Expression of Cnox-2, a HOM/HOX homeobox gene in hydra, is correlated with axial pattern formation

Epigenomic landscape of enhancer elements during Hydra head organizer formation

BACKGROUND

Axis patterning during development is accompanied by large-scale gene expression changes. These are brought about by changes in the histone modifications leading to dynamic alterations in chromatin architecture. The cis regulatory DNA elements also play an important role towards modulating gene expression in a context-dependent manner. Hydra belongs to the phylum Cnidaria where the first asymmetry in the body plan was observed and the oral-aboral axis originated. Wnt signaling has been shown to determine the head organizer function in the basal metazoan Hydra.

RESULTS

To gain insights into the evolution of cis regulatory elements and associated chromatin signatures, we ectopically activated the Wnt signaling pathway in Hydra and monitored the genome-wide alterations in key histone modifications. Motif analysis of putative intergenic enhancer elements from Hydra revealed the conservation of bilaterian cis regulatory elements that play critical roles in development. Differentially regulated enhancer elements were identified upon ectopic activation of Wnt signaling and found to regulate many head organizer specific genes. Enhancer activity of many of the identified cis regulatory elements was confirmed by luciferase reporter assay. Quantitative chromatin immunoprecipitation analysis upon activation of Wnt signaling further confirmed the enrichment of H3K27ac on the enhancer elements of Hv_Wnt5a, Hv_Wnt11 and head organizer genes Hv_Bra1, CnGsc and Hv_Pitx1. Additionally, perturbation of the putative H3K27me3 eraser activity using a specific inhibitor affected the ectopic activation of Wnt signaling indicating the importance of the dynamic changes in the H3K27 modifications towards regulation of the genes involved in the head organizer activity.

CONCLUSIONS

The activation-associated histone marks H3K4me3, H3K27ac and H3K9ac mark chromatin in a similar manner as seen in bilaterians. We identified intergenic cis regulatory elements which harbor sites for key transcription factors involved in developmental processes. Differentially regulated enhancers exhibited motifs for many zinc-finger, T-box and ETS related TFs whose homologs have a head specific expression in Hydra and could be a part of the pioneer TF network in the patterning of the head. The ability to differentially modify the H3K27 residue is critical for the patterning of Hydra axis revealing a dynamic acetylation/methylation switch to regulate gene expression and chromatin architecture.

Box, stalked, and upside-down? Draft genomes from diverse jellyfish (Cnidaria, Acraspeda) lineages: Alatina alata (Cubozoa), Calvadosia cruxmelitensis (Staurozoa), and Cassiopea xamachana (Scyphozoa)

BACKGROUND

Anthozoa, Endocnidozoa, and Medusozoa are the 3 major clades of Cnidaria. Medusozoa is further divided into 4 clades, Hydrozoa, Staurozoa, Cubozoa, and Scyphozoa-the latter 3 lineages make up the clade Acraspeda. Acraspeda encompasses extraordinary diversity in terms of life history, numerous nuisance species, taxa with complex eyes rivaling other animals, and some of the most venomous organisms on the planet. Genomes have recently become available within Scyphozoa and Cubozoa, but there are currently no published genomes within Staurozoa and Cubozoa.

FINDINGS

Here we present 3 new draft genomes of Calvadosia cruxmelitensis (Staurozoa), Alatina alata (Cubozoa), and Cassiopea xamachana (Scyphozoa) for which we provide a preliminary orthology analysis that includes an inventory of their respective venom-related genes. Additionally, we identify synteny between POU and Hox genes that had previously been reported in a hydrozoan, suggesting this linkage is highly conserved, possibly dating back to at least the last common ancestor of Medusozoa, yet likely independent of vertebrate POU-Hox linkages.

CONCLUSIONS

These draft genomes provide a valuable resource for studying the evolutionary history and biology of these extraordinary animals, and for identifying genomic features underlying venom, vision, and life history traits in Acraspeda.

Cellular and Molecular Mechanisms of Hydra Regeneration

Regeneration of lost body parts is essential to regain the fitness of the organism for successful living. In the animal kingdom, organisms from different clades exhibit varied regeneration abilities. Hydra is one of the few organisms that possess tremendous regeneration potential, capable of regenerating complete organism from small tissue fragments or even from dissociated cells. This peculiar property has made this genus one of the most invaluable model organisms for understanding the process of regeneration. Multiple studies in Hydra led to the current understanding of gross morphological changes, basic cellular dynamics, and the role of molecular signalling such as the Wnt signalling pathway. However, cell-to-cell communication by cell adhesion, role of extracellular components such as extracellular matrix (ECM), and nature of cell types that contribute to the regeneration process need to be explored in depth. Additionally, roles of developmental signalling pathways need to be elucidated to enable more comprehensive understanding of regeneration in Hydra. Further research on cross communication among extracellular, cellular, and molecular signalling in Hydra will advance the field of regeneration biology. Here, we present a review of the existing literature on Hydra regeneration biology and outline the future perspectives.

DNA repair enzyme APE1 from evolutionarily ancient Hydra reveals redox activity exclusively found in mammalian APE1

Only mammalian apurinic/apyrimidinic endonuclease1 (APE1) has been reported to possess both DNA repair and redox activities. C terminal of the protein is required for base excision repair, while the redox activity resides in the N terminal due to cysteine residues at specific positions. APE1s from other organisms studied so far lack the redox activity in spite of having the N terminal domain. We find that APE1 from the Cnidarian Hydra exhibits both endonuclease and redox activities similar to mammalian APE1. We further show the presence of the three indispensable cysteines in Hydra APE1 for redox activity by site directed mutagenesis. Importance of redox domain but not the repair domain of APE1 in regeneration has been demonstrated by using domain-specific inhibitors. Our findings clearly demonstrate that the redox function of APE1 evolved very early in metazoan evolution and is not a recent acquisition in mammalian APE1 as believed so far.

A new paradigm for animal symmetry

My aim in this article is to soften certain rigid concepts concerning the radial and bilateral symmetry of the animal body plan, and to offer a more flexible framework of thinking for them, based on recent understandings of how morphogenesis is regulated by the mosaically acting gene regulatory networks. Based on general principles of the genetic regulation of morphogenesis, it can be seen that the difference between the symmetry of the whole body and that of minor anatomical structures is only a question of a diverse timing during development. I propose that the animal genome, as such, is capable of expressing both radial and bilateral symmetries, and deploys them according to the functional requirements which must be satisfied by both the anatomical structure and body as a whole. Although it may seem paradoxical, this flexible view of symmetry, together with the idea that symmetry is strongly determined by function, bolsters the concept that the presence of the two main symmetries in the animal world is not due to chance: they are necessary biological patterns emerging in evolution.

Cnidarians and the evolutionary origin of the nervous system

Cnidarians are widely regarded as one of the first organisms in animal evolution possessing a nervous system. Conventional histological and electrophysiological studies have revealed a considerable degree of complexity of the cnidarian nervous system. Thanks to expressed sequence tags and genome projects and the availability of functional assay systems in cnidarians, this simple nervous system is now genetically accessible and becomes particularly valuable for understanding the origin and evolution of the genetic control mechanisms underlying its development. In the present review, the anatomical and physiological features of the cnidarian nervous system and the interesting parallels in neurodevelopmental mechanisms between Cnidaria and Bilateria are discussed.

Conserved and novel gene expression between regeneration and asexual fission in Nematostella vectensis

Due to work in model systems (e.g., flies and mice), the molecular mechanisms of embryogenesis are known in exquisite detail. However, these organisms are incapable of asexual reproduction and possess limited regenerative abilities. Thus, the mechanisms of alternate developmental trajectories and their relation to embryonic mechanisms remain understudied. Because these developmental trajectories are present in a diverse group of animal phyla spanning the metazoan phylogeny, including cnidarians, annelids, and echinoderms, they are likely to have played a major role in animal evolution. The starlet sea anemone Nematostella vectensis, an emerging model system, undergoes larval development, asexual fission, and complete bi-directional regeneration in the field and laboratory. In order to investigate to what extent embryonic patterning mechanisms are utilized during alternate developmental trajectories, we examined expression of developmental regulatory genes during regeneration and fission. When compared to previously reported embryonic expression patterns, we found that all genes displayed some level of expression consistent with embryogenesis. However, five of seven genes investigated also displayed striking differences in gene expression between one or more developmental trajectory. These results demonstrate that alternate developmental trajectories utilize distinct molecular mechanisms upstream of major developmental regulatory genes such as fox, otx, and Hox-like.

Morphallactic regeneration as revealed by region-specific gene expression in the digestive tract of Enchytraeus japonensis (Oligochaeta, Annelida)

Enchytraeus japonensis is a small oligochaete, which primarily reproduces asexually by fragmentation and regeneration. For precise analysis of the pattern formation during regeneration, we isolated three region-specific genes (EjTuba, mino, and horu) expressed in the digestive tract. In growing worms, the expression of EjTuba in the head and mino in the trunk region just posterior to the head were observed in defined body segments, while the expression areas of EjTuba in the trunk and horu were proportional to the total number of body segments. In the regeneration process, expression of these genes disappeared once and recovered to their original pattern by day 7. In abnormal regeneration such as a bipolar head, mino was still expressed in the region next to both the normal and the ectopic heads. These results suggest that there is morphallactic as well as epimorphic or inductive regulation of the body patterning during regeneration of E. japonensis.

Changing hydrozoan bauplans by silencing Hox-like genes

Regulatory genes of the Antp class have been a major factor for the invention and radiation of animal bauplans. One of the most diverse animal phyla are the Cnidaria, which are close to the root of metazoan life and which often appear in two distinct generations and a remarkable variety of body forms. Hox-like genes have been known to be involved in axial patterning in the Cnidaria and have been suspected to play roles in the genetic control of many of the observed bauplan changes. Unfortunately RNAi mediated gene silencing studies have not been satisfactory for marine invertebrate organisms thus far. No direct evidence supporting Hox-like gene induced bauplan changes in cnidarians have been documented as of yet. Herein, we report a protocol for RNAi transfection of marine invertebrates and demonstrate that knock downs of Hox-like genes in Cnidaria create substantial bauplan alterations, including the formation of multiple oral poles (“heads”) by Cnox-2 and Cnox-3 inhibition, deformation of the main body axis by Cnox-5 inhibition and duplication of tentacles by Cnox-1 inhibition. All phenotypes observed in the course of the RNAi studies were identical to those obtained by morpholino antisense oligo experiments and are reminiscent of macroevolutionary bauplan changes. The reported protocol will allow routine RNAi studies in marine invertebrates to be established.

Head regeneration in wild-type hydra requires de novo neurogenesis

Because head regeneration occurs in nerve-free hydra mutants, neurogenesis was regarded as dispensable for this process. Here, in wild-type hydra, we tested the function of the ParaHox gsx homolog gene, cnox-2,which is a specific marker for bipotent neuronal progenitors, expressed in cycling interstitial cells that give rise to apical neurons and gastric nematoblasts (i.e. sensory mechanoreceptor precursors). cnox-2 RNAi silencing leads to a dramatic downregulation of hyZic, prdl-a, gscand cnASH, whereas hyCOUP-TF is upregulated. cnox-2indeed acts as an upstream regulator of the neuronal and nematocyte differentiation pathways, as cnox-2(-) hydra display a drastic reduction in apical neurons and gastric nematoblasts, a disorganized apical nervous system and a decreased body size. During head regeneration, the locally restricted de novo neurogenesis that precedes head formation is cnox-2 dependent: cnox-2 expression is induced in neuronal precursors and differentiating neurons that appear in the regenerating tip; cnox-2 RNAi silencing reduces this de novo neurogenesis and delays head formation. Similarly, the disappearance of cnox-2+cells in sf-1 mutants also correlates with head regeneration blockade. Hence in wild-type hydra, head regeneration requires the cnox-2 neurogenic function. When neurogenesis is missing, an alternative, slower and less efficient, head developmental program is possibly activated.

Pre-bilaterian origins of the Hox cluster and the Hox code: evidence from the sea anemone, Nematostella vectensis

BACKGROUND

Hox genes were critical to many morphological innovations of bilaterian animals. However, early Hox evolution remains obscure. Phylogenetic, developmental, and genomic analyses on the cnidarian sea anemone Nematostella vectensis challenge recent claims that the Hox code is a bilaterian invention and that no "true" Hox genes exist in the phylum Cnidaria.

METHODOLOGY/PRINCIPAL FINDINGS

Phylogenetic analyses of 18 Hox-related genes from Nematostella identify putative Hox1, Hox2, and Hox9+ genes. Statistical comparisons among competing hypotheses bolster these findings, including an explicit consideration of the gene losses implied by alternate topologies. In situ hybridization studies of 20 Hox-related genes reveal that multiple Hox genes are expressed in distinct regions along the primary body axis, supporting the existence of a pre-bilaterian Hox code. Additionally, several Hox genes are expressed in nested domains along the secondary body axis, suggesting a role in "dorsoventral" patterning.

CONCLUSIONS/SIGNIFICANCE

A cluster of anterior and posterior Hox genes, as well as ParaHox cluster of genes evolved prior to the cnidarian-bilaterian split. There is evidence to suggest that these clusters were formed from a series of tandem gene duplication events and played a role in patterning both the primary and secondary body axes in a bilaterally symmetrical common ancestor. Cnidarians and bilaterians shared a common ancestor some 570 to 700 million years ago, and as such, are derived from a common body plan. Our work reveals several conserved genetic components that are found in both of these diverse lineages. This finding is consistent with the hypothesis that a set of developmental rules established in the common ancestor of cnidarians and bilaterians is still at work today.

Why polyps regenerate and we don't: towards a cellular and molecular framework for Hydra regeneration

The basis for Hydra's enormous regeneration capacity is the "stem cellness" of its epithelium which continuously undergoes self-renewing mitotic divisions and also has the option to follow differentiation pathways. Now, emerging molecular tools have shed light on the molecular processes controlling these pathways. In this review I discuss how the modular tissue architecture may allow continuous replacement of cells in Hydra. I also describe the discovery and regulation of factors controlling the transition from self-renewing epithelial stem cells to differentiated cells.

Expression of a Gsx parahox gene, Cnox-2, in colony ontogeny in Hydractinia (Cnidaria: Hydrozoa)

The ontogeny of colonial animals is markedly distinct from that of solitary animals, yet no regulatory genes have thus far been implicated in colonial development. In cnidarians, colony ontogeny is characterized by the production of a nexus of vascular stolons, from which the feeding and reproductive structures, called polyps, are budded. Here we describe and characterize the Gsx parahox gene, Cnox-2, in the colonial cnidarian Hydractinia symbiolongicarpus of the class Hydrozoa. Cnox-2 is expressed in prominent components of the colony-wide patterning system; in the epithelia of distal stolon tips and polyp bud rudiments. Both are regions of active morphogenetic activity, characterized by cytologically and behaviorally distinct epithelia. Experimental induction and elimination of stolonal tips result in up- and down-regulation, respectively, of Cnox-2 expression. In the developing polyp, Cnox-2 expression remains uniformly high throughout the period of axial differentiation. The differential oral-aboral Cnox-2 expression in the epithelia of the mature polyp, previously described for this and another hydrozoan, arises after oral structures have completed development. Differential Cnox-2 expression is, thus, associated with key aspects of patterning of both the colony and the polyp, a finding that is particularly striking given that polyp and colony form are dissociable in the evolution of Hydrozoa.

Minimal ProtoHox cluster inferred from bilaterian and cnidarian Hox complements

Bilaterian animals have a Hox gene cluster essential for patterning the main body axis, and a ParaHox gene cluster. Comparison of Hox and ParaHox genes has led workers to postulate that both clusters originated from the duplication of an ancient cluster named ProtoHox, which contained up to four genes with at least the precursors of anterior and posterior Hox/ParaHox genes. However, the way in which genes diversified within the ProtoHox, Hox and ParaHox clusters remains unclear because no systematic study of non-bilaterian animals exists. Here we characterize the full Hox/ParaHox gene complements and genomic organization in two cnidarian species (Nematostella vectensis and Hydra magnipapillata), and suggest a ProtoHox cluster simpler than originally thought on the basis of three arguments. First, both species possess bilaterian-like anterior Hox genes, but their non-anterior genes do not appear as counterparts of either bilaterian central or posterior genes; second, two clustered ParaHox genes, Gsx and a gene related to Xlox and Cdx, are found in Nematostella vectensis; and third, we do not find clear phylogenetic support for a common origin of bilaterian Cdx and posterior genes, which might therefore have appeared after the ProtoHox cluster duplication. Consequently, the ProtoHox cluster might have consisted of only two anterior genes. Non-anterior genes could have appeared independently in the Hox and ParaHox clusters, possibly after the separation of bilaterians and cnidarians.

Asymmetric expression of the BMP antagonists chordin and gremlin in the sea anemone Nematostella vectensis: implications for the evolution of axial patterning

The evolutionary origin of the anterior-posterior and the dorsoventral body axes of Bilateria is a long-standing question. It is unclear how the main body axis of Cnidaria, the sister group to the Bilateria, is related to the two body axes of Bilateria. The conserved antagonism between two secreted factors, BMP2/4 (Dpp in Drosophila) and its antagonist Chordin (Short gastrulation in Drosophila) is a crucial component in the establishment of the dorsoventral body axis of Bilateria and could therefore provide important insight into the evolutionary origin of bilaterian axes. Here, we cloned and characterized two BMP ligands, dpp and GDF5-like as well as two secreted antagonists, chordin and gremlin, from the basal cnidarian Nematostella vectensis. Injection experiments in zebrafish show that the ventralizing activity of NvDpp mRNA is counteracted by NvGremlin and NvChordin, suggesting that Gremlin and Chordin proteins can function as endogenous antagonists of NvDpp. Expression analysis during embryonic and larval development of Nematostella reveals asymmetric expression of all four genes along both the oral-aboral body axis and along an axis perpendicular to this one, the directive axis. Unexpectedly, NvDpp and NvChordin show complex and overlapping expression on the same side of the embryo, whereas NvGDF5-like and NvGremlin are both expressed on the opposite side. Yet, the two pairs of ligands and antagonists only partially overlap, suggesting complex gradients of BMP activity along the directive axis but also along the oral-aboral axis. We conclude that a molecular interaction between BMP-like molecules and their secreted antagonists was already employed in the common ancestor of Cnidaria and Bilateria to create axial asymmetries, but that there is no simple relationship between the oral-aboral body axis of Nematostella and one particular body axis of Bilateria.

Complex colony-level organization of the deep-sea siphonophoreBargmannia elongata(Cnidaria, Hydrozoa) is directionally asymmetric and arises by the subdivision of pro-buds

Siphonophores are free-swimming colonial hydrozoans (Cnidaria) composed of asexually produced multicellular zooids. These zooids, which are homologous to solitary animals, are functionally specialized and arranged in complex species-specific patterns. The coloniality of siphonophores provides an opportunity to study the major transitions in evolution that give rise to new levels of biological organization, but siphonophores are poorly known because they are fragile and live in the open ocean. The organization and development of the deep-sea siphonophore Bargmannia elongata is described here using specimens collected with a remotely operated underwater vehicle. Each bud gives rise to a precise, directionally asymmetric sequence of zooids through a stereotypical series of subdivisions, rather than to a single zooid as in most other hydrozoans. This initial description of development in a deep-sea siphonophore provides an example of how precise colony-level organization can arise, and illustrates that the morphological complexity of cnidarians is greater than is often assumed.

Formation of the head organizer in hydra involves the canonical Wnt pathway

Stabilization of β-catenin by inhibiting the activity of glycogen synthase kinase-3β has been shown to initiate axis formation or axial patterning processes in many bilaterians. In hydra, the head organizer is located in the hypostome, the apical portion of the head. Treatment of hydra with alsterpaullone, a specific inhibitor of glycogen synthase kinase-3β,results in the body column acquiring characteristics of the head organizer, as measured by transplantation experiments, and by the expression of genes associated with the head organizer. Hence, the role of the canonical Wnt pathway for the initiation of axis formation was established early in metazoan evolution.

Ancient signals: peptides and the interpretation of positional information in ancestral metazoans

Understanding the 'tool kit' that builds the most fundamental aspects of animal complexity requires data from the basal animals. Among the earliest diverging animal phyla are the Cnidaria which are the first in having a defined body plan including an axis, a nervous system and a tissue layer construction. Here I revise our understanding of patterning mechanism in cnidarians with special emphasis on the nature of positional signals in Hydra as perhaps the best studied model organism within this phylum. I show that (i) peptides play a major role as positional signals and in cell-cell communication; (ii) that intracellular signalling pathways in Hydra leading to activation of target genes are shared with all multicellular animals; (iii) that homeobox genes translate the positional signals; and (iv) that the signals are integrated by a complex genetic regulatory machinery that includes both novel cis regulatory elements as well as taxon specific target genes. On the basis of these results I present a model for the regulatory interactions required for axis formation in Hydra.

The Trox-2 Hox/ParaHox gene of Trichoplax (Placozoa) marks an epithelial boundary

Hox and ParaHox genes are implicated in axial patterning of cnidarians and bilaterians, and are thought to have originated by tandem duplication of a single "ProtoHox" gene followed by duplication of the resultant gene cluster. It is unclear what the ancestral role of Hox/ParaHox genes was before the divergence of Cnidaria and Bilateria, or what roles the postulated ProtoHox gene(s) played. Here we describe the full coding region, spatial expression and function of Trox-2, the single Hox/ParaHox-type gene identified in Trichoplax adhaerens (phylum Placozoa) and either a candidate ProtoHox or a ParaHox gene. Trox-2 is expressed in a ring around the periphery of Trichoplax, in small cells located between the outer margins of the upper and lower epithelial cell layers. Inhibition of Trox-2 function, either by uptake of morpholino antisense oligonucleotides or by RNA interference, causes complete cessation of growth and binary fission. We speculate that Trox-2 functions within a hitherto unrecognized population of possibly multipotential peripheral stem cells that contribute to differentiated cells at the epithelial boundary of Trichoplax.

Hox and paraHox genes from the anthozoan Parazoanthus parasiticus

We surveyed the genome of the Caribbean zoanthid Parazoanthus parasiticus for Hox and paraHox genes, and examined gene expression patterns for sequences we uncovered. Two Hox genes and three paraHox genes were identified in our surveys. The Hox genes belong to anterior and posterior classes. In phylogenetic analyses, the anterior Hox sequence formed an anthozoan-specific cluster that appears to be a second class of cnidarian anterior Hox gene. The presence of an anterior Gsx-like paraHox gene supports the hypothesis that duplication of a protoHox gene family preceded the divergence of the Cnidaria and bilaterians. The presence of two Mox class paraHox genes in P. parasiticus deserves further attention. Expression analysis using RT-PCR, indicated that one Mox gene and the anterior paraHox gene are not expressed in adult tissue, whereas the other three sequences are expressed in both dividing and unitary polyps. Dividing polyps showed slightly lower Ppox1 (i.e., Mox) expression levels. Our data add to the number of published anthozoan sequences, and provide additional detail concerning the evolutionary significance of cnidarian Hox and paraHox genes.

Early evolution of a homeobox gene: the parahox gene Gsx in the Cnidaria and the Bilateria

Homeobox transcription factors are commonly involved in developmental regulation in diverse eukaryotes, including plants, animals, and fungi. The origin of novel homeobox genes is thought to have contributed to many evolutionary innovations in animals. We perform a molecular phylogenetic analysis of cnox2, the best studied homeobox gene from the phylum Cnidaria, a very ancient lineage of animals. Among three competing hypotheses, our analysis decisively favors the hypothesis that cnox2 is orthologous to the gsx gene of Bilateria, thereby establishing the existence of this specific homeobox gene in the eumetazoan stem lineage, some 650-900 million years ago. We assayed the expression of gsx in the planula larva and polyp of the sea anemone Nematostella vectensis using in situ hybridization and reverse transcriptase polymerase chain reaction. The gsx ortholog of Nematostella, known as anthox2, is expressed at high levels in the posterior planula and the corresponding "head" region of the polyp. It cannot be detected in the anterior planula or the corresponding "foot" region of the polyp. We have attempted to reconstruct the evolution of gsx spatiotemporal expression in cnidarians and bilaterians using a phylogenetic framework. Because of the surprisingly high degree of variability in gsx expression within the Cnidaria, it is currently not possible to infer unambiguously the ancestral cnidarian condition or the ancestral eumetazoan condition for gsx expression.

The biology of foetal development and injury

This oversimplified view of foetal development and the risk to injury aims to highlight the following: assuming there is a normal environment, gene expression will generate a protein chain that should fold to the expected stereological shape to function normally. Here we must take into consideration the important role played by external (environmental) factors. Abnormal organogenesis or foetal injury are, in all likelihood, due to abnormal genes or genes expressing themselves 'out of sinc', that is to say 'outside' their time allocated for expression. This type of injury is difficult to correct. Morphogenesis, or the continuous remodelling of formed organ/systems can be more amenable to correction since the basic layout of the organ already exists. The common denominator to organogenesis and morphogenesis, at cellular/tissue levels, is the harmonic interplay between cell multiplication, cell migration, cell differentiation, cell death, the generation of intercellular matrix and its resorption. All of this must take place at the 'right time'. Any departure from it may lead to injury, whether clinically detectable or not.

Coral biodiversity and evolution: recent molecular contributions

Tropical reefs are among the most diverse ecosystems. Corals, as the most prominent members and framework builders of these communities, deserve special attention, especially in light of the recent decline of coral reefs worldwide. The diversity of corals at various levels has been the subject of many studies, and has traditionally been investigated using morphological characters. This approach has proved insufficient, owing to several ecological and life-history traits of corals. The use of molecular/biochemical approaches has been propelling this discipline forward at an ever-increasing rate for the past decade or so. Reticulate evolution in corals, which has challenged traditional views on the ecology, evolution, and biodiversity of these organisms, is only one example of the results of molecular studies supporting the development of new concepts. We review recent literature reporting studies of the biodiversity, ecology, and evolution of corals in which molecular methods have been employed. We anticipate that in the coming years, an increasing number of studies in molecular biology will generate new and exciting ideas regarding the biology of corals.

Patterning and cell differentiation inHydra: novel genes and the limits to conservation

In the last few years more than 100 genes have been identified from Hydra, and well over 80 have been characterized. Since most genes are homologs of genes found in bilaterians, the genetic mechanisms for axial patterning and cell differentiation are evolutionarily conserved. This constitutes something of a paradox. If key developmental-control genes are the same in Hydra and all other organisms, how does one account for the marked differences in development and morphology of the different animal groups? How are taxon-specific features encoded? To examine whether in Hydra, in addition to conserved mechanisms, there are genetic features that control uniquely taxon-specific (Hydra/Hydrozoa/Cnidaria) aspects, we used an experimental strategy that does not require sequence data from related taxa. By means of this unbiased ("knowledge-independent") approach we have identified genes from Hydra encoding signal molecules and effector genes with no sequence similarity to genes in other organisms. When tested functionally, the novel genes were found to be essential for axial patterning and differentiation of Hydra-specific characteristics. Experimental analysis of the cis-regulatory apparatus of these novel genes reveals target sites for novel trans-acting factors. The use of unbiased screening approaches for several other organisms also reveals a large number of novel and taxon-specific genes of as yet unknown function. Thus, comparative data alone may not be sufficient for gaining a full understanding of the development of taxon-specific characteristics.

Is the evolution of Cnox-2 Hox/ParaHox genes "multicolored" and "polygenealogical?"

Understanding the evolution of metazoan bauplans is linked to understanding the evolution of Hox and ParaHox genes. At the base of metazoan radiation we see in both cases a quite confusing picture yet. Here Cnox-2 is one of the best studied diploblast Hox genes. Homologs of this gene are known from Placozoa and several Cnidaria. In those cases where full length gene sequences, or at least full length homeobox sequences, are available the relationship to Hox genes from triploblastic animals as well as the classification to Hox or ParaHox genes can be controversially discussed. The existing data on possible gene functions also reveal a quite heterogeneous picture. It seems conceivable that part of the "multicolored" picture relates to a "polygenealogical" origin of the Cnox-2 gene.

Cnidarian and bilaterian promoters can direct GFP expression in transfected hydra

Complete sexual development is not easily amenable to experimentation in hydra. Therefore, the analysis of gene function and gene regulation requires the introduction of exogenous DNA in a large number of cells of the hydra polyps and the significant expression of reporter constructs in these cells. We present here the procedure whereby we coupled DNA injection into the gastric cavity to electroporation of the whole animal in order to efficiently transfect hydra polyps. We could detect GFP fluorescence in both endodermal and ectodermal cell layers of live animals and in epithelial as well as interstitial cell types of dissociated hydra. In addition, we could confirm GFP protein expression by showing colocalisation between GFP fluorescence and anti-GFP immunofluorescence. Finally, when a FLAG epitope was inserted in-frame with the GFP coding sequence, GFP fluorescence also colocalised with anti-FLAG immunofluorescence. This GFP expression in hydra cells was directed by various promoters, either homologous, like the hydra homeobox cnox-2 gene promoter, or heterologous, like the two nematode ribosomal protein S5 and L28 gene promoters, and the chicken beta-actin gene promoter. This strategy provides new tools for dissecting developmental molecular mechanisms in hydra; more specifically, the genetic regulations that take place in endodermal cells at the time budding or regeneration is initiated.

The radial-symmetric hydra and the evolution of the bilateral body plan: an old body became a young brain

The radial symmetric cnidarians are regarded as being close to the common metazoan ancestor before bilaterality evolved. It is proposed that a large fraction of the body of this gastrula-like organism gave rise to the head of more evolved organisms. The trunk was added later in evolution from an unfolding of a narrow zone between the tentacles and the blastoporus. This implies that, counter intuitively, the foot of the hydra corresponds to the most anterior part (forebrain and heart) while the opening of the gastric column gave rise to the anus. Two fundamentally different modes of midline formation evolved. In vertebrates, the organiser attracts cells from the both sides of the marginal zone. These leave the organiser as a unified band. The midline is formed sequentially from anterior to posterior. In insects, the midline forms opposite a dorsal repelling center, i.e., on the ventral side. This can occur more or less simultaneously over the whole anteroposterior extension.

Current problems with the zootype and the early evolution of Hox genes

"Hox cluster type" genes have sparked intriguing attempts to unite all metazoan animals by a shared pattern of expression and genomic organization of a specific set of regulatory genes. The basic idea, the zootype concept, claims the conservation of a specific set of "Hox cluster type genes" in all metazoan animals, i.e., in the basal diploblasts as well as in the derived triploblastic animals. Depending on the data used and the type of analysis performed, different opposing views have been taken on this idea. We review here the sum of data currently available in a total evidence analysis, which includes morphological and the most recent molecular data. This analysis highlights several problems with the idea of a simple "Hox cluster type" synapomorphy between the diploblastic and triploblastic animals and suggests that the "zootype differentiation" of the Hox cluster most likely is an invention of the triploblasts. The view presented is compatible with the idea that early Hox gene evolution started with a single proto-Hox (possibly a paraHox) gene. J. Exp. Zool. (Mol. Dev. Evol.) 291:169-174, 2001.

Conservation of Hox/ParaHox-related genes in the early development of a cnidarian

To clarify the relationship between axial patterning in cnidarians and bilaterians, we have investigated the embryonic development of the hydrozoan Podocoryne carnea. The expression of Hox-like homeobox genes was analyzed by RT-PCR and in situ hybridization. Cnox1-Pc, an anterior Hox gene, is a maternal message. It is present throughout larval development, first weakly in all blastomeres and later restricted mostly to the anterior pole of the planula. Gsx, an anterior ParaHox gene, is first seen in the anterior endoderm but also extends into posterior regions. Cnox4-Pc, an orphan Hox-like gene, is expressed in the egg as a ring-shaped cloud around the germinal vesicle. After fertilization, the message remains in most animal blastomeres. When the embryo elongates in late blastula, staining is restricted to a few cells at the posterior pole where gastrulation will start. However, once gastrulation starts, the Cnox4-Pc signal disappears and is absent in later stages of larval development. Phylogenetic analysis shows that not all cnidarian Hox-like genes have recognizable orthologues in bilaterian groups. However, the expression analysis of Cnox1-Pc and Gsx correlates to some extent with the expression pattern of cognate genes of bilaterians, confirming the conservation of genes involved in organizing animal body plans and their putative common ancestral origin.

Polyps, peptides and patterning

Peptides serve as important signalling molecules in development and differentiation in the simple metazoan Hydra. A systematic approach (The Hydra Peptide Project) has revealed that Hydra contains several hundreds of peptide signalling molecules, some of which are neuropeptides and others emanate from epithelial cells. These peptides control biological processes as diverse as muscle contraction, neuron differentiation, and the positional value gradient. Signal peptides cause changes in cell behaviour by controlling target genes such as matrix metalloproteases. The abundance of peptides in Hydra raises the question of whether, in early metazoan evolution, cell-cell communication was based mainly on these small molecules rather than on the growth-factor-like cytokines that control differentiation and development in higher animals.

Ancient origin of the Hox gene cluster

The Hox gene cluster has a crucial function in body patterning during animal development. How and when this gene cluster originated is being clarified by recent data from Cnidaria, a basal animal phylum. The characterization of Hox-like genes from Hydra, sea anemones and jellyfish has revealed that a Hox gene cluster is extremely ancient, having originated even before the divergence of these basal animals.

The novel peptide HEADY specifies apical fate in a simple radially symmetric metazoan

One of the first steps in animal development is axis formation, during which an uneven distribution of signals and/or transcription factors results in the establishment of polarity in the embryo. Hydra, one of the simplest metazoan animals, shows characteristics of a permanent embryo. Even adult polyps have a striking capacity to regenerate, suggesting that molecular mechanisms underlying de novo pattern formation are permanently active and self regulatory. Here we show that HEADY, a short, amidated peptide, plays a central role in the specification of apical fate in this simple metazoan. The HEADY gene, whose transcripts accumulate at the apical organizing center, is required for specification of apical fate, as disruption of HEADY function by dsRNA mediated interference (RNAi) results in severe defects in head formation. Conversely, an instructive role of HEADY in head specification is demonstrated by the application of synthetic HEADY peptide, which induces formation of secondary axes with head morphology. Thus, the HEADY peptide acts as developmental switch to pattern the apical-basal axis of Hydra, providing a first insight into how initial asymmetry is specified in lower metazoan animals.

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.

Expression of a Scr/Hox5 gene in the larval central nervous system of the gastropod Haliotis, a non-segmented spiralian lophotrochozoan

Hox genes encode a set of evolutionarily conserved transcription factors that regulate anteroposterior patterning mechanisms in insects and vertebrates and are expressed along this axis in a range of bilaterians. Here we present the developmental expression of a Scr/Hox5 gene in the gastropod mollusc Haliotis. In Haliotis, embryogenesis yields a non-feeding trochophore larva that subsequently develops into the veliger larva, which possesses many of the characteristics of the adult body plan. Quantitative RT-PCR analysis reveals that this gene, which is called Hru-Hox5, is first expressed in the trochophore larva. Hru-Hox5 transcript prevalence increases continually through larval development until metamorphic competence develops in the veliger and then again over the first four days of metamorphosis. In situ hybridization reveals that larval expression of Hru-Hox5 is restricted primarily to the primordial and newly formed branchial ganglia, located between the anterior cerebral-pleuropedal ganglionic complex and the posterior visceral ganglia. The expression of Hru-Hox5 in the central region of the abalone CNS is similar to that observed for its orthologue (Lox20) in the leech, suggesting that Hox5 genes were used, along with other Hox genes, to pattern the CNS of the ancestral spiralian lophotrochozoan.

Lemon encodes an unusual receptor protein-tyrosine kinase expressed during gametogenesis in Hydra

In a screen for receptor protein-tyrosine kinase (RTK) genes expressed during gametogenesis in the cnidarian Hydra vulgaris, we isolated a cDNA encoding Lemon, an RTK with unusual features. Lemon is orthologous to Drosophila Dtrk, chicken Klg, and human colon carcinoma kinase-4. These genes constitute an RTK class characterized by a conserved transmembrane sequence, the presence of extracellular immunoglobulin-like repeats, and the absence of the DFG motif in the kinase domain. We provide evidence that Lemon is a component of an unusual RTK signal transduction mechanism that may involve transmembrane domain-mediated interactions and may not be dependent on its own catalytic activity. Lemon transcription is dynamically regulated in interstitial cells during asexual budding and gametogenesis. Transcriptional up-regulation occurs early in spermatogenesis and oogenesis concurrent with the local accumulation of interstitial cells in the body column of sexual polyps.

Evolution of Antp-class genes and differential expression of Hydra Hox/paraHox genes in anterior patterning

The conservation of developmental functions exerted by Antp-class homeoproteins in protostomes and deuterostomes suggested that homologs with related functions are present in diploblastic animals. Our phylogenetic analyses showed that Antp-class homeodomains belong either to non-Hox or to Hox/paraHox families. Among the 13 non-Hox families, 9 have diploblastic homologs, Msx, Emx, Barx, Evx, Tlx, NK-2, and Prh/Hex, Not, and Dlx, reported here. Among the Hox/paraHox, poriferan sequences were not found, and the cnidarian sequences formed at least five distinct cnox families. Two are significantly related to the paraHox Gsx (cnox-2) and the mox (cnox-5) sequences, whereas three display some relatedness to the Hox paralog groups 1 (cnox-1), 9/10 (cnox-3) and the paraHox cdx (cnox-4). Intermediate Hox/paraHox genes (PG 3 to 8 and lox) did not have clear cnidarian counterparts. In Hydra, cnox-1, cnox-2, and cnox-3 were not found chromosomally linked within a 150-kb range and displayed specific expression patterns in the adult head. During regeneration, cnox-1 was expressed as an early gene whatever the polarity, whereas cnox-2 was up-regulated later during head but not foot regeneration. Finally, cnox-3 expression was reestablished in the adult head once it was fully formed. These results suggest that the Hydra genes related to anterior Hox/paraHox genes are involved at different stages of apical differentiation. However, the positional information defining the oral/aboral axis in Hydra cannot be correlated strictly to that characterizing the anterior-posterior axis in vertebrates or arthropods.

Expression of a novel receptor tyrosine kinase gene and a paired-like homeobox gene provides evidence of differences in patterning at the oral and aboral ends of hydra

Axial patterning of the aboral end of the hydra body column was examined using expression data from two genes. One, shin guard, is a novel receptor protein-tyrosine kinase gene expressed in the ectoderm of the peduncle, the end of the body column adjacent to the basal disk. The other gene, manacle, is a paired-like homeobox gene expressed in differentiating basal disk ectoderm. During regeneration of the aboral end, expression of manacle precedes that of shin guard. This result is consistent with a requirement for induction of peduncle tissue by basal disk tissue. Our data contrast with data on regeneration of the oral end. During oral end regeneration, markers for tissue of the tentacles, which lie below the extreme oral end (the hypostome), are detected first. Later, markers for the hypostome itself appear at the regenerating tip, with tentacle markers displaced to the region below. Additional evidence that tissue can form basal disk without passing through a stage as peduncle tissue comes from LiCl-induced formation of patches of ectopic basal disk tissue. While manacle is ectopically expressed during formation of basal disk patches, shin guard is not. The genes examined also provide new information on development of the aboral end in buds. Although adult hydra are radially symmetrical, expression of both genes in the bud's aboral end is initially asymmetrical, appearing first on the side of the bud closest to the parent's basal disk. The asymmetry can be explained by differences in positional information in the body column tissue that evaginates to form a bud. As predicted by this hypothesis, grafts reversing the orientation of evaginating body column tissue also reverse the orientation of asymmetrical gene expression.

Sweet Tooth, a novel receptor protein-tyrosine kinase with C-type lectin-like extracellular domains

A gene encoding a novel type of receptor protein-tyrosine kinase was identified in Hydra vulgaris. The extracellular portion of this receptor (which we have named Sweet Tooth) contains four C-type lectin-like domains (CTLDs). Comparison of the sequences of these domains with the sequences of the carbohydrate recognition domains of various vertebrate C-type lectins shows that Sweet Tooth CTLD1 and CTLD4 have amino acids in common with those shown to be involved in carbohydrate binding by the lectins. Comparison of sequences encoding CTLD1 from the Sweet Tooth genes from different species of Hydra shows variation in some of the conserved residues that participate in carbohydrate binding in C-type lectins. The Sweet Tooth gene is expressed widely in the Hydra polyp, and expression is particularly high in the endoderm of the tentacles. Treatment of polyps with peptides corresponding to sequences in the Sweet Tooth CTLDs results in the disintegration of the animal. These same peptides do not block adhesion or morphogenesis of Hydra cell aggregates.

Bilaterian origins: significance of new experimental observations

Several recent laboratory observations that bear on the origin of the Bilateria are reviewed and interpreted in light of our set-aside cell theory for bilaterian origins. We first discuss new data concerning the phylogeny of bilaterian phyla. Next, we use systematic, molecular, and paleontological lines of evidence to argue that the latest common ancestor of echinoderms plus hemichordates used a maximal indirect mode of development. Furthermore, the latest common ancestor of molluscs and annelids was also indirectly developing. Finally, we discuss new data on Hox gene expression patterns which suggest that both sea urchins and polychaete annelids use Hox genes in a very similar fashion. Neither utilizes the complete Hox complex in the development of the larva per se, while the Hox complex is expressed in the set-aside cells from which the adult body plan is formed. Our current views on the ancestry of the bilaterians are summarized in phylogenetic terms, incorporating the characters discussed in this paper.

A gene whose major transcript encodes only the substrate-binding domain of a protein-tyrosine kinase

We have identified a novel protein-tyrosine kinase gene family in the simple multicellular animal Hydra vulgaris that consists of at least three members. Two of the genes encode receptor protein-tyrosine kinases. The third member of the family is unusual in that in non-sexual animals, the only transcripts that it produces encode polypeptides lacking all or nearly all of the ATP-binding lobe. Characterization of multiple cDNA clones and hybridization mapping of genomic DNA indicate that the gene, which we have termed Hinterteil (Hint), undergoes alternative cis-splicing, alternative trans-splicing, and alternative polyadenylation. In-situ hybridization analysis shows that expression of the gene is upregulated during spermatogenesis. Sexual males also produce an additional Hint transcript that is larger than the transcript seen in non-sexual animals, but still not large enough to encode a receptor.

Cngsc, a homologue of goosecoid, participates in the patterning of the head, and is expressed in the organizer region of Hydra

We have isolated Cngsc, a hydra homologue of goosecoid gene. The homeodomain of Cngsc is identical to the vertebrate (65-72%) and Drosophila (70%) orthologues. When injected into the ventral side of an early Xenopus embryo, Cngsc induces a partial secondary axis. During head formation, Cngsc expression appears prior to, and directly above, the zone where the tentacles will emerge, but is not observed nearby when the single apical tentacle is formed. This observation indicates that the expression of the gene is not necessary for the formation of a tentacle per se. Rather, it may be involved in defining the border between the hypostome and the tentacle zone. When Cngsc(+) tip of an early bud is grafted into the body column, it induces a secondary axis, while the adjacent Cngsc(−) region has much weaker inductive capacities. Thus, Cngsc is expressed in a tissue that acts as an organizer. Cngsc is also expressed in the sensory neurons of the tip of the hypostome and in the epithelial endodermal cells of the upper part of the body column. The plausible roles of Cngsc in organizer function, head formation and anterior neuron differentiation are similar to roles goosecoid plays in vertebrates and Drosophila. It suggests widespread evolutionary conservation of the function of the gene.

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.

CnOtx, a member of the Otx gene family, has a role in cell movement in hydra

Otx genes have been identified in a variety of organisms and are commonly associated with the patterning of anterior structures. In some vertebrates, Otx genes are also expressed in the prechordal mesoderm, where they may have a role in cell movement. Here we report the characterization of CnOtx, an Otx gene in hydra, thereby providing evidence that Otx genes appeared early in metazoan evolution. CnOtx is expressed at high levels in developing buds and aggregates, where it appears to have a role in the cell movements that are involved in the formation of new axes. Further, the gene is expressed at a low level throughout the body column of hydra. This latter pattern may reflect a role for CnOtx in specifying tissue as competent to be anterior, although the gene does not have a direct role in the formation of the head.

Ancient origins of axial patterning genes: Hox genes and ParaHox genes in the Cnidaria

Among the bilaterally symmetrical, triploblastic animals (the Bilateria), a conserved set of developmental regulatory genes are known to function in patterning the anterior-posterior (AP) axis. This set includes the well-studied Hox cluster genes, and the recently described genes of the ParaHox cluster, which is believed to be the evolutionary sister of the Hox cluster (Brooke et al. 1998). The conserved role of these axial patterning genes in animals as diverse as frogs and flies is believed to reflect an underlying homology (i.e., all bilaterians derive from a common ancestor which possessed an AP axis and the developmental mechanisms responsible for patterning the axis). However, the origin and early evolution of Hox genes and ParaHox genes remain obscure. Repeated attempts have been made to reconstruct the early evolution of Hox genes by analyzing data from the triphoblastic animals, the Bilateria (Schubert et al. 1993; Zhang and Nei 1996). A more precise dating of Hox origins has been elusive due to a lack of sufficient information from outgroup taxa such as the phylum Cnidaria (corals, hydras, jellyfishes, and sea anemones). In combination with outgroup taxa, another potential source of information about Hox origins is outgroup genes (e.g., the genes of the ParaHox cluster). In this article, we present cDNA sequences of two Hox-like genes (anthox2 and anthox6) from the sea anemone, Nematostella vectensis. Phylogenetic analysis indicates that anthox2 (= Cnox2) is homologous to the GSX class of ParaHox genes, and anthox6 is homologous to the anterior class of Hox genes. Therefore, the origin of Hox genes and ParaHox genes occurred prior to the evolutionary split between the Cnidaria and the Bilateria and predated the evolution of the anterior-posterior axis of bilaterian animals. Our analysis also suggests that the central Hox class was invented in the bilaterian lineage, subsequent to their split from the Cnidaria.

The planarian HOM/HOX homeobox genes (Plox) expressed along the anteroposterior axis

In the freshwater planarian Dugesia japonica, five cDNAs for HOM/HOX homeobox genes were cloned and sequenced. Together with sequence data on HOM/HOX homeobox genes of platyhelminthes deposited in databases, comparison of the deduced amino acid sequences revealed that planarians have at least seven HOM/HOX homeobox genes, Plox1 to Plox7 (planarian HOM/HOX homeobox genes). Whole-mount in situ hybridization and RT-PCR revealed that Plox4 and Plox5 were increasingly expressed along a spatial gradient in the posterior region of intact animals. During regeneration, Plox5 was expressed only in the posterior region of regenerating body pieces, suggesting that the gene is involved in the anteroposterior patterning in planarians. Plox5 was not found to be expressed in a blastema-specific manner, which contradicts a previous report (J. R. Bayascas, E. Castillo, A. M. Muños-Mármol, and E. Saló. Development 124, 141-148, 1997). X-ray irradiation experiments showed that Plox5 was expressed at least in some cells other than neoblasts, but that the induction of Plox5 expression during regeneration might require neoblasts.

Interactions between the foot and the head patterning systems in Hydra vulgaris

The Cnidarian, hydra, is an appealing model system for studying the basic processes underlying pattern formation. Classical studies have elucidated much basic information regarding the role of development gradients, and theoretical models have been quite successful at describing experimental results. However, most experiments and computer simulations have dealt with isolated patterning events such as the dynamics of head regeneration. More global events such as interactions among the head, bud, and foot patterning systems have not been extensively addressed. The characterization of monoclonal antibodies with position-specific labeling patterns and the recent cloning and characterization of genes expressed in position-specific manners now provide the tools for investigating global interactions between patterning systems. In particular, changes in the axial positional value gradient may be monitored in response to experimental perturbation. Rather than studying isolated patterning events, this approach allows us to study patterning over the entire animal. The studies reported here focus on interactions between the foot and the head patterning systems in Hydra vulgaris following induction of a foot in close proximity to a head, axial grafting of a foot closer to the head, or doubling the amount of basal tissue by lateral grafting of an additional peduncle-foot onto host animals. Resulting positional value changes as monitored by antigen (TS19) and gene (ks1 and CnNK-2) expression were assessed in the foot, head, and intervening tissue. The results of the experiments indicate that positional values changed rapidly, in a matter of hours, and that there were reciprocal interactions between the foot and the head patterning systems. Theoretical interpretations of the results in the form of computer simulations based on the reaction-diffusion model are presented and predict many, but not all, of the experimental observations. Since the lateral grafting experiment cannot, at present, be simulated, it is discussed in light of what has been learned from the axial grafting experiments and their simulations.

Interactions between the foot and bud patterning systems in Hydra vulgaris

In the freshwater coelenterate, hydra, asexual reproduction via budding occurs at the base of the gastric region about two-thirds of the distance from the head to the foot. Developmental gradients of head and foot activation and inhibition originating from these organizing centers have long been assumed to control budding in hydra. Much has been learned over the years about these developmental gradients and axial pattern formation, and in particular, the inhibitory influence of the head on budding is well documented. However, understanding of the role of the foot and potential interactions between the foot, bud, and head patterning systems is lacking. The purpose of this study was to investigate the role of the foot in the initiation of new axis formation during budding by manipulating the foot and monitoring effects on the onset of first bud evagination and the time necessary to reach the 50% budding point. Several experimental situations were examined: the lower peduncle and foot (PF) were injured or removed, a second PF was laterally grafted onto animals either basally (below the budding zone) or apically (above the budding zone), or both the head and PF were removed simultaneously. When the PF was injured or removed, the onset of first bud evagination was delayed and/or the time until the 50% budding point was reached was longer. The effects were more pronounced when the manipulation was performed closer to the anticipated onset of budding. When PF tissue was doubled, precocious bud evagination was induced, regardless of graft location. Removal of the PF at the same time as decapitation reduced the inductive effect of decapitation on bud evagination. These results are discussed in light of potential signals from the foot or interactions between the foot and head patterning systems that might influence bud axis initiation.

Colony integration and the expression of the Hox gene, Cnox-2, in Hydractinia symbiolongicarpus (Cnidaria: Hydrozoa)

The stolonal mat is an anatomical feature correlated with increased colonial integration in several lineages of the cnidarian class Hydrozoa. Cnox-2 is a Hox gene known to be expressed in the body column of the cnidarian polyp. We report the pattern of Cnox-2 expression in both the stolonal mat and free stolons of the hydroid Hydractinia symbiolongicarpus. The gene is found to have high levels of expression in the mat similar to that found in the basal portion of the polyp, but it is not detectably expressed in those regions of free stolons where polyps are budded. These findings suggest that the stolonal mat arose via an expansion of the basal ectoderm of the polyp.