Table of contents

Wingless and insect segmentation

In insects, there are two different modes of segmentation. In the higher dipteran insects (like Drosophila), segmentation takes place almost simultaneously in the syncytial blastoderm. By contrast, in the orthopteran insects [like Schistocerca (grasshopper)] anterior segments form almost simultaneously in the cellular blastoderm and then the remaining posterior part elongates to form segments sequentially from the posterior proliferative zone. Although most of orthopteran orthologs of the Drosophila segmentation genes may be involved in orthopteran segmentation, little is known about the role of these genes. Segmentation processes of Gryllus bimaculatus have been investigated, focusing on its orthologs of the Drosophila segment-polarity genes, G. bimaculatus wingless (Gbwg), armadillo (Gbarm) and hedgehog (Gbhh). Gbhh and Gbwg were observed to be expressed in the each anterior segment and the posterior proliferative zone. In order to know their roles, RNA interference (RNAi) was used. No significant effects of RNAi for Gbwg and Gbhh on segmentation were observed, probably due to functional replacement by another member of the corresponding gene families. Embryos obtained by RNAi for Gbarm exhibited abnormal anterior segments and lack of the abdomen. These results suggest that GbWg/GbArm signaling is involved in the posterior sequential segmentation in the G. bimaculatus embryos, while Gbwg, Gbarm and Gbhh are likely to act as the segment-polarity genes in the anterior segmentation similarly as in Drosophila (Miyawaki, 2004).

Gbwg is expressed continuously in the posterior-most region of G. bimaculatus embryos. This expression pattern is a common feature in the intermediate- and short-germ insects. The posterior elongation takes place between the T3 segment and the posterior-most region of embryos. Judged from the phenotypes of the GbarmRNAi embryos, it is reasonable to propose that the canonical Wnt/Wg pathway is involved in the posterior sequential elongation and segmentation. Since intermediate phenotypes, for example, lack of the T3-A5 segments, were not observed even in mild RNAi conditions, GbWg in the posterior growth zone should be responsible for the phenotypes of the GbarmRNAi embryos, and is likely to act as an organizing signal in posterior sequential segmentation (Miyawaki, 2004).

There are several lines of evidence supporting the idea that Wg/Wnt may play essential roles in pattern formation through a process of cell-cell interaction. In Hydra, Wnt expressed in the putative head organizer may be involved in axis formation. Furthermore, a recent advance in the study of vertebrate somitogenesis reveals that the somites appear by the progressive anterior conversion of a temporally periodic pattern into a spatially periodic pattern, requiring the expression of homologs of the Drosophila pair-rule gene hairy and segment-polarity gene wingless. Recently, Wnt3a was reported to play major roles in the segmentation clock controlling somitogenesis in vertebrates. In this case, Wnt/ß-catenin signaling pathway is known to link to the segmentation clock through negative-feedback loop with Axin2. The existence of this mechanism in vertebrates makes the clock model for short and intermediate-germ insects plausible. Since the posterior segments are formed sequentially with about 2 h periodicity in cricket embryos, it is probable therefore that a segmentation clock operates in the insect embryos (Miyawaki, 2004).

Although the molecular mechanisms directing anteroposterior patterning of the Drosophila embryo (long-germband mode) are well understood, how these mechanisms were evolved from an ancestral mode of insect embryogenesis remains largely unknown. In order to gain insight into mechanisms of evolution in insect embryogenesis, the expression and function of the orthologue of Drosophila caudal (cad) was examined in the intermediate-germband cricket Gryllus bimaculatus. A posterior (high) to anterior (low) gradient in the levels of Gryllus bimaculatus cad (Gbcad) transcript is formed in the early-stage embryo, and then Gb' cad is expressed in the posterior growth zone until the posterior segmentation is completed. Reduction of Gb' cad expression level by RNA interference results in deletion of the gnathum, thorax, and abdomen in embryos, remaining only anterior head. The gnathal and thoracic segments are formed by Gb' cad probably through the transcriptional regulation of gap genes including Gb' hunchback and Gb' Krüppel. Furthermore, Gb'cad is found to be involved in the posterior elongation, acting as a downstream gene in the Wingless/Armadillo signalling pathways. These findings indicate that Gb'cad does not function as it does in Drosophila, suggesting that regulatory and functional changes of cad occurred during insect evolution. The Wg/Cad pathway in the posterior pattern formation may be common in short- and intermediate-germband embryogenesis. During the evolutionary transition from short- or intermediate- to long-germband embryogenesis, an ancestral cell-cell signalling system including Wg/Arm signalling may have been replaced by a diffusion system of transcription factors as found in Drosophila. Since Wnt/Cdx pathways are involved in the posterior patterning of vertebrates, such mechanisms may be conserved in animals that undergo sequential segmentation from the posterior growth zone (Shinmyo, 2005).

In the long germ insect Drosophila, all body segments are determined almost simultaneously at the blastoderm stage under the control of the anterior, the posterior, and the terminal genetic system. Most other arthropods (and similarly also vertebrates) develop more slowly as short germ embryos, where only the anterior body segments are specified early in embryogenesis. The body axis extends later by the sequential addition of new segments from the growth zone or the tail bud. The mechanisms that initiate or maintain the elongation of the body axis (axial growth) are poorly understood. The terminal system in the short germ insect Tribolium was functionally analyzed. Unexpectedly, Torso signaling is required for setting up or maintaining a functional growth zone and at the anterior for the extraembryonic serosa. Thus, as in Drosophila, fates at both poles of the blastoderm embryo depend on terminal genes, but different tissues are patterned in Tribolium. Short germ development as seen in Tribolium likely represents the ancestral mode of how the primary body axis is set up during embryogenesis. It is therefore concluded that the ancient function of the terminal system mainly was to define a growth zone and that in phylogenetically derived insects like Drosophila, Torso signaling became restricted to the determination of terminal body structures (Schoppmeier, 2005).

In Drosophila, the anterior- and posterior-most terminal body regions of the embryo depend on the maternal terminal-group genes. One of them, the torso-like (tsl) gene is expressed in somatic follicle cells located at the anterior and posterior pole of the oocyte. In the embryo, tsl contributes to the local activation of the receptor tyrosine kinase Torso at the egg poles. The signal is transduced to the nucleus via a Ras-Raf-MAP-K/Erk phosphorylation cascade, and leads to the expression of the zygotic target genes tailless (tll) and huckebein (hkb) at the posterior terminus of the embryo. Failure to activate Torso signaling results in defects in the head skeleton and loss of all segments posterior to abdominal segment 7, in addition to loss of the hindgut and posterior midgut anlagen (Schoppmeier, 2005 and references therein).

Whether an anteriorly acting terminal system is a general feature of all insects has been challenged because under certain conditions, Torso function at the anterior is dispensable for head development in Drosophila. This hypothesis is supported by the expression of the Tribolium ortholog of tll at the posterior, but not at the anterior pole of blastoderm stage embryos. Thus, in Tribolium, posterior terminal cells appear to be determined before the onset of abdomen formation. It is unknown, however, whether these cells specify posterior fate after axis elongation and abdomen formation is completed or whether they also contribute to earlier steps of segmentation (Schoppmeier, 2005).

The orthologs of the key components of the Torso pathway have been isolated in the short germ beetle Tribolium torso (Tc-tor) and torso-like (Tc-tsl). As in Drosophila, Tc-torso mRNA is maternally inherited by the embryo and expressed ubiquitously in freshly laid eggs, and Tc-tsl is expressed during oogenesis anteriorly and posteriorly in the follicle cells of the oocyte (Schoppmeier, 2005).

Knocking down the function of Tc-torso or Tc-tsl using parental RNA interference leads to identical embryonic phenotypes. Whereas the head and the anterior thorax are unaffected, unexpectedly the most extreme Tc-torsoRNAi and Tc-tslRNAi embryos lack all structures that develop during postblastodermal abdominal growth. Thus, the head and thoracic segments that form in torso or tsl RNAi embryos likely represent the structures, which are determined already during the Tribolium blastoderm stage. Less strongly affected embryos fail to form the full number of abdominal segments (Schoppmeier, 2005).

To determine whether the Tc-torso RNAi phenotype does not reflect a late function of maintaining abdominal fate prior to cuticularization, the expression of Engrailed protein was examined in Tc-torsoRNAi embryos at a stage when abdominal segments should already have developed. Indeed, in strongly affected embryos, Engrailed stripes corresponding to the head and thorax, but not to abdominal segments, are present (Schoppmeier, 2005).

The emergence of segments was visualized in embryos with impaired Torso signaling by analyzing the Tc-even-skipped (Tc-eve) expression pattern. In wild-type embryos, Tc-eve is initially expressed in a double segmental pattern that later resolves into secondary segmental stripes. Tc-tsl RNAi does not interfere with the formation of the first two primary Tc-eve stripes that give rise to the gnathal and the first thoracic (T1) segments. However, although the third primary Tc-eve expression domain (Tc-eve stripe 3) forms normally, this domain does not resolve into segmental stripes, and no additional primary eve-stripes form. In the wild-type, Tc-eve stripe 3 covers the region where the second (T2) and third thoracic (T3) segment will develop. Although Tc-eve stripe 3 does not split in Tc-tsl RNAi embryos, this domain gives rise to the second thoracic segment. Thus, Torso signaling is required for the initiation of axial growth or maintaining the segmentation process (Schoppmeier, 2005).

As revealed by DAPI staining and by morphology, posterior invagination of cells is abolished in both Torso- and tsl RNAi embryos, and as a consequence, no posterior pit forms. To understand how Torso signaling is propagated at the posterior pole and to test whether downstream gene activity is affected in the growth zone in Tc-torsoRNAi embryos, the activity of the Map-kinase and the expression of Tc-wingless (Tc-wg), Tc-tailless (Tc-tll), Tc-caudal (Tc-cad), and Tc-forkhead (Tc-fkh) RNA was analyzed in early embryos (Schoppmeier, 2005).

The active state of the Torso receptor is transduced to the nucleus via the Ras-Raf signal transduction pathway and leads to the activation of zygotic target genes. The activity of this pathway can be visualized with an antibody that recognizes ErkPP but does not discriminate between the different pathways that involve ErkPP signaling. In nontreated embryos, ErkPP can be detected in a subpopulation of the serosa, a single row of cells at the border of the serosa and the embryonic anlage; at the rims of the mesoderm; and at the posterior pole. In torsoRNAi embryos posterior ErkPP expression is lost, further indicating that ErkPP is involved in propagating terminal signaling. ErkPP expression in the serosa is mildly affected whereas the other sites where ErkPP activity is detected in the wild-type are normal. ErkPP activity in the amnion appears not to be reduced; however, the amnion itself does also not form properly. Whether this is a direct or indirect consequence of Torso reduction is unclear (Schoppmeier, 2005).

In addition to segmental stripes, a terminal wingless (wg) expression domain first seen at the blastoderm stage is present throughout the phase of body elongation in the growth zone of the wild-type. In Tc-torsoRNAi embryos, the posterior terminal Tc-wg domain is missing at the blastoderm stage, as well as in older embryos corresponding in age to wild-type embryos undergoing body axis extension. Drosophila-torso mutant embryos also lack the posterior terminal wg expression domain, indicating, that the dependence of wg on torso is conserved. The segmental wg stripes that were built prior to the growth process, form close to the posterior end. This shows that a presegmented region (PSR) normally separating the last segment formed at the posterior end of the embryo is strongly reduced or absent in torsoRNAi embryos. The absence of Tc-cad and Tc-tll in torsoRNAi embryos establishes these genes as potential targets of terminal signaling also in Tribolium (Schoppmeier, 2005).

In the Drosophila segmentation hierarchy, periodic expression of pair-rule genes translates gradients of regional information from maternal and gap genes into the segmental expression of segment polarity genes. In Tribolium, homologs of almost all the eight canonical Drosophila pair-rule genes are expressed in pair-rule domains, but only five have pair-rule functions. even-skipped, runt and odd-skipped act as primary pair-rule genes, while the functions of paired (prd) and sloppy-paired (slp) are secondary. Since secondary pair-rule genes directly regulate segment polarity genes in Drosophila, Tc-prd and Tc-slp were analyzed to determine the extent to which this paradigm is conserved in Tribolium. It was found that the role of prd is conserved between Drosophila and Tribolium; it is required in both insects to activate engrailed in odd-numbered parasegments and wingless (wg) in even-numbered parasegments. Similarly, slp is required to activate wg in alternate parasegments and to maintain the remaining wg stripes in both insects. However, the parasegmental register for Tc-slp is opposite that of Drosophila slp1. Thus, while prd is functionally conserved, the fact that the register of slp function has evolved differently in the lineages leading to Drosophila and Tribolium reveals an unprecedented flexibility in pair-rule patterning (Choe, 2007; full text of article).

wingless/Wnt family are essential to development in virtually all metazoans. In short-germ insects, including the red flour beetle (Tribolium castaneum), the segment-polarity function of wg is conserved. Wnt signaling is also implicated in posterior patterning and germband elongation, but despite its expression in the posterior growth zone, Wnt1/wg alone is not responsible for these functions. Tribolium contains additional Wnt family genes that are also expressed in the growth zone. After depleting Tc-WntD/8, a small percentage of embryos were found lacking abdominal segments. Additional removal of Tc-Wnt1 significantly enhanced the penetrance of this phenotype. Seeking alternative methods to deplete Wnt signal, RNAi with other components of the Wnt pathway including wntless (wls), porcupine (porc), and pangolin (pan). Tc-wls RNAi caused segmentation defects similar to Tc-Wnt1 RNAi, but not Tc-WntD/8 RNAi, indicating that Tc-WntD/8 function is Tc-wls independent. Depletion of Tc-porc and Tc-pan produced embryos resembling double Tc-Wnt1,Tc-WntD/8 RNAi embryos, suggesting that Tc-porc is essential for the function of both ligands, which signal through the canonical pathway. This is the first evidence of functional redundancy between Wnt ligands in posterior patterning in short-germ insects. This Wnt function appears to be conserved in other arthropods and vertebrates (Bolognesi, 2008). >

Wnt/β-catenin and Hedgehog (Hh) signaling are essential for transmitting signals across cell membranes in animal embryos. Early patterning of the principal insect model, Drosophila melanogaster, occurs in the syncytial blastoderm, where diffusion of transcription factors obviates the need for signaling pathways. However, in the cellularized growth zone of typical short germ insect embryos, signaling pathways are predicted to play a more fundamental role. Indeed, the Wnt/β-catenin pathway is required for posterior elongation in most arthropods, although which target genes are activated in this context remains elusive. This study used the short germ beetle Tribolium castaneum to investigate two Wnt and Hh signaling centers located in the head anlagen and in the growth zone of early embryos. Wnt/β-catenin signaling was found to act upstream of Hh in the growth zone, whereas the opposite interaction occurs in the head. The target gene sets of the Wnt/β-catenin and Hh pathways were determined; the growth zone signaling center activates a much greater number of genes and the Wnt and Hh target gene sets are essentially non-overlapping. The Wnt pathway activates key genes of all three germ layers, including pair-rule genes, and Tc-caudal (see Drosophila caudal) and Tc-twist (see Drosophila twist). Furthermore, the Wnt pathway is required for hindgut development and Tc-senseless (see Drosophila senseless) as a novel hindgut patterning gene required in the early growth zone. At the same time, Wnt acts on growth zone metabolism and cell division, thereby integrating growth with patterning. Posterior Hh signaling activates several genes potentially involved in a proteinase cascade of unknown function (Oberhofer, 2014).

Wingless and insect wing patterns

The morphological and functional evolution of appendages has played a critical role in animal evolution, but the developmental genetic mechanisms underlying appendage diversity are not understood. Given that homologous appendage development is controlled by the same Hox gene in different organisms, and that Hox genes are transcription factors, diversity may evolve from changes in the regulation of Hox target genes. Two impediments to understanding the role of Hox genes in morphological evolution have been the limited number of organisms in which Hox gene function can be studied and the paucity of known Hox-regulated target genes. An analysis was carried out of Hindsight, a butterfly homeotic mutant in which portions of the ventral hindwing pattern are transformed to ventral forewing identity, and the regulation of target genes by the Ultrabithorax (Ubx) gene product was compared in Lepidopteran and Dipteran hindwings. Ubx gene expression is lost from patches of cells in developing Hindsight hindwings, which correlates with changes in wing pigmentation, color pattern elements, and scale morphology. This mutant was used to study how regulation of target genes by Ubx protein differs between species. Drosophila Serum response factor (blistered), Achaete-Scute Complex, and wingless are repressed in Drosophila halteres. Portions of the expression pattern of Lepidopteran homologs of these genes are not repressed in butterfly hindwings. Unlike the expression patterns of the homologous genes in halteres, butterfly wg is not repressed along the posterior margin in the hindwing, nor is butterfly SRF repressed in intervein regions, and the AS-C homologs are not repressed in cells flanking the dorsal-ventral boundary. These differences in the regulation of wg, SRF and AS-C between Drosophila halteres and butterfly hindwings suggest that these genes became repressed by Ubx when an ancestral hindwing evolved into a haltere in the dipteran lineage, with a concomitant reduction of appendage size, loss of margin bristles, and changes in shape. Two additional exampes of Ubx-regulated differences in gene expression between fly and butterfly flight appendages were found. (1) wg is expressed in two stripes in butterfly forewings that roughly correspond to the future location of the proximal band elements. This protein of the wg pattern is absent from butterfly hindwings and has not counterpart in flies and represents a novel feature regulated by Ubx in butterflies. (2) Dll is expressed along the margin of both butterfly wings and the Drosophila forewing, but this expression is modified in halteres and may be regulated by Ubx. Changes in Hox-regulated target gene sets are, in general, likely to underlie the morphological divergence of homologous structures between animals (Weatherbee, 1999).

Wingless and insect leg development

Insects can be grouped into two main categories, holometabolous and hemimetabolous, according to the extent of their morphological change during metamorphosis. The three thoracic legs, for example, are known to develop through two overtly different pathways: holometabolous insects make legs through their imaginal discs, while hemimetabolous legs develop from their leg buds. Thus, how the molecular mechanisms of leg development differ from each other is an intriguing question. In the holometabolous long-germ insect, these mechanisms have been extensively studied using Drosophila melanogaster. However, little is known about the mechanism in the hemimetabolous insect. Leg development of the hemimetabolous short-germ insect, Gryllus bimaculatus (cricket), has been studied focusing on expression patterns of the three key signaling molecules, hedgehog, wingless and decapentaplegic, which are essential during leg development in Drosophila. In Gryllus embryos, expression of hh is restricted in the posterior half of each leg bud, while dpp and wg are expressed in the dorsal and ventral sides of its anterior/posterior (A/P) boundary, respectively. Their expression patterns are essentially comparable with those of the three genes in Drosophila leg imaginal discs, suggesting the existence of the common mechanism for leg pattern formation. However, expression pattern of dpp is significantly divergent among Gryllus, Schistocerca (grasshopper) and Drosophila embryos, while expression patterns of hh and wg are conserved. Furthermore, the divergence is found between the pro/mesothoracic and metathoracic Gryllus leg buds. These observations imply that the divergence in the dpp expression pattern may correlate with diversity of leg morphology (Niwa, 2000).

In the allocation phase of Drosophila 5h embryos, wg and hh are expressed in a stripe along the A/P compartment boundary and in the posterior region of each segment, respectively. However, dpp is expressed throughout the dorsal region and then in the dorsal side of the wg stripe. Later, the expression changes to give two thin stripes running anteroposteriorly along the length of the embryo. Wg, but not Dpp, is responsible for initial specification of the limb primordia with a distal identity and for induction of Dll. A model for the allocation of the limb primordium (the G-H model) is presented. A stripe of Wg induces the limb primordium expressing Dll. Repression of Dll by Dpp from the dorsal side and by Spitz (Drosophila EGF) from the ventral side limits the limb formation only in the lateral position. Then, Dpp specifies proximal cell identity in the primordium in a concentration-dependent manner. In Gryllus and Schistocerca embryos, expression of wg is detected in a stripe along the A/P compartment boundary of the body segment. In Gryllus embryos, expression of dpp is first detected along the periphery of the germ band. Similar expression patterns have been observed in Tribolium. Although the cricket and grasshopper belong to the same Orthoptera, the expression patterns of Sadpp are more complicated than those of Gbdpp. In Schistocerca embryos at early stages, Sadpp is expressed in two partial stripes in each hemisegment, intrasegmentally and intersegmentally, paralleling the D/V axis. The intrasegmental stripes extend along both dorsal and ventral sides of the presumptive leg field. Early expression patterns of Gbdpp resemble those of Dmdpp or Tcdpp more closely than those of Sadpp. Thus, the wg expression pattern appears conserved in the allocation phase, while early expression patterns of dpp seems divergent even in the Orthoptera. Thus, more data are necessary to judge whether the G-H model is also applicable as a model for initiation of limb formation in other insects (Niwa, 2000 and references therein).

In Phase 2, in the Drosophila leg imaginal disc, hh is expressed in the posterior compartment of the disc, determining the A/P pattern, and induces dpp and wg expression in the dorsal and ventral side of the A/P boundary, respectively. They act cooperatively in a concentration-dependent manner to organize the P/D axis and induce expression of Dll at the center of the disc. In Gryllus and Schistocerca limb buds, since hh and wg are expressed in the posterior and the ventral side of the A/P boundary, respectively, their functions during limb development should be conserved among the fly, cricket, beetle and grasshopper. However, expression patterns of Gbdpp are considerably different from those of Drosophila dpp: Gbdpp expression is limited to a dorsal stripe, transiently around the time of limb bud emerging, at stage 6-7. At this time, expression of Dll was found in the distal tip of the limb bud. This transient expression pattern also occurs in Schistocerca embryos. In Drosophila, removal of Dpp signaling prior to the second larval instar results in loss of Dll expression, while later removal of Dpp does not affect Dll expression, indicating that Dpp is required for the initiation but not maintenance of Dll transcription. Thus, it is reasonable to consider that transient dpp expression is enough to induce expression of Dll, which is required for the P/D leg pattern formation (Niwa, 2000 and references therein).

To understand the mechanism of regeneration, many experiments have been carried out with hemimetabolous insects, since their nymphs possess the ability to regenerate amputated legs. Patterns of hedgehog, wingless, and decapentaplegic expression were examined during leg regeneration of the cricket Gryllus bimaculatus. The observed expression patterns are essentially consistent with the predictions derived from the boundary model modified by Campbell and Tomlinson (CTBM). Thus, it is concluded that the formation of the proximodistal axis of a regenerating leg is triggered at a site where ventral wg-expressing cells abut dorsal dpp-expressing cells in the anteroposterior (A/P) boundary, as postulated in the CTBM (Mito, 2002).

In the cricket leg, the single layer of surface epidermal cells forms precise patterns of structures, including bristles and spines, in the overlying cuticle. The regional specialization of the leg epidermal cells is evident along the three major axes of the leg, which include the anteroposterior (A/P), dorsoventral (D/V), and proximodistal (P/D) axes. The P/D axis relates to the distance from the body trunk, while the A/P and D/V axes unite to form the single circumferential axis. When a metathoracic leg of a Gryllus nymph in the third instar is amputated at the tibia, the distal missing part is completely recovered after about 30-35 days through four molts subsequent to the amputation. Just after the amputation, a trachea running along the P/D axis, reticulate fat bodies, and muscles are observed in sagittal sections. By 6 h after amputation, wounded muscles already start to degenerate, while hemocytes aggregate in the wound to form a scab. By day 2, epidermal cells migrate over the wound surface, and epidermal continuity is restored underneath the scab. Cell proliferation can be detected in epidermis lining the scab during this process. By day 5, the wound epidermis thickens to form a regeneration bud, or blastema, and cell proliferation is greatly activated in the blastema. Cells in the blastema lose their differentiated character and start to grow. By day 7, the blastema becomes the primordia of the tibia and tarsus concomitant with muscle recovery. By day 10, the boundary of the tibia-tarsus is visible in the blastema. Finally, all of the structures that normally lie distal to the point of amputation are restored (Mito, 2002).

In normally developing cricket leg buds, hh i expressed in the posterior (P) compartment, while wg and dpp are expressed in the ventral (V) side and dorsal (D) side of the anteroposterior (A/P) boundary, respectively. In a normal leg at the stage corresponding to the regeneration samples, hybridization signals for hh are weakly detected in epidermal cells located in the posterior region, whereas the expressions of wg and dpp are not observed. In contrast, the induced expressions of hh, wg, and dpp are observed in the blastemata of regenerating legs. The expression signals of hh are localized on the posterior side of the leg epidermis. The localization of the En protein was examined in cryosections with the monoclonal antibody mAb4D9. Signals were detected in both sagittal and transverse sections, indicating that En is localized in the posterior half of epidermis and supporting the results for hh. In the transverse sections, the En expression domain looks slightly broader than that of hh (Mito, 2002).

The expression pattern of wg is clearly observed in the ventral region of the blastema with a distal-to-proximal gradient in the signal intensity. The signals of the dpp expression are much weaker than the wg signals. Furthermore, there was variation in the expression patterns. Since such variation was not observed in the wg expression pattern, it is considered that the expression pattern of dpp is dynamically changed, as observed during leg development. The observed expression patterns of dpp were classified mainly into three types: Type I, with signals restricted in dorsodistal epithelial cells of the blastema, where intense non-specific signals appear in the trachea due to longer staining reactions; Type II, with signals observed in dorsal and distal epithelial cells, and weakly in ventral cells; and Type III, with signals so weak that no pattern is discernible (n=24). Type I expression patterns are observed in the early stages, while Type II patterns are observed even in the later stages (~4 days). Therefore, it is reasonable to consider that the expression pattern of dpp changes from Type I to Type II as the regeneration proceeds (Mito, 2002).

The expression patterns of wg and dpp in the blastema are comparable to those in the leg bud of the cricket embryo. In particular, the discrete expression of dpp (Type I) observed in the blastema is also observed in the dorsal side along the A/P boundary in the cricket leg bud, which differs from the expression of dpp in the leg imaginal disc. However, a major difference between the leg bud and blastemata is the size of the wg/dpp expression boundary: the boundary becomes a line in the blastema, similar to the apical ectodermal ridge of vertebrate limb buds, rather than a point in the insect leg bud. After wound healing, the restoration of the epidermal continuity results in the formation of a D/V boundary where dpp-expressing epidermal cells abut wg-expressing cells, which possibly initiates the formation of the P/D axis in the regeneration blastema (Mito, 2002).

The Drosophila genes wingless and decapentaplegic comprise the top level of a hierarchical gene cascade involved in proximal-distal (PD) patterning of the legs. It remains unclear, whether this cascade is common to the appendages of all arthropods. Here, wg and dpp are studied in the millipede Glomeris marginata, a representative of the Myriapoda. Glomeris wg (Gm-wg) is expressed along the ventral side of the appendages compatible with functioning during the patterning of both the PD and dorsal-ventral (DV) axes. Gm-wg may also be involved in sensory organ formation in the gnathal appendages by inducing the expression of Distal-less (Dll) and H15 in the organ primordia. Expression of Glomeris dpp (Gm-dpp) is found at the tip of the trunk legs as well as weakly along the dorsal side of the legs in early stages. Taking data from other arthropods into account, these results may be interpreted in favor of a conserved mode of WG/DPP signaling. Apart from the main PD axis, many arthropod appendages have additional branches (e.g., endites). It is debated whether these extra branches develop their PD axis via the same mechanism as the main PD axis, or whether branch-specific mechanisms exist. Gene expression in possible endite homologs in Glomeris argues for the latter alternative. All available data argue in favor of a conserved role of WG/DPP morphogen gradients in guiding the development of the main PD axis. Additional branches in multibranched (multiramous) appendage types apparently do not utilize the WG/DPP signaling system for their PD development. This further supports recent work on crustaceans and insects, that lead to similar conclusions (Prpic, 2004).

ß-Catenin regulates cell adhesion and cellular differentiation during development, and misregulation of ß-catenin contributes to numerous forms of cancer in humans. This study describes C. elegans conditional alleles of mom-2/Wnt, mom-4/Tak1, and wrm-1/ß-catenin. These reagents were used to examine the regulation of WRM-1/ß-catenin during a Wnt-signaling-induced asymmetric cell division. While WRM-1 protein initially accumulates in the nuclei of all cells, signaling promotes the retention of WRM-1 in nuclei of responding cells. Both PRY-1/Axin and the nuclear exportin homolog IMB-4/CRM-1 antagonize signaling. These findings reveal how Wnt signals direct the asymmetric localization of ß-catenin during polarized cell division (Nakamura, 2005).

A possible insight into the connection between cortical and nuclear signaling events comes from preliminary findings on the cortical localization of WRM-1. In the course of these studies, a faint localization of WRM-1::GFP to the cell cortex was seen during each mitosis. Interestingly, in the EMS cell (the 4-cell stage blastomere in C. elegans ), WRM-1::GFP is lost along the posterior cortex proximal to the signaling cell P2, while staining is maintained along the anterior cortex of the dividing EMS cell. This cortical localization is also visible at later stages and in developing larvae. These preliminary studies suggest that MOM-5/Frizzled is required for cortical association, while cortical release correlates with signaling via MOM-2/Wnt. Although these observations require further investigation, they suggest an interesting model that could explain how signaling at the cortex could drive nuclear WRM-1 asymmetries. Importantly, this model could also explain the difference between the penetrance of the endoderm defects seen in mom-2/Wnt mutants (~60% gutless) and mom-5/Fz mutants (only 5% gutless), and the surprising finding that the lower penetrance gutless phenotype of mom-5 is epistatic to mom-2 (Nakamura, 2005).

According to this model, P2/EMS signaling alters the affinity of WRM-1 for the posterior cortex of EMS and simultaneously activates WRM-1 for downstream signaling. This activation could be direct (e.g., by phosphorylation of WRM-1) or indirect (e.g., by modification of a WRM-1-interacting protein such as LIT-1). For simplicity in this discussion, the direct activation model will be considered. At steady state, only a small percentage of WRM-1 protein localizes at the cortex and this level drops during signaling, suggesting that cortical association may reflect a dynamic process that is modulated by signaling. Cortical signaling events also ensure that the mitotic apparatus of the cell is oriented such that division produces one nucleus that is more proximal to the posterior cortex and thus exposed to higher concentrations of an activated and less cortically associated form of WRM-1. At the beginning of telophase, WRM-1 accumulates in both nascent nuclei via a mechanism that depends on the kinases MOM-4 and LIT-1. During late telophase, and shortly after cytokinesis, IMB-4/CRM-1-dependent export begins to reduce WRM-1 nuclear levels in MS. However, in E, the signal-dependent release of an activated form of WRM-1 from the cortex induces a net nuclear retention of WRM-1. Finally, retention of WRM-1 in the nascent E (endoderm-restricted precursor) nucleus correlates with a simultaneous CRM-1-dependent nuclear export of POP-1 (Nakamura, 2005).

This model explains the phenotypic differences between mom-2 and mom-5 mutants. In mom-2 mutants, MOM-5 sequesters WRM-1 at the posterior cortex, reducing WRM-1 nuclear retention in E, and resulting in the higher penetrance of the mom-2 endoderm defect. In mom-5 mutants or in mom-2; mom-5 double mutants, signaling from P2 via the parallel SRC-1 tyrosine kinase pathway can activate WRM-1, which is then free to enter the nucleus and promote POP-1 nuclear export. Since SRC-1 has little effect on WRM-1 localization, these findings suggest that SRC-1 may instead alter WRM-1 activity (Nakamura, 2005).

The details of the mechanism that drives the reciprocal nuclear accumulation of WRM-1 and POP-1 are still not clear. The finding that the nuclear accumulation of WRM-1 partially depends on POP-1 suggests that WRM-1 and POP-1 may directly compete for nuclear export factors or nuclear/cytoplasmic retention sites. For example, WRM-1-dependent phosphorylation of POP-1 might increase the affinity of POP-1 for CRM-1, perhaps by promoting the interaction of POP-1 with PAR-5/14-3-3. This could lead to a direct competition that displaces WRM-1 from the export machinery in responding cells. Alternatively, signaling may alter the relative affinity of WRM-1 and/or POP-1 for binding to mutually exclusive partners in the nucleus or in the cytoplasm, causing a simultaneous and codependent shift in the net balance of their nuclear/cytoplasmic retention (Nakamura, 2005).

In summary, this study has analyzed the regulation of a ß-catenin homolog, WRM-1, during a polarized cell division in C. elegans. The findings suggest that WRM-1 is subject to regulation at multiple levels, and begin to place the surprising genetic complexity of P2/EMS signaling into a cell-biological context. Furthermore, the findings suggest that Wnt signaling can control the nuclear accumulation of ß-catenin and may also influence its cortical distribution. These modes of regulation may be of particular importance when Wnt signaling induces a polarized, asymmetric cell division (Nakamura, 2005).

Wingless and parasegmental organization in invertebrates

Spiders belong to the chelicerates, which is a basal arthropod group. To shed more light on the evolution of the segmentation process, orthologs of the Drosophila segment polarity genes engrailed, wingless/Wnt and cubitus interruptus have been recovered from the spider Cupiennius salei. The spider has two engrailed genes. The expression of Cs-engrailed-1 is reminiscent of engrailed expression in insects and crustaceans, suggesting that this gene is regulated in a similar way. This is different for the second spider engrailed gene, Cs-engrailed-2, which is expressed at the posterior cap of the embryo from which stripes split off, suggesting a different mode of regulation. Nevertheless, the Cs-engrailed-2 stripes eventually define the same border as the Cs-engrailed-1 stripes. The spider wingless/Wnt genes are expressed in different patterns from their orthologs in insects and crustaceans. The Cs-wingless gene is expressed in iterated stripes just anterior to the engrailed stripes, but is not expressed in the most ventral region of the germ band. However, Cs-Wnt5-1 appears to act in this ventral region. Cs-wingless and Cs-Wnt5-1 together seem to perform the role of insect wingless. Although there are differences, the wingless/Wnt-expressing cells and en-expressing cells seem to define an important boundary that is conserved among arthropods. This boundary may match the parasegmental compartment boundary and is even visible morphologically in the spider embryo. An additional piece of evidence for a parasegmental organization comes from the expression domains of the Hox genes that are confined to the boundaries, as molecularly defined by the engrailed and wingless/Wnt genes. Parasegments, therefore, are presumably important functional units and conserved entities in arthropod development and form an ancestral character of arthropods. The lack of engrailed and wingless/Wnt-defined boundaries in other segmented phyla does not support a common origin of segmentation (Damen, 2002).

There is an ongoing discussion of whether segmentation in different phyla has a common origin. The presumably conserved segment-polarity network and the organization into parasegments can be seen as an ancestral character for arthropods. In the closely related onychophorans, engrailed expression points to a comparable organization. However, segment polarity gene orthologs are apparently not involved in body segmentation in other segmented phyla. In annelids, engrailed is expressed in segmentally iterated spots in the CNS and in mesodermal cells, but is probably not involved in body segmentation as in arthropods. The establishment of segment polarity in leeches is independent of cell interactions along the anteroposterior axis; this is in contrast to the situation in arthropods, where anterior and posterior fates of the segments are specified by intercellular signaling between wg- and en-expressing cells. Furthermore, there are no indications that the annelid embryo is constructed from units like the parasegment. In the leech, progeny of particular teloblasts overlap with respect to segmental boundaries and do not form genealogical units as in crustaceans. Some key aspects of arthropod segmentation are thus not present in annelids. The segmentation of annelids and arthropods, therefore, seems to be brought about by different mechanisms. This is an important argument against a common origin of segmentation in annelids and arthropods. In chordates it is also doubtful whether engrailed plays a role in somitogenesis. engrailed but not wingless is expressed in reiterated pattern in the somites of the cephalochordate amphioxus, which suggests that the segment polarity gene network as present in arthropods is not conserved. Furthermore, vertebrate engrailed orthologs do not play a role in somite formation or maintenance of the somite boundaries. This points to a different mode of segmentation in vertebrates and arthropods, and does not support a common origin of segmentation. However additional evidence is required to prove this (Damen, 2002).

Segment formation is critical to arthropod development, yet there is still relatively little known about this process in most arthropods. The expression patterns of the genes even-skipped, engrailed, and wingless in a centipede, Lithobius atkinsoni, were examined. Despite some differences when compared with the patterns in insects and crustaceans, the expression of these genes in the centipede suggests that their basic roles are conserved across the mandibulate arthropods. For example, unlike the seven pair-rule stripes of eve expression in the Drosophila embryonic germband, the centipede eve gene is expressed strongly in the posterior of the embryo, and in only a few stripes between newly formed segments. Nonetheless, this pattern likely reflects a conserved role for eve in the process of segment formation, within the different context of a short-germband mode of embryonic development. In the centipede, the genes wingless and engrailed are expressed in stripes along the middle and posterior of each segment, respectively, similar to their expression in Drosophila. The adjacent expression of the engrailed and wingless stripes suggests that the regulatory relationship between the two genes may be conserved in the centipede, and thus this pathway may be a fundamental mechanism of segmental development in most arthropods (Hughes, 2002).

Wnts in other invertebrate species

During Caenorhabditis elegans development, the HSN neurons and the right Q neuroblast and its descendants undergo long-range anteriorly directed migrations. Both of these migrations require EGL-20, a C. elegans Wnt homolog. Through a canonical Wnt signaling pathway, EGL-20/Wnt transcriptionally activates the Hox gene mab-5 in the left Q neuroblast and its descendants, causing the cells to migrate posteriorly. CAM-1, a Ror receptor tyrosine kinase (RTK) family member, inhibits EGL-20 signaling. Excess EGL-20, like loss of cam-1, causes the HSNs to migrate too far anteriorly. Excess CAM-1, like loss of egl-20, shifts the final positions of the HSNs posteriorly and causes the left Q neuroblast descendants to migrate anteriorly. The reversal in the migration of the left Q neuroblast and its descendants results from a failure to express mab-5, an egl-20 mutant phenotype. These data suggest that CAM-1 negatively regulates EGL-20 (Forrester, 2004).

Arthropods are the most diverse and speciose group of organisms on earth. A key feature in their successful radiation is the ease with which various appendages become readily adapted to new functions in novel environments. Arthropod limbs differ radically in form and function, from unbranched walking legs to multi-branched swimming paddles. To uncover the developmental and genetic mechanisms underlying this diversification in form, it was asked whether a three-signal model of limb growth based on Drosophila experiments applies to the development of arthropod limbs with variant shape. A Wnt-1 ortholog (Tlwnt-1) was cloned from Triops longicaudatus, a basal crustacean with a multibranched limb. The mRNA in situ hybridization pattern during larval development was examined to determine whether changes in wg expression are correlated with innovation in limb form. During larval growth and segmentation Tlwnt-1 is expressed in a segmentally reiterated pattern in the trunk. Unexpectedly, this pattern is restricted to the ventral portion of the epidermis. During early limb formation the single continuous stripe of Tlwnt-1 expression in each segment becomes ventrolaterally restricted into a series of shorter stripes. Some but not all of these shorter stripes correspond to what becomes the ventral side of a developing limb branch. It is concluded that the Drosophila model of limb development cannot explain all types of arthropod proximodistal outgrowths, and that the multi-branched limb of Triops develops from an early reorganization of the ventral body wall. In Triops, Tlwnt-1 plays a semiconservative role similar to that played by Drosophila wingless in segmentation and limb formation: morphological innovation in limb form arises in part through an early modulation in the expression of the Tlwnt-1 gene (Nulsen, 1999).

At hatching, all newly formed Triops segments express Tlwnt-1 transcripts in the anterior of each segment and En in a single row of cells in the posterior of each segment. However, Tlwnt-1 transcripts are expressed only ventrally during segmentation, whereas in Drosophila, wg transcripts are detected encircling the embryo, albeit slightly weaker dorsally. The absence of dorsal Tlwnt-1 transcripts was unexpected and suggests that there is a difference in the way in which ventral versus dorsal tissue is patterned in Triops. The phenotype of Drosophila wg mutation has not suggested a difference in wg function dorsally. However, the maintenance of wg activity in dorsal and ventral epidermis requires separate regulatory mechanisms. The lack of Tlwnt-1 transcripts in the dorsal half of each Triops segment is unlikely to be an artifact, because no dorsal staining is seen in both the carapace and the posterior ring. Nor is it proposed that this difference is due to a shift in timing, because this pattern is observed in all newly forming segments as well. Is there any other evidence for the dissociation of dorsal and ventral segmentation in arthropods? Most arthropods bear one pair of limbs per segment, yet it has been observed that the posterior abdominal segments in Triops are variable in this regard. They can bear from one to seven limbs with the number increasing posteriorly. A discrepancy in the number of dorsal and ventral En stripes is observed in these posterior segments, suggesting a mechanistic separation of dorsal from ventral segmentation. Fossil evidence for decoupled dorsal and ventral segmentation has also been reported for euthycarcinoids and Fuxianhuia. However, in all these cases only the posterior-most segments are not congruent in pattern from ventral to dorsal sides, whereas Tlwnt-1 transcripts are never observed dorsally, even during the formation of the more anterior segments (Nulsen, 1999).

Millipedes provide another possible example of dorsal/ventral dissociation, since these animals bear two pairs of limbs ventrally for each dorsal segment. However, in this case the variance between dorsal and ventral segmentation is thought to arise as a later fusion event of two segment primordia dorsally. The lack of Tlwnt-1 expression dorsally suggests that it does not play a role in either the delineation or direction of dorsal cell fates in Triops. The dorsal cells may employ a different development mechanism in order to grow and/or maintain segment polarity. In this regard, it will be interesting to determine whether other Triops wnt family members have dorsal expression patterns (Nulsen, 1999).

Does Tlwnt-1 regulate segment formation? Another interesting feature of the Triops Tlwnt-1 expression pattern is the terminal ring. A terminal ring of wg expression has now been observed in Drosophila, Tribolium and Triops. The Drosophila terminal ring expression has always been attributed to the precursor cells to the proctodeum, which also stain with wg later. However, the proctodeal staining can be clearly distinguished from the terminal ring in Triops. wg is a growth factor known to play a role in proliferation in the notum of the wing and the formation of Malphigian tubules in Drosophila. It is reasonable to expect that it plays a similar role in the proliferation of segments from posterior in Triops. Triops segments become delineated from the posterior of the larva in the region of the posterior ring. Similar to the 'progress zone' model in the chick limb bud, cells may be actively proliferating from the posterior. The posterior Tlwnt-1 expressing cells could be the source of a morphogen necessary for the function of the growth zone. High levels of Tlwnt-1 posteriorly could function either as a signal for mitosis, or as an inhibitor for differentiation. Evidence for the existence of posterior morphogens has also been suggested from experimental manipulations on many insect embryos. Once the segments are found, the more anterior, segmentally reiterated ventral stripes of Tlwnt-1 may then play a role in the maintenance and polarity of each newly formed segment (Nulsen, 1999 and references therein).

Tlwnt-1 expression during 'late' appendage development exhibits parallels to the Drosophila uniramous paradigm. Expression patterns of several genes are conserved in the development of branches of the multibranched limb of Triops and the uniramous limb of Drosophila. In Drosophila limb development, wg is required in an anterior ventral sector of the leg imaginal disc, en in the posterior of the disc and Dll at the center of the disc, where it promotes proximodistal outgrowth. The interaction between these genes has led to a model of limb development in Drosophila termed the uniramous paradigm. During the development of the multibranched limbs of Triops, Tlwnt-1 is restricted to an anteroventral portion of most of the ventral branches: En is posterior and Dll is detected in each of the developing limb branches. Thus, Triops Tlwnt-1, En and Dll expression patterns show striking molecular parallels between the patterning of an individual branch in a multi-branched limb and the patterning of a uniramous Drosophila limb. This suggests that Triops limb branches are patterned individually, each branch consisting of its own set of orthogonal axes and supports the hypothesis that multiple branches in a multibranched limb are patterned as a molecular reiteration of the key elements utilized to pattern a Drosophila limb (Nulsen, 1999 and references therein).

However, drawing a strict parallel is problematic: Tlwnt-1 is not restricted to the anteroventral sector of all limb branches. Notably, in the most dorsal branch, the epipod, Tlwnt-1 transcripts are detected in a one-cell-wide row encircling the entire branch. Another variation in the Tlwnt-1 expression pattern during limb outgrowth occurs in the exopod and gnathobase of the developing trunk limbs. These two branches exhibit Tlwnt-1 transcripts not only in a ventral sector of the branch but also in a dorsal one. A similar expression pattern is observed in the developing mandibles and maxillae. Interestingly, the mandible is branched and the maxilla unbranched, yet both exhibit similar expression patterns. It is clear from these data that the three-way intersection model of limb development generated from Drosophila experiments is not the only mechanism for elaboration of a proximodistal axis. Proximodistal outgrowths can develop in the absence of a restriction of wg to the ventral sector. The variants of Tlwnt-1 expression seen in different limb branches provide molecular evidence that individual multibranched limb branches are not all formed by a simple iterative process. Rather, in this crustacean, individual limb branches have unique characteristics to their patterning, apparent from the earliest stages of development. It appears that each limb modification, whether it be a flattened, lobate dorsal epipod or a chewing appendage such as the mandible, is correlated with a change in Tlwnt-1 expression (Nulsen, 1999 and references therein).

Evidence has been provided that crustacean dorsal epipods and insect wings are homologous structures, based on the expression of the Drosophila wing patterning genes apterous and nubbin in one of the two dorsal epipods of the branchiopod crustacean Artemia franciscana and the malacostracan Pacifastacus leniusculus. The Tlwnt-1 expression pattern in the Triops epipod appears to provide additional support for this hypothesis. The Tlwnt-1 expression in the epipod is similar to the wg expression pattern in the middle stages of Drosophila wing development, where wg transcripts are detected along the entire wing margin. An alternative interpretation, however, would be that this pattern of Tlwnt-1 expression functions to generate a branch shape that is more flattened and lobate and on this basis has been selected for independently in both crustacean and insect lineages. An examination of the wnt expression pattern in the rod-shaped dorsal epipod of the malacostracan crustacean would be informative (Nulsen, 1999 and references therein).

The Tlwnt-1 pattern during 'early' limb development deviates from the uniramous paradigm The most striking deviation from the Drosophila uniramous paradigm concerns the initial specification of the limb primordia from the ventral body wall. In Drosophila the developing limb primordia occupies at most one-fifth of the dorso-ventral extent of the body wall. By contrast, the limb field of the Triops larva consists of nearly the entire ventral body wall. It is interesting to speculate, based on the Wnt and Dll expression patterns, that Triops develops eight separate legs on each hemisegment, which later in development fuse to form one swimming appendage. However, the interpretation that there is one large limb primordia that subsequently divides into eight parts is preferred. Morphologically, the ventral body wall first protrudes as a single epithelial ridge. From this ridge the eight limb branches subsequently protrude. At the first signs of limb differentiation, when the limbs protrude from the ventral body wall, Tlwnt-1 transcripts are still observed in a nearly continuous stripe. P/D elongation of branches occurs before Tlwnt-1 is restricted to the ventral portion of each branch. The disruption of the continuous Tlwnt-1 stripe does not occur until the lobes are clearly distinguishable. Similarly, Dll protein can only be detected in the Triops limb lobes after P/D elongation of the branches. In contrast, in the Drosophila imaginal disc, P/D elongation requires that wg be restricted to a discrete ventral domain of expression before Dll expression proceeds, and this temporal and spatial expression is required for P/D elongation. These data suggest an additional mechanism for the P/D axis formation in Triops, as compared to Drosophila (Nulsen, 1999 and references therein).

The dynamic expression of Tlwnt-1 argues that innovation in limb form found in Triops is likely due to an early change in D/V patterning. The Drosophila paradigm relies on an initial specification of the imaginal primordia via an early interaction between D/V and A/P patterning genes. How does Triops establish multiple branches from one set of A/P and D/V coordinates? It is proposed that limbs with many branches, such as those seen in the trunk swimming appendages of the branchiopod crustacean Triops longicaudatus, result from an increase in the size of the limb primordia allocated during dorsal-ventral axis formation in the earliest stages of development. A change in how the D/V axis is patterned results in the production of a much larger limb primordia. Tlwnt-1 transcripts are subsequently repressed in particular groups of cells, within this enlarged primordia. An expression analysis of conserved D/V patterning genes, as well as the expression of wg and Dll in arthropods with biramous limbs, will help test this hypothesis (Nulsen, 1999).

Members of the Wnt/wingless family of secreted proteins act as short-range inducers and long-range organizers during axis formation, organogenesis and tumorigenesis in many developing tissues. Wnt signaling pathways are conserved in nematodes, insects and vertebrates. Despite its developmental significance, the evolutionary origin of Wnt signaling is unclear. Described here is the molecular characterization of members of the Wnt signaling pathway (Wnt, Dishevelled, GSK3, beta-Catenin and Tcf/Lef) in Hydra, a member of the evolutionarily old metazoan phylum Cnidaria. Wnt and Tcf are expressed in the putative Hydra head organizer, the upper part of the hypostome. Wnt, beta-Catenin and Tcf are transcriptionally upregulated when head organizers are established early in bud formation and head regeneration. Wnt and Tcf expression domains also define head organizers created by de novo pattern formation in aggregates. These results indicate that Wnt signaling may be involved in axis formation in Hydra and support the idea that it was central in the evolution of axial differentiation in early multicellular animals (Hobmayer, 2000).

The current form of a provisional DNA sequence-based regulatory gene network is presented that explains in outline how endomesodermal specification in the sea urchin embryo is controlled. The model of the network is in a continuous process of revision and growth as new genes are added and new experimental results become available; see End-mes: Gene Network Update for the latest version. The network contains over 40 genes at present, many newly uncovered in the course of this work, and most encoding DNA-binding transcriptional regulatory factors. The architecture of the network was approached initially by construction of a logic model that integrated the extensive experimental evidence now available on endomesoderm specification. The internal linkages between genes in the network have been determined functionally, by measurement of the effects of regulatory perturbations on the expression of all relevant genes in the network. Five kinds of perturbation have been applied: (1) use of morpholino antisense oligonucleotides targeted to many of the key regulatory genes in the network; (2) transformation of other regulatory factors into dominant repressors by construction of Engrailed repressor domain fusions; (3) ectopic expression of given regulatory factors, from genetic expression constructs and from injected mRNAs; (4) blockade of the ß-catenin/Tcf pathway by introduction of mRNA encoding the intracellular domain of cadherin, and (5) blockade of the Notch signaling pathway by introduction of mRNA encoding the extracellular domain of the Notch receptor. The network model predicts the cis-regulatory inputs that link each gene into the network. Therefore, its architecture is testable by cis-regulatory analysis. Strongylocentrotus purpuratus and Lytechinus variegatus genomic BAC recombinants that include a large number of the genes in the network have been sequenced and annotated (Davidson, 2002).

Tests of the cis-regulatory predictions of the model are greatly facilitated by interspecific computational sequence comparison, which affords a rapid identification of likely cis-regulatory elements in advance of experimental analysis. The network specifies genomically encoded regulatory processes between early cleavage and gastrula stages. These control the specification of the micromere lineage and of the initial veg2 endomesodermal domain, the blastula-stage separation of the central veg2 mesodermal domain (i.e., the secondary mesenchyme progenitor field) from the peripheral veg2 endodermal domain, the stabilization of specification state within these domains, and activation of some downstream differentiation genes. Each of the temporal-spatial phases of specification is represented in a subelement of the network model that treats regulatory events within the relevant embryonic nuclei at particular stages (Davidson, 2002).

A gene encoding Wnt8 (see Drosophila Wnt8), a ligand that activates the ß-catenin/Tcf system, is expressed in the same prospective endomesodermal cells in which the autonomous maternal system initially causes ß-catenin nuclearization. This observation implies an autoreinforcing Tcf control loop, which is set up within the endomesodermal domain once this is defined. This loop is necessary, for if it is blocked by introduction of a negatively acting form of the Wnt8 ligand, so is endomesoderm specification. The inferred Wnt8 loop conforms to the 'community effect' concept of Gurdon, i.e., a requirement for intercellular signaling within a field of cells in a given state of specification that is necessary for the maintenance and the further developmental progression of that state (Davidson, 2002).

The origin of animal segmentation, the periodic repetition of anatomical structures along the anteroposterior axis, is a long-standing issue that has been recently revived by comparative developmental genetics. In particular, a similar extensive morphological segmentation (or metamerism) is commonly recognized in annelids and arthropods. Mostly based on this supposedly homologous segmentation, these phyla have been united for a long time into the clade Articulata. However, recent phylogenetic analysis has dismissed the Articulata and thus challenged the segmentation homology hypothesis. In Platynereis, engrailed and wingless orthologs are expressed in continuous ectodermal stripes on either side of the segmental boundary before, during, and after its formation; this expression pattern suggests that these genes are involved in segment formation. The striking similarities of engrailed and wingless expressions in Platynereis and arthropods may be due to evolutionary convergence or common heritage. In agreement with similarities in segment ontogeny and morphological organization in arthropods and annelids, these results are interpreted as molecular evidence of a segmented ancestor of protostomes (Prud'homme, 2003).

During posterior growth, both during normal juvenile segment formation and after caudal regeneration, Pdu-en is expressed in ectodermal circular stripes in developing segments. This segmental expression appears in continuous rings of cells immediately after the growth zone has produced them (in younger, posterior-most segments) and persists in differentiating (more anterior) segments. The pattern is more complicated on the ventral face, since, in addition to the continuous segmental expression, Pdu-en is expressed in mesodermal groups of cells and in forming ganglia of the ventral nerve cord. A longitudinal section shows that the segmental stripes of expression occur long before segmental coelomic cavities or segmental boundaries are visible. As segments mature, it becomes apparent that continuous segmental stripes of Pdu-en expression are always restricted to the anterior-most row of epidermal cells within a segment immediately posterior to the forming segmental groove corresponding to the actual segmental boundary. These segmental grooves are the only ones to form and do not seem to shift during segment differentiation, as indicated by the relative position of an appendage marker, distal-less. Hence, this expression pattern suggests that during postlarval growth in Platynereis, engrailed is involved both in the establishment of the segmental boundaries in the ectoderm and in the specification of particular cell types in the mesoderm and the central nervous system (Prud'homme, 2003).

Pdu-wnt1 is also expressed early in ectodermal stripes in each developing segment both during normal juvenile segment formation and after caudal regeneration, although the signal level is much weaker compared to that in Pdu-en. Pdu-wnt1 is expressed in the posterior-most ectodermal cells of each developing trunk segment, immediately anterior to the segmental boundary. In contrast with Pdu-en, the thickness of Pdu-wnt1 stripes increases in proportion with the segment length. Pdu-wnt1 is also expressed in the posterior part and in an anterior-proximal spot of the parapodia, as well as in the proctodaeum (Prud'homme, 2003).

Based on morphological landmarks (i.e., segmental grooves), these results suggest that Pdu-en and Pdu-wnt1 are expressed in adjacent domains on either side of the segmental boundary and play a role in the formation and maintenance of this boundary. According to these observations, Pdu-en and Pdu-wnt1 are most likely expressed in directly neighboring cells. However, due to technical difficulties with double in situ stainings, it has not been been possible to ascertain this point (Prud'homme, 2003).

The primary (animal-vegetal) (AV) and secondary (oral-aboral) (OA) axes of sea urchin embryos are established by distinct regulatory pathways. However, because experimental perturbations of AV patterning also invariably disrupt OA patterning and radialize the embryo, these two axes must be mechanistically linked. This study shows that FoxQ2, which is progressively restricted to the animal plate during cleavage stages, provides this linkage. When AV patterning is prevented by blocking the nuclear function of beta-catenin, the animal plate where FoxQ2 is expressed expands throughout the future ectoderm, and expression of nodal, which initiates OA polarity, is blocked. Surprisingly, nodal transcription and OA differentiation are rescued simply by inhibiting FoxQ2 translation. Therefore, restriction of FoxQ2 to the animal plate is a crucial element of canonical Wnt signaling that coordinates patterning along the AV axis with the initiation of OA specification (Yaguchi, 2008).

Planarians have high regenerative ability, which is dependent on pluripotent adult somatic stem cells called neoblasts. Recently, canonical Wnt/β-catenin signaling was shown to be required for posterior specification, and Hedgehog signaling was shown to control anterior-posterior polarity via activation of the Djwnt1/P-1 gene at the posterior end of planarians. Thus, various signaling molecules play an important role in planarian stem cell regulation. However, the molecular mechanisms directly involved in stem cell differentiation have remained unclear. This study demonstrates that one of the planarian LIM-homeobox genes, Djislet, is required for the differentiation of Djwnt1/P-1-expressing cells from stem cells at the posterior end. RNA interference (RNAi)-treated planarians of Djislet [Djislet(RNAi)] show a tail-less phenotype. Thus, it is speculated that Djislet might be involved in activation of the Wnt signaling pathway in the posterior blastema. When the expression pattern of Djwnt1/P-1 was carefully examined by quantitative real-time PCR during posterior regeneration, two phases of Djwnt1/P-1 expression were found: the first phase was detected in the differentiated cells in the old tissue in the early stage of regeneration and then a second phase was observed in the cells derived from stem cells in the posterior blastema. Interestingly, Djislet is expressed in stem cell-derived DjPiwiA- and Djwnt1/P-1-expressing cells, and Djislet(RNAi) only perturbed the second phase. Thus, it is proposed that Djislet might act to trigger the differentiation of cells expressing Djwnt1/P-1 from stem cells (Hayashi, 2011).

Planarian yorkie/YAP functions to integrate adult stem cell proliferation, organ homeostasis and maintenance of axial patterning

During adult homeostasis and regeneration, the freshwater planarian must accomplish a constant balance between cell proliferation and cell death, while also maintaining proper tissue and organ size and patterning. How these ordered processes are precisely modulated remains relatively unknown. This study shows that planarians use the downstream effector of the Hippo signaling cascade, yorkie (yki; YAP in vertebrates) to control a diverse set of pleiotropic processes in organ homeostasis, stem cell regulation, regeneration and axial patterning. yki functions to maintain the homeostasis of the planarian excretory (protonephridial) system and to limit stem cell proliferation, but does not affect the differentiation process or cell death. Finally, Yki acts synergistically with WNT/beta-catenin signaling to repress head determination by limiting the expression domains of posterior WNT genes and that of the WNT-inhibitor notum. Together, these data show that yki is a key gene in planarians that integrates stem cell proliferation control, organ homeostasis, and the spatial patterning of tissues (Lin, 2014).

Wnt genes in amphioxus and the evolution of somitogenesis

The amphioxus tail bud is similar to the amphibian tail bud in having an epithelial organization without a mesenchymal component. Three amphioxus Wnt genes (AmphiWnt3, AmphiWnt5, and AmphiWnt6) have been characterized; their early expression around the blastopore can subsequently be traced into the tail bud. In vertebrate embryos, there is a similar progression of expression domains for Wnt3, Wnt5, and Wnt6 genes from the blastopore lip (or its equivalent) to the tail bud. In amphioxus, AmphiWnt3, AmphiWnt5, and AmphiWnt6 are each expressed in a specific subregion of the tail bud, tentatively suggesting that a combinatorial code of developmental gene expression may help generate specific tissues during posterior elongation and somitogenesis. In spite of similarities within their tail buds, vertebrate and amphioxus embryos differ markedly in the relation between the tail bud and the nascent somites: vertebrates have a relatively extensive zone of unsegmented mesenchyme (i.e., presomitic mesoderm) intervening between the tail bud and the forming somites, whereas the amphioxus tail bud gives rise to new somites directly. It is likely that presomitic mesoderm is a vertebrate innovation made possible by developmental interconversions between epithelium and mesenchyme that first became prominent at the dawn of vertebrate evolution (Schubert, 2001).

Wnt-Ror signaling to SIA and SIB neurons directs anterior axon guidance and nerve ring placement in C. elegans

Wnt signaling through Frizzled proteins guides posterior cells and axons in C. elegans into different spatial domains. This study demonstrates an essential role for Wnt signaling through Ror tyrosine kinase homologs in the most prominent anterior neuropil, the nerve ring. A genetic screen uncovered cwn-2, the C. elegans homolog of Wnt5, as a regulator of nerve ring placement. In cwn-2 mutants, all neuronal structures in and around the nerve ring are shifted to an abnormal anterior position. cwn-2 is required at the time of nerve ring formation; it is expressed by cells posterior of the nerve ring, but its precise site of expression is not critical for its function. In nerve ring development, cwn-2 acts primarily through the Wnt receptor CAM-1 (Ror), together with the Frizzled protein MIG-1, with parallel roles for the Frizzled protein CFZ-2. The identification of CAM-1 as a CWN-2 receptor contrasts with CAM-1 action as a non-receptor in other C. elegans Wnt pathways. Cell-specific rescue of cam-1 and cell ablation experiments reveal a crucial role for the SIA and SIB neurons in positioning the nerve ring, linking Wnt signaling to specific cells that organize the anterior nervous system (Kemmerdell, 2009).

CWN-2 has an essential role in nerve ring placement. The results suggest that CWN-2 is a ligand for the CAM-1 (Ror) receptor in the SIA and SIB neurons, perhaps with MIG-1 (Frizzled) as a co-receptor. In the absence of this signaling pathway, many axons and cell bodies in the nerve ring are displaced towards the anterior. The similar effects of Wnt pathway mutations and genetic ablations suggest that SIA and SIB neurons direct normal nerve ring placement. Additional nerve ring guidance genes that act at least partly parallel to cwn-2, cam-1 and mig-1 are the Frizzled gene cfz-2, the Wnt gene cwn-1, and the Robo gene sax-3 (Kemmerdell, 2009).

cwn-2 is required at a discrete time in development, but the site of cwn-2 expression is relatively unimportant. The rescue of cwn-2 mutants by uniform expression or misexpression echoes the rescue of egl-20 and lin-44 Wnt defects by cDNAs expressed from heat-shock promoters, and suggests that C. elegans Wnts can sometimes function as non-spatial cues. For example, CWN-2 could stimulate axon outgrowth of SIA and SIB at a particular time, with spatial information provided by the distribution of receptors or by other guidance cues near the nerve ring, such as UNC-6 and SLT-1. Alternatively, cwn-2 activity could be spatially limited by cell-specific post-translational pathways or by extracellular Wnt-binding proteins. Finally, additional Wnts, such as CWN-1, might contribute spatial information when CWN-2 is misexpressed: disrupting cwn-2 alone may not eliminate the overall posteriorly biased pattern of Wnt expression. Indeed, in the posterior body, overlapping functions of lin-44, egl-20 and cwn-1 can mask the effects of misexpressing a single Wnt (Kemmerdell, 2009).

CAM-1 has been proposed to act as an extracellular inhibitor of Wnts owing to its non-cell-autonomous action in vulval development and the apparent dispensability of its intracellular domain. However, the CAM-1-related protein Ror2 is an established tyrosine kinase receptor for mammalian Wnts, although kinase-independent functions are also known for vertebrate Rors. Nerve ring development initially appeared not to require the intracellular domain of CAM-1, but many double mutants that included the Frizzleds mig-1, cfz-2 and lin-17 and the LRP-like mig-13 uncovered a requirement for the intracellular domain. Together with a specific requirement for cam-1 expression in the SIA and SIB neurons, these results support a receptor function of CAM-1 in nerve ring development. The overlapping expression and rescue of cam-1 and mig-1 in SIA and SIB matches the genetic results suggesting that they act together in a common process, perhaps as co-receptors for CWN-2. In mammalian osteocytes and lung epithelial cells, Frizzled and Ror or Ryk receptors can function together in a signaling complex. The relevant cellular sites of action for cfz-2, lin-17 and mig-13 are unknown, and expression of cfz-2 in SIA and SIB neurons did not rescue cfz-2 mutants, suggesting that cfz-2 has primary functions outside of SIA and SIB. It is too early to determine whether CFZ-2, LIN-17 and MIG-13 might also be CAM-1 co-receptors (Kemmerdell, 2009).

One interesting implication of the use of multiple Wnt receptors is that spatially and temporally restricted receptor expression might be as important in development as restricted ligand expression. Rather than responding passively to an instructive Wnt cue, developing neurons can shape their response to Wnts through their receptor complement. They can also shape the response of more-distant cells by capturing Wnt ligands, as shown for CAM-1 near the vulva (Kemmerdell, 2009).

Cell-type-specific rescue of cam-1, mig-1 and sax-3 and cell ablation experiments revealed an important role for SIA and SIB neurons in nerve ring placement. Several models could explain cwn-2 effects on SIA and SIB. First, cwn-2 could act in a traditional Wnt patterning role to determine SIA and SIB cell fates; SIA and SIB would then organize nerve ring development through other molecular pathways. However, several SIA and SIB markers are expressed normally in cwn-2 mutants, arguing against a cell fate change (Kemmerdell, 2009).

The model that cwn-2 directly affects axon guidance of SIA and SIB neurons, which in turn instruct the positioning of the nerve ring. SIA and SIB neurons occupy a position near the base of the nerve ring, where they might detect CWN-2, as well as the ventral attractant UNC-6 and the anterior repellent SLT-1. In wild-type animals, the nerve ring axon trajectories of SIA and SIB neurons are unusually complex, consistent with a special patterning role. In cwn-2 and cam-1 mutants, the disruption of axon trajectories in SIA and SIB neurons is more complicated than in other cell types: SIA and SIB have guidance defects at many positions, whereas other neurons simply move to an anterior location. It is suggested that the guidance of SIA and SIB neurons is under the direct control of CWN-2, which generates a temporally precise and spatially less precise signal to form a nerve ring at the correct location. Other nerve ring neurons follow SIA and SIB neurons to this location if possible; if SIA and SIB neurons are misguided or absent, the nerve ring shifts to a more anterior position that might be a default position, or one specified by another guidance cue (Kemmerdell, 2009).

Table of contents

wingless continued: Biological Overview | Transcriptional regulation |Targets of Activity | Protein Interactions | mRNA Transport | Developmental Biology | Effects of Mutation | References

Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.

The Interactive Fly resides on the
Society for Developmental Biology's Web server.