Toll Genes Have an Ancestral Role in Axis Elongation

Activation of the tick Toll pathway to control infection of Ixodes ricinus by the apicomplexan parasite Babesia microti

The vector competence of blood-feeding arthropods is influenced by the interaction between pathogens and the immune system of the vector. The Toll and IMD (immune deficiency) signaling pathways play a key role in the regulation of innate immunity in both the Drosophila model and blood-feeding insects. However, in ticks (chelicerates), immune determination for pathogen acquisition and transmission has not yet been fully explored. Here, we have mapped homologs of insect Toll and IMD pathways in the European tick Ixodes ricinus, an important vector of human and animal diseases. We show that most genes of the Toll pathway are well conserved, whereas the IMD pathway has been greatly reduced. We therefore investigated the functions of the individual components of the tick Toll pathway and found that, unlike in Drosophila, it was specifically activated by Gram-negative bacteria. The activation of pathway induced the expression of defensin (defIR), the first identified downstream effector gene of the tick Toll pathway. Borrelia, an atypical bacterium and causative agent of Lyme borreliosis, bypassed Toll-mediated recognition in I. ricinus and also resisted systemic effector molecules when the Toll pathway was activated by silencing its repressor cactus via RNA interference. Babesia, an apicomplexan parasite, also avoided Toll-mediated recognition. Strikingly, unlike Borrelia, the number of Babesia parasites reaching the salivary glands during tick infection was significantly reduced by knocking down cactus. The simultaneous silencing of cactus and dorsal resulted in greater infections and underscored the importance of tick immunity in regulating parasite infections in these important disease vectors.

A taxon-restricted duplicate of Iroquois3 is required for patterning the spider waist

The chelicerate body plan is distinguished from other arthropod groups by its division of segments into 2 tagmata: the anterior prosoma ("cephalothorax") and the posterior opisthosoma ("abdomen"). Little is understood about the genetic mechanisms that establish the prosomal-opisthosomal (PO) boundary. To discover these mechanisms, we created high-quality genomic resources for the large-bodied spider Aphonopelma hentzi. We sequenced specific territories along the antero-posterior axis of developing embryos and applied differential gene expression analyses to identify putative regulators of regional identity. After bioinformatic screening for candidate genes that were consistently highly expressed in only 1 tagma (either the prosoma or the opisthosoma), we validated the function of highly ranked candidates in the tractable spider model Parasteatoda tepidariorum. Here, we show that an arthropod homolog of the Iroquois complex of homeobox genes is required for proper formation of the boundary between arachnid tagmata. The function of this homolog had not been previously characterized, because it was lost in the common ancestor of Pancrustacea, precluding its investigation in well-studied insect model organisms. Knockdown of the spider copy of this gene, which we designate as waist-less, in P. tepidariorum resulted in embryos with defects in the PO boundary, incurring discontinuous spider germ bands. We show that waist-less is required for proper specification of the segments that span the prosoma-opisthosoma boundary, which in adult spiders corresponds to the narrowed pedicel. Our results demonstrate the requirement of an ancient, taxon-restricted paralog for the establishment of the tagmatic boundary that defines Chelicerata.

Neofunctionalization of Toll Signaling in Insects: From Immunity to Dorsoventral Patterning

Toll signaling plays a crucial role in pathogen defense throughout the animal kingdom. It was discovered, however, for its function in dorsoventral (DV) axis formation in Drosophila. In all other insects studied so far, but not outside the insects, Toll is also required for DV patterning. However, in insects more distantly related to Drosophila, Toll's patterning role is frequently reduced and substituted by an expanded influence of BMP signaling, the pathway implicated in DV axis formation in all major metazoan lineages. This suggests that Toll was integrated into an ancestral BMP-based patterning system at the base of the insects or during insect evolution. The observation that Toll signaling has an immune function in the extraembryonic serosa, an early differentiating tissue of most insect embryos, suggests a scenario of how Toll was co-opted from an ancestral immune function for its new role in axis formation.

The Impact of Whole Genome Duplication on the Evolution of the Arachnids

The proliferation of genomic resources for Chelicerata in the past 10 years has revealed that the evolution of chelicerate genomes is more dynamic than previously thought, with multiple waves of ancient whole genome duplications affecting separate lineages. Such duplication events are fascinating from the perspective of evolutionary history because the burst of new gene copies associated with genome duplications facilitates the acquisition of new gene functions (neofunctionalization), which may in turn lead to morphological novelties and spur net diversification. While neofunctionalization has been invoked in several contexts with respect to the success and diversity of spiders, the overall impact of whole genome duplications on chelicerate evolution and development remains imperfectly understood. The purpose of this review is to examine critically the role of whole genome duplication on the diversification of the extant arachnid orders, as well as assess functional datasets for evidence of subfunctionalization or neofunctionalization in chelicerates. This examination focuses on functional data from two focal model taxa: the spider Parasteatoda tepidariorum, which exhibits evidence for an ancient duplication, and the harvestman Phalangium opilio, which exhibits an unduplicated genome. I show that there is no evidence that taxa with genome duplications are more successful than taxa with unduplicated genomes. I contend that evidence for sub- or neofunctionalization of duplicated developmental patterning genes in spiders is indirect or fragmentary at present, despite the appeal of this postulate for explaining the success of groups like spiders. Available expression data suggest that the condition of duplicated Hox modules may have played a role in promoting body plan disparity in the posterior tagma of some orders, such as spiders and scorpions, but functional data substantiating this postulate are critically missing. Spatiotemporal dynamics of duplicated transcription factors in spiders may represent cases of developmental system drift, rather than neofunctionalization. Developmental system drift may represent an important, but overlooked, null hypothesis for studies of paralogs in chelicerate developmental biology. To distinguish between subfunctionalization, neofunctionalization, and developmental system drift, concomitant establishment of comparative functional datasets from taxa exhibiting the genome duplication, as well as those that lack the paralogy, is sorely needed.

Appendage abnormalities in spiders induced by an alternating temperature protocol in the context of recent advances in molecular spider embryology

In the literature there are numerous reports of developmental deformities in arthropods collected in their natural habitat. Since such teratogenically affected individuals are found purely by chance, the causes of their defects are unknown. Numerous potential physical, mechanical, chemical, and biological teratogens have been considered and tested in the laboratory. Thermal shocks, frequently used in teratological research on the spider Eratigena atrica, have led to deformities on both the prosoma and the opisthosoma. In the 2020/2021 breeding season, by applying alternating temperatures (14 °C and 32 °C, changed every 12 h) for the first 10 days of embryonic development, we obtained 212 postembryos (out of 3,007) with the following anomalies: oligomely, heterosymely, bicephaly, schistomely, symely, polymely, complex anomalies, and others. From these we selected six spiders with defects on the prosoma and two with short appendages on the pedicel for further consideration. The latter cases seem particularly interesting because appendages do not normally develop on this body part, viewed as the first segment of the opisthosoma, and appear to represent examples of atavism. In view of the ongoing development of molecular techniques and recent research on developmental mechanisms in spiders, we believe the observed phenotypes may result, at least in part, from the erroneous suppression or expression of segmentation or appendage patterning genes. We consider "knockdown" experiments described in the literature as a means for generating hypotheses about the sources of temperature-induced body abnormalities in E. atrica.

Striped Expression of Leucine-Rich Repeat Proteins Coordinates Cell Intercalation and Compartment Boundary Formation in the Early Drosophila Embryo

Planar polarity is a commonly observed phenomenon in which proteins display a consistent asymmetry in their subcellular localization or activity across the plane of a tissue. During animal development, planar polarity is a fundamental mechanism for coordinating the behaviors of groups of cells to achieve anisotropic tissue remodeling, growth, and organization. Therefore, a primary focus of developmental biology research has been to understand the molecular mechanisms underlying planar polarity in a variety of systems to identify conserved principles of tissue organization. In the early Drosophila embryo, the germband neuroectoderm epithelium rapidly doubles in length along the anterior-posterior axis through a process known as convergent extension (CE); it also becomes subdivided into tandem tissue compartments through the formation of compartment boundaries (CBs). Both processes are dependent on the planar polarity of proteins involved in cellular tension and adhesion. The enrichment of actomyosin-based tension and adherens junction-based adhesion at specific cell-cell contacts is required for coordinated cell intercalation, which drives CE, and the creation of highly stable cell-cell contacts at CBs. Recent studies have revealed a system for rapid cellular polarization triggered by the expression of leucine-rich-repeat (LRR) cell-surface proteins in striped patterns. In particular, the non-uniform expression of Toll-2, Toll-6, Toll-8, and Tartan generates local cellular asymmetries that allow cells to distinguish between cell-cell contacts oriented parallel or perpendicular to the anterior-posterior axis. In this review, we discuss (1) the biomechanical underpinnings of CE and CB formation, (2) how the initial symmetry-breaking events of anterior-posterior patterning culminate in planar polarity, and (3) recent advances in understanding the molecular mechanisms downstream of LRR receptors that lead to planar polarized tension and junctional adhesion.

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

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

FGF signalling is involved in cumulus migration in the common house spider Parasteatoda tepidariorum

Cell migration is a fundamental component during the development of most multicellular organisms. In the early spider embryo, the collective migration of signalling cells, known as the cumulus, is required to set the dorsoventral body axis. Here, we show that FGF signalling plays an important role during cumulus migration in the spider Parasteatoda tepidariorum. Spider embryos with reduced FGF signalling show reduced or absent cumulus migration and display dorsoventral patterning defects. Our study reveals that the transcription factor Ets4 regulates the expression of several FGF signalling components in the cumulus. In conjunction with a previous study, we show that the expression of fgf8 in the germ-disc is regulated via the Hedgehog signalling pathway. We also demonstrate that FGF signalling influences the BMP signalling pathway activity in the region around cumulus cells. Finally, we show that FGFR signalling might also influence cumulus migration in basally branching spiders and we propose that fgf8 might act as a chemo-attractant to guide cumulus cells towards the future dorsal pole of the spider embryo.

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.

Cell mechanics and cell-cell recognition controls by Toll-like receptors in tissue morphogenesis and homeostasis

Signal transduction by the Toll-like receptors (TLRs) is conserved and essential for innate immunity in metazoans. The founding member of the TLR family, Drosophila Toll-1, was initially identified for its role in dorsoventral axis formation in early embryogenesis. The Drosophila genome encodes nine TLRs that display dynamic expression patterns during development, suggesting their involvement in tissue morphogenesis and homeostasis. Recent progress on the developmental functions of TLRs beyond dorsoventral patterning has revealed not only their diverse functions in various biological processes, but also unprecedented molecular mechanisms in directly regulating cell mechanics and cell-cell recognition independent of the canonical signal transduction pathway involving transcriptional regulation of target genes. In this review, I feature and discuss the non-immune functions of TLRs in the control of epithelial tissue homeostasis, tissue morphogenesis, and cell-cell recognition between cell populations with different cell identities.

Toll-9 interacts with Toll-1 to mediate a feedback loop during apoptosis-induced proliferation in Drosophila

Drosophila Toll-1 and all mammalian Toll-like receptors regulate innate immunity. However, the functions of the remaining eight Toll-related proteins in Drosophila are not fully understood. Here, we show that Drosophila Toll-9 is necessary and sufficient for a special form of compensatory proliferation after apoptotic cell loss (undead apoptosis-induced proliferation [AiP]). Mechanistically, for AiP, Toll-9 interacts with Toll-1 to activate the intracellular Toll-1 pathway for nuclear translocation of the NF-κB-like transcription factor Dorsal, which induces expression of the pro-apoptotic genes reaper and hid. This activity contributes to the feedback amplification loop that operates in undead cells. Given that Toll-9 also functions in loser cells during cell competition, we define a general role of Toll-9 in cellular stress situations leading to the expression of pro-apoptotic genes that trigger apoptosis and apoptosis-induced processes such as AiP. This work identifies conceptual similarities between cell competition and AiP.

Toll signalling promotes blastema cell proliferation during cricket leg regeneration via insect macrophages

Hemimetabolous insects, such as the two-spotted cricket Gryllus bimaculatus, can recover lost tissues, in contrast to the limited regenerative abilities of human tissues. Following cricket leg amputation, the wound surface is covered by the wound epidermis, and plasmatocytes, which are insect macrophages, accumulate in the wound region. Here, we studied the function of Toll-related molecules identified by comparative RNA sequencing during leg regeneration. Of the 11 Toll genes in the Gryllus genome, expression of Toll2-1, Toll2-2 and Toll2-5 was upregulated during regeneration. RNA interference (RNAi) of Toll, Toll2-1, Toll2-2, Toll2-3 or Toll2-4 produced regeneration defects in more than 50% of crickets. RNAi of Toll2-2 led to a decrease in the ratio of S- and M-phase cells, reduced expression of JAK/STAT signalling genes, and reduced accumulation of plasmatocytes in the blastema. Depletion of plasmatocytes in crickets using clodronate also produced regeneration defects, as well as fewer proliferating cells in the regenerating legs. Plasmatocyte depletion also downregulated the expression of Toll and JAK/STAT signalling genes in the regenerating legs. These results suggest that Spz-Toll-related signalling in plasmatocytes promotes leg regeneration through blastema cell proliferation by regulating the Upd-JAK/STAT signalling pathway.

Expression and Function of Toll Pathway Components in the Early Development of the Wasp Nasonia vitripennis

The Toll signaling pathway is the main source of embryonic DV polarity in the fly Drosophila melanogaster. This pathway appears to have been co-opted from an ancestral innate immunity system within the insects and has been deployed in different ways among insect taxa. Here we report the expression and function of homologs of the important components of the D. melanogaster Toll pathway in the wasp Nasonia vitripennis. We found homologs for all the components; many components had one or more additional paralogs in the wasp relative the fly. We also found significant deviations in expression patterns of N. vitripennis homologs. Finally, we provide some preliminary functional analyses of the N. vitripennis homologs, where we find a mixture of conservation and divergence of function.

The T-box gene optomotor-blind organizes proximodistal leg patterning in the beetle Tribolium castaneum by repressing dorsal Dpp pathway activity

Leg axis formation in Drosophila is organized by Wingless (Wg) and Decapentaplegic (Dpp) that control a number of downstream factors to pattern the dorsoventral (DV) and proximodistal (PD) axis. The T-box genes are important downstream factors mainly involved in dorsoventral leg axis formation. The ventral side is specified by H15 and midline, whereas optomotor-blind (omb) and Dorsocross (Doc1) are factors to specify dorsal cell fates. We show here that omb also organizes PD leg axis patterning in the beetle Tribolium castaneum. In the legs, Tc-omb is expressed along the dorsal side and represses ventral factors like wg and H15. Intriguingly, removing Tc-omb function leads to the activation of the Dpp pathway along the dorsal side of the legs, thus mimicking normal dpp expression in Drosophila. Dpp activity along the dorsal side leads to altered expression of proximal-distal patterning genes such as Distal-less (Dll) and dachshund (dac). Our results indicate a cell-autonomous activation of Dll and repression of dac by dpp. These findings are compatible with the cross-regulatory "cascade model" of proximal-distal leg imaginal disc patterning of Drosophila.

In search of conserved principles of planar cell polarization

The making of an embryo and its internal organs entails the spatial coordination of cellular activities. This manifests during tissue morphogenesis as cells change shape, rearrange and divide along preferential axis and during cell differentiation. Cells live in a polarized field and respond to it by polarizing their cellular activities in the plane of the tissue by a phenomenon called planar cell polarization. This phenomenon is ubiquitous in animals and depends on a few conserved planar cell polarity (PCP) pathways. All PCP pathways share two essential characteristics: the existence of local interactions between protein complexes present at the cell surface leading to their asymmetric distribution within cells; a supracellular graded cue that aligns these cellular asymmetries at the tissue level. Here, we discuss the potential common principles of planar cell polarization by comparing the local and global mechanisms employed by the different PCP pathways identified so far. The focus of the review is on the logic of the system rather than the molecules per se.

The evolution of the metazoan Toll receptor family and its expression during protostome development

BACKGROUND

Toll-like receptors (TLRs) play a crucial role in immunity and development. They contain leucine-rich repeat domains, one transmembrane domain, and one Toll/IL-1 receptor domain. TLRs have been classified into V-type/scc and P-type/mcc TLRs, based on differences in the leucine-rich repeat domain region. Although TLRs are widespread in animals, detailed phylogenetic studies of this gene family are lacking. Here we aim to uncover TLR evolution by conducting a survey and a phylogenetic analysis in species across Bilateria. To discriminate between their role in development and immunity we furthermore analyzed stage-specific transcriptomes of the ecdysozoans Priapulus caudatus and Hypsibius exemplaris, and the spiralians Crassostrea gigas and Terebratalia transversa.

RESULTS

We detected a low number of TLRs in ecdysozoan species, and multiple independent radiations within the Spiralia. V-type/scc and P-type/mcc type-receptors are present in cnidarians, protostomes and deuterostomes, and therefore they emerged early in TLR evolution, followed by a loss in xenacoelomorphs. Our phylogenetic analysis shows that TLRs cluster into three major clades: clade α is present in cnidarians, ecdysozoans, and spiralians; clade β in deuterostomes, ecdysozoans, and spiralians; and clade γ is only found in spiralians. Our stage-specific transcriptome and in situ hybridization analyses show that TLRs are expressed during development in all species analyzed, which indicates a broad role of TLRs during animal development.

CONCLUSIONS

Our findings suggest that a clade α TLR gene (TLR-Ca) and a clade β/γ TLR gene (TLR-Cβ/γ) were already present in the cnidarian-bilaterian common ancestor. However, although TLR-Ca was conserved in cnidarians, TLR-Cβ/γ was lost during the early evolution of these taxa. Moreover, TLR-Cβ/γ duplicated to generate TLR-Cβ and TLR-Cγ in the lineage to the last common protostome-deuterostome ancestor. TLR-Ca, TLR-Cβ and TLR-Cγ further expanded generating the three major TLR clades. While all three clades radiated in several spiralian lineages, specific TLRs clades have been presumably lost in other lineages. Furthermore, the expression of the majority of these genes during protostome ontogeny suggests a likely role in development.

The Toll gene in Drosophila pattern formation

Toll-like receptors (TLRs) play a crucial role in innate immunity in animals. Their discovery was rewarded a Nobel Prize to Jules Hoffmann and Bruce Beutler in 2011. The name Toll stems from a Drosophila mutant that was isolated in 1980 by Eric Wieschaus and myself as a byproduct of our screen for segmentation genes in Drosophila for which we received the Nobel Prize in 1995. It was named Toll due to its amazing dominant phenotype displayed in embryos from Toll/+ females. The analysis of Toll by Kathryn Anderson in my laboratory in Tübingen and subsequently in her own laboratory in Berkeley singled out Toll as a central component of the complex pathway regulating dorsoventral polarity and pattern of the Drosophila embryo. The Drosophila Toll story provides a striking example for the value of curiosity-driven research in providing fundamental insights that later gain strong impact on applied medical research.

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.

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 reaction-diffusion network model predicts a dual role of Cactus/IκB to regulate Dorsal/NFκB nuclear translocation in Drosophila

Dorsal-ventral patterning of the Drosophila embryo depends on the NFκB superfamily transcription factor Dorsal (Dl). Toll receptor activation signals for degradation of the IκB inhibitor Cactus (Cact), leading to a ventral-to-dorsal nuclear Dl gradient. Cact is critical for Dl nuclear import, as it binds to and prevents Dl from entering the nuclei. Quantitative analysis of cact mutants revealed an additional Cact function to promote Dl nuclear translocation in ventral regions of the embryo. To investigate this dual Cact role, we developed a predictive model based on a reaction-diffusion regulatory network. This network distinguishes non-uniform Toll-dependent Dl nuclear import and Cact degradation, from the Toll-independent processes of Cact degradation and reversible nuclear-cytoplasmic Dl flow. In addition, it incorporates translational control of Cact levels by Dl. Our model successfully reproduces wild-type data and emulates the Dl nuclear gradient in mutant dl and cact allelic combinations. Our results indicate that the dual role of Cact depends on the dynamics of Dl-Cact trimers along the dorsal-ventral axis: In the absence of Toll activation, free Dl-Cact trimers retain Dl in the cytoplasm, limiting the flow of Dl into the nucleus; in ventral-lateral regions, Dl-Cact trimers are recruited by Toll activation into predominant signaling complexes and promote Dl nuclear translocation. Simulations suggest that the balance between Toll-dependent and Toll-independent processes are key to this dynamics and reproduce the full assortment of Cact effects. Considering the high evolutionary conservation of these pathways, our analysis should contribute to understanding NFκB/c-Rel activation in other contexts such as in the vertebrate immune system and disease.

In Drosophila, Toll pathway establishes spatially distinct gene expression territories that define the embryonic dorsal-ventral axis. Toll activation leads to degradation of the IκB inhibitor Cactus, releasing the NFκB superfamily transcription factor Dorsal for nuclear entry. Recently, quantitative analysis of cact mutants revealed that Cact displays an additional function to promote Dl nuclear translocation in ventral regions of the embryo. To understand this novel activity, we developed a predictive theoretical model that shows that the kinetics of Dorsal-Cactus complex formation prior to their recruitment to Toll-signaling complexes is an essential regulatory hub. Cactus controls the balance between the recruitment of these complexes by active Toll receptor and association-dissociation events that generate free Dorsal for direct nuclear import.

Formation of polarized contractile interfaces by self-organized Toll-8/Cirl GPCR asymmetry

Interfaces between cells with distinct genetic identities elicit signals to organize local cell behaviors driving tissue morphogenesis. The Drosophila embryonic axis extension requires planar polarized enrichment of myosin-II powering oriented cell intercalations. Myosin-II levels are quantitatively controlled by GPCR signaling, whereas myosin-II polarity requires patterned expression of several Toll receptors. How Toll receptors polarize myosin-II and how this involves GPCRs remain unknown. Here, we report that differential expression of a single Toll receptor, Toll-8, polarizes myosin-II through binding to the adhesion GPCR Cirl/latrophilin. Asymmetric expression of Cirl is sufficient to enrich myosin-II, and Cirl localization is asymmetric at Toll-8 expression boundaries. Exploring the process dynamically, we reveal that Toll-8 and Cirl exhibit mutually dependent planar polarity in response to quantitative differences in Toll-8 expression between neighboring cells. Collectively, we propose that the cell surface protein complex Toll-8/Cirl self-organizes to generate local asymmetric interfaces essential for planar polarization of contractility.

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Asymmetric expression of a single Toll receptor leads to Myo-II polarization

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The adhesion GPCR Cirl binds to Toll-8 mediating Toll-8-induced Myo-II polarization

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Toll-8 boundaries generate a Cirl interfacial asymmetry that can polarize Myo-II

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Differences in Toll-8 levels lead to interdependent Toll-8 and Cirl planar polarity

Toll receptors remodel epithelia by directing planar-polarized Src and PI3K activity

Toll-like receptors are essential for animal development and survival, with conserved roles in innate immunity, tissue patterning, and cell behavior. The mechanisms by which Toll receptors signal to the nucleus are well characterized, but how Toll receptors generate rapid, localized signals at the cell membrane to produce acute changes in cell polarity and behavior is not known. We show that Drosophila Toll receptors direct epithelial convergent extension by inducing planar-polarized patterns of Src and PI3-kinase (PI3K) activity. Toll receptors target Src activity to specific sites at the membrane, and Src recruits PI3K to the Toll-2 complex through tyrosine phosphorylation of the Toll-2 cytoplasmic domain. Reducing Src or PI3K activity disrupts planar-polarized myosin assembly, cell intercalation, and convergent extension, whereas constitutive Src activity promotes ectopic PI3K and myosin cortical localization. These results demonstrate that Toll receptors direct cell polarity and behavior by locally mobilizing Src and PI3K activity.

Striking parallels between dorsoventral patterning in Drosophila and Gryllus reveal a complex evolutionary history behind a model gene regulatory network

Dorsoventral pattering relies on Toll and BMP signalling in all insects studied so far, with variations in the relative contributions of both pathways. Drosophila and the beetle Tribolium share extensive dependence on Toll, while representatives of more distantly related lineages like the wasp Nasonia and bug Oncopeltus rely more strongly on BMP signalling. Here, we show that in the cricket Gryllus bimaculatus, an evolutionarily distant outgroup, Toll has, like in Drosophila, a direct patterning role for the ventral half of the embryo. In addition, Toll polarises BMP signalling, although this does not involve the conserved BMP inhibitor Sog/Chordin. Finally, Toll activation relies on ovarian patterning mechanisms with striking similarity to Drosophila. Our data suggest two surprising hypotheses: (1) that Toll's patterning function in Gryllus and Drosophila is the result of convergent evolution or (2) a Drosophila-like system arose early in insect evolution and was extensively altered in multiple independent lineages.

The Evolution of Sox Gene Repertoires and Regulation of Segmentation in Arachnids

The Sox family of transcription factors regulates many processes during metazoan development, including stem cell maintenance and nervous system specification. Characterizing the repertoires and roles of these genes can therefore provide important insights into animal evolution and development. We further characterized the Sox repertoires of several arachnid species with and without an ancestral whole-genome duplication and compared their expression between the spider Parasteatoda tepidariorum and the harvestman Phalangium opilio. We found that most Sox families have been retained as ohnologs after whole-genome duplication and evidence for potential subfunctionalization and/or neofunctionalization events. Our results also suggest that Sox21b-1 likely regulated segmentation ancestrally in arachnids, playing a similar role to the closely related SoxB gene, Dichaete, in insects. We previously showed that Sox21b-1 is required for the simultaneous formation of prosomal segments and sequential addition of opisthosomal segments in P. tepidariorum. We studied the expression and function of Sox21b-1 further in this spider and found that although this gene regulates the generation of both prosomal and opisthosomal segments, it plays different roles in the formation of these tagmata reflecting their contrasting modes of segmentation and deployment of gene regulatory networks with different architectures.

A conserved role for arrow in posterior axis patterning across Arthropoda

Segmentation is a key characteristic of Arthropoda that is linked to the evolutionary success of this lineage. It has previously been shown in both vertebrates and short germ insects that posterior segmentation requires canonical Wnt (cWnt) signaling, which maintains the expression of Caudal and the posterior growth zone; disruption of cWnt signaling incurs posterior truncations in these lineages due to the loss of the tail bud. However, comparable datasets for Wnt signaling are limited outside of holometabolous insects, due to incomparable phenotypic spectra and inefficacy of gene misexpression methods in certain model species. We applied RNA interference (RNAi) against the Wnt co-receptor arrow (arr), a key member of the cWnt signaling pathway in holometabolous insects and vertebrates, to examine posterior axis elongation of the cobweb spider Parasteatoda tepidariorum (short germ embryogenesis; one Wnt8 homolog), the cricket Gryllus bimaculatus (intermediate germ; one Wnt8 homolog), and the milkweed bug Oncopeltus fasciatus (short germ; two Wnt8 homologs). Knockdown of arr in insects resulted in posterior truncations affecting the gnathos through the abdomen in O. fasciatus, whereas posterior truncations only affected the T3 segment through the abdomen in G. bimaculatus. Spider embryos with disrupted arr expression exhibited defects along the entire axis, including segmentation defects throughout the germband. RNA-Seq-based differential gene expression analysis of severe Ptep-arr loss-of-function phenotypes at two developmental stages was used to confirm that knockdown of Ptep-arr results in systemic disruption of the Wnt pathway. Intriguingly, we found that knockdown of arr did not abrogate Wnt8 expression in any of the three species, with cad expression additionally retained in severe loss-of-function phenotypes in the cricket and the spider. Together with data from a holometabolous insect, our results suggest that cWnt signaling is not required for maintenance of Wnt8 expression across Arthropoda. These outcomes underscore the diagnostic power of differential gene expression analyses in characterizing catastrophic phenotypes in emerging model species.

Spatiotemporal variation in cell proliferation patterns during arthropod axial elongation

An elongated and segmented body plan is a common morphological characteristic of all arthropods and is probably responsible for their high adaptation ability to diverse environments. Most arthropods form their bodies by progressively adding segments, resembling vertebrate somitogenesis. This sequential segmentation relies on a molecular clock that operates in the posterior region of the elongating embryo that combines dynamically with cellular behaviors and tissue rearrangements, allowing the extension of the developing body along its main embryonic axis. Even though the molecular mechanisms involved in elongation and segment formation have been found to be conserved in a considerable degree, cellular processes such as cell division are quite variable between different arthropods. In this study, we show that cell proliferation in the beetle Tribolium castaneum has a nonuniform spatiotemporal patterning during axial elongation. We found that dividing cells are preferentially oriented along the anterior-posterior axis, more abundant and posteriorly localized during thoracic segments formation and that this cell proliferation peak was triggered at the onset of axis elongation. This raise in cell divisions, in turn, was correlated with an increase in the elongation rate, but not with changes in cell density. When DNA synthesis was inhibited over this period, both the area and length of thoracic segments were significantly reduced but not of the first abdominal segment. We discuss the variable participation that different cell division patterns and cell movements may have on arthropod posterior growth and their evolutionary contribution.

Differential cell adhesion implemented by Drosophila Toll corrects local distortions of the anterior-posterior compartment boundary

Maintaining lineage restriction boundaries in proliferating tissues is vital to animal development. A long-standing thermodynamics theory, the differential adhesion hypothesis, attributes cell sorting phenomena to differentially expressed adhesion molecules. However, the contribution of the differential adhesion system during tissue morphogenesis has been unsubstantiated despite substantial theoretical support. Here, we report that Toll-1, a transmembrane receptor protein, acts as a differentially expressed adhesion molecule that straightens the fluctuating anteroposterior compartment boundary in the abdominal epidermal epithelium of the Drosophila pupa. Toll-1 is expressed across the entire posterior compartment under the control of the selector gene engrailed and displays a sharp expression boundary that coincides with the compartment boundary. Toll-1 corrects local distortions of the boundary in the absence of cable-like Myosin II enrichment along the boundary. The reinforced adhesion of homotypic cell contacts, together with pulsed cell contraction, achieves a biased vertex sliding action by resisting the separation of homotypic cell contacts in boundary cells. This work reveals a self-organizing system that integrates a differential adhesion system with pulsed contraction of cells to maintain lineage restriction boundaries.

Evolutionary History of the Toll-Like Receptor Gene Family across Vertebrates

Adaptation to a wide range of pathogenic environments is a major aspect of the ecological adaptations of vertebrates during evolution. Toll-like receptors (TLRs) are ancient membrane-bound sensors in animals and are best known for their roles in detecting and defense against invading pathogenic microorganisms. To understand the evolutionary history of the vertebrate TLR gene family, we first traced the origin of single-cysteine cluster TLRs that share the same protein architecture with vertebrate TLRs in early-branching animals and then analyzed all members of the TLR family in over 200 species covering all major vertebrate clades. Our results indicate that although the emergence of single-cysteine cluster TLRs predates the separation of bilaterians and cnidarians, most vertebrate TLR members originated shortly after vertebrate emergence. Phylogenetic analyses divided 1,726 vertebrate TLRs into 8 subfamilies, and TLR3 may represent the most ancient subfamily that emerged before the branching of deuterostomes. Our analysis reveals that purifying selection predominated in the evolution of all vertebrate TLRs, with mean dN/dS (ω) values ranging from 0.082 for TLR21 in birds to 0.434 for TLR11 in mammals. However, we did observe patterns of positive selection acting on specific codons (527 of 60,294 codons across all vertebrate TLRs, 8.7‰), which are significantly concentrated in ligand-binding extracellular domains and suggest host–pathogen coevolutionary interactions. Additionally, we found stronger positive selection acting on nonviral compared with viral TLRs, indicating the more essential nonredundant function of viral TLRs in host immunity. Taken together, our findings provide comprehensive insight into the complex evolutionary processes of the vertebrate TLR gene family, involving gene duplication, pseudogenization, purification, and positive selection.

Elongation during segmentation shows axial variability, low mitotic rates, and synchronized cell cycle domains in the crustacean, Thamnocephalus platyurus

Background

Segmentation in arthropods typically occurs by sequential addition of segments from a posterior growth zone. However, the amount of tissue required for growth and the cell behaviors producing posterior elongation are sparsely documented.

Results

Using precisely staged larvae of the crustacean, Thamnocephalus platyurus, we systematically examine cell division patterns and morphometric changes associated with posterior elongation during segmentation. We show that cell division occurs during normal elongation but that cells in the growth zone need only divide ~ 1.5 times to meet growth estimates; correspondingly, direct measures of cell division in the growth zone are low. Morphometric measurements of the growth zone and of newly formed segments suggest tagma-specific features of segment generation. Using methods for detecting two different phases in the cell cycle, we show distinct domains of synchronized cells in the posterior trunk. Borders of cell cycle domains correlate with domains of segmental gene expression, suggesting an intimate link between segment generation and cell cycle regulation.

Conclusions

Emerging measures of cellular dynamics underlying posterior elongation already show a number of intriguing characteristics that may be widespread among sequentially segmenting arthropods and are likely a source of evolutionary variability. These characteristics include: the low rates of posterior mitosis, the apparently tight regulation of cell cycle at the growth zone/new segment border, and a correlation between changes in elongation and tagma boundaries.

Cellular, molecular, and biophysical control of epithelial cell intercalation

Convergent extension is a conserved mechanism for elongating tissues. In the Drosophila embryo, convergent extension is driven by planar polarized cell intercalation and is a paradigm for understanding the cellular, molecular, and biophysical mechanisms that establish tissue structure. Studies of convergent extension in Drosophila have provided key insights into the force-generating molecules that promote convergent extension in epithelial tissues, as well as the global systems of spatial information that systematically organize these cell behaviors. A general framework has emerged in which asymmetrically localized proteins involved in cytoskeletal tension and cell adhesion direct oriented cell movements, and spatial signals provided by the Toll, Tartan, and Teneurin receptor families break planar symmetry to establish and coordinate planar cell polarity throughout the tissue. In this chapter, we describe the cellular, molecular, and biophysical mechanisms that regulate cell intercalation in the Drosophila embryo, and discuss how research in this system has revealed conserved biological principles that control the organization of multicellular tissues and animal body plans.

Functional analysis of engrailed in Tribolium segmentation

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

Fog signaling has diverse roles in epithelial morphogenesis in insects

The Drosophila Fog pathway represents one of the best-understood signaling cascades controlling epithelial morphogenesis. During gastrulation, Fog induces apical cell constrictions that drive the invagination of mesoderm and posterior gut primordia. The cellular mechanisms underlying primordia internalization vary greatly among insects and recent work has suggested that Fog signaling is specific to the fast mode of gastrulation found in some flies. On the contrary, here we show in the beetle Tribolium, whose development is broadly representative for insects, that Fog has multiple morphogenetic functions. It modulates mesoderm internalization and controls a massive posterior infolding involved in gut and extraembryonic development. In addition, Fog signaling affects blastoderm cellularization, primordial germ cell positioning, and cuboidal-to-squamous cell shape transitions in the extraembryonic serosa. Comparative analyses with two other distantly related insect species reveals that Fog's role during cellularization is widely conserved and therefore might represent the ancestral function of the pathway.

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.

An LRR Receptor-Teneurin System Directs Planar Polarity at Compartment Boundaries

Epithelial cells dynamically self-organize in response to extracellular spatial cues relayed by cell-surface receptors. During convergent extension in Drosophila, Toll-related receptors direct planar polarized cell rearrangements that elongate the head-to-tail axis. However, many cells establish polarity in the absence of Toll receptor activity, indicating the presence of additional spatial cues. Here we demonstrate that the leucine-rich-repeat receptor Tartan and the teneurin Ten-m provide critical polarity signals at epithelial compartment boundaries. The Tartan and Ten-m extracellular domains interact in vitro, and Tartan promotes Ten-m localization to compartment boundaries in vivo. We show that Tartan and Ten-m are necessary for the planar polarity and organization of compartment boundary cells. Moreover, ectopic stripes of Tartan and Ten-m are sufficient to induce myosin accumulation at stripe boundaries. These results demonstrate that the Tartan/Ten-m and Toll receptor systems together create a high-resolution network of spatial cues that guides cell behavior during convergent extension.

Dynamic expression of Drosophila segmental cell surface-encoding genes and their pair-rule regulators

Drosophila segmentation is regulated by a complex network of transcription factors that include products of the pair-rule genes (PRGs). PRGs are expressed in early embryos in the primorida of alternate segmental units, establishing the repeated, segmental body plan of the fly. Despite detailed analysis of the regulatory logic among segmentation genes, the relationship between these genes and the morphological formation of segments is still poorly understood, since regulation of transcription factor expression is not sufficient to explain how segments actually form and are maintained. Cell surface proteins containing Leucine rich repeats (LRR) play a variety of roles in development, and those expressed in segmental patterns likely impact segment morphogenesis. Here we explore the relationships between the PRG network and segmentally expressed LRR-encoding (sLRR) genes. We examined expression of Toll2, Toll6, Toll7, Toll8 and tartan (trn) in wild type or PRG mutant embryos. Expression of each sLRR-encoding gene is dynamic, but each has a unique register along the anterior-posterior axis. The registers for different sLRRs are off-set from one another resulting in a continually changing set of overlapping expression patterns among the sLRR-encoding genes themselves and between the sLRR-encoding genes and the PRGs. Accordingly, each sLRR-encoding gene is regulated by a unique combination of PRGs. These findings suggest that one role of the PRG network is to promote segmentation by establishing a cell surface code: each row of cells in the two-segment-wide primordia expresses a unique combination of sLRRs, thereby translating regulatory information from the PRGs to direct segment morphogenesis.

Mechanical Force-Driven Adherens Junction Remodeling and Epithelial Dynamics

During epithelial tissue development, repair, and homeostasis, adherens junctions (AJs) ensure intercellular adhesion and tissue integrity while allowing for cell and tissue dynamics. Mechanical forces play critical roles in AJs' composition and dynamics. Recent findings highlight that beyond a well-established role in reinforcing cell-cell adhesion, AJ mechanosensitivity promotes junctional remodeling and polarization, thereby regulating critical processes such as cell intercalation, division, and collective migration. Here, we provide an integrated view of mechanosensing mechanisms that regulate cell-cell contact composition, geometry, and integrity under tension and highlight pivotal roles for mechanosensitive AJ remodeling in preserving epithelial integrity and sustaining tissue dynamics.

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.

Evolution of the bilaterian mouth and anus

It is widely held that the bilaterian tubular gut with mouth and anus evolved from a simple gut with one major gastric opening. However, there is no consensus on how this happened. Did the single gastric opening evolve into a mouth, with the anus forming elsewhere in the body (protostomy), or did it evolve into an anus, with the mouth forming elsewhere (deuterostomy), or did it evolve into both mouth and anus (amphistomy)? These questions are addressed by the comparison of developmental fates of the blastopore, the opening of the embryonic gut, in diverse animals that live today. Here we review comparative data on the identity and fate of blastoporal tissue, investigate how the formation of the through-gut relates to the major body axes, and discuss to what extent evolutionary scenarios are consistent with these data. Available evidence indicates that stem bilaterians had a slit-like gastric opening that was partially closed in subsequent evolution, leaving open the anus and most likely also the mouth, which would favour amphistomy. We discuss remaining difficulties, and outline directions for future research.

A revised understanding of Tribolium morphogenesis further reconciles short and long germ development

In Drosophila melanogaster, the germband forms directly on the egg surface and solely consists of embryonic tissue. In contrast, most insect embryos undergo a complicated set of tissue rearrangements to generate a condensed, multilayered germband. The ventral side of the germband is embryonic, while the dorsal side is thought to be an extraembryonic tissue called the amnion. While this tissue organisation has been accepted for decades and has been widely reported in insects, its accuracy has not been directly tested in any species. Using live cell tracking and differential cell labelling in the short germ beetle Tribolium castaneum, I show that most of the cells previously thought to be amnion actually give rise to large parts of the embryo. This process occurs via the dorsal-to-ventral flow of cells and contributes to germband extension (GBE). In addition, I show that true 'amnion' cells in Tribolium originate from a small region of the blastoderm. Together, my findings show that development in the short germ embryos of Tribolium and the long germ embryos of Drosophila is more similar than previously proposed. Dorsal-to-ventral cell flow also occurs in Drosophila during GBE, and I argue that the flow is driven by a conserved set of underlying morphogenetic events in both species. Furthermore, the revised Tribolium fate map that I present is far more similar to that of Drosophila than the classic Tribolium fate map. Lastly, my findings show that there is no qualitative difference between the tissue structure of the cellularised blastoderm and the short/intermediate germ germband. As such, the same tissue patterning mechanisms could function continuously throughout the cellularised blastoderm and germband stages, and easily shift between them over evolutionary time.

RNA interference-mediated silencing of genes involved in the immune responses of the soybean pod borer Leguminivora glycinivorella (Lepidoptera: Olethreutidae)

RNA interference (RNAi) technology may be useful for developing new crop protection strategies against the soybean pod borer (SPB; Leguminivora glycinivorella), which is a critical soybean pest in northeastern Asia. Immune-related genes have been recently identified as potential RNAi targets for controlling insects. However, little is known about these genes or mechanisms underlying their expression in the SPB. In this study, we completed a transcriptome-wide analysis of SPB immune-related genes. We identified 41 genes associated with SPB microbial recognition proteins, immune-related effectors or signalling molecules in immune response pathways (e.g., Toll and immune deficiency pathways). Eleven of these genes were selected for a double-stranded RNA artificial feeding assay. The down-regulated expression levels of LgToll-5-1a and LgPGRP-LB2a resulted in relatively high larval mortality rates and abnormal development. Our data represent a comprehensive genetic resource for immune-related SPB genes, and may contribute to the elucidation of the mechanism regulating innate immunity in Lepidoptera species. Furthermore, two immune-related SPB genes were identified as potential RNAi targets, which may be used in the development of RNAi-mediated SPB control methods.

Embryonic expression of a Long Toll (Loto) gene in the onychophorans Euperipatoides kanangrensis and Cephalofovea clandestina

Recent research has shown that Toll genes, and in particular a newly defined class of Toll genes, the so-called Long Toll Genes (Loto genes), are crucial factors in embryogenesis. In arthropods, they are involved in axis formation via a process called convergent extension (CE). A hallmark of Loto genes is their relatively (compared to other Toll genes) high number of leucine-rich repeat elements (LRRs) coupled with the fact that they are expressed in transverse stripes in all segments, or a subset of segments, patterns that are reminiscent of classical segmentation genes such as the pair-rule genes. Onychophorans represent a close outgroup to the arthropods; however, their embryonic development differs substantially. It is unclear if convergent extension contributes to onychophoran germ band formation and, if so, whether Loto genes are involved in governing this process. This study identifies a single onychophoran Toll gene from a sequenced embryonic transcriptome in two onychophoran species. The identified gene shows sequence and expression pattern characteristics of Loto genes. However, its expression pattern also comprises some general differences to arthropod Loto genes that are involved in CE.

Evidence for the temporal regulation of insect segmentation by a conserved sequence of transcription factors

Long-germ insects, such as the fruit fly Drosophila melanogaster, pattern their segments simultaneously, whereas short-germ insects, such as the beetle Tribolium castaneum, pattern their segments sequentially, from anterior to posterior. While the two modes of segmentation at first appear quite distinct, much of this difference might simply reflect developmental heterochrony. We now show here that, in both Drosophila and Tribolium, segment patterning occurs within a common framework of sequential Caudal, Dichaete, and Odd-paired expression. In Drosophila these transcription factors are expressed like simple timers within the blastoderm, while in Tribolium they form wavefronts that sweep from anterior to posterior across the germband. In Drosophila, all three are known to regulate pair-rule gene expression and influence the temporal progression of segmentation. We propose that these regulatory roles are conserved in short-germ embryos, and that therefore the changing expression profiles of these genes across insects provide a mechanistic explanation for observed differences in the timing of segmentation. In support of this hypothesis we demonstrate that Odd-paired is essential for segmentation in Tribolium, contrary to previous reports.

Evo-Devo: Universal Toll Pass for the Extension Highway?

The distinction between long-germ and short-germ insects is a classic one in evo-devo, yet a common genetic mechanism may underlie germband extension in all insects, even all arthropods.

Sea anemone model has a single Toll-like receptor that can function in pathogen detection, NF-κB signal transduction, and development

In organisms from insects to vertebrates, Toll-like receptors (TLRs) are primary pathogen detectors that activate downstream pathways, specifically those that direct expression of innate immune effector genes. TLRs also have roles in development in many species. The sea anemone Nematostella vectensis is a useful cnidarian model to study the origins of TLR signaling because its genome encodes a single TLR and homologs of many downstream signaling components, including the NF-κB pathway. We have characterized the single N. vectensis TLR (Nv-TLR) and demonstrated that it can activate canonical NF-κB signaling in human cells. Furthermore, we show that the intracellular Toll/IL-1 receptor (TIR) domain of Nv-TLR can interact with the human TLR adapter proteins MAL and MYD88. We demonstrate that the coral pathogen Vibrio coralliilyticus causes a rapidly lethal disease in N. vectensis and that heat-inactivated V. coralliilyticus and bacterial flagellin can activate a reconstituted Nv-TLR-to-NF-κB pathway in human cells. By immunostaining of anemones, we show that Nv-TLR is expressed in a subset of cnidocytes and that many of these Nv-TLR-expressing cells also express Nv-NF-κB. Additionally, the nematosome, which is a Nematostella-specific multicellular structure, expresses Nv-TLR and many innate immune pathway homologs and can engulf V. coralliilyticus Morpholino knockdown indicates that Nv-TLR also has an essential role during early embryonic development. Our characterization of this primitive TLR and identification of a bacterial pathogen for N. vectensis reveal ancient TLR functions and provide a model for studying the molecular basis of cnidarian disease and immunity.

Conservation and variation in pair-rule gene expression and function in the intermediate-germ beetle Dermestes maculatus

A set of pair-rule (PR) segmentation genes (PRGs) promotes the formation of alternate body segments in Drosophila melanogaster Whereas Drosophila embryos are long-germ, with segments specified more or less simultaneously, most insects add segments sequentially as the germband elongates. The hide beetle Dermestes maculatus represents an intermediate between short- and long-germ development, ideal for comparative study of PRGs. We show that eight of nine Drosophila PRG orthologs are expressed in stripes in Dermestes Functional results parse these genes into three groups: Dmac-eve, -odd and -run play roles in both germband elongation and PR patterning; Dmac-slp and -prd function exclusively as complementary, classic PRGs, supporting functional decoupling of elongation and segment formation; and orthologs of ftz, ftz-f1, h and opa show more variable function in Dermestes and other species. While extensive cell death generally prefigured Dermestes PRG RNAi-mediated cuticle defects, an organized region with high mitotic activity near the margin of the segment addition zone is likely to have contributed to truncation of eve^RNAi embryos. Our results suggest general conservation of clock-like regulation of PR stripe addition in sequentially segmenting species while highlighting regulatory rewiring involving a subset of PRG orthologs.

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

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

Dynamic patterning by the Drosophila pair-rule network reconciles long-germ and short-germ segmentation

Drosophila segmentation is a well-established paradigm for developmental pattern formation. However, the later stages of segment patterning, regulated by the "pair-rule" genes, are still not well understood at the system level. Building on established genetic interactions, I construct a logical model of the Drosophila pair-rule system that takes into account the demonstrated stage-specific architecture of the pair-rule gene network. Simulation of this model can accurately recapitulate the observed spatiotemporal expression of the pair-rule genes, but only when the system is provided with dynamic "gap" inputs. This result suggests that dynamic shifts of pair-rule stripes are essential for segment patterning in the trunk and provides a functional role for observed posterior-to-anterior gap domain shifts that occur during cellularisation. The model also suggests revised patterning mechanisms for the parasegment boundaries and explains the aetiology of the even-skipped null mutant phenotype. Strikingly, a slightly modified version of the model is able to pattern segments in either simultaneous or sequential modes, depending only on initial conditions. This suggests that fundamentally similar mechanisms may underlie segmentation in short-germ and long-germ arthropods.