Embryonic expression of the single Tribolium engrailed homolog

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.

Functional Divergence of the Tribolium castaneum engrailed and invected Paralogs

Engrailed (en) and invected (inv) encode paralogous transcription factors found as a closely linked tandem duplication within holometabolous insects. Drosophila en mutants segment normally, then fail to maintain their segments. Loss of Drosophila inv is viable, while loss of both genes results in asegmental larvae. Surprisingly, the knockdown of Oncopeltus inv can result in the loss or fusion of the entire abdomen and en knockdowns in Tribolium show variable degrees of segmental loss. The consequence of losing or knocking down both paralogs on embryogenesis has not been studied beyond Drosophila. To further investigate the relative functions of each paralog and the mechanism behind the segmental loss, Tribolium double and single knockdowns of en and inv were analyzed. The most common cuticular phenotype of the double knockdowns was small, limbless, and open dorsally, with all but a single, segmentally iterated row of bristles. Less severe knockdowns had fused segments and reduced appendages. The Tribolium paralogs appear to act synergistically: the knockdown of either Tribolium gene alone was typically less severe, with all limbs present, whereas the most extreme single knockdowns mimic the most severe double knockdown phenotype. Morphological abnormalities unique to either single gene knockdown were not found. inv expression was not affected in the Tribolium en knockdowns, but hh expression was unexpectedly increased midway through development. Thus, while the segmental expression of en/inv is broadly conserved within insects, the functions of en and inv are evolving independently in different lineages.

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

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

Serial Homology and Segment Identity in the Arthropod Head

The anterior-most unit of the crown-group arthropod body plan includes three segments, the pre-gnathal segments, that contain three neuromeres that together comprise the brain. Recent work on the development of this anterior region has shown that its three units exhibit many developmental differences to the more posterior segments, to the extent that they should not be considered serial homologs. Building on this revised understanding of the development of the pre-gnathal segments, we suggest a novel scenario for arthropod head evolution. We posit an expansion of an ancestral single-segmented head at the transition from Radiodonta to Deuteropoda in the arthropod stem group. The expanded head subdivided into three segmental units, each maintaining some of the structures of the ancestral head. This scenario is consistent with what we know of head evolution from the fossil record and helps reconcile some of the debates about early arthropod evolution.

Lack of evidence for conserved parasegmental grooves in arthropods

In the arthropod model species Drosophila melanogaster, a dipteran fly, segmentation of the anterior–posterior body axis is under control of a hierarchic gene cascade. Segmental boundaries that form morphological grooves are established posteriorly within the segmental expression domain of the segment-polarity gene (SPG) engrailed (en). More important for the development of the fly, however, are the parasegmental boundaries that are established at the interface of en expressing cells and anteriorly adjacent wingless (wg) expressing cells. In Drosophila, both segmental and transient parasegmental grooves form. The latter are positioned anterior to the expression of en. Although the function of the SPGs in establishing and maintaining segmental and parasegmental boundaries is highly conserved among arthropods, parasegmental grooves have only been reported for Drosophila, and a spider (Cupiennius salei). Here, we present new data on en expression, and re-evaluate published data, from four distantly related spiders, including Cupiennius, and a distantly related chelicerate, the harvestman Phalangium opilio. Gene expression analysis of en genes in these animals does not corroborate the presence of parasegmental grooves. Consequently, our data question the general presence of parasegmental grooves in arthropods.

Development of the Pre-gnathal Segments in the Milkweed Bug Oncopeltus fasciatus Suggests They Are Not Serial Homologs of Trunk Segments

The three anterior-most segments in arthropods contain the ganglia that make up the arthropod brain. These segments, the pre-gnathal segments (PGS), are known to exhibit many developmental differences to other segments, believed to reflect their divergent morphology. We have analyzed the expression and function of the genes involved in the conserved segment-polarity network, including genes from the Wnt and Hedgehog pathways, in the PGS, compared with the trunk segments, in the hemimetabolous insect Oncopeltus fasciatus. Gene function was tested by manipulating expression through RNA interference against components of the two pathways. We show that there are fundamental differences in the expression patterns of the segment polarity genes, in the timing of their expression and in the interactions among them in the process of pre-gnathal segment generation, relative to all other segments. We argue that given these differences, the PGS should not be considered serially homologous to trunk segments. This realization raises important questions about the differing evolutionary ancestry of different regions of the arthropod head.

A pair-rule function of odd-skipped in germband stages of Tribolium development

While pair-rule patterning has been observed in most insects examined, the orthologs of Drosophila pair-rule genes have shown divergent roles in insect segmentation. In the beetle Tribolium castaneum, while odd-skipped (Tc-odd) was expressed as a series of pair-rule stripes, RNAi-mediated knockdown of Tc-odd (Tc-odd^RNAi) resulted in severely truncated, almost asegmental phenotypes rather than the classical pair-rule phenotypes observed in germbands and larval cuticles. However, considering that most segments arise later in germband stages of Tribolium development, the roles of Tc-odd in segmentation of growing germbands could not be analyzed properly in the truncated Tc-odd^RNAi germbands. Here, we investigated the segmentation function of Tc-odd in germband stages of Tribolium development by analyzing Tc-odd^RNAi embryos that resumed germband extension. In the larval cuticles of Tc-odd^RNAi embryos, normal mandibular and maxillary and loss of the labial segments were consistent in the head, whereas a broad range of segmentation defects including loss or fusion of thoracic and/or abdominal segments was observed in the trunk. Interestingly, a group of Tc-odd^RNAi germbands showed pair-rule-like defects in the segmental stripes of the segment-polarity genes, engrailed, hedgehog, or wingless, in the abdominal regions. While the pair-rule genes even-skipped, runt, odd, and paired were misregulated in the growing Tc-odd^RNAi germbands, paired expression required for odd-numbered segment formation was largely abolished, which might cause the pair-rule-like defects. Taken together, these findings suggest that Tc-odd can function as a pair-rule gene in the germband stages of Tribolium development.

even-skipped acts as a pair-rule gene in germ band stages of Tribolium development

The pair-rule gene even-skipped (eve) is essential for insect segmentation, yet its function varies among insect clades. While loss of eve results in typical pair-rule phenotypes in Drosophila, knock-down of eve orthologs shows segmental, gap-like, or asegmental phenotypes in non-Drosophila insects. In Tribolium, knock-down of the eve ortholog (Tc-eve) resulted in a graded phenotypic series ranging from strong to weak, the most informative of which was intermediate phenotypes. The strong knock-down embryos displayed asegmental phenotypes and severely disorganized germ bands which have prevented determination of Tc-eve function in later stages. In order to understand the segmentation function of Tc-eve during later germ band elongation stages, we analyzed intermediate Tc-eve^RNAi embryos in which germ band elongation was less affected. Most intermediate Tc-eve^RNAi germ bands displayed segmentation defects with a double segmental periodicity in the abdomen. In these intermediate embryos, Tc-engrailed (Tc-en) stripes were ectopically expanded into large bands with a double segmental periodicity, while the remaining Tc-en stripes between the expanded Tc-en stripes were absent or barely formed. The expanded Tc-en stripes seemed to be activated by primary Tc-eve stripes and Tc-paired, both of which failed to resolve into secondary segmental stripes. The absence of Tc-en stripes appeared to be a consequence of the absence of the secondary stripes of Tc-runt that were required for the activation of Tc-en stripes. These results suggest that Tc-eve functions as a pair-rule gene at least in the germ band stages of Tribolium development.

Functional analysis of engrailed in Tribolium segmentation

The segment-polarity gene engrailed is required for segmentation in the early Drosophila embryo. Loss of Engrailed function results in segmentation defects that vary in severity from pair-rule phenotypes to a lawn phenotype lacking in obvious of segmentation. During segmentation, Engrailed is expressed in stripes with a single segmental periodicity in Drosophila, which is conserved in all arthropods examined so far. To define segments, the segmental stripes of Engrailed induce the segmental stripes of wingless at each parasegmental boundary. However, segmentation functions of orthologs of engrailed in non-Drosophila arthropods have yet to be reported. Here, we analyzed functions of the Tribolium ortholog of engrailed (Tc-engrailed) during embryonic segmentation. Larval cuticles with Tc-engrailed being knocked down had segmentation phenotypes including incomplete segment formation and loss of a group of segments. In agreement with the cuticle segmentation defects, segments developed incompletely and irregularly or did not form in Tribolium germbands where Tc-engrailed was knocked down. Furthermore, knock-down of Tc-engrailed did not properly express the segmental stripes of wingless in Tribolium germbands. Taken together with the conserved expression patterns of Engrailed in arthropod segmentation, our data suggest that Tc-engrailed is required for embryonic segmentation in Tribolium, and the genetic mechanism of Engrailed inducing wingless expression is conserved at least between Drosophila and Tribolium.

Evolution of limb development in cephalopod mollusks

Cephalopod mollusks evolved numerous anatomical novelties, including arms and tentacles, but little is known about the developmental mechanisms underlying cephalopod limb evolution. Here we show that all three axes of cuttlefish limbs are patterned by the same signaling networks that act in vertebrates and arthropods, although they evolved limbs independently. In cuttlefish limb buds, Hedgehog is expressed anteriorly. Posterior transplantation of Hedgehog-expressing cells induced mirror-image limb duplications. Bmp and Wnt signals, which establish dorsoventral polarity in vertebrate and arthropod limbs, are similarly polarized in cuttlefish. Inhibition of Bmp2/4 dorsally caused ectopic expression of Notum, which marks the ventral sucker field, and ectopic sucker development. Cuttlefish also show proximodistal regionalization of Hth, Exd, Dll, Dac, Sp8/9, and Wnt expression, which delineates arm and tentacle sucker fields. These results suggest that cephalopod limbs evolved by parallel activation of a genetic program for appendage development that was present in the bilaterian common ancestor.

A Role for buttonhead in the Early Head and Trunk Development in the Beetle Tribolium castaneum

Thehead gap gene buttonhead (btd) is required for the patterning of head segments in the early Drosophila embryo. Mutant phenotypes of btd display a gap-like phenotype in which antennal, intercalary, mandibular and the anterior portion of the maxillary segmentsare eliminated. In agreement with the phenotypes, btd is expressed in a stripe covering the head segments at the blastoderm stage. During the early phase of the germband extension, btd is expressed in stripes with single segmental periodicity, which is required for the formation of the peripheral nervous system. In contrast to the key role of btd in Drosophila embryonic development, it has been suggested that Tribolium ortholog of btd (Tc-btd) is dispensable for embryonic head development. In order for better understanding of the requirement of Tc-btd in the early Tribolium embryo, we re-analyzed the expression patterns and functions of Tc-btd during embryonic segmentation. Tc-btd is expressed in segmental stripes at the stages of blastoderm and germband elongation. Up to 28.3% of embryos in which Tc-btd is knocked down displays the loss of antennal, mandibular and the pregnathal regions in the head, with abdominal segments being disrupted in the trunk. Our findings suggest that Tc-btd is required for the head and trunk development in the early Tribolium embryo.

Establishment of molecular genetic approaches to study gene expression and function in an invasive hemipteran, Halyomorpha halys

Hemiptera is a large clade of insects understudied in terms of developmental biology. Halyomorpha halys, the Brown Marmorated Stink Bug (BMSB, referred to throughout as H. halys), is an invasive hemipteran pest of the mid-Atlantic region of the USA that has rapidly spread to other regions in recent years, devastating a wide range of crops using a piercing and sucking mechanism. Its phylogenetic position, polyphagous habits, and rapid spread in the USA suggested that H. halys would be an ideal system to broaden our knowledge of developmental mechanisms in insects. We and others previously generated transcriptome sequences from different life stages of this insect. Here, we describe tools to examine gene expression patterns in whole-mount H. halys embryos and to test the response of H. halys to RNA interference (RNAi). We show that spatial and temporal patterns of gene expression in H. halys can be effectively monitored by both immunostaining and in situ hybridization. We also show that delivery of dsRNA to adult females knocks down gene function in offspring, using the homeotic gene Sex combs reduced (Scr). Knockdown of Hh-Scr resulted in dramatic malformations of the mouthparts, demonstrating for the first time that RNAi is effective in this species. Our results suggest that, despite difficulties with long-term laboratory culture of H. halys, this species shows promise as a developmental system.

Contribution of cell proliferation to axial elongation in the red flour beetle Tribolium castaneum

Most arthropods generate their posterior bodies by adding segments periodically, as the embryo grows, from a posteriorly located region called the segment addition zone. This mode of segmentation is shared with vertebrates and relies on oscillatory mechanisms, where the temporal periodicity of a clock is translated into repetitive spatial patterns. This ordered anterior-to-posterior pattern is achieved at the same time as the tissue elongates, opening the question of the functional coordination between the mechanisms of segmental patterning and posterior growth. The study of these processes in different arthropods has played an important role in unravelling some of the molecular mechanisms of segment formation. However, the behavior of cells during elongation and how cellular processes affect this segmental patterning has been poorly studied. Cell proliferation together with cell rearrangements are presumed to be the major forces driving axis elongation in the red flour beetle Tribolium castaneum. However, there still no strong evidence about the role and distribution of cell proliferation within the embryo. In this study, we propose to address these questions by using whole embryo cultures and pharmacological manipulation. We show that considerable cell proliferation occurs during germband elongation, measured by incorporation of the nucleoside analog of thymidine 5-Ethynyl-2’-deoxyuridine, EdU. Moreover, proliferating cells appeared to be spread along the elongating embryo with a posterior bias at early segmentation. In addition, when we blocked cell division, treated germbands were always shorter than controls and in some cases not able to fully elongate, even when control embryos already started to retract and leg buds are evident. Finally, we found that the absence of cell proliferation has no apparent effect on segmental patterning, as evidenced by Tc-engrailed (Tc-en) gene expression.

Regulation and function of odd-paired in Tribolium segmentation

The pair-rule gene odd-paired (opa) is required for the patterning of alternate segment boundaries in the early Drosophila embryo. Mutant phenotypes of opa display a typical pair-rule phenotype in which most of each odd-numbered denticle belt is eliminated. However, among the nine Drosophila pair-rule genes, opa is the only gene that is not expressed in stripes with double segmental periodicity; its transcript and protein are expressed in a broad domain within segmenting embryos. While expression patterns of orthologs of opa have been analyzed in several arthropod species, their regulation and function in segmentation were largely unknown. Here, we analyzed the expression patterns, regulation, and function of the Tribolium ortholog of opa (Tc-opa). Tc-opa is expressed in segmental stripes in the early stages of segmentation and then is expressed in a broad domain at the growth zone of elongating germbands where new segments form. This broad expression of Tc-opa is processed into segmental stripes once the trunk has become segmented. Tc-opa expression is regulated positively and negatively by even-skipped and odd-skipped, respectively. However, knock-down of Tc-opa does not affect embryonic segmentation. Our findings suggest that Tc-opa expression is regulated by the pair-rule gene network even though its requirement for segmentation is uncertain in Tribolium.

Linking gene regulation to cell behaviors in the posterior growth zone of sequentially segmenting arthropods

Virtually all arthropods all arthropods add their body segments sequentially, one by one in an anterior to posterior progression. That process requires not only segment specification but typically growth and elongation. Here we review the functions of some of the key genes that regulate segmentation: Wnt, caudal, Notch pathway, and pair-rule genes, and discuss what can be inferred about their evolution. We focus on how these regulatory factors are integrated with growth and elongation and discuss the importance and challenges of baseline measures of growth and elongation. We emphasize a perspective that integrates the genetic regulation of segment patterning with the cellular mechanisms of growth and elongation.

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.

Dermestes maculatus: an intermediate-germ beetle model system for evo-devo

BACKGROUND

Understanding how genes change during evolution to direct the development of diverse body plans is a major goal of the evo-devo field. Achieving this will require the establishment of new model systems that represent key points in phylogeny. These new model systems must be amenable to laboratory culture, and molecular and functional approaches should be feasible. To date, studies of insects have been best represented by the model system Drosophila melanogaster. Given the enormous diversity represented by insect taxa, comparative studies within this clade will provide a wealth of information about the evolutionary potential and trajectories of alternative developmental strategies.

RESULTS

Here we established the beetle Dermestes maculatus, a member of the speciose clade Coleoptera, as a new insect model system. We have maintained a continuously breeding culture in the lab and documented Dermestes maculatus embryogenesis using nuclear and phalloidin staining. Anterior segments are specified during the blastoderm stage before gastrulation, and posterior segments are added sequentially during germ band elongation. We isolated and studied the expression and function of the pair-rule segmentation gene paired in Dermestes maculatus. In this species, paired is expressed in stripes during both blastoderm and germ band stages: four primary stripes arise prior to gastrulation, confirming an intermediate-germ mode of development for this species. As in other insects, these primary stripes then split into secondary stripes. To study gene function, we established both embryonic and parental RNAi. Knockdown of Dmac-paired with either method resulted in pair-rule-like segmentation defects, including loss of Engrailed expression in alternate stripes.

CONCLUSIONS

These studies establish basic approaches necessary to use Dermestes maculatus as a model system. Methods are now available for use of this intermediate-germ insect for future studies of the evolution of regulatory networks controlling insect segmentation, as well as of other processes in development and homeostasis. Consistent with the role of paired in long-germ Drosophila and shorter-germ Tribolium, paired functions as a pair-rule segmentation gene in Dermestes maculatus. Thus, paired retains pair-rule function in insects with different modes of segment addition.

Engrailed homeoproteins in visual system development

Engrailed is a homeoprotein transcription factor. This family of transcription factors is characterized by their DNA-binding homeodomain and some members, including Engrailed, can transfer between cells and regulate protein translation in addition to gene transcription. Engrailed is intimately involved in the development of the vertebrate visual system. Early expression of Engrailed in dorsal mesencephalon contributes to the development and organization of a visual structure, the optic tectum/superior colliculus. This structure is an important target for retinal ganglion cell axons that carry visual information from the retina. Engrailed regulates the expression of Ephrin axon guidance cues in the tectum/superior colliculus. More recently it has been reported that Engrailed itself acts as an axon guidance cue in synergy with the Ephrin system and is proposed to enhance retinal topographic precision.

The evolving role of the orphan nuclear receptor ftz-f1, a pair-rule segmentation gene

Segmentation is a critical developmental process that occurs by different mechanisms in diverse taxa. In insects, there are three common modes of embryogenesis-short-, intermediate-, and long-germ development-which differ in the number of segments specified at the blastoderm stage. While genes involved in segmentation have been extensively studied in the long-germ insect Drosophila melanogaster (Dm), it has been found that their expression and function in segmentation in short- and intermediate-germ insects often differ. Drosophila ftz-f1 encodes an orphan nuclear receptor that functions as a maternally expressed pair-rule segmentation gene, responsible for the formation of alternate body segments during Drosophila embryogenesis. Here we investigated the expression and function of ftz-f1 in the short-germ beetle, Tribolium castaneum (Tc). We found that Tc-ftz-f1 is expressed in stripes in Tribolium embryos. These stripes overlap alternate Tc-Engrailed (Tc-En) stripes, indicative of a pair-rule expression pattern. To test whether Tc-ftz-f1 has pair-rule function, we utilized embryonic RNAi, injecting double-stranded RNA corresponding to Tc-ftz-f1 coding or non-coding regions into early Tribolium embryos. Knockdown of Tc-ftz-f1 produced pair-rule segmentation defects, evidenced by loss of expression of alternate En stripes. In addition, a later role for Tc-ftz-f1 in cuticle formation was revealed. These results identify a new pair-rule gene in Tribolium and suggest that its role in segmentation may be shared among holometabolous insects. Interestingly, while Tc-ftz-f1 is expressed in pair-rule stripes, the gene is ubiquitously expressed in Drosophila embryos. Thus, the pair-rule function of ftz-f1 is conserved despite differences in expression patterns of ftz-f1 genes in different lineages. This suggests that ftz-f1 expression changed after the divergence of lineages leading to extant beetles and flies, likely due to differences in cis-regulatory sequences. We propose that the dependence of Dm-Ftz-F1 on interaction with the homeodomain protein Ftz which is expressed in stripes in Drosophila, loosened constraints on Dm-ftz-f1 expression, allowing for ubiquitous expression of this pair-rule gene in Drosophila.

Tribolium embryo morphogenesis: may the force be with you

Development of multicellular organisms depends on patterning and growth mechanisms encoded in the genome, but also on the physical properties and mechanical interactions of the constituent cells that interpret these genetic cues. This fundamental biological problem requires integrated studies at multiple levels of biological organization: from genes, to cell behaviors, to tissue morphogenesis. We have recently combined functional genetics with live imaging approaches in embryos of the insect Tribolium castaneum, in order to understand their remarkable transformation from a uniform single-layered blastoderm into a condensed multi-layered embryo covered by extensive extra-embryonic tissues. We first developed a quick and reliable methodology to fluorescently label various cell components in entire Tribolium embryos. Live imaging of labeled embryos at single cell resolution provided detailed descriptions of cell behaviors and tissue movements during normal embryogenesis. We then compared cell and tissue dynamics between wild-type and genetically perturbed embryos that exhibited altered relative proportions of constituent tissues. This systematic comparison led to a qualitative model of the molecular, cellular and tissue interactions that orchestrate the observed epithelial rearrangements. We expect this work to establish the Tribolium embryo as a powerful and attractive model system for biologists and biophysicists interested in the molecular, cellular and mechanical control of tissue morphogenesis.

Cell and tissue dynamics during Tribolium embryogenesis revealed by versatile fluorescence labeling approaches

Studies on new arthropod models such as the beetle Tribolium castaneum are shifting our knowledge of embryonic patterning and morphogenesis beyond the Drosophila paradigm. In contrast to Drosophila, Tribolium embryos exhibit the short-germ type of development and become enveloped by extensive extra-embryonic membranes, the amnion and serosa. The genetic basis of these processes has been the focus of active research. Here, we complement genetic approaches with live fluorescence imaging of Tribolium embryos to make the link between gene function and morphogenetic cell behaviors during blastoderm formation and differentiation, germband condensation and elongation, and extra-embryonic development. We first show that transient labeling methods result in strong, homogeneous and persistent expression of fluorescent markers in Tribolium embryos, labeling the chromatin, membrane, cytoskeleton or combinations thereof. We then use co-injection of fluorescent markers with dsRNA for live imaging of embryos with disrupted caudal gene function caused by RNA interference. Using these approaches, we describe and compare cell and tissue dynamics in Tribolium embryos with wild-type and altered fate maps. We find that Tribolium germband condensation is effected by cell contraction and intercalation, with the latter being dependent on the anterior-posterior patterning system. We propose that germband condensation drives initiation of amnion folding, whereas expansion of the amniotic fold and closure of the amniotic cavity are likely driven by contraction of an actomyosin cable at the boundary between the amnion and serosa. Our methodology provides a comprehensive framework for testing quantitative models of patterning, growth and morphogenetic mechanisms in Tribolium and other arthropod species.

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.

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.

When are cellular oscillators sufficient for sequential segmentation?

Sequential segmentation during embryogenesis involves the generation of a repeated pattern along the embryo, which is concurrently undergoing axial elongation by cell division. Most mathematical models of sequential segmentation involve inherent cellular oscillators, acting as a segmentation clock. The cellular oscillation is assumed to be governed by the cell's physiological age or by its interaction with an external morphogen gradient. Here, we address the issue of when cellular oscillators alone are sufficient for predicting segmentation, and when a morphogen gradient is required. The key to resolving this issue lies in how cells determine positional information in the model--this is directly related to the distribution of cell divisions responsible for axial elongation. Mathematical models demonstrate that if axial elongation occurs through cell divisions restricted to the posterior end of the unsegmented region, a cell can obtain its positional information from its physiological age, and therefore cellular oscillators will suffice. Alternatively, if axial elongation occurs through cell divisions distributed throughout the unsegmented region, then positional information can be obtained through another mechanism, such as a morphogen gradient. Two alternative ways to establish a morphogen gradient in tissue with distributed cell divisions are presented--one with diffusion and the other without diffusion. Our model produces segment polarity and a distribution of segment size from the anterior-to-posterior ends, as observed in some systems. Furthermore, the model predicts segment deletions when there is an interruption in cell division, just as seen in heat shock experiments, as well as the growth and final shrinkage of the presomitic mesoderm during somitogenesis.

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.

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.

Do teashirt family genes specify trunk identity? Insights from the single tiptop/teashirt homolog of Tribolium castaneum

The Drosophila teashirt gene acts in concert with the homeotic selector (Hox) genes to specify trunk (thorax and abdomen) identity. There has been speculation that this trunk-specifying function might be very ancient, dating back to the common ancestor of insects and vertebrates. However, other evidence suggests that the role of teashirt in trunk identity is not well conserved even within the Insecta. To address this issue, we have analyzed the function of Tc-tiotsh, the lone teashirt family member in the red flour beetle, Tribolium castaneum. Although Tc-tiotsh is important for aspects of both embryonic and imaginal development including some trunk features, we find no evidence that it acts as a trunk identity gene. We discuss this finding in the context of recent insights into the evolution and function of the Drosophila teashirt family genes.

Hedgehog signaling pathway function conserved in Tribolium segmentation

In Drosophila, maintenance of parasegmental boundaries and formation of segmental grooves depend on interactions between segment polarity genes. Wingless and Engrailed appear to have similar roles in both short and long germ segmentation, but relatively little is known about the extent to which Hedgehog signaling is conserved. In a companion study to the Tribolium genome project, we analyzed the expression and function of hedgehog, smoothened, patched, and cubitus interruptus orthologs during segmentation in Tribolium. Their expression was largely conserved between Drosophila and Tribolium. Parental RNAi analysis of positive regulators of the pathway (Tc-hh, Tc-smo, or Tc-ci) resulted in small spherical cuticles with little or no evidence of segmental grooves. Segmental Engrailed expression in these embryos was initiated but not maintained. Wingless-independent Engrailed expression in the CNS was maintained and became highly compacted during germ band retraction, providing evidence that derivatives from every segment were present in these small spherical embryos. On the other hand, RNAi analysis of a negative regulator (Tc-ptc) resulted in embryos with ectopic segmental grooves visible during germband elongation but not discernible in the first instar larval cuticles. These transient grooves formed adjacent to Engrailed expressing cells that encircled wider than normal wg domains in the Tc-ptc RNAi embryos. These results suggest that the en–wg–hh gene circuit is functionally conserved in the maintenance of segmental boundaries during germ band retraction and groove formation in Tribolium and that the segment polarity genes form a robust genetic regulatory module in the segmentation of this short germ insect.

The engrailed-expressing secondary head spots in the embryonic crayfish brain: examples for a group of homologous neurons in Crustacea and Hexapoda?

Hexapoda have been traditionally seen as the closest relatives of the Myriapoda (Tracheata hypothesis) but molecular studies have challenged this hypothesis and rather have suggested a close relationship of hexapods and crustaceans (Tetraconata hypothesis). In this new debate, data on the structure and development of the arthropod nervous system contribute important new data ("neurophylogeny"). Neurophylogenetic studies have already provided several examples for individually identifiably neurons in the ventral nerve cord that are homologous between insects and crustaceans. In the present report, we have analysed the emergence of Engrailed-expressing cells in the embryonic brain of a parthenogenetic crayfish, the marbled crayfish (Marmorkrebs), and have compared our findings to the pattern previously reported from insects. Our data suggest that a group of six Engrailed-expressing neurons in the optic anlagen, the so-called secondary head spot cells can be homologised between crayfish and the grasshopper. In the grasshopper, these cells are supposed to be involved in establishing the primary axon scaffold of the brain. Our data provide the first example for a cluster of brain neurons that can be homologised between insects and crustaceans and show that even at the level of certain cell groups, brain structures are evolutionary conserved in these two groups.

The evolution of hexapod engrailed-family genes: evidence for conservation and concerted evolution

Phylogenetic analyses imply that multiple engrailed-family gene duplications occurred during hexapod evolution, a view supported by previous reports of only a single engrailed-family gene in members of the grasshopper genus Schistocerca and in the beetle Tribolium castaneum. Here, we report the cloning of a second engrailed-family gene from Schistocerca gregaria and present evidence for two engrailed-family genes from four additional hexapod species. We also report the existence of a second engrailed-family gene in the Tribolium genome. We suggest that the engrailed and invected genes of Drosophila melanogaster have existed as a conserved gene cassette throughout holometabolous insect evolution. In total 11 phylogenetically diverse hexapod orders are now known to contain species that possess two engrailed-family paralogues, with in each case only one paralogue encoding the RS-motif, a characteristic feature of holometabolous insect invected proteins. We propose that the homeoboxes of hexapod engrailed-family paralogues are evolving in a concerted fashion, resulting in gene trees that overestimate the frequency of gene duplication. We present new phylogenetic analyses using non-homeodomain amino acid sequence that support this view. The S. gregaria engrailed-family paralogues provide strong evidence that concerted evolution might in part be explained by recurrent gene conversion. Finally, we hypothesize that the RS-motif is part of a serine-rich domain targeted for phosphorylation.

The Tribolium castaneum ortholog of Sex combs reduced controls dorsal ridge development

In insects, the boundary between the embryonic head and thorax is formed by the dorsal ridge, a fused structure composed of portions of the maxillary and labial segments. However, the mechanisms that promote development of this unusual structure remain a mystery. In Drosophila, mutations in the Hox genes Sex combs reduced and Deformed have been reported to cause abnormal dorsal ridge formation, but the significance of these abnormalities is not clear. We have identified three mutant allele classes of Cephalothorax, the Tribolium castaneum (red flour beetle) ortholog of Sex combs reduced, each of which has a different effect on dorsal ridge development. By using Engrailed expression to monitor dorsal ridge development in these mutants, we demonstrate that Cephalothorax promotes the fusion and subsequent dorsolateral extension of the maxillary and labial Engrailed stripes (posterior compartments) during dorsal ridge formation. Molecular and genetic analysis of these alleles indicates that the N terminus of Cephalothorax is important for the fusion step, but is dispensable for Engrailed stripe extension. Thus, we find that specific regions of Cephalothorax are required for discrete steps in dorsal ridge formation.

The evolution of arthropod heads: reconciling morphological, developmental and palaeontological evidence

Understanding the head is one of the great challenges in the fields of comparative anatomy, developmental biology, and palaeontology of arthropods. Numerous conflicting views and interpretations are based on an enormous variety of descriptive and experimental approaches. The interpretation of the head influences views on phylogenetic relationships within the Arthropoda as well as outgroup relationships. Here, we review current hypotheses about head segmentation and the nature of head structures from various perspectives, which we try to combine to gain a deeper understanding of the arthropod head. Though discussion about arthropod heads shows some progress, unquestioned concepts (e.g., a presegmental acron) are still a source of bias. Several interpretations are no longer tenable based on recent results from comparative molecular developmental studies, improved morphological investigations, and new fossils. Current data indicate that the anterior arthropod head comprises three elements: the protocerebral/ocular region, the deutocerebral/antennal/cheliceral segment, and the tritocerebral/pedipalpal/second antennal/intercalary segment. The labrum and the mouth are part of the protocerebral/ocular region. Whether the labrum derives from a former pair of limbs remains an open question, but a majority of data support its broad homology across the Euarthropoda. From the alignment of head segments between onychophorans and euarthropods, we develop the concept of "primary" and "secondary antennae" in Recent and fossil arthropods, posit that "primary antennae" are retained in some fossil euarthropods below the crown group level, and propose that Trilobita are stem lineage representatives of the Mandibulata.

The roles of wingless and decapentaplegic in axis and appendage development in the red flour beetle, Tribolium castaneum

Axis patterning and appendage development have been well studied in Drosophila melanogaster, a species in which both limb and segment morphogenesis are derived. In Drosophila, positional information from genes important in anteroposterior and dorsoventral axis formation, including wingless (wg) and decapentaplegic (dpp), is required for allocating and patterning the appendage primordia. We used RNA interference to characterize the functions of wg and dpp in the red flour beetle, Tribolium castaneum, which retains more ancestral modes of limb and segment morphogenesis. We also characterized the expression of potential targets of the WG and DPP signaling pathways in these embryos. Tribolium embryos in which dpp had been downregulated had defects in the dorsalmost body wall, but did not appear to have been globally repatterned and had normal appendages. Downregulation of wg led to the loss of segment boundaries, gnathal and thoracic appendages, and lateral head lobes, and to changes in the expression of dpp, Distal-less, and Engrailed. The functions of wg varied along both the anteroposterior and dorsoventral axes of the embryo. Phylogenetic comparisons indicate that the role of WNT signaling in segment boundary formation is evolutionarily old, but that its role in appendage allocation originated in the common ancestor of holometabolous insects.

Maintenance of segment and appendage primordia by the Tribolium gene knödel

For homeotic and segment-polarity genes in Drosophila, a switch in gene regulation has been described that distinguishes patterning and maintenance phases. Maintenance of segment and organ primordia involves secondary patterning and differentiation steps, as well as survival factors regulating proliferation and organ size. In a screen for embryonic lethal mutations in the flour beetle Tribolium castaneum, we have recovered two alleles of the knödel gene, which result in short, bag-like embryos. These embryos have severely reduced appendages and differentiate a cuticle that lacks most overt signs of segmentation. In addition, they lack bristles and display defects in the nervous system. Early patterning in knödel mutant embryos is normal up to the extended germ band stage, as indicated by the formation of regular even-skipped (Tc'eve) and wingless (Tc'wg) stripes. Afterwards, however, these patterns degenerate. Similarly, proximo-distal growth and patterning of limbs are nearly normal initially, but limb primordia shrink, and proximo-distal patterns degenerate, during subsequent stages. knödel could be a segment polarity gene required for segment border maintenance in both trunk and appendages. Alternatively, it may have a more general role in tissue or organ maintenance.

A pair-rule gene circuit defines segments sequentially in the short-germ insect Tribolium castaneum

In Drosophila, a hierarchy of maternal, gap, pair-rule, and segment polarity gene interactions regulates virtually simultaneous blastoderm segmentation. For the last decade, studies have focused on revealing the extent to which Drosophila segmentation mechanisms are conserved in other arthropods where segments are added sequentially from anterior to posterior in a cellular environment. Despite our increased knowledge of individual segmentation genes, details of their interactions in non-Drosophilid insects are not well understood. We analyzed the Tribolium orthologs of Drosophila pair-rule genes, which display pair-rule expression patterns. Tribolium castaneum paired (Tc-prd) and sloppy-paired (Tc-slp) genes produced pair-rule phenotypes when their transcripts were severely reduced by RNA interference. In contrast, similar analysis of T. castaneum even-skipped (Tc-eve), runt (Tc-run), or odd-skipped (Tc-odd) genes produced severely truncated, almost completely asegmental phenotypes. Analysis of interactions between pair-rule components revealed that Tc-eve, Tc-run, and Tc-odd form a three-gene circuit to regulate one another as well as their downstream targets, Tc-prd and Tc-slp. The complement of primary pair-rule genes in Tribolium differs from Drosophila in that it includes Tc-odd but not Tc-hairy. This gene circuit defines segments sequentially in double segment periodicity. Furthermore, this single mechanism functions in the early blastoderm stage and subsequently during germ-band elongation. The periodicity of the Tribolium pair-rule gene interactions reveals components of the genetic hierarchy that are regulated in a repetitive circuit or clock-like mechanism. This pair-rule gene circuit provides insight into short-germ segmentation in Tribolium that may be more generally applicable to segmentation in other arthropods.

Short and long germ segmentation: unanswered questions in the evolution of a developmental mode

The insect body plan is very well conserved, yet the developmental mechanisms of segmentation are surprisingly varied. Less evolutionarily derived insects undergo short germ segmentation where only the anterior segments are specified before gastrulation whereas the remaining posterior segments are formed during a later secondary growth phase. In contrast, derived long germ insects such as Drosophila specify their entire bodies essentially simultaneously. These fundamental embryological differences imply potentially divergent molecular patterning events. Numerous studies have focused on comparing the expression and function of the homologs of Drosophila segmentation genes between Drosophila and different short and long germ insects. Here we review these comparative data with special emphasis on understanding how short germ insects generate segments and how this ancestral mechanism may have been modified in derived long germ insects such as Drosophila. We break down the larger issue of short versus long germ segmentation into its component developmental problems and structure our discussion in order to highlight the unanswered questions in the evolution of insect segmentation.

Expression of 'segmentation' genes during larval and juvenile development in the polychaetes Capitella sp. I and H. elegans

Polychaete annelids and arthropods are both segmented protostome invertebrates. To investigate whether the segmented body plan of these two phyla share a common molecular ground pattern, we report the developmental expression of orthologues of the arthropod segment polarity genes engrailed (en), hedgehog (hh), and wingless (wg/Wnt1) in larval and juvenile stages of the polychaete annelid Capitella sp. I and en in a second polychaete, Hydroides elegans. Temporally, neither Wnt1 nor hh are detected in the segmented region of the larval body until after morphological segmentation is apparent. Expression of CapI-Wnt1 is limited to a ring of ectoderm marking the future anus during larval segmentation. CapI-hh is expressed in a ring of the hindgut internal to that of CapI-Wnt1, as well as in a subset of ventral nerve cord neurons, anterior gut tissue, and mesoderm. In both H. elegans and Capitella sp. I, en is expressed in a spatially and temporally dynamic manner in segmentally iterated structures as well as a population of cells that migrate internally from ectoderm to mesoderm, possibly representing a population of ecto-mesodermal precursors. Significantly, the expression patterns we report for wg, en, and hh orthologues in Capitella sp. I and for en in larval development of H. elegans are not comparable to the highly conserved ectodermal segment polarity pattern observed in arthropods at any life history stage, consistent with distinct origins of segmentation between annelids and arthropods.

even-skipped is not a pair-rule gene but has segmental and gap-like functions in Oncopeltus fasciatus, an intermediate germband insect

The pair-rule gene even-skipped is required for the initiation of metameric pattern in Drosophila. But Drosophila segmentation is evolutionarily derived and is not representative of most insects. Therefore, in order to shed light on the evolution of insect segmentation, homologs of the pair-rule gene even-skipped have been studied in several insect taxa. However, most of these studies have reported the expression eve but not its function. We report the isolation, expression and function of the homolog of Drosophila even-skipped from the intermediate germband insect Oncopeltus fasciatus. We find that in Oncopeltus, even-skipped striped expression initiates in a segmental and not pair-rule pattern. Weak RNAi suppression of Oncopeltus even-skipped shows no apparent pair-rule like phenotype, while stronger RNAi suppression shows deletion of nearly the entire body. These results suggest that in Oncopeltus, even-skipped is not acting as a pair-rule gene. In almost all insects, prior to its striped expression, even-skipped is expressed in a conserved broad gap-like domain but its function has been largely ignored. We find that this early broad domain is required for activation of the gap genes hunchback and Krüppel. Given the large RNAi deletion phenotype and its regulation of hunchback and Krüppel, even-skipped seems to act as an über-gap gene in Oncopeltus, indicating that it may have both upstream and downstream roles in segmentation.

Tribolium castaneum twist: gastrulation and mesoderm formation in a short-germ beetle

Mesoderm formation has been extensively analyzed in the long-germ insect Drosophila melanogaster. In Drosophila, both the invagination and specification of the mesoderm is controlled by twist. Here we present a detailed description of mesoderm formation and twist regulation for the short-germ beetle Tribolium castaneum. In contrast to Drosophila, (1) the presumptive mesodermal cells of Tribolium are part of a mitotic domain and divide prior to ventral furrow formation, (2) ventral furrow formation progresses from posterior to anterior, (3) the number of cell layers within the furrow changes from multilayered in caudal to single layered in cephalic regions, and (4) there is a continuous production of mesodermal cells after gastrulation as new segments arise from the posterior growth zone. Tribolium twist (Tc-twist) is initially expressed in all presumptive mesodermal cells; however, after invagination, expression is maintained only in particular locations, which include the anterior compartments of the cephalic segments and a patch of cells at the posterior rim of the growth zone. The growth zone is multilayered with its inner cell layer being continuous with the mesoderm of the newly forming segments where twist expression is re-initiated anterior to the emerging engrailed stripes. A genomic region of Tc-twist was identified which drives ventral expression of a reporter construct in Drosophila. The expression of this Tc-twist construct in the background of Drosophila maternal mutations suggests that the dorsoventral system regulates Tc-twist, but that differences exist in regulation of the Dm-twist and Tc-twist genes by the terminal system.

Gene Network Polymorphism Is the Raw Material of Natural Selection: The Selfish Gene Network Hypothesis

Population genetics, the mathematical theory of modern evolutionary biology, defines evolution as the alteration of the frequency of distinct gene variants (alleles) differing in fitness over the time. The major problem with this view is that in gene and protein sequences we can find little evidence concerning the molecular basis of phenotypic variance, especially those that would confer adaptive benefit to the bearers. Some novel data, however, suggest that a large amount of genetic variation exists in the regulatory region of genes within populations. In addition, comparison of homologous DNA sequences of various species shows that evolution appears to depend more strongly on gene expression than on the genes themselves. Furthermore, it has been demonstrated in several systems that genes form functional networks, whose products exhibit interrelated expression profiles. Finally, it has been found that regulatory circuits of development behave as evolutionary units. These data demonstrate that our view of evolution calls for a new synthesis. In this article I propose a novel concept, termed the selfish gene network hypothesis, which is based on an overall consideration of the above findings. The major statements of this hypothesis are as follows. (1) Instead of individual genes, gene networks (GNs) are responsible for the determination of traits and behaviors. (2) The primary source of microevolution is the intraspecific polymorphism in GNs and not the allelic variation in either the coding or the regulatory sequences of individual genes. (3) GN polymorphism is generated by the variation in the regulatory regions of the component genes and not by the variance in their coding sequences. (4) Evolution proceeds through continuous restructuring of the composition of GNs rather than fixing of specific alleles or GN variants.

Divergent segmentation mechanism in the short germ insectTriboliumrevealed bygiantexpression and function

Segmentation is well understood in Drosophila, where all segments are determined at the blastoderm stage. In the flour beetle Tribolium castaneum, as in most insects, the posterior segments are added at later stages from a posteriorly located growth zone, suggesting that formation of these segments may rely on a different mechanism. Nevertheless, the expression and function of many segmentation genes seem conserved between Tribolium and Drosophila. We have cloned the Tribolium ortholog of the abdominal gap gene giant. As in Drosophila, Tribolium giant is expressed in two primary domains, one each in the head and trunk. Although the position of the anterior domain is conserved, the posterior domain is located at least four segments anterior to that of Drosophila. Knockdown phenotypes generated with morpholino oligonucleotides, as well as embryonic and parental RNA interference, indicate that giant is required for segment formation and identity also in Tribolium. In giant-depleted embryos,the maxillary and labial segment primordia are normally formed but assume thoracic identity. The segmentation process is disrupted only in postgnathal metamers. Unlike Drosophila, segmentation defects are not restricted to a limited domain but extend to all thoracic and abdominal segments, many of which are specified long after giant expression has ceased. These data show that giant in Tribolium does not function as in Drosophila, and suggest that posterior gap genes underwent major regulatory and functional changes during the evolution from short to long germ embryogenesis.