The expression and function of the achaete-scute genes in Tribolium castaneum reveals conservation and variation in neural pattern formation and cell fate specification

Single-cell RNA sequencing of mid-to-late stage spider embryos: new insights into spider development

BACKGROUND

The common house spider Parasteatoda tepidariorum represents an emerging new model organism of arthropod evolutionary and developmental (EvoDevo) studies. Recent technical advances have resulted in the first single-cell sequencing (SCS) data on this species allowing deeper insights to be gained into its early development, but mid-to-late stage embryos were not included in these pioneering studies.

RESULTS

Therefore, we performed SCS on mid-to-late stage embryos of Parasteatoda and characterized resulting cell clusters by means of in-silico analysis (comparison of key markers of each cluster with previously published information on these genes). In-silico prediction of the nature of each cluster was then tested/verified by means of additional in-situ hybridization experiments with additional markers of each cluster.

CONCLUSIONS

Our data show that SCS data reliably group cells with similar genetic fingerprints into more or less distinct clusters, and thus allows identification of developing cell types on a broader level, such as the distinction of ectodermal, mesodermal and endodermal cell lineages, as well as the identification of distinct developing tissues such as subtypes of nervous tissue cells, the developing heart, or the ventral sulcus (VS). In comparison with recent other SCS studies on the same species, our data represent later developmental stages, and thus provide insights into different stages of developing cell types and tissues such as differentiating neurons and the VS that are only present at these later stages.

Gene expression mapping of the neuroectoderm across phyla - conservation and divergence of early brain anlagen between insects and vertebrates

Gene expression has been employed for homologizing body regions across bilateria. The molecular comparison of vertebrate and fly brains has led to a number of disputed homology hypotheses. Data from the fly Drosophila melanogaster have recently been complemented by extensive data from the red flour beetle Tribolium castaneum with its more insect-typical development. In this review, we revisit the molecular mapping of the neuroectoderm of insects and vertebrates to reconsider homology hypotheses. We claim that the protocerebrum is non-segmental and homologous to the vertebrate fore- and midbrain. The boundary between antennal and ocular regions correspond to the vertebrate mid-hindbrain boundary while the deutocerebrum represents the anterior-most ganglion with serial homology to the trunk. The insect head placode is shares common embryonic origin with the vertebrate adenohypophyseal placode. Intriguingly, vertebrate eyes develop from a different region compared to the insect compound eyes calling organ homology into question. Finally, we suggest a molecular re-definition of the classic concepts of archi- and prosocerebrum.

Multi-scale dissection of wing transparency in the clearwing butterfly Phanus vitreus

Optical transparency is rare in terrestrial organisms, and often originates through loss of pigmentation and reduction in scattering. The coloured wings of some butterflies and moths have repeatedly evolved transparency, offering examples of how they function optically and biologically. Because pigments are primarily localized in the scales that cover a colourless wing membrane, transparency has often evolved through the complete loss of scales or radical modification of their shape. Whereas bristle-like scales have been well documented in glasswing butterflies, other scale modifications resulting in transparency remain understudied. The butterfly Phanus vitreus achieves transparency while retaining its scales and exhibiting blue/cyan transparent zones. Here, we investigate the mechanism of wing transparency in P. vitreus by light microscopy, focused ion beam milling, microspectrophotometry and optical modelling. We show that transparency is achieved via loss of pigments and vertical orientation in normal paddle-like scales. These alterations are combined with an anti-reflective nipple array on portions of the wing membrane being more exposed to light. The blueish coloration of the P. vitreus transparent regions is due to the properties of the wing membrane, and local scale nanostructures. We show that scale retention in the transparent patches might be explained by these perpendicular scales having hydrophobic properties.

Transcriptomic exploration of the Coleopteran wings reveals insight into the evolution of novel structures associated with the beetle elytron

The acquisition of novel traits is central to organismal evolution, yet the molecular mechanisms underlying this process are elusive. The beetle forewings (elytra) are evolutionarily modified to serve as a protective shield, providing a unique opportunity to study these mechanisms. In the past, the orthologs of genes within the wing gene network from Drosophila studies served as the starting point when studying the evolution of elytra (candidate genes). Although effective, candidate gene lists are finite and only explore genes conserved across species. To go beyond candidate genes, we used RNA sequencing and explored the wing transcriptomes of two Coleopteran species, the red flour beetle (Tribolium castaneum) and the Japanese stag beetle (Dorcus hopei). Our analysis revealed sets of genes enriched in Tribolium elytra (57 genes) and genes unique to the hindwings, which possess more "typical" insect wing morphologies (29 genes). Over a third of the hindwing-enriched genes were "candidate genes" whose functions were previously analyzed in Tribolium, demonstrating the robustness of our sequencing. Although the overlap was limited, transcriptomic comparison between the beetle species found a common set of genes, including key wing genes, enriched in either elytra or hindwings. Our RNA interference analysis for elytron-enriched genes in Tribolium uncovered novel genes with roles in forming various aspects of morphology that are unique to elytra, such as pigmentation, hardening, sensory development, and vein formation. Our analyses deepen our understanding of how gene network evolution facilitated the emergence of the elytron, a unique structure critical to the evolutionary success of beetles.

Leg length and bristle density, both necessary for water surface locomotion, are genetically correlated in water striders

Access to hitherto unexploited ecological opportunities is associated with phenotypic evolution and often results in significant lineage diversification. Yet our understanding of the mechanisms underlying such adaptive traits remains limited. Water striders have been able to exploit the water-air interface, primarily facilitated by changes in the density of hydrophobic bristles and a significant increase in leg length. These two traits are functionally correlated and are both necessary for generating efficient locomotion on the water surface. Whether bristle density and leg length have any cellular or developmental genetic mechanisms in common is unknown. Here, we combine comparative genomics and transcriptomics with functional RNA interference assays to examine the developmental genetic and cellular mechanisms underlying the patterning of the bristles and the legs in Gerris buenoi and Mesovelia mulsanti, two species of water striders. We found that two duplication events in the genes beadex and taxi led to a functional expansion of the paralogs, which affected bristle density and leg length. We also identified genes for which no function in bristle development has been previously described in other insects. Interestingly, most of these genes play a dual role in regulating bristle development and leg length. In addition, these genes play a role in regulating cell division. This result suggests that cell division may be a common mechanism through which these genes can simultaneously regulate leg length and bristle density. We propose that pleiotropy, through which gene function affects the development of multiple traits, may play a prominent role in facilitating access to unexploited ecological opportunities and species diversification.

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.

Insights into the genetic regulatory network underlying neurogenesis in the parthenogenetic marbled crayfish Procambarus virginalis

Nervous system development has been intensely studied in insects (especially Drosophila melanogaster), providing detailed insights into the genetic regulatory network governing the formation and maintenance of the neural stem cells (neuroblasts) and the differentiation of their progeny. Despite notable advances over the last two decades, neurogenesis in other arthropod groups remains by comparison less well understood, hampering finer resolution of evolutionary cell type transformations and changes in the genetic regulatory network in some branches of the arthropod tree of life. Although the neurogenic cellular machinery in malacostracan crustaceans is well described morphologically, its genetic molecular characterization is pending. To address this, we established an in situ hybridization protocol for the crayfish Procambarus virginalis and studied embryonic expression patterns of a suite of key genes, encompassing three SoxB group transcription factors, two achaete-scute homologs, a Snail family member, the differentiation determinants Prospero and Brain tumor, and the neuron marker Elav. We document cell type expression patterns with notable similarities to insects and branchiopod crustaceans, lending further support to the homology of hexapod-crustacean neuroblasts and their cell lineages. Remarkably, in the crayfish head region, cell emigration from the neuroectoderm coupled with gene expression data points to a neuroblast-independent initial phase of brain neurogenesis. Further, SoxB group expression patterns suggest an involvement of Dichaete in segmentation, in concordance with insects. Our target gene set is a promising starting point for further embryonic studies, as well as for the molecular genetic characterization of subregions and cell types in the neurogenic systems in the adult crayfish brain.

Functional analysis of sense organ specification in the Tribolium castaneum larva reveals divergent mechanisms in insects

Abstract

Insects and other arthropods utilise external sensory structures for mechanosensory, olfactory, and gustatory reception. These sense organs have characteristic shapes related to their function, and in many cases are distributed in a fixed pattern so that they are identifiable individually. In Drosophila melanogaster, the identity of sense organs is regulated by specific combinations of transcription factors. In other arthropods, however, sense organ subtypes cannot be linked to the same code of gene expression. This raises the questions of how sense organ diversity has evolved and whether the principles underlying subtype identity in D. melanogaster are representative of other insects. Here, we provide evidence that such principles cannot be generalised, and suggest that sensory organ diversification followed the recruitment of sensory genes to distinct sensory organ specification mechanism.

Results

We analysed sense organ development in a nondipteran insect, the flour beetle Tribolium castaneum, by gene expression and RNA interference studies. We show that in contrast to D. melanogaster, T. castaneum sense organs cannot be categorised based on the expression or their requirement for individual or combinations of conserved sense organ transcription factors such as cut and pox neuro, or members of the Achaete-Scute (Tc ASH, Tc asense), Atonal (Tc atonal, Tc cato, Tc amos), and neurogenin families (Tc tap). Rather, our observations support an evolutionary scenario whereby these sensory genes are required for the specification of sense organ precursors and the development and differentiation of sensory cell types in diverse external sensilla which do not fall into specific morphological and functional classes.

Conclusions

Based on our findings and past research, we present an evolutionary scenario suggesting that sense organ subtype identity has evolved by recruitment of a flexible sensory gene network to the different sense organ specification processes. A dominant role of these genes in subtype identity has evolved as a secondary effect of the function of these genes in individual or subsets of sense organs, probably modulated by positional cues.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-00948-y.

Epigenomic landscape of enhancer elements during Hydra head organizer formation

BACKGROUND

Axis patterning during development is accompanied by large-scale gene expression changes. These are brought about by changes in the histone modifications leading to dynamic alterations in chromatin architecture. The cis regulatory DNA elements also play an important role towards modulating gene expression in a context-dependent manner. Hydra belongs to the phylum Cnidaria where the first asymmetry in the body plan was observed and the oral-aboral axis originated. Wnt signaling has been shown to determine the head organizer function in the basal metazoan Hydra.

RESULTS

To gain insights into the evolution of cis regulatory elements and associated chromatin signatures, we ectopically activated the Wnt signaling pathway in Hydra and monitored the genome-wide alterations in key histone modifications. Motif analysis of putative intergenic enhancer elements from Hydra revealed the conservation of bilaterian cis regulatory elements that play critical roles in development. Differentially regulated enhancer elements were identified upon ectopic activation of Wnt signaling and found to regulate many head organizer specific genes. Enhancer activity of many of the identified cis regulatory elements was confirmed by luciferase reporter assay. Quantitative chromatin immunoprecipitation analysis upon activation of Wnt signaling further confirmed the enrichment of H3K27ac on the enhancer elements of Hv_Wnt5a, Hv_Wnt11 and head organizer genes Hv_Bra1, CnGsc and Hv_Pitx1. Additionally, perturbation of the putative H3K27me3 eraser activity using a specific inhibitor affected the ectopic activation of Wnt signaling indicating the importance of the dynamic changes in the H3K27 modifications towards regulation of the genes involved in the head organizer activity.

CONCLUSIONS

The activation-associated histone marks H3K4me3, H3K27ac and H3K9ac mark chromatin in a similar manner as seen in bilaterians. We identified intergenic cis regulatory elements which harbor sites for key transcription factors involved in developmental processes. Differentially regulated enhancers exhibited motifs for many zinc-finger, T-box and ETS related TFs whose homologs have a head specific expression in Hydra and could be a part of the pioneer TF network in the patterning of the head. The ability to differentially modify the H3K27 residue is critical for the patterning of Hydra axis revealing a dynamic acetylation/methylation switch to regulate gene expression and chromatin architecture.

Length-dependent accumulation of double-stranded RNAs in plastids affects RNA interference efficiency in the Colorado potato beetle

Transplastomic potato plants expressing double-stranded RNA (dsRNA) targeted against essential genes of the Colorado potato beetle (CPB) can be lethal to larvae by triggering an RNA interference (RNAi) response. High accumulation levels of dsRNAs in plastids are crucial to confer an efficient RNAi response in the insects. However, whether length and sequence of the dsRNA determine the efficacy of RNAi and/or influence the level of dsRNA accumulation in plastids is not known. We compared the RNAi efficacy of different lengths of dsRNA targeted against the CPB β-Actin gene (ACT) by feeding in vitro-synthesized dsRNAs to larvae. We showed that, while the 60 bp dsRNA induced only a relatively low RNAi response in CPB, dsRNAs of 200 bp and longer caused high mortality and similar larval growth retardation. When the dsRNAs were expressed from the plastid (chloroplast) genome of potato plants, we found that their accumulation were negatively correlated with length. The level of dsRNA accumulation was positively associated with the observed mortality, suppression of larval growth, and suppression of target gene expression. Importantly, transplastomic potato plants expressing the 200 bp dsRNA were better protected from CPB than plants expressing the 297 bp dsRNA, the best-performing line in our previous study. Our results suggest that the length of dsRNAs is an important factor that influences their accumulation in plastids and thus determines the strength of the insecticidal RNAi effect. Our findings will aid the design of optimized dsRNA expression constructs for plant protection by plastid-mediated RNAi.

Adult neurogenesis in crayfish: Origin, expansion, and migration of neural progenitor lineages in a pseudostratified neuroepithelium

Two decades after the discovery of adult-born neurons in the brains of decapod crustaceans, the deutocerebral proliferative system (DPS) producing these neural lineages has become a model of adult neurogenesis in invertebrates. Studies on crayfish have provided substantial insights into the anatomy, cellular dynamics, and regulation of the DPS. Contrary to traditional thinking, recent evidence suggests that the neurogenic niche in the crayfish DPS lacks self-renewing stem cells, its cell pool being instead sustained via integration of hemocytes generated by the innate immune system. Here, we investigated the origin, division and migration patterns of the adult-born neural progenitor (NP) lineages in detail. We show that the niche cell pool is not only replenished by hemocyte integration but also by limited numbers of symmetric cell divisions with some characteristics reminiscent of interkinetic nuclear migration. Once specified in the niche, first generation NPs act as transit-amplifying intermediate NPs that eventually exit and produce multicellular clones as they move along migratory streams toward target brain areas. Different clones may migrate simultaneously in the streams but occupy separate tracks and show spatio-temporally flexible division patterns. Based on this, we propose an extended DPS model that emphasizes structural similarities to pseudostratified neuroepithelia in other arthropods and vertebrates. This model includes hemocyte integration and intrinsic cell proliferation to synergistically counteract niche cell pool depletion during the animal's lifespan. Further, we discuss parallels to recent findings on mammalian adult neurogenesis, as both systems seem to exhibit a similar decoupling of proliferative replenishment divisions and consuming neurogenic divisions.

An ancestral apical brain region contributes to the central complex under the control of foxQ2 in the beetle Tribolium

The genetic control of anterior brain development is highly conserved throughout animals. For instance, a conserved anterior gene regulatory network specifies the ancestral neuroendocrine center of animals and the apical organ of marine organisms. However, its contribution to the brain in non-marine animals has remained elusive. Here, we study the function of the Tc-foxQ2 forkhead transcription factor, a key regulator of the anterior gene regulatory network of insects. We characterized four distinct types of Tc-foxQ2 positive neural progenitor cells based on differential co-expression with Tc-six3/optix, Tc-six4, Tc-chx/vsx, Tc-nkx2.1/scro, Tc-ey, Tc-rx and Tc-fez1. An enhancer trap line built by genome editing marked Tc-foxQ2 positive neurons, which projected through the primary brain commissure and later through a subset of commissural fascicles. Eventually, they contributed to the central complex. Strikingly, in Tc-foxQ2 RNAi knock-down embryos the primary brain commissure did not split and subsequent development of midline brain structures stalled. Our work establishes foxQ2 as a key regulator of brain midline structures, which distinguish the protocerebrum from segmental ganglia. Unexpectedly, our data suggest that the central complex evolved by integrating neural cells from an ancestral anterior neuroendocrine center.

The achaete-scute complex contains a single gene that controls bristle development in the semi-aquatic bugs

The semi-aquatic bugs (Heteroptera, Gerromorpha) conquered water surfaces worldwide and diversified to occupy puddles, ponds, streams, lakes, mangroves and even oceans. Critical to this lifestyle is the evolution of sets of hairs that allow these insects to maintain their body weight on the water surface and protect the animals against wetting and drowning. In addition, the legs of these insects are equipped with various grooming combs that are important for cleaning and tidying the hair layers for optimal functional efficiency. Here we show that the hairs covering the legs of water striders represent innervated bristles. Genomic and transcriptomic analyses revealed that in water striders the achaete–scute complex, known to control bristle development in flies, contains only the achaete–scute homologue (ASH) gene owing to the loss of the gene asense. Using RNA interference, we show that ASH plays a pivotal role in the development of both bristles and grooming combs in water striders. Our data suggest that the ASH locus may have contributed to the adaptation to water surface lifestyle through shaping the hydrophobic bristles that prevent water striders from wetting and allow them to exploit water surface tension.

Characterization of the bHLH family of transcriptional regulators in the acoel S. roscoffensis and their putative role in neurogenesis

Background

The basic helix-loop-helix (bHLH) family of transcription factors is one of the largest superfamilies of regulatory transcription factors and is widely used in eukaryotic organisms. They play an essential role in a range of metabolic, physiological, and developmental processes, including the development of the nervous system (NS). These transcription factors have been studied in many metazoans, especially in vertebrates but also in early branching metazoan clades such as the cnidarians and sponges. However, currently very little is known about their expression in the most basally branching bilaterian group, the xenacoelomorphs. Recently, our laboratory has characterized the full complement of bHLH in the genome of two members of the Xenacoelomorpha, the xenoturbellid Xenoturbella bocki and the acoel Symsagittifera roscoffensis. Understanding the patterns of bHLH gene expression in members of this phylum (in space and time) provides critical new insights into the conserved roles of the bHLH and their putative specificities in this group. Our focus is on deciphering the specific roles that these genes have in the process of neurogenesis.

Results

Here, we analyze the developmental expression of the whole complement of bHLH genes identified in the acoel S. roscoffensis. Based on their expression patterns, several members of bHLH class A appear to have specific conserved roles in neurogenesis, while other class A genes (as well as members of other classes) have likely taken on more generalized functions. All gene expression patterns are described in embryos and early juveniles.

Conclusion

Our results suggest that the main roles of the bHLH genes of S. roscoffensis are evolutionarily conserved, with a specific subset dedicated to patterning the nervous system: SrAscA, SrAscB, SrHes/Hey, SrNscl, SrSrebp, SrE12/E47 and SrOlig.

Electronic supplementary material

The online version of this article (10.1186/s13227-018-0097-y) contains supplementary material, which is available to authorized users.

Genome-wide identification of Tribolium dorsoventral patterning genes

The gene regulatory network controlling dorsoventral axis formation in insects has undergone drastic evolutionary changes. In Drosophila, a stable long-range gradient of Toll signalling specifies ventral cell fates and restricts BMP signalling to the dorsal half of the embryo. In Tribolium, however, Toll signalling is transient and only indirectly controls BMP signalling. In order to gain unbiased insights into the Tribolium network, we performed comparative transcriptome analyses of embryos with various dorsoventral pattering defects produced by parental RNAi for Toll and BMP signalling components. We also included embryos lacking the mesoderm (produced by Tc-twist RNAi) and characterized similarities and differences between Drosophila and Tribolium twist loss-of-function phenotypes. Using stringent conditions, we identified over 750 differentially expressed genes and analysed a subset with altered expression in more than one knockdown condition. We found new genes with localized expression and showed that conserved genes frequently possess earlier and stronger phenotypes than their Drosophila orthologues. For example, the leucine-rich repeat (LRR) protein Tartan, which has only a minor influence on nervous system development in Drosophila, is essential for early neurogenesis in Tribolium and the Tc-zinc-finger homeodomain protein 1 (Tc-zfh1), the orthologue of which plays a minor role in Drosophila muscle development, is essential for maintaining early Tc-twist expression, indicating an important function for mesoderm specification.

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.

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.

The 'ventral organs' of Pycnogonida (Arthropoda) are neurogenic niches of late embryonic and post-embryonic nervous system development

Early neurogenesis in arthropods has been in the focus of numerous studies, its cellular basis, spatio-temporal dynamics and underlying genetic network being by now comparably well characterized for representatives of chelicerates, myriapods, hexapods and crustaceans. By contrast, neurogenesis during late embryonic and/or post-embryonic development has received less attention, especially in myriapods and chelicerates. Here, we apply (i) immunolabeling, (ii) histology and (iii) scanning electron microscopy to study post-embryonic ventral nerve cord development in Pseudopallene sp., a representative of the sea spiders (Pycnogonida), the presumable sister group of the remaining chelicerates. During early post-embryonic development, large neural stem cells give rise to additional ganglion cell material in segmentally paired invaginations in the ventral ectoderm. These ectodermal cell regions - traditionally designated as 'ventral organs' - detach from the surface into the interior and persist as apical cell clusters on the ventral ganglion side. Each cluster is a post-embryonic neurogenic niche that features a tiny central cavity and initially still houses larger neural stem cells. The cluster stays connected to the underlying ganglionic somata cortex via an anterior and a posterior cell stream. Cell proliferation remains restricted to the cluster and streams, and migration of newly produced cells along the streams seems to account for increasing ganglion cell numbers in the cortex. The pycnogonid cluster-stream-systems show striking similarities to the life-long neurogenic system of decapod crustaceans, and due to their close vicinity to glomerulus-like neuropils, we consider their possible involvement in post-embryonic (perhaps even adult) replenishment of olfactory neurons - as in decapods. An instance of a potentially similar post-embryonic/adult neurogenic system in the arthropod outgroup Onychophora is discussed. Additionally, we document two transient posterior ganglia in the ventral nerve cord of Pseudopallene sp. and evaluate this finding in light of the often discussed reduction of a segmented 'opisthosoma' during pycnogonid evolution.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

The two Tribolium E(spl) genes show evolutionarily conserved expression and function during embryonic neurogenesis

Tribolium castaneum is a well-characterised model insect, whose short germ-band mode of embryonic development is characteristic of many insect species and differs from the exhaustively studied Drosophila. Mechanisms of early neurogenesis, however, show significant conservation with Drosophila, as a characteristic pattern of neuroblasts arises from neuroectoderm proneural clusters in response to the bHLH activator Ash, a homologue of Achaete-Scute. Here we study the expression and function of two other bHLH proteins, the bHLH-O repressors E(spl)1 and E(spl)3. Their Drosophila homologues are expressed in response to Notch signalling and antagonize the activity of Achaete-Scute proteins, thus restricting the number of nascent neuroblasts. E(spl)1 and 3 are the only E(spl) homologues in Tribolium and both show expression in the cephalic and ventral neuroectoderm during embryonic neurogenesis, as well as a dynamic pattern of expression in other tissues. Their expression starts early, soon after Ash expression and is dependent on both Ash and Notch activities. They act redundantly, since a double E(spl) knockdown (but not single knockdowns) results in neurogenesis defects similar to those caused by Notch loss-of-function. A number of other activities have been evolutionarily conserved, most notably their ability to interact with proneural proteins Scute and Daughterless.

Changes in anterior head patterning underlie the evolution of long germ embryogenesis

Early embryonic stages differ significantly among related animal taxa while subsequent development converges at the conserved phylotypic stage before again diverging. Although this phenomenon has long been observed, its underlying genetic mechanisms remain enigmatic. The dipteran Drosophila melanogaster develops as a long germ embryo where the head anlagen form a cap at the anterior pole of the blastoderm. Consequently, the anterior and terminal maternal systems give crucial input for head patterning. However, in the short germ beetle Tribolium castaneum, as in most insects, the head anlagen is located at a ventral position distant from the anterior pole of the blastoderm. In line with these divergent embryonic anlagen, several differences in the axis formation between the insects have been discovered. We now ask to what extent patterning and morphogenesis of the anterior median region (AMR) of the head, including clypeolabral and stomodeal anlagen, differ among these insects. Unexpectedly, we find that Tc-huckebein is not a terminal gap gene and, unlike its Drosophila ortholog, is not involved in Tribolium head development. Instead, Tc-six3 acts upstream of Tc-crocodile and Tc-cap'n'collar to pattern posterior and anterior parts of the AMR, respectively. We further find that instead of huckebein, Tc-crocodile is required for stomodeum development by activating Tc-forkhead. Finally, a morphogenetic movement not found in Drosophila shapes the embryonic head of Tribolium. Apparently, with anterior displacement of the head anlagen during long germ evolution of Drosophila, the ancestral regulation by the bilaterian anterior control gene six3 was replaced by the anterior and terminal maternal systems, which were further elaborated by adding bicoid, tailless and huckebein as anterior regionalization genes.

Distribution and function of splash, an achaete-scute homolog in the adult olfactory organ of the Caribbean spiny lobster Panulirus argus

achaete-scute complex (ASC) genes, which encode basic helix-loop-helix transcription factors, regulate embryonic and adult neurogenesis in many animals. In adult arthropods, including crustaceans, ASC homologs have been identified but rarely functionally characterized. We took advantage of the recently identified crustacean homolog, splash (spiny lobster achaete scute homolog), in the olfactory organ of the Caribbean spiny lobster Panulirus argus to examine its role in adult neurogenesis. We tested the hypothesis that splash is associated with but not restricted to sensory neuron formation in the olfactory organ, the antennular lateral flagellum (LF), of adult spiny lobsters. We demonstrated splash labeling in epithelial cells across LF developmental zones (i.e., proliferation and mature zones), in auxiliary cells surrounding dendrites of olfactory receptor neurons (ORNs), and in immature and mature ORNs, but not in granulocytes or chromatophores. Since ORN proliferation varies with molt stage, we examined splash expression across molt stages and found that molt stage affected splash expression in the ORN mature zone but not in the proliferation zone. In vivo incorporation of bromodeoxyuridine (BrdU) showed no correlation in the cellular pattern of splash expression and BrdU labeling. The intensity of splash labeling was dramatically enhanced in the proliferation zones following LF damage, suggesting enhanced splash expression during repair and/or regeneration. We conclude that splash is not closely associated with the formation of sensory neurons under normal physiological conditions, and we propose that splash is involved in repair and regeneration. We also propose that splash has additional roles other than neurogenesis in adult crustaceans.

Neurogenesis in the water flea Daphnia magna (Crustacea, Branchiopoda) suggests different mechanisms of neuroblast formation in insects and crustaceans

Within euarthropods, the morphological and molecular mechanisms of early nervous system development have been analysed in insects and several representatives of chelicerates and myriapods, while data on crustaceans are fragmentary. Neural stem cells (neuroblasts) generate the nervous system in insects and in higher crustaceans (malacostracans); in the remaining euarthropod groups, the chelicerates (e.g. spiders) and myriapods (e.g. millipedes), neuroblasts are missing. In the latter taxa, groups of neural precursors segregate from the neuroectoderm and directly differentiate into neurons and glial cells. In all euarthropod groups, achaete-scute homologues are required for neuroblast/neural precursor group formation. In the insects Drosophila melanogaster and Tribolium castaneum achaete-scute homologues are initially expressed in clusters of cells (proneural clusters) in the neuroepithelium but expression becomes restricted to the future neuroblast. Subsequently genes such as snail and prospero are expressed in the neuroblasts which are required for asymmetric division and differentiation. In contrast to insects, malacostracan neuroblasts do not segregate into the embryo but remain in the outer neuroepithelium, similar to vertebrate neural stem cells. It has been suggested that neuroblasts are present in another crustacean group, the branchiopods, and that they also remain in the neuroepithelium. This raises the questions how the molecular mechanisms of neuroblast selection have been modified during crustacean and insect evolution and if the segregation or the maintenance of neuroblasts in the neuroepithelium represents the ancestral state. Here we take advantage of the recently published Daphnia pulex (branchiopod) genome and identify genes in Daphnia magna that are known to be required for the selection and asymmetric division of neuroblasts in the fruit fly D. melanogaster. We unambiguously identify neuroblasts in D. magna by molecular marker gene expression and division pattern. We show for the first time that branchiopod neuroblasts divide in the same pattern as insect and malacostracan neuroblasts. Furthermore, in contrast to D. melanogaster, neuroblasts are not selected from proneural clusters in the branchiopod. Snail rather than ASH is the first gene to be expressed in the nascent neuroblasts suggesting that ASH is not required for the selection of neuroblasts as in D. melanogaster. The prolonged expression of ASH in D. magna furthermore suggests that it is involved in the maintenance of the neuroblasts in the neuroepithelium. Based on these and additional data from various representatives of arthropods we conclude that the selection of neural precursors from proneural clusters as well as the segregation of neural precursors represents the ancestral state of neurogenesis in arthropods. We discuss that the derived characters of malacostracans and branchiopods - the absence of neuroblast segregation and proneural clusters - might be used to support or reject the possible groupings of paraphyletic crustaceans.

Insect Tc-six4 marks a unit with similarity to vertebrate placodes

Cranial placodes are specialized ectodermal regions in the developing vertebrate head that give rise to both neural and non-neural cell types of the neuroendocrine system and the sense organs of the visual, olfactory and acoustic systems. The cranial placodes develop from a panplacodal region which is specifically marked by genes of the eyes absent/eya and two "six homeobox" family members (sine oculis/six1 and six4). It had been believed that cranial placodes are evolutionary novelties of vertebrates. However, data from non-vertebrate chordates suggest that placode-like structures evolved in the chordate ancestor already. Here, we identify a morphological structure in the embryonic head of the beetle Tribolium castaneum with placode-like features. It is marked by the orthologs of the panplacodal markers Tc-six4, Tc-eya and Tc-sine oculis/six1 (Tc-six1) and expresses several genes known to be involved in adenohypophyseal placode development in vertebrates. Moreover, it contributes to both epidermal and neural tissues. We identify Tc-six4 as a specific marker for this structure that we term the insect head placode. Finally, we reveal the regulatory gene network of the panplacodal genes Tc-six4, Tc-eya and Tc-six1 and identify them as head epidermis patterning genes. Our finding of a placode-like structure in an insect suggests that a placode precursor was already present in the last common ancestor of bilaterian animals.

The members of bHLH transcription factor superfamily are required for female reproduction in the red flour beetle, Tribolium castaneum

Proteins containing the basic Helix-Loop-Helix (bHLH) domain function as transcription factors and play important roles during the development of various metazoans including insects, nematodes and vertebrates. Insect genomes contain more than 50 bHLH transcription factors, but the function of only a few of these proteins in regulation of female reproduction is known. Using RNA interference, we have tested knock-down in the expression of genes coding for bHLH transcription factors in newly emerged adult females to determine their function in regulation of female reproduction in the red flour beetle, Tribolium castaneum. Knock-down in the expression of genes coding for four bHLH transcription factors (TcSRC, TcSim1, TcAsh and TcDaughterless) caused mortality in the female beetles. In addition, knocking-down the expression of 16 bHLH genes affected oogenesis and knock-down in the expression of 13 genes affected embryogenesis. Two genes TcSide1 and TcSpineless are required for both oogenesis and embryogenesis. Thus, the data reported here showed that 31 bHLH transcription factors are required for female survival, reproduction and embryogenesis.

An arthropod cis-regulatory element functioning in sensory organ precursor development dates back to the Cambrian

Background

An increasing number of publications demonstrate conservation of function of cis-regulatory elements without sequence similarity. In invertebrates such functional conservation has only been shown for closely related species. Here we demonstrate the existence of an ancient arthropod regulatory element that functions during the selection of neural precursors. The activity of genes of the achaete-scute (ac-sc) family endows cells with neural potential. An essential, conserved characteristic of proneural genes is their ability to restrict their own activity to single or a small number of progenitor cells from their initially broad domains of expression. This is achieved through a process called lateral inhibition. A regulatory element, the sensory organ precursor enhancer (SOPE), is required for this process. First identified in Drosophila, the SOPE contains discrete binding sites for four regulatory factors. The SOPE of the Drosophila asense gene is situated in the 5' UTR.

Results

Through a manual comparison of consensus binding site sequences we have been able to identify a SOPE in UTR sequences of asense-like genes in species belonging to all four arthropod groups (Crustacea, Myriapoda, Chelicerata and Insecta). The SOPEs of the spider Cupiennius salei and the insect Tribolium castaneum are shown to be functional in transgenic Drosophila. This would place the origin of this regulatory sequence as far back as the last common ancestor of the Arthropoda, that is, in the Cambrian, 550 million years ago.

Conclusions

The SOPE is not detectable by inter-specific sequence comparison, raising the possibility that other ancient regulatory modules in invertebrates might have escaped detection.

Metamorphosis and adult development of the mushroom bodies of the red flour beetle, Tribolium castaneum

The insect mushroom bodies play important roles in a number of higher processing functions such as sensory integration, higher level olfactory processing, and spatial and associative learning and memory. These functions have been established through studies in a handful of tractable model systems, of which only the fruit fly Drosophila melanogaster has been readily amenable to genetic manipulations. The red flour beetle Tribolium castaneum has a sequenced genome and has been subject to the development of molecular tools for the ready manipulation of gene expression; however, little is known about the development and organization of the mushroom bodies of this insect. The present account bridges this gap by demonstrating that the organization of the Tribolium mushroom bodies is strikingly like that of the fruit fly, with the significant exception that the timeline of neurogenesis is shifted so that the last population of Kenyon cells is born entirely after adult eclosion. Tribolium Kenyon cells are generated by two large neuroblasts per hemisphere and segregate into an early-born delta lobe subpopulation followed by clear homologs of the Drosophila gamma, alpha'/beta' and alpha/beta lobe subpopulations, with the larval-born cohorts undergoing dendritic reorganization during metamorphosis. BrdU labeling and immunohistochemical staining also reveal that a proportion of individual Tribolium have variable numbers of mushroom body neuroblasts. If heritable, this variation represents a unique opportunity for further studies of the genetic control of brain region size through the control of neuroblast number and cell cycle dynamics.

Molecular cloning and characterization of homologs of achaete-scute and hairy-enhancer of split in the olfactory organ of the spiny lobster Panulirus argus

The olfactory organ of the Caribbean spiny lobster Panulirus argus maintains lifelong proliferation and turnover of olfactory receptor neurons (ORNs). Towards examining the molecular basis of this adult neurogenesis, we search for expression of homologs of proneural, neurogenic, and pre-pattern genes in this olfactory organ. We report here a homolog of the proneural Achaete-Scute family, called splash (spiny lobster achaete-scute homolog), and a homolog of the pre-pattern and neurogenic hairy-enhancer of split family, called splhairy (spiny lobster hairy). Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) indicates a molt stage dependence of the levels of expression of splash and splhairy mRNA in the olfactory organ, with higher expression in premolt than in postmolt or intermolt animals, which is positively correlated with rates of neurogenesis. splash and splhairy mRNA are expressed not only in the olfactory organ but also in other tissues, albeit at lower levels, irrespective of molt stage. We conclude that the expression of achaete-scute and hairy-enhancer of split in the proliferation zone of the olfactory organ of spiny lobsters and their enhanced expression in premolt animals suggest that they play a role in the proliferation of ORNs and that their expression in regions of the olfactory organ populated by mature ORNs and in other tissues suggests that they have additional functions.

Evolution of the achaete-scute complex in insects: convergent duplication of proneural genes

Proneural genes encode transcriptional activators of the basic Helix-loop-helix class that are involved in neuronal specification and differentiation. We have used the recent availability of genome sequences of multiple distant insect species to study the evolution of a family of proneural genes, the achaete-scute genes, and to examine their genomic organization and evolution. We document independent evolution of multiple copies of achaete-scute homologues and argue that this might have contributed to morphological diversity in Diptera and Lepidoptera.

Identification and expression of the achaete-scute complex in the silkworm, Bombyx mori

Recently, the study of achaete-scute (AS-C) homologues has contributed enormously to understanding of gene duplication and function evolution, particularly in Diptera. We identified four AS-C homologue genes in the silkworm, Bombyx mori, referred to as BmASH, BmASH2, BmASH3, and Bmase. The complex displayed tandem array structure in the genome. Analysis of spatial expression profiles showed that they all were expressed in obviously higher levels in wing disc than in other tissues, suggesting that they might play important roles in the development of the wing. Furthermore, we found that their expression profiles in the wing discs were mostly correlated with the development of the scales, especially the BmASH gene. RNA interference results further indicated that BmASH was necessary for scale formation in silkworm wing.

HLH-3 is a C. elegans Achaete/Scute protein required for differentiation of the hermaphrodite-specific motor neurons

Basic helix-loop-helix (bHLH) proteins of the Achaete/Scute (Ac/Sc) family are required for neurogenesis in both Drosophila and vertebrates. These transcription factors are commonly referred to as 'proneural' factors, as they promote neural fate in many contexts. Although Ac/Sc proteins have been studied in Hydra, jellyfish, many insects, and several vertebrates, the role of these proteins in Caenorhabditis elegans neurogenesis is relatively uncharacterized. The C. elegans genome consists of three Ac/Sc genes, previously identified as hlh-3, hlh-6, and hlh-14. Here, we characterize the role of hlh-3 in nervous system development. Although hlh-3 appears to be expressed in all neural precursors, we find that hlh-3 null mutants have a mostly functional nervous system. However, these mutants are egg-laying defective, resulting from a block in differentiation of the HSN motor neurons. Detectable HSNs have misdirected axon projection, which appears to result from a lack of netrin signaling within the HSNs. Thus, our findings suggest a novel link between Ac/Sc bHLH proteins and the expression of genes required for proper interpretation of axon guidance cues. Lastly, based on sequence identity, expression pattern, and a role in neural differentiation, hlh-3 is most likely an ortholog of Drosophila asense.

Self-regulatory circuits in dorsoventral axis formation of the short-germ beetle Tribolium castaneum

The rel/NF-kappaB transcription factor Dorsal controls dorsoventral (DV) axis formation in Drosophila. A stable nuclear gradient of Dorsal directly regulates approximately 50 target genes. In Tribolium castaneum (Tc), a beetle with an ancestral type of embryogenesis, the Dorsal nuclear gradient is not stable, but rapidly shrinks and disappears. We find that negative feedback accounts for this dynamic behavior: Tc-Dorsal and one of its target genes activate transcription of the IkB homolog Tc-cactus, terminating Dorsal function. Despite its transient role, Tc-Dorsal is strictly required to initiate DV polarity, as in Drosophila. However, unlike in Drosophila, embryos lacking Tc-Dorsal display a periodic pattern of DV cell fates along the AP axis, indicating that a self-organizing ectodermal patterning system operates independently of mesoderm or maternal DV polarity cues. Our results also elucidate how extraembryonic tissues are organized in short-germ embryos, and how patterning information is transmitted from the early embryo to the growth zone.

Filling the gap between identified neuroblasts and neurons in crustaceans adds new support for Tetraconata

The complex spatio-temporal patterns of development and anatomy of nervous systems play a key role in our understanding of arthropod evolution. However, the degree of resolution of neural processes is not always detailed enough to claim homology between arthropod groups. One example is neural precursors and their progeny in crustaceans and insects. Pioneer neurons of crustaceans and insects show some similarities that indicate homology. In contrast, the differentiation of insect and crustacean neuroblasts (NBs) shows profound differences and their homology is controversial. For Drosophila and grasshoppers, the complete lineage of several NBs up to formation of pioneer neurons is known. Apart from data on median NBs no comparable results exist for Crustacea. Accordingly, it is not clear where the crustacean pioneer neurons come from and whether there are NBs lateral to the midline homologous to those of insects. To fill this gap, individual NBs in the ventral neuroectoderm of the crustacean Orchestia cavimana were labelled in vivo with a fluorescent dye. A partial neuroblast map was established and for the first time lineages from individual NBs to identified pioneer neurons were established in a crustacean. Our data strongly suggest homology of NBs and their lineages, providing further evidence for a close insect-crustacean relationship.

The tumor suppressors Brat and Numb regulate transit-amplifying neuroblast lineages in Drosophila

In both vertebrates and insects, neurons typically arise from neural stem cells or terminally dividing intermediate progenitors. Here, we describe another mode of neurogenesis where neural stem cells generate secondary precursors that undergo multiple rounds of self-renewing transit-amplifying divisions. We identify the Posterior Asense-Negative (PAN) neuroblasts, which do not express the transcription factors Asense or Prospero. PAN neuroblasts rely on the segregating determinants Numb and Brat to generate smaller, secondary neuroblasts that in turn give rise to ganglion mother cells (GMCs) and neurons throughout larval development. In brat or numb mutants, misspecified secondary neuroblasts are unable to produce differentiated progeny and initiate tumor-like overgrowth. In prospero mutants, however, tumors arise from GMCs while secondary neuroblasts are correctly specified. Our data describe a transit-amplifying lineage in the Drosophila nervous system and suggest that different vulnerabilities in intermediate cell types can affect the outcome of tumor suppressor loss in stem cell lineages.

Analysis of four achaete-scute homologs in Bombyx mori reveals new viewpoints of the evolution and functions of this gene family

Background

achaete-scute complexe (AS-C) has been widely studied at genetic, developmental and evolutional levels. Genes of this family encode proteins containing a highly conserved bHLH domain, which take part in the regulation of the development of central nervous system and peripheral nervous system. Many AS-C homologs have been isolated from various vertebrates and invertebrates. Also, AS-C genes are duplicated during the evolution of Diptera. Functions besides neural development controlling have also been found in Drosophila AS-C genes.

Results

We cloned four achaete-scute homologs (ASH) from the lepidopteran model organism Bombyx mori, including three proneural genes and one neural precursor gene. Proteins encoded by them contained the characteristic bHLH domain and the three proneural ones were also found to have the C-terminal conserved motif. These genes regulated promoter activity through the Class A E-boxes in vitro. Though both Bm-ASH and Drosophila AS-C have four members, they are not in one by one corresponding relationships. Results of RT-PCR and real-time PCR showed that Bm-ASH genes were expressed in different larval tissues, and had well-regulated expressional profiles during the development of embryo and wing/wing disc.

Conclusion

There are four achaete-scute homologs in Bombyx mori, the second insect having four AS-C genes so far, and these genes have multiple functions in silkworm life cycle. AS-C gene duplication in insects occurs after or parallel to, but not before the taxonomic order formation during evolution.

Sog/Chordin is required for ventral-to-dorsal Dpp/BMP transport and head formation in a short germ insect

Bone morphogenetic protein (BMP) signaling plays a major role in dorsoventral patterning in vertebrates and in Drosophila. Remarkably, in Tribolium, a beetle with an ancestral type of insect development, early BMP/dpp exhibits differential expression along the anteroposterior axis. However, the BMP/Dpp inhibitor Sog/chordin is expressed ventrally and establishes a dorsal domain of BMP/Dpp activity by transporting BMPs toward the dorsal side, like in Drosophila. Loss of Tribolium Sog not only abolishes dorsoventral polarity in the ectoderm, but also leads to the complete absence of the CNS. This phenotype suggests that sog is the main BMP antagonist in Tribolium, in contrast to vertebrates and Drosophila, which possess redundant antagonists. Surprisingly, Sog also is required for head formation in Tribolium, as are the BMP antagonists in vertebrates. Thus, in Tribolium, the system of BMP and its antagonists is less complex than in Drosophila or vertebrates and combines features from both, suggesting that it might represent an ancestral state.

An ancient transcriptional regulatory linkage

Changes in gene regulatory networks are a major engine for creating developmental novelty during evolution. Conversely, regulatory linkages that survive for very long evolutionary periods might be characteristic of ancient and abstract functions of fundamental utility to all metazoans. The proneural genes, which encode a distinctive family of basic helix-loop-helix (bHLH) transcriptional activators, act to promote neural cell fates in the ectoderm of diverse species. Here we report that these genes have been associated for at least 600-700 million years--since before the cnidarian/bilaterian divergence--with a high-affinity binding site for Hairy/Enhancer of split (Hes) repressor proteins. We suggest that the systematic identification of such ancient and conserved connections will be a powerful means of uncovering the primordial functions of transcription factors and signaling systems.

The origin and evolution of stereotyped patterns of macrochaetes on the nota of cyclorraphous Diptera

A long-standing problem in evolutionary biology is how genetic variation arises within populations and evolves to make species anatomically different. Many of the morphological differences in body plans between animal groups are thought to result from changes in gene expression during development. The rules governing the structure and evolution of cis-regulatory gene sequences are unknown, however, and the evolution of traits between closely related species remains relatively unexplored at a molecular level. To study the evolution of gene regulation, it is necessary to find a tractable trait that varies between species and for which the genetic regulation is well known in at least one of the species. The stereotyped, two-dimensional pattern of bristles on the thorax of Drosophila has been intensively investigated and is due to a precise spatial expression of proneural genes. Other species of flies have different bristle patterns and so comparisons between them provide a good paradigm for the study of changes in gene regulation. Here, we review the current state of understanding of these changes.

Genetics of graviperception in animals

Gravity is a constant stimulus for life on Earth and most organisms have evolved structures to sense gravitational force and incorporate its influence into their behavioral repertoire. Here we focus attention on animals and their diverse structures for perceiving and responding to the gravitational vector-one of the few static reference stimuli for any mobile organism. We discuss vertebrate, arthropod, and nematode models from the perspective of the role that genetics is playing in our understanding of graviperception in each system. We describe the key sensory structures in each class of organism and present what is known about the genetic control of development of these structures and the molecular signaling pathways operating in the mature organs. We also discuss the role of large genetic screens in identifying specific genes with roles in mechanosensation and graviperception.

The expression pattern of genes involved in early neurogenesis suggests distinct and conserved functions in the diplopod Glomeris marginata

We have shown recently that the expression and function of proneural genes is conserved in chelicerates and myriapods, although groups of neural precursors are specified in the ventral neuroectoderm of these arthropod groups, rather than single cells as in insects and crustaceans. We present additional evidence that the pattern of neurogenesis seen in chelicerates and in previously analyzed myriapod species is representative of both arthropod groups, by analysing the formation of neural precursors in the diplopod Archispirostreptus sp. This raises the question as to what extent the genetic network has been modified to result in different modes of neurogenesis in the arthropod group. To find out which components of the neural genetic network might account for the different mode of neural precursor formation in chelicerates and myriapods, we identified genes in the diplopod Glomeris marginata that are known to be involved in early neurogenesis in Drosophila and studied their expression pattern. In Drosophila, early neurogenesis is controlled by proneural genes that encode HLH transcription factors. These genes belong to two major subfamilies, the achaete-scute group and the atonal group. Different proneural proteins activate both a common neural programme and distinct neuronal subtype-specific target genes. We show that the expression pattern of homologs of the Drosophila proneural genes daughterless, atonal, and Sox B1 are partially conserved in Glomeris mariginata. While the expression of the pan-neural gene snail is conserved in the ventral neuroectoderm of G. marginata, we found an additional expression domain in the ventral midline. We conclude that, although the components of the genetic network involved in specification of neural precursors seem to be conserved in chelicerates, myriapods, and Drosophila, the function of some of the genes might have changed during evolution.

Conservation of upstream regulators of scute on the notum of cyclorraphous Diptera

Bristles on the notum of many cyclorraphous flies are arranged into species-specific stereotyped patterns. Differences in the spatial expression of the proneural gene scute correlate with the positions of bristles in those species looked at so far. However, the examination of a number of genes encoding trans-regulatory factors, such as pannier, stripe, u-shaped, caupolican and wingless, indicates that they are expressed in conserved domains on the prospective notum. This suggests that the function of a trans-regulatory network of genes is relatively unchanged in derived Diptera, and that many differences are likely to be due to changes in cis-regulatory sequences of scute. In contrast, in Anopheles gambiae, a basal species with no stereotyped bristle pattern, the expression patterns of pannier and wingless are not conserved, and expression of AgASH, the Anopheles proneural gene, does not correlate in a similar manner with the bristle pattern. We discuss the possibility that independently acting cis-regulatory sequences at the scute locus may have arisen in the lineage giving rise to cyclorraphous flies.

Specification of neural precursor identity in the geophilomorph centipede Strigamia maritima

Despite differences in the formation of neural precursors, all arthropod species analyzed so far generate about 30 single precursors (insects/crustaceans) or precursor groups (chelicerates/myriapods) per hemi-segment. In Drosophila, each precursor has a distinct identity conferred by segment polarity and dorso-ventral patterning genes that subdivide the ventral neuroectoderm into a grid-like structure. Temporal patterning mechanisms generate additional diversity after delamination from the neuroectoderm. Previous work shows that the genetic network involved in recruitment and specification of neural precursors is conserved in arthropods. However, comparative studies on generation of precursor diversity are few and partial. Here, we test whether aspects of the Drosophila model may apply in the geophilomorph centipede Strigamia maritima. We describe precursor formation, based on morphology and on Delta and Notch expression. We then show that in S. maritima, hunchback and Krüppel are expressed in subsets of neural precursors generating distinct temporal expression domains within the plane of the neuroectoderm. This expression pattern suggests that temporal changes in spatial patterning cues may result in the ordered production of different neural identities. We suggest that temporal patterning mechanisms were present in the last common ancestor of arthropods, although the regulatory interactions of transcription factors might have diverged in the lineage leading to insects.

Evolution of early development of the nervous system: a comparison between arthropods

Large numbers of cells with unique neuronal specificity are generated during development of the central nervous system of animals. Here we discuss the events that generate cell diversity during early development of the ventral nerve cord of different arthropod groups. Neural precursors are generated in a spatial array in the epithelium of each hemisegment over a period of time. Spatial cues within the epithelium are thought to evolve as embryogenesis proceeds. This spatiotemporal information might generate diversity among the neural precursors in all arthropod groups, although the mechanisms regulating the positioning of individual precursors have diverged. However, distinct strategies for the generation of neuronal diversity have evolved in the different arthropod lineages that appear to correlate with specific modes of ontogenesis. We hypothesize that an evolutionary trend towards reduced cell numbers and possibly rapid embryogenesis in insects has culminated in the appearance of stereotyped neuroblast lineages.

The identification and expression of achaete-scute genes in the branchiopod crustacean Triops longicaudatus

The achaete-scute (ac/sc) genes are a highly conserved family of transcription factors that play important roles in the development of neural cells in both vertebrates and invertebrates. As such, the study of arthropod ac/sc gene expression during neurogenesis has become a model system for investigating the evolution of neural patterning. To date, ac/sc gene expression has been investigated in insects, chelicerates, and myriapods. Here we present the identification of two ac/sc genes from the branchiopod crustacean Triops longicaudatus. Triops longicaudatus achaete-scute homologs1 and 2 (Tl-ASH1 and Tl-ASH2) exhibit dynamic and distinct expression profiles during Triops neurogenesis. Tl-ASH1 expression initiates in nearly all cells of the neurogenic region and subsequently in clusters of cells evenly spaced along the length of the developing limbs. In contrast, Tl-ASH2 initiates expression after Tl-ASH1. In the CNS, only a subset of Tl-ASH1 cells appears to express Tl-ASH2. Similarly, in the PNS individual Tl-ASH2 positive cells appear to arise from the clusters of Tl-ASH1 expressing cells. Shortly after activating Tl-ASH2 expression, these cells enlarge and divide. The expression dynamics of ac/sc genes in Triops parallel those observed in insects and contrasts with those found in chelicerates and myriapods.

Ultrabithorax is required for membranous wing identity in the beetle Tribolium castaneum

The two pairs of wings that are characteristic of ancestral pterygotes (winged insects) have often undergone evolutionary modification. In the fruitfly, Drosophila melanogaster, differences between the membranous forewings and the modified hindwings (halteres) depend on the Hox gene Ultrabithorax (Ubx). The Drosophila forewings develop without Hox input, while Ubx represses genes that are important for wing development, promoting haltere identity. However, the idea that Hox input is important to the morphologically specialized wing derivatives such as halteres, and not the more ancestral wings, requires examination in other insect orders. In beetles, such as Tribolium castaneum, it is the forewings that are modified (to form elytra), while the hindwings retain a morphologically more ancestral identity. Here we show that in this beetle Ubx 'de-specializes' the hindwings, which are transformed to elytra when the gene is knocked down. We also show evidence that elytra result from a Hox-free state, despite their diverged morphology. Ubx function in the hindwing seems necessary for a change in the expression of spalt, iroquois and achaete-scute homologues from elytron-like to more typical wing-like patterns. This counteracting effect of Ubx in beetle hindwings represents a previously unknown mode of wing diversification in insects.

The Tribolium columnar genes reveal conservation and plasticity in neural precursor patterning along the embryonic dorsal–ventral axis

The Drosophila columnar genes are key regulators of neural precursor formation and patterning along the dorsal-ventral axis of the developing CNS and include ventral nerve cord defective (vnd), intermediate nerve cord defective (ind), muscle segment homeodomain (msh), and Epidermal growth factor receptor (Egfr). To investigate the evolution of neural pattern formation, we identified and determined the expression patterns of Tribolium vnd, ind, and msh, and found that they are expressed in the medial, intermediate, and lateral columns of the developing CNS, respectively, in patterns similar, but not identical, to their Drosophila orthologs. The pattern of Egfr activity suggests that the genetic regulatory mechanisms that initiate Tc-vnd expression are similar in Drosophila and Tribolium, whereas those that initiate Tc-ind have diverged. RNAi analyses of gene function show that Tc-vnd and Tc-ind promote the formation of medial and intermediate column neural precursors and that vnd-mediated repression of ind establishes the boundary between the medial and intermediate columns. These data suggest that columnar gene expression and function underlie neural pattern formation in Drosophila, Tribolium, and potentially all insects, but that subtle spatiotemporal differences in expression of these genes may produce species-specific morphological differences.