The evolution of arthropod segmentation mechanisms

Modelling the evolution of novelty: a review

Evolution has been an inventive process since its inception, about 4 billion years ago. It has generated an astounding diversity of novel mechanisms and structures for adaptation to the environment, for competition and cooperation, and for organisation of the internal and external dynamics of the organism. How does this novelty come about? Evolution builds with the tools available, and on top of what it has already built - therefore, much novelty consists in repurposing old functions in a different context. In the process, the tools themselves evolve, allowing yet more novelty to arise. Despite evolutionary novelty being the most striking observable of evolution, it is not accounted for in classical evolutionary theory. Nevertheless, mathematical and computational models that illustrate mechanisms of evolutionary innovation have been developed. In the present review, we present and compare several examples of computational evo-devo models that capture two aspects of novelty: 'between-level novelty' and 'constructive novelty.' Novelty can evolve between predefined levels of organisation to dynamically transcode biological information across these levels - as occurs during development. Constructive novelty instead generates a level of biological organisation by exploiting the lower level as an informational scaffold to open a new space of possibilities - an example being the evolution of multicellularity. We propose that the field of computational evo-devo is well-poised to reveal many more exciting mechanisms for the evolution of novelty. A broader theory of evolutionary novelty may well be attainable in the near future.

Developmental dynamics is revealed in the early Cambrian arthropod Chuandianella ovata

Segmentation and tagmatization have contributed to the preeminent success of arthropods since their first appearance in the Cambrian. However, the exact mechanism of segmentogenesis is still insufficiently known in living and extinct groups. Here, we describe the postembryonic development of a Waptiid arthropod Chuandianella ovata from the early Cambrian Chengjiang biota, South China. The new data illuminate a complex dynamic pattern of anamorphosis and epimorphosis, and a three-step process of segmentogenesis, i.e., the elongation of the terminal segment, delineation of an incipient segment, and full separation of a new segment. Compensatory growth is accomplished by rapid growth of new segments and/or generation of additional segments, which results in the trimorphism of the posterior tagma. Such complex developmental dynamics has rarely been known in the arthropod fossil record and its presence in early history helps to understand the rapid diversification of arthropods in the early Cambrian.

The evolution of the gene regulatory networks patterning the Drosophila Blastoderm

The Drosophila blastoderm gene regulatory network is one of the best studied networks in biology. It is composed of a series of tiered sub-networks that act sequentially to generate a primary segmental pattern. Many of these sub-networks have been studied in other arthropods, allowing us to reconstruct how each of them evolved over the transition from the arthropod ancestor to the situation seen in Drosophila today. I trace the evolution of each of these networks, showing how some of them have been modified significantly in Drosophila relative to the ancestral state while others are largely conserved across evolutionary timescales. I compare the putative ancestral arthropod segmentation network with that found in Drosophila and discuss how and why it has been modified throughout evolution, and to what extent this modification is unusual.

The genomics of phenotypically differentiated Asellus aquaticus cave, surface stream and lake ecotypes

Organisms well suited for the study of ecotype formation have wide distribution ranges, where they adapt to multiple drastically different habitats repeatedly over space and time. Here we study such ecotypes in a Crustacean model, Asellus aquaticus, a commonly occurring isopod found in freshwater habitats as diverse as streams, caves and lakes. Previous studies focusing on cave vs. surface ecotypes have attributed depigmentation, eye loss and prolonged antennae to several south European cave systems. Likewise, surveys across multiple Swedish lakes have identified the presence of dark-pigmented "reed" and light-pigmented "stonewort" ecotypes, which can be found within the same lake. In this study, we sequenced the first draft genome of A. aquaticus, and subsequently use this to map reads and call variants in surface stream, cave and two lake ecotypes. In addition, the draft genome was combined with a RADseq approach to perform a quantitative trait locus (QTL) mapping study using a laboratory bred F₂ and F₄ cave × surface intercross. We identified genomic regions associated with body pigmentation, antennae length and body size. Furthermore, we compared genome-wide differentiation between natural populations and found several genes potentially associated with these habitats. The assessment of the cave QTL regions in the light-dark comparison of lake populations suggests that the regions associated with cave adaptation are also involved with genomic differentiation in the lake ecotypes. These demonstrate how troglomorphic adaptations can be used as a model for related ecotype formation.

Two sets of candidate crustacean wing homologues and their implication for the origin of insect wings

The origin of insect wings is a biological mystery that has fascinated scientists for centuries. Identification of tissues homologous to insect wings from lineages outside of Insecta will provide pivotal information to resolve this conundrum. Here, through expression and clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) functional analyses in Parhyale, we show that a gene network similar to the insect wing gene network (preWGN) operates both in the crustacean terga and in the proximal leg segments, suggesting that the evolution of a preWGN precedes the emergence of insect wings, and that from an evo-devo perspective, both of these tissues qualify as potential crustacean wing homologues. Combining these results with recent wing origin studies in insects, we discuss the possibility that both tissues are crustacean wing homologues, which supports a dual evolutionary origin of insect wings (that is, novelty through a merger of two distinct tissues). These outcomes have a crucial impact on the course of the intellectual battle between the two historically competing wing origin hypotheses.

The embryonic expression pattern of a second, hitherto unrecognized, paralog of the pair-rule gene sloppy-paired in the beetle Tribolium castaneum

In the fly Drosophila melanogaster, a hierarchic segmentation gene cascade patterns the anterior-posterior body axis of the developing embryo. Within this cascade, the pair-rule genes (PRGs) transform the more uniform patterning of the higher-level genes into a metameric pattern that first represents double-segmental units, and then, in a second step, represents a true segmental pattern. Within the PRG network, primary PRGs regulate secondary PRGs that are directly involved in the regulation of the next lower level, the segment-polarity genes (SPGs). While the complement of primary PRGs is different in Drosophila and the beetle Tribolium, another arthropod model organism, both paired (prd) and sloppy-paired (slp), acts as secondary PRGs. In earlier studies, the interaction of PRGs and the role of the single slp ortholog in Tribolium have been investigated in some detail revealing conserved and diverged aspects of PRG function. In this study, I present the identification and the analysis of embryonic expression patterns of a second slp gene (called slp2) in Tribolium. While the previously identified gene, slp, is expressed in a typical PRG pattern, expression of slp2 is more similar to that of the downstream-acting SPGs, and shows expression similarities to slp2 in Drosophila. The previously reported differences between the function of slp in Drosophila and Tribolium may partially account for the function of the newly identified second slp paralog in Tribolium, and it may therefore be advised to conduct further studies on PRG function in the beetle.

Candidate gene screen for potential interaction partners and regulatory targets of the Hox gene labial in the spider Parasteatoda tepidariorum

The Hox gene labial (lab) governs the formation of the tritocerebral head segment in insects and spiders. However, the morphology that results from lab action is very different in the two groups. In insects, the tritocerebral segment (intercalary segment) is reduced and lacks appendages, whereas in spiders the corresponding segment (pedipalpal segment) is a proper segment including a pair of appendages (pedipalps). It is likely that this difference between lab action in insects and spiders is mediated by regulatory targets or interacting partners of lab. However, only a few such genes are known in insects and none in spiders. We have conducted a candidate gene screen in the spider Parasteatoda tepidariorum using as candidates Drosophila melanogaster genes known to (potentially) interact with lab or to be expressed in the intercalary segment. We have studied 75 P. tepidariorum genes (including previously published and duplicated genes). Only 3 of these (proboscipedia-A (pb-A) and two paralogs of extradenticle (exd)) showed differential expression between leg and pedipalp. The low success rate points to a weakness of the candidate gene approach when it is applied to lineage specific organs. The spider pedipalp has no counterpart in insects, and therefore relying on insect data apparently cannot identify larger numbers of factors implicated in its specification and formation. We argue that in these cases a de novo approach to gene discovery might be superior to the candidate gene approach.

Establishment of the mayfly Cloeon dipterum as a new model system to investigate insect evolution

The great capability of insects to adapt to new environments promoted their extraordinary diversification, resulting in the group of Metazoa with the largest number of species distributed worldwide. To understand this enormous diversity, it is essential to investigate lineages that would allow the reconstruction of the early events in the evolution of insects. However, research on insect ecology, physiology, development and evolution has mostly focused on few well-established model species. The key phylogenetic position of mayflies within Paleoptera as the sister group of the rest of winged insects and life history traits of mayflies make them an essential order to understand insect evolution. Here, we describe the establishment of a continuous culture system of the mayfly Cloeon dipterum and a series of experimental protocols and omics resources that allow the study of its development and its great regenerative capability. Thus, the establishment of Cloeon as an experimental platform paves the way to understand genomic and morphogenetic events that occurred at the origin of winged insects.

A re-inducible gap gene cascade patterns the anterior-posterior axis of insects in a threshold-free fashion

Gap genes mediate the division of the anterior-posterior axis of insects into different fates through regulating downstream hox genes. Decades of tinkering the segmentation gene network of Drosophila melanogaster led to the conclusion that gap genes are regulated (at least initially) through a threshold-based mechanism, guided by both anteriorly- and posteriorly-localized morphogen gradients. In this paper, we show that the response of the gap gene network in the beetle Tribolium castaneum upon perturbation is consistent with a threshold-free ‘Speed Regulation’ mechanism, in which the speed of a genetic cascade of gap genes is regulated by a posterior morphogen gradient. We show this by re-inducing the leading gap gene (namely, hunchback) resulting in the re-induction of the gap gene cascade at arbitrary points in time. This demonstrates that the gap gene network is self-regulatory and is primarily under the control of a posterior regulator in Tribolium and possibly other short/intermediate-germ insects.

Growth zone segmentation in the milkweed bug Oncopeltus fasciatus sheds light on the evolution of insect segmentation

Background

One of the best studied developmental processes is the Drosophila segmentation cascade. However, this cascade is generally considered to be highly derived and unusual, with segments being patterned simultaneously, rather than the ancestral sequential segmentation mode. We present a detailed analysis of the segmentation cascade of the milkweed bug Oncopletus fasciatus, an insect with a more primitive segmentation mode, as a comparison to Drosophila, with the aim of reconstructing the evolution of insect segmentation modes.

Results

We document the expression of 12 genes, representing different phases in the segmentation process. Using double staining we reconstruct the spatio-temporal relationships among these genes. We then show knock-down phenotypes of representative genes in order to uncover their roles and position in the cascade.

Conclusions

We conclude that sequential segmentation in the Oncopeltus germband includes three slightly overlapping phases: Primary pair-rule genes generate the first segmental gene expression in the anterior growth zone. This pattern is carried anteriorly by a series of secondary pair-rule genes, expressed in the transition between the growth zone and the segmented germband. Segment polarity genes are expressed in the segmented germband with conserved relationships. Unlike most holometabolous insects, this process generates a single-segment periodicity, and does not have a double-segment pattern at any stage. We suggest that the evolutionary transition to double-segment patterning lies in mutually exclusive expression patterns of secondary pair-rule genes. The fact that many aspects of the putative Oncopeltus segmentation network are similar to those of Drosophila, is consistent with a simple transition between sequential and simultaneous segmentation.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1293-z) contains supplementary material, which is available to authorized users.

A damped oscillator imposes temporal order on posterior gap gene expression in Drosophila

Insects determine their body segments in two different ways. Short-germband insects, such as the flour beetle Tribolium castaneum, use a molecular clock to establish segments sequentially. In contrast, long-germband insects, such as the vinegar fly Drosophila melanogaster, determine all segments simultaneously through a hierarchical cascade of gene regulation. Gap genes constitute the first layer of the Drosophila segmentation gene hierarchy, downstream of maternal gradients such as that of Caudal (Cad). We use data-driven mathematical modelling and phase space analysis to show that shifting gap domains in the posterior half of the Drosophila embryo are an emergent property of a robust damped oscillator mechanism, suggesting that the regulatory dynamics underlying long- and short-germband segmentation are much more similar than previously thought. In Tribolium, Cad has been proposed to modulate the frequency of the segmentation oscillator. Surprisingly, our simulations and experiments show that the shift rate of posterior gap domains is independent of maternal Cad levels in Drosophila. Our results suggest a novel evolutionary scenario for the short- to long-germband transition and help explain why this transition occurred convergently multiple times during the radiation of the holometabolan insects.

Different insect species exhibit one of two distinct modes of determining their body segments (known as segmentation) during development: they either use a molecular oscillator to position segments sequentially, or they generate segments simultaneously through a hierarchical gene-regulatory cascade. The sequential mode is ancestral, while the simultaneous mode has been derived from it independently several times during evolution. In this paper, we present evidence suggesting that simultaneous segmentation also involves an oscillator in the posterior end of the embryo of the vinegar fly, Drosophila melanogaster. This surprising result indicates that both modes of segment determination are much more similar than previously thought. Such similarity provides an important step towards our understanding of the frequent evolutionary transitions observed between sequential and simultaneous segmentation.

Blastoderm segmentation in Oncopeltus fasciatus and the evolution of insect segmentation mechanisms

Segments are formed simultaneously in the blastoderm of the fly Drosophila melanogaster through a hierarchical cascade of interacting transcription factors. Conversely, in many insects and in all non-insect arthropods most segments are formed sequentially from the posterior. We have looked at segmentation in the milkweed bug Oncopeltus fasciatus. Posterior segments are formed sequentially, through what is probably the ancestral arthropod mechanism. Formation of anterior segments bears many similarities to the Drosophila segmentation mode. These segments appear nearly simultaneously in the blastoderm, via a segmentation cascade that involves orthologues of Drosophila gap genes working through a functionally similar mechanism. We suggest that simultaneous blastoderm segmentation evolved at or close to the origin of holometabolous insects, and formed the basis for the evolution of the segmentation mode seen in Drosophila. We discuss the changes in segmentation mechanisms throughout insect evolution, and suggest that the appearance of simultaneous segmentation as a novel feature of holometabolous insects may have contributed to the phenomenal success of this group.

Non-canonical dorsoventral patterning in the moth midge Clogmia albipunctata

Background

Bone morphogenetic proteins (BMPs) are of central importance for dorsal–ventral (DV) axis specification. They are core components of a signalling cascade that includes the BMP ligand decapentaplegic (DPP) and its antagonist short gastrulation (SOG) in Drosophila melanogaster. These components are very ancient, with orthologs involved in DV patterning in both protostomes and deuterostomes. Despite such strong conservation, recent comparative work in insects has revealed interesting differences in the way the patterning function of the DV system is achieved in different species.

Results

In this paper, we characterise the expression patterns of the principal components of the BMP DV patterning system, as well as its signalling outputs and downstream targets, in the non-cyclorrhaphan moth midge Clogmia albipunctata (Diptera: Psychodidae). We previously reported ventral expression patterns of dpp in the pole regions of C. albipunctata blastoderm embryos. Strikingly, we also find ventral sog and posteriorly restricted tkv expression, as well as expanded polar activity of pMad. We use our results from gene knock-down by embryonic RNA interference to propose a mechanism of polar morphogen shuttling in C. albipunctata. We compare these results to available data from other species and discuss scenarios for the evolution of DV signalling in the holometabolan insects.

Conclusions

A comparison of gene expression patterns across hemipteran and holometabolan insects reveals that expression of upstream signalling factors in the DV system is very variable, while signalling output is highly conserved. This has two major implications: first, as long as ligand shuttling and other upstream regulatory mechanisms lead to an appropriately localised activation of BMP signalling at the dorsal midline, it is of less importance exactly where the upstream components of the DV system are expressed. This, in turn, explains why the early-acting components of the DV patterning system in insects exhibit extensive amounts of developmental systems drift constrained by highly conserved downstream signalling output.

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.

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.

In silico evo-devo: reconstructing stages in the evolution of animal segmentation

BACKGROUND

The evolution of animal segmentation is a major research focus within the field of evolutionary-developmental biology. Most studied segmented animals generate their segments in a repetitive, anterior-to-posterior fashion coordinated with the extension of the body axis from a posterior growth zone. In the current study we ask which selection pressures and ordering of evolutionary events may have contributed to the evolution of this specific segmentation mode.

RESULTS

To answer this question we extend a previous in silico simulation model of the evolution of segmentation by allowing the tissue growth pattern to freely evolve. We then determine the likelihood of evolving oscillatory sequential segmentation combined with posterior growth under various conditions, such as the presence or absence of a posterior morphogen gradient or selection for determinate growth. We find that posterior growth with sequential segmentation is the predominant outcome of our simulations only if a posterior morphogen gradient is assumed to have already evolved and selection for determinate growth occurs secondarily. Otherwise, an alternative segmentation mechanism dominates, in which divisions occur in large bursts through the entire tissue and all segments are created simultaneously.

CONCLUSIONS

Our study suggests that the ancestry of a posterior signalling centre has played an important role in the evolution of sequential segmentation. In addition, it suggests that determinate growth evolved secondarily, after the evolution of posterior growth. More generally, we demonstrate the potential of evo-devo simulation models that allow us to vary conditions as well as the onset of selection pressures to infer a likely order of evolutionary innovations.

Reversion of developmental mode in insects: evolution from long germband to short germband in the polyembrionic wasp Macrocentrus cingulum Brischke

Germband size in insects has played a central role in our understanding of insect patterning mechanisms and their evolution. The polarity of evolutionary change in insect patterning has been viewed so far as the unidirectional shift from the ancestral short germband patterning of basal hemimetabolous insects to the long germband patterning observed in most modern Holometabola. However, some orders of holometabolic insects display both short and long germband development, though the absence of a clear phylogenetic context does not permit definite conclusions on the polarity of change. Derived hymenoptera, that is, bees and wasps, represent a classical textbook example of long germband development. Yet, in some wasps putative short germband development has been described correlating with lifestyle changes, namely with evolution of endoparasitism and polyembryony. To address the potential reversion from long to short germband, we focused on the family Braconidae, which displays ancestral long germband development, and examined the derived polyembryonic braconid Macrocentrus cingulum. Using SEM analysis of M. cingulum embryogenesis coupled with analyses of embryonic patterning markers, we show that this wasp evolved short germband embryogenesis secondarily, in a way that is reminiscent of embryogenesis in the beetle Tribolium castaneum. This work shows that the evolution of germband size in insects is a reversible process that may correlate with other life-history traits and suggests broader implications on the mechanisms and evolvability of insect development.

Segmentation of the millipede trunk as suggested by a homeotic mutant with six extra pairs of gonopods

BACKGROUND

The mismatch between dorsal and ventral trunk features along the millipede trunk was long a subject of controversy, largely resting on alternative interpretations of segmentation. Most models of arthropod segmentation presuppose a strict sequential antero-posterior specification of trunk segments, whereas alternative models involve the early delineation of a limited number of 'primary segments' followed by their sequential stereotypic subdivision into 2n definitive segments. The 'primary segments' should be intended as units identified by molecular markers, rather than as overt morphological entities. Two predictions were suggested to test the plausibility of multiple-duplication models of segmentation: first, a specific pattern of evolvability of segment number in those arthropod clades in which segment number is not fixed (e.g., epimorphic centipedes and millipedes); second, the occurrence of discrete multisegmental patterns due to early, initially contiguous positional markers.

RESULTS

We describe a unique case of a homeotic millipede with 6 extra pairs of ectopic gonopods replacing walking legs on rings 8 (leg-pairs 10-11), 15 (leg-pairs 24-25) and 16 (leg-pairs 26-27); we discuss the segmental distribution of these appendages in the framework of alternative models of segmentation and present an interpretation of the origin of the distribution of the additional gonopods.The anterior set of contiguous gonopods (those normally occurring on ring 7 plus the first set of ectopic ones on ring 8) is reiterated by the posterior set (on rings 15-16) after exactly 16 leg positions along the AP body axis. This suggests that a body section including 16 leg pairs could be a module deriving from 4 cycles of regular binary splitting of an embryonic 'primary segment'.

CONCLUSIONS

A very likely early determination of the sites of the future metamorphosis of walking legs into gonopods and a segmentation process according to the multiplicative model may provide a detailed explanation for the distribution of the extra gonopods in the homeotic specimen. The hypothesized steps of segmentation are similar in both a normal and the studied homeotic specimen. The difference between them would consist in the size of the embryonic trunk region endowed with a positional marker whose presence will later determine the replacement of walking legs by gonopods.

An analysis of segmentation dynamics throughout embryogenesis in the centipede Strigamia maritima

BACKGROUND

Most segmented animals add segments sequentially as the animal grows. In vertebrates, segment patterning depends on oscillations of gene expression coordinated as travelling waves in the posterior, unsegmented mesoderm. Recently, waves of segmentation gene expression have been clearly documented in insects. However, it remains unclear whether cyclic gene activity is widespread across arthropods, and possibly ancestral among segmented animals. Previous studies have suggested that a segmentation oscillator may exist in Strigamia, an arthropod only distantly related to insects, but further evidence is needed to document this.

RESULTS

Using the genes even skipped and Delta as representative of genes involved in segment patterning in insects and in vertebrates, respectively, we have carried out a detailed analysis of the spatio-temporal dynamics of gene expression throughout the process of segment patterning in Strigamia. We show that a segmentation clock is involved in segment formation: most segments are generated by cycles of dynamic gene activity that generate a pattern of double segment periodicity, which is only later resolved to the definitive single segment pattern. However, not all segments are generated by this process. The most posterior segments are added individually from a localized sub-terminal area of the embryo, without prior pair-rule patterning.

CONCLUSIONS

Our data suggest that dynamic patterning of gene expression may be widespread among the arthropods, but that a single network of segmentation genes can generate either oscillatory behavior at pair-rule periodicity or direct single segment patterning, at different stages of embryogenesis.

Mechanisms and constraints shaping the evolution of body plan segmentation

Segmentation of the major body axis into repeating units is arguably one of the major inventions in the evolution of animal body plan pattering. It is found in current day vertebrates, annelids and arthropods. Most segmented animals seem to use a clock-and-wavefront type mechanism in which oscillations emanating from a posterior growth zone become transformed into an anterior posterior sequence of segments. In contrast, few animals such as Drosophila use a complex gene regulatory hierarchy to simultaneously subdivide their entire body axis into segments. Here I discuss how in silico models simulating the evolution of developmental patterning can be used to investigate the forces and constraints that helped shape these two developmental modes. I perform an analysis of a series of previous simulation studies, exploiting the similarities and differences in their outcomes in relation to model characteristics to elucidate the circumstances and constraints likely to have been important for the evolution of sequential and simultaneous segmentation modes. The analysis suggests that constraints arising from the involved growth process and spatial patterning signal--posterior elongation producing a propagating wavefront versus a tissue wide morphogen gradient--and the evolutionary history--ancestral versus derived segmentation mode--strongly shaped both segmentation mechanisms. Furthermore, this implies that these patterning types are to be expected rather than random evolutionary outcomes and supports the likelihood of multiple parallel evolutionary origins.

Expression of pair rule gene orthologs in the blastoderm of a myriapod: evidence for pair rule-like mechanisms?

Background

A hallmark of Drosophila segmentation is the stepwise subdivision of the body into smaller and smaller units, and finally into the segments. This is achieved by the function of the well-understood segmentation gene cascade. The first molecular sign of a segmented body appears with the action of the pair rule genes, which are expressed as transversal stripes in alternating segments. Drosophila development, however, is derived, and in most other arthropods only the anterior body is patterned (almost) simultaneously from a pre-existing field of cells; posterior segments are added sequentially from a posterior segment addition zone. A long-standing question is to what extent segmentation mechanisms known from Drosophila may be conserved in short-germ arthropods. Despite the derived developmental modes, it appears more likely that conserved mechanisms can be found in anterior patterning.

Results

Expression analysis of pair rule gene orthologs in the blastoderm of the pill millipede Glomeris marginata (Myriapoda: Diplopoda) suggests that these genes are generally involved in segmenting the anterior embryo. We find that the Glomeris pairberry-1 ( pby-1) gene is expressed in a pair rule pattern that is also found in insects and a chelicerate, the mite Tetraynchus urticae. Other Glomeris pair rule gene orthologs are expressed in double segment wide domains in the blastoderm, which at subsequent stages split into two stripes in adjacent segments.

Conclusions

The expression patterns of the millipede pair rule gene orthologs resemble pair rule patterning in Drosophila and other insects, and thus represent evidence for the presence of an ancestral pair rule-like mechanism in myriapods. We discuss the possibilities that blastoderm patterning may be conserved in long-germ and short-germ arthropods, and that a posterior double segmental mechanism may be present in short-germ arthropods.

Notch/Delta signalling is not required for segment generation in the basally branching insect Gryllus bimaculatus

Arthropods and vertebrates display a segmental body organisation along all or part of the anterior-posterior axis. Whether this reflects a shared, ancestral developmental genetic mechanism for segmentation is uncertain. In vertebrates, segments are formed sequentially by a segmentation 'clock' of oscillating gene expression involving Notch pathway components. Recent studies in spiders and basal insects have suggested that segmentation in these arthropods also involves Notch-based signalling. These observations have been interpreted as evidence for a shared, ancestral gene network for insect, arthropod and bilaterian segmentation. However, because this pathway can play multiple roles in development, elucidating the specific requirements for Notch signalling is important for understanding the ancestry of segmentation. Here we show that Delta, a ligand of the Notch pathway, is not required for segment formation in the cricket Gryllus bimaculatus, which retains ancestral characteristics of arthropod embryogenesis. Segment patterning genes are expressed before Delta in abdominal segments, and Delta expression does not oscillate in the pre-segmental region or in formed segments. Instead, Delta is required for neuroectoderm and mesectoderm formation; embryos missing these tissues are developmentally delayed and show defects in segment morphology but normal segment number. Thus, what initially appear to be 'segmentation phenotypes' can in fact be due to developmental delays and cell specification errors. Our data do not support an essential or ancestral role of Notch signalling in segment generation across the arthropods, and show that the pleiotropy of the Notch pathway can confound speculation on possible segmentation mechanisms in the last common bilaterian ancestor.

Travelling and splitting of a wave of hedgehog expression involved in spider-head segmentation

During development segmentation is a process that generates a spatial periodic pattern. Peak splitting of waves of gene expression is a mathematically predicted, simple strategy accounting for this type of process, but it has not been well characterized biologically. Here we show temporally repeated splitting of gene expression into stripes that is associated with head axis growth in the spider Achaearanea embryo. Preceding segmentation, a wave of hedgehog homologue gene expression is observed to travel posteriorly during development stage 6. This stripe, co-expressing an orthodenticle homologue, undergoes two cycles of splitting and shifting accompanied by convergent extension, serving as a generative zone for the head segments. The two orthodenticle and odd-paired homologues are identified as targets of Hedgehog signalling, and evidence suggests that their activities mediate feedback to maintain the head generative zone and to promote stripe splitting in this zone. We propose that the 'stripe-splitting' strategy employs genetic components shared with Drosophila blastoderm subdivision, which are required for participation in an autoregulatory signalling network.

Expression of myriapod pair rule gene orthologs

Background

Segmentation is a hallmark of the arthropods; most knowledge about the molecular basis of arthropod segmentation comes from work on the fly Drosophila melanogaster. In this species a hierarchic cascade of segmentation genes subdivides the blastoderm stepwise into single segment wide regions. However, segmentation in the fly is a derived feature since all segments form virtually simultaneously. Conversely, in the vast majority of arthropods the posterior segments form one at a time from a posterior pre-segmental zone. The pair rule genes (PRGs) comprise an important level of the Drosophila segmentation gene cascade and are indeed the first genes that are expressed in typical transverse stripes in the early embryo. Information on expression and function of PRGs outside the insects, however, is scarce.

Results

Here we present the expression of the pair rule gene orthologs in the pill millipede Glomeris marginata (Myriapoda: Diplopoda). We find evidence that these genes are involved in segmentation and that components of the hierarchic interaction of the gene network as found in insects may be conserved. We further provide evidence that segments are formed in a single-segment periodicity rather than in pairs of two like in another myriapod, the centipede Strigamia maritima. Finally we show that decoupling of dorsal and ventral segmentation in Glomeris appears already at the level of the PRGs.

Conclusions

Although the pair rule gene network is partially conserved among insects and myriapods, some aspects of PRG interaction are, as suggested by expression pattern analysis, convergent, even within the Myriapoda. Conserved expression patterns of PRGs in insects and myriapods, however, may represent ancestral features involved in segmenting the arthropod ancestor.

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.

Cloning and characterization of Bmrunt from the silkworm Bombyx mori during embryonic development

Pair-rule genes (genes that are expressed only in alternate segments, odd or even) play an important role in translating the broad gradients of upstream genes into dual segment periodicity for body plan patterning in Drosophila. However, homologues of pair-rule genes show a remarkable diversity of expression patterns and functions in other insects. We cloned the homologue of runt in the silkworm Bombyx mori, an intermediate germband-type insect. Whole-mount in situ hybridization revealed three stripes arose one by one before gastrulation at the blastoderm stage. Five additional stripes were then generated sequentially as the growth zone elongated. Eight stripes appeared in a pair-rule manner with two-segment periodicity, each of which was confined to the posterior of an odd-numbered parasegment. The weaker segmental secondary stripes emerged de novo in even-numbered parasegments. The Bmrunt transcript vanished before blastokinesis and was then expressed again in the whole embryo. RNA interference for Bmrunt caused severely truncated, almost completely asegmental defects. This cadual-like phenotype suggests that Bmrunt does not function as a pair-rule gene in silkworm segmentation. Bmrunt is required for formation of most body segments and axis elongation in B. mori.

Notch signaling during larval and juvenile development in the polychaete annelid Capitella sp. I

Notch signaling is involved in a large range of developmental processes, and has been functionally implicated in body plan segmentation in two of the three diverse segmented taxa, the vertebrates and arthropods. Here we investigate expression of Notch, Delta, and hes gene homologues during larval and juvenile development in the polychaete annelid Capitella sp. I., a member of the third group of segmented animals. During larval stages, CapI-Notch, CapI-Delta, CapI-hes2, and CapI-hes3 transcripts are initially detected in broad ectodermal domains in future segments as well as in the brain and foregut; later, CapI-Notch, CapI-Delta, and CapI-hes2 transcripts are detected in the presumptive chaetal sacs. In contrast, CapI-hes1 has a segmentally reiterated pattern in a restricted region of the mesoderm in each presumptive segment. CapI-Notch, CapI-Delta, CapI-hes2, and CapI-hes3 and CapI-hes1 are all expressed in the terminal growth zone that generates post-metamorphic segments, however, CapI-hes1 has a non-overlapping complementary expression pattern to that of CapI-Notch and CapI-Delta. CapI-Delta and CapI-Notch transcripts are localized to already formed segments, with posterior boundaries that correlate with the posterior boundary of the nascent segment, while CapI-hes1 lies posterior to CapI-Notch and CapI-Delta. The localization of CapI-Notch, CapI-Delta, and CapI-hes transcripts correlate with areas of rapid cell proliferation in Capitella, which include the brain, foregut, and terminal growth zone.

Evolutionary conservation and divergence of the segmentation process in arthropods

A fundamental characteristic of the arthropod body plan is its organization in metameric units along the anterior-posterior axis. The segmental organization is laid down during early embryogenesis. Our view on arthropod segmentation is still strongly influenced by the huge amount of data available from the fruit fly Drosophila melanogaster (the Drosophila paradigm). However, the simultaneous formation of the segments in Drosophila is a derived mode of segmentation. Successive terminal addition of segments from a posteriorly localized presegmental zone is the ancestral mode of arthropod segmentation. This review focuses on the evolutionary conservation and divergence of the genetic mechanisms of segmentation within arthropods. The more downstream levels of the segmentation gene network (e.g., segment polarity genes) appear to be more conserved than the more upstream levels (gap genes, Notch/Delta signaling). Surprisingly, the basally branched arthropod groups also show similarities to mechanisms used in vertebrate somitogenesis. Furthermore, it has become clear that the activation of pair rule gene orthologs is a key step in the segmentation of all arthropods. Important findings of conserved and diverged aspects of segmentation from the last few years now allow us to draw an evolutionary scenario on how the mechanisms of segmentation could have evolved and led to the present mechanisms seen in various insect groups including dipterans like Drosophila.

Drosophila Ten-a is a maternal pair-rule and patterning gene

The Ten-a gene of Drosophila melanogaster encodes several alternative variants of a full length member of the Odz/Tenm protein family. A number of Ten-a mutants created by inexact excisions of a resident P-element insertion are embryonic lethal, but show no pair-rule phenotype. In contrast, these mutants, and deficiencies removing Ten-a, do enhance the segmentation phenotype of a weak allele of the paralog gene odz (or Ten-m) to the odz amorphic phenotype. Germ line clone derived Ten-a(-) embryos display a pair-rule phenotype which phenocopies that of odz. Post segmentation eye patterning phenotypes of Ten-a mutants establish it as a pleiotropic patterning co-partner of odz.

Interrogating genomes with combinatorial artificial transcription factor libraries: asking zinc finger questions

Artificial transcription factors (ATFs) are proteins designed to specifically bind and regulate genes. Because of their DNA-binding selectivity and modular organization, arrays of zinc finger (ZF) domains have traditionally been used to build the ATF's DNA-binding domains. ATFs have been designed and constructed to regulate a variety of therapeutic targets. Recently, novel combinatorial technologies have been developed to induce expression of any gene of interest or to modify cellular phenotypes. Large repertoires of ATFs have been generated by recombination of all available sequence-specific ZF lexicons. These libraries comprise millions of ATFs with unique DNA-binding specificities. The ATFs are produced by combinatorial assembly of three- and six-ZF building blocks and are linked to activator or repressor domains. Upon delivery into a cell population, any gene in the human genome can potentially be regulated. ATF library members generate genome-wide, experimental perturbations of gene expression, resulting in a phenotypically diverse population, or cellular library. A variety of phenotypic screenings can be applied to select for cells exhibiting a phenotype of interest. The ATFs are then used as genetic probes to identify the targeted genes responsible for the phenotypic switch. In this review we will summarize several applications of ATF library screenings in gene discovery, biotechnology, and disease therapeutics.

Characterization of Notch-class gene expression in segmentation stem cells and segment founder cells in Helobdella robusta (Lophotrochozoa; Annelida; Clitellata; Hirudinida; Glossiphoniidae)

To understand the evolution of segmentation, we must compare segmentation in all three major groups of eusegmented animals: vertebrates, arthropods, and annelids. The leech Helobdella robusta is an experimentally tractable annelid representative, which makes segments in anteroposterior progression from a posterior growth zone consisting of 10 identified stem cells. In vertebrates and some arthropods, Notch signaling is required for normal segmentation and functions via regulation of hes-class genes. We have previously characterized the expression of an hes-class gene (Hro-hes) during segmentation in Helobdella, and here, we characterize the expression of an H. robusta notch homolog (Hro-notch) during this process. We find that Hro-notch is transcribed in the segmental founder cells (blast cells) and their stem-cell precursors (teloblasts), as well as in other nonsegmental tissues. The mesodermal and ectodermal lineages show clear differences in the levels of Hro-notch expression. Finally, Hro-notch is shown to be inherited by newly born segmental founder cells as well as transcribed by them before their first cell division.

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

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

Trunk segment numbers and sequential segmentation in myriapods

Sequential segmentation from a posterior "proliferative zone" is considered to be the primitive mechanism of segmentation in arthropods. Several studies of embryonic and post-embryonic development and gene expression suggest that this occurs in all major arthropod taxa. Sequential segmentation is often associated with the idea of posterior production of body units that accumulate along the main body axis. However, the precise mechanism of sequential segmentation has not been identified yet, and, while searching for the genetic circuitry able to generate a first periodic pattern in the embryo, we can at least outline the distinctive role in segmentation of a proliferative zone. A perusal of myriapod segmentation patterns suggests that these patterns result from multi-layered developmental processes, where gene expression and epigenetic mechanisms interact in a nonstrictly hierarchical way. The posterior zone is possibly a zone of periodic signal production, but, in general, the resulting segmental pattern is not completely attributable to the activity of the signal generator. In this sense, a posterior proliferative zone would be more a "segmental organizer" than a "segment generator."

Pair rule gene orthologs in spider segmentation

The activation of pair rule genes is the first indication of the metameric organization of the Drosophila embryo and thus forms a key step in the segmentation process. There are two classes of pair rule genes in Drosophila: the primary pair rule genes that are directly activated by the maternal and gap genes and the secondary pair rule genes that rely on input from the primary pair rule genes. Here we analyze orthologs of Drosophila primary and secondary pair rule orthologs in the spider Cupiennius salei. The expression patterns of the spider pair rule gene orthologs can be subdivided in three groups: even-skipped and runt-1 expression is in stripes that start at the posterior end of the growth zone and their expression ends before the stripes reach the anterior end of the growth zone, while hairy and pairberry-3 stripes also start at the posterior end, but do not cease in the anterior growth zone. Stripes of odd-paired, odd-skipped-related-1, and sloppy paired are only found in the anterior portion of the growth zone. The various genes thus seem to be active during different phases of segment specification. It is notable that the spider orthologs of the Drosophila primary pair rule genes are active more posterior in the growth zone and thus during earlier phases of segment specification than most orthologs of Drosophila secondary pair rule genes, indicating that parts of the hierarchy might be conserved between flies and spiders. The spider ortholog of the Drosophila pair rule gene fushi tarazu is not expressed in the growth zone, but is expressed in a Hox-like fashion. The segmentation function of fushi tarazu thus appears to be a newly acquired role of the gene in the lineage of the mandibulate arthropods.

Suppressor of Hairless and Presenilin phenotypes imply involvement of canonical Notch-signalling in segmentation of the spider Cupiennius salei

Arthropods, vertebrates, and annelids all have a segmented body. Our recent discovery of involvement of Notch-signalling in spider segmentation revived the discussion on the origin of segmented body plans and suggests the sharing of a common genetic program in a common ancestor. Here, we analysed the spider homologues of the Suppressor of Hairless and Presenilin genes, which encode components of the canonical Notch-pathway, to further explore the role of Notch-signalling in spider segmentation. RNAi silencing of two spider Suppressor of Hairless homologues and the spider Presenilin homologue causes severe segmentation phenotypes. The most prominent defect is the consistent breakdown of segmentation after the formation of three (Suppressor of Hairless) or five (Presenilin) opisthosomal segments. These phenotypes indicate that Notch-signalling during spider segmentation likely involves the canonical pathway via Presenilin and Suppressor of Hairless. Furthermore, it implies that Notch-signalling influences both the formation and patterning of the spider segments: it is required for the specification of the posterior segments and for proper specification of the segment boundaries. We argue that alternative, partly redundant, pathways might act in the formation of the anterior segments that are not active in the posterior segments. This suggests that at least some differences exist in the specification of anterior and posterior segments of the spider, a finding that may be valid for most short germ arthropods. Our data provide additional evidence for the similarities of Notch-signalling in spider segmentation and vertebrate somitogenesis and strengthen our previous notion that the formation of the segments in arthropods and vertebrates might have shared a genetic program in a common ancestor.