Three maternal coordinate systems cooperate in the patterning of the Drosophila head

Three foxg1 paralogues in lampreys and gnathostomes-brothers or cousins?

Foxg1 is a key regulator of the early development of the vertebrate forebrain and sensory organs. In this study, we describe for the first time three foxg1 paralogues in lamprey, representative of one of two basally diverged lineages of vertebrates-the agnathans. We also first describe three foxg1 genes in sterlet-representative of one of the evolutionarily ancient clades of gnathostomes. According to the analysis of local genomic synteny, three foxg1 genes of agnathans and gnathostomes have a common origin as a result of two rounds of genomic duplications in the early evolution of vertebrates. At the same time, it is difficult to reliably establish pairwise orthology between foxg1 genes of agnathans and gnathostomes based on the analysis of phylogeny and local genomic synteny, as well as our studies of the spatiotemporal expression of foxg1 genes in the river lamprey Lampetra fluviatilis and the sterlet Acipenser ruthenus. Thus, the appearance of three foxg1 paralogues in agnathans and gnathostomes could have occurred either as a result of two rounds of duplication of the vertebrate common ancestor genome (2R hypothesis) or as a result of the first common round followed by subsequent independent polyploidizations in two evolutionary lineages (1R hypothesis).

Shaping the scaling characteristics of gap gene expression patterns in Drosophila

How patterns are formed to scale with tissue size remains an unresolved problem. Here we investigate embryonic patterns of gap gene expression along the anterior-posterior (AP) axis in Drosophila. We use embryos that greatly differ in length and, importantly, possess distinct length-scaling characteristics of the Bicoid (Bcd) gradient. We systematically analyze the dynamic movements of gap gene expression boundaries in relation to both embryo length and Bcd input as a function of time. We document the process through which such dynamic movements drive both an emergence of a global scaling landscape and evolution of boundary-specific scaling characteristics. We show that, despite initial differences in pattern scaling characteristics that mimic those of Bcd in the anterior, such characteristics of final patterns converge. Our study thus partitions the contributions of Bcd input and regulatory dynamics inherent to the AP patterning network in shaping embryonic pattern's scaling characteristics.

Empty-spiracles is maternally expressed and essential for neurodevelopment and early embryo determination in Rhodnius prolixus

Since empty-spiracles (ems) was identified and characterized in Drosophila melanogaster as a head-gap gene, several studies have been carried out in other insect orders to confirm its evolutionary conserved function. Using the blood-sucking bug Rhodnius prolixus as biological model, we found an ems transcript with three highly conserved regions: Box-A, Box-B, and the homeodomain. R. prolixus embryos silenced by parental RNAi for two of these ems conserved regions showed both maternal and zygotic defects. Rp-emsB fragment results in early lethal embryogenesis, with eggs without any embryonic structure inside. Rp-emsB expression pattern is only maternally expressed and localized in the ovary tropharium, follicular cells, and in the unfertilized female pronucleus. Rp-emsA fragment is zygotically expressed during early blastoderm formation until late developmental stages in two main patterns: anterior in the antennal segment, and in a segmentary in the neuroblast and tracheal pits. R. prolixus knockdown embryos for Rp-emsA showed an incomplete larval hatching, reduced heads, and severe neuromotor defects. Furthermore, in situ hybridization revealed a spatial and temporal expression pattern that highly correlates with Rp-ems observed function. Here,Rp-ems function in R. prolixus development was validated, showing that empty-spiracles does not act as a true head-gap gene, but it is necessary for proper head development and crucial for early embryo determination and neurodevelopment.

A comprehensive study of arthropod and onychophoran Fox gene expression patterns

Fox genes represent an evolutionary old class of transcription factor encoding genes that evolved in the last common ancestor of fungi and animals. They represent key-components of multiple gene regulatory networks (GRNs) that are essential for embryonic development. Most of our knowledge about the function of Fox genes comes from vertebrate research, and for arthropods the only comprehensive gene expression analysis is that of the fly Drosophila melanogaster. For other arthropods, only selected Fox genes have been investigated. In this study, we provide the first comprehensive gene expression analysis of arthropod Fox genes including representative species of all main groups of arthropods, Pancrustacea, Myriapoda and Chelicerata. We also provide the first comprehensive analysis of Fox gene expression in an onychophoran species. Our data show that many of the Fox genes likely retained their function during panarthropod evolution highlighting their importance in development. Comparison with published data from other groups of animals shows that this high degree of evolutionary conservation often dates back beyond the last common ancestor of Panarthropoda.

The Genetic Network of Forkhead Gene Family in Development of Brown Planthoppers

We identified 18 distinct Fox genes in the genome of the brown planthopper, Nilaparvata lugens, and further found a novel insect-specific subfamily that we temporarily named FoxT. A total of 16 genes were highly expressed in the eggs, while NlFoxL2 and NlFoxT are female- and male-specific genes, respectively. Large scale RNAi and RNA-seq analyses were used to reveal the functions and potential targets of NlFoxs. In the eggs, NlFoxA, NlFoxN1 and NlFoxN2 are indispensable to early embryogenesis by regulating different target genes; NlFoxG and NlFoxQ co-regulate NlSix3 for brain development; and NlFoxC, NlFoxJ1 and NlFoxP have complementary effects on late embryogenesis. Moreover, NlFoxA, NlFoxNl and NlFoxQ have pleiotropism. NlFoxA and NlFoxQ regulate the expression of NlCHS1 and cuticular proteins, respectively, thereby participating in the formation of cuticles. NlFoxN1, which regulates the expression of NlKrt9 is involved in the formation of intermediate filament frameworks. Our previous studies have revealed that NlFoxL2 and NlFoxO play important roles in chorion formation and wing polyphenism. Altogether, N. lugens Fox genes exhibit functional diversity in embryonic development and organogenesis. This comprehensive study combines genomics, transcriptomics and phenomics, thereby constructing a complex genetic network that spans the entire life cycle of the brown planthopper.

Panarthropod tiptop/teashirt and spalt orthologs and their potential role as "trunk"-selector genes

BACKGROUND

In the vinegar fly Drosophila melanogaster, the homeodomain containing transcription factor Teashirt (Tsh) appears to specify trunk identity in concert with the function of the Hox genes. While in Drosophila there is a second gene closely related to tsh, called tiptop (tio), in other arthropods species only one copy exists (called tio/tsh). The expression of tsh and tio/tsh, respectively, is surprisingly similar among arthropods suggesting that its function as trunk selector gene may be conserved. Other research, for example on the beetle Tribolium castaneum, questions even conservation of Tsh function among insects. The zinc-finger transcription factor Spalt (Sal) is involved in the regulation of Drosophila tsh, but this regulatory interaction does not appear to be conserved in Tribolium either. Whether the function and interaction of tsh and sal as potential trunk-specifiers, however, is conserved is still unclear because comparative studies on sal expression (except for Tribolium) are lacking, and functional data are (if at all existing) restricted to Insecta.

RESULTS

Here, we provide additional data on arthropod tsh expression, show the first data on onychophoran tio/tsh expression, and provide a comprehensive investigation on sal expression patterns in arthropods and an onychophoran.

CONCLUSIONS

Our data support the idea that tio/tsh genes are involved in the development of "trunk" segments by regulating limb development. Our data suggest further that the function of Sal is indeed unlikely to be conserved in trunk vs head development like in Drosophila, but early expression of sal is in line with a potential homeotic function, at least in Arthropoda.

Tissue-Specific Actions of Pax6 on Proliferation and Differentiation Balance in Developing Forebrain Are Foxg1 Dependent

Differences in the growth and maturation of diverse forebrain tissues depend on region-specific transcriptional regulation. Individual transcription factors act simultaneously in multiple regions that develop very differently, raising questions about the extent to which their actions vary regionally. We found that the transcription factor Pax6 affects the transcriptomes and the balance between proliferation and differentiation in opposite directions in the diencephalon versus cerebral cortex. We tested several possible mechanisms to explain Pax6's tissue-specific actions and found that the presence of the transcription factor Foxg1 in the cortex but not in the diencephalon was most influential. We found that Foxg1 is responsible for many of the differences in cell cycle gene expression between the diencephalon and cortex and, in cortex lacking Foxg1, Pax6's action on the balance of proliferation versus differentiation becomes diencephalon like. Our findings reveal a mechanism for generating regional forebrain diversity in which one transcription factor completely reverses the actions of another.

Formation and subdivision of the head field in the centipede Strigamia maritima, as revealed by the expression of head gap gene orthologues and hedgehog dynamics

Background

There have been few studies of head patterning in non-insect arthropods, and even in the insects, much is not yet understood. In the fly Drosophila three head gap genes, orthodenticle (otd), buttonhead (btd) and empty spiracles (ems) are essential for patterning the head. However, they do not act through the same pair-rule genes that pattern the trunk from the mandibular segment backwards. Instead they act through the downstream factors collier (col) and cap‘n’collar (cnc), and presumably other unknown factors. In the beetle Tribolium, these same gap and downstream genes are also expressed during early head development, but in more restricted domains, and some of them have been shown to be of minor functional importance. In the spider Parasteatoda tepidariorum, hedgehog (hh) and otd have been shown to play an important role in head segmentation.

Results

We have investigated the expression dynamics of otx (otd), SP5/btd, ems, and the downstream factors col, cnc and hh during early head development of the centipede Strigamia maritima. Our results reveal the process of head condensation and show that the anteroposterior sequence of specific gene expression is conserved with that in insects. SP5/btd and otx genes are expressed prior to and during head field formation, whereas ems is not expressed until after the initial formation of the head field, in an emerging gap between SP5/btd and otx expression. Furthermore, we observe an early domain of Strigamia hh expression in the head field that splits to produce segmental stripes in the ocular, antennal and intercalary segments.

Conclusions

The dynamics of early gene expression in the centipede show considerable similarity with that in the beetle, both showing more localised expression of head gap genes than occurs in the fly. This suggests that the broad overlapping domains of head gap genes observed in Drosophila are derived in this lineage. We also suggest that the splitting of the early hh segmental stripes may reflect an ancestral and conserved process in arthropod head patterning. A remarkably similar stripe splitting process has been described in a spider, and in the Drosophila head hh expression starts from a broad domain that transforms into three stripes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-017-0082-x) contains supplementary material, which is available to authorized users.

Evolutionary conservation and conversion of Foxg1 function in brain development

Among the forkhead box protein family, Foxg1 is a unique transcription factor that plays pleiotropic and non-redundant roles in vertebrate brain development. The emergence of the telencephalon at the rostral end of the neural tube and its subsequent expansion that is mediated by Foxg1 was a key reason for the vertebrate brain to acquire higher order information processing, where Foxg1 is repetitively used in the sequential events of telencephalic development to control multi-steps of brain circuit formation ranging from cell cycle control to neuronal differentiation in a clade- and species-specific manner. The objective of this review is to discuss how the evolutionary changes in cis- and trans-regulatory network that is mediated by a single transcription factor has contributed to determining the fundamental vertebrate brain structure and its divergent roles in instructing species-specific neuronal circuitry and functional specialization.

Repression activity of Tailless on h 1 and eve 1 pair-rule stripes

We investigated the hypothesis that several transcriptional repressors are necessary to set the boundaries of anterior pair-rule stripes in Drosophila. Specifically, we tested whether Tailless (Tll) is part of a repression mechanism that correctly sets the anterior boundaries of hairy 1 (h 1) and even-skipped 1 (eve 1) stripes. Single mutant tll embryos displayed subtle deviations from the normal positions of h 1 and eve 1 stripes. Moreover, we observed stronger stripe deviations in embryos lacking both Tll and Sloppy-paired 1 (Slp 1), a common repressor for anterior pair-rule stripes. Using h 1 and eve 1 reporter constructs in the genetic assays, we provided further evidence that interference with normal mechanisms of stripe expression is mediated by Tll repression. Indeed, Tll represses both h 1 and eve 1 reporter stripes when misexpressed. Investigating the expression of other anterior gap genes in different genetic backgrounds and in the misexpression assays strengthened Tll direct repression in the regulation of h 1 and eve 1. Our results are consistent with tll being a newly-identified component of a combinatorial network of repressor genes that control pair-rule stripe formation in the anterior blastoderm of Drosophila.

Positional information, positional error, and readout precision in morphogenesis: a mathematical framework

The concept of positional information is central to our understanding of how cells determine their location in a multicellular structure and thereby their developmental fates. Nevertheless, positional information has neither been defined mathematically nor quantified in a principled way. Here we provide an information-theoretic definition in the context of developmental gene expression patterns and examine the features of expression patterns that affect positional information quantitatively. We connect positional information with the concept of positional error and develop tools to directly measure information and error from experimental data. We illustrate our framework for the case of gap gene expression patterns in the early Drosophila embryo and show how information that is distributed among only four genes is sufficient to determine developmental fates with nearly single-cell resolution. Our approach can be generalized to a variety of different model systems; procedures and examples are discussed in detail.

Tc-knirps plays different roles in the specification of antennal and mandibular parasegment boundaries and is regulated by a pair-rule gene in the beetle Tribolium castaneum

Background

The Drosophila larval head is evolutionarily derived at the genetic and morphological level. In the beetle Tribolium castaneum, development of the larval head more closely resembles the ancestral arthropod condition. Unlike in Drosophila, a knirps homologue (Tc-kni) is required for development of the antennae and mandibles. However, published Tc-kni data are restricted to cuticle phenotypes and Tc-even-skipped and Tc-wingless stainings in knockdown embryos. Hence, it has remained unclear whether the entire antennal and mandibular segments depend on Tc-kni function, and whether the intervening intercalary segment is formed completely. We address these questions with a detailed examination of Tc-kni function.

Results

By examining the expression of marker genes in RNAi embryos, we show that Tc-kni is required only for the formation of the posterior parts of the antennal and mandibular segments (i.e. the parasegmental boundaries). Moreover, we find that the role of Tc-kni is distinct in these segments: Tc-kni is required for the initiation of the antennal parasegment boundary, but only for the maintenance of the mandibular parasegmental boundary. Surprisingly, Tc-kni controls the timing of expression of the Hox gene Tc-labial in the intercalary segment, although this segment does form in the absence of Tc-kni function. Unexpectedly, we find that the pair-rule gene Tc-even-skipped helps set the posterior boundary of Tc-kni expression in the mandible. Using the mutant antennaless, a likely regulatory Null mutation at the Tc-kni locus, we provide evidence that our RNAi studies represent a Null situation.

Conclusions

Tc-kni is required for the initiation of the antennal and the maintenance of the mandibular parasegmental boundaries. Tc-kni is not required for specification of the anterior regions of these segments, nor the intervening intercalary segment, confirming that Tc-kni is not a canonical ‘gap-gene’. Our finding that a gap gene orthologue is regulated by a pair rule gene adds to the view that the segmentation gene hierarchies differ between Tribolium and Drosophila upstream of the pair rule gene level. In Tribolium, as in Drosophila, head and trunk segmentation gene networks cooperate to pattern the mandibular segment, albeit involving Tc-kni as novel component.

Dynamic interpretation of maternal inputs by the Drosophila segmentation gene network

Patterning of body parts in multicellular organisms relies on the interpretation of transcription factor (TF) concentrations by genetic networks. To determine the extent by which absolute TF concentration dictates gene expression and morphogenesis programs that ultimately lead to patterns in Drosophila embryos, we manipulate maternally supplied patterning determinants and measure readout concentration at the position of various developmental markers. When we increase the overall amount of the maternal TF Bicoid (Bcd) fivefold, Bcd concentrations in cells at positions of the cephalic furrow, an early morphological marker, differ by a factor of 2. This finding apparently contradicts the traditional threshold-dependent readout model, which predicts that the Bcd concentrations at these positions should be identical. In contrast, Bcd concentration at target gene expression boundaries is nearly unchanged early in development but adjusts dynamically toward the same twofold change as development progresses. Thus, the Drosophila segmentation gene network responds faithfully to Bcd concentration during early development, in agreement with the threshold model, but subsequently partially adapts in response to altered Bcd dosage, driving segmentation patterns toward their WT positions. This dynamic response requires other maternal regulators, such as Torso and Nanos, suggesting that integration of maternal input information is not achieved through molecular interactions at the time of readout but through the subsequent collective interplay of the network.

Modeling the evolution of gene regulatory networks for spatial patterning in embryo development

A central question in evolutionary biology concerns the transition between discrete numbers of units (e.g. vertebrate digits, arthropod segments). How do particular numbers of units, robust and characteristic for one species, evolve into another number for another species? Intermediate phases with a diversity of forms have long been theorized, but these leave little fossil or genomic data. We use evolutionary computations (EC) of a gene regulatory network (GRN) model to investigate how embryonic development is altered to create new forms. The trajectories are epochal and non-smooth, in accord with both the observed stability of species and the evolvability between forms.

Candidate gene screen in the red flour beetle Tribolium reveals six3 as ancient regulator of anterior median head and central complex development

Several highly conserved genes play a role in anterior neural plate patterning of vertebrates and in head and brain patterning of insects. However, head involution in Drosophila has impeded a systematic identification of genes required for insect head formation. Therefore, we use the red flour beetle Tribolium castaneum in order to comprehensively test the function of orthologs of vertebrate neural plate patterning genes for a function in insect head development. RNAi analysis reveals that most of these genes are indeed required for insect head capsule patterning, and we also identified several genes that had not been implicated in this process before. Furthermore, we show that Tc-six3/optix acts upstream of Tc-wingless, Tc-orthodenticle1, and Tc-eyeless to control anterior median development. Finally, we demonstrate that Tc-six3/optix is the first gene known to be required for the embryonic formation of the central complex, a midline-spanning brain part connected to the neuroendocrine pars intercerebralis. These functions are very likely conserved among bilaterians since vertebrate six3 is required for neuroendocrine and median brain development with certain mutations leading to holoprosencephaly.

Whole-embryo modeling of early segmentation in Drosophila identifies robust and fragile expression domains

Segmentation of the Drosophila melanogaster embryo results from the dynamic establishment of spatial mRNA and protein patterns. Here, we exploit recent temporal mRNA and protein expression measurements on the full surface of the blastoderm to calibrate a dynamical model of the gap gene network on the entire embryo cortex. We model the early mRNA and protein dynamics of the gap genes hunchback, Kruppel, giant, and knirps, taking as regulatory inputs the maternal Bicoid and Caudal gradients, plus the zygotic Tailless and Huckebein proteins. The model captures the expression patterns faithfully, and its predictions are assessed from gap gene mutants. The inferred network shows an architecture based on reciprocal repression between gap genes that can stably pattern the embryo on a realistic geometry but requires complex regulations such as those involving the Hunchback monomer and dimers. Sensitivity analysis identifies the posterior domain of giant as among the most fragile features of an otherwise robust network, and hints at redundant regulations by Bicoid and Hunchback, possibly reflecting recent evolutionary changes in the gap-gene network in insects.

Huckebein is part of a combinatorial repression code in the anterior blastoderm

The hierarchy of the segmentation cascade responsible for establishing the Drosophila body plan is composed by gap, pair-rule and segment polarity genes. However, no pair-rule stripes are formed in the anterior regions of the embryo. This lack of stripe formation, as well as other evidence from the literature that is further investigated here, led us to the hypothesis that anterior gap genes might be involved in a combinatorial mechanism responsible for repressing the cis-regulatory modules (CRMs) of hairy (h), even-skipped (eve), runt (run), and fushi-tarazu (ftz) anterior-most stripes. In this study, we investigated huckebein (hkb), which has a gap expression domain at the anterior tip of the embryo. Using genetic methods we were able to detect deviations from the wild-type patterns of the anterior-most pair-rule stripes in different genetic backgrounds, which were consistent with Hkb-mediated repression. Moreover, we developed an image processing tool that, for the most part, confirmed our assumptions. Using an hkb misexpression system, we further detected specific repression on anterior stripes. Furthermore, bioinformatics analysis predicted an increased significance of binding site clusters in the CRMs of h 1, eve 1, run 1 and ftz 1when Hkb was incorporated in the analysis, indicating that Hkb plays a direct role in these CRMs. We further discuss that Hkb and Slp1, which is the other previously identified common repressor of anterior stripes, might participate in a combinatorial repression mechanism controlling stripe CRMs in the anterior parts of the embryo and define the borders of these anterior stripes.

Second order regulator Collier directly controls intercalary-specific segment polarity gene expression

In Drosophila, trunk metamerization is established by a cascade of segmentation gene activities: the gap genes, the pair rule genes, and the segment polarity genes. In the anterior head, metamerization requires also gap-like genes and segment polarity genes. However, because the pair rule genes are not active in this part of the embryo, the question on which gene activities are fulfilling the role of the second order regulator genes still remains to be solved. Here we provide first molecular evidence that the Helix-Loop-Helix-COE transcription factor Collier fulfills this role by directly activating the expression of the segment polarity gene hedgehog in the posterior part of the intercalary segment. Collier thereby occupies a newly identified binding site within an intercalary-specific cis-regulatory element. Moreover, we identified a direct physical association between Collier and the basic-leucine-zipper transcription factor Cap'n'collar B, which seems to restrict the activating input of Collier to the posterior part of the intercalary segment and to lead to the attenuation of hedgehog expression in the intercalary lobes at later stages.

A novel mutation of HOXA10 in a Chinese woman with a Mullerian duct anomaly

BACKGROUND

Müllerian duct anomalies consist of a set of congenital structural malformations that occur when the Müllerian ducts do not develop properly during embryonic life. Their molecular genetic basis is poorly understood.

METHODS

In this study, we conducted mutation analysis of the HOXA10 gene in a cohort of 109 Chinese women with Müllerian duct anomalies.

RESULTS

We identified a novel mutation (Y57C) in one patient with a didelphic uterus. The mutation affected the transcriptional regulation capacity of HOXA10.

CONCLUSIONS

Our study showed that mutation of HOXA10 gene may contribute to the development of Müllerian duct anomalies and confirmed that HOXA10 is an important transcription factor in reproductive tract development.

A novel role for the AAA ATPase spastin as a HOXA10 transcriptional corepressor in Ishikawa endometrial cells

Homeobox A10 (HOXA10), a transcription factor required for uterine development and embryo receptivity, functions downstream of estrogen and progesterone in uterine endometrium. HOXA10 represses endometrial expression of empty spiracles homeobox 2 (EMX2), the human ortholog of Drosophila empty spiracles. The ATPases associated with various cellular activities (AAA) ATPase spastin has a well-characterized role in neurotransmitter trafficking. In this study, we characterize a novel role of spastin in transcriptional regulation. We identified spastin as a novel component of the HOXA10 transcriptional complex in Ishikawa nuclear extracts by immunoprecipitation and mass spectrophotometry. Using EMX2 as a model endometrial HOXA10 target gene, we show that the HOXA10-spastin corepressor complex bound the EMX2 promoter in chromatin immunoprecipitation assays. HOXA10 has been previously shown to repress endometrial EMX2 expression. We further observed that, although cotransfection of HOXA10 and spastin continued to repress endometrial EMX2-luciferase expression, the repression was reversed when spastin small interfering RNA was cotransfected with HOXA10. Mutations in the nuclear localization signal sequences of spastin abrogated not only its nuclear translocation but also its colocalization with HOXA10 as well as reversed EMX2-luciferase repression. Here, we describe a novel role for the AAA ATPase spastin in Ishikawa cells as a HOXA10 corepressor of EMX2. Uterine EMX2 levels are inversely related to embryo implantation rates. HOXA10 acts downstream of progesterone and has been shown to facilitate embryo implantation through regulation of endometrial EMX2 expression. Endometrial spastin, therefore, likely has a novel function downstream of estrogen and progesterone in implantation biology as a cofactor of HOXA10.

Unique establishment of procephalic head segments is supported by the identification of cis-regulatory elements driving segment-specific segment polarity gene expression in Drosophila

Anterior head segmentation is governed by different regulatory mechanisms than those that control trunk segmentation in Drosophila. For segment polarity genes, both initial mode of activation as well as cross-regulatory interactions among them differ from the typical genetic circuitry in the trunk and are unique for each of the procephalic segments. In order to better understand the segment-specific gene network responsible for the procephalic expression of the earliest active segment polarity genes wingless and hedgehog, we started to identify and analyze cis-regulatory DNA elements of these genes. For hedgehog, we could identify a cis-regulatory element, ic-CRE, that mediates expression specifically in the posterior part of the intercalary segment and requires promoter-specific interaction for its function. The intercalary stripe is the last part of the metameric hedgehog expression pattern that appears during embryonic development, which probably reflects the late and distinct establishment of this segment. The identification of a cis-regulatory element that is specific for one head segment supports the mutant-based observation that the expression of segment polarity genes is governed by a unique gene network in each of the procephalic segments. This provides further indication that the anterior-most head segments represent primary segments, which are set up independently, in contrast to the secondary segments of the trunk, which resemble true repetitive units.

The gap gene network

Gap genes are involved in segment determination during the early development of the fruit fly Drosophila melanogaster as well as in other insects. This review attempts to synthesize the current knowledge of the gap gene network through a comprehensive survey of the experimental literature. I focus on genetic and molecular evidence, which provides us with an almost-complete picture of the regulatory interactions responsible for trunk gap gene expression. I discuss the regulatory mechanisms involved, and highlight the remaining ambiguities and gaps in the evidence. This is followed by a brief discussion of molecular regulatory mechanisms for transcriptional regulation, as well as precision and size-regulation provided by the system. Finally, I discuss evidence on the evolution of gap gene expression from species other than Drosophila. My survey concludes that studies of the gap gene system continue to reveal interesting and important new insights into the role of gene regulatory networks in development and evolution.

Genetics, development and composition of the insect head--a beetle's view

Many questions regarding evolution and ontogeny of the insect head remain open. Likewise, the genetic basis of insect head development is poorly understood. Recently, the investigation of gene expression data and the analysis of patterning gene function have revived interest in insect head development. Here, we argue that the red flour beetle Tribolium castaneum is a well suited model organism to spearhead research with respect to the genetic control of insect head development. We review recent molecular data and discuss its bearing on early development and morphogenesis of the head. We present a novel hypothesis on the ontogenetic origin of insect head sutures and review recent insights into the question on the origin of the labrum. Further, we argue that the study of developmental genes may identify the elusive anterior non-segmental region and present some evidence in favor of its existence. With respect to the question of evolution of patterning we show that the head Anlagen of the fruit fly Drosophila melanogaster and Tribolium differ considerably and we review profound differences of their genetic regulation. Finally, we discuss which insect model species might help us to answer the open questions concerning the genetic regulation of head development and its evolution.

Imaging of transgenic cricket embryos reveals cell movements consistent with a syncytial patterning mechanism

The mode of insect embryogenesis varies among species, reflecting adaptations to different life history strategies [1, 2]. In holometabolous insects, which include the model systems, such as the fruit fly and the red flour beetle, a large proportion of the blastoderm produces an embryo, whereas hemimetabolous embryos generally arise from a small region of the blastoderm [3]. Despite their importance in evolutionary studies, information of early developmental dynamics of hemimetabolous insects remains limited. Here, to clarify how maternal and gap gene products act in patterning the embryo of basal hemimetabolous insects, we analyzed the dynamic segmentation process in transgenic embryos of an intermediate-germ insect species, the cricket, Gryllus bimaculatus. Our data based on live imaging of fluorescently labeled embryonic cells and nuclei suggest that the positional specification of the cellular blastoderm may be established in the syncytium, where maternally derived gradients could act fundamentally in a way that is similar to that of Drosophila, namely throughout the egg. Then, the blastoderm cells move dynamically, retaining their positional information to form the posteriorly localized germ anlage. Furthermore, we find that the anterior head region of the cricket embryo is specified by orthodenticle in a cellular environment earlier than the gnathal and thoracic regions. Our findings imply that the syncytial mode of the early segmentation in long-germ insects evolved from a dynamic syncytial-to-cellular mode found in the present study, accompanied by a heterochronic shift of gap gene action.

The bicoid morphogen system

Several fundamental concepts of developmental biology have emerged from studies on the early development of the Drosophila melanogaster embryo. In the late 1980s, studies on Bicoid provided the first solid experimental evidence for the existence of morphogenetic gradients and their implication in axial patterning. Bicoid has since stimulated further research, bringing together developmental and cell biologists, physicists and theoreticians to address fundamental biological questions. These include mechanistic aspects of transcriptional and translational control, molecular and functional aspects of evolution and, more recently with the development of quantitative approaches, the robustness of axial patterning in a systems biology view. However, recent studies provide data which lead to contradictory interpretations. Here, we discuss these recent observations, highlighting the data helping to understand how anterior patterning is achieved under the control of Bicoid and point to novel challenges for future studies.

Ems and Nkx6 are central regulators in dorsoventral patterning of the Drosophila brain

In central nervous system development, the identity of neural stem cells (neuroblasts) critically depends on the precise spatial patterning of the neuroectoderm in the dorsoventral (DV) axis. Here, we uncover a novel gene regulatory network underlying DV patterning in the Drosophila brain, and show that the cephalic gap gene empty spiracles (ems) and the Nk6 homeobox gene (Nkx6) encode key regulators. The regulatory network implicates novel interactions between these and the evolutionarily conserved homeobox genes ventral nervous system defective (vnd), intermediate neuroblasts defective (ind) and muscle segment homeobox (msh). We show that Msh cross-repressively interacts with Nkx6 to sustain the boundary between dorsal and intermediate neuroectoderm in the tritocerebrum (TC) and deutocerebrum (DC), and that Vnd positively regulates Nkx6 by suppressing Msh. Remarkably, Ems is required to activate Nkx6, ind and msh in the TC and DC, whereas later Nkx6 and Ind act together to repress ems in the intermediate DC. Furthermore, the initially overlapping expression of Ems and Vnd in the ventral/intermediate TC and DC resolves into complementary expression patterns due to cross-repressive interaction. These results indicate that the anteroposterior patterning gene ems controls the expression of DV genes, and vice versa. In addition, in contrast to regulation in the ventral nerve cord, cross-inhibition between homeodomain factors (between Ems and Vnd, and between Nkx6 and Msh) is essential for the establishment and maintenance of discrete DV gene expression domains in the Drosophila brain. This resembles the mutually repressive relationship between pairs of homeodomain proteins that pattern the vertebrate neural tube in the DV axis.

Antagonistic action of Bicoid and the repressor Capicua determines the spatial limits of Drosophila head gene expression domains

Bicoid (Bcd) is the anterior determinant in Drosophila. Accordingly, loss of Bcd causes loss of head and thorax and their replacement with posterior structures. bcd mRNA is maternally deposited at the anterior pole and Bcd forms an anterior-to-posterior (AP) concentration gradient. The expression of a series of zygotic head genes is thought to be differentially regulated by distinct threshold concentrations of the Bcd gradient. Thereby Bcd functions as a morphogen, instructing fields of cells to take on specific fates. Here, we show that spatial limits of anterior genes are also set in the absence of a Bcd gradient and depend on factors of the maternal terminal system. The receptor tyrosine kinase Torso (Tor), a key component of this system, is active in the pole regions of the embryo. Its activity downregulates the maternally deposited repressor Capicua (Cic), leaving high Cic activity in the central regions and decreasingly lower Cic activities toward the poles. We show that the positions of posterior boundaries of Bcd target genes are dependent not only on Bcd, but also on Tor-mediated Cic activity. The results indicate that Cic can mediate repression through distinct binding sites within a Bcd responsive enhancer and that gene activation by Bcd is antagonized by Cic. The activating and repressive effects of Bcd and Cic, respectively, are integrated by the Bcd target gene enhancer. We conclude that the spatial domains of head gene expression are determined by Bcd in concert with Tor-dependent repressors.

Formation of the insect head involves lateral contribution of the intercalary segment, which depends on Tc-labial function

The insect head is composed of several segments. During embryonic development, the segments fuse to form a rigid head capsule where obvious segmental boundaries are lacking. Hence, the assignment of regions of the insect head to specific segments is hampered, especially with respect to dorsal (vertex) and lateral (gena) parts. We show that upon Tribolium labial (Tc-lab) knock down, the intercalary segment is deleted but not transformed. Furthermore, we find that the intercalary segment contributes to lateral parts of the head cuticle in Tribolium. Based on several additional mutant and RNAi phenotypes that interfere with gnathal segment development, we show that these segments do not contribute to the dorsal head capsule apart from the dorsal ridge. Opposing the classical view but in line with findings in the vinegar fly Drosophila melanogaster and the milkweed bug Oncopeltus fasciatus, we propose a "bend and zipper" model for insect head capsule formation.

Anterior-posterior positional information in the absence of a strong Bicoid gradient

The Bicoid (Bcd) transcription factor is distributed as a long-range concentration gradient along the anterior posterior (AP) axis of the Drosophila embryo. Bcd is required for the activation of a series of target genes, which are expressed at specific positions within the gradient. Here we directly tested whether different concentration thresholds within the Bcd gradient establish the relative positions of its target genes by flattening the gradient and systematically varying expression levels. Genome-wide expression profiles were used to estimate the total number of Bcd target genes, and a general correlation was found between the Bcd concentration required for activation and the positions where target genes are expressed in wild-type embryos. However, concentrations required for target gene activation in embryos with flattened Bcd were consistently lower than those present at each target gene's position in the wild-type gradient, suggesting that Bcd is in excess at every position along the AP axis. Also, several Bcd target genes were positioned in correctly ordered stripes in embryos with flattened Bcd, and we suggest that these stripes are normally regulated by interactions between Bcd and the terminal patterning system. Our findings argue strongly against the strict interpretation of the Bcd morphogen hypothesis, and support the idea that target gene positioning involves combinatorial interactions that are mediated by the binding site architecture of each gene's cis-regulatory elements.

Divergent functions of orthodenticle, empty spiracles and buttonhead in early head patterning of the beetle Tribolium castaneum (Coleoptera)

The head gap genes orthodenticle (otd), empty spiracles (ems) and buttonhead (btd) are required for metamerization and segment specification in Drosophila. We asked whether the function of their orthologs is conserved in the red flour beetle Tribolium castaneum which in contrast to Drosophila develops its larval head in a way typical for insects. We find that depending on dsRNA injection time, two functions of Tc-orthodenticle1 (Tc-otd1) can be identified. The early regionalization function affects all segments formed during the blastoderm stage while the later head patterning function is similar to Drosophila. In contrast, both expression and function of Tc-empty spiracles (Tc-ems) are restricted to the posterior part of the ocular and the anterior part of the antennal segment and Tc-buttonhead (Tc-btd) is not required for head cuticle formation at all. We conclude that the gap gene like roles of ems and btd are not conserved while at least the head patterning function of otd appears to be similar in fly and beetle. Hence, the ancestral mode of insect head segmentation remains to be discovered. With this work, we establish Tribolium as a model system for arthropod head development that does not suffer from the Drosophila specific problems like head involution and strongly reduced head structures.

Expression of otd orthologs in the amphipod crustacean, Parhyale hawaiensis

The arthropod head is a complex metameric structure. In insects, orthodenticle (otd) functions as a 'head gap gene' and plays a significant role in patterning and development of the anterior head ectoderm, the protocerebrum, and the ventral midline. In this study, we characterize the structure and developmental deployment of two otd paralogs in the amphipod crustacean, Parhyale hawaiensis. Photd1 is initially expressed at gastrulation through germband stages in a bilaterally symmetric, restricted region of the anterior head ectoderm and also in a single column of cells along the ventral midline. Late in embryogenesis, Photd1 is expressed within the developing anterior brain and the expression along the embryonic midline has become restricted to a stereotypic group of segmentally reiterated cells. The second ortholog Photd2, however, has a unique temporal-spatial expression pattern and is not detected until after the head lobes have been organized in the developing ectoderm of the germband during late germband stages. Anteriorly, Photd2 is coincident with the Photd1 head expression domain; however, Photd2 is not detected along the ventral midline during formation of the germband and only appears in the ventral midline late in embryonic development in a restricted group of cells distinct from those expressing Photd1. The early expression of Photd1 in the anterior head ectoderm is consistent with a role as a head gap gene. The more posterior expression of Photd1 is suggestive of a role in patterning the embryonic ventral midline. Photd2 expression appears too late to play a role in early head patterning but may contribute to latter patterning in restricted regions of both the head and the ventral midline. The comparative analysis of otd reveals the divergence of gene expression and gene function associated with duplication of this important developmental gene.

Evolutionary flexibility of pair-rule patterning revealed by functional analysis of secondary pair-rule genes, paired and sloppy-paired in the short-germ insect, Tribolium castaneum

In the Drosophila segmentation hierarchy, periodic expression of pair-rule genes translates gradients of regional information from maternal and gap genes into the segmental expression of segment polarity genes. In Tribolium, homologs of almost all the eight canonical Drosophila pair-rule genes are expressed in pair-rule domains, but only five have pair-rule functions. even-skipped, runt and odd-skipped act as primary pair-rule genes, while the functions of paired (prd) and sloppy-paired (slp) are secondary. Since secondary pair-rule genes directly regulate segment polarity genes in Drosophila, we analyzed Tc-prd and Tc-slp to determine the extent to which this paradigm is conserved in Tribolium. We found that the role of prd is conserved between Drosophila and Tribolium; it is required in both insects to activate engrailed in odd-numbered parasegments and wingless (wg) in even-numbered parasegments. Similarly, slp is required to activate wg in alternate parasegments and to maintain the remaining wg stripes in both insects. However, the parasegmental register for Tc-slp is opposite that of Drosophila slp1. Thus, while prd is functionally conserved, the fact that the register of slp function has evolved differently in the lineages leading to Drosophila and Tribolium reveals an unprecedented flexibility in pair-rule patterning.

Orthodenticle and empty spiracles genes are expressed in a segmental pattern in chelicerates

Members of the orthodenticle (otd/Otx) and empty spiracles (ems/Emx) gene families are head gap genes that encode homeodomain-containing DNA-binding proteins. Although numerous studies show their central role in developmental processes in brain specification, a surprisingly high number of other developmental processes have been shown to involve their expression. In this paper, we report the identification and expression of ems and otd in two chelicerate species: a scorpion, Euscorpius flavicaudis (Chactidae, Scorpiona, Arachnida, Euchelicerata) and a spider, Tegenaria saeva (Aranea, Arachnida, Euchelicerata). We show that both ems and otd are expressed not only in an anterior head domain but also along the entire anterior-posterior axis during embryonic development. The expression patterns for both genes are typically segmental and concern neurectodermal territories. During patterning of the opisthosoma, ems and otd are expressed in the lateral ectoderm just anterior to the limb bud primordia giving rise to respiratory organs and spinnerets (spider). This common pattern found in two divergent species thus appears to be a conserved character of chelicerates. These results are discussed in terms of evolutionary origin of respiratory organs and/or functional pathway recruitment.

Endocrine regulation of HOX genes

Hox genes have a well-characterized role in embryonic development, where they determine identity along the anteroposterior body axis. Hox genes are expressed not only during embryogenesis but also in the adult, where they are necessary for functional differentiation. Despite the known function of these genes as transcription factors, few regulatory mechanisms that drive Hox expression are known. Recently, several hormones and their cognate receptors have been shown to regulate Hox gene expression and thereby mediate development in the embryo as well as functional differentiation in the adult organism. Estradiol, progesterone, testosterone, retinoic acid, and vitamin D have been shown to regulate Hox gene expression. In the embryo, the endocrine system directs axial Hox gene expression; aberrant Hox gene expression due to exposure to endocrine disruptors contributes to the teratogenicity of these compounds. In the adult, endocrine regulation of Hox genes is necessary to enable such diverse functions as hematopoiesis and reproduction; endocrinopathies can result in dysregulated HOX gene expression affecting physiology. By regulating HOX genes, hormonal signals utilize a conserved mechanism that allows generation of structural and functional diversity in both developing and adult tissues. This review discusses endocrine Hox regulation and its impact on physiology and human pathology.

Modeling segmental patterning in Drosophila: Maternal and gap genes

We propose a new mathematical model describing the establishment of maternal and gap proteins segmental patterning along the antero-posterior axis of the Drosophila early embryo. This model is based on experimental data and, without recurring to pre-defined activation thresholds, predicts qualitatively and quantitatively the expression patterns of the maternal and gap proteins, as well as the expression patterns of proteins resulting from mRNA ectopic expression and from some loss-of-function mutations. We conclude that the gap genes segmental patterning and consequent spatial organization of the embryo is determined by three main factors: (1) the initial positioning of the maternal bicoid and torso mRNA inside the egg, and subsequent diffusion of the corresponding proteins; (2) the structure of the genetic regulatory network; (3) the role of conservation laws in the establishment of steady and non-uniform spatial distributions of non-diffusing proteins.

Insights into the urbilaterian brain: conserved genetic patterning mechanisms in insect and vertebrate brain development

Recent molecular genetic analyses of Drosophila melanogaster and mouse central nervous system (CNS) development revealed strikingly similar genetic patterning mechanisms in the formation of the insect and vertebrate brain. Thus, in both insects and vertebrates, the correct regionalization and neuronal identity of the anterior brain anlage is controlled by the cephalic gap genes otd/Otx and ems/Emx, whereas members of the Hox genes are involved in patterning of the posterior brain. A third intermediate domain on the anteroposterior axis of the vertebrate and insect brain is characterized by the expression of the Pax2/5/8 orthologues, suggesting that the tripartite ground plans of the protostome and deuterostome brains share a common evolutionary origin. Furthermore, cross-phylum rescue experiments demonstrate that insect and mammalian members of the otd/Otx and ems/Emx gene families can functionally replace each other in embryonic brain patterning. Homologous genes involved in dorsoventral regionalization of the CNS in vertebrates and insects show remarkably similar patterning and orientation with respect to the neurogenic region (ventral in insects and dorsal in vertebrates). This supports the notion that a dorsoventral body axis inversion occurred after the separation of protostome and deuterostome lineages in evolution. Taken together, these findings demonstrate conserved genetic patterning mechanisms in insect and vertebrate brain development and suggest a monophyletic origin of the brain in protostome and deuterostome bilaterians.

Dynamical analysis of regulatory interactions in the gap gene system of Drosophila melanogaster

Genetic studies have revealed that segment determination in Drosophila melanogaster is based on hierarchical regulatory interactions among maternal coordinate and zygotic segmentation genes. The gap gene system constitutes the most upstream zygotic layer of this regulatory hierarchy, responsible for the initial interpretation of positional information encoded by maternal gradients. We present a detailed analysis of regulatory interactions involved in gap gene regulation based on gap gene circuits, which are mathematical gene network models used to infer regulatory interactions from quantitative gene expression data. Our models reproduce gap gene expression at high accuracy and temporal resolution. Regulatory interactions found in gap gene circuits provide consistent and sufficient mechanisms for gap gene expression, which largely agree with mechanisms previously inferred from qualitative studies of mutant gene expression patterns. Our models predict activation of Kr by Cad and clarify several other regulatory interactions. Our analysis suggests a central role for repressive feedback loops between complementary gap genes. We observe that repressive interactions among overlapping gap genes show anteroposterior asymmetry with posterior dominance. Finally, our models suggest a correlation between timing of gap domain boundary formation and regulatory contributions from the terminal maternal system.

Groucho-dependent repression by Sloppy-paired 1 differentially positions anterior pair-rule stripes in the Drosophila embryo

The Drosophila body plan is composed of a linear array of cephalic, thoracic, and abdominal segments along the anterior posterior axis. The number and positions of individual segments are established by a transcriptional network comprised of maternal effect, gap, pair-rule, and segment polarity genes. The sloppy-paired (slp) locus contains two genes (slp1 and slp2) that are expressed in overlapping striped patterns in the presumptive thorax and abdomen. Previous studies suggest that these genes function at the pair-rule and segment polarity levels to establish the spacing and polarity of thoracic and abdominal segments. One of these genes (slp1) is also expressed in a broad anterior domain that appears before the striped patterns. There are severe cephalic defects in slp1 mutants, including the complete loss of the mandibular segment, but the molecular roles played by Slp1 in anterior patterning are not clear. Here, we present evidence that the anterior Slp1 domain acts as a gradient to differentially repress the anteriormost stripes of several different pair-rule genes. This repressive gradient contributes to the precise spatial arrangement of anterior pair-rule stripe borders required for expression of the first engrailed stripe and the formation of the mandibular segment. These results suggest that Slp1 functions as a gap gene-like repressor, in addition to its roles at the pair-rule and segment polarity levels of the hierarchy. The Slp1 protein contains a protein motif (EH1) which mediates binding to the transcriptional corepressor Groucho (Gro). We show that this domain is required for Slp1-mediated repression in vivo.

The Role ofHOXGenes in Human Implantation

The endometrium undergoes an ordered process of differentiation leading to receptivity to embryonic implantation. HOX genes direct this development in a fashion similar to that in which they direct embryonic development, including development of the reproductive tract. HOXA10 and HOXA11 expression increases during the menstrual cycle, increasing drastically in the midluteal phase, at the time of implantation. This expression is regulated by sex steroid hormones. This expression is necessary for implantation of the blastocyst as demonstrated by the decreased implantation rates in women with altered HOX expression. HOX genes are markers of endometrial receptivity. The possibility of augmenting HOX gene expression with gene therapy to improve implantation has promise for the future.

The urbilaterian brain: developmental insights into the evolutionary origin of the brain in insects and vertebrates

Classical phylogenetic, neuroanatomical and neuroembryological studies propose an independent evolutionary origin of the brains of insects and vertebrates. Contrasting with this, data from three sets of molecular and genetic analyses indicate that the developmental program of brains of insects and vertebrates might be highly conserved and suggest a monophyletic origin of the brain of protostomes and deuterostomes. First, recent results of molecular phylogeny imply that none of the currently living animals correspond to evolutionary intermediates between protostomes and deuterostomes, thus making it impossible to infer the morphological organization of an ancestral bilaterian brain from living specimens. Second, recent molecular genetic evidence provides support for the body axis inversion hypothesis, which implies that a dorsoventral inversion of the body axis occurred in protostomes versus deuterostomes, leading to the inverted location of neurogenic regions in these animal groups. Third, recent developmental genetic analyses are uncovering the existence of structurally and functionally homologous genes that have comparable and interchangeable functions in early brain development in insect and vertebrate model systems. Thus, development of the anteriormost part of the embryonic brain in both insects and vertebrates depends upon the otd/Otx and ems/Emx genes; development of the posterior part of the embryonic brain in both insects and vertebrates involves homologous control genes of the Hox cluster. These findings, which demonstrate the conserved expression and function of key patterning genes involved in embryonic brain development in insects and vertebrates support the hypothesis that the brains of protostomes and deuterostomes are of monophyletic, urbilaterian origin.

Transcriptional repression of peri-implantation EMX2 expression in mammalian reproduction by HOXA10

HOXA10 is necessary for mammalian reproduction; however, its transcriptional targets are not completely defined. EMX2, a divergent homeobox gene, is necessary for urogenital tract development. In these studies we identify and characterize the regulation of EMX2 by HOXA10. By using Northern analysis and in situ hybridization, we found that EMX2 is expressed in the adult urogenital tract in an inverse temporal pattern from HOXA10, suggestive of a negative regulatory relationship. Constitutive expression of HOXA10 diminished EMX2 mRNA, whereas blocking HOXA10 through the use of antisense resulted in high EMX2 mRNA expression. Deletional analysis of the EMX2 5' regulatory region revealed that a 150-bp element mediated transcriptional repression when cotransfected with pcDNA3.1/HOXA10 in transient-transfection assays. Binding of HOXA10 protein to this element was demonstrated by electrophoretic mobility shift assay and further localized to a consensus HOXA10 binding site within this element by DNase I footprinting. Site-directed mutagenesis abolished binding, as well as the negative transcriptional regulation. Transcriptional activation of empty spiracles, the Drosophila ortholog of EMX2, by Abdominal-B (HOXA10 ortholog) has been previously demonstrated. These findings demonstrate conservation of the transcription factor-target gene relationship, although the direction of regulation is reversed with possible evolutionary implications.

BEST: a novel computational approach for comparing gene expression patterns from early stages of Drosophila melanogaster development

Embryonic gene expression patterns are an indispensable part of modern developmental biology. Currently, investigators must visually inspect numerous images containing embryonic expression patterns to identify spatially similar patterns for inferring potential genetic interactions. The lack of a computational approach to identify pattern similarities is an impediment to advancement in developmental biology research because of the rapidly increasing amount of available embryonic gene expression data. Therefore, we have developed computational approaches to automate the comparison of gene expression patterns contained in images of early stage Drosophila melanogaster embryos (prior to the beginning of germ-band elongation); similarities and differences in gene expression patterns in these early stages have extensive developmental effects. Here we describe a basic expression search tool (BEST) to retrieve best matching expression patterns for a given query expression pattern and a computational device for gene interaction inference using gene expression pattern images and information on the associated genotypes and probes. Analysis of a prototype collection of Drosophila gene expression pattern images is presented to demonstrate the utility of these methods in identifying biologically meaningful matches and inferring gene interactions by direct image content analysis. In particular, the use of BEST searches for gene expression patterns is akin to that of BLAST searches for finding similar sequences. These computational developmental biology methodologies are likely to make the great wealth of embryonic gene expression pattern data easily accessible and to accelerate the discovery of developmental networks.

Interaction of gap genes in the Drosophila head: tailless regulates expression of empty spiracles in early embryonic patterning and brain development

Unlike gap genes in the trunk region of Drosophila embryos, gap genes in the head were presumed not to regulate each other's transcription. Here, we show that in tailless (tll) loss-of-function mutants the empty spiracles (ems) expression domain in the head expands, whereas it retracts in tll gain-of-function embryos. We have identified a 304bp element in the ems-enhancer which is sufficient to drive expression in the head and brain and which contains two TLL and two BCD binding sites. Transgenic reporter gene lines containing mutations of the TLL binding sites demonstrate that tll directly inhibits the expression of ems in the early embryonic head and the protocerebral brain anlage. These results are the first demonstration of direct transcriptional regulation between gap genes in the head.

Common developmental genetic mechanisms for patterning invertebrate and vertebrate brains

Recent genetic studies on embryonic brain development in the fly Drosophila melanogaster together with investigations on early morphogenesis and patterning in the embryonic brain of the mouse revealed developmental mechanisms that are strikingly similar in insects and mammals. The homeotic (Hox) genes are expressed in a virtually colinear anteroposterior pattern in the developing posterior brain of insects and mammals, where they are required for the specification of segmental neuronal identity. The otd/Otx cephalic gap genes are expressed in the anterior brain of insects and mammals and are of central importance for its formation because in both phyla loss of otd/Otx2 causes the loss of the entire rostral brain. Specific Pax genes are involved in numerous aspects of brain development in both phyla. These developmental genetic findings reveal a striking evolutionary conservation of cephalic gap gene, homeotic gene, and Pax gene action in embryonic brain development that extends beyond gene structure to encompass patterned expression and function. This comparative evidence indicates that the genetic programs which direct embryonic brain development are remarkably conserved and lends further support to the hypothesis that a common molecular bauplan underlies brain development in invertebrates and vertebrates. In consequence, it seems increasingly likely that both modern brain types share their evolutionary origin in a common ancestral bilaterian brain which was established before the protostome-deuterostome divergence over 600 million years ago.

Cellular and molecular aspects of striatal development

The striatum is a key component of the basal ganglia and there is considerable evidence that it has an important role in motor, cognitive and limbic functions. However, very little is known about how this forebrain structure develops. This review considers the role of cellular and molecular mechanisms involved in the development of the striatum, and the potential application of this knowledge to the understanding of the pathology and treatment of primary disease of this structure.

Different ways to make a head

In Drosophila, the establishment of the larval head and thorax depends on the transcription factors BICOID and HUNCHBACK, and on signalling mediated by the receptor tyrosine kinase TORSO. Genetic experiments described in two recent papers(1, 2) demonstrate that these factors can, to a large extent, replace each other, revealing a surprising degree of plasticity in establishing larval anterior structures. The commutability of developmental factors might in part reflect the evolutionary history of the system. BioEssays 23:8-11, 2001.

Hmx: an evolutionary conserved homeobox gene family expressed in the developing nervous system in mice and Drosophila

Three homeobox genes, one from Drosophila melanogaster (Drosophila Hmx gene) and two from mouse (murine Hmx2 and Hmx3) were isolated and the full-length cDNAs and corresponding genomic structures were characterized. The striking homeodomain similarity encoded by these three genes to previously identified genes in sea urchin, chick and human, as well as the recently cloned murine Hmx1 gene, and the low homology to other homeobox genes indicate that the Hmx genes comprise a novel gene family. The widespread existence of Hmx genes in the animal kingdom suggests that this gene family is of ancient origin. Drosophila Hmx was mapped to the 90B5 region of Chromosome 3 and at early embryonic stages is primarily expressed in distinct areas of the neuroectoderm and subsets of neuroblasts in the developing fly brain. Later its expression continues in rostral areas of the brain in a segmented pattern, suggesting a putative role in the development of the Drosophila central nervous system. During evolution, mouse Hmx2 and Hmx3 may have retained a primary function in central nervous system development as suggested by their expression in the postmitotic cells of the neural tube, as well as in the hypothalamus, the mesencephalon, metencephalon and discrete regions in the myelencephalon during embryogenesis. Hmx1 has diverged from other Hmx members by its expression in the dorsal root, sympathetic and vagal nerve (X) ganglia. Aside from their expression in the developing nervous system, all three Hmx genes display expression in sensory organ development, and in the adult uterus. Hmx2 and Hmx3 show identical expression in the otic vesicle, whereas Hmx1 is strongly expressed in the developing eye. Transgenic mouse lines were generated to examine the DNA regulatory elements controlling Hmx2 and Hmx3. Transgenic constructs spanning more than 31 kb of genomic DNA gave reproducible expression patterns in the developing central and peripheral nervous systems, eye, ear and other tissues, yet failed to fully recapitulate the endogenous expression pattern of either Hmx2 or Hmx3, suggesting both the presence and absence of certain critical enhancers in the transgenes, or the requirement of proximal enhancers to work synergistically.

High bicoid levels render the terminal system dispensable for Drosophila head development

In Drosophila, the gradient of the Bicoid (Bcd) morphogen organizes the anteroposterior axis while the ends of the embryo are patterned by the maternal terminal system. At the posterior pole, expression of terminal gap genes is mediated by the local activation of the Torso receptor tyrosine kinase (Tor). At the anterior, terminal gap genes are also activated by the Tor pathway but Bcd contributes to their activation. Here we present evidence that Tor and Bcd act independently on common target genes in an additive manner. Furthermore, we show that the terminal maternal system is not required for proper head development, since high levels of Bcd activity can functionally rescue the lack of terminal system activity at the anterior pole. This observation is consistent with a recent evolution of an anterior morphogenetic center consisting of Bcd and anterior Tor function.