaphorase histochemistry to detect NO synthesizing cells we describe the appearance of several individually identifiable neurons. At embryonic stage 50% four NADPH-diaphorase positive neurons can be detected in each brain hemisphere. In addition to the labeling of differentiating neurons, NADPH-diaphorase staining appears also in distinct proliferative cell clusters. At embryonic stage 70% the general organization of NADPH-diaphorase activity starts to resemble the adult brain. The immunocytochemical detection of NO-induced accumulation of cGMP starts at embryonic stage 45% resulting in the staining of large neuronal populations in all brain areas. During embryonic stages 50-70%, the number of cGMP-immunoreactive cells increases from 200 to several hundred in each brain hemisphere. Since all NADPH-diaphorase positive local interneurons of the adult antennal lobe express GABA-immunoreactivity, we also report on the earliest appearance of GABA-immunoreactivity in the embryonic antennal lobe. Thus, we present a first developmental investigation of nitrergic and GABAergic transmitter phenotypes in the brain of the embryonic grasshopper.

Morphological and molecular data argue for the labrum being non-apical, articulated, and the appendage of the intercalary segment in the locust

Our analysis of head segmentation in the locust embryo reveals that the labrum is not apical as often interpreted but constitutes the topologically fused appendicular pair of appendages of the third head metamere. Using molecular, immunocytochemical and retrograde axonal staining methods we show that this metamere, the intercalary segment, is innervated by the third brain neuromere-the tritocerebrum. Evidence for the appendicular nature of the labrum is firstly, the presence of an engrailed stripe within its posterior epithelium as is typical of all appendages in the early embryo. Secondly, the labrum is innervated by a segmental nerve originating from the third brain neuromere (the tritocerebrum). Immunocytochemical staining with Lazarillo and horseradish peroxidase antibodies reveal that sensory neurons on the labrum contribute to the segmental (tritocerebral) nerve via the labral nerve in the same way as for the appendages immediately anterior (antenna) and posterior (mandible) on the head. All but one of the adult and embryonic motoneurons innervating the muscles of the labrum have their cell bodies and dendrites located completely within the tritocerebral neuromere and putatively derive from engrailed expressing tritocerebral neuroblasts. Molecular evidence (repo) suggests the labrum is not only appendicular but also articulated, comprising two jointed elements homologous to the coxa and trochanter of the leg.

A single cell analysis of engrailed expression in the early embryonic brain of the grasshopper Schistocerca gregaria: ontogeny and identity of the secondary headspot cells

The expression pattern of the segment polarity gene engrailed was studied at the single cell level in the protocerebrum of the early embryonic brain of the grasshopper Schistocerca gregaria, the neuromere containing the secondary headspot cells. The engrailed protein is first expressed in the protocerebrum at about 22% of embryogenesis by a group of identified neuroblasts bordering the antennal lobe. The number of immunoreactive neuroblasts increases up to 26% of embryogenesis and then rapidly declines so that by 30% only the three most posterior remain immunoreactive. These three neuroblasts become incorporated into the developing antennal lobe of the deutocerebrum. Subsequently, there is a progressive re-expression of the engrailed protein in the protocerebrum by the so-called six secondary headspot cells. These are the first born sibling progeny of three identified protocerebral neuroblasts which themselves expressed the engrailed protein prior to generating their lineages, and so represents a reacquisition of engrailed expression within identified clones. The secondary headspot cells are neurons which direct axonal processes into the developing optic tract and so contribute to the primary axon scaffold of the brain. From our analysis of their ontogeny, we conclude that the secondary headspot cells do not represent a segmental border in the brain.

The pars intercerebralis of the locust brain: a developmental and comparative study

The anterior midline of the brain, also known as the pars intercerebralis, contains the largest collection of neurosecretory cells in the central nervous system of the grasshopper. In this study, we use immunocytochemical, intracellular staining, and histological methods to establish the ontogenies of the various cell types in the brain midline, and show how these cells contribute to the pars intercerebralis of the adult brain. We show that the adult pars intercerebralis develops from three distinct embryonic cell groups: (1) the median neurosecretory cells, which derive from a subset of neuroblasts in the protocerebral hemispheres, and which project axons to the corpora cardiaca; (2) the paired primary commissure pioneers, which derive directly from the mesectoderm of the dorsal median domain and whose axons project to the ventral nerve cord via the midline tract; and (3) the six progeny of the median precursor in the dorsal median domain, which share a common axonal projection with the primary commissure pioneers. Since the adult pars intercerebralis is a fusion product of these independent cellular components, it can only be understood in terms of its origins in the embryonic brain. When the expression pattern of the TERM-1 antigen is compared in subsets of median neurosecretory cells in a wide range of insect orders, the results suggests a common organizational Bauplan for the pars intercerebralis. This hypothesis is supported by the identification of putative homologs of the grasshopper primary commissure pioneers in all these insects.

Building the antennal lobe: engrailed expression reveals a contribution from protocerebral neuroblasts in the grasshopper Schistocerca gregaria

The expression pattern of the engrailed protein was studied in neuroblasts which delaminate at the border of the protocerebrum and antennal lobe of the deutocerebrum in the early embryonic brain of the grasshopper. The antennal lobe is a complex structure comprising both glomerular and non-glomerular components, a cellular organization which distinguishes it from the striate-like neuropil comprising the remainder of the deutocerebrum. Early in embryogenesis engrailed expression in the protocerebrum is restricted to a compact block of neuroblasts located at its interface with the antennal lobe. Subsequently engrailed expression in these cells disappears in a stepwise manner from anterior to posterior so that by 37% of embryogenesis only a single row of three engrailed positive neuroblasts and their progeny remains. Contemporaneously engrailed expression reappears in a group of more anterior progeny deriving from neuroblasts which are no longer immunoreactive. The three remaining engrailed positive neuroblasts then become separated from their non-immunoreactive neighbours by an invagination of the perineurium called the lateral cleft and come to lie completely within the developing antennal lobe. These cells then direct columns of immunoreactive progeny centrifugally towards the centre of the lobe. Such a protocerebral contribution to the antennal lobe suggests that the evolution and ontogeny of this brain region need to be reconsidered.

Building a brain: developmental insights in insects

Understanding the cellular, molecular and genetic mechanisms involved in building the brain remains one of the most challenging problems of neurobiology. In this article, we review recent work on the developmental mechanisms that generate the embryonic brain in insects. We compare some of the early developmental events that occur in the insect brain with those that operate during brain development in vertebrates and find that numerous parallels are present at both the cellular and the molecular levels. Thus, the roles of glial cells in prefiguring axon pathways, the function of pioneer neurons in establishing axon pathways, and the formation of a primary axon scaffolding are features of embryonic brain development in both insects and vertebrates. Moreover, at the molecular genetic level homologous regulatory genes control morphogenesis, regionalization and patterning during embryonic brain development in both insects and vertebrates. This indicates that there might be universal mechanisms for brain development, and that knowledge gained from Drosophila and other insects is relevant to our understanding of brain development in other more complex organisms, including man.

Structure and development of larval antennae in embryos ofLytta viridanaLeConte (Coleoptera: Meloidae)

At time of hatch (252–264 h at 25 ± 0.5 °C), each antenna in Lytta viridana has three flagellomeres, three extrinsic muscles, and 25 sensilla of five different types, including a large composite sensillum of 19 sensory units on flagellomere II. Each antenna evaginates from epidermis on either side the stomodaeum beginning at 16% of embryogenesis. At 21%, a cell near its apex divides into two pioneer neurons that move into its lumen and project their axons to the brain by 29%. Sensillar stem cells begin to emerge at 23%, those of the appendix within a large embryonic placode and, from 26 to 48%, divide asymmetrically to generate the neurons and accessory cells of each sensillum. Sensillar axonogenesis begins at 34%, the first axons contact the brain at 35%, and antennal glomeruli begin to form within the deutocerebra at 57%. At 35%, the trichogen cell of each sensillum begins to grow out and larval cuticle is deposited about these, beginning at 57%. Upon withdrawal of trichogen cytoplasm from within the appendix at 81%, the dendrites of each sensory unit grow into it and begin to branch. Functional aspects are addressed and the observations compared with the limited information available on embryos of other insects.