Tribolium Wnts: evidence for a larger repertoire in insects with overlapping expression patterns that suggest multiple redundant functions in embryogenesis

Temporal control of RNAi reveals both robust and labile feedback loops in the segmentation clock of the red flour beetle

Animals from all major clades have evolved a segmented trunk, reflected in the human spine or the insect segments. These units emerge during embryogenesis from a posterior segment addition zone (SAZ), where repetitive gene activity is regulated by a mechanism described by the clock and wavefront/speed gradient model. In the red flour beetle Tribolium castaneum, RNA interference (RNAi) has been used to continuously knock down the function of primary pair-rule genes (pPRGs), caudal or Wnt pathway components, which has led to the complete breakdown of segmentation. However, it has remained untested, if this breakdown was reversible by bringing the missing gene function back to the system. To fill this gap, we established a transgenic system in T. castaneum, which allows blocking an ongoing RNAi effect with temporal control by expressing a viral inhibitor of RNAi via heat shock. We show that the T. castaneum segmentation machinery was able to reestablish after RNAi targeting the pPRGs Tc-eve, Tc-odd, and Tc-runt was blocked. However, we observed no rescue after blocking RNAi targeting Wnt pathway components. We conclude that the insect segmentation system contains both robust feedback loops that can reestablish and labile feedback loops that break down irreversibly. This combination may reconcile conflicting needs of the system: Labile systems controlling initiation and maintenance of the SAZ ensure that only one SAZ is formed. Robust feedback loops confer developmental robustness toward external disturbances.

WntA and Wnt4 during the regeneration of internal organs in the holothurian Eupentacta fraudatrix

BACKGROUND

Over the past few years, it has been established that wnt genes are involved in the regenerative processes of holothurians. The wnt4 gene was identified as one of the most active genes in Eupentacta fraudatrix regeneration using differential gene expression analysis and qPCR of individual genes. Also, the wntA gene was found in holothurians, which is present only in invertebrates and can perform unique functions.

RESULTS

In this regard, both these genes and proteins were studied in this work. During regeneration, the Wnt4 protein is found in the cells of the coelomic and ambulacral epithelium, retractor muscles, and radial nerves. Single cells with this protein are also found in the connective tissue of the developing aquapharyngeal bulb and in the hypoderm of the body wall. Cells with WntA are found exclusively in the hypoderm of the body wall.

CONCLUSION

We assume that both genes are involved in regeneration, but Wnt4 coordinates the formation of the epithelial tissue structure, while WntA maintains the state of the intercellular substance of the body wall.

New insights into mesoderm and endoderm development, and the nature of the onychophoran blastopore

BACKGROUND

Early during onychophoran development and prior to the formation of the germ band, a posterior tissue thickening forms the posterior pit. Anterior to this thickening forms a groove, the embryonic slit, that marks the anterior-posterior orientation of the developing embryo. This slit is by some authors considered the blastopore, and thus the origin of the endoderm, while others argue that the posterior pit represents the blastopore. This controversy is of evolutionary significance because if the slit represents the blastopore, then this would support the amphistomy hypothesis that suggests that a slit-like blastopore in the bilaterian ancestor evolved into protostomy and deuterostomy.

RESULTS

In this paper, we summarize our current knowledge about endoderm and mesoderm development in onychophorans and provide additional data on early endoderm- and mesoderm-determining marker genes such as Blimp, Mox, and the T-box genes.

CONCLUSION

We come to the conclusion that the endoderm of onychophorans forms prior to the development of the embryonic slit, and thus that the slit is not the primary origin of the endoderm. It is thus unlikely that the embryonic slit represents the blastopore. We suggest instead that the posterior pit indeed represents the lips of the blastopore, and that the embryonic slit (and surrounding tissue) represents a morphologically superficial archenteron-like structure. We conclude further that both endoderm and mesoderm development are under control of conserved gene regulatory networks, and that many of the features found in arthropods including the model Drosophila melanogaster are likely derived.

Prediction and analysis of cis-regulatory elements in Dorsal and Ventral patterning genes of Tribolium castaneum and its comparison with Drosophila melanogaster

Insect embryonic development and morphology are characterized by their anterior-posterior and dorsal-ventral (DV) patterning. In Drosophila embryos, DV patterning is mediated by a dorsal protein gradient which activates twist and snail proteins, the important regulators of DV patterning. To activate or repress gene expression, some regulatory proteins bind in clusters to their target gene at sites known as cis-regulatory elements or enhancers. To understand how variations in gene expression in different lineages might lead to different phenotypes, it is necessary to understand enhancers and their evolution. Drosophila melanogaster has been widely studied to understand the interactions between transcription factors and the transcription factor binding sites. Tribolium castaneum is an upcoming model animal which is catching the interest of biologists and the research on the enhancer mechanisms in the insect's axes patterning is still in infancy. Therefore, the current study was designed to compare the enhancers of DV patterning in the two insect species. The sequences of ten proteins involved in DV patterning of D. melanogaster were obtained from Flybase. The protein sequences of T. castaneum orthologous to those obtained from D. melanogaster were acquired from NCBI BLAST, and these were then converted to DNA sequences which were modified by adding 20 kb sequences both upstream and downstream to the gene. These modified sequences were used for further analysis. Bioinformatics tools (Cluster-Buster and MCAST) were used to search for clusters of binding sites (enhancers) in the modified DV genes. The results obtained showed that the transcription factors in Drosophila melanogaster and Tribolium castaneum are nearly identical; however, the number of binding sites varies between the two species, indicating transcription factor binding site evolution, as predicted by two different computational tools. It was observed that dorsal, twist, snail, zelda, and Supressor of Hairless are the transcription factors responsible for the regulation of DV patterning in the two insect species.

Genome-wide identification and expression profiling of Wnt gene family in Neocaridina denticulata sinensis

Wnt proteins are a class of hydrophobic secreted glycoproteins involved in diverse important biological processes, such as tissue formation and regeneration, embryonic development and innate immunity. The Wnt gene family has an early origin and is present in all deuterostomes. In the process of evolution, the phenomenon of gene expansion, contraction and adaptive evolution occurs in the Wnt gene family. In the current study, eleven Wnt genes (NdWnt1-2, NdWnt4-7, NdWnt9-11, NdWnt16, and NdWntA) belonging to different subfamilies were obtained based on the genomic and transcriptomic data of Neocaridina denticulata sinensis. Then the expression patterns of all NdWnts were analyzed in various tissues, at different developmental stages and under different stresses. The expression profiles of NdWnts at different developmental stages showed that most NdWnt genes were initially expressed at gastrula stage, and the expression of NdWnt5 and NdWnt16 throughout all developmental stages. The spatial expression of NdWnt genes presented tissue specificity. They were mainly expressed in four tissues, namely gill, intestines, ovary and eyestalk. After Vibrio parahemolyticus infection and under copper exposure, the expression levels of five NdWnts (NdWnt1, NdWnt5, NdWnt10, NdWnt16 and NdWntA) were variable. Our findings enrich the research on the Wnt gene family of N. denticulata sinensis and provide valuable insights into relationship between structure and function of Wnt genes in crustaceans.

The Wnt gene family in Tenebrio molitor and other coleopterans

The Wnt gene family is involved in a wide range of developmental processes. Despite its significance, the evolution and function of Wnt genes remain largely unclear. Here, an exhaustive survey of Wnt genes was conducted in Tenebrio molitor and 17 other beetle genomes. A total of 146 Wnt genes were identified, creating a comprehensive coleopteran Wnt gene catalog. Comparative genomics indicates that dynamic evolutionary patterns of Wnt gene loss and duplication occurred in Coleoptera, leading to the diverse Wnt gene repertoire in various beetles. A striking loss of particular Wnt gene subfamilies occurs in Coleoptera. Remarkably, Wnt gene duplication was discovered for the first time in insects. Further analysis of Wnt gene expression in T. molitor indicates that each Wnt gene, including the duplicated ones, has a unique spatial or temporal expression pattern. The current study provides valuable insight into the evolution and functional validation of Wnt genes in Coleoptera.

Genome-Wide Identification and Expression Profiling of the Wnt Gene Family in Three Rice Planthoppers: Sogatella furcifera, Laodelphax striatellus, and Nilaparvata lugens

The Wnt gene family plays essential roles in regulating many developmental processes, including the maintenance of stem cells, cell division, and cell migration. The number of Wnt genes varies among species. Due to the diversity and importance of their functions, the Wnt gene family has gained extensive research interest in various animal species from invertebrates to vertebrates. However, knowledge of the Wnt gene family is limited in rice planthoppers. Three planthopper species, the white-backed planthopper (Sogatella furcifera Horvath), the small brown planthopper (Laodelphax striatellus Fallén) and the brown planthopper (Nilaparvata lugens Stål) (Hemiptera: Delphacidae), are devastating specialist pests of rice and cause serious damage to rice plants. To better study the evolution and function of the Wnt gene family in rice planthoppers, we identified 8 Wnt family genes in three rice planthoppers with both genomic and extensive transcriptomic resources available. We conducted a systematic analysis of the three kinds of rice planthoppers and analyzed the dynamic patterns of gene conservation, as well as Wnt gene loss and duplication. The expression profiles in different developmental stages of S. furcifera and different adult organs and tissues of L. striatellus provide preliminary functional implications for the Wnt genes in rice planthopper. This study presents the first genome-wide study of the Wnt gene family in rice planthoppers, and our findings provide insights into Wnt function and evolution in rice planthoppers.

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.

Embryonic expression patterns of Wnt genes in the RTA-clade spider Cupiennius salei

Spiders represent widely used model organisms for chelicerate and even arthropod development and evolution. Wnt genes are important and evolutionary conserved factors that control and regulate numerous developmental processes. Recent studies comprehensively investigated the complement and expression of spider Wnt genes revealing conserved as well as diverged aspects of their expression and thus (likely) function among different groups of spiders representing Mygalomorphae (tarantulas), and both main groups of Araneae (true spiders) (Haplogynae/Synspermiata and Entelegynae). The allegedly most modern/derived group of entelegyne spiders is represented by the RTA-clade of which no comprehensive data on Wnt expression were available prior to this study. Here, we investigated the embryonic expression of all Wnt genes of the RTA-clade spider Cupiennius salei. We found that most of the Wnt expression patterns are conserved between Cupiennius and other spiders, especially more basally branching species. Surprisingly, most differences in Wnt gene expression are seen in the common model spider Parasteatoda tepidariorum (a non-RTA clade entelegyne species). These results show that data and conclusions drawn from research on one member of a group of animals (or any other organism) cannot necessarily be extrapolated to the group as a whole, and instead highlight the need for comprehensive taxon sampling.

Genome-wide identification and expression profiling analysis of Wnt family genes affecting adipocyte differentiation in cattle

The Wnt family features conserved glycoproteins that play roles in tissue regeneration, animal development and cell proliferation and differentiation. For its functional diversity and importance, this family has been studied in several species, but not in the Bovinae. Herein we identified 19 Wnt genes in cattle, and seven other species of Bovinae, and described their corresponding protein properties. Phylogenetic analysis clustered the 149 Wnt proteins in Bovinae, and 38 Wnt proteins from the human and mouse into 12 major clades. Wnt genes from the same subfamilies shared similar protein motif compositions and exon–intron patterns. Chromosomal distribution and collinearity analysis revealed that they were conservative in cattle and five species of Bovinae. RNA-seq data analysis indicated that Wnt genes exhibited tissue-specific expression in cattle. qPCR analysis revealed a unique expression pattern of each gene during bovine adipocytes differentiation. Finally, the comprehensive analysis indicated that Wnt2B may regulate adipose differentiation by activating FZD5, which is worthy of further study. Our study presents the first genome-wide study of the Wnt gene family in Bovinae, and lays the foundation for further functional characterization of this family in bovine adipocytes differentiation.

Extensive loss of Wnt genes in Tardigrada

Background

Wnt genes code for ligands that activate signaling pathways during development in Metazoa. Through the canonical Wnt (cWnt) signaling pathway, these genes regulate important processes in bilaterian development, such as establishing the anteroposterior axis and posterior growth. In Arthropoda, Wnt ligands also regulate segment polarity, and outgrowth and patterning of developing appendages. Arthropods are part of a lineage called Panarthropoda that includes Onychophora and Tardigrada. Previous studies revealed potential roles of Wnt genes in regulating posterior growth, segment polarity, and growth and patterning of legs in Onychophora. Unlike most other panarthropods, tardigrades lack posterior growth, but retain segmentation and appendages. Here, we investigated Wnt genes in tardigrades to gain insight into potential roles that these genes play during development of the highly compact and miniaturized tardigrade body plan.

Results

We analyzed published genomes for two representatives of Tardigrada, Hypsibius exemplaris and Ramazzottius varieornatus. We identified single orthologs of Wnt4, Wnt5, Wnt9, Wnt11, and WntA, as well as two Wnt16 paralogs in both tardigrade genomes. We only found a Wnt2 ortholog in H. exemplaris. We could not identify orthologs of Wnt1, Wnt6, Wnt7, Wnt8, or Wnt10. We identified most other components of cWnt signaling in both tardigrade genomes. However, we were unable to identify an ortholog of arrow/Lrp5/6, a gene that codes for a Frizzled co-receptor of Wnt ligands. Additionally, we found that some other animals that have lost several Wnt genes and are secondarily miniaturized, like tardigrades, are also missing an ortholog of arrow/Lrp5/6. We analyzed the embryonic expression patterns of Wnt genes in H. exemplaris during developmental stages that span the establishment of the AP axis through segmentation and leg development. We detected expression of all Wnt genes in H. exemplaris besides one of the Wnt16 paralogs. During embryo elongation, expression of several Wnt genes was restricted to the posterior pole or a region between the anterior and posterior poles. Wnt genes were expressed in distinct patterns during segmentation and development of legs in H. exemplaris, rather than in broadly overlapping patterns.

Conclusions

Our results indicate that Wnt signaling has been highly modified in Tardigrada. While most components of cWnt signaling are conserved in tardigrades, we conclude that tardigrades have lost Wnt1, Wnt6, Wnt7, Wnt8, and Wnt10, along with arrow/Lrp5/6. Our expression data may indicate a conserved role of Wnt genes in specifying posterior identities during establishment of the AP axis. However, the loss of several Wnt genes and the distinct expression patterns of Wnt genes during segmentation and leg development may indicate that combinatorial interactions among Wnt genes are less important during tardigrade development compared to many other animals. Based on our results, and comparisons to previous studies, we speculate that the loss of several Wnt genes in Tardigrada may be related to a reduced number of cells and simplified development that accompanied miniaturization and anatomical simplification in this lineage.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01954-y.

Widespread retention of ohnologs in key developmental gene families following whole-genome duplication in arachnopulmonates

Whole-genome duplications (WGDs) have occurred multiple times during animal evolution, including in lineages leading to vertebrates, teleosts, horseshoe crabs, and arachnopulmonates. These dramatic events initially produce a wealth of new genetic material, generally followed by extensive gene loss. It appears, however, that developmental genes such as homeobox genes, signaling pathway components and microRNAs are frequently retained as duplicates (so-called ohnologs) following WGD. These not only provide the best evidence for WGD, but an opportunity to study its evolutionary consequences. Although these genes are well studied in the context of vertebrate WGD, similar comparisons across the extant arachnopulmonate orders are patchy. We sequenced embryonic transcriptomes from two spider species and two amblypygid species and surveyed three important gene families, Hox, Wnt, and frizzled, across these and 12 existing transcriptomic and genomic resources for chelicerates. We report extensive retention of putative ohnologs, further supporting the ancestral arachnopulmonate WGD. We also found evidence of consistent evolutionary trajectories in Hox and Wnt gene repertoires across three of the six arachnopulmonate orders, with interorder variation in the retention of specific paralogs. We identified variation between major clades in spiders and are better able to reconstruct the chronology of gene duplications and losses in spiders, amblypygids, and scorpions. These insights shed light on the evolution of the developmental toolkit in arachnopulmonates, highlight the importance of the comparative approach within lineages, and provide substantial new transcriptomic data for future study.

A chelicerate Wnt gene expression atlas: novel insights into the complexity of arthropod Wnt-patterning

The Wnt genes represent a large family of secreted glycoprotein ligands that date back to early animal evolution. Multiple duplication events generated a set of 13 Wnt families of which 12 are preserved in protostomes. Embryonic Wnt expression patterns (Wnt-patterning) are complex, representing the plentitude of functions these genes play during development. Here, we comprehensively investigated the embryonic expression patterns of Wnt genes from three species of spiders covering both main groups of true spiders, Haplogynae and Entelegynae, a mygalomorph species (tarantula), as well as a distantly related chelicerate outgroup species, the harvestman Phalangium opilio. All spiders possess the same ten classes of Wnt genes, but retained partially different sets of duplicated Wnt genes after whole genome duplication, some of which representing impressive examples of sub- and neo-functionalization. The harvestman, however, possesses a more complete set of 11 Wnt genes but with no duplicates. Our comprehensive data-analysis suggests a high degree of complexity and evolutionary flexibility of Wnt-patterning likely providing a firm network of mutational protection. We discuss the new data on Wnt gene expression in terms of their potential function in segmentation, posterior elongation, and appendage development and critically review previous research on these topics. We conclude that earlier research may have suffered from the absence of comprehensive gene expression data leading to partial misconceptions about the roles of Wnt genes in development and evolution.

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.

Utilizing a chromosomal-length genome assembly to annotate the Wnt signaling pathway in the Asian citrus psyllid, Diaphorina citri

The Asian citrus psyllid, Diaphorina citri, is an insect vector that transmits Candidatus Liberibacter asiaticus, the causal agent of the Huanglongbing (HLB), or citrus greening disease. This disease has devastated Florida's citrus industry, and threatens California's industry as well as other citrus producing regions around the world. To find novel solutions to the disease, a better understanding of the vector is needed. The D. citri genome has been used to identify and characterize genes involved in Wnt signaling pathways. Wnt signaling is utilized for many important biological processes in metazoans, such as patterning and tissue generation. Curation based on RNA sequencing data and sequence homology confirms 24 Wnt signaling genes within the D. citri genome, including homologs for beta-catenin, Frizzled receptors, and seven Wnt-ligands. Through phylogenetic analysis, we classify D. citri Wnt ligands as Wg/Wnt1, Wnt5, Wnt6, Wnt7, Wnt10, Wnt11, and WntA. The D. citri version 3.0 genome with chromosomal length scaffolds reveals a conserved Wnt1-Wnt6-Wnt10 gene cluster with a gene configuration like that in Drosophila melanogaster. These findings provide greater insight into the evolutionary history of D. citri and Wnt signaling in this important hemipteran vector. Manual annotation was essential for identifying high quality gene models. These gene models can be used to develop molecular systems, such as CRISPR and RNAi, which target and control psyllid populations to manage the spread of HLB. Manual annotation of Wnt signaling pathways was done as part of a collaborative community annotation project.

Porcupine/Wntless-dependent trafficking of the conserved WntA ligand in butterflies

Wnt ligands are key signaling molecules in animals, but little is known about the evolutionary dynamics and mode of action of the WntA orthologs, which are not present in the vertebrates or in Drosophila. Here we show that the WntA subfamily evolved at the base of the Bilateria + Cnidaria clade, and conserved the thumb region and Ser209 acylation site present in most other Wnts, suggesting WntA requires the core Wnt secretory pathway. WntA proteins are distinguishable from other Wnts by a synapomorphic Iso/Val/Ala216 amino-acid residue that replaces the otherwise ubiquitous Thr216 position. WntA embryonic expression is conserved between beetles and butterflies, suggesting functionality, but the WntA gene was lost three times within arthropods, in podoplean copepods, in the cyclorrhaphan fly radiation, and in ensiferan crickets and katydids. Finally, CRISPR mosaic knockouts (KOs) of porcupine and wntless phenocopied the pattern-specific effects of WntA KOs in the wings of Vanessa cardui butterflies. These results highlight the molecular conservation of the WntA protein across invertebrates, and imply it functions as a typical Wnt ligand that is acylated and secreted through the Porcupine/Wntless secretory pathway.

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.

Integrating evolutionarily novel horns within the deeply conserved insect head

BACKGROUND

How novel traits integrate within ancient trait complexes without compromising ancestral functions is a foundational challenge in evo-devo. The insect head represents an ancient body region patterned by a deeply conserved developmental genetic network, yet at the same time constitutes a hot spot for morphological innovation. However, the mechanisms that facilitate the repeated emergence, integration, and diversification of morphological novelties within this body region are virtually unknown. Using horned Onthophagus beetles, we investigated the mechanisms that instruct the development of the dorsal adult head and the formation and integration of head horns, one of the most elaborate classes of secondary sexual weapons in the animal kingdom.

RESULTS

Using region-specific RNAseq and gene knockdowns, we (i) show that the head is compartmentalized along multiple axes, (ii) identify striking parallels between morphological and transcriptional complexity across regions, yet (iii) fail to identify a horn-forming gene module. Instead, (iv) our results support that sex-biased regulation of a shared transcriptional repertoire underpins the formation of horned and hornless heads. Furthermore, (v) we show that embryonic head patterning genes frequently maintain expression within the dorsal head well into late post-embryonic development, thereby possibly facilitating the repurposing of such genes within novel developmental contexts. Lastly, (vi) we identify novel functions for several genes including three embryonic head patterning genes in the integration of both posterior and anterior head horns.

CONCLUSIONS

Our results illuminate how the adult insect head is patterned and suggest mechanisms capable of integrating novel traits within ancient trait complexes in a sex- and species-specific manner. More generally, our work underscores how significant morphological innovation in developmental evolution need not require the recruitment of new genes, pathways, or gene networks but instead may be scaffolded by pre-existing developmental machinery.

FoxB, a new and highly conserved key factor in arthropod dorsal-ventral (DV) limb patterning

Forkhead box (Fox) transcription factors evolved early in animal evolution and represent important components of conserved gene regulatory networks (GRNs) during animal development. Most of the researches concerning Fox genes, however, are on vertebrates and only a relatively low number of studies investigate Fox gene function in invertebrates. In addition to this shortcoming, the focus of attention is often restricted to a few well-characterized Fox genes such as FoxA (forkhead), FoxC (crocodile) and FoxQ2. Although arthropods represent the largest and most diverse animal group, most other Fox genes have not been investigated in detail, not even in the arthropod model species Drosophila melanogaster. In a general gene expression pattern screen for panarthropod Fox genes including the red flour beetle Tribolium castaneum, the pill millipede Glomeris marginata, the common house spider Parasteatoda tepidariorum, and the velvet worm Euperipatoides kanangrensis, we identified a Fox gene with a highly conserved expression pattern along the ventral ectoderm of arthropod and onychophoran limbs. Functional investigation of FoxB in Parasteatoda reveals a hitherto unrecognized important function of FoxB upstream of wingless (wg) and decapentaplegic (dpp) in the GRN orchestrating dorsal–ventral limb patterning.

Evolution of limb development in cephalopod mollusks

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

Genome-Wide Identification and Expression Profiling of Wnt Family Genes in the Silkworm, Bombyx mori

Wnt is a family of conserved glycoproteins that participate in a variety of important biological processes including embryo development, cell proliferation and differentiation, and tissue regeneration. The Wnt family is a metazoan novelty found in all animal phyla. Studies have revealed that the number of Wnt genes varies among species, presumably due to reproduction and loss of genes during evolution. However, a comprehensive inventory of Wnt genes in Lepidoptera is lacking. In this study, we identified the repertoire of Wnt genes in the silkworm and seven other species of Lepidoptera and obtained eight Wnt genes (Wnt1, Wnt5–Wnt7, Wnt9–Wnt11, and WntA) in each species. Four of these Wnt genes are clustered in two orientations (5′-Wnt9-Wnt1-Wnt6-Wnt10-3′ and 5′-Wnt10-Wnt6-Wnt1-Wnt9-3′) in both moths and butterflies. Transcript analysis of Wnt in silkworm embryonic stages showed that each BmWnt gene had a unique expression pattern during embryological development. Analysis of a larval stage revealed differential expression of Wnt family members in diverse tissues. Our study provides an overview of the Wnt family in Lepidoptera and will inspire further functional study of the Wnt genes in the silkworm.

Gene expression analysis of potential morphogen signalling modifying factors in Panarthropoda

Background

Morphogen signalling represents a key mechanism of developmental processes during animal development. Previously, several evolutionary conserved morphogen signalling pathways have been identified, and their players such as the morphogen receptors, morphogen modulating factors (MMFs) and the morphogens themselves have been studied. MMFs are factors that regulate morphogen distribution and activity. The interactions of MMFs with different morphogen signalling pathways such as Wnt signalling, Hedgehog (Hh) signalling and Decapentaplegic (Dpp) signalling are complex because some of the MMFs have been shown to interact with more than one signalling pathway, and depending on genetic context, to have different, biphasic or even opposing function. This complicates the interpretation of expression data and functional data of MMFs and may be one reason why data on MMFs in other arthropods than Drosophila are scarce or totally lacking.

Results

As a first step to a better understanding of the potential roles of MMFs in arthropod development, we investigate here the embryonic expression patterns of division abnormally delayed (dally), dally-like protein (dlp), shifted (shf) and secreted frizzled-related protein 125 (sFRP125) and sFRP34 in the beetle Tribolium castaneum, the spider Parasteatoda tepidariorum, the millipede Glomeris marginata and the onychophoran Euperipatoides kanangrensis. This pioneer study represents the first comprehensive comparative data set of these genes in panarthropods.

Conclusions

Expression profiles reveal a high degree of diversity, suggesting that MMFs may represent highly evolvable nodes in otherwise conserved gene regulatory networks. Conserved aspects of MMF expression, however, appear to concern function in segmentation and limb development, two of the key topics of evolutionary developmental research.

Electronic supplementary material

The online version of this article (10.1186/s13227-018-0109-y) contains supplementary material, which is available to authorized users.

Wnt evolution and function shuffling in liberal and conservative chordate genomes

BACKGROUND

What impact gene loss has on the evolution of developmental processes, and how function shuffling has affected retained genes driving essential biological processes, remain open questions in the fields of genome evolution and EvoDevo. To investigate these problems, we have analyzed the evolution of the Wnt ligand repertoire in the chordate phylum as a case study.

RESULTS

We conduct an exhaustive survey of Wnt genes in genomic databases, identifying 156 Wnt genes in 13 non-vertebrate chordates. This represents the most complete Wnt gene catalog of the chordate subphyla and has allowed us to resolve previous ambiguities about the orthology of many Wnt genes, including the identification of WntA for the first time in chordates. Moreover, we create the first complete expression atlas for the Wnt family during amphioxus development, providing a useful resource to investigate the evolution of Wnt expression throughout the radiation of chordates.

CONCLUSIONS

Our data underscore extraordinary genomic stasis in cephalochordates, which contrasts with the liberal and dynamic evolutionary patterns of gene loss and duplication in urochordate genomes. Our analysis has allowed us to infer ancestral Wnt functions shared among all chordates, several cases of function shuffling among Wnt paralogs, as well as unique expression domains for Wnt genes that likely reflect functional innovations in each chordate lineage. Finally, we propose a potential relationship between the evolution of WntA and the evolution of the mouth in chordates.

Wnt gene family members and their expression profiling in Litopenaeus vannamei

The Wnt gene family encodes secreted glycoproteins involved in a wide variety of biological processes, including embryo development, cell proliferation and differentiation, and tissue regeneration. The Wnt pathway exists in all metazoan animals, however, the relevant research is rare in crustaceans. Here we described 12 Wnt genes representing 12 Wnt gene subfamilies in the Pacific white shrimp, Litopenaeus vannamei. Based on homolog annotations and phylogenetic analyses, we named these 12 Wnt genes as LvWnt1, LvWnt2, LvWnt4-11, LvWnt16, and LvWntA. All the corresponding LvWnt proteins shared a conserved Wnt1 domain and 22 conserved cysteine residues. LvWnt1 and LvWnt6 were adjacent in a scaffold in the shrimp genome. Furthermore, we performed expression analyses of LvWnt genes at different developmental stages, during the molting process, in different tissues and after different pathogenic infection. We showed that each LvWnt gene had a unique expression pattern at different developmental stages but only a few of them expressed in adult shrimp. All the investigated LvWnt genes were initially expressed at the gastrula or limb bud embryo stages. Among them, LvWnt8 was specifically high expressed only in early embryos. LvWntA and LvWnt5 displayed high and similar expression profiles during the molting process, and LvWnt6 and LvWnt16 were specifically expressed in the thoracic ganglion, ventral nerve, intestines and gill tissues, respectively. We also found the expression of LvWntA, LvWnt5, LvWnt6, LvWnt9, and LvWnt16 were varied in the different tissues after infected with Staphylococcus aureus, Vibrio parahaemolyticus and white spot syndrome virus (WSSV), which indicated that they might participate in immune response in L. vannamei. This study provided an insight into the repertoire of the Wnt gene structure and expression in shrimps, and furthermore, might promote the understanding of development, growth and immune response of shrimps and crustaceans.

The Roles of the Wnt-Antagonists Axin and Lrp4 during Embryogenesis of the Red Flour Beetle Tribolium castaneum

In both vertebrates and invertebrates, the Wnt-signaling pathway is essential for numerous processes in embryogenesis and during adult life. Wnt activity is fine-tuned at various levels by the interplay of a number of Wnt-agonists (Wnt ligands, Frizzled-receptors, Lrp5/6 coreceptors) and Wnt-antagonists (among them Axin, Secreted frizzled and Lrp4) to define anterior–posterior polarity of the early embryo and specify cell fate in organogenesis. So far, the functional analysis of Wnt-pathway components in insects has concentrated on the roles of Wnt-agonists and on the Wnt-antagonist Axin. We depict here additional features of the Wnt-antagonist Axin in the flour beetle Tribolium castaneum. We show that Tc-axin is dynamically expressed throughout embryogenesis and confirm its essential role in head development. In addition, we describe an as yet undetected, more extreme Tc-axin RNAi-phenotype, the ectopic formation of posterior abdominal segments in reverse polarity and a second hindgut at the anterior. For the first time, we describe here that an lrp4 ortholog is involved in axis formation in an insect. The Tribolium Lrp4 ortholog is ubiquitously expressed throughout embryogenesis. Its downregulation via maternal RNAi results in the reduction of head structures but not in axis polarity reversal. Furthermore, segmentation is impaired and larvae develop with a severe gap-phenotype. We conclude that, as in vertebrates, Tc-lrp4 functions as a Wnt-inhibitor in Tribolium during various stages of embryogenesis. We discuss the role of both components as negative modulators of Wnt signaling in respect to axis formation and segmentation in Tribolium.

Speed regulation of genetic cascades allows for evolvability in the body plan specification of insects

During the anterior-posterior fate specification of insects, anterior fates arise in a nonelongating tissue (called the "blastoderm"), and posterior fates arise in an elongating tissue (called the "germband"). However, insects differ widely in the extent to which anterior-posterior fates are specified in the blastoderm versus the germband. Here we present a model in which patterning in both the blastoderm and germband of the beetle Tribolium castaneum is based on the same flexible mechanism: a gradient that modulates the speed of a genetic cascade of gap genes, resulting in the induction of sequential kinematic waves of gap gene expression. The mechanism is flexible and capable of patterning both elongating and nonelongating tissues, and hence converting blastodermal to germband fates and vice versa. Using RNAi perturbations, we found that blastodermal fates could be shifted to the germband, and germband fates could be generated in a blastoderm-like morphology. We also suggest a molecular mechanism underlying our model, in which gradient levels regulate the switch between two enhancers: One enhancer is responsible for sequential gene activation, and the other is responsible for freezing temporal rhythms into spatial patterns. This model is consistent with findings in Drosophila melanogaster, where gap genes were found to be regulated by two nonredundant "shadow" enhancers.

Cleavage modification did not alter blastomere fates during bryozoan evolution

BACKGROUND

Stereotypic cleavage patterns play a crucial role in cell fate determination by precisely positioning early embryonic blastomeres. Although misplaced cell divisions can alter blastomere fates and cause embryonic defects, cleavage patterns have been modified several times during animal evolution. However, it remains unclear how evolutionary changes in cleavage impact the specification of blastomere fates. Here, we analyze the transition from spiral cleavage - a stereotypic pattern remarkably conserved in many protostomes - to a biradial cleavage pattern, which occurred during the evolution of bryozoans.

RESULTS

Using 3D-live imaging time-lapse microscopy (4D-microscopy), we characterize the cell lineage, MAPK signaling, and the expression of 16 developmental genes in the bryozoan Membranipora membranacea. We found that the molecular identity and the fates of early bryozoan blastomeres are similar to the putative homologous blastomeres in spiral-cleaving embryos.

CONCLUSIONS

Our work suggests that bryozoans have retained traits of spiral development, such as the early embryonic fate map, despite the evolution of a novel cleavage geometry. These findings provide additional support that stereotypic cleavage patterns can be modified during evolution without major changes to the molecular identity and fate of embryonic blastomeres.

Molecular control of gut formation in the spider Parasteatoda tepidariorum

The development of a digestive system is an essential feature of bilaterians. Studies of the molecular control of gut formation in arthropods have been studied in detail in the fruit fly Drosophila melanogaster. However, little is known in other arthropods, especially in noninsect arthropods. To better understand the evolution of arthropod alimentary system, we investigate the molecular control of gut development in the spider Parasteatoda tepidariorum (Pt), the primary chelicerate model species for developmental studies. Orthologs of the ectodermal genes Pt-wingless (Pt-wg) and Pt-hedgehog (Pt-hh), of the endodermal genes, Pt-serpent (Pt-srp) and Pt-hepatocyte-nuclear factor-4 (Pt-hnf4) and of the mesodermal gene Pt-twist (Pt-twi) are expressed in the same germ layers during spider gut development as in D. melanogaster. Thus, our expression data suggest that the downstream molecular components involved in gut development in arthropods are conserved. However, Pt-forkhead (Pt-fkh) expression and function in spiders is considerably different from its D. melanogaster ortholog. Pt-fkh is expressed before gastrulation in a cell population that gives rise to endodermal and mesodermal precursors, suggesting a possible role for this factor in specification of both germ layers. To test this hypothesis, we knocked down Pt-fkh via RNA interference. Pt-fkh RNAi embryos not only fail to develop a proper gut, but also lack the mesodermal Pt-twi expressing cells. Thus, in spiders Pt-fkh specifies endodermal and mesodermal germ layers. We discuss the implications of these findings for the evolution and development of gut formation in Ecdysozoans.

Transcriptome sequencing reveals maelstrom as a novel target gene of the terminal-system in the red flour beetle Tribolium castaneum

Terminal regions of the Drosophila embryo are patterned by the localized activation of the Torso-RTK pathway, which promotes the down-regulation of Capicua. In the short-germ beetle Tribolium, the function of the terminal system appears to be rather different, as the pathway promotes axis elongation and in addition, is required for patterning the extraembryonic serosa at the anterior. Here we show that Torso signalling induces gene expression by relieving CAPICUA-mediated repression also in Tribolium. Given that the majority of Torso target genes remain to be identified, we established a differential gene-expression screen. A subset of 50 putative terminal target genes was screened for functions in early embryonic patterning. Of those, 13 genes show early terminal expression domains and also phenotypes were related to terminal patterning. Among others, we found the PIWI-interacting RNA factor Maelstrom to be crucial for early embryonic polarization. Tc-mael is required for proper serosal size regulation and head morphogenesis. Moreover, Tc-mael promotes growth-zone formation and axis elongation. Our results suggest that posterior patterning by Torso may be realized through Maelstrom depended activation of posterior wnt-domains.

Weevil endosymbiont dynamics is associated with a clamping of immunity

BACKGROUND

Insects subsisting on nutritionally unbalanced diets have evolved long-term mutualistic relationships with intracellular symbiotic bacteria (endosymbionts). The endosymbiont population load undergoes changes along with insect development. In the cereal weevil Sitophilus oryzae, the midgut endosymbionts Sodalis pierantonius drastically multiply following adult metamorphosis and rapidly decline until total elimination when the insect achieves its cuticle synthesis. Whilst symbiont load was shown to timely meet insect metabolic needs, little is known about the host molecular and immune processes underlying this dynamics.

METHODS

We performed RNA sequencing analysis on weevil midguts at three representative phases of the endosymbiont dynamics (i.e. increase, climax and decrease). To screen genes which transcriptional changes are specifically related to symbiont dynamics and not to the intrinsic development of the midgut, we further have monitored by RT-qPCR sixteen gene transcript levels in symbiotic and artificially non-symbiotic (aposymbiotic) weevils. We also localized the endosymbionts during the elimination process by fluorescence microscopy.

RESULTS

Functional analysis of the host differentially expressed genes by RNA sequencing showed that the main transcriptional changes occur during endosymbiont growth phase and affect cell proliferation, apoptosis, autophagy, phagocytosis, and metabolism of fatty acids and nucleic acids. We also showed that symbiont dynamics alters the expression of several genes involved in insect development. Our results strengthened the implication of apoptosis and autophagy processes in symbiont elimination and recycling. Remarkably, apart from the coleoptericin A that is known to target endosymbionts and controls their division and location, no gene coding antimicrobial peptide was upregulated during the symbiont growth and elimination phases.

CONCLUSION

We show that endosymbiont dynamics parallels numerous transcriptional changes in weevil developing adults and affects several biological processes, including metabolism and development. It also triggers cell apoptosis, autophagy and gut epithelial cell swelling and delamination. Strikingly, immunity is repressed during the whole process, presumably avoiding tissue inflammation and allowing insects to optimize nutrient recovery from recycled endosymbiont.

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.

Caudal regulates the spatiotemporal dynamics of pair-rule waves in Tribolium

In the short-germ beetle Tribolium castaneum, waves of pair-rule gene expression propagate from the posterior end of the embryo towards the anterior and eventually freeze into stable stripes, partitioning the anterior-posterior axis into segments. Similar waves in vertebrates are assumed to arise due to the modulation of a molecular clock by a posterior-to-anterior frequency gradient. However, neither a molecular candidate nor a functional role has been identified to date for such a frequency gradient, either in vertebrates or elsewhere. Here we provide evidence that the posterior gradient of Tc-caudal expression regulates the oscillation frequency of pair-rule gene expression in Tribolium. We show this by analyzing the spatiotemporal dynamics of Tc-even-skipped expression in strong and mild knockdown of Tc-caudal, and by correlating the extension, level and slope of the Tc-caudal expression gradient to the spatiotemporal dynamics of Tc-even-skipped expression in wild type as well as in different RNAi knockdowns of Tc-caudal regulators. Further, we show that besides its absolute importance for stripe generation in the static phase of the Tribolium blastoderm, a frequency gradient might serve as a buffer against noise during axis elongation phase in Tribolium as well as vertebrates. Our results highlight the role of frequency gradients in pattern formation.

One of the most popular problems in development is how the anterior-posterior axis of vertebrates, arthropods and annelids is partitioned into segments. In vertebrates, and recently shown in the beetle Tribolium castaneum, segments are demarcated by means of gene expression waves that propagate from posterior to anterior as the embryo elongates. These waves are assumed to arise due to the regulation of a molecular clock by a frequency gradient. However, to date, neither a candidate nor a functional role has been identified for such a frequency gradient. Here we provide evidence that a static expression gradient of caudal regulates pair-rule oscillations during blastoderm stage in Tribolium. In such a static setup, a frequency gradient is essential to convert clock oscillations into a striped pattern. We further show that a frequency gradient might be essential even in the presence of axis elongation as a buffer against noise. Our work also provides the best evidence to date that Caudal acts as a type of morphogen gradient in the blastoderm of short-germ arthropods; however, Caudal seems to convey positional information through frequency regulation of pair-rule oscillations, rather than through threshold concentration levels in the gradient.

Wnt signaling underlies evolution and development of the butterfly wing pattern symmetry systems

Most butterfly wing patterns are proposed to be derived from a set of conserved pattern elements known as symmetry systems. Symmetry systems are so-named because they are often associated with parallel color stripes mirrored around linear organizing centers that run between the anterior and posterior wing margins. Even though the symmetry systems are the most prominent and diverse wing pattern elements, their study has been confounded by a lack of knowledge regarding the molecular basis of their development, as well as the difficulty of drawing pattern homologies across species with highly derived wing patterns. Here we present the first molecular characterization of symmetry system development by showing that WntA expression is consistently associated with the major basal, discal, central, and external symmetry system patterns of nymphalid butterflies. Pharmacological manipulations of signaling gradients using heparin and dextran sulfate showed that pattern organizing centers correspond precisely with WntA, wingless, Wnt6, and Wnt10 expression patterns, thus suggesting a role for Wnt signaling in color pattern induction. Importantly, this model is supported by recent genetic and population genomic work identifying WntA as the causative locus underlying wing pattern variation within several butterfly species. By comparing the expression of WntA between nymphalid butterflies representing a range of prototypical symmetry systems, slightly deviated symmetry systems, and highly derived wing patterns, we were able to infer symmetry system homologies in several challenging cases. Our work illustrates how highly divergent morphologies can be derived from modifications to a common ground plan across both micro- and macro-evolutionary time scales.

Analysis of the Wnt gene repertoire in an onychophoran provides new insights into the evolution of segmentation

BACKGROUND

The Onychophora are a probable sister group to Arthropoda, one of the most intensively studied animal phyla from a developmental perspective. Pioneering work on the fruit fly Drosophila melanogaster and subsequent investigation of other arthropods has revealed important roles for Wnt genes during many developmental processes in these animals.

RESULTS

We screened the embryonic transcriptome of the onychophoran Euperipatoides kanangrensis and found that at least 11 Wnt genes are expressed during embryogenesis. These genes represent 11 of the 13 known subfamilies of Wnt genes.

CONCLUSIONS

Many onychophoran Wnt genes are expressed in segment polarity gene-like patterns, suggesting a general role for these ligands during segment regionalization, as has been described in arthropods. During early stages of development, Wnt2, Wnt4, and Wnt5 are expressed in broad multiple segment-wide domains that are reminiscent of arthropod gap and Hox gene expression patterns, which suggests an early instructive role for Wnt genes during E. kanangrensis segmentation.

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

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

Insights into insect wing origin provided by functional analysis of vestigial in the red flour beetle, Tribolium castaneum

Despite accumulating efforts to unveil the origin of insect wings, it remains one of the principal mysteries in evolution. Currently, there are two prominent models regarding insect wing origin: one connecting the origin to the paranotal lobe and the other to the proximodorsal leg branch (exite). However, neither hypothesis has been able to surpass the other. To approach this conundrum, we focused our analysis on vestigial (vg), a critical wing gene initially identified in Drosophila. Our investigation in Tribolium (Coleoptera) has revealed that, despite the well-accepted view of vg as an essential wing gene, there are two groups of vg-dependent tissues in the "wingless" first thoracic segment (T1). We show that one of these tissues, the carinated margin, also depends on other factors essential for wing development (such as Wingless signal and apterous), and has nubbin enhancer activity. In addition, our homeotic mutant analysis shows that wing transformation in T1 originates from both the carinated margin and the other vg-dependent tissue, the pleural structures (trochantin and epimeron). Intriguingly, these two tissues may actually be homologous to the two proposed wing origins (paranotal lobes and exite bearing proximal leg segments). Therefore, our findings suggest that the vg-dependent tissues in T1 could be wing serial homologs present in a more ancestral state, thus providing compelling functional evidence for the dual origin of insect wings.

Expression patterns of Wnt genes in the venom claws of centipedes

The venom claws of centipedes, also known as forcipules, represent an evolutionary novelty that must have arisen in the centipede stem species, as they are not found in any other myriapods. The developmental-genetic changes that are involved in the origin of novelties are of considerable interest. It has previously been shown that centipede forcipules have a unique Hox code. However, this is a combinatorial code: no single Hox gene has a forcipule-specific expression. Here, we focus on Wnt genes. Two genes of this family show forcipule-specific expression in the "model centipede" Strigamia maritima: Wnt7 and Wnt11. For Wnt7, this forcipular expression zone seems to be a new one, which has arisen in evolution subsequently to other expression zones of the same gene. However, for Wnt11, the forcipule-specific expression probably arose by reduction of a more general pattern that originally included most or all of the limbs of an ancestral myriapod. Thus the developmental-genetic basis of the evolutionary change that turned the first pair of walking legs into venom claws is complex, involving different types of change in expression pattern. This sort of complexity is likely to be the case regarding evolutionary changes in morphology in general. Whether the origins of those features that can be considered as novelties are different in terms of their developmental-genetic basis from more routine evolutionary changes remains an open question.

Expression of wnt and frizzled genes during early sea star development

The Wnt signaling pathway is highly conserved across metazoa and has pleiotropic functions in the development of many animals. Binding of a secreted Wnt ligand to its Frizzled (Fz) receptor activates Dishevelled, which then drives one of three major signaling cascades, canonical (β-catenin), calcium, or planar cell polarity signaling. These pathways have distinct developmental effects and function in different processes in different organisms. Here we report the expression of six wnt and three fz genes during embryogenesis of the sea star, Patiria miniata, as a first step in uncovering the roles of Wnt signaling in the development of this organism. wnt3, wnt4, wnt8, and wnt16 are expressed in nested domains in the endoderm and lateral ectoderm from blastula through late gastrula stages; wnt2 and wnt5 are expressed in the mesoderm and anterior endoderm. Expression of different fz paralogs is detected in the mesoderm; posterior endoderm and ectoderm; and anterior ectoderm. Taken together, this suggests that Wnt signaling can occur throughout most of the embryo and may therefore play multiple roles during sea star development.

Differential deployment of paralogous Wnt genes in the mouse and chick embryo during development

Genes encoding Wnt ligands are crucial in body patterning and are highly conserved among metazoans. Given their conservation at the protein-coding level, it is likely that changes in where and when these genes are active are important in generating evolutionary variations. However, we lack detailed knowledge about how their deployment has diverged. Here, we focus on four Wnt subfamilies (Wnt2, Wnt5, Wnt7, and Wnt8) in mammalian and avian species, consisting of a paralogous gene pair in each, believed to have duplicated in the last common ancestor of vertebrates. We use three-dimensional imaging to capture expression patterns in detail and carry out systematic comparisons. We find evidence of greater divergence between these subgroup paralogues than the respective orthologues, consistent with some level of subfunctionalization/neofunctionalization in the common vertebrate ancestor that has been conserved. However, there were exceptions; in the case of chick Wnt2b, individual sites were shared with both mouse Wnt2 and Wnt2b. We also find greater divergence, between paralogues and orthologues, in some subfamilies (Wnt2 and Wnt8) compared to others (Wnt5 and Wnt7) with the more highly similar expression patterns showing more extensive expression in more structures in the embryo. Wnt8 genes were most restricted and most divergent. Major sites of expression for all subfamilies include CNS, limbs, and facial region, and in general there were more similarities in gene deployment in these territories with divergent patterns featuring more in organs such as heart and gut. A detailed comparison of gene expression patterns in the limb showed similarities in overall combined domains across species with notable differences that may relate to lineage-specific morphogenesis.

RNA localization in the honeybee (Apis mellifera) oocyte reveals insights about the evolution of RNA localization mechanisms

Subcellular localization of RNAs is a critical biological process for generation of cellular asymmetries for many cell types and a critical step in axis determination during the early development of animals. We have identified transcripts localized to the anterior and posterior of honeybee oocyte using laser capture microscopy and microarray analysis. Analysis of orthologous transcripts in Drosophila indicates that many do not show a conserved pattern of localization. By microinjecting fluorescently labeled honeybee transcripts into Drosophila egg chambers we show that these RNAs become localized in a similar manner to their localization in honeybee oocytes, indicating conservation of the localization machinery. Thus while the mechanisms for localizing RNA are conserved, the complement of localized RNAs are not. We propose that this complement of localized RNAs may change relatively rapidly through the loss or evolution of signal sequences detected by the conserved localization machinery, and show this has occurred in one transcript that is localized in a novel way in the honeybee. Our proposal, that the acquisition of novel RNA localization is relatively easy to evolve, has implications for the evolution of symmetry breaking mechanisms that trigger axis formation and development in animal embryos.

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

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

[Wnt signaling pathway and the Evo-Devo of deuterostome axis]

A series of signal transduction pathways have been found to regulate the polarity establishment and formation of animal primary body axis. Among them, Wnt signaling pathway is extremely conserved and several key components in the pathway have been identified in the demosponge lineage. This implies that it is one of the earliest pathways involved in the ancestral metazoan axis development and might play an important role in specification and development of posterior and ventral fate of animal axis. Recently, with the establishment of functional experiments in vitro, the body plan formation has been found to be affected, in varying degrees, by many genes in the Wnt signaling pathway, such as members of wnt gene family, maternal gene beta-catenin and some transcription factor encoding genes. In this review, we analyzed the evolutionary origin of the wnt gene family involved in development of metazoan body plans, and then made a brief review on the roles of canonical Wnt/beta-catenin signaling in the polarity establishment and formation of primary body axis in diverse deuterostomes including sea urchin, amphioxus, zebrafish, frog, and mouse.

A context-dependent combination of Wnt receptors controls axis elongation and leg development in a short germ insect

Short germ embryos elongate their primary body axis by consecutively adding segments from a posteriorly located growth zone. Wnt signalling is required for axis elongation in short germ arthropods, including Tribolium castaneum, but the precise functions of the different Wnt receptors involved in this process are unclear. We analysed the individual and combinatorial functions of the three Wnt receptors, Frizzled-1 (Tc-Fz1), Frizzled-2 (Tc-Fz2) and Frizzled-4 (Tc-Fz4), and their co-receptor Arrow (Tc-Arr) in the beetle Tribolium. Knockdown of gene function and expression analyses revealed that Frizzled-dependent Wnt signalling occurs anteriorly in the growth zone in the presegmental region (PSR). We show that simultaneous functional knockdown of the Wnt receptors Tc-fz1 and Tc-fz2 via RNAi resulted in collapse of the growth zone and impairment of embryonic axis elongation. Although posterior cells of the growth zone were not completely abolished, Wnt signalling within the PSR controls axial elongation at the level of pair-rule patterning, Wnt5 signalling and FGF signalling. These results identify the PSR in Tribolium as an integral tissue required for the axial elongation process, reminiscent of the presomitic mesoderm in vertebrates. Knockdown of Tc-fz1 alone interfered with the formation of the proximo-distal and the dorso-ventral axes during leg development, whereas no effect was observed with single Tc-fz2 or Tc-fz4 RNAi knockdowns. We identify Tc-Arr as an obligatory Wnt co-receptor for axis elongation, leg distalisation and segmentation. We discuss how Wnt signalling is regulated at the receptor and co-receptor levels in a dose-dependent fashion.