The segmentation cascade in the centipede Strigamia maritima: involvement of the Notch pathway and pair-rule gene homologues

Delta of Exopalaemon carinicauda: molecular characterization, expression in different tissues and developmental stages, and its SNPs association analysis with development

BACKGROUND

The Notch signaling pathway plays a significant role in the gene regulatory network of development of vertebrate and invertebrate. However, as a ligand for the Notch signaling pathway, the mechanism of Delta in the development of Exopalaemon carinicauda is still unclear.

METHODS AND RESULTS

The Delta's molecular characteristics, tissue distribution and their association with development in E. carinicauda were studied by RACE (rapid amplification of cDNA end), qRT-PCR (quantitative Real-time PCR) and SNP (single nucleotide polymorphism), respectively. The delta in E. carinicauda had a full-length cDNA of 2807 bp and its Delta of 808 amino-acid residue had the highest identity with the Delta of Homarus americanus (identity = 76.63%). Delta had the highest expression in the ovary, and its expression varied with different stages of embryonic, larval, and ovarian development. After delta RNA interference (with a highest interference efficiency of 66% at 24 h), the expression of Notch signaling pathway genes and developmental related genes was significantly reduced, and the ovarian development was significantly delayed. Further study found that there were 4 SNPs (ds1-4) in delta cDNA, of which two (ds2 T1521G caused a mutation Asn422Lys and ds3 G1674A caused a mutation Tyr473Cys in the EGF-like domain) were associated with the development of E. carinicauda. The Gonadosomatic Index (GSI) of the ds2 TT genotypes was 37.28% and 134.60% higher than E. carinicauda of GT and GG genotype respectively (P < 0.05).

CONCLUSION

Our research indicated that delta was involved in the development of E. carinicauda and provided new insights for molecular breeding with SNP markers in E. carinicauda.

The organizing role of Wnt signaling pathway during arthropod posterior growth

Wnt signaling pathways are recognized for having major roles in tissue patterning and cell proliferation. In the last years, remarkable progress has been made in elucidating the molecular and cellular mechanisms that underlie sequential segmentation and axial elongation in various arthropods, and the canonical Wnt pathway has emerged as an essential factor in these processes. Here we review, with a comparative perspective, the current evidence concerning the participation of this pathway during posterior growth, its degree of conservation among the different subphyla within Arthropoda and its relationship with the rest of the gene regulatory network involved. Furthermore, we discuss how this signaling pathway could regulate segmentation to establish this repetitive pattern and, at the same time, probably modulate different cellular processes precisely coupled to axial elongation. Based on the information collected, we suggest that this pathway plays an organizing role in the formation of the body segments through the regulation of the dynamic expression of segmentation genes, via controlling the caudal gene, at the posterior region of the embryo/larva, that is necessary for the correct sequential formation of body segments in most arthropods and possibly in their common segmented ancestor. On the other hand, there is insufficient evidence to link this pathway to axial elongation by controlling its main cellular processes, such as convergent extension and cell proliferation. However, conclusions are premature until more studies incorporating diverse arthropods are carried out.

Patterning with clocks and genetic cascades: Segmentation and regionalization of vertebrate versus insect body plans

Oscillatory and sequential processes have been implicated in the spatial patterning of many embryonic tissues. For example, molecular clocks delimit segmental boundaries in vertebrates and insects and mediate lateral root formation in plants, whereas sequential gene activities are involved in the specification of regional identities of insect neuroblasts, vertebrate neural tube, vertebrate limb, and insect and vertebrate body axes. These processes take place in various tissues and organisms, and, hence, raise the question of what common themes and strategies they share. In this article, we review 2 processes that rely on the spatial regulation of periodic and sequential gene activities: segmentation and regionalization of the anterior-posterior (AP) axis of animal body plans. We study these processes in species that belong to 2 different phyla: vertebrates and insects. By contrasting 2 different processes (segmentation and regionalization) in species that belong to 2 distantly related phyla (arthropods and vertebrates), we elucidate the deep logic of patterning by oscillatory and sequential gene activities. Furthermore, in some of these organisms (e.g., the fruit fly Drosophila), a mode of AP patterning has evolved that seems not to overtly rely on oscillations or sequential gene activities, providing an opportunity to study the evolution of pattern formation mechanisms.

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.

Time and space in segmentation

Arthropod segmentation and vertebrate somitogenesis are leading fields in the experimental and theoretical interrogation of developmental patterning. However, despite the sophistication of current research, basic conceptual issues remain unresolved. These include: (i) the mechanistic origins of spatial organization within the segment addition zone (SAZ); (ii) the mechanistic origins of segment polarization; (iii) the mechanistic origins of axial variation; and (iv) the evolutionary origins of simultaneous patterning. Here, I explore these problems using coarse-grained models of cross-regulating dynamical processes. In the morphogenetic framework of a row of cells undergoing axial elongation, I simulate interactions between an 'oscillator', a 'switch' and up to three 'timers', successfully reproducing essential patterning behaviours of segmenting systems. By comparing the output of these largely cell-autonomous models to variants that incorporate positional information, I find that scaling relationships, wave patterns and patterning dynamics all depend on whether the SAZ is regulated by temporal or spatial information. I also identify three mechanisms for polarizing oscillator output, all of which functionally implicate the oscillator frequency profile. Finally, I demonstrate significant dynamical and regulatory continuity between sequential and simultaneous modes of segmentation. I discuss these results in the context of the experimental literature.

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.

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.

Embryonic development and secondary axis induction in the Brazilian white knee tarantula Acanthoscurria geniculata, C. L. Koch, 1841 (Araneae; Mygalomorphae; Theraphosidae)

Tarantulas represent some of the heaviest and most famous spiders. However, there is little information about the embryonic development of these spiders or their relatives (infraorder Mygalomorphae) and time-lapse recording of the embryonic development is entirely missing. I here describe the complete development of the Brazilian white knee tarantula, Acanthoscurria geniculata, in fixed and live embryos. The establishment of the blastoderm, the formation, migration and signalling of the cumulus and the shape changes that occur in the segment addition zone are analysed in detail. In addition, I show that there might be differences in the contraction process of early embryos of different theraphosid spider species. A new embryonic reference transcriptome was generated for this study and was used to clone and analyse the expression of several important developmental genes. Finally, I show that embryos of A. geniculata are amenable to tissue transplantation and bead insertion experiments. Using these functional approaches, I induced axis duplication in embryos via cumulus transplantation and ectopic activation of BMP signalling. Overall, the mygalomorph spider A. geniculata is a useful laboratory system to analyse evolutionary developmental questions, and the availability of such a system will help understanding conserved and divergent aspects of spider/chelicerate development.

Arthropod segmentation

There is now compelling evidence that many arthropods pattern their segments using a clock-and-wavefront mechanism, analogous to that operating during vertebrate somitogenesis. In this Review, we discuss how the arthropod segmentation clock generates a repeating sequence of pair-rule gene expression, and how this is converted into a segment-polarity pattern by ‘timing factor’ wavefronts associated with axial extension. We argue that the gene regulatory network that patterns segments may be relatively conserved, although the timing of segmentation varies widely, and double-segment periodicity appears to have evolved at least twice. Finally, we describe how the repeated evolution of a simultaneous (Drosophila-like) mode of segmentation within holometabolan insects can be explained by heterochronic shifts in timing factor expression plus extensive pre-patterning of the pair-rule genes.

Shifting roles of Drosophila pair-rule gene orthologs: segmental expression and function in the milkweed bug Oncopeltus fasciatus

The discovery of pair-rule genes (PRGs) in Drosophila revealed the existence of an underlying two-segment-wide prepattern directing embryogenesis. The milkweed bug Oncopeltus fasciatus, a hemimetabolous insect, is a more representative arthropod: most of its segments form sequentially after gastrulation. Here, we report the expression and function of orthologs of the complete set of nine Drosophila PRGs in Oncopeltus Seven Of-PRG-orthologs are expressed in stripes in the primordia of every segment, rather than every other segment; Of-runt is PR-like and several orthologs are also expressed in the segment addition zone. RNAi-mediated knockdown of Of-odd-skipped, paired and sloppy-paired impacted all segments, with no indication of PR-like register. We confirm that Of-E75A is expressed in PR-like stripes, although it is not expressed in this way in Drosophila, demonstrating the existence of an underlying PR-like prepattern in Oncopeltus These findings reveal that a switch occurred in regulatory circuits, leading to segment formation: while several holometabolous insects are 'Drosophila-like', using PRG orthologs for PR patterning, most Of-PRGs are expressed segmentally in Oncopeltus, a more basally branching insect. Thus, an evolutionarily stable phenotype - segment formation - is directed by alternate regulatory pathways in diverse species.

The mlpt/Ubr3/Svb module comprises an ancient developmental switch for embryonic patterning

Small open reading frames (smORFs) encoding 'micropeptides' exhibit remarkable evolutionary complexity. Conserved peptides encoded by mille-pattes (mlpt)/polished rice (pri)/tarsal less (tal) are essential for embryo segmentation in Tribolium but, in Drosophila, function in terminal epidermal differentiation and patterning of adult legs. Here, we show that a molecular complex identified in Drosophila epidermal differentiation, comprising Mlpt peptides, ubiquitin-ligase Ubr3 and transcription factor Shavenbaby (Svb), represents an ancient developmental module required for early insect embryo patterning. We find that loss of segmentation function for this module in flies evolved concomitantly with restriction of Svb expression in early Drosophila embryos. Consistent with this observation, artificially restoring early Svb expression in flies causes segmentation defects that depend on mlpt function, demonstrating enduring potency of an ancestral developmental switch despite evolving embryonic patterning modes. These results highlight the evolutionary plasticity of conserved molecular complexes under the constraints of essential genetic networks.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

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.

Around the clock: gradient shape and noise impact the evolution of oscillatory segmentation dynamics

BACKGROUND

Segmentation, the subdivision of the major body axis into repeated elements, is considered one of the major evolutionary innovations in bilaterian animals. In all three segmented animal clades, the predominant segmentation mechanism is sequential segmentation, where segments are generated one by one in anterior-posterior order from a posterior undifferentiated zone. In vertebrates and arthropods, sequential segmentation is thought to arise from a clock-and-wavefront-type mechanism, where oscillations in the posterior growth zone are transformed into a segmental prepattern in the anterior by a receding wavefront. Previous evo-devo simulation studies have demonstrated that this segmentation type repeatedly arises, supporting the idea of parallel evolutionary origins in these animal clades. Sequential segmentation has been studied most extensively in vertebrates, where travelling waves have been observed that reflect the slowing down of oscillations prior to their cessation and where these oscillations involve a highly complex regulatory network. It is currently unclear under which conditions this oscillator complexity and slowing should be expected to evolve, how they are related and to what extent similar properties should be expected for sequential segmentation in other animal species.

RESULTS

To investigate these questions, we extend a previously developed computational model for the evolution of segmentation. We vary the slope of the posterior morphogen gradient and the strength of gene expression noise. We find that compared to a shallow gradient, a steep morphogen gradient allows for faster evolution and evolved oscillator networks are simpler. Furthermore, under steep gradients, damped oscillators often evolve, whereas shallow gradients appear to require persistent oscillators which are regularly accompanied by travelling waves, indicative of a frequency gradient. We show that gene expression noise increases the likelihood of evolving persistent oscillators under steep gradients and of evolving frequency gradients under shallow gradients. Surprisingly, we find that the evolutions of oscillator complexity and travelling waves are not correlated, suggesting that these properties may have evolved separately.

CONCLUSIONS

Based on our findings, we suggest that travelling waves may have evolved in response to shallow morphogen gradients and gene expression noise. These two factors may thus also be responsible for the observed differences between different species within both the arthropod and chordate phyla.

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.

The Evolution of Gene Regulatory Networks that Define Arthropod Body Plans

Our understanding of the genetics of arthropod body plan development originally stems from work on Drosophila melanogaster from the late 1970s and onward. In Drosophila, there is a relatively detailed model for the network of gene interactions that proceeds in a sequential-hierarchical fashion to define the main features of the body plan. Over the years, we have a growing understanding of the networks involved in defining the body plan in an increasing number of arthropod species. It is now becoming possible to tease out the conserved aspects of these networks and to try to reconstruct their evolution. In this contribution, we focus on several key nodes of these networks, starting from early patterning in which the main axes are determined and the broad morphological domains of the embryo are defined, and on to later stage wherein the growth zone network is active in sequential addition of posterior segments. The pattern of conservation of networks is very patchy, with some key aspects being highly conserved in all arthropods and others being very labile. Many aspects of early axis patterning are highly conserved, as are some aspects of sequential segment generation. In contrast, regional patterning varies among different taxa, and some networks, such as the terminal patterning network, are only found in a limited range of taxa. The growth zone segmentation network is ancient and is probably plesiomorphic to all arthropods. In some insects, it has undergone significant modification to give rise to a more hardwired network that generates individual segments separately. In other insects and in most arthropods, the sequential segmentation network has undergone a significant amount of systems drift, wherein many of the genes have changed. However, it maintains a conserved underlying logic and function.

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.

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.

On the evolution of bilaterality

Bilaterality – the possession of two orthogonal body axes – is the name-giving trait of all bilaterian animals. These body axes are established during early embryogenesis and serve as a three-dimensional coordinate system that provides crucial spatial cues for developing cells, tissues, organs and appendages. The emergence of bilaterality was a major evolutionary transition, as it allowed animals to evolve more complex body plans. Therefore, how bilaterality evolved and whether it evolved once or several times independently is a fundamental issue in evolutionary developmental biology. Recent findings from non-bilaterian animals, in particular from Cnidaria, the sister group to Bilateria, have shed new light into the evolutionary origin of bilaterality. Here, we compare the molecular control of body axes in radially and bilaterally symmetric cnidarians and bilaterians, identify the minimal set of traits common for Bilateria, and evaluate whether bilaterality arose once or more than once during evolution.

Delta-Notch signalling in segmentation

Modular body organization is found widely across multicellular organisms, and some of them form repetitive modular structures via the process of segmentation. It's vastly interesting to understand how these regularly repeated structures are robustly generated from the underlying noise in biomolecular interactions. Recent studies from arthropods reveal similarities in segmentation mechanisms with vertebrates, and raise the possibility that the three phylogenetic clades, annelids, arthropods and chordates, might share homology in this process from a bilaterian ancestor. Here, we discuss vertebrate segmentation with particular emphasis on the role of the Notch intercellular signalling pathway. We introduce vertebrate segmentation and Notch signalling, pointing out historical milestones, then describe existing models for the Notch pathway in the synchronization of noisy neighbouring oscillators, and a new role in the modulation of gene expression wave patterns. We ask what functions Notch signalling may have in arthropod segmentation and explore the relationship between Notch-mediated lateral inhibition and synchronization. Finally, we propose open questions and technical challenges to guide future investigations into Notch signalling in segmentation.

Development and evolution of segmentation assessed by geometric morphometrics: The centipede Strigamia maritima as a case study

Using the centipede model species Strigamia maritima as a subject of study, we illustrate the potential of geometric morphometrics for investigating the development and evolution of segmentation, with a specific focus on post-embryonic segmental patterning. We show how these techniques can contribute detailed descriptive data for comparative purposes, but also precious information on some features of the developmental system that are considered relevant for the evolvability of a segmented body architecture, such as developmental stability and canalization. Morphometric analyses allow to separately investigate several sources of phenotypic variation along a segmented body axis, like constitutive and random segment heteronomy, both within and among individuals. Specifically, in S. maritima, the segmental pattern of ventral sclerite shapes mirrors that of their bilateral fluctuating asymmetry and among-individual variation in associating the most anterior and most posterior segments in diverging from the central ones. Also, among segments, there seems to be a correlation between fluctuating asymmetry and shape variation among individuals, suggesting that canalization and developmental stability are somehow associated. Overall, these associations might stem from a joint influence of the segmental position on the two processes of developmental buffering.

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.

A molecular view of onychophoran segmentation

This paper summarizes our current knowledge on the expression and assumed function of Drosophila and (other) arthropod segmentation gene orthologs in Onychophora, a closely related outgroup to Arthropoda. This includes orthologs of the so-called Drosophila segmentation gene cascade including the Hox genes, as well as other genetic factors and pathways involved in non-drosophilid arthropods. Open questions about and around the topic are addressed, such as the definition of segments in onychophorans, the unclear regulation of conserved expression patterns downstream of non-conserved factors, and the potential role of mesodermal patterning in onychophoran segmentation.

Dynamics of growth zone patterning in the milkweed bug Oncopeltus fasciatus

We describe the dynamic process of abdominal segment generation in the milkweed bug Oncopeltus fasciatus. We present detailed morphological measurements of the growing germband throughout segmentation. Our data are complemented by cell division profiles and expression patterns of key genes, including invected and even-skipped as markers for different stages of segment formation. We describe morphological and mechanistic changes in the growth zone and in nascent segments during the generation of individual segments and throughout segmentation, and examine the relative contribution of newly formed versus existing tissue to segment formation. Although abdominal segment addition is primarily generated through the rearrangement of a pool of undifferentiated cells, there is nonetheless proliferation in the posterior. By correlating proliferation with gene expression in the growth zone, we propose a model for growth zone dynamics during segmentation in which the growth zone is functionally subdivided into two distinct regions: a posterior region devoted to a slow rate of growth among undifferentiated cells, and an anterior region in which segmental differentiation is initiated and proliferation inhibited.

Summary: A detailed analysis of posterior segment addition in an insect reveals that the growth zone is divided into two functional domains responsible for growth and differentiation.

Gene expression analysis reveals that Delta/Notch signalling is not involved in onychophoran segmentation

Delta/Notch (Dl/N) signalling is involved in the gene regulatory network underlying the segmentation process in vertebrates and possibly also in annelids and arthropods, leading to the hypothesis that segmentation may have evolved in the last common ancestor of bilaterian animals. Because of seemingly contradicting results within the well-studied arthropods, however, the role and origin of Dl/N signalling in segmentation generally is still unclear. In this study, we investigate core components of Dl/N signalling by means of gene expression analysis in the onychophoran Euperipatoides kanangrensis, a close relative to the arthropods. We find that neither Delta or Notch nor any other investigated components of its signalling pathway are likely to be involved in segment addition in onychophorans. We instead suggest that Dl/N signalling may be involved in posterior elongation, another conserved function of these genes. We suggest further that the posterior elongation network, rather than classic Dl/N signalling, may be in the control of the highly conserved segment polarity gene network and the lower-level pair-rule gene network in onychophorans. Consequently, we believe that the pair-rule gene network and its interaction with Dl/N signalling may have evolved within the arthropod lineage and that Dl/N signalling has thus likely been recruited independently for segment addition in different phyla.

The Wnt and Delta-Notch signalling pathways interact to direct pair-rule gene expression via caudal during segment addition in the spider Parasteatoda tepidariorum

In short germ arthropods, posterior segments are added sequentially from a growth zone or segment addition zone (SAZ) during embryogenesis. Studies in spiders such as the common house spider, Parasteatoda tepidariorum, have provided insights into the gene regulatory network (GRN) that underlies the development of the SAZ, and revealed the involvement of two important signalling pathways. It was shown that Wnt8 maintains a pool of undifferentiated cells in the SAZ, but this ligand is also required for dynamic Delta (Dl) expression associated with the formation of new segments. However, it remains unclear how these pathways interact during SAZ formation and subsequently regulate segment addition. Here we show that Delta-Notch signalling is required for Wnt8 expression in posterior SAZ cells, but represses the expression of this Wnt gene in anterior SAZ cells. We also found that these two signalling pathways are required for the expression of the spider orthologues of the segmentation genes even-skipped (eve) and runt-1 (run-1), at least in part via the transcription factor encoded by caudal (cad). Moreover, it appears that dynamic expression of eve in this spider does not require a feedback loop with run-1, as is found in the pair-rule circuit of the beetle Tribolium. Taken together, our results suggest that the development of posterior segments in Parasteatoda is directed by dynamic interactions between Wnt8 and Delta-Notch signalling that are read out by cad, which is necessary but not sufficient to regulate the expression of the pair-rule genes eve and run-1. Our study therefore provides new insights towards better understanding the evolution and developmental regulation of segmentation in other arthropods including insects.

Analyses of interactions among pair-rule genes and the gap gene Krüppel in Bombyx segmentation

In the short-germ insect Tribolium, a pair-rule gene circuit consisting of the Tribolium homologs of even-skipped, runt, and odd-skipped (Tc-eve, Tc-run and Tc-odd, respectively) has been implicated in segment formation. To examine the application of the model to other taxa, I studied the expression and function of pair-rule genes in Bombyx mori, together with a Bombyx homolog of Krüppel (Bm-Kr), a known gap gene. Knockdown embryos of Bombyx homologs of eve, run and odd (Bm-eve, Bm-run and Bm-odd) exhibited asegmental phenotypes similar to those of Tribolium knockdowns. However, pair-rule gene interactions were similar to those of both Tribolium and Drosophila, which, different from Tribolium, shows a hierarchical segmentation mode. Additionally, the Bm-odd expression pattern shares characteristics with those of Drosophila pair-rule genes that receive upstream regulatory input. On the other hand, Bm-Kr knockdowns exhibited a large posterior segment deletion as observed in short-germ insects. However, a detailed analysis of these embryos indicated that Bm-Kr modulates expression of pair-rule genes like in Drosophila, although the mechanisms appear to be different. This suggested hierarchical interactions between Bm-Kr and pair-rule genes. Based on these results, I concluded that the pair-rule gene circuit model that describes Tribolium development is not applicable to Bombyx.

Functional analysis of centipede development supports roles for Wnt genes in posterior development and segment generation

The genes of the Wnt family play important and highly conserved roles in posterior growth and development in a wide range of animal taxa. Wnt genes also operate in arthropod segmentation, and there has been much recent debate regarding the relationship between arthropod and vertebrate segmentation mechanisms. Due to its phylogenetic position, body form, and possession of many (11) Wnt genes, the centipede Strigamia maritima is a useful system with which to examine these issues. This study takes a functional approach based on treatment with lithium chloride, which causes ubiquitous activation of canonical Wnt signalling. This is the first functional developmental study performed in any of the 15,000 species of the arthropod subphylum Myriapoda. The expression of all 11 Wnt genes in Strigamia was analyzed in relation to posterior development. Three of these genes, Wnt11, Wnt5, and WntA, were strongly expressed in the posterior region and, thus, may play important roles in posterior developmental processes. In support of this hypothesis, LiCl treatment of S. maritima embryos was observed to produce posterior developmental defects and perturbations in AbdB and Delta expression. The effects of LiCl differ depending on the developmental stage treated, with more severe effects elicited by treatment during germband formation than by treatment at later stages. These results support a role for Wnt signalling in conferring posterior identity in Strigamia. In addition, data from this study are consistent with the hypothesis of segmentation based on a "clock and wavefront" mechanism operating in this species.

Changing cell behaviours during beetle embryogenesis correlates with slowing of segmentation

Segmented animals are found in major clades as phylogenetically distant as vertebrates and arthropods. Typically, segments form sequentially in what has been thought to be a regular process, relying on a segmentation clock to pattern budding segments and posterior mitosis to generate axial elongation. Here we show that segmentation in Tribolium has phases of variable periodicity during which segments are added at different rates. Furthermore, elongation during a period of rapid posterior segment addition is driven by high rates of cell rearrangement, demonstrated by differential fates of marked anterior and posterior blastoderm cells. A computational model of this period successfully reproduces elongation through cell rearrangement in the absence of cell division. Unlike current models of steady-state sequential segmentation and elongation from a proliferative growth zone, our results indicate that cell behaviours are dynamic and variable, corresponding to differences in segmentation rate and giving rise to morphologically distinct regions of the embryo.

The complete mitochondrial genome of the geophilomorph centipede Strigamia maritima

Strigamia maritima (Myriapoda; Chilopoda) is a species from the soil-living order of geophilomorph centipedes. The Geophilomorpha is the most speciose order of centipedes with over a 1000 species described. They are notable for their large number of appendage bearing segments and are being used as a laboratory model to study the embryological process of segmentation within the myriapods. Using a scaffold derived from the recently published genome of Strigamia maritima that contained multiple mitochondrial protein-coding genes, here we report the complete mitochondrial genome of Strigamia, the first from any geophilomorph centipede. The mitochondrial genome of S. maritima is a circular molecule of 14,938 base pairs, within which we could identify the typical mitochondrial genome complement of 13 protein-coding genes and 2 ribosomal RNA genes. Sequences resembling 16 of the 22 transfer RNA genes typical of metazoan mitochondrial genomes could be identified, many of which have clear deviations from the standard ‘cloverleaf’ secondary structures of tRNA. Phylogenetic trees derived from the concatenated alignment of protein-coding genes of S. maritima and >50 other metazoans were unable to resolve the Myriapoda as monophyletic, but did support a monophyletic group of chilopods: Strigamia was resolved as the sister group of the scolopendromorph Scolopocryptos sp. and these two (Geophilomorpha and Scolopendromorpha), along with the Lithobiomorpha, formed a monophyletic group the Pleurostigmomorpha. Gene order within the S. maritima mitochondrial genome is unique compared to any other arthropod or metazoan mitochondrial genome to which it has been compared. The highly unusual organisation of the mitochondrial genome of Strigamia maritima is in striking contrast with the conservatively evolving nuclear genome: sampling of more members of this order of centipedes will be required to see whether this unusual organization is typical of the Geophilomorpha or results from a more recent reorganisation in the lineage leading to Strigamia.

Genome evolution: groping in the soil interstices

Centipedes are a very old lineage of terrestrial animals. The first completely sequenced myriapod genome reveals that the blind centipede Strigamia maritima has no gene for light-sensory proteins, lacks the canonical circadian clock and possesses unusual features related to chemosensory perception.

A metameric origin for the annelid pygidium?

BACKGROUND

Segmented body organizations are widely represented in the animal kingdom. Whether the last common bilaterian ancestor was already segmented is intensely debated. Annelids display broad morphological diversity but many species are among the most homonomous metameric animals. The front end (prostomium) and tail piece (pygidium) of annelids are classically described as non-segmental. However, the pygidium structure and development remain poorly studied.

RESULTS

Using different methods of microscopy, immunolabelling and a number of molecular markers, we describe the neural and mesodermal structures of the pygidium of Platynereis dumerilii. We establish that the pygidium possesses a complicated nervous system with a nerve ring and a pair of sensory ganglia, a complex intrinsic musculature, a large terminal circular blood sinus and an unusual unpaired torus-shaped coelomic cavity. We also describe some earlier steps of pygidial development and pygidial structure of mature animals after epitokous transformation.

CONCLUSIONS

We describe a much more complex organization of the pygidium of P. dumerilii than previously suggested. Many of the characteristics are strikingly similar to those found in the trunk segments, opening the debate on whether the pygidium and trunk segments derive from the same ancestral metameric unit. We analyze these scenarios in the context of two classical theories on the origin of segmentation: the cyclomeric/archicoelomate concept and the colonial theory. Both theories provide possible explanations for the partial or complete homology of trunk segments and pygidium.

Evidence for the plasticity of arthropod signal transduction pathways

Metazoans are known to contain a limited, yet highly conserved, set of signal transduction pathways that instruct early developmental patterning mechanisms. Genomic surveys that have compared gene conservation in signal transduction pathways between various insects and Drosophila support the conclusion that these pathways are conserved in evolution. However, the degree to which individual components of signal transduction pathways vary among more divergent arthropods is not known. Here, we report our results of a survey of the genome of the two-spotted spider mite Tetranychus urticae, using a set of 294 Drosophila orthologs of genes that function in signal transduction. We find a third of all genes surveyed absent from the spider mite genome. We also identify several novel duplications that have not been previously reported for a chelicerate. In comparison with previous insect surveys, Tetranychus contains a decrease in overall gene conservation, as well as an unusual ratio of ligands to receptors and other modifiers. These findings suggest that gene loss and duplication among components of signal transduction pathways are common among arthropods and suggest that signal transduction pathways in arthropods are more evolutionarily labile than previously hypothesized.

The first myriapod genome sequence reveals conservative arthropod gene content and genome organisation in the centipede Strigamia maritima

Myriapods (e.g., centipedes and millipedes) display a simple homonomous body plan relative to other arthropods. All members of the class are terrestrial, but they attained terrestriality independently of insects. Myriapoda is the only arthropod class not represented by a sequenced genome. We present an analysis of the genome of the centipede Strigamia maritima. It retains a compact genome that has undergone less gene loss and shuffling than previously sequenced arthropods, and many orthologues of genes conserved from the bilaterian ancestor that have been lost in insects. Our analysis locates many genes in conserved macro-synteny contexts, and many small-scale examples of gene clustering. We describe several examples where S. maritima shows different solutions from insects to similar problems. The insect olfactory receptor gene family is absent from S. maritima, and olfaction in air is likely effected by expansion of other receptor gene families. For some genes S. maritima has evolved paralogues to generate coding sequence diversity, where insects use alternate splicing. This is most striking for the Dscam gene, which in Drosophila generates more than 100,000 alternate splice forms, but in S. maritima is encoded by over 100 paralogues. We see an intriguing linkage between the absence of any known photosensory proteins in a blind organism and the additional absence of canonical circadian clock genes. The phylogenetic position of myriapods allows us to identify where in arthropod phylogeny several particular molecular mechanisms and traits emerged. For example, we conclude that juvenile hormone signalling evolved with the emergence of the exoskeleton in the arthropods and that RR-1 containing cuticle proteins evolved in the lineage leading to Mandibulata. We also identify when various gene expansions and losses occurred. The genome of S. maritima offers us a unique glimpse into the ancestral arthropod genome, while also displaying many adaptations to its specific life history.

Arthropods are the most abundant animals on earth. Among them, insects clearly dominate on land, whereas crustaceans hold the title for the most diverse invertebrates in the oceans. Much is known about the biology of these groups, not least because of genomic studies of the fruit fly Drosophila, the water flea Daphnia, and other species used in research. Here we report the first genome sequence from a species belonging to a lineage that has previously received very little attention—the myriapods. Myriapods were among the first arthropods to invade the land over 400 million years ago, and survive today as the herbivorous millipedes and venomous centipedes, one of which—Strigamia maritima—we have sequenced here. We find that the genome of this centipede retains more characteristics of the presumed arthropod ancestor than other sequenced insect genomes. The genome provides access to many aspects of myriapod biology that have not been studied before, suggesting, for example, that they have diversified receptors for smell that are quite different from those used by insects. In addition, it shows specific consequences of the largely subterranean life of this particular species, which seems to have lost the genes for all known light-sensing molecules, even though it still avoids light.

History of a prolific family: the Hes/Hey-related genes of the annelid Platynereis

BACKGROUND

The Hes superfamily or Hes/Hey-related genes encompass a variety of metazoan-specific bHLH genes, with somewhat fuzzy phylogenetic relationships. Hes superfamily members are involved in a variety of major developmental mechanisms in metazoans, notably in neurogenesis and segmentation processes, in which they often act as direct effector genes of the Notch signaling pathway.

RESULTS

We have investigated the molecular and functional evolution of the Hes superfamily in metazoans using the lophotrochozoan Platynereis dumerilii as model. Our phylogenetic analyses of more than 200 Metazoan Hes/Hey-related genes revealed the presence of five families, three of them (Hes, Hey and Helt) being pan-metazoan. Those families were likely composed of a unique representative in the last common metazoan ancestor. The evolution of the Hes family was shaped by many independent lineage specific tandem duplication events. The expression patterns of 13 of the 15 Hes/Hey-related genes in Platynereis indicate a broad functional diversification. Nevertheless, a majority of these genes are involved in two crucial developmental processes in annelids: neurogenesis and segmentation, resembling functions highlighted in other animal models.

CONCLUSIONS

Combining phylogenetic and expression data, our study suggests an unusual evolutionary history for the Hes superfamily. An ancestral multifunctional annelid Hes gene may have undergone multiples rounds of duplication-degeneration-complementation processes in the lineage leading to Platynereis, each gene copies ensuring their maintenance in the genome by subfunctionalisation. Similar but independent waves of duplications are at the origin of the multiplicity of Hes genes in other metazoan lineages.

Classification of transient behaviours in a time-dependent toggle switch model

BACKGROUND

Waddington's epigenetic landscape is an intuitive metaphor for the developmental and evolutionary potential of biological regulatory processes. It emphasises time-dependence and transient behaviour. Nowadays, we can derive this landscape by modelling a specific regulatory network as a dynamical system and calculating its so-called potential surface. In this sense, potential surfaces are the mathematical equivalent of the Waddingtonian landscape metaphor. In order to fully capture the time-dependent (non-autonomous) transient behaviour of biological processes, we must be able to characterise potential landscapes and how they change over time. However, currently available mathematical tools focus on the asymptotic (steady-state) behaviour of autonomous dynamical systems, which restricts how biological systems are studied.

RESULTS

We present a pragmatic first step towards a methodology for dealing with transient behaviours in non-autonomous systems. We propose a classification scheme for different kinds of such dynamics based on the simulation of a simple genetic toggle-switch model with time-variable parameters. For this low-dimensional system, we can calculate and explicitly visualise numerical approximations to the potential landscape. Focussing on transient dynamics in non-autonomous systems reveals a range of interesting and biologically relevant behaviours that would be missed in steady-state analyses of autonomous systems. Our simulation-based approach allows us to identify four qualitatively different kinds of dynamics: transitions, pursuits, and two kinds of captures. We describe these in detail, and illustrate the usefulness of our classification scheme by providing a number of examples that demonstrate how it can be employed to gain specific mechanistic insights into the dynamics of gene regulation.

CONCLUSIONS

The practical aim of our proposed classification scheme is to make the analysis of explicitly time-dependent transient behaviour tractable, and to encourage the wider use of non-autonomous models in systems biology. Our method is applicable to a large class of biological processes.

Dual mode of embryonic development is highlighted by expression and function of Nasonia pair-rule genes

Embryonic anterior-posterior patterning is well understood in Drosophila, which uses 'long germ' embryogenesis, in which all segments are patterned before cellularization. In contrast, most insects use 'short germ' embryogenesis, wherein only head and thorax are patterned in a syncytial environment while the remainder of the embryo is generated after cellularization. We use the wasp Nasonia (Nv) to address how the transition from short to long germ embryogenesis occurred. Maternal and gap gene expression in Nasonia suggest long germ embryogenesis. However, the Nasonia pair-rule genes even-skipped, odd-skipped, runt and hairy are all expressed as early blastoderm pair-rule stripes and late-forming posterior stripes. Knockdown of Nv eve, odd or h causes loss of alternate segments at the anterior and complete loss of abdominal segments. We propose that Nasonia uses a mixed mode of segmentation wherein pair-rule genes pattern the embryo in a manner resembling Drosophila at the anterior and ancestral Tribolium at the posterior. DOI: http://dx.doi.org/10.7554/eLife.01440.001.

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.

Early embryonic determination of the sexual dimorphism in segment number in geophilomorph centipedes

Background

Most geophilomorph centipedes show intraspecific variability in the number of leg-bearing segments. This intraspecific variability generally has a component that is related to sex, with females having on average more segments than males. Neither the developmental basis nor the adaptive role of this dimorphism is known.

Results

To determine when this sexual dimorphism in segment number is established, we have followed the development of Strigamia maritima embryos from the onset of segmentation to the first post-embryonic stage where we could determine the sex morphologically. We find that males and females differ in segment number by Stage 6.1, a point during embryogenesis when segment addition pauses while the embryo undergoes large-scale movements. We have confirmed this pattern by establishing a molecular method to determine the sex of single embryos, utilising duplex PCR amplification for Y chromosomal and autosomal sequences. This confirms that male embryos have a modal number of 43 segments visible at Stage 6, while females have 45. In our Strigamia population, adult males have a modal number of 47 leg-bearing segments, and females have 49. This implies that the sexual dimorphism in segment number is determined before the addition of the last leg-bearing segments and the terminal genital segments.

Conclusions

Sexual dimorphism in segment number is not associated with terminal segment differentiation, but must instead be related to some earlier process during segment patterning. The dimorphism may be associated with a difference in the rate and/or duration of segment addition during the main phase of rapid segment addition that precedes embryonic Stage 6. This suggests that the adaptive role, if any, of the dimorphism is likely to be related to segment number per se, and not to sexual differentiation of the terminal region.

Deciphering the onychophoran 'segmentation gene cascade': Gene expression reveals limited involvement of pair rule gene orthologs in segmentation, but a highly conserved segment polarity gene network

The hallmark of the arthropods is their segmented body, although origin of segmentation, however, is unresolved. In order to shed light on the origin of segmentation we investigated orthologs of pair rule genes (PRGs) and segment polarity genes (SPGs) in a member of the closest related sister-group to the arthropods, the onychophorans. Our gene expression data analysis suggests that most of the onychophoran PRGs do not play a role in segmentation. One possible exception is the even-skipped (eve) gene that is expressed in the posterior end of the onychophoran where new segments are likely patterned, and is also expressed in segmentation-gene typical transverse stripes in at least a number of newly formed segments. Other onychophoran PRGs such as runt (run), hairy/Hes (h/Hes) and odd-skipped (odd) do not appear to have a function in segmentation at all. Onychophoran PRGs that act low in the segmentation gene cascade in insects, however, are potentially involved in segment-patterning. Most obvious is that from the expression of the pairberry (pby) gene ortholog that is expressed in a typical SPG-pattern. Since this result suggested possible conservation of the SPG-network we further investigated SPGs (and associated factors) such as Notum in the onychophoran. We find that the expression patterns of SPGs in arthropods and the onychophoran are highly conserved, suggesting a conserved SPG-network in these two clades, and indeed also in an annelid. This may suggest that the common ancestor of lophotrochozoans and ecdysozoans was already segmented utilising the same SPG-network, or that the SPG-network was recruited independently in annelids and onychophorans/arthropods.

Evolution of the pair rule gene network: Insights from a centipede

Comparative studies have examined the expression and function of homologues of the Drosophila melanogaster pair rule and segment polarity genes in a range of arthropods. The segment polarity gene homologues have a conserved role in the specification of the parasegment boundary, but the degree of conservation of the upstream patterning genes has proved more variable. Using genomic resources we identify a complete set of pair rule gene homologues from the centipede Strigamia maritima, and document a detailed time series of expression during trunk segmentation. We find supportive evidence for a conserved hierarchical organisation of the pair rule genes, with a division into early- and late-activated genes which parallels the functional division into primary and secondary pair rule genes described in insects. We confirm that the relative expression of sloppy-paired and paired with respect to wingless and engrailed at the parasegment boundary is conserved between myriapods and insects; suggesting that functional interactions between these genes might be an ancient feature of arthropod segment patterning. However, we find that the relative expression of a number of the primary pair rule genes is divergent between myriapods and insects. This corroborates suggestions that the evolution of upper tiers in the segmentation gene network is more flexible. Finally, we find that the expression of the Strigamia pair rule genes in periodic patterns is restricted to the ectoderm. This suggests that any direct role of these genes in segmentation is restricted to this germ layer, and that mesoderm segmentation is either dependent on the ectoderm, or occurs through an independent mechanism.