Molecules match morphology: mitochondrial DNA supports Bayer's Lytreia - Bebryce - Heterogorgia (Alcyonacea : Octocorallia) clade hypothesis

Several studies attempting to clarify the taxonomy and systematics of Octocorallia have highlighted the important role of molecular characters in corroborating or rejecting previous hypotheses based on morphological variation. One such hypothesis is that of a close phylogenetic relationship between the genera Lytreia, Bebryce and Heterogorgia proposed by Bayer based on morphological studies of the genera. Herein, we tested Bayer’s hypothesis using the mitochondrial marker mshI. We deduced a molecular phylogeny including members of the families Gorgoniidae and ‘Paramuriceidae’ that corroborated the existence of Bayer’s Lytreia–Bebryce–Heterogorgia clade. In addition, we provide a morphological assessment of the three genera as well as diagnoses for each of them. We also discuss, based on the phylogenetic results obtained, the evolution of sclerite morphology within Bayer’s Lytreia–Bebyce–Heterogorgia clade. Finally, we propose a Tethyan origin for the Lytreia–Bebryce–Heterogorgia clade.

Character-based DNA barcoding: a superior tool for species classification

In zoonosis research only correct assigned host-agent-vector associations can lead to success. If most biological species on Earth, from agent to host and from procaryotes to vertebrates, are still undetected, the development of a reliable and universal diversity detection tool becomes a conditio sine qua non. In this context, in breathtaking speed, modern molecular-genetic techniques have become acknowledged tools for the classification of life forms at all taxonomic levels. While previous DNA-barcoding techniques were criticised for several reasons (Moritz and Cicero, 2004; Rubinoff et al., 2006a, b; Rubinoff, 2006; Rubinoff and Haines, 2006) a new approach, the so called CAOS-barcoding (Character Attribute Organisation System), avoids most of the weak points. Traditional DNA-barcoding approaches are based on distances, i. e. they use genetic distances and tree construction algorithms for the classification of species or lineages. The definition of limit values is enforced and prohibits a discrete or clear assignment. In comparison, the new character-based barcoding (CAOS-barcoding; DeSalle et al., 2005; DeSalle, 2006; Rach et al., 2008) works with discrete single characters and character combinations which permits a clear, unambiguous classification. In Hannover (Germany) we are optimising this system and developing a semiautomatic high-throughput procedure for hosts, agents and vectors being studied within the Zoonosis Centre of the "Stiftung Tierärztliche Hochschule Hannover". Our primary research is concentrated on insects, the most successful and species-rich animal group on Earth (every fourth animal is a bug). One subgroup, the winged insects (Pterygota), represents the outstanding majority of all zoonosis relevant animal vectors.

Non-coding RNA annotation of the genome of Trichoplax adhaerens

A detailed annotation of non-protein coding RNAs is typically missing in initial releases of newly sequenced genomes. Here we report on a comprehensive ncRNA annotation of the genome of Trichoplax adhaerens, the presumably most basal metazoan whose genome has been published to-date. Since blast identified only a small fraction of the best-conserved ncRNAs--in particular rRNAs, tRNAs and some snRNAs--we developed a semi-global dynamic programming tool, GotohScan, to increase the sensitivity of the homology search. It successfully identified the full complement of major and minor spliceosomal snRNAs, the genes for RNase P and MRP RNAs, the SRP RNA, as well as several small nucleolar RNAs. We did not find any microRNA candidates homologous to known eumetazoan sequences. Interestingly, most ncRNAs, including the pol-III transcripts, appear as single-copy genes or with very small copy numbers in the Trichoplax genome.

Multiple dicer genes in the early-diverging metazoa

Dicer proteins are highly conserved, are present in organisms ranging from plants to metazoans, and are essential components of the RNA interference pathway. Although the complement of Dicer proteins has been investigated in many "higher" metazoans, there has been no corresponding characterization of Dicer proteins in any early-branching metazoan. We cloned partial cDNAs of genes belonging to the Dicer family from the anthozoan cnidarian Nematostella vectensis and two distantly related haplotypes (species lineages) of the Placozoa (Trichoplax adhaerens 16S haplotype 1 [H1] and Placozoa sp. [H2]). We also identified Dicer genes in the hydrozoan Hydra magnipapillata and the demosponge Amphimedon queenslandica with the use of publicly available sequence databases. Two Dicer genes are present in each cnidarian species, whereas five Dicer genes each are found in the Porifera and Placozoa. Phylogenetic analyses comparing these and other metazoan Dicers suggest an ancient duplication event of a "Proto-Dicer" gene. We show that the Placozoa is the only known metazoan phylum which contains both representatives of this duplication event and that the multiple Dicer genes of the "basal" metazoan phyla represent lineage-specific duplications. There is a striking diversity of Dicer genes in basal metazoans, in stark contrast to the single Dicer gene found in most higher metazoans. This new data has allowed us to formulate new hypotheses regarding the evolution of metazoan Dicer proteins and their possible functions in the early diverging metazoan phyla. We theorize that the multiple placozoan Dicer genes fulfill a specific biological requirement, such as an immune defense strategy against viruses.

Concatenated analysis sheds light on early metazoan evolution and fuels a modern "urmetazoon" hypothesis

For more than a century, the origin of metazoan animals has been debated. One aspect of this debate has been centered on what the hypothetical “urmetazoon” bauplan might have been. The morphologically most simply organized metazoan animal, the placozoan Trichoplax adhaerens, resembles an intriguing model for one of several “urmetazoon” hypotheses: the placula hypothesis. Clear support for a basal position of Placozoa would aid in resolving several key issues of metazoan-specific inventions (including, for example, head–foot axis, symmetry, and coelom) and would determine a root for unraveling their evolution. Unfortunately, the phylogenetic relationships at the base of Metazoa have been controversial because of conflicting phylogenetic scenarios generated while addressing the question. Here, we analyze the sum of morphological evidence, the secondary structure of mitochondrial ribosomal genes, and molecular sequence data from mitochondrial and nuclear genes that amass over 9,400 phylogenetically informative characters from 24 to 73 taxa. Together with mitochondrial DNA genome structure and sequence analyses and Hox-like gene expression patterns, these data (1) provide evidence that Placozoa are basal relative to all other diploblast phyla and (2) spark a modernized “urmetazoon” hypothesis.

Following one of the basic principles in evolutionary biology that complex life forms derive from more primitive ancestors, it has long been believed that the higher animals, the Bilateria, arose from simpler (diploblastic) organisms such as the cnidarians (corals, polyps, and jellyfishes). A large number of studies, using different datasets and different methods, have tried to determine the most ancestral animal group as well as the ancestor of the higher animals. Here, we use “total evidence” analysis, which incorporates all available data (including morphology, genome, and gene expression data) and come to a surprising conclusion. The Bilateria and Cnidaria (together with the other diploblastic animals) are in fact sister groups: that is, they evolved in parallel from a very simple common ancestor. We conclude that the higher animals (Bilateria) and lower animals (diploblasts), probably separated very early, at the very beginning of metazoan animal evolution and independently evolved their complex body plans, including body axes, nervous system, sensory organs, and other characteristics. The striking similarities in several complex characters (such as the eyes) resulted from both lineages using the same basic genetic tool kit, which was already present in the common ancestor. The study identifies Placozoa as the most basal diploblast group and thus a living fossil genome that nicely demonstrates, not only that complex genetic tool kits arise before morphological complexity, but also that these kits may form similar morphological structures in parallel.

Total evidence analyses reveal a surprise: Higher animals did not evolve from any known lower animal group.

An even "newer" animal phylogeny

Metazoa are one of the great monophyletic groups of organisms. They comprise several major groups of organisms readily recognizable based on their anatomy. These major groups include the Bilateria (animals with bilateral symmetry), Cnidaria (jellyfish, corals and other closely related animals), Porifera (sponges), Ctenophores (comb jellies) and a phylum currently made up of a single species, the Placozoa. Attempts to systematize the relationships of these major groups as well as to determine relationships within the groups have been made for nearly two centuries. Many of the attempts have led to frustration, because of a lack of resolution between and within groups. Other attempts have led to "a new animal phylogeny". Now, a study by Dunn et al., using the expresssed sequence tag (EST) approach to obtaining high-throughput large phylogenetic matrices, presents an "even newer" animal phylogeny. There are two major aspects of this study that should be of interest to the general biological community. First, the methods used by the authors to generate their phylogenetic hypotheses call for close examination. Second, the relationships of animal taxa in their resultant trees also prompt further discussion.

Placozoa and the evolution of Metazoa and intrasomatic cell differentiation

The multicellular Metazoa evolved from single-celled organisms (Protozoa) and usually - but not necessarily - consist of more cells than Protozoa. In all cases, and thus by definition, Metazoa possess more than one somatic cell type, i.e. they show-in sharp contrast to protists-intrasomatic differentiation. Placozoa have the lowest degree of intrasomatic variation; the number of somatic cell types according to text books is four (but see also Jakob W, Sagasser S, Dellaporta S, Holland P, Kuhn K, and Schierwater B. The Trox-2 Hox/ParaHox gene of Trichoplax (Placozoa) marks an epithelial boundary. Dev Genes Evol 2004;214:170-5). For this and several other reasons Placozoa have been regarded by many as the most basal metazoan phylum. Thus, the morphologically most simply organized metazoan animal, the placozoan Trichoplax adhaerens, resembles a unique model system for cell differentiation studies and also an intriguing model for a prominent "urmetazoon" hypotheses-the placula hypothesis. A basal position of Placozoa would provide answers to several key issues of metazoan-specific inventions (including for example different lines of somatic cell differentiation leading to organ development and axis formation) and would determine a root for unraveling their evolution. However, the phylogenetic relationships at the base of Metazoa are controversial and a basal position of Placozoa is not generally accepted (e.g. Schierwater B, DeSalle R. Can we ever identify the Urmetazoan? Integr Comp Biol 2007;47:670-76; DeSalle R, Schierwater B. An even "newer" animal phylogeny. Bioessays 2008;30:1043-47). Here we review and discuss (i) long-standing morphological evidence for the simple placozoan bauplan resembling an ancestral metazoan stage, (ii) some rapidly changing alternative hypotheses derived from molecular analyses, (iii) the surprising idea that triploblasts (Bilateria) and diploblasts may be sister groups, and (iv) the presence of genes involved in cell differentiation and signaling pathways in the placozoan genome.

The early ANTP gene repertoire: insights from the placozoan genome

The evolution of ANTP genes in the Metazoa has been the subject of conflicting hypotheses derived from full or partial gene sequences and genomic organization in higher animals. Whole genome sequences have recently filled in some crucial gaps for the basal metazoan phyla Cnidaria and Porifera. Here we analyze the complete genome of Trichoplax adhaerens, representing the basal metazoan phylum Placozoa, for its set of ANTP class genes. The Trichoplax genome encodes representatives of Hox/ParaHox-like, NKL, and extended Hox genes. This repertoire possibly mirrors the condition of a hypothetical cnidarian-bilaterian ancestor. The evolution of the cnidarian and bilaterian ANTP gene repertoires can be deduced by a limited number of cis-duplications of NKL and “extended Hox” genes and the presence of a single ancestral “ProtoHox” gene.

Character-based DNA barcoding allows discrimination of genera, species and populations in Odonata

DNA barcoding has become a promising means for identifying organisms of all life stages. Currently, phenetic approaches and tree-building methods have been used to define species boundaries and discover 'cryptic species'. However, a universal threshold of genetic distance values to distinguish taxonomic groups cannot be determined. As an alternative, DNA barcoding approaches can be 'character based', whereby species are identified through the presence or absence of discrete nucleotide substitutions (character states) within a DNA sequence. We demonstrate the potential of character-based DNA barcodes by analysing 833 odonate specimens from 103 localities belonging to 64 species. A total of 54 species and 22 genera could be discriminated reliably through unique combinations of character states within only one mitochondrial gene region (NADH dehydrogenase 1). Character-based DNA barcodes were further successfully established at a population level discriminating seven population-specific entities out of a total of 19 populations belonging to three species. Thus, for the first time, DNA barcodes have been found to identify entities below the species level that may constitute separate conservation units or even species units. Our findings suggest that character-based DNA barcoding can be a rapid and reliable means for (i) the assignment of unknown specimens to a taxonomic group, (ii) the exploration of diagnosability of conservation units, and (iii) complementing taxonomic identification systems.

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.

Implications of cnidarian gene expression patterns for the origins of bilaterality is the glass half full or half empty?

The past two years have seen a dramatic increase in the available data on gene sequence and gene expression for cnidarians and other "lower" Metazoa, and a flurry of recent papers has drawn on these to address the origins of bilaterality. Cnidarian homologs of many genes that play key roles in the specification of both the A/P and D/V axes of bilaterians have been characterized, and their patterns of expression determined. Some of these expression patterns are consistent with the possibility of conservation of function between Cnidaria and Bilateria, but others clearly differ. Moreover, in some cases very different interpretations have been made on the basis of the same, or similar, data. In part, these differences reflect the inevitable uncertainties associated with the depth of the divergence between cnidarians and bilaterians. In this article, we briefly summarize the cnidarian data on gene expression and organization relevant to axis formation, the varying interpretations of these data, and where they conflict. Our conclusion is that the presently available data do not allow us to unequivocally homologize the single overt axis of cnidarians with either of the bilaterian axes.

Key transitions in animal evolution

In order to address this subject further and to assess progress in the examination of animal origins and transitions, an international group of scientists was convened at the Society for Comparative Biology meeting in January 2007.

Can we ever identify the Urmetazoan?

Unraveling the root of the metazoan tree of life has been a difficult task since the time of Haeckel and the invention of phylogenetics. Even considerable amounts of recent molecular data have not provided a generally accepted answer. Here, we review the major problems of this phylogenetic conundrum and provide some directions for solving it.

Cnidarian milestones in metazoan evolution

Cnidarians display most of the characters considered as milestones of metazoan evolution. Whereas a tissue-level organization was probably already present in the multicellular common ancestor of all animals, the Urmetazoa, the emergence of important animal features such as bilateral symmetry, triploblasty, a polarized nervous system, sense organs (eyes, statocysts), and a (chitinous or calcium-based) continuous skeleton can be traced back before the divergence between cnidarians and bilaterians. Modularity and metamery might be also regarded as two faces of the same medal, likely involving conserved molecular mechanisms ruling animal body architectures through regional specification of iterated units. Available evidence indicates that the common ancestor of cnidarians and bilaterians, the UrEumetazoa, was a surprisingly complex animal with nerve cell differentiation. We suggest that paedomorphic events in descendants of this ancestor led to the array of diversity seen in the main extant animal phyla. The use of molecular analyses and identifying the genetic determinants of anatomical organizations can provide an integrative test of hypotheses of homologies and independent evidence of the evolutionary relationships among extant taxa.

Ancient complexity of the non-Hox ANTP gene complement in the anthozoan Nematostella vectensis: implications for the evolution of the ANTP superclass

The origin and evolution of ANTP superclass genes has raised controversial discussions. While recent evidence suggests that a true Hox cluster emerged after the cnidarian bilaterian split, the origin of the ANTP superclass as a whole remains unclear. Based on analyses of bilaterian genomes, it seems very likely that clustering has once been a characteristic of all ANTP homeobox genes and that their ancestors have emerged through several series of cis-duplications from the same genomic region. Since the diploblastic Cnidaria possess orthologs of some non-Hox ANTP genes, at least some steps of the expansion of this hypothetical homeobox gene array must have occurred in the last common ancestor of both lineages--but it is unknown to what extent. By screening the unassembled Nematostella genome, we have identified unambiguous orthologs to almost all non-Hox ANTP genes which are present in Bilateria--with the exception of En, Tlx and (possibly) Vax. Furthermore, Nematostella possesses ANTP genes that are missing in some bilaterian lineages, like the rough gene or NK7. In addition, several ANTP homeobox gene families have been independently duplicated in Nematostella. We conclude that the last cnidarian/bilaterian ancestor already harboured the almost full complement of non-Hox ANTP genes before the Hox system evolved.

Solution to the phylogenetic enigma of Tetraplatia, a worm-shaped cnidarian

Tetraplatia is a genus containing two species of pelagic cnidarians of curious morphology. Their vermiform shape and four swimming flaps are difficult to relate to the features of other cnidarians, thus obscuring their phylogenetic affinities. Since their discovery in the mid-1800s, a number of prominent cnidarian workers have weighed in on this conundrum, some arguing that they are aberrant hydrozoans and others concluding that they are unusual scyphozoans. Current taxonomic practice conforms to the latter view. However, data presented here from the large and small subunits of the nuclear ribosome leave little doubt that Tetraplatia is in fact a hydrozoan genus. Indeed, its precise phylogenetic position is within Narcomedusae, as some authors had previously deduced based on structural characters. The distinctive body plan of Tetraplatia is remarkable because it appears to have a recent origin, in contrast to the prevailing pattern of metazoan history.

Ancient linkage of a POU class 6 and an anterior hox-like gene in cnidaria: implications for the evolution of homeobox genes

Linkage analyses in metazoan genomes suggest two ancestral arrays for the majority of homeobox genes. The related homeobox genes and chromosomal regions that are dispersed in extant species derived possibly from only two single common ancestor regions. One proposed ancestral array, designated as ANTP mega-array, contains most of the ANTP class homeobox genes; the second, named the contraHox super-paralogon, would consist of the classes PRD, POU, LIM, CUT, prospero, TALE and SIX. Here, we report the tight linkage of a POU class 6 gene to an anterior Hox-like gene in the hydrozoan Eleutheria dichotoma and discuss its possible significance for the evolution of homeobox genes. POU class 6 genes also seem to be ancestrally linked to the HoxC and A clusters in vertebrates, despite POU homeobox genes belonging to the contraHox paralogon. Hence, the much tighter linkage of a POU class 6 gene to an anterior Hox-like gene in a cnidarian is possibly the evolutionary echo of an ancestral genomic region from which most metazoan homeobox classes emerged.

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.

Mitochondrial genome of Trichoplax adhaerens supports placozoa as the basal lower metazoan phylum

Mitochondrial genomes of multicellular animals are typically 15- to 24-kb circular molecules that encode a nearly identical set of 12-14 proteins for oxidative phosphorylation and 24-25 structural RNAs (16S rRNA, 12S rRNA, and tRNAs). These genomes lack significant intragenic spacers and are generally without introns. Here, we report the complete mitochondrial genome sequence of the placozoan Trichoplax adhaerens, a metazoan with the simplest known body plan of any animal, possessing no organs, no basal membrane, and only four different somatic cell types. Our analysis shows that the Trichoplax mitochondrion contains the largest known metazoan mtDNA genome at 43,079 bp, more than twice the size of the typical metazoan mtDNA. The mitochondrion's size is due to numerous intragenic spacers, several introns and ORFs of unknown function, and protein-coding regions that are generally larger than those found in other animals. Not only does the Trichoplax mtDNA have characteristics of the mitochondrial genomes of known metazoan outgroups, such as chytrid fungi and choanoflagellates, but, more importantly, it shares derived features unique to the Metazoa. Phylogenetic analyses of mitochondrial proteins provide strong support for the placement of the phylum Placozoa at the root of the Metazoa.

A low diversity of ANTP class homeobox genes in Placozoa

Homeobox genes of the ANTP and PRD classes play important roles in body patterning of metazoans, and a large diversity of these genes have been described in bilaterian animals and cnidarians. Trichoplax adhaerens (Phylum Placozoa) is a small multicellular marine animal with one of the simplest body organizations of all metazoans, showing no symmetry and a small number of distinct cell types. Only two ANTP class genes have been described from Trichoplax: the Hox/ParaHox gene Trox-2 and a gene related to the Not family. Here we report an extensive screen for ANTP class genes in Trichoplax, leading to isolation of three additional ANTP class genes. These can be assigned to the Dlx, Mnx and Hmx gene families. Sequencing approximately 12-20 kb around each gene indicates that none are part of tight gene clusters, and in situ hybridization reveals that at least two have spatially restricted expression around the periphery of the animal. The low diversity of ANTP class genes isolated in Trichoplax can be reconciled with the low anatomical complexity of this animal, although the finding that these genes are assignable to recognized gene families is intriguing.

Medusozoan phylogeny and character evolution clarified by new large and small subunit rDNA data and an assessment of the utility of phylogenetic mixture models

A newly compiled data set of nearly complete sequences of the large subunit of the nuclear ribosome (LSU or 28S) sampled from 31 diverse medusozoans greatly clarifies the phylogenetic history of Cnidaria. These data have substantial power to discern among many of the competing hypotheses of relationship derived from prior work. Moreover, LSU data provide strong support at key nodes that were equivocal based on other molecular markers. Combining LSU sequences with those of the small subunit of the nuclear ribosome (SSU or 18S), we present a detailed working hypothesis of medusozoan relationships and discuss character evolution within this diverse clade. Stauromedusae, comprising the benthic, so-called stalked jellyfish, appears to be the sister group of all other medusozoans, implying that the free-swimming medusa stage, the motor nerve net, and statocysts of ecto-endodermal origin are features derived within Medusozoa. Cubozoans, which have had uncertain phylogenetic affinities since the elucidation of their life cycles, form a clade-named Acraspeda-with the scyphozoan groups Coronatae, Rhizostomeae, and Semaeostomeae. The polyps of both cubozoans and hydrozoans appear to be secondarily simplified. Hydrozoa is comprised by two well-supported clades, Trachylina and Hydroidolina. The position of Limnomedusae within Trachylina indicates that the ancestral hydrozoan had a biphasic life cycle and that the medusa was formed via an entocodon. Recently hypothesized homologies between the entocodon and bilaterian mesoderm are therefore suspect. Laingiomedusae, which has often been viewed as a close ally of the trachyline group Narcomedusae, is instead shown to be unambiguously a member of Hydroidolina. The important model organisms of the Hydra species complex are part of a clade, Aplanulata, with other hydrozoans possessing direct development not involving a ciliated planula stage. Finally, applying phylogenetic mixture models to our data proved to be of little additional value over a more traditional phylogenetic approach involving explicit hypothesis testing and bootstrap analyses under multiple optimality criteria. [18S; 28S; Cubozoa; Hydrozoa; medusa; molecular systematics; polyp; Scyphozoa; Staurozoa.].

Axial patterning and diversification in the cnidaria predate the Hox system

Across the animal kingdom, Hox genes are organized in clusters whose genomic organization reflects their central roles in patterning along the anterior/posterior (A/P) axis . While a cluster of Hox genes was present in the bilaterian common ancestor, the origins of this system remain unclear (cf. ). With new data for two representatives of the closest extant phylum to the Bilateria, the sea anemone Nematostella and the hydromedusa Eleutheria, we argue here that the Cnidaria predate the evolution of the Hox system. Although Hox-like genes are present in a range of cnidarians, many of these are paralogs and in neither Nematostella nor Eleutheria is an equivalent of the Hox cluster present. With the exception of independently duplicated genes, the cnidarian genes are unlinked and in several cases are flanked by non-Hox genes. Furthermore, the cnidarian genes are expressed in patterns that are inconsistent with the Hox paradigm. We conclude that the Cnidaria/Bilateria split occurred before a definitive Hox system developed. The spectacular variety in morphological and developmental characteristics shown by extant cnidarians demonstrates that there is no obligate link between the Hox system and morphological diversity in the animal kingdom and that a canonical Hox system is not mandatory for axial patterning.

My favorite animal, Trichoplax adhaerens

Trichoplax adhaerens is more simply organized than any other living metazoan. This tiny marine animal looks like a irregular "hairy plate" ("tricho plax") with a simple upper and lower epithelium and some loose cells in between. After its original description by F.E. Schulze 1883, it attracted particular attention as a potential candidate representing the basic and ancestral state of metazoan organization. The lack of any kind of symmetry, organs, nerve cells, muscle cells, basal lamina and extracellular matrix originally left little doubt about the basal position of T. adhaerens. Nevertheless, the interest of zoologists and evolutionary biologists suddenly vanished for more than half a century when Trichoplax was claimed to be an aberrant hydrozoan planula larva. Recently, Trichoplax has been rediscovered as a key species for unraveling early metazoan evolution. For example, research on regulatory genes and whole genome sequencing promise insights into the genetics underlying the origin and development of basal metazoan phyla. Trichoplax offers unique potential for understanding the minimal requirements of metazoan animal organization.

Molecular biomarkers and adaptation to environmental stress in moon jelly (Aurelia spp.)

We describe a strategy that identifies molecular biomarkers and links the study of abiotic stress to evolutionary history. By utilizing the moon jellyfish Aurelia spp. as a model, we identified genes differentially regulated in response to the chemical stressor tributyltin by means of complementary DNA subtraction analyses. Expression of 3 out of 25 identified candidate genes, one oxidative stress gene, one heat shock (hsp70) gene, and one GTP-binding gene, was quantified under laboratory conditions and in field tests using semiquantitative reverse transcriptase polymerase chain reaction. Differential expression patterns were found following exposure to tributyltin and temperature treatments. The findings suggest that the identified genes are involved in response to chemical as well as heat- induced stress and may serve as biomarkers for monitoring marine habitats. Gene regulatory patterns combined with phylogenetic inferences of the hsp70 gene support a possible role of ecologically driven divergence within the genus Aurelia. We show that added information on genetic variability can raise the predictive power of molecular biomarkers in studies of individual stress response.

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.