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Ocular coloboma (OC) is a congenital disorder caused by the incomplete closure of the embryonic ocular fissure. OC can present as a simple anomaly or, in more complex forms, be associated with additional ocular abnormalities. It can occur in isolation or as part of a broader syndrome, exhibiting considerable genetic heterogeneity. Diagnostic yield for OC remains below 30%, indicating the need for further genetic exploration. Mutations in the Wnt receptor FZD5, which is expressed throughout eye development, have been linked to both isolated and complex forms of coloboma. These mutations often result in a dominant-negative effect, where the mutated FZD5 protein disrupts WNT signaling by sequestering WNT ligands. Here, we describe a case of syndromic bilateral OC with additional features such as microcornea, bone developmental anomalies, and mild intellectual disability. Whole exome sequencing revealed a homozygous rare missense variant in FZD5. Consistent with a loss-of-function effect, overexpressing of fzd5 mRNA harboring the missense variant in zebrafish embryos does not influence embryonic development, whereas overexpression of wild-type fzd5 mRNA results in body axis duplications. However, in vitro TOPFlash assays revealed that the missense variant only caused partial loss-of-function, behaving as a hypomorphic mutation. We further showed that the mutant protein still localized to the cell membrane and maintained proper conformation when modeled in silico, suggesting that the impairment lies in signal transduction. This hypothesis is further supported by the fact that the variant affects a highly conserved amino acid known to be crucial for protein-protein interactions., Competing Interests: Declarations Competing interests The authors declare no competing interests. Rights retention statement This research was funded in part, by the Wellcome Trust [Seed Award in Sciences 213928/Z/18/Z to FC]. For the purpose of open access, the authors have applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission., (© 2024. The Author(s).)

Neurons on the left and right sides of the nervous system often show asymmetric properties, but how such differences arise is poorly understood. Genetic screening in zebrafish revealed that loss of function of the transmembrane protein Cachd1 resulted in right-sided habenula neurons adopting left-sided identity. Cachd1 is expressed in neuronal progenitors, functions downstream of asymmetric environmental signals, and influences timing of the normally asymmetric patterns of neurogenesis. Biochemical and structural analyses demonstrated that Cachd1 can bind simultaneously to Lrp6 and Frizzled family Wnt co-receptors. Consistent with this, lrp6 mutant zebrafish lose asymmetry in the habenulae, and epistasis experiments support a role for Cachd1 in modulating Wnt pathway activity in the brain. These studies identify Cachd1 as a conserved Wnt receptor-interacting protein that regulates lateralized neuronal identity in the zebrafish brain.

Asymmetries are essential for proper organization and function of organ systems. Genetic studies in bilaterians have shown signaling through the Nodal/Smad2 pathway plays a key, conserved role in the establishment of body asymmetries. Although the main molecular players in the network for the establishment of left-right asymmetry (LRA) have been deeply described in deuterostomes, little is known about the regulation of Nodal signaling in spiralians. Here, we identified orthologs of the egf-cfc gene, a master regulator of the Nodal pathway in vertebrates, in several invertebrate species, which includes the first evidence of its presence in non-deuterostomes. Our functional experiments indicate that despite being present, egf-cfc does not play a role in the establishment of LRA in gastropods. However, experiments in zebrafish suggest that a single amino acid mutation in the egf-cfc gene in at least the common ancestor of chordates was the necessary step to induce a gain of function in LRA regulation. This study shows that the egf-cfc gene likely appeared in the ancestors of deuterostomes and "protostomes", before being adopted as a mechanism to regulate the Nodal pathway and the establishment of LRA in some lineages of deuterostomes., (© The Author(s) 2022. Published by Oxford University Press on behalf of Society for Molecular Biology and Evolution.)

Appropriate patterning of the retina during embryonic development is assumed to underlie the establishment of spatially localised specialisations that mediate the perception of specific visual features. For example, in zebrafish, an area involved in high acuity vision (HAA) is thought to be present in the ventro-temporal retina. Here, we show that the interplay of the transcription factor Rx3 with Fibroblast Growth Factor and Hedgehog signals initiates and restricts foxd1 expression to the prospective temporal retina, initiating naso-temporal regionalisation of the retina. Abrogation of Foxd1 results in the loss of temporal and expansion of nasal retinal character, and consequent absence of the HAA. These structural defects correlate with severe visual defects, as assessed in optokinetic and optomotor response assays. In contrast, optokinetic responses are unaffected in the opposite condition, in which nasal retinal character is lost at the expense of expanded temporal character. Our study indicates that the establishment of temporal retinal character during early retinal development is required for the specification of the HAA, and suggests a prominent role of the temporal retina in controlling specific visual functions., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2022. Published by The Company of Biologists Ltd.)

The vertebrate eye primordium consists of a pseudostratified neuroepithelium, the optic vesicle (OV), in which cells acquire neural retina or retinal pigment epithelium (RPE) fates. As these fates arise, the OV assumes a cup shape, influenced by mechanical forces generated within the neural retina. Whether the RPE passively adapts to retinal changes or actively contributes to OV morphogenesis remains unexplored. We generated a zebrafish Tg(E1- bhlhe40 :GFP) line to track RPE morphogenesis and interrogate its participation in OV folding. We show that, in virtual absence of proliferation, RPE cells stretch and flatten, thereby matching the retinal curvature and promoting OV folding. Localized interference with the RPE cytoskeleton disrupts tissue stretching and OV folding. Thus, extreme RPE flattening and accelerated differentiation are efficient solutions adopted by fast-developing species to enable timely optic cup formation. This mechanism differs in amniotes, in which proliferation drives RPE expansion with a much-reduced need of cell flattening., Competing Interests: TM, ML, NT, MM, MC, FC, PB No competing interests declared, (© 2021, Moreno-Mármol et al.)

Efficient and accurate DNA replication is particularly critical in stem and progenitor cells for successful proliferation and survival. The replisome, an amalgam of protein complexes, is responsible for binding potential origins of replication, unwinding the double helix, and then synthesizing complimentary strands of DNA. According to current models, the initial steps of DNA unwinding and opening are facilitated by the CMG complex, which is composed of a GINS heterotetramer that connects Cdc45 with the mini-chromosome maintenance (Mcm) helicase. In this work, we provide evidence that in the absence of GINS function DNA replication is cell autonomously impaired, and we also show that gins1 and gins2 mutants exhibit elevated levels of apoptosis restricted to actively proliferating regions of the central nervous system (CNS). Intriguingly, our results also suggest that the rapid cell cycles during early embryonic development in zebrafish may not require the function of the canonical GINS complex as neither zygotic Gins1 nor Gins2 isoforms seem to be present during these stages., (Copyright © 2020 Varga, Csályi, Bertyák, Menyhárd, Poole, Cerveny, Kövesdi, Barátki, Rouse, Vad, Hawkins, Stickney, Cavodeassi, Schwarz, Young and Wilson.)

In this brief commentary, we provide some of our thoughts and opinions on the current and future use of zebrafish to model human eye disease, dissect pathological progression and advance in our understanding of the genetic bases of microphthalmia, andophthalmia and coloboma (MAC) in humans. We provide some background on eye formation in fish and conservation and divergence across vertebrates in this process, discuss different approaches for manipulating gene function and speculate on future research areas where we think research using fish may prove to be particularly effective.

Mutations in effectors of the hedgehog signaling pathway are responsible for a wide variety of ocular developmental anomalies. These range from massive malformations of the brain and ocular primordia, not always compatible with postnatal life, to subtle but damaging functional effects on specific eye components. This review will concentrate on the effects and effectors of the major vertebrate hedgehog ligand for eye and brain formation, Sonic hedgehog (SHH), in tissues that constitute the eye directly and also in those tissues that exert indirect influence on eye formation. After a brief overview of human eye development, the many roles of the SHH signaling pathway during both early and later morphogenetic processes in the brain and then eye and periocular primordia will be evoked. Some of the unique molecular biology of this pathway in vertebrates, particularly ciliary signal transduction, will also be broached within this developmental cellular context.

The vertebrate eye originates from the eye field, a domain of cells specified by a small number of transcription factors. In this study, we show that Tcf7l1a is one such transcription factor that acts cell-autonomously to specify the eye field in zebrafish. Despite the much-reduced eye field in tcf7l1a mutants, these fish develop normal eyes revealing a striking ability of the eye to recover from a severe early phenotype. This robustness is not mediated through genetic compensation at neural plate stage; instead, the smaller optic vesicle of tcf7l1a mutants shows delayed neurogenesis and continues to grow until it achieves approximately normal size. Although the developing eye is robust to the lack of Tcf7l1a function, it is sensitised to the effects of additional mutations. In support of this, a forward genetic screen identified mutations in hesx1 , cct5 and gdf6a , which give synthetically enhanced eye specification or growth phenotypes when in combination with the tcf7l1a mutation., Competing Interests: RY, TH, FC, HS, QS, LL, CW, BC, JL, EA, AK, IS, JR, CT, IB, MA, EB, GG, SW No competing interests declared, (© 2019, Young et al.)

The neural component of the zebrafish eye derives from a small group of cells known as the eye/retinal field. These cells, positioned in the anterior neural plate, rearrange extensively and generate the optic vesicles (OVs). Each vesicle subsequently folds over itself to form the double-layered optic cup, from which the mature eye derives. During this transition, cells of the OV are progressively specified toward three different fates: the retinal pigment epithelium (RPE), the neural retina, and the optic stalk. Recent studies have shown that folding of the zebrafish OV into a cup is in part driven by basal constriction of the cells of the future neural retina. During folding, however, RPE cells undergo an even more dramatic shape conversion that seems to entail the acquisition of unique properties. How these changes occur and whether they contribute to optic cup formation is still poorly understood. Here we will review present knowledge on RPE morphogenesis and discuss potential mechanisms that may explain such transformation using examples taken from embryonic Drosophila tissues that undergo similar shape changes. We will also put forward a hypothesis for optic cup folding that considers an active contribution from the RPE.

The original version of this Article contained an error in ref. 39, which incorrectly cited 'Fristrom, D. & Fristrom, J. W. in The Development of Drosophila melanogaster (eds. Bate, M. & Martinez-Arias, A.) II, (Cold spring harbor laboratory press, 1993)'. The correct reference is 'Condic, M.L, Fristrom, D. & Fristrom, J.W. Apical cell shape changes during Drosophila imaginal leg disc elongation: a novel morphogenetic mechanism. Development 111: 23-33 (1991)'. Furthermore, the last sentence of the fourth paragraph of the introduction incorrectly omitted citation of work by Rupprecht et al. The correct citation is given below. These errors have now been corrected in both the PDF and HTML versions of the Article. Rupprecht, J.F., Ong, K.H., Yin, J., Huang, A., Dinh, H.H., Singh, A.P., Zhang, S., Yu, W. & Saunders, T.E. Geometric constraints alter cell arrangements within curved epithelial tissues. Mol. Biol. Cell 28, 3582-3594 (2017).

As animals develop, tissue bending contributes to shape the organs into complex three-dimensional structures. However, the architecture and packing of curved epithelia remains largely unknown. Here we show by means of mathematical modelling that cells in bent epithelia can undergo intercalations along the apico-basal axis. This phenomenon forces cells to have different neighbours in their basal and apical surfaces. As a consequence, epithelial cells adopt a novel shape that we term "scutoid". The detailed analysis of diverse tissues confirms that generation of apico-basal intercalations between cells is a common feature during morphogenesis. Using biophysical arguments, we propose that scutoids make possible the minimization of the tissue energy and stabilize three-dimensional packing. Hence, we conclude that scutoids are one of nature's solutions to achieve epithelial bending. Our findings pave the way to understand the three-dimensional organization of epithelial organs.

Over the last thirty years, fish models, such as the zebrafish and medaka, have become essential to pursue developmental studies and model human disease. Community efforts have led to the generation of wide collections of mutants, a complete sequence of their genomes, and the development of sophisticated genetic tools, enabling the manipulation of gene activity and labelling and tracking of specific groups of cells during embryonic development. When combined with the accessibility and optical clarity of fish embryos, these approaches have made of them an unbeatable model to monitor developmental processes in vivo and in real time. Over the last few years, live-imaging studies in fish have provided fascinating insights into tissue morphogenesis and organogenesis. This review will illustrate the advantages of fish models to pursue morphogenetic studies by highlighting the findings that, in the last decade, have transformed our understanding of eye morphogenesis., Competing Interests: The author declares no conflict of interest.

The molecular bases of vertebrate eye formation have been extensively investigated during the past 20 years. This has resulted in the definition of the backbone of the gene regulatory networks controlling the different steps of eye development and has further highlighted a substantial conservation of these networks among vertebrates. Yet, the precise morphogenetic events allowing the formation of the optic cup from a small group of cells within the anterior neural plate are still poorly understood. It is also unclear if the morphogenetic events leading to eyes of very similar shape are indeed comparable among all vertebrates or if there are any species-specific peculiarities. Improved imaging techniques have enabled to follow how the eye forms in living embryos of a few vertebrate models, whereas the development of organoid cultures has provided fascinating tools to recapitulate tissue morphogenesis of other less accessible species. Here, we will discuss what these advances have taught us about eye morphogenesis, underscoring possible similarities and differences among vertebrates. We will also discuss the contribution of cell shape changes to this process and how morphogenetic and patterning mechanisms integrate to assemble the final architecture of the eye.

Maintaining neurogenesis in growing tissues requires a tight balance between progenitor cell proliferation and differentiation. In the zebrafish retina, neuronal differentiation proceeds in two stages with embryonic retinal progenitor cells (RPCs) of the central retina accounting for the first rounds of differentiation, and stem cells from the ciliary marginal zone (CMZ) being responsible for late neurogenesis and growth of the eye. In this study, we analyse two mutants with small eyes that display defects during both early and late phases of retinal neurogenesis. These mutants carry lesions in gdf6a, a gene encoding a BMP family member previously implicated in dorsoventral patterning of the eye. We show that gdf6a mutant eyes exhibit expanded retinoic acid (RA) signalling and demonstrate that exogenous activation of this pathway in wild-type eyes inhibits retinal growth, generating small eyes with a reduced CMZ and fewer proliferating progenitors, similar to gdf6a mutants. We provide evidence that RA regulates the timing of RPC differentiation by promoting cell cycle exit. Furthermore, reducing RA signalling in gdf6a mutants re-establishes appropriate timing of embryonic retinal neurogenesis and restores putative stem and progenitor cell populations in the CMZ. Together, our results support a model in which dorsally expressed gdf6a limits RA pathway activity to control the transition from proliferation to differentiation in the growing eye., (© 2016. Published by The Company of Biologists Ltd.)

Background: Although left-right asymmetries are common features of nervous systems, their developmental bases are largely unknown. In the zebrafish epithalamus, dorsal habenular neurons adopt medial (dHbm) and lateral (dHbl) subnuclear character at very different frequencies on the left and right sides. The left-sided parapineal promotes the elaboration of dHbl character in the left habenula, albeit by an unknown mechanism. Likewise, the genetic pathways acting within habenular neurons to control their asymmetric differentiated character are unknown., Results: In a forward genetic screen for mutations that result in loss of habenular asymmetry, we identified two mutant alleles of tcf7l2, a gene that encodes a transcriptional regulator of Wnt signaling. In tcf7l2 mutants, most neurons on both sides differentiate with dHbl identity. Consequently, the habenulae develop symmetrically, with both sides adopting a pronounced leftward character. Tcf7l2 acts cell automously in nascent equipotential neurons, and on the right side, it promotes dHbm and suppresses dHbl differentiation. On the left, the parapineal prevents this Tcf7l2-dependent process, thereby promoting dHbl differentiation., Conclusions: Tcf7l2 is essential for lateralized fate selection by habenular neurons that can differentiate along two alternative pathways, thereby leading to major neural circuit asymmetries., (Copyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.)

Wnts are essential for a multitude of processes during embryonic development and adult homeostasis. The molecular structure of the Wnt pathway is extremely complex, and it keeps growing as new molecular components and novel interactions are uncovered. Recent studies have advanced our understanding on how the diverse molecular outcomes of the Wnt pathway are integrated during organ development, an integration that is also essential, although mechanistically poorly understood, during the formation of the anterior part of the nervous system, the forebrain. In this article, the author has summarized these findings and discussed their implications for forebrain development. A special emphasis has been put forth on studies performed in the zebrafish as this model system has been instrumental for our current understanding of forebrain patterning., (© 2013 Wiley Periodicals, Inc.)

Using high-resolution live imaging in zebrafish, we show that presumptive eye cells acquire apicobasal polarity and adopt neuroepithelial character prior to other regions of the neural plate. Neuroepithelial organization is first apparent at the margin of the eye field, whereas cells at its core have mesenchymal morphology. These core cells subsequently intercalate between the marginal cells contributing to the bilateral expansion of the optic vesicles. During later evagination, optic vesicle cells shorten, drawing their apical surfaces laterally relative to the basal lamina, resulting in further laterally directed evagination. The early neuroepithelial organization of the eye field requires Laminin1, and ectopic Laminin1 can redirect the apicobasal orientation of eye field cells. Furthermore, disrupting cell polarity through combined abrogation of the polarity protein Pard6γb and Laminin1 severely compromises optic vesicle evagination. Our studies elucidate the cellular events underlying early eye morphogenesis and provide a framework for understanding epithelialization and complex tissue formation., (Copyright © 2013 The Authors. Published by Elsevier Inc. All rights reserved.)

During forebrain morphogenesis, there is extensive reorganisation of the cells destined to form the eyes, telencephalon and diencephalon. Little is known about the molecular mechanisms that regulate region-specific behaviours and that maintain the coherence of cell populations undergoing specific morphogenetic processes. In this study, we show that the activity of the Eph/Ephrin signalling pathway maintains segregation between the prospective eyes and adjacent regions of the anterior neural plate during the early stages of forebrain morphogenesis in zebrafish. Several Ephrins and Ephs are expressed in complementary domains in the prospective forebrain and combinatorial abrogation of their activity results in incomplete segregation of the eyes and telencephalon and in defective evagination of the optic vesicles. Conversely, expression of exogenous Ephs or Ephrins in regions of the prospective forebrain where they are not usually expressed changes the adhesion properties of the cells, resulting in segregation to the wrong domain without changing their regional fate. The failure of eye morphogenesis in rx3 mutants is accompanied by a loss of complementary expression of Ephs and Ephrins, suggesting that this pathway is activated downstream of the regional fate specification machinery to establish boundaries between domains undergoing different programmes of morphogenesis.

ER-to-Golgi transport of proteins destined for the extracellular space or intracellular compartments depends on the COPII vesicle coat and is constitutive in all translationally active cells. Nevertheless, there is emerging evidence that this process is regulated on a cell- and tissue-specific basis, which means that components of the COPII coat will be of differential importance to certain cell types. The COPII coat consists of an inner layer, Sec23/24 and an outer shell, Sec13/31. We have shown previously that knock-down of Sec13 results in concomitant loss of Sec31. In zebrafish and cultured human cells this leads to impaired trafficking of large cargo, namely procollagens, and is causative for defects in craniofacial and gut development. It is now widely accepted that the outer COPII coat is key to the architecture and stability of ER export vesicles containing large, unusual cargo proteins. Here, we investigate zebrafish eye development following Sec13 depletion. We find that photoreceptors degenerate or fail to develop from the onset. Impaired collagen trafficking from the retinal pigment epithelium and defects in overall retinal lamination also seen in Sec13-depleted zebrafish might have been caused by increased apoptosis and reduced topical proliferation in the retina. Our data show that the outer layer of the COPII coat is also necessary for the transport of large amounts of cargo proteins, in this case rhodopsin, rather than just large cargo as previously thought.

Our knowledge of the general mechanisms controlling the formation of the vertebrate central nervous system has advanced tremendously in the last decade. Here, we discuss the impact of the combined use of cell manipulation, in vivo imaging and genetics in the zebrafish on recent progress in understanding how signaling processes progressively control regionalization of the central nervous system. We highlight the unresolved issues and speculate upon the fundamental role the zebrafish will continue having in answering them.

During tissue morphogenesis and differentiation, cells must self-renew while contemporaneously generating daughters that contribute to the growing tissue. How tissues achieve this precise balance between proliferation and differentiation is, in most instances, poorly understood. This is in part due to the difficulties in dissociating the mechanisms that underlie tissue patterning from those that regulate proliferation. In the migrating posterior lateral line primordium (PLLP), proliferation is predominantly localised to the leading zone. As cells emerge from this zone, they periodically organise into rosettes that subsequently dissociate from the primordium and differentiate as neuromasts. Despite this reiterative loss of cells, the primordium maintains its size through regenerative cell proliferation until it reaches the tail. In this study, we identify a null mutation in the Wnt-pathway transcription factor Lef1 and show that its activity is required to maintain proliferation in the progenitor pool of cells that sustains the PLLP as it undergoes migration, morphogenesis and differentiation. In absence of Lef1, the leading zone becomes depleted of cells during its migration leading to the collapse of the primordium into a couple of terminal neuromasts. We show that this behaviour resembles the process by which the PLLP normally ends its migration, suggesting that suppression of Wnt signalling is required for termination of neuromast production in the tail. Our data support a model in which Lef1 sustains proliferation of leading zone progenitors, maintaining the primordium size and defining neuromast deposition rate.

It is currently unclear how intrinsic and extrinsic mechanisms cooperate to control the progression from self-renewing to neurogenic divisions in retinal precursor cells. Here, we use the zebrafish flotte lotte (flo) mutant, which carries a mutation in the elys (ahctf1) gene, to study the relationship between cell cycle progression and neuronal differentiation by investigating how proliferating progenitor cells transition towards differentiation in a retinal stem cell niche termed the ciliary marginal zone (CMZ). In zebrafish embryos without Elys, CMZ cells retain the capacity to proliferate but lose the ability to enter their final neurogenic divisions to differentiate as neurons. However, mosaic retinae composed of wild-type and flo cells show that despite inherent cell cycle defects, flo mutant cells progress from proliferation to differentiation when in the vicinity of wild-type retinal neurons. We propose that the differentiated retinal environment limits the proliferation of precursors emerging from the CMZ in a manner that explains the spatial organisation of cells in the CMZ and ensures that proliferative retinal progenitors are driven towards differentiation.

During embryonic development, pattern formation must be tightly synchronized with tissue morphogenesis to coordinate the establishment of the spatial identities of cells with their movements. In the vertebrate retina, patterning along the dorsal-ventral and nasal-temporal (anterior-posterior) axes is required for correct spatial representation in the retinotectal map. However, it is unknown how specification of axial cell positions in the retina occurs during the complex process of early eye morphogenesis. Studying zebrafish embryos, we show that morphogenetic tissue rearrangements during eye evagination result in progenitor cells in the nasal half of the retina primordium being brought into proximity to the sources of three fibroblast growth factors, Fgf8/3/24, outside the eye. Triple-mutant analysis shows that this combined Fgf signal fully controls nasal retina identity by regulating the nasal transcription factor Foxg1. Surprisingly, nasal-temporal axis specification occurs very early along the dorsal-ventral axis of the evaginating eye. By in vivo imaging GFP-tagged retinal progenitor cells, we find that subsequent eye morphogenesis requires gradual tissue compaction in the nasal half and directed cell movements into the temporal half of the retina. Balancing these processes drives the progressive alignment of the nasal-temporal retina axis with the anterior-posterior body axis and is controlled by a feed-forward effect of Fgf signaling on Foxg1-mediated cell cohesion. Thus, the mechanistic coupling and dynamic synchronization of tissue patterning with morphogenetic cell behavior through Fgf signaling leads to the graded allocation of cell positional identity in the eye, underlying retinotectal map formation., Competing Interests: The authors have declared that no competing interests exist.

Background: Key molecules involved in notochord differentiation and function have been identified through genetic analysis in zebrafish and mice, but MEK1 and 2 have so far not been implicated in this process due to early lethality (Mek1-/-) and functional redundancy (Mek2-/-) in the knockout animals., Results: Here, we reveal a potential role for Mek1/2 during notochord development by using the small molecule Mek1/2 inhibitor U0126 which blocks phosphorylation of the Mek1/2 target gene Erk1/2 in vivo. Applying the inhibitor from early gastrulation until the 18-somite stage produces a specific and consistent phenotype with lack of dark pigmentation, shorter tail and an abnormal, undulated notochord. Using morphological analysis, in situ hybridization, immunhistochemistry, TUNEL staining and electron microscopy, we demonstrate that in treated embryos the chordamesoderm to notochord transition is disrupted and identify disorganization in the medial layer of the perinotochordal basement mebrane as the probable cause of the undulations and bulges in the notochord. We also examined and excluded FGF as the upstream signal during this process., Conclusion: Using the small chemical U0126, we have established a novel link between MAPK-signaling and notochord differentiation. Our phenotypic analysis suggests a potential connection between the MAPK-pathway, the COPI-mediated intracellular transport and/or the copper-dependent posttranslational regulatory processes during notochord differentiation.

During regional patterning of the anterior neural plate, a medially positioned domain of cells is specified to adopt retinal identity. These eye field cells remain coherent as they undergo morphogenetic events distinct from other prospective forebrain domains. We show that two branches of the Wnt signaling pathway coordinate cell fate determination with cell behavior during eye field formation. Wnt/beta-catenin signaling antagonizes eye specification through the activity of Wnt8b and Fz8a. In contrast, Wnt11 and Fz5 promote eye field development, at least in part, through local antagonism of Wnt/beta-catenin signaling. Additionally, Wnt11 regulates the behavior of eye field cells, promoting their cohesion. Together, these results allow us to postulate a model in which Wnt11 and Fz5 signaling promotes early eye development through the coordinated antagonism of signals that suppress retinal identity and promotion of coherence of eye field cells.

During development, the imaginal wing disc of Drosophila is subdivided along the proximal-distal axis into different territories that will give rise to body wall (notum and mesothoracic pleura) and appendage (wing hinge and wing blade). Expression of the Iroquois complex (Iro-C) homeobox genes in the most proximal part of the disc defines the notum, since Iro-C(-) cells within this territory acquire the identity of the adjacent distal region, the wing hinge. Here we analyze how the expression of Iro-C is confined to the notum territory. Neither Wingless signalling, which is essential for wing development, nor Vein-dependent EGFR signalling, which is needed to activate Iro-C, appear to delimit Iro-C expression. We show that a main effector of this confinement is the TGFbeta homolog Decapentaplegic (Dpp), a molecule known to pattern the disc along its anterior-posterior axis. At early second larval instar, the Dpp signalling pathway functions only in the wing and hinge territories, represses Iro-C and confines its expression to the notum territory. Later, Dpp becomes expressed in the most proximal part of the notum and turns off Iro-C in this region. This downregulation is associated with the subdivision of the notum into medial and lateral regions.