Evolution of paired domains: isolation and sequencing of jellyfish and hydra Pax genes related to Pax-5 and Pax-6

PAX9 functions as a tumor suppressor gene for cervical cancer via modulating cell proliferation and apoptosis

To investigate the effect of PAX9 on the progression of cervical cancer (CC). PAX9 expression was quantified in CC tissues and adjacent normal tissues, as well as human CC cell lines and human cervical epithelial cells (HCerEpiC). PAX9-overexpression lentiviral vectors were transfected into CC cell lines, followed by the measurement of proliferation and apoptosis and the quantification of apoptosis-related proteins. In vivo, mice were subcutaneously injected with CaSki cells transfected with PAX9-overexpression lentiviral vectors and control vectors. Then, the volume and weight of tumors were measured followed by hematoxylin and eosin (HE) staining, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining, and immunohistochemistry. PAX9 expression in the CC tissues was lower than that in the adjacent normal tissues, which was correlated with the FIGO stage, tumor size, infiltration depth, parametrium invasion, lympho-vascular space invasion tumor-positive lymph nodes, and prognosis. Furthermore, PAX9 in CC cell lines was also lower than in HCerEpiC. PAX9 inhibits the CC cell proliferation and promotes the apoptosis, with the up-regulations of caspase-3, poly(ADP-ribose) polymerase (PARP), and Bax and the down-regulation of Bcl-2. In vivo experiments demonstrated that in the PAX9 group, the tumor weight and volume were lower than those in the vector group accompanying the decreased Ki-67, cleaved-caspase-3, and Bax expressions and the increased TUNEL and Bcl-2 expression. PAX9 was lowly expressed in the CC tissues and associated with the clinicopathological characteristics and prognosis. PAX9 could inhibit proliferation of CC cell lines and promote the apoptosis, thus suppressing the tumor growth in vivo, indicating its potential therapeutic role for CC treatment.

Conserved Patterns in Developmental Processes and Phases, Rather than Genes, Unite the Highly Divergent Bilateria

Bilateria are the predominant clade of animals on Earth. Despite having evolved a wide variety of body plans and developmental modes, they are characterized by common morphological traits. By default, researchers have tried to link clade-specific genes to these traits, thus distinguishing bilaterians from non-bilaterians, by their gene content. Here we argue that it is rather biological processes that unite Bilateria and set them apart from their non-bilaterian sisters, with a less complex body morphology. To test this hypothesis, we compared proteomes of bilaterian and non-bilaterian species in an elaborate computational pipeline, aiming to search for a set of bilaterian-specific genes. Despite the limited confidence in their bilaterian specificity, we nevertheless detected Bilateria-specific functional and developmental patterns in the sub-set of genes conserved in distantly related Bilateria. Using a novel multi-species GO-enrichment method, we determined the functional repertoire of genes that are widely conserved among Bilateria. Analyzing expression profiles in three very distantly related model species-D. melanogaster, D. rerio and C. elegans-we find characteristic peaks at comparable stages of development and a delayed onset of expression in embryos. In particular, the expression of the conserved genes appears to peak at the phylotypic stage of different bilaterian phyla. In summary, our study illustrate how development connects distantly related Bilateria after millions of years of divergence, pointing to processes potentially separating them from non-bilaterians. We argue that evolutionary biologists should return from a purely gene-centric view of evolution and place more focus on analyzing and defining conserved developmental processes and periods.

The Pax gene family: Highlights from cephalopods

Pax genes play important roles in Metazoan development. Their evolution has been extensively studied but Lophotrochozoa are usually omitted. We addressed the question of Pax paralog diversity in Lophotrochozoa by a thorough review of available databases. The existence of six Pax families (Pax1/9, Pax2/5/8, Pax3/7, Pax4/6, Paxβ, PoxNeuro) was confirmed and the lophotrochozoan Paxβ subfamily was further characterized. Contrary to the pattern reported in chordates, the Pax2/5/8 family is devoid of homeodomain in Lophotrochozoa. Expression patterns of the three main pax classes (pax2/5/8, pax3/7, pax4/6) during Sepia officinalis development showed that Pax roles taken as ancestral and common in metazoans are modified in S. officinalis, most likely due to either the morphological specificities of cephalopods or to their direct development. Some expected expression patterns were missing (e.g. pax6 in the developing retina), and some expressions in unexpected tissues have been found (e.g. pax2/5/8 in dermal tissue and in gills). This study underlines the diversity and functional plasticity of Pax genes and illustrates the difficulty of using probable gene homology as strict indicator of homology between biological structures.

Light and the evolution of vision

It might seem a little ridiculous to cover the period over which vision evolved, perhaps 1.5 billion years, in only 3000 words. Yet, if we examine the photoreceptor molecules of the most basic eukaryote protists and even before that, in those of prokaryote bacteria and cyanobacteria, we see how similar they are to those of mammalian rod and cone photoreceptor opsins and the photoreceptive molecules of light sensitive ganglion cells. This shows us much with regard the development of vision once these proteins existed, but there is much more to discover about the evolution of even more primitive vision systems.

The RED domain of Paired is specifically required for Drosophila accessory gland maturation

The evolutionarily conserved paired domain consists of the N-terminal PAI and the C-terminal RED domains, each containing a helix–turn–helix motif capable of binding DNA. Despite its conserved sequence, the physiological functions of the RED domain remain elusive. Here, we constructed a prd transgene expressing a truncated Paired (Prd) protein without the RED domain, and examined its rescue ability in prd mutants. We found that the RED domain is specifically required for the expression of Acp26Aa and sex peptide in male accessory glands, and the induction of female post-mating response. Our data thus identified an important physiological function for the evolutionarily conserved RED domain.

Transcriptome analysis of Nautilus and pygmy squid developing eye provides insights in lens and eye evolution

Coleoid cephalopods like squids have a camera-type eye similar to vertebrates. On the other hand, Nautilus (Nautiloids) has a pinhole eye that lacks lens and cornea. Since pygmy squid and Nautilus are closely related species they are excellent model organisms to study eye evolution. Having being able to collect Nautilus embryos, we employed next-generation RNA sequencing using Nautilus and pygmy squid developing eyes. Their transcriptomes were compared and analyzed. Enrichment analysis of Gene Ontology revealed that contigs related to nucleic acid binding were largely up-regulated in squid, while the ones related to metabolic processes and extracellular matrix-related genes were up-regulated in Nautilus. These differences are most likely correlated with the complexity of tissue organization in these species. Moreover, when the analysis focused on the eye-related contigs several interesting patterns emerged. First, contigs from both species related to eye tissue differentiation and morphogenesis as well as to cilia showed best hits with their Human counterparts, while contigs related to rabdomeric photoreceptors showed the best hit with their Drosophila counterparts. This bolsters the idea that eye morphogenesis genes have been generally conserved in evolution, and compliments other studies showing that genes involved in photoreceptor differentiation clearly follow the diversification of invertebrate (rabdomeric) and vertebrate (ciliated) photoreceptors. Interestingly some contigs showed as good a hit with Drosophila and Human homologues in Nautilus and squid samples. One of them, capt/CAP1, is known to be preferentially expressed in Drosophila developing eye and in vertebrate lens. Importantly our analysis also provided evidence of gene duplication and diversification of their function in both species. One of these genes is the Neurofibromatosis 1 (NF1/Nf1), which in mice has been implicated in lens formation, suggesting a hitherto unsuspected role in the evolution of the lens in molluscs.

Molecular paleoscience: systems biology from the past

Experimental paleomolecular biology, paleobiochemistry, and paleogenetics are closely related emerging fields that infer the sequences of ancient genes and proteins from now-extinct organisms, and then resurrect them for study in the laboratory. The goal of paleogenetics is to use information from natural history to solve the conundrum of modern genomics: How can we understand deeply the function of biomolecular structures uncovered and described by modern chemical biology? Reviewed here are the first 20 cases where biomolecular resurrections have been achieved. These show how paleogenetics can lead to an understanding of the function of biomolecules, analyze changing function, and put meaning to genomic sequences, all in ways that are not possible with traditional molecular biological studies.

The Pax2/5/8 gene egl-38 coordinates organogenesis of the C. elegansegg-laying system

Organogenesis requires coordinated development between different tissues and cells. The Pax family of transcription factors coordinates multiple developmental events in organs including the kidney, thyroid and the eye. Studying Pax factors in different organisms should identify unifying characteristics of organ development with implications to both development and disease. Here we investigate the function of the Pax2/5/8 transcription factor EGL-38 in coordinating development of the C. elegans egg-laying system. A functional egg-laying system requires cell fate specification events in the epithelial cells of the vulva as well as the mesodermal cells in the uterus of the somatic gonad. Using gene expression studies, genetic mutant analysis and genetic mosaics, we show that egl-38 has functions in both tissues of the organ to promote its development. We incorporate these results together with previous results to propose that EGL-38 plays multiple roles in the development of the egg-laying system, acting to both promote cell fate and to coordinate the development between different cell types. As the Pax2 gene performs similar roles in the development of the mammalian kidney, we show that coordinating organogenesis is a conserved function for Pax2/5/8 transcription factors.

Pax6 and eye development in Arthropoda

The arthropod compound eye is one of the three main types of eyes observed in the animal kingdom. Comparison of the eyes seen in Insecta, Crustacea, Myriapoda and Chelicerata reveals considerable variation in terms of overall cell number, cell positioning, and photoreceptor rhabdomeres, yet, molecular data suggest there may be unexpected similarities. We review here the role of Pax6 in eye development and evolution and the relationship of Pax6 with other retinal determination genes and signaling pathways. We then discuss how the study of changes in Pax6 primary structure, in the gene networks controlled by Pax6 and in the relationship of Pax6 with signaling pathways may contribute to our insight into the relative role of conserved molecular-genetic mechanisms and emergence of evolutionary novelty in shaping the ommatidial eyes seen in the Arthropoda.

Dorsoventral patterning in hemichordates: insights into early chordate evolution

We have compared the dorsoventral development of hemichordates and chordates to deduce the organization of their common ancestor, and hence to identify the evolutionary modifications of the chordate body axis after the lineages split. In the hemichordate embryo, genes encoding bone morphogenetic proteins (Bmp) 2/4 and 5/8, as well as several genes for modulators of Bmp activity, are expressed in a thin stripe of ectoderm on one midline, historically called "dorsal." On the opposite midline, the genes encoding Chordin and Anti-dorsalizing morphogenetic protein (Admp) are expressed. Thus, we find a Bmp-Chordin developmental axis preceding and underlying the anatomical dorsoventral axis of hemichordates, adding to the evidence from Drosophila and chordates that this axis may be at least as ancient as the first bilateral animals. Numerous genes encoding transcription factors and signaling ligands are expressed in the three germ layers of hemichordate embryos in distinct dorsoventral domains, such as pox neuro, pituitary homeobox, distalless, and tbx2/3 on the Bmp side and netrin, mnx, mox, and single-minded on the Chordin-Admp side. When we expose the embryo to excess Bmp protein, or when we deplete endogenous Bmp by small interfering RNA injections, these expression domains expand or contract, reflecting their activation or repression by Bmp, and the embryos develop as dorsalized or ventralized limit forms. Dorsoventral patterning is independent of anterior/posterior patterning, as in Drosophila but not chordates. Unlike both chordates and Drosophila, neural gene expression in hemichordates is not repressed by high Bmp levels, consistent with their development of a diffuse rather than centralized nervous system. We suggest that the common ancestor of hemichordates and chordates did not use its Bmp-Chordin axis to segregate epidermal and neural ectoderm but to pattern many other dorsoventral aspects of the germ layers, including neural cell fates within a diffuse nervous system. Accordingly, centralization was added in the chordate line by neural-epidermal segregation, mediated by the pre-existing Bmp-Chordin axis. Finally, since hemichordates develop the mouth on the non-Bmp side, like arthropods but opposite to chordates, the mouth and Bmp-Chordin axis may have rearranged in the chordate line, one relative to the other.

Getting the Proto-Pax by the Tail

Pax genes encode transcription factors governing the determination of different cell types and even organs in the development of multicellular animals. Pax proteins are characterized by the presence of three evolutionarily conserved elements: two DNA-binding domains, the paired domain (PD) and paired-type homeodomain (PtHD), and the short octopeptide sequence (OP) located between PD and PtHD. PD is the defining feature of this class of genes, while OP and/or PtHD may be divergent or absent in some members of the family. Phylogenetic analyses of the PD and PtHD sequences do not distinguish which particular type of the extant Pax genes more resembles the ancestral type. Here we present evidence for the existence of a fourth evolutionarily conserved domain in the Pax proteins, the paired-type homeodomain tail (PHT). Our data also imply that the hypothetical proto-Pax protein most probably exhibited a complex structure, PD-OP-PtHD-PHT, which has been retained in the extant proteins Pax3/7 of the ascidia and lancelet, and Pax7 of the vertebrates. Finally, based on structural considerations, a scenario for the evolutionary emergence of the proto-Pax gene is proposed.

Developmental expression of transcription factor genes in a demosponge: insights into the origin of metazoan multicellularity

Demosponges are considered part of the most basal evolutionary lineage in the animal kingdom. Although the sponge body plan fundamentally differs from that of other metazoans, their development includes many of the hallmarks of bilaterian and eumetazoan embryogenesis, namely fertilization followed by a period of cell division yielding distinct cell populations, which through a gastrulation-like process become allocated into different cell layers and patterned within these layers. These observations suggest that the last common ancestor (LCA) to all living animals was developmentally more sophisticated than is widely appreciated and used asymmetric cell division and morphogen gradients to establish localized populations of specified cells within the embryo. Here we demonstrate that members of a range of transcription factor gene classes, many of which appear to be metazoan-specific, are expressed during the development of the demosponge Reniera, including ANTP, Pax, POU, LIM-HD, Sox, nuclear receptor, Fox (forkhead), T-box, Mef2, and Ets genes. Phylogenetic analysis of these genes suggests that not only the origin but the diversification of some of the major developmental metazoan transcription factor classes took place before sponges diverged from the rest of the Metazoa. Their expression during demosponge development suggests that, as in today's sophisticated metazoans, these genes may have functioned in the regulatory network of the metazoan LCA to control cell specification and regionalized gene expression during embryogenesis.

The Trichoplax PaxB Gene: A Putative Proto-PaxA/B/C Gene Predating the Origin of Nerve and Sensory Cells

Pax genes play key regulatory roles in embryonic and sensory organ development in metazoans but their evolution and ancestral functions remain widely unresolved. We have isolated a Pax gene from Placozoa, beside Porifera the only metazoan phylum that completely lacks nerve and sensory cells or organs. These simplest known metazoans also lack any kind of symmetry, organs, extracellular matrix, basal lamina, muscle cells, and main body axis. The isolated Pax gene from Trichoplax adhaerens harbors a paired domain, an octapeptide, and a full-length homeodomain. It displays structural features not only of PaxB and Pax2/5/8-like genes but also of PaxC and Pax6 genes. Conserved splice sites between Placozoa, Cnidaria, and triploblasts, mark the ancient origin of intron structures. Phylogenetic analyses demonstrate that the Trichoplax PaxB gene, TriPaxB, is basal not only to all other known PaxB genes but also to PaxA and PaxC genes and their relatives in triploblasts (namely Pax2/5/8, Pax4/6, and Poxneuro). TriPaxB is expressed in distinct cell patches near the outer edge of the animal body, where undifferentiated and possibly multipotent cells are found. This expression pattern indicates a developmental role in cell-type specification and/or differentiation, probably in specifying-determining fiber cells, which are regarded as proto-neural/muscle cells in Trichoplax. While PaxB, Pax2/5/8, and Pax6 genes have been linked to nerve cell and sensory system/organ development in virtually all animals investigated so far, our study suggests that Pax genes predate the origin of nerve and sensory cells.

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.

sine oculis in basal Metazoa

We report the recovery of homologs of Six1/2/sine oculis (so), a homeodomain-containing member of the Six-gene family, from a diverse set of basal Metazoa, including representatives of the poriferan classes Demospongia, Calcarea and Hexactinellida, the cnidarian classes Hydrozoa, Scyphozoa and Anthozoa, as well as a ctenophore. so sequences were also recovered from a platyhelminth, an echiurid and two bivalve molluscs, members of the super-phyletic group Lophotrochozoa. In the case of the platyhelminth, multiple distinct so sequences were recovered, as well as a member of the related group Six4/5/D-Six4. Extended sequences of the so gene were recovered from the demosponge, Haliclona sp., and the scyphozoan Aurelia aurita via PCR, and 3' RACE. The affinities of all recovered sequences were assessed using a parsimony analysis based on both nucleic and amino acid sequence and using successive character weighting. Our results indicate that so is highly conserved across the animal kingdom. Preliminary expression data for Aurelia reveal that transcripts of the so homolog are present in the manubrium as well as in the rhopalia, which contain the statocyst and eyes, in the free-swimming ephyra and juvenile stages of these jellyfish.

Role of Pax genes in eye evolution: a cnidarian PaxB gene uniting Pax2 and Pax6 functions

PaxB from Tripedalia cystophora, a cubomedusan jellyfish possessing complex eyes (ocelli), was characterized. PaxB, the only Pax gene found in this cnidarian, is expressed in the larva, retina, lens, and statocyst. PaxB contains a Pax2/5/8-type paired domain and octapeptide, but a Pax6 prd-type homeodomain. Pax2/5/8-like properties of PaxB include a DNA binding specificity of the paired domain, activation and inhibitory domains, and the ability to rescue spa(pol), a Drosophila Pax2 eye mutant. Like Pax6, PaxB activates jellyfish crystallin and Drosophila rhodopsin rh6 promoters and induces small ectopic eyes in Drosophila. Pax6 has been considered a "master" control gene for eye development. Our data suggest that the ancestor of jellyfish PaxB, a PaxB-like protein, was the primordial Pax protein in eye evolution and that Pax6-like genes evolved in triploblasts after separation from Cnidaria, raising the possibility that cnidarian and sophisticated triploblastic eyes arose independently.

DNA-binding characteristics of cnidarian Pax-C and Pax-B proteins in vivo and in vitro: no simple relationship with the Pax-6 and Pax-2/5/8 classes

Cnidarians are the simplest animals in which distinct eyes are present. We have previously suggested that cnidarian Pax-Cam might represent a precursor of the Pax-6 class. Here we show that when expressed in Drosophila imaginal discs, Pax-Cam chimeric proteins containing the C-terminal region of EY were capable of eye induction and driving expression of a reporter gene under the control of a known EY target (the sine oculis gene). Whilst these results are consistent with a Pax-6-like function for Pax-Cam, in band shift experiments we were unable to distinguish the DNA-binding behaviour of the Pax-Cam Paired domain from that of a second Acropora Pax protein, Pax-Bam. The ability of a Pax-Bam/EY chimera to also induce eye formation in leg imaginal discs, together with the in vitro data, cast doubt on previously assumed direct relationships between cnidarian Pax genes and the Pax-6 and Pax-2/5/8 classes of bilateral animals.

Mutational analysis of the Acropora millepora PaxD paired domain highlights the importance of the linker region for DNA binding

Pax transcription factors are found in animals, from simple sponges to insects and vertebrates. The defining feature of Pax proteins is the DNA-binding paired domain (PD), which consists of two helix-turn-helix subdomains, joined with a linker region. Despite high specificity in vivo, the paired domains of different Pax proteins bind similar consensus DNA sequences in vitro. Using bandshift techniques, we show here that the paired domain of the Acropora millepora PaxD protein, which unambiguously belongs to the Pax3/7 group, does not bind to three defined paired domain-binding sites. Domain swapping experiments and site-directed mutagenesis identified two amino acid residues in the linker region of the paired domain as critical to DNA binding; G70 and S71 are highly conserved in Pax proteins, but differ in PaxD (L70 and N71). The PaxD data thus highlight the importance of the linker region, and particularly G70 and S71, in DNA binding by Pax proteins.

Expression of Pax258 in the gastropod statocyst: insights into the antiquity of metazoan geosensory organs

Most animals have sensory systems that allow them to balance and orient relative to the pull of gravity. Structures responsible for these functions range from very simple statocysts found in many aquatic invertebrates to the complex inner ear of mammals. Previous studies suggest that the specialized mechanosensory structures responsible for balance in vertebrates and insects may be homologous based on the requirement and expression of group II Pax genes (i.e., Pax-2/5/8 genes). Here we report the expression of a Pax-258 gene in the statocysts and other chemosensory and mechanosensory cells during the development of the gastropod mollusk Haliotis asinina, a member of the Lophotrochozoa. Based on the phylogenetic distribution of geosensory systems and the consistent expression of Pax-258 in the cells that form these systems, we propose that Pax-258, along with POU-III and -IV genes, has an ancient and conserved role in the formation of structures responsible for balance and geotaxis in eumetazoans.

Pax6 in the sepiolid squid Euprymna scolopes: evidence for a role in eye, sensory organ and brain development

The cloning of a Pax6 orthologue from the sepiolid squid Euprymna scolopes and its developmental expression pattern are described. The data are consistent with the presence of a single gene encoding a protein with highly conserved DNA-binding paired and homeodomains. A detailed expression analysis by in situ hybridization and immunodetection revealed Pax6 mRNA and protein with predominantly nuclear localization in the developing eye, olfactory organ, brain lobes (optic lobe, olfactory lobe, peduncle lobe, superior frontal lobe and dorsal basal lobe), arms and mantle, suggestive of a role in eye, brain, and sensory organ development.

Photoreceptors of cnidarians

Cnidarians are the most primitive present-day invertebrates to have multicellular light-detecting organs, called ocelli (eyes). These photodetectors include simple eyespots, pigment cups, complex pigment cups with lenses, and camera-type eyes with a cornea, lens, and retina. Ocelli are composed of sensory photoreceptor cells interspersed among nonsensory pigment cells. The photoreceptor cells are bipolar, the apical end forming a light-receptor process and the basal end forming an axon. These axons synapse with second-order neurons that may form ocular nerves. A cilium with a 9 + 2 arrangement of microtubules projects from the receptor-cell process. Depending on the species, the membrane covering the cilium shows several variations, including evaginating microvilli. In the cubomedusae stacks of membranes fill the apical regions of the photoreceptor cells. Pigment cells are rich in pigment granules, and in some animals the distal regions of these cells form tubular processes that project into the cavity of the ocellus. Microvilli may extend laterally from these tubular processes and interdigitate with the microvilli from the ciliary membranes of photoreceptor cells. Photoreceptor cells respond to changes in light intensity with graded potentials that are directly proportional to the range of the changes in light intensity. In the Hydrozoa these cells may be electrically coupled to each other through gap junctions. Light affects the behavioral activities of cnidarians, including diel vertical migration, responses to rapid changes in light intensity, and reproduction. Medusae with the most highly modified photoreceptors demonstrate the most complex photic behaviors. The sophisticated visual system of the cubomedusan jellyfish Carybdea marsupialis is described. Extraocular photosensitivity is widespread throughout the cnidarians, with neurons, epithelial cells, and muscle cells mediating light detection. Rhodopsin-like and opsin-like proteins are present in the photoreceptor cells of the complex eyes of some cubomedusae and in some neurons of hydras. Neurons expressing glutamate, serotonin, γ-aminobutyric acid, and RFamide (Arg-Phe-amide) are found in close proximity to the complex eyes of cubomedusae; these neurotransmitters may function in the photic system of the jellyfish. Pax genes are expressed in cnidarians; these genes may control many developmental pathways, including eye development. The photobiology of cnidarians is similar in many ways to that of higher multicellular animals.

Identification of essential amino acid changes in paired domain evolution using a novel combination of evolutionary analysis and in vitro and in vivo studies

Pax genes are defined by the presence of a paired box that encodes a DNA-binding domain of 128 amino acids. They are involved in the development of the central nervous system, organogenesis, and oncogenesis. The known Pax genes are divided into five groups within two supergroups. By means of a novel combination of evolutionary analysis, in vitro binding assays and in vivo functional analyses, we have identified the key residues that determine the differing DNA-binding properties of the two supergroups and of the Pax-2, 5, 8 and Pax-6 subgroups within supergroup I. The differences in binding properties between the two supergroups are largely caused by amino acid changes at residues 20 and 121 of the paired domain. Although the paired domains of the Pax-2, 5, 8 and the Pax-6 group differ by >19 amino acids, their distinct DNA-binding properties are determined almost completely by a single amino acid change. Thus, a small number of amino acid changes can account in large part for the divergence in binding properties among the known paired domains. Our approach for selecting candidate sites responsible for the functional divergence between genes should also be useful for studying other gene families.

Pax genes and eye organogenesis

Pax6 is a highly conserved gene that controls eye development in all species where it has been tested. In spite of this common 'master control regulator', the eyes of different animals are morphologically very different and it is believed that they have evolved independently multiple times through evolution. Recent works looking at eye development in 'primitive' species offer some explanation as to the surprising amount of conservation in genetic and morphogenetic pathways involved in eye development. These studies not only implicate the Pax genes but also the So/Six gene family in playing a crucial ancestral role in visual system development.

Highly conserved amino acids in Pax and Ets proteins are required for DNA binding and ternary complex assembly

Combinatorial association of DNA-binding proteins on composite binding sites enhances their nucleotide sequence specificity and functional synergy. As a paradigm for these interactions, Pax-5 (BSAP) assembles ternary complexes with Ets proteins on the B cell-specific mb-1 promoter through interactions between their respective DNA-binding domains. Pax-5 recruits Ets-1 to bind the promoter, but not the closely related Ets protein SAP1a. Here we show that, while several different mutations increase binding of SAP1a to an optimized Ets binding site, only conversion of Val68 to an acidic amino acid facilitates ternary complex assembly with Pax-5 on the mb-1 promoter. This suggests that enhanced DNA binding by SAP1a is not sufficient for recruitment by Pax-5, but instead involves protein-protein interactions mediated by the acidic side chain. Recruitment of Ets proteins by Pax-5 requires Gln22 within the N-terminal beta-hairpin motif of its paired domain. The beta-hairpin also participates in recognition of a subset of Pax-5-binding sites. Thus, Pax-5 incorporates protein-protein interaction and DNA recognition functions in a single motif. The Caenorhabditis elegans Pax protein EGL-38 also binds specifically to the mb-1 promoter and recruits murine Ets-1 or the C.elegans Ets protein T08H4.3, but not the related LIN-1 protein. Together, our results define specific amino acid requirements for Pax-Ets ternary complex assembly and show that the mechanism is conserved between evolutionarily related proteins of diverse animal species. Moreover, the data suggest that interactions between Pax and Ets proteins are an important mechanism that regulates fundamental biological processes in worms and humans.

Isolation of Cladonema Pax-B genes and studies of the DNA-binding properties of cnidarian Pax paired domains

Pax genes encode nuclear transcription factors that are involved in developmental control. They contain a conserved DNA-binding domain, the paired domain. The DNA-binding specificity of paired domains is directly related to the gene regulation function of Pax proteins. Pax genes were previously divided into five groups on the basis of a phylogenetic analysis of paired domains. In this study, two highly similar cnidarian Pax-B genes from Cladonema californicum, a jellyfish with eyes, were found and sequenced. In an effort to understand the function of the cnidarian Pax genes isolated in this and a previous study, we characterized the consensus DNA sequences bound by the cnidarian paired domains using a PCR-based method and electrophoretic mobility shift assays. The consensus DNA sequences obtained are very similar to those bound by mammalian Pax proteins. Comparison of known consensus sequences indicates that they are all partially palindromic, but this characteristic is most prominent in cnidarians, which suggests that the DNA sequences bound by the ancestral paired domain could have been palindromic. Also, cnidarian paired domains, like those of Pax-2/5/8, possess a broader binding specificity than other paired domains, which implies that the common ancestor of Pax-2/5/8 and Pax-4/6 paired domains could also have had a similar broad DNA-binding specificity. Thus far, a definitive Pax-6 gene has not been found in several cnidarian species examined, which is consistent with a later origin of the Pax-6 gene and raises two possibilities: the Pax genes of cnidarians are multifunctional and control two or more developmental pathways, including eye development, or they use a Pax-independent pathway for eye development. Whether this pathway does exist and is unique to cnidarians or it whether it represents a true master control under which Pax-6 was later included remains to be determined.

The paired homeodomain transcription factor Pax-2 is expressed in the endocrine pancreas and transactivates the glucagon gene promoter

Glucagon gene expression is controlled by at least four DNA elements within the promoter; G2, G3, and G4 confer islet-specific expression, while G1 restricts glucagon transcription to alpha cells. Two islet-specific complexes are formed on G3, the insulin-responsive element of the glucagon gene; one of these corresponds to the paired homeodomain protein Pax-6, a major glucagon gene transactivator that plays a crucial role in alpha cell development. We describe here the identification of the second complex as Pax-2, another member of the paired box family. Pax-2 is known to be crucial for the development of the urogenital tract and of the central nervous system, but its presence in the endocrine pancreas has not been reported. We detected Pax-2 gene expression by RT-PCR; in islets, Pax-2 is present as two alternative splicing isoforms, Pax-2A and Pax-2B, whereas in the glucagon- and insulin-producing cell lines alphaTC1 and Min6, a distinct isoform, Pax-2D2, is found in addition to Pax-2B. Both islet-specific isoforms bind to the enhancer element G3 and to the alpha-specific promoter element G1 that also interacts with Pax-6. Pax-2A and Pax-2B dose-dependently activate transcription from the G3 and the G1 elements both in heterologous and in glucagon-producing cells. Our data indicate that Pax-2 is the third paired domain protein present in the endocrine pancreas and that one of its roles may be the regulation of glucagon gene expression.

Homeobox gene diversification in the calcareous sponge, Sycon raphanus

Knowledge of the developmental mechanisms in living basal metazoan phyla is crucial for understanding the genetic bases of morphological evolution in early animal history. We looked for homeobox genes in the calcareous sponge, Sycon raphanus, using the polymerase chain reaction. Partial sequences of eight homeoboxes were recovered, five of which are assignable to the NK-2 class of homeoboxes. The three remaining sequences are related members of a new class of homeoboxes, the Sycox class, showing limited similarity to bilaterian Lbx, Hlx, HEX, En, and Cad classes. Among the five NK-2 class homeoboxes are four closely related sequences occupying a divergent position within the class, the remaining one on the contrary showing high sequence similarity with members of the NK-2 family, a particular subgroup within the NK-2 class, previously known only from the Bilateria. This suggests that diversification of the NK-2 class occurred early in metazoan history. Altogether, the results reveal an unexpected diversification of homeobox genes in S. raphanus.

Xenopus Pax-2/5/8 orthologues: novel insights into Pax gene evolution and identification of Pax-8 as the earliest marker for otic and pronephric cell lineages

Pax genes are a family of transcription factors playing fundamental roles during organogenesis. We have recently demonstrated the expression of Pax-2 during Xenopus embryogenesis [Heller N, Brändli AW (1997): Mech Dev 69: 83-104]. Here we report the cloning and characterization of Xenopus Pax-5 and Pax-8, two orthologues of the Pax-2/5/8 gene family. Molecular phylogenetic analysis indicates that the amphibian Pax-2/5/8 genes are close relatives of their mammalian counterparts and that all vertebrate Pax-2/5/8 genes are derived from a single ancestral gene. Xenopus Pax-2/5/8 genes are expressed in spatially and temporally overlapping patterns during development of at least seven distinct tissues. Most strikingly, Xenopus Pax-8 was identified as the earliest marker of the prospective otic placode and of the intermediate mesoderm, indicating that Pax-8 may play a central role in auditory and excretory system development. Comparison of the expression patterns of fish, amphibian, and mammalian Pax-2/5/8 genes revealed that the tissue specificity of Pax-2/5/8 gene family expression is overall evolutionarily conserved. The expression domains of individual orthologues can however vary in a species-specific manner. For example, the thyroid glands of mammals express Pax-8, while in Xenopus Pax-2 is expressed instead. Our findings indicate that differential silencing of Pax-2/5/8 gene expression may have occurred after the different classes of vertebrates began to evolve separately.

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.

A novel Pax-6 binding site in rodent B1 repetitive elements: coevolution between developmental regulation and repeated elements?

Pax-6 encodes a transcription factor that is important in the development of eye and CNS. Identification of Pax-6 target genes is crucial for understanding the gene regulatory network in these developmental processes. Using an in-vitro approach of cyclic amplification of the protein binding sequences (CAPBS), we isolated a PAX6 binding sequence from a human single-copy (sc) DNA library. Characterization of this PAX6 binding sequence revealed a 15bp region (hGCalpha1BLs5) that is sufficient for PAX6 specific binding. From a homology search in the GenBank, we found that an hGCalpha1BLs5-like Pax-6 binding site exists in 21 genes (16 from rodent), 15 of which were shown to be able to bind Pax-6 in vitro. Interestingly, some of these sites occur in B1 repetitive elements. Although hGCalpha1BLs5 is highly similar to a region in B1 repetitive elements, PAX6 does not bind to the consensus sequence in B1. However, a single-step mutation in some B1 elements can lead to a gain of function for PAX6 binding. This experimental evidence and phylogenetic analysis raise an interesting speculation for the coevolution between PAX6 regulation and repeat elements. Since a (Pax-6-binding) null B1 element can be re-activated by even a single-step mutation, it has the potential to recruit gene targets for Pax-6 if it is inserted into the regulatory region, and therefore may play a role for evolutionary modification of Pax-6 regulation.

The coral Acropora: what it can contribute to our knowledge of metazoan evolution and the evolution of developmental processes

The diploblastic Cnidaria form one of the most ancient metazoan phyla and thus provide a useful outgroup for comparative studies of the molecular control of development in the more complex, and more often studied, triploblasts. Among cnidarians, the reef building coral Acropora is a particularly appropriate choice for study. Acropora belongs to the Anthozoa, which several lines of evidence now indicate is the basal class within the phylum Cnidaria, and has the practical advantages that its reproduction is predictable, external and accessible and that the base content of its genome is not strongly biased. The Acropora system has already provided insights into ancestral linkages of homeobox genes and the evolution of the Pax genes, and has the potential to provide further new perspectives on the age, role in development, and evolution of these and other gene families.

Origin of anterior patterning. How old is our head?

Most animals that display a bilateral symmetry (bilaterians) share homologous regulatory genes involved in head development. Recently, homologues of several of these genes have been cloned from animals that are radially organized, such as coral, sea anemones, jellyfish or hydra (cnidarians). Surprisingly, some of these are expressed apically and/or during apical patterning in hydrozoans, suggesting that head patterning is much older than previously thought.

Silencing of developmental genes in Hydra

Numerous developmental control genes have been isolated in a variety of organisms by either homology cloning or system-specific strategies. Functional genetic tests, however, are available for only a few model organisms and particularly are missing in a number of animals that occupy key positions for understanding the evolution of development and gene function. Double-stranded RNA-mediated interference (RNAi) opens a way to perform functional studies in such "nongenetic" organisms. Here we show that RNAi can be used to test the function of developmental genes in the cnidarian Hydra, a classical model for developmental studies. Introduction of double-stranded RNA corresponding to the head-specific gene ks1 caused strong depletion of ks1 transcripts. ks1 loss-of-function polyps exhibited severe defects in head formation, indicating an important role of ks1 in Hydra head development. Our results demonstrate for the first time efficient gene silencing in Hydra. RNAi provides an entry point for a variety of functional studies and a direct approach for analyzing the hierarchy of regulatory genes in Hydra, which until now has not been amenable to loss-of-function genetics.

Synergism between Pax-8 and lim-1 in embryonic kidney development

Pax genes encode a family of highly conserved DNA-binding transcription factors. These proteins play key roles in regulating a number of vertebrate and invertebrate developmental processes. Mutations in Pax-6 result in eye defects in flies, mice, and humans, and ectopic expression of this gene can trigger the development of ectopic compound eyes in flies. Likewise, mutation of other Pax genes in vertebrates results in the failure of specific differentiation programs-Pax-1 causes skeletal defects; Pax-2, kidney defects; Pax-3 or Pax-7, neural crest defects; Pax-4, pancreatic beta-cell defects; Pax-5, B-cell defects; Pax-8, thyroid defects; and Pax-9, tooth defects. Although this class of genes is obviously required for the normal differentiation of a number of distinct organ systems, they have not previously been demonstrated to be capable of directing the embryonic development of organs in vertebrates. In this report, it is demonstrated that Pax-8 plays such a role in the establishment of the Xenopus embryonic kidney, the pronephros. However, in order to efficiently direct cells to form pronephric kidneys, XPax-8 requires cofactors, one of which may be the homeobox transcription factor Xlim-1. These two genes are initially expressed in overlapping domains in late gastrulae, and cells expressing both genes will go on to form the kidney. Ectopic expression of either gene alone has a moderate effect on pronephric patterning, while coexpression of XPax-8 plus Xlim-1 results in the development of embryonic kidneys of up to five times normal complexity and also leads to the development of ectopic pronephric tubules. This effect was synergistic rather than additive. XPax-2 can also synergize with Xlim-1, but the expression profile of this gene indicates that it normally functions later in pronephric development than does XPax-8. Together these data indicate that the interaction between XPax-8 and Xlim-1 is a key early step in the establishment of the pronephric primordium.

Early animal evolution: emerging views from comparative biology and geology

The Cambrian appearance of fossils representing diverse phyla has long inspired hypotheses about possible genetic or environmental catalysts of early animal evolution. Only recently, however, have data begun to emerge that can resolve the sequence of genetic and morphological innovations, environmental events, and ecological interactions that collectively shaped Cambrian evolution. Assembly of the modern genetic tool kit for development and the initial divergence of major animal clades occurred during the Proterozoic Eon. Crown group morphologies diversified in the Cambrian through changes in the genetic regulatory networks that organize animal ontogeny. Cambrian radiation may have been triggered by environmental perturbation near the Proterozoic-Cambrian boundary and subsequently amplified by ecological interactions within reorganized ecosystems.

Developmental expression of Pax1/9 genes in urochordate and hemichordate gills: insight into function and evolution of the pharyngeal epithelium

The epithelium of the pharynx contributes to the formation of gills in hemichordates, urochordates, cephalochordates and primitive vertebrates, and is therefore a key structure for understanding developmental mechanisms underlying the establishment of chordate body plans. Pax1- and Pax9-related genes encode transcription factors which are expressed in the pharyngeal region of cephalochordates as well as in the vertebrate pharyngeal pouch epithelium that forms the thymus and parathyroid glands. To explore the molecular basis underlying the occurrence and modifications of the pharyngeal epithelium during evolution, we isolated cDNA clones for Pax1- and Pax9-related genes of urochordates (HrPax1/9 of Halocynthia roretzi and CiPax1/9 of Ciona intestinalis) and a hemichordate (PfPax1/9 of Ptychodera flava) from gill cDNA libraries. Each gene is present as a single copy per haploid genome. All of the cDNAs encode typical paired domains and octapeptides but not a homeodomain, as is also true of other Pax1- and Pax9-related genes. Molecular phylogenetic analysis based on comparison of the paired domain amino-acid sequences suggests that HrPax1/9, CiPax1/9 and PfPax1/9 belong to the Pax1/9 subfamily, and that they are descendants of a single precursor of Pax1/Pax9. Screening of HrPax1/9 cDNA clones yielded six different types of transcripts which were generated by alternative splicing. Northern blot, RT-PCR/Southern and in situ hybridization analyses revealed that HrPax1/9, CiPax1/9 and PfPax1/9 are not expressed during early embryogenesis but are expressed in the epithelia of differentiating gills, suggesting that these genes encode gill-specific transcription factors. The Pax1/9 genes therefore might provide the first developmental genetic corroboration of hypotheses of organ-level homology that unifies hemichordates, urochordates and cephalochordates.

Isolation and expression of a Pax-6 gene in the regenerating and intact Planarian Dugesia(G)tigrina

The Pax-6 gene encodes a transcription factor containing both a paired and a homeodomain and is highly conserved among Metazoa. In both vertebrates and invertebrates, Pax-6 is required for eye morphogenesis, development of parts of the central nervous system, and, in some phyla, for the development of olfactory sense organs. Ectopic expression of Pax-6 from insects, mammals, cephalopods, and ascidians induces ectopic eyes in Drosophila, suggesting that Pax-6 may be a universal master control gene for eye morphogenesis. Platyhelminthes are an ancient phylum, originating from the base of spiralian protostomes, that bear primitive eyes, consisting of a group of rhabdomeric photoreceptor cells enclosed in a cup of pigment cells. The analysis of Pax-6 and its expression pattern should provide insights into the ancestral function of Pax-6 in eye morphogenesis. We have identified the Pax-6 gene of the planarian Dugesia(G)tigrina (Platyhelminthes; Turbellaria; Tricladida). This gene shares significant sequence identity and conserved genomic organization with Pax-6 proteins from other phyla. Phylogenetic analysis indicates that it clusters with the other Pax-6 genes, but in the most basal position. DtPax-6 is expressed as a single transcript in both regenerating and fully grown eyes, and electron microscopy studies show strong expression in the perykarion of both photoreceptor and pigment cells. Very low levels of expression also are detectable in other body regions. Because a bona fide Pax-6 homolog so far has not been detected in diploblastic animals, we speculate that Pax-6 may be typical for triploblasts and that the appearance of additional Pax genes may have coincided with increasingly complex body plans.

Retinoic acid X receptor in the diploblast, Tripedalia cystophora

Nuclear hormone receptors comprise a characteristic family of transcription factors found in vertebrates, insects and nematodes. Here we show by cDNA and gene cloning that a Cnidarian, Tripedalia cystophora, possesses a retinoid receptor (jRXR) with remarkable homology to vertebrate retinoic acid X receptors (RXRs). Like vertebrate RXRs, jRXR binds 9-cis retinoic acid (Kd = 4 x 10(-10) M) and binds to the DNA sequence, PuGGTCA as a monomer in vitro. jRXR also heterodimerizes with Xenopus TR beta on a thyroid responsive element of a direct repeat separated by 4 bp. A jRXR binding half-site capable of interacting with (His6)jRXR fusion protein was identified in the promoters of three T. cystophora crystallin genes that are expressed highly in the eye lens of this jellyfish. Because crystallin gene expression is regulated by retionoid signaling in vertebrates, the jellyfish crystallin genes are candidate in vivo targets for jRXR. Finally, an antibody prepared against (His6)jRXR showed that full-length jRXR is expressed at all developmental stages of T. cystophora except the ephydra, where a smaller form replaces is. These data show that Cnidaria, a diploblastic phylum ancestral to the triploblastic invertebrate and subsequent vertebrate lineages, already have an RXR suggesting that RXR is an early component of the regulatory mechanisms of metazoa.

Isolation and developmental expression of the amphioxus Pax-6 gene (AmphiPax-6): insights into eye and photoreceptor evolution

Pax-6 genes have been identified from a broad range of invertebrate and vertebrate animals and shown to be always involved in early eye development. Therefore, it has been proposed that the various types of eyes evolved from a single eye prototype, by a Pax-6-dependent mechanism. Here we describe the characterization of a cephalochordate Pax-6 gene. The single amphioxus Pax-6 gene (AmphiPax-6) can produce several alternatively spliced transcripts, resulting in proteins with markedly different amino and carboxy termini. The amphioxus Pax-6 proteins are 92% identical to mammalian Pax-6 proteins in the paired domain and 100% identical in the homeodomain. Expression of AmphiPax-6 in the anterior epidermis of embryos may be related to development of an olfactory epithelium. Expression is also detectable in Hatschek's left diverticulum as it forms the preoral ciliated pit, part of which gives rise to the homolog of the vertebrate anterior pituitary. A zone of expression in the anterior neural plate of early embryos is carried into the cerebral vesicle (a probable diencephalic homolog) during neurulation. This zone includes cells that will differentiate into the lamellar body, a presumed homolog of the vertebrate pineal eye. In neurulae, AmphiPax-6 is also expressed in ventral cells at the anterior tip of the nerve cord; these cells are precursors of the photoreceptive neurons of the frontal eye, the presumed homolog of the vertebrate paired eyes. However, AmphiPax-6 expression was not detected in two additional types of photoreceptors, the Joseph cells or the organs of Hesse, which are evidently relatively recent adaptations (ganglionic photoreceptors) and appear to be rare exceptions to the general rule that animal photoreceptors develop from a genetic program triggered by Pax-6.

prdl-a, a gene marker for hydra apical differentiation related to triploblastic paired-like head-specific genes

Two homeobox genes, prdl-a and prdl-b, which were isolated from a Hydra vulgaris cDNA library, encode paired-like class homeodomains highly related to that of the aristaless-related genes. In adult polyps, prdl-b is a marker for synchronously dividing nematoblasts while prdl-a displays an expression restricted to the the nerve cell lineage of the head region. During budding and apical regeneration, an early and transient prdl-a expression was observed in endodermal cells of the stump at a time when the head organizer is established. When apical regeneration was delayed upon concomittant budding, prdl-a expression was found to be altered in the stump. Furthermore, a specific anti-prdl-a protein immunoserum revealed that prdl-a was overexpressed in adult polyps of the Chlorohydra viridissima multiheaded mutant, with an expression domain extending below the tentacle ring towards the body column. Accordingly, prdl-a DNA-binding activity was enhanced in nuclear extracts from this mutant. These results suggest that prdl-a responds to apical forming signals and might thus be involved in apical specification. When a marine hydrozoan (Podocorynae carnea) was used, the anti-prdl-a antibody showed cross-reactivity with cells located around the oral region, indicating that prdl-a function is shared by other cnidaria. The ancestral role for prdl-a-related genes in the molecular definition of the head (or oral-surrounding region) is discussed.

Organisation of the human PAX4 gene and its exclusion as a candidate for the Wolcott-Rallison syndrome

Neonatal diabetes mellitus is a rare condition, the causes of which are mostly unknown. One well defined though very rare entity is the autosomal recessive Wolcott-Rallison syndrome, in which permanent neonatal diabetes, osteopenia, and epiphyseal dysplasia occur. Only five previous families have been reported, and here we describe the second in which parental consanguinity was present. The proband was born to first cousin parents and died at 2 years from the sequelae of poorly controlled diabetes. To test the hypothesis that mutation of PAX4, required in the mouse for pancreatic islet beta cell development, might cause WRS, the structure of the human PAX4 gene was deduced and DNA from two unrelated WRS patients sequenced. No PAX4 mutation was present, though the entire coding region was sequenced in both patients. It therefore appears unlikely that PAX4 is involved in the aetiology of Wolcott-Rallison syndrome, though it remains a good candidate for other forms of neonatal diabetes mellitus.

Tripartite organization of the ancestral chordate brain and the antiquity of placodes: insights from ascidian Pax-2/5/8, Hox and Otx genes

Ascidians and vertebrates belong to the Phylum Chordata and both have dorsal tubular central nervous systems. The structure of the ascidian neural tube is extremely simple, containing less than 400 cells, among which less than 100 cells are neurons. Recent studies suggest that, despite its simple organization, the mechanisms patterning the ascidian neural tube are similar to those of the more complex vertebrate brain. Identification of homologous regions between vertebrate and ascidian nervous systems, however, remains to be resolved. Here we report the expression of HrPax-258 gene: an ascidian homologue of vertebrate Pax-2, Pax-5 and Pax-8 genes. Molecular phylogenetic analyses indicate that HrPax-258 is descendant from a single precursor gene that gave rise to the three vertebrate genes. The expression pattern of HrPax-258 suggests that this subfamily of Pax genes has conserved roles in regional specification of the brain. Comparison with expression of ascidian Otx (Hroth) and a Hox gene (HrHox1) by double-staining in situ hybridizations indicate that the ascidian brain region can be subdivided into three regions; the anterior region marked by Hroth probably homologous to the vertebrate forebrain and midbrain, the middle region marked by HrPax-258 probably homologous to the vertebrate anterior hindbrain (and maybe also midbrain) and the posterior region marked by Hox genes which is homologous to the vertebrate hindbrain and spinal cord. Later expression of HrPax-258 in atrial primordia implies that basal chordates such as ascidians have already acquired a sensory organ that develops from epidermal thickenings (placodes) and expresses HrPax-258; we suggest it is homologous to the vertebrate ear. Therefore, placodes are not likely to be a newly acquired feature in vertebrates, but may have already been possessed by the earliest chordates.