New perspectives on eye development and the evolution of eyes and photoreceptors.

Recent experiments on the genetic control of eye development have opened up a completely new perspective on eye evolution. The demonstration that targeted expression of one and the same master control gene, that is, Pax6 can induce the formation of ectopic eyes in both insects and vertebrates, necessitates a reconsideration of the dogma of a polyphyletic origin of the various eye types in all the animal phyla. The involvement of Pax6 and six1 and six3 genes, which encode highly conserved transcription factors, in the genetic control of eye development in organisms ranging from planarians to humans argues strongly for a monophyletic origin of the eye. Because transcription factors can control the expression of any target gene provided it contains the appropriate gene regulatory elements, the conservation of the genetic control of eye development by Pax6 among all bilaterian animals is not due to functional constraints but a consequence of its evolutionary history. The prototypic eyes postulated by Darwin to consist of two cells only, a photoreceptor and a pigment cell, were accidentally controlled by Pax6 and the subsequent evolution of the various eye types occurred by building onto this original genetic program. A hypothesis of intercalary evolution is proposed that assumes that the eye morphogenetic pathway is progressively modified by intercalation of genes between the master control genes on the top of the hierarchy and the structural genes like rhodopsin at the bottom. The recruitment of novel genes into the eye morphogenetic pathway can be due to at least two different genetic mechanisms, gene duplication and enhancer fusion.In tracing back the evolution of eyes beyond bilaterians, we find highly developed eyes in some box-jellyfish as well as in some Hydrozoans. In Hydrozoans the same orthologous six genes (six1 and six3) are required for eye regeneration as in planarians, and in the box jellyfish Tripedalia a pax B gene, which may be a precursor of Pax6, was found to be expressed in the eyes. In contrast to the adults, which have highly evolved eyes, the Planula larva of Tripedalia has single- celled photoreceptors similar to some unicellular protists. For the origin of photoreceptor cells in metazoa, I propose two hypotheses, one based on cellular differentiation and a more speculative one based on symbiosis. The former assumes that photoreceptor cells originated from a colonial protist in which all the cells were photosensitive and subsequent cellular differentiation to give rise to photoreceptor cells. The symbiont hypothesis, which I call the Russian doll model, assumes that photosensitivity arose first in photosynthetic cyanobacteria that were subsequently taken up into red algae as primary chloroplasts. The red algae in turn were taken up by dinoflagellates as secondary chloroplasts and in some species evolved into the most sophisticated eye organelles, as found, for example, in some dinoflagellates like Erythropsis and Warnovia, which lack chloroplasts. Because dinoflagellates are commonly found as symbionts in cnidarians, the dinoflagellates may have transferred their photoreceptor genes to cnidarians. In cnidarians such as Tripedalia the step from photoreceptor organelles to multicellular eyes has occurred. These two hypotheses, the cellular differentiation and the symbiont hypothesis, are not mutually exclusive and are the subject of further investigations.

Transgenic planarian lines obtained by electroporation using transposon-derived vectors and an eye-specific GFP marker.

To generate transgenic planarians we used a set of versatile vectors for animal transgenesis based on the promiscuous transposons, mariner, Hermes and piggyBac, and a universal enhanced GFP (EGFP) marker system with three Pax6 dimeric binding sites, the 3xP3-EGFP developed by Berghammer et al. [Berghammer, A. J., Klinger, M. & Wimmer, E. A. (1999) Nature 402, 370-371]. This marker is expressed specifically in the eyes of various arthropod taxa. Upon microinjection into the parenchyma of adult planarians and subsequent electroporation, these vectors transpose efficiently into the planarian genome. One of the cell types transformed are the totipotent "neoblast" stem cells present in the adults, representing 30% of total cells. The neoblast represents a unique cell type with the capacity to proliferate and to differentiate into all somatic cell types as well as into germ cells. All three transposon vectors have high transformation efficiency, but only Hermes and piggyBac show stable integration. The mariner vector is frequently lost presumably because of the presence of active mariner-type transposons in the genome of the Girardia tigrina. Transformed animals are mosaics containing both transformed and untransformed neoblasts. These differentiate to form EGFP-positive and -negative photoreceptor cells. Such mosaicism is maintained through several cycles of regeneration induced by decapitation or asexual reproduction. Transformed neoblasts also contribute to the germ line, and can give rise to pure transgenic planarian lines in which EGFP is expressed in all photoreceptor cells after sexual reproduction. The presence of the transgenes was confirmed by PCR, plasmid rescue assay, inverse PCR, and Southern blotting. Our results with the 3xP3-EGFP marker confirm the presence of Pax6 activity in the differentiated photoreceptor cells of planarian eyes. Transgenesis will be an important tool to dissect developmental molecular mechanisms in planarian regeneration, development and stem cell biology, and may also be an entry point to analyze the biology of parasitic Platyhelminthes.

Functions of the medial frontal cortex in the processing of conflict and errors.

A principal function of the medial frontal cortex, in particular the anterior cingulate cortex (ACC), is to monitor action. The error-related negativity (ERN, or N(E)), an event-related brain potential, reflects medial frontal action-monitoring processes. Specifically, the error-detection theory of the ERN states that the ERN reflects ACC processing that is directly related to detecting the error. This theory predicts that ERN and ACC activity should increase directly with the dissimilarity of the error from the correct response, with similarity defined with respect to the common movement features of the responses. In contrast, the conflict-detection theory claims that ACC and ERN activity represent the detection of response conflict. This theory predicts that the activity should increase directly with the similarity of the error and the correct response. To test these theories, we investigated the effects of response similarity and conflict on the ERN, using a task that involved hand and foot movements. ERN activity was largest under conditions of high response conflict, where the error was similar to the correct response. This finding favors the conflict-detection theory over the error-detection theory, although the ERN was not associated with posterror slowing, as predicted by proponents of both theories. Discrepancies between our results and those of past studies may stem from the use in previous studies of four-finger response tasks which are subject to unique physiological and biomechanical constraints. We conclude that the ERN reflects medial frontal activity involved in the detection or affective processing of response conflict.

The eyeless homeodomain is dispensable for eye development in Drosophila.

Pax-6 genes, known to be essential for eye development, encode an evolutionarily conserved transcription factor with two DNA-binding domains. To corroborate the contribution of each DNA-binding domain to eye formation, we generated truncated forms of the Drosophila Pax-6 gene eyeless and tested their capacity to rescue the ey(2) mutant. Surprisingly, EY deleted of the homeodomain rescued the ey(2) mutant and triggered ectopic eyes morphogenesis. In contrast, EY lacking the paired domain failed to rescue the ey(2) mutant, led to truncation of appendages, and repressed Distal-less when misexpressed. This result suggests distinct functions mediated differentially by the two DNA-binding domains of eyeless.

Differential expression and function of the Drosophila Pax6 genes eyeless and twin of eyeless in embryonic central nervous system development.

We analyzed the expression and function of eyeless (ey) and twin of eyeless (toy) in the embryonic central nervous system (CNS) of Drosophila. Both genes are differentially expressed in specific neuronal subsets (but not in glia) in every CNS neuromere, and in the brain, specific cell populations co-expressing both proteins define a longitudinal domain which is intercalated between broad exclusive expression domains of ey and toy. Studies of genetic null alleles and dsRNA interference did not reveal any gross neuroanatomical effects of ey, toy, or ey/toy elimination in the embryonic CNS. In contrast, targeted misexpression of ey, but not of toy, resulted in profound axonal abnormalities in the embryonic ventral nerve cord and brain.

Molecular basis for the inhibition of Drosophila eye development by Antennapedia.

Hox genes encoding homeodomain transcriptional regulators are known to specify the body plan of multicellular organisms and are able to induce body plan transformations when misexpressed. These findings led to the hypothesis that duplication events and misexpression of Hox genes during evolution have been necessary for generating the observed morphological diversity found in metazoans. It is known that overexpressing Antennapedia (Antp) in the head induces antenna-to-leg as well as head-to-thorax transformation and eye reduction. At present, little is known about the exact molecular mechanism causing these phenotypes. The aim of this study is to understand the basis of inhibition of eye development. We demonstrate that Antp represses the activity of the eye regulatory cascade. By ectopic expression, we show that Antp antagonizes the activity of the eye selector gene eyeless. Using both in vitro and in vivo experiments, we demonstrate that this inhibitory mechanism involves direct protein-protein interactions between the DNA-binding domains of EY and ANTP, resulting in mutual inhibition.

Characterization and expression analysis of an ancestor-type Pax gene in the hydrozoan jellyfish Podocoryne carnea.

We characterized a Pax gene from the hydrozoan Podocoryne carnea. It is most similar to cnidarian Pax-B genes and encodes a paired domain, a homeodomain and an octapeptide. Expression analysis demonstrates the presence of Pax-B transcripts in eggs, the ectoderm of the planula larva and in a few scattered cells in the apical polyp ectoderm. In developing and mature medusae, Pax-B is localized in particular endodermal cells, oriented toward the outside. Pax-B is not expressed in muscle cells. However, if isolated striated muscle tissue is activated for transdifferentiation, the gene is expressed within 1 h, before new cell types, such as smooth muscle and nerve cells, have formed. The expression data indicate that Pax-B is involved in nerve cell differentiation.

Pax gene diversity in the basal cnidarian Acropora millepora (Cnidaria, Anthozoa): implications for the evolution of the Pax gene family.

Pax genes encode a family of transcription factors, many of which play key roles in animal embryonic development but whose evolutionary relationships and ancestral functions are unclear. To address these issues, we are characterizing the Pax gene complement of the coral Acropora millepora, an anthozoan cnidarian. As the simplest animals at the tissue level of organization, cnidarians occupy a key position in animal evolution, and the Anthozoa are the basal class within this diverse phylum. We have identified four Pax genes in Acropora: two (Pax-Aam and Pax-Bam) are orthologs of genes identified in other cnidarians; the others (Pax-Cam and Pax-Dam) are unique to Acropora. Pax-Aam may be orthologous with Drosophila Pox neuro, and Pax-Bam clearly belongs to the Pax-2/5/8 class. The Pax-Bam Paired domain binds specifically and preferentially to Pax-2/5/8 binding sites. The recently identified Acropora gene Pax-Dam belongs to the Pax-3/7 class. Clearly, substantial diversification of the Pax family occurred before the Cnidaria/higher Metazoa split. The fourth Acropora Pax gene, Pax-Cam, may correspond to the ancestral vertebrate Pax gene and most closely resembles Pax-6. The expression pattern of Pax-Cam, in putative neurons, is consistent with an ancestral role of the Pax family in neural differentiation and patterning. We have determined the genomic structure of each Acropora Pax gene and show that some splice sites are shared both between the coral genes and between these and Pax genes in triploblastic metazoans. Together, these data support the monophyly of the Pax family and indicate ancient origins of several introns.

Searching for the prototypic eye genetic network: Sine oculis is essential for eye regeneration in planarians.

We have identified a sine oculis gene in the planarian Girardia tigrina (Platyhelminthes; Turbellaria; Tricladida). The planarian sine oculis gene (Gtso) encodes a protein with a sine oculis (Six) domain and a homeodomain that shares significant sequence similarity with so proteins assigned to the Six-2 gene family. Gtso is expressed as a single transcript in both regenerating and fully developed eyes. Whole-mount in situ hybridization studies show exclusive expression in photoreceptor cells. Loss of function of Gtso by RNA interference during planarian regeneration inhibits eye regeneration completely. Gtso is also essential for maintenance of the differentiated state of photoreceptor cells. These results, combined with the previously demonstrated expression of Pax-6 in planarian eyes, suggest that the same basic gene regulatory circuit required for eye development in Drosophila and mouse is used in the prototypic eye spots of platyhelminthes and, therefore, is truly conserved during evolution.

Prefrontal-cingulate interactions in action monitoring.

We found that medial frontal cortex activity associated with action monitoring (detecting errors and behavioral conflict) depended on activity in the lateral prefrontal cortex. We recorded the error-related negativity (ERN), an event-related brain potential proposed to reflect anterior cingulate action monitoring, from individuals with lateral prefrontal damage or age-matched or young control participants. In controls, error trials generated greater ERN activity than correct trials. In individuals with lateral prefrontal damage, however, correct-trial ERN activity was equal to error-trial ERN activity. Lateral prefrontal damage also affected corrective behavior. Thus the lateral prefrontal cortex seemed to interact with the anterior cingulate cortex in monitoring behavior and in guiding compensatory systems.

The Drosophila homeobox gene optix is capable of inducing ectopic eyes by an eyeless-independent mechanism.

optix is a new member of the Six/so gene family from Drosophila that contains both a six domain and a homeodomain. Because of its high amino acid sequence similarity with the mouse Six3 gene, optix is considered to be the orthologous gene from Drosophila rather than sine oculis, as previously believed. optix expression was detected in the eye, wing and haltere imaginal discs. Ectopic expression of optix leads to the formation of ectopic eyes suggesting that optix has important functions in eye development. Although optix and sine oculis belong to the same gene family (Six/so) and share a high degree of amino acid sequence identity, there are a number of factors which suggest that their developmental roles are different: (1) the expression patterns of optix and sine oculis are clearly distinct; (2) sine oculis acts downstream of eyeless, whereas optix is expressed independently of eyeless; (3) sine oculis functions synergistically with eyes absent in eye development whereas optix does not; (4) ectopic expression of optix alone, but not of sine oculis can induce ectopic eyes in the antennal disc. These results suggest that optix is involved in eye morphogenesis by an eyeless-independent mechanism.

Functional expression of a locust visual pigment in transgenic Drosophila melanogaster.

The cDNA encoding a visual pigment of the locust Schistocerca gregaria has been inserted into the germline of the ninaE mutant of Drosophila melanogaster by P-element-mediated transformation. Functional expression has been documented by recording light-regulated electroretinograms in transgenic flies. The spectral properties of the expressed visual pigment were determined with detergent-solubilized material, prepared from the eyecups of the transgenic D. melanogaster. The recombinant locust pigment, as well as the genuine pigment of the fruitfly (Rh1) that served as a control for transformation/expression, showed photoreversibility between the pigment and metapigment forms. The absorptions of the difference spectra identify the locust visual pigment as a short wavelength-absorbing, blue-light-sensitive photoreceptor. The absorption maxima are similar to those recorded on living locust animals. These results show that, although locust visual pigments contain 11-cis retinal as chromophore, the expressed protein is able to adopt 3-hydroxyretinal that is provided by the transgenic fruitflies. The electrophysiological recordings reveal that the locust visual pigment is able to induce phototransduction in the fruitfly. The reported results have two important consequences: On the one hand, the binding site of the locust opsin is apparently able to interact with the 3-hydroxyretinal from Drosophila in a way that the biological signal generated by the photoisomerization of the chromophore can be used by the protein to adopt a physiologically active conformation. On the other hand, despite the relatively large phylogenetic distance between both insect species, the extent of conservation between the protein domains thought to be involved in G-protein activation is striking.

Notch signaling and the determination of appendage identity.

The Notch signaling pathway defines an evolutionarily conserved cell-cell interaction mechanism that throughout development controls the ability of precursor cells to respond to developmental signals. Here we show that Notch signaling regulates the expression of the master control genes eyeless, vestigial, and Distal-less, which in combination with homeotic genes induce the formation of eyes, wings, antennae, and legs. Therefore, Notch is involved in a common regulatory pathway for the determination of the various Drosophila appendages.

Genetic control of development of the mushroom bodies, the associative learning centers in the Drosophila brain, by the eyeless, twin of eyeless, and Dachshund genes.

Mushroom bodies (MBs) are the centers for olfactory associative learning and elementary cognitive functions in the Drosophila brain. By high-resolution neuroanatomy, we show that eyeless (ey), twin of eyeless, and dachshund (dac), which are implicated in eye development, also are expressed in the developing MBs. Mutations of ey completely disrupted the MB neuropils, and a null mutation of dac resulted in marked disruption and aberrant axonal projections. Genetic analyses demonstrated that, whereas ey and dac synergistically control the structural development of the MBs, the two genes are regulated independently in the course of MB development. These data argue for a distinct combinatorial code of regulatory genes for MBs as compared with eye development and suggest conserved roles of Pax6 homologs in the genetic programs of the olfactory learning centers of complex brains.

Action-monitoring dysfunction in obsessive-compulsive disorder.

Evidence suggests that a hyperactive frontal-striatal-thalamic-frontal circuit is associated with the symptoms of obsessive-compulsive disorder (OCD), but there is little agreement about the function of the exaggerated activity. We report electrophysiological evidence suggesting that part of this system monitors events and generates error signals when the events conflict with an individual's internal standards or goals. Nine individuals with OCD and 9 age-, sex-, and education-matched control participants performed a speeded reaction time task. The error-related negativity, an event-related brain potential component that reflects action-monitoring processes, was enhanced in the individuals with OCD. The magnitude of this enhancement correlated with symptom severity. Dipole modeling suggested that the locus of the enhancement corresponded to medial frontal regions, possibly the anterior cingulate cortex.

Gene duplication and recruitment of a specific tropomyosin into striated muscle cells in the jellyfish Podocoryne carnea.

Cnidaria are the most basal animal phylum in which smooth and striated muscle cells have evolved. Since the ultrastructure of the mononucleated striated muscle is similar to that of higher animals, it is of interest to compare the striated muscle of Cnidaria at the molecular level to that of triploblastic phyla. We have used tropomyosins, a family of actin binding proteins to address this question. Throughout the animal kingdom, a great diversity of tropomyosin isoforms is found in non-muscle cells but only a few conserved tropomyosins are expressed in muscle cells. Muscle tropomyosins are all similar in length and share conserved termini. Two cnidarian tropomyosins have been described previously but neither of them is expressed in striated muscle cells. Here, we have characterized a new tropomyosin gene Tpm2 from the hydrozoan Podocoryne carnea. Expression analysis by RT-PCR and by whole mount in situ hybridization demonstrate that Tpm2 is exclusively expressed in striated muscle cells of the medusa. The Tpm2 protein is shorter in length than its counterparts from higher animals and differs at both amino and carboxy termini from striated muscle isoforms of higher animals. Interestingly, Tpm2 differs considerably from Tpm1 (only 19% identity) which was described previously in Podocoryne carnea. This divergence indicates a functional separation of cytoskeletal and striated muscle tropomyosins in cnidarians. These data contribute to our understanding of the evolution of the tropomyosin gene family and demonstrate the recruitment of tropomyosin into hydrozoan striated muscles during metazoan evolution. J. Exp. Zool. (Mol. Dev. Evol.) 285:378-386, 1999.

Reverse homeosis in homeotically reconstructed ribbonworms.

Homeosis is the replacement of one body part by another, which may be caused by either developmental or genetic variations. It is particularly obvious in segmented animals, like insects, in which one body segment may be transformed into another. However, homeosis also occurs in animals without overt segmentation that also have detailed positional information specifying their body plan. By grafting, we have artificially generated homeotic ribbonworms of the species Lineus ruber with a duplicated ocellar region replacing the postocellar region anterior to the brain. Such chimeric animals are capable of complete morphogenetic regulation of the anterior-posterior (A-P) pattern. The missing postocellar region is restored by intercalary regeneration, and the anterior duplicated ocellar region is eliminated by a process called transgeneration. Thus, homeosis is reversed, and a completely normal pattern along the A-P axis is restored. This reverse homeosis involves the elimination of the syngeneic eyes and the survival of the grafted allogeneic eye region. LsPax-6, the Lineus sanguineus ortholog of the mammalian Pax-6 gene, which is considered to be a master control gene for eye morphogenesis, is expressed specifically in regenerating, regenerated, and intact eye regions. Our data show that ribbonworm eyes are either maintained or they regress according to their position along the A-P axis, even though there are no obvious segmental boundaries. This system allows us to test the function of LsPax-6 protein not only during eye regeneration but also during maintenance and regression of the eyes.

The anterior cingulate cortex lends a hand in response selection.

The anterior cingulate cortex is involved in decisions between conflicting response tendencies. This executive function seems to involve separate pathways for manual and verbal responses.