Signaling circuitries in development: insights from the retinal determination gene network

The Drosophila T-box transcription factor Midline functions within the Notch-Delta signaling pathway to specify sensory organ precursor cell fates and regulates cell survival within the eye imaginal disc

We report that the T-box transcription factor Midline (Mid), an evolutionary conserved homolog of the vertebrate Tbx20 protein, functions within the Notch-Delta signaling pathway essential for specifying the fates of sensory organ precursor (SOP) cells. These findings complement an established history of research showing that Mid regulates the cell-fate specification of diverse cell types within the developing heart, epidermis and central nervous system. Tbx20 has been detected in unique neuronal and epithelial cells of embryonic eye tissues in both mice and humans. However, the mechanisms by which either Mid or Tbx20 function to regulate cell-fate specification or other critical aspects of eye development including cell survival have not yet been elucidated. We have also gathered preliminary evidence suggesting that Mid may play an indirect, but vital role in selecting SOP cells within the third-instar larval eye disc by regulating the expression of the proneural gene atonal. During subsequent pupal stages, Mid specifies SOP cell fates as a member of the Notch-Delta signaling hierarchy and is essential for maintaining cell viability by inhibiting apoptotic pathways. We present several new hypotheses that seek to understand the role of Mid in regulating developmental processes downstream of the Notch receptor that are critical for specifying unique cell fates, patterning the adult eye and maintaining cellular homeostasis during eye disc morphogenesis.

Chromatin occupancy patterns of the ETS repressor Yan: a mechanism for buffering gene expression against noise?

Developmental programs are driven by transcription factors that coordinate precise patterns of gene expression. While recent publications have described the importance of coordinated action of transcriptional activators at multiple cis-regulatory modules or enhancers, the contribution of sequence-specific repressors to overall regulation and robustness of gene expression has been difficult to ascertain. The Ets transcriptional repressor Yan functions as part of a conserved network downstream of receptor tyrosine kinase (RTK) signaling in Drosophila. This network displays switch-like responsiveness to RTK signaling, with the transition from a high-Yan to a low-Yan state induced by mitogen-activated protein kinase (MAPK)-mediated phosphorylation and inactivation of Yan. The ability of Yan to self-associate through a conserved sterile α motif (SAM) is essential for Yan's repressive ability, and has been suggested to allow spreading of Yan repressive complexes along chromatin. Such a mechanism has the potential to confer both signal responsiveness and robustness to the Yan network. To explore this spreading model, we compared the genome-wide chromatin binding profiles of wild-type vs. monomeric Yan. Consistent with the starting prediction, we found that wild type chromatin occupancy at genes encoding crucial developmental regulators and core signaling pathway components occurs as clusters of peaks that "spread" over multiple kilobases. However monomeric Yan, which fails to rescue a yan null mutation and displays significantly impaired repressive ability, exhibits a broadly similar occupancy profile to that of wild-type Yan, with multi-kilobase binding at developmentally important genes. This unexpected result suggests that SAM-mediated self-association does not mediate Yan recruitment to DNA or chromatin spreading, and raises the questions of why developmentally important genes require extensive Yan chromatin occupancy and how SAM-mediated polymerization might contribute to active repressive mechanisms in this context. In this Extra View article we discuss potential mechanisms by which Yan self-association and extended chromatin occupancy may contribute to robust regulation of gene expression.

The "eyes absent" (eya) gene in the eye-bearing hydrozoan jellyfish Cladonema radiatum: conservation of the retinal determination network

Eyes absent (Eya) is a member of the Retinal Determination Gene Network (RDGN), a set of genes responsible for eye specification in Drosophila. Eya is a dual function protein, working as a transcription factor in the nucleus and as a tyrosine phosphatase in the cytoplasm. It had been shown that Pax and Six family genes, main components of the RDGN, are present in the hydrozoan Cladonema radiatum and that they are expressed in the eye. However, nothing had been known about the Eya family in hydrozoan jellyfish. Here we report the presence of an Eya homologue (CrEya) in Cladonema. Real-time PCR analysis and in situ hybridization showed that CrEya is expressed in the eye. Furthermore, the comprehensive survey of eukaryote genomes revealed that the acquisition of the N-terminal transactivation domain, including the EYA Domain 2 and its adjacent sequence shared by all eumetazoans, happened early in evolution, before the separation of Cnidaria and Bilateria. Our results uncover the evolution of the two domains and show a conservation of the expression pattern of the Eya gene between Cnidaria and Bilateria, which, together with previous data, supports the hypothesis of the monophyletic origin of metazoans eyes. We additionally show that CrEya is also expressed in the oocytes, where two other members of the RDGN, CrPaxB, and Six4/5-Cr, are known to be expressed. These data suggest that several members of the RDGN have begun to be localized also into the different context of egg development early in the course of metazoan evolution.

Comparative motif discovery combined with comparative transcriptomics yields accurate targetome and enhancer predictions

The identification of transcription factor binding sites, enhancers, and transcriptional target genes often relies on the integration of gene expression profiling and computational cis-regulatory sequence analysis. Methods for the prediction of cis-regulatory elements can take advantage of comparative genomics to increase signal-to-noise levels. However, gene expression data are usually derived from only one species. Here we investigate tissue-specific cross-species gene expression profiling by high-throughput sequencing, combined with cross-species motif discovery. First, we compared different methods for expression level quantification and cross-species integration using Tag-seq data. Using the optimal pipeline, we derived a set of genes with conserved expression during retinal determination across Drosophila melanogaster, Drosophila yakuba, and Drosophila virilis. These genes are enriched for binding sites of eye-related transcription factors including the zinc-finger Glass, a master regulator of photoreceptor differentiation. Validation of predicted Glass targets using RNA-seq in homozygous glass mutants confirms that the majority of our predictions are expressed downstream from Glass. Finally, we tested nine candidate enhancers by in vivo reporter assays and found eight of them to drive GFP in the eye disc, of which seven colocalize with the Glass protein, namely, scrt, chp, dpr10, CG6329, retn, Lim3, and dmrt99B. In conclusion, we show for the first time the combined use of cross-species expression profiling with cross-species motif discovery as a method to define a core developmental program, and we augment the candidate Glass targetome from a single known target gene, lozenge, to at least 62 conserved transcriptional targets.

Transcriptome analysis of the planarian eye identifies ovo as a specific regulator of eye regeneration

Among the millions of invertebrate species with visual systems, the genetic basis of eye development and function is well understood only in Drosophila melanogaster. We describe an eye transcriptome for the planarian Schmidtea mediterranea. Planarian photoreceptors expressed orthologs of genes required for phototransduction and microvillus structure in Drosophila and vertebrates, and optic pigment cells expressed solute transporters and melanin synthesis enzymes similar to those active in the vertebrate retinal pigment epithelium. Orthologs of several planarian eye genes, such as bestrophin-1 and Usher syndrome genes, cause eye defects in mammals when perturbed and were not previously described to have roles in invertebrate eyes. Five previously undescribed planarian eye transcription factors were required for normal eye formation during head regeneration. In particular, a conserved, transcription-factor-encoding ovo gene was expressed from the earliest stages of eye regeneration and was required for regeneration of all cell types of the eye.

The lineage-specific gene ponzr1 is essential for zebrafish pronephric and pharyngeal arch development

The Homeobox (Hox) and Paired box (Pax) gene families are key determinants of animal body plans and organ structure. In particular, they function within regulatory networks that control organogenesis. How these conserved genes elicit differences in organ form and function in response to evolutionary pressures is incompletely understood. We molecularly and functionally characterized one member of an evolutionarily dynamic gene family, plac8 onzin related protein 1 (ponzr1), in the zebrafish. ponzr1 mRNA is expressed early in the developing kidney and pharyngeal arches. Using ponzr1-targeting morpholinos, we show that ponzr1 is required for formation of the glomerulus. Loss of ponzr1 results in a nonfunctional glomerulus but retention of a functional pronephros, an arrangement similar to the aglomerular kidneys found in a subset of marine fish. ponzr1 is integrated into the pax2a pathway, with ponzr1 expression requiring pax2a gene function, and proper pax2a expression requiring normal ponzr1 expression. In addition to pronephric function, ponzr1 is required for pharyngeal arch formation. We functionally demonstrate that ponzr1 can act as a transcription factor or co-factor, providing the first molecular mode of action for this newly described gene family. Together, this work provides experimental evidence of an additional mechanism that incorporates evolutionarily dynamic, lineage-specific gene families into conserved regulatory gene networks to create functional organ diversity.

Nemo phosphorylates Eyes absent and enhances output from the Eya-Sine oculis transcriptional complex during Drosophila retinal determination

The retinal determination gene network comprises a collection of transcription factors that respond to multiple signaling inputs to direct Drosophila eye development. Previous genetic studies have shown that nemo (nmo), a gene encoding a proline-directed serine/threonine kinase, can promote retinal specification through interactions with the retinal determination gene network, although the molecular point of cross-talk was not defined. Here, we report that the Nemo kinase positively and directly regulates Eyes absent (Eya). Genetic assays show that Nmo catalytic activity enhances Eya-mediated ectopic eye formation and potentiates induction of the Eya-Sine oculis (So) transcriptional targets dachshund and lozenge. Biochemical analyses demonstrate that Nmo forms a complex with and phosphorylates Eya at two consensus mitogen-activated protein kinase (MAPK) phosphorylation sites. These same sites appear crucial for Nmo-mediated activation of Eya function in vivo. Thus, we propose that Nmo phosphorylation of Eya potentiates its transactivation function to enhance transcription of Eya-So target genes during eye specification and development.

dlx and sp6-9 Control optic cup regeneration in a prototypic eye

Optic cups are a structural feature of diverse eyes, from simple pit eyes to camera eyes of vertebrates and cephalopods. We used the planarian prototypic eye as a model to study the genetic control of optic cup formation and regeneration. We identified two genes encoding transcription factors, sp6-9 and dlx, that were expressed in the eye specifically in the optic cup and not the photoreceptor neurons. RNAi of these genes prevented formation of visible optic cups during regeneration. Planarian regeneration requires an adult proliferative cell population with stem cell-like properties called the neoblasts. We found that optic cup formation occurred only after migration of progressively differentiating progenitor cells from the neoblast population. The eye regeneration defect caused by dlx and sp6-9 RNAi can be explained by a failure to generate these early optic cup progenitors. Dlx and Sp6-9 genes function as a module during the development of diverse animal appendages, including vertebrate and insect limbs. Our work reveals a novel function for this gene pair in the development of a fundamental eye component, and it utilizes these genes to demonstrate a mechanism for total organ regeneration in which extensive cell movement separates new cell specification from organ morphogenesis.

Some invertebrates, such as planarians and Hydra, can regenerate fully after amputations that remove large parts of the body. We investigated how cells in the body of planarians provide new cells for eye regeneration after complete head removal. Planarians possess highly potent regenerative cells (neoblasts) in a compartment inside the worm, and these cells must be present in a body fragment for it to regenerate. We identify a pair of transcription factors, sp6-9 and dlx, that are expressed in the optic cup, and use expression of these genes as markers to demonstrate that lineage restriction of eye cells during regeneration begins within the neoblast compartment. dlx and sp6-9 are essential for formation of optic cup progenitors, and inhibition of these genes with RNA interference results in eyes that lack optic cups after regeneration. During eye development in both flies and vertebrates, progenitors form within a patterned epithelium. Interestingly, planarian eye precursors only aggregate once they have stopped cycling and undergone extensive migration. At this stage they already express markers of the terminally differentiated state. Therefore, we identify a mechanism for eye formation during regeneration and a novel function for a conserved gene pair in eye regeneration.

Yki/YAP, Sd/TEAD and Hth/MEIS control tissue specification in the Drosophila eye disc epithelium

During animal development, accurate control of tissue specification and growth are critical to generate organisms of reproducible shape and size. The eye-antennal disc epithelium of Drosophila is a powerful model system to identify the signaling pathway and transcription factors that mediate and coordinate these processes. We show here that the Yorkie (Yki) pathway plays a major role in tissue specification within the developing fly eye disc epithelium at a time when organ primordia and regional identity domains are specified. RNAi-mediated inactivation of Yki, or its partner Scalloped (Sd), or increased activity of the upstream negative regulators of Yki cause a dramatic reorganization of the eye disc fate map leading to specification of the entire disc epithelium into retina. On the contrary, constitutive expression of Yki suppresses eye formation in a Sd-dependent fashion. We also show that knockdown of the transcription factor Homothorax (Hth), known to partner Yki in some developmental contexts, also induces an ectopic retina domain, that Yki and Scalloped regulate Hth expression, and that the gain-of-function activity of Yki is partially dependent on Hth. Our results support a critical role for Yki- and its partners Sd and Hth - in shaping the fate map of the eye epithelium independently of its universal role as a regulator of proliferation and survival.

Regulation of ocellar specification and size by twin of eyeless and homothorax

The retinal determination gene network (RDGN) constitutes a paradigm of a gene network controlling organ specification and growth. In this study, we probed the RDGN in the Drosophila ocelli, a set of simple eyes located on the fly's dorsal head, by studying the expression, regulation, and function of toy, hth, eya, and so, members of the Pax6, Meis, Eya, and Six gene families. Our results highlight the role of the pax6 gene toy, together with the hh signaling pathway, in the initiation of eya and so expression; the engagement of eya and so in a feedback loop necessary for their full expression; and the interplay between hh signaling and hth as a mechanism of organ size control, as general regulatory steps in the specification of visual organs.

Differential selection within the Drosophila retinal determination network and evidence for functional divergence between paralog pairs

The retinal determination (RD) network in Drosophila comprises 14 known nuclear proteins that include DNA-binding proteins, transcriptional coactivators, kinases, and phosphatases. The composition of the network varies considerably throughout the animal kingdom, with the network in several basal insects having fewer members and with vertebrates having potentially significantly higher numbers of RD genes. One important contributing factor for the variation in gene number within the network is gene duplication. For example, 10 members of the RD network in Drosophila are derived from duplication events. Here we present an analysis of the coding regions of the five pairs of duplicate genes from within the RD network of several different Drosophila species. We demonstrate that there is differential selection across the coding regions of all RD genes. Additionally, some of the most significant differences in ratios of non-silent-to-silent site substitutions (d(N)/d(S)) between paralog pairs are found within regions that have no ascribed function. Previous structure/function analyses of several duplicate genes have identified areas within one gene that contain novel activities when compared with its paralog. The evolutionary analysis presented here identifies these same areas in the paralogs as being under high levels of relaxed selection. We suggest that sequence divergence between paralogs and selection signatures can be used as a reasonable predictor of functional changes in rapidly evolving motifs.

Drosophila CtBP regulates proliferation and differentiation of eye precursors and complexes with Eyeless, Dachshund, Dan, and Danr during eye and antennal development

Specification factors regulate cell fate in part by interacting with transcriptional co-regulators like CtBP to regulate gene expression. Here, we demonstrate that CtBP forms a complex or complexes with the Drosophila melanogaster Pax6 homolog Eyeless (Ey), and with Distal antenna (Dan), Distal antenna related (Danr), and Dachshund to promote eye and antennal specification. Phenotypic analysis together with molecular data indicate that CtBP interacts with Ey to prevent overproliferation of eye precursors. In contrast, CtBP,dan,danr triple mutant adult eyes have significantly fewer ommatidia than CtBP single or dan,danr double mutants, suggesting that the CtBP/Dan/Danr complex functions to recruit ommatidia from the eye precursor pool. Furthermore, CtBP single and to a greater extent CtBP,dan,danr triple mutants affect the establishment and maintenance of the R8 precursor, which is the founding ommatidial cell. Thus, CtBP interacts with different eye specification factors to regulate gene expression appropriate for proliferative vs. differentiative stages of eye development.

Retinal determination the beginning of eye development

The road to producing an eye begins with the decision to commit a population of cells to adopting an eye tissue fate, the process of retinal determination. Over the past decade and a half, a network of transcription factors has been found to mediate this process in all seeing animals. This retinal determination network is known to regulate not only tissue fate but also cell proliferation, pattern formation, compartment boundary establishment, and even retinal cell specification. The compound eye of the fruit fly, Drosophila melanogaster, has proven to be an excellent experimental system to study the mechanisms by which this network regulates organogenesis and tissue patterning. In fact the founding members of most of the gene families that make up this network were first isolated in Drosophila based on loss-of-function phenotypes that affect the eye. This chapter will highlight the history of discovery of the retinal determination network and will draw attention to the molecular and biochemical mechanisms that underlie our understanding of how the fate of the retina is determined.

Cell fate determination factor Dachshund reprograms breast cancer stem cell function

The cell fate determination factor Dachshund was cloned as a dominant inhibitor of the hyperactive epidermal growth factor receptor ellipse. The expression of Dachshund is lost in human breast cancer associated with poor prognosis. Breast tumor-initiating cells (TIC) may contribute to tumor progression and therapy resistance. Here, endogenous DACH1 was reduced in breast cancer cell lines with high expression of TIC markers and in patient samples of the basal breast cancer phenotype. Re-expression of DACH1 reduced new tumor formation in serial transplantations in vivo, reduced mammosphere formation, and reduced the proportion of CD44(high)/CD24(low) breast tumor cells. Conversely, lentiviral shRNA to DACH1 increased the breast (B)TIC population. Genome-wide expression studies of mammary tumors demonstrated DACH1 repressed a molecular signature associated with stem cells (SOX2, Nanog, and KLF4) and genome-wide ChIP-seq analysis identified DACH1 binding to the promoter of the Nanog, KLF4, and Lin28 genes. KLF4/c-Myc and Oct4/Sox2 antagonized DACH1 repression of BTIC. Mechanistic studies demonstrated DACH1 directly repressed the Nanog and Sox2 promoters via a conserved domain. Endogenous DACH1 regulates BTIC in vitro and in vivo.

Smed454 dataset: unravelling the transcriptome of Schmidtea mediterranea

Background

Freshwater planarians are an attractive model for regeneration and stem cell research and have become a promising tool in the field of regenerative medicine. With the availability of a sequenced planarian genome, the recent application of modern genetic and high-throughput tools has resulted in revitalized interest in these animals, long known for their amazing regenerative capabilities, which enable them to regrow even a new head after decapitation. However, a detailed description of the planarian transcriptome is essential for future investigation into regenerative processes using planarians as a model system.

Results

In order to complement and improve existing gene annotations, we used a 454 pyrosequencing approach to analyze the transcriptome of the planarian species Schmidtea mediterranea Altogether, 598,435 454-sequencing reads, with an average length of 327 bp, were assembled together with the ~10,000 sequences of the S. mediterranea UniGene set using different similarity cutoffs. The assembly was then mapped onto the current genome data. Remarkably, our Smed454 dataset contains more than 3 million novel transcribed nucleotides sequenced for the first time. A descriptive analysis of planarian splice sites was conducted on those Smed454 contigs that mapped univocally to the current genome assembly. Sequence analysis allowed us to identify genes encoding putative proteins with defined structural properties, such as transmembrane domains. Moreover, we annotated the Smed454 dataset using Gene Ontology, and identified putative homologues of several gene families that may play a key role during regeneration, such as neurotransmitter and hormone receptors, homeobox-containing genes, and genes related to eye function.

Conclusions

We report the first planarian transcript dataset, Smed454, as an open resource tool that can be accessed via a web interface. Smed454 contains significant novel sequence information about most expressed genes of S. mediterranea. Analysis of the annotated data promises to contribute to identification of gene families poorly characterized at a functional level. The Smed454 transcriptome data will assist in the molecular characterization of S. mediterranea as a model organism, which will be useful to a broad scientific community.

Dorsal eye selector pannier (pnr) suppresses the eye fate to define dorsal margin of the Drosophila eye

Axial patterning is crucial for organogenesis. During Drosophila eye development, dorso-ventral (DV) axis determination is the first lineage restriction event. The eye primordium begins with a default ventral fate, on which the dorsal eye fate is established by expression of the GATA-1 transcription factor pannier (pnr). Earlier, it was suggested that loss of pnr function induces enlargement in the dorsal eye due to ectopic equator formation. Interestingly, we found that in addition to regulating DV patterning, pnr suppresses the eye fate by downregulating the core retinal determination genes eyes absent (eya), sine oculis (so) and dacshund (dac) to define the dorsal eye margin. We found that pnr acts downstream of Ey and affects the retinal determination pathway by suppressing eya. Further analysis of the "eye suppression" function of pnr revealed that this function is likely mediated through suppression of the homeotic gene teashirt (tsh) and is independent of homothorax (hth), a negative regulator of eye. Pnr expression is restricted to the peripodial membrane on the dorsal eye margin, which gives rise to head structures around the eye, and pnr is not expressed in the eye disc proper that forms the retina. Thus, pnr has dual function, during early developmental stages pnr is involved in axial patterning whereas later it promotes the head specific fate. These studies will help in understanding the developmental regulation of boundary formation of the eye field on the dorsal eye margin.

Expression of Gpr177, a Wnt trafficking regulator, in mouse embryogenesis

Wls/Evi/Srt encoding a multipass transmembrane protein has been identified as a regulator for proper sorting and secretion of Wnt in flies. We have previously demonstrated that Gpr177 is the mouse ortholog required for axis determination. Gpr177 is a transcriptional target of Wnt that is activated to assist its subcellular distribution in a feedback regulatory loop. We, therefore, proposed that reciprocal regulation of Wnt and Gpr177 is essential for the Wnt-dependent developmental and pathogenic processes. Here, we examine the expression pattern of Gpr177 in mouse development. Gpr177 is expressed in a variety of tissues and cell types during organogenesis. Furthermore, Gpr177 is a glycoprotein primarily accumulating in the Golgi apparatus in signal-producing cells. The glycosylation of Gpr177 is necessary for proper transportation in the secretory pathway. Our findings suggest that the Gpr177-mediated regulation of Wnt is crucial for organogenesis in health and disease.

Temporal anatomy of an epigenetic switch in cell programming: the white-opaque transition of C. albicans

The human pathogen Candida albicans undergoes a well-defined switch between two distinct cell types, named 'white' and 'opaque'. White and opaque cells differ in metabolic preferences, mating behaviours, cellular morphologies and host interactions. Each cell type is stable through many generations; switching between them is rare, stochastic and occurs without any known changes in the primary sequence of the genome; thus the switch is epigenetic. The white-opaque switch is regulated by a transcriptional circuit, composed of four regulators arranged in a series of interlocking feedback loops. To understand how switching occurs, we investigated the order of regulatory changes that occur during the switch from the opaque to the white cell type. Surprisingly, changes in key transcriptional regulators occur gradually, extending over several cell divisions with little cell-to-cell variation. Additional experiments, including perturbations to regulator concentrations, refine the signature of the commitment point. Transcriptome analysis reveals that opaque cells begin to globally resemble white cells well before they irreversibly commit to switching. We propose that these characteristics of the switching process permit C. albicans to 'test the waters' before making an all-or-none decision.

Flexibly deployed Pax genes in eye development at the early evolution of animals demonstrated by studies on a hydrozoan jellyfish

Pax transcription factors are involved in a variety of developmental processes in bilaterians, including eye development, a role typically assigned to Pax-6. Although no true Pax-6 gene has been found in nonbilateral animals, some jellyfish have eyes with complex structures. In the cubozoan jellyfish Tripedalia, Pax-B, an ortholog of vertebrate Pax-2/5/8, had been proposed as a regulator of eye development. Here we have isolated three Pax genes (Pax-A, Pax-B, and Pax-E) from Cladonema radiatum, a hydrozoan jellyfish with elaborate eyes. Cladonema Pax-A is strongly expressed in the retina, whereas Pax-B and Pax-E are highly expressed in the manubrium, the feeding and reproductive organ. Misexpression of Cladonema Pax-A induces ectopic eyes in Drosophila imaginal discs, whereas Pax-B and Pax-E do not. Furthermore, Cladonema Pax-A paired domain protein directly binds to the 5' upstream region of eye-specific Cladonema opsin genes, whereas Pax-B does not. Our data suggest that Pax-A, but not Pax-B or Pax-E, is involved in eye development and/or maintenance in Cladonema. Phylogenetic analysis indicates that Pax-6, Pax-B, and Pax-A belong to different Pax subfamilies, which diverged at the latest before the Cnidaria-Bilateria separation. We argue that our data, showing the involvement of Pax genes in hydrozoan eye development as in bilaterians, supports the monophyletic evolutionary origin of all animal eyes. We then propose that during the early evolution of animals, distinct classes of Pax genes, which may have played redundant roles at that time, were flexibly deployed for eye development in different animal lineages.

Enforcing biphasic eye development in a directly developing insect by transient knockdown of single eye selector genes

The visual system of indirectly developing insects such as Drosophila passes through two phases of development. Larval eyes form in the embryo, whereas the adult compound eyes form during metamorphosis. Comparative evidence implies that this biphasic mode of visual system development evolved from the continuously developing eye of directly developing insects. We investigated the developmental basis of this evolutionary transformation in a directly developing insect taking advantage of the time-limited nature of systemic RNAi in the grasshopper Schistocerca americana. Transient knockdown of the homologs of the early retinal genes eyes absent (eya) or sine oculis (so) both induced long-term arrest of eye development in grasshopper nymphs. Eye development, however, resumed after knockdown expiry. This finding sheds first light on the molecular regulation of postembryonic eye development in directly developing insects and unravels an inherent capacity of the underlying gene regulatory network to accommodate for partitioning visual system development into discrete phases, as in indirectly developing insects.

Co-option of an anteroposterior head axis patterning system for proximodistal patterning of appendages in early bilaterian evolution

The enormous diversity of extant animal forms is a testament to the power of evolution, and much of this diversity has been achieved through the emergence of novel morphological traits. The origin of novel morphological traits is an extremely important issue in biology, and a frequent source of this novelty is co-option of pre-existing genetic systems for new purposes (Carroll et al., 2008). Appendages, such as limbs, fins and antennae, are structures common to many animal body plans which must have arisen at least once, and probably multiple times, in lineages which lacked appendages. We provide evidence that appendage proximodistal patterning genes are expressed in similar registers in the anterior embryonic neurectoderm of Drosophila melanogaster and Saccoglossus kowalevskii (a hemichordate). These results, in concert with existing expression data from a variety of other animals suggest that a pre-existing genetic system for anteroposterior head patterning was co-opted for patterning of the proximodistal axis of appendages of bilaterian animals.

Gene duplication and the origins of morphological complexity in pancrustacean eyes, a genomic approach

BACKGROUND

Duplication and divergence of genes and genetic networks is hypothesized to be a major driver of the evolution of complexity and novel features. Here, we examine the history of genes and genetic networks in the context of eye evolution by using new approaches to understand patterns of gene duplication during the evolution of metazoan genomes. We hypothesize that 1) genes involved in eye development and phototransduction have duplicated and are retained at higher rates in animal clades that possess more distinct types of optical design; and 2) genes with functional relationships were duplicated and lost together, thereby preserving genetic networks. To test these hypotheses, we examine the rates and patterns of gene duplication and loss evident in 19 metazoan genomes, including that of Daphnia pulex - the first completely sequenced crustacean genome. This is of particular interest because the pancrustaceans (hexapods+crustaceans) have more optical designs than any other major clade of animals, allowing us to test specifically whether the high amount of disparity in pancrustacean eyes is correlated with a higher rate of duplication and retention of vision genes.

RESULTS

Using protein predictions from 19 metazoan whole-genome projects, we found all members of 23 gene families known to be involved in eye development or phototransduction and deduced their phylogenetic relationships. This allowed us to estimate the number and timing of gene duplication and loss events in these gene families during animal evolution. When comparing duplication/retention rates of these genes, we found that the rate was significantly higher in pancrustaceans than in either vertebrates or non-pancrustacean protostomes. Comparing patterns of co-duplication across Metazoa showed that while these eye-genes co-duplicate at a significantly higher rate than those within a randomly shuffled matrix, many genes with known functional relationships in model organisms did not co-duplicate more often than expected by chance.

CONCLUSIONS

Overall, and when accounting for factors such as differential rates of whole-genome duplication in different groups, our results are broadly consistent with the hypothesis that genes involved in eye development and phototransduction duplicate at a higher rate in Pancrustacea, the group with the greatest variety of optical designs. The result that these genes have a significantly high number of co-duplications and co-losses could be influenced by shared functions or other unstudied factors such as synteny. Since we did not observe co-duplication/co-loss of genes for all known functional modules (e.g. specific regulatory networks), the interactions among suites of known co-functioning genes (modules) may be plastic at the temporal scale of analysis performed here. Other factors in addition to gene duplication - such as cis-regulation, heterotopy, and co-option - are also likely to be strong factors in the diversification of eye types.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

Two themes on the assembly of the Drosophila eye

Cells are sequentially recruited during formation of the Drosophila compound eye. A few simple rules are reiteratively utilized to control successive steps of eye assembly. Two themes emerge: the interplay between cell signaling and competence determines diversity of cell types and selective cell adhesion determines spatial patterns of cells. Cell signaling through competence creates signaling relays, which sequentially trigger differentiation of all cell types. Selective cell adhesion, on the other hand, provides forces to drive cells into energy-favored spatial configurations. Organ formation is nevertheless a complex process. The complexity lies in the spatial, temporal, and quantitative precision of gene expression. Many challenging questions remain.

Lhx2 links the intrinsic and extrinsic factors that control optic cup formation

A crucial step in eye organogenesis is the transition of the optic vesicle into the optic cup. Several transcription factors and extracellular signals mediate this transition, but whether a single factor links them into a common genetic network is unclear. Here, we provide evidence that the LIM homeobox gene Lhx2, which is expressed in the optic neuroepithelium, fulfils such a role. In Lhx2(-/-) mouse embryos, eye field specification and optic vesicle morphogenesis occur, but development arrests prior to optic cup formation in both the optic neuroepithelium and lens ectoderm. This is accompanied by failure to maintain or initiate the expression patterns of optic-vesicle-patterning and lens-inducing determinants. Of the signaling pathways examined, only BMP signaling is noticeably altered and Bmp4 and Bmp7 mRNAs are undetectable. Lhx2(-/-) optic vesicles and lens ectoderm upregulate Pax2, Fgf15 and Sox2 in response to BMP treatments, and Lhx2 genetic mosaics reveal that transcription factors, including Vsx2 and Mitf, require Lhx2 cell-autonomously for their expression. Our data indicate that Lhx2 is required for optic vesicle patterning and lens formation in part by regulating BMP signaling in an autocrine manner in the optic neuroepithelium and in a paracrine manner in the lens ectoderm. We propose a model in which Lhx2 is a central link in a genetic network that coordinates the multiple pathways leading to optic cup formation.

Eye evolution: common use and independent recruitment of genetic components

Animal eyes can vary in complexity ranging from a single photoreceptor cell shaded by a pigment cell to elaborate arrays of these basic units, which allow image formation in compound eyes of insects or camera-type eyes of vertebrates. The evolution of the eye requires involvement of several distinct components-photoreceptors, screening pigment and genes orchestrating their proper temporal and spatial organization. Analysis of particular genetic and biochemical components shows that many evolutionary processes have participated in eye evolution. Multiple examples of co-option of crystallins, Galpha protein subunits and screening pigments contrast with the conserved role of opsins and a set of transcription factors governing eye development in distantly related animal phyla. The direct regulation of essential photoreceptor genes by these factors suggests that this regulatory relationship might have been already established in the ancestral photoreceptor cell.

Glial cell development and function in the Drosophila visual system

In the developing nervous system, building a functional neuronal network relies on coordinating the formation, specification and survival to diverse neuronal and glial cell subtypes. The establishment of neuronal connections further depends on sequential neuron-neuron and neuron-glia interactions that regulate cell-migration patterns and axon guidance. The visual system of Drosophila has a highly regular, retinotopic organization into reiterated interconnected synaptic circuits. It is therefore an excellent invertebrate model to investigate basic cellular strategies and molecular determinants regulating the different developmental processes that lead to network formation. Studies in the visual system have provided important insights into the mechanisms by which photoreceptor axons connect with their synaptic partners within the optic lobe. In this review, we highlight that this system is also well suited for uncovering general principles that underlie glial cell biology. We describe the glial cell subtypes in the visual system and discuss recent findings about their development and migration. Finally, we outline the pivotal roles of glial cells in mediating neural circuit assembly, boundary formation, neural proliferation and survival, as well as synaptic function.

Inhibition of Eyes Absent Homolog 4 expression induces malignant peripheral nerve sheath tumor necrosis

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive sarcomas without effective therapeutics. Bioinformatics was used to identify potential therapeutic targets. Paired Box (PAX), Eyes Absent (EYA), Dachsund (DACH), and Sine Oculis (SIX) genes, which form a regulatory interactive network in drosophila, were found to be dysregulated in human MPNST cell lines and solid tumors. We identified a decrease in DACH1 expression, and increases in expression of PAX6, EYA1, EYA2, EYA4, and SIX1- 4. Consistent with the observation that half of MPNSTs develop in neurofibromatosis type 1 patients, subsequent to NF1 mutation, we found that exogenous expression of the NF1-GAP related domain (GRD) normalized DACH1 expression. EYA4 mRNA was elevated more than 100-fold as estimated by quantitative real time PCR in most MPSNT cell lines. In vitro, suppression of EYA4 expression using shRNA reduced cell adhesion and migration and caused cellular necrosis without affecting cell proliferation or apoptotic cell death. MPNST cells expressing sh-EYA4 either failed to form tumors in nude mice or formed very small tumors, with extensive necrosis but similar levels of proliferation and apoptosis as control cells. Our findings identify a role for EYA4 and possibly interacting SIX and DACH proteins in MPNSTs and suggest the EYA4 pathway as a rational therapeutic target.

The cell fate determination factor DACH1 is expressed in estrogen receptor-alpha-positive breast cancer and represses estrogen receptor-alpha signaling

The Dachshund (dac) gene, initially cloned as a dominant inhibitor of the Drosophila hyperactive EGFR mutant ellipse, encodes a key component of the cell fate determination pathway involved in Drosophila eye development. Analysis of more than 2,200 breast cancer samples showed improved survival by some 40 months in patients whose tumors expressed DACH1. Herein, DACH1 and estrogen receptor-alpha (ERalpha) expressions were inversely correlated in human breast cancer. DACH1 bound and inhibited ERalpha function. Nuclear DACH1 expression inhibited estradiol (E(2))-induced DNA synthesis and cellular proliferation. DACH1 bound ERalpha in immunoprecipitation-Western blotting, associated with ERalpha in chromatin immunoprecipitation, and inhibited ERalpha transcriptional activity, requiring a conserved DS domain. Proteomic analysis identified proline, glutamic acid, and leucine rich protein 1 (PELP1) as a DACH1-binding protein. The DACH1 COOH terminus was required for binding to PELP1. DACH1 inhibited induction of ERalpha signaling. E(2) recruited ERalpha and disengaged corepressors from DACH1 at an endogenous ER response element, allowing PELP1 to serve as an ERalpha coactivator. DACH1 expression, which is lost in poor prognosis human breast cancer, functions as an endogenous inhibitor of ERalpha function.

Activation of Six1 target genes is required for sensory placode formation

In vertebrates, cranial placodes form crucial parts of the sensory nervous system in the head. All cranial placodes arise from a common territory, the preplacodal region, and are identified by the expression of Six1/4 and Eya1/2 genes, which control different aspects of sensory development in invertebrates as well as vertebrates. While So and Eya can induce ectopic eyes in Drosophila, the ability of their vertebrate homologues to induce placodes in non-placodal ectoderm has not been explored. Here we show that Six1 and Eya2 are involved in ectodermal patterning and cooperate to induce preplacodal gene expression, while repressing neural plate and neural crest fates. However, they are not sufficient to induce ectopic sensory placodes in future epidermis. Activation of Six1 target genes is required for expression of preplacodal genes, for normal placode morphology and for placode-specific Pax protein expression. These findings suggest that unlike in the fly where the Pax6 homologue Eyeless acts upstream of Six and Eya, the regulatory relationships between these genes are reversed in early vertebrate placode development.

Transcription factor choice in the Hippo signaling pathway: homothorax and yorkie regulation of the microRNA bantam in the progenitor domain of the Drosophila eye imaginal disc

The accurate control of cell proliferation and survival is critical for animal development. The Hippo tumor suppressor pathway regulates both of these parameters by controlling the nuclear availability of the transcriptional coactivator Yorkie (Yki), which regulates downstream target genes together with Scalloped (Sd), a DNA-binding protein. Here we provide evidence that Yki can also regulate target genes in conjunction with Homothorax (Hth) and Teashirt (Tsh), two DNA-binding transcription factors expressed in the uncommitted progenitor cells of the Drosophila eye imaginal disc. Clonal analyses demonstrate that Hth and Tsh promote cell proliferation and protect eye progenitor cells from apoptosis. Genetic epistasis experiments suggest that Hth and Tsh execute these functions with Yki, in part by up-regulating the microRNA bantam. A physical interaction between Hth and Yki can be detected in cell culture, and we show that Hth and Yki are bound to a DNA sequence approximately 14 kb upstream of the bantam hairpin in eye imaginal disc cells, arguing that this regulation is direct. These data suggest that the Hippo pathway uses different DNA-binding transcription factors depending on the cellular context. In the eye disc, Hth and Tsh provide spatial information to this pathway, promoting cell proliferation and survival in the progenitor domain.

Pattern formation in the Drosophila eye disc

Differentiation of the Drosophila compound eye from the eye imaginal disc is a progressive process: columns of cells successively differentiate in a posterior to anterior sequence, clusters of cells form at regularly spaced intervals within each column, and individual photoreceptors differentiate in a defined order within each cluster. The progression of differentiation across the eye disc is driven by a positive autoregulatory loop of expression of the secreted molecule Hedgehog, which is temporally delayed by the intercalation of a second signal, Spitz. Hedgehog refines the spatial position at which each column initiates its differentiation by inducing secondary signals that act over different ranges to control the expression of positive and negative regulators. The position of clusters within each column is controlled by secreted inhibitory signals from clusters in the preceding column, and a single founder neuron, R8, is singled out within each cluster by Notch-mediated lateral inhibition. R8 then sequentially recruits surrounding cells to differentiate by producing a short-range signal, Spitz, which induces a secondary short-range signal, Delta. Intrinsic transcription factors act in combination with these two signals to produce cell-type diversity within the ommatidium. The Hedgehog and Spitz signals are transported along the photoreceptor axons and reused within the brain as long-range and local cues to trigger the differentiation and assembly of target neurons.

Interactions with the Abelson tyrosine kinase reveal compartmentalization of eyes absent function between nucleus and cytoplasm

Eyes absent (Eya), named for its role in Drosophila eye development but broadly conserved in metazoa, possesses dual functions as a transcriptional coactivator and protein tyrosine phosphatase. Although Eya's transcriptional activity has been extensively characterized, the physiological requirements for its phosphatase activity remain obscure. In this study, we provide insight into Eya's participation in phosphotyrosine-mediated signaling networks by demonstrating cooperative interactions between Eya and the Abelson (Abl) tyrosine kinase during development of the Drosophila larval visual system. Mechanistically, Abl-mediated phosphorylation recruits Eya to the cytoplasm, where in vivo studies reveal a requirement for its phosphatase function. Thus, we propose a model in which, in addition to its role as a transcription factor, Eya functions as a cytoplasmic protein tyrosine phosphatase.

The SM protein Car/Vps33A regulates SNARE-mediated trafficking to lysosomes and lysosome-related organelles

The SM proteins Vps33A and Vps33B are believed to act in membrane fusions in endosomal pathways, but their specific roles are controversial. In Drosophila, Vps33A is the product of the carnation (car) gene. We generated a null allele of car to test its requirement for trafficking to different organelles. Complete loss of car function is lethal during larval development. Eye-specific loss of Car causes late, light-independent degeneration of photoreceptor cells. Earlier in these cells, two distinct phenotypes were detected. In young adults, autophagosomes amassed indicating that their fusion with lysosomes requires Car. In eye discs, endocytosed receptors and ligands accumulate in Rab7-positive prelysosomal compartments. The requirement of Car for late endosome-to-lysosome fusion in imaginal discs is specific as early endosomes are unaffected. Furthermore, lysosomal delivery is not restored by expression of dVps33B. This specificity reflects the distinct pattern of binding to different Syntaxins in vitro: dVps33B predominantly binds the early endosomal Avl and Car to dSyntaxin16. Consistent with a role in Car-mediated fusion, dSyntaxin16 is not restricted to Golgi membranes but also present on lysosomes.

Tyrosine dephosphorylation of H2AX modulates apoptosis and survival decisions

Life and death fate decisions allow cells to avoid massive apoptotic death in response to genotoxic stress. While the regulatory mechanisms and signaling pathways controlling DNA repair and apoptosis are well characterized, the precise molecular strategies that determine the ultimate choice of DNA repair and survival or apoptotic cell death remain incompletely understood. Here, we report that a protein tyrosine phosphatase, Eya, is involved in promoting efficient DNA repair rather than apoptosis in response to genotoxic stress in specific tissue/cell types by executing a damage-signal dependent dephosphorylation of an H2AX C-terminal tyrosine phosphate (Y142). This post-translational modification determines the relative recruitment of either DNA repair or pro-apoptotic factors to the tail of γH2AX and allows it to function as an active determinant of repair/survival versus apoptotic responses to DNA damage, revealing an additional phosphorylation-dependent mechanism that modulates survival/apoptotic decisions during mammalian organogenesis.

Genomics and the evolutionary origins of nervous system complexity

Advances in genomics are leading to increased understanding of the evolution of complexity, especially by beginning to bridge genotype and phenotype. Here, using examples from nervous system evolution, we define general patterns of increased complexity seen across levels of biological organization. We also explore specific evolutionary mechanisms that increase complexity, namely those that increase the number of biological units (parts) in a system. We provide specific neurobiological examples of increased complexity in genes, gene networks, cell types, and tissues/organs. These examples illustrate that while a variety of different mechanisms increase biological complexity, they can be understood in a generalized comparative framework.

The six family of homeobox genes in development and cancer

The homeobox gene superfamily encodes transcription factors that act as master regulators of development through their ability to activate or repress a diverse range of downstream target genes. Numerous families exist within the homeobox gene superfamily, and are classified on the basis of conservation of their homeodomains as well as additional motifs that contribute to DNA binding and to interactions with other proteins. Members of one such family, the Six family, form a transcriptional complex with Eya and Dach proteins, and together these proteins make up part of the retinal determination network first identified in Drosophila. This network is highly conserved in both invertebrate and vertebrate species, where it influences the development of numerous organs in addition to the eye, primarily through regulation of cell proliferation, survival, migration, and invasion. Mutations in Six, Eya, and Dach genes have been identified in a variety of human genetic disorders, demonstrating their critical role in human development. In addition, aberrant expression of Six, Eya, and Dach occurs in numerous human tumors, and Six1, in particular, plays a causal role both in tumor initiation and in metastasis. Emerging evidence for the importance of Six family members and their cofactors in numerous human tumors suggests that targeting of this complex may be a novel and powerful means to inhibit both tumor growth and progression.

Retinal determination genes as targets and possible effectors of extracellular signals

Retinal determination genes are sufficient to specify eyes in ectopic locations, raising the question of how these master regulatory genes define an eye developmental field. Genetic mosaic studies establish that expression of the retinal determination genes eyeless, teashirt, homothorax, eyes absent, sine oculis, and dachshund are each regulated by combinations of Dpp, Hh, N, Wg, and Ras signals in Drosophila. Dpp and Hh control eyeless, teashirt, sine oculis, and dachshund expression, Dpp and Ras control homothorax, and all the signaling pathways affect eyes absent expression. These results suggest that eye-specific development uses retinal determination gene expression to relay positional information to eye target genes, because the distinct, overlapping patterns of retinal determination gene expression reflect the activities of the extracellular signaling pathways.

CLOCK expression identifies developing circadian oscillator neurons in the brains of Drosophila embryos

BACKGROUND

The Drosophila circadian oscillator is composed of transcriptional feedback loops in which CLOCK-CYCLE (CLK-CYC) heterodimers activate their feedback regulators period (per) and timeless (tim) via E-box mediated transcription. These feedback loop oscillators are present in distinct clusters of dorsal and lateral neurons in the adult brain, but how this pattern of expression is established during development is not known. Since CLK is required to initiate feedback loop function, defining the pattern of CLK expression in embryos and larvae will shed light on oscillator neuron development.

RESULTS

A novel CLK antiserum is used to show that CLK expression in the larval CNS and adult brain is limited to circadian oscillator cells. CLK is initially expressed in presumptive small ventral lateral neurons (s-LNvs), dorsal neurons 2 s (DN2s), and dorsal neuron 1 s (DN1s) at embryonic stage (ES) 16, and this CLK expression pattern persists through larval development. PER then accumulates in all CLK-expressing cells except presumptive DN2s during late ES 16 and ES 17, consistent with the delayed accumulation of PER in adult oscillator neurons and antiphase cycling of PER in larval DN2s. PER is also expressed in non-CLK-expressing cells in the embryonic CNS starting at ES 12. Although PER expression in CLK-negative cells continues in ClkJrk embryos, PER expression in cells that co-express PER and CLK is eliminated.

CONCLUSION

These data demonstrate that brain oscillator neurons begin development during embryogenesis, that PER expression in non-oscillator cells is CLK-independent, and that oscillator phase is an intrinsic characteristic of brain oscillator neurons. These results define the temporal and spatial coordinates of factors that initiate Clk expression, imply that circadian photoreceptors are not activated until the end of embryogenesis, and suggest that PER functions in a different capacity before oscillator cell development is initiated.

Context dependence of proneural bHLH proteins

A key point in neural development is the commitment of progenitor cells to a specific neural fate. In all animals studied, proneural proteins - transcription factors of the basic helix-loop-helix (bHLH) family - are central to this process. The function of these factors is strongly influenced by the spatial and temporal context in which they are expressed. It is important to understand the molecular mechanisms by which developmental context interacts with and modifies the intrinsic functions and properties of the proneural proteins. Recent insights have been obtained in Drosophila and vertebrates from analysis of how bHLH proteins interact with other transcription factors to regulate target genes.

Drosophila nemo promotes eye specification directed by the retinal determination gene network

Drosophila nemo (nmo) is the founding member of the Nemo-like kinase (Nlk) family of serine-threonine kinases. Previous work has characterized nmo's role in planar cell polarity during ommatidial patterning. Here we examine an earlier role for nmo in eye formation through interactions with the retinal determination gene network (RDGN). nmo is dynamically expressed in second and third instar eye imaginal discs, suggesting additional roles in patterning of the eyes, ocelli, and antennae. We utilized genetic approaches to investigate Nmo's role in determining eye fate. nmo genetically interacts with the retinal determination factors Eyeless (Ey), Eyes Absent (Eya), and Dachshund (Dac). Loss of nmo rescues ey and eya mutant phenotypes, and heterozygosity for eya modifies the nmo eye phenotype. Reducing nmo also rescues small-eye defects induced by misexpression of ey and eya in early eye development. nmo can potentiate RDGN-mediated eye formation in ectopic eye induction assays. Moreover, elevated Nmo alone can respecify presumptive head cells to an eye fate by inducing ectopic expression of dac and eya. Together, our genetic analyses reveal that nmo promotes normal and ectopic eye development directed by the RDGN.

Cell-type-specific transcription of prospero is controlled by combinatorial signaling in the Drosophila eye

In Drosophila, Notch and Egfr signaling regulate the determination of many cell types, and yet how these common signals generate cell-specific transcription is not well understood. In the compound eye, prospero (pros) is transcribed specifically in R7 photoreceptors and cone cells. We show that the transcription of pros is activated by two visual-specific transcription selectors, Glass and Sine Oculis, that bind to an enhancer and promote its activation. Together with the pre-patterning transcription factor Lozenge, these factors work in a highly combinatorial manner, such that cells missing any one factor transcribe pros only weakly, if at all. However, the factors are not sufficient to activate the enhancer because of an additional requirement for both Notch and Egfr signals. The loss of Notch signaling produces a ;salt and pepper' effect, with some cells expressing near-normal levels and others expressing no detectable pros at all; thus, the signaling loss does not produce a uniformly reduced level of transcription activity in cells. This suggests a probabilistic mechanism, in which Notch signals influence the likelihood that the enhancer is inactive or fully active in any given cell. The activity level, therefore, is dictated by the proper combination of highly cooperative selector and pre-pattern factors present in the cell.

Assembly of the cnidarian camera-type eye from vertebrate-like components

Animal eyes are morphologically diverse. Their assembly, however, always relies on the same basic principle, i.e., photoreceptors located in the vicinity of dark shielding pigment. Cnidaria as the likely sister group to the Bilateria are the earliest branching phylum with a well developed visual system. Here, we show that camera-type eyes of the cubozoan jellyfish, Tripedalia cystophora, use genetic building blocks typical of vertebrate eyes, namely, a ciliary phototransduction cascade and melanogenic pathway. Our findings indicative of parallelism provide an insight into eye evolution. Combined, the available data favor the possibility that vertebrate and cubozoan eyes arose by independent recruitment of orthologous genes during evolution.

Central projections of photoreceptor axons originating from ectopic eyes in Drosophila

Ectopic expression of the retinal determination gene eyeless (ey) induces the formation of supernumerary eyes on antennae, legs, wings, and halteres. These ectopic eyes form ommatidia that contain photoreceptors and accessory cells and respond to light. Here, we demonstrate that ectopic eyes on antennae and legs extend axonal projections to the central nervous system. Furthermore, electroretinograms and morphological evidence indicate that the photoreceptor axons of at least the antennal ectopic eyes can form completely constituted ectopic synapses with foreign postsynaptic elements and suggest that transmission at these sites may be functional. However, the ectopic axons do not connect to their correct optic lobe targets and do not project deeply into the neuropile, but rather form synapses at superficial positions in the neuropils. By means of confocal and electron microscopy we show that these ectopic synapses resemble normal synapses, albeit with some distinct morphological differences. Our data strongly suggest that the developmental programs controlling photoreceptor synaptogenesis and visual map formation depend to a considerable extent on presynaptic and thus photoreceptor-autonomous steps. Our data also suggest that photoreceptor axon projections and the establishment of the highly stereotypical neural circuitry in the optic lobe, the normal target neuropil, may depend on target-specific cues that appear to be absent from the antennal lobe and thoracic ganglion.