RESUMO
CaV1 and CaV2 voltage-gated calcium channels evolved from an ancestral CaV1/2 channel via gene duplication somewhere near the stem animal lineage. The divergence of these channel types led to distinguishing functional properties that are conserved among vertebrates and bilaterian invertebrates and contribute to their unique cellular roles. One key difference pertains to their regulation by calmodulin (CaM), wherein bilaterian CaV1 channels are uniquely subject to pronounced, buffer-resistant Ca2+/CaM-dependent inactivation, permitting negative feedback regulation of calcium influx in response to local cytoplasmic Ca2+ rises. Early diverging, nonbilaterian invertebrates also possess CaV1 and CaV2 channels, but it is unclear whether they share these conserved functional features. The most divergent animals to possess both CaV1 and CaV2 channels are placozoans such as Trichoplax adhaerens, which separated from other animals over 600 million years ago shortly after their emergence. Hence, placozoans can provide important insights into the early evolution of CaV1 and CaV2 channels. Here, we build upon previous characterization of Trichoplax CaV channels by determining the cellular expression and ion-conducting properties of the CaV1 channel orthologue, TCaV1. We show that TCaV1 is expressed in neuroendocrine-like gland cells and contractile dorsal epithelial cells. In vitro, this channel conducts dihydropyridine-insensitive, high-voltage-activated Ca2+ currents with kinetics resembling those of rat CaV1.2 but with left-shifted voltage sensitivity for activation and inactivation. Interestingly, TCaV1, but not TCaV2, exhibits buffer-resistant Ca2+/CaM-dependent inactivation, indicating that this functional divergence evolved prior to the emergence of bilaterian animals and may have contributed to their unique adaptation for cytoplasmic Ca2+ signaling within various cellular contexts.
Assuntos
Canais de Cálcio , Calmodulina , Evolução Molecular , Placozoa , Animais , Cálcio/metabolismo , Canais de Cálcio/genética , Canais de Cálcio/metabolismo , Calmodulina/genética , Calmodulina/metabolismo , Retroalimentação Fisiológica , Placozoa/classificação , Placozoa/genética , Placozoa/metabolismo , RatosRESUMO
Infiltration of the endothelial layer of the blood-brain barrier by leukocytes plays a critical role in health and disease. When passing through the endothelial layer during the diapedesis process lymphocytes can either follow a paracellular route or a transcellular one. There is a debate whether these two processes constitute one mechanism, or they form two evolutionary distinct migration pathways. We used artificial intelligence, phylogenetic analysis, HH search, ancestor sequence reconstruction to investigate further this intriguing question. We found that the two systems share several ancient components, such as RhoA protein that plays a critical role in controlling actin movement in both mechanisms. However, some of the key components differ between these two transmigration processes. CAV1 genes emerged during Trichoplax adhaerens, and it was only reported in transcellular process. Paracellular process is dependent on PECAM1. PECAM1 emerged from FASL5 during Zebrafish divergence. Lastly, both systems employ late divergent genes such as ICAM1 and VECAM1. Taken together, our results suggest that these two systems constitute two different mechanical sensing mechanisms of immune cell infiltrations of the brain, yet these two systems are connected. We postulate that the mechanical properties of the cellular polarity is the main driving force determining the migration pathway. Our analysis indicates that both systems coevolved with immune cells, evolving to a higher level of complexity in association with the evolution of the immune system.
Assuntos
Células Endoteliais/metabolismo , Leucócitos/metabolismo , Proteínas/genética , Migração Transcelular de Célula/genética , Transcriptoma , Migração Transendotelial e Transepitelial/genética , Animais , Evolução Biológica , Barreira Hematoencefálica/citologia , Barreira Hematoencefálica/metabolismo , Caenorhabditis elegans/classificação , Caenorhabditis elegans/citologia , Caenorhabditis elegans/genética , Caenorhabditis elegans/metabolismo , Galinhas/classificação , Galinhas/genética , Galinhas/metabolismo , Ciona intestinalis/classificação , Ciona intestinalis/citologia , Ciona intestinalis/genética , Ciona intestinalis/metabolismo , Drosophila melanogaster/classificação , Drosophila melanogaster/citologia , Drosophila melanogaster/genética , Drosophila melanogaster/metabolismo , Células Endoteliais/citologia , Perfilação da Expressão Gênica , Regulação da Expressão Gênica , Ontologia Genética , Humanos , Leucócitos/citologia , Camundongos , Pan troglodytes/classificação , Pan troglodytes/genética , Pan troglodytes/metabolismo , Petromyzon/classificação , Petromyzon/genética , Petromyzon/metabolismo , Filogenia , Placozoa/classificação , Placozoa/citologia , Placozoa/genética , Placozoa/metabolismo , Proteínas/classificação , Proteínas/metabolismo , Anêmonas-do-Mar/classificação , Anêmonas-do-Mar/citologia , Anêmonas-do-Mar/genética , Anêmonas-do-Mar/metabolismo , Tubarões/classificação , Tubarões/genética , Tubarões/metabolismo , Peixe-Zebra/classificação , Peixe-Zebra/genética , Peixe-Zebra/metabolismoRESUMO
Placozoa are small disc-shaped animals, representing the simplest known, possibly ancestral, organization of free-living animals. With only six morphological distinct cell types, without any recognized neurons or muscle, placozoans exhibit fast effector reactions and complex behaviors. However, little is known about electrogenic mechanisms in these animals. Here, we showed the presence of rapid action potentials in four species of placozoans (Trichoplax adhaerens [H1 haplotype], Trichoplax sp.[H2], Hoilungia hongkongensis [H13], and Hoilungia sp. [H4]). These action potentials are sodium-dependent and can be inducible. The molecular analysis suggests the presence of 5-7 different types of voltage-gated sodium channels, which showed substantial evolutionary radiation compared to many other metazoans. Such unexpected diversity of sodium channels in early-branched metazoan lineages reflect both duplication events and parallel evolution of unique behavioral integration in these nerveless animals.
Assuntos
Placozoa/metabolismo , Canais de Sódio/metabolismo , Sódio/metabolismo , Potenciais de Ação , Motivos de Aminoácidos , Sequência de Aminoácidos , Animais , Evolução Molecular , Variação Genética , Modelos Moleculares , Filogenia , Placozoa/classificação , Placozoa/genética , Conformação Proteica , Canais de Sódio/química , Canais de Sódio/genéticaRESUMO
Peptidic messengers constitute a highly diversified group of intercellular messengers widely distributed in nature that regulate a great number of physiological processes in Metazoa. Being crucial for life, it seem that they have appeared in the ancestral group from which Metazoa evolved, and were highly conserved along the evolutionary process. Peptides act mainly through G-protein coupled receptors (GPCRs), a family of transmembrane molecules. GPCRs are also widely distributed in nature being present in metazoan, but also in Choanoflagellata and Fungi. Among GPCRs, the Allatotropin/Orexin (AT/Ox) family is particularly characterized by the presence of the DRW motif in the second intracellular loop (IC Loop 2), and seems to be present in Cnidaria, Placozoa and in Bilateria, suggesting that it was present in the common ancestor of Metazoa. Looking for the evolutionary history of this GPCRs we searched for corresponding sequences in public databases. Our results suggest that AT/Ox receptors were highly conserved along evolutionary process, and that they are characterized by the presence of the E/DRWYAI motif at the IC Loop 2. Phylogenetic analyses show that AT/Ox family of receptors reflects evolutionary relationships that agree with current phylogenetic understanding in Actinopterygii and Sauropsida, including also the largely discussed position of Testudines.
Assuntos
Hormônios de Inseto/genética , Neuropeptídeos/genética , Orexinas/genética , Animais , Evolução Biológica , Classificação/métodos , Cnidários/classificação , Cnidários/genética , Bases de Dados Genéticas , Evolução Molecular , Hormônios de Inseto/metabolismo , Neuropeptídeos/metabolismo , Orexinas/metabolismo , Filogenia , Placozoa/classificação , Placozoa/genética , Receptores Acoplados a Proteínas G/genética , Análise de Sequência de DNA , Vertebrados/genéticaRESUMO
The enigmatic phylum Placozoa is harboring an unknown number of cryptic species and has become a challenge for modern systematics. Only recently, a second species has been described [1], while the presence of more than a hundred additional species has been suggested [2]. The original placozoan species Trichoplax adhaerens[3], the second species Hoilungia hongkongensis[1] and all yet undescribed species are morphologically indistinguishable (i.e. no species diagnostic characters are available [4]). Here, we report on a new placozoan species, Polyplacotoma mediterranea gen. nov., spec. nov., which differs from other placozoans in its completely different morphological habitus, including long polytomous body branches and a maximum body length of more than 10 mm. Polyplacotoma mediterranea also necessitates a different view of placozoan mitochondrial genetics. P. mediterranea harbors a highly compact mitochondrial genome with overlapping mitochondrial tRNA and protein coding genes. Furthermore, the new species lacks typical placozoan features, including the cox1 micro exon and cox1 barcode intron. As phylogenetic analyses suggest a sister group relationship of P. mediterranea to all other placozoans, this new species may also be relevant for studies addressing the relationships at the base of the metazoan tree of life.
Assuntos
Genoma Mitocondrial , Filogenia , Placozoa/classificação , Animais , Itália , Placozoa/citologia , Placozoa/ultraestruturaRESUMO
The phylogenetic placement of the morphologically simple placozoans is crucial to understanding the evolution of complex animal traits. Here, we examine the influence of adding new genomes from placozoans to a large dataset designed to study the deepest splits in the animal phylogeny. Using site-heterogeneous substitution models, we show that it is possible to obtain strong support, in both amino acid and reduced-alphabet matrices, for either a sister-group relationship between Cnidaria and Placozoa, or for Cnidaria and Bilateria as seen in most published work to date, depending on the orthologues selected to construct the matrix. We demonstrate that a majority of genes show evidence of compositional heterogeneity, and that support for the Cnidaria + Bilateria clade can be assigned to this source of systematic error. In interpreting these results, we caution against a peremptory reading of placozoans as secondarily reduced forms of little relevance to broader discussions of early animal evolution.
Assuntos
Composição de Bases/genética , Viés , Cnidários/classificação , Cnidários/genética , Filogenia , Placozoa/classificação , Placozoa/genética , Animais , Teorema de Bayes , Funções Verossimilhança , Metagenoma , Modelos GenéticosAssuntos
Animais de Laboratório , Ctenóforos , Modelos Animais , Placozoa , Poríferos , Animais , Animais de Laboratório/anatomia & histologia , Animais de Laboratório/classificação , Animais de Laboratório/fisiologia , Ctenóforos/anatomia & histologia , Ctenóforos/classificação , Ctenóforos/fisiologia , Ciência dos Animais de Laboratório , Características de História de Vida , Placozoa/anatomia & histologia , Placozoa/classificação , Placozoa/fisiologia , Poríferos/anatomia & histologia , Poríferos/classificação , Poríferos/fisiologiaRESUMO
The phylum Placozoa officially consists of only a single described species, Trichoplax adhaerens, although several lineages can be separated by molecular markers, geographical distributions and environmental demands. The placozoan 16S haplotype H2 (Trichoplax sp. H2) is the most robust and cosmopolitan lineage of placozoans found to date. In this study, its genome was found to be distinct but highly related to the Trichoplax adhaerens reference genome, for remarkably unique reasons. The pattern of variation and allele distribution between the two lineages suggests that both originate from a single interbreeding event in the wild, dating back at least several decades ago, and both seem not to have engaged in sexual reproduction since. We conclude that populations of certain placozoan haplotypes remain stable for long periods without bisexual reproduction. Furthermore, allelic variation within and between the two Trichoplax lineages indicates that successful bisexual reproduction between related placozoan lineages might serve to either counter accumulated negative somatic mutations or to cope with changing environmental conditions. On the other hand, enrichment of neutral or beneficial somatic mutations by vegetative reproduction, combined with rare sexual reproduction, could instantaneously boost genetic variation, generating novel ecotypes and eventually species.
Assuntos
Animais Selvagens/genética , Loci Gênicos/genética , Placozoa/classificação , Placozoa/genética , Dinâmica Populacional , Reprodução Assexuada/genética , Adaptação Biológica/genética , Animais , Ecótipo , Frequência do Gene , Haplótipos/genética , Mutação/genética , Nucleotídeos/genética , Filogenia , Polimorfismo de Nucleotídeo Único , População/genética , Sequências Repetitivas de Ácido Nucleico/genética , Seleção Genética/genética , Sintenia/genéticaRESUMO
The biannual international workshop entitled "The diversification of early emerging metazoans: A window into animal evolution?" took place at the Evangelische Akademie Tutzing, Germany, 11-14. September 2017. It was organized by Thomas Bosch (Kiel), Thomas Holstein (Heidelberg), and Ulrich Technau (Vienna), and it was sponsored by the Deutsche Forschungsgemeinschaft (DFG). The meeting gathered over 140 researchers to discuss the contribution of non-bilaterian metazoan models (Porifera, Ctenophora, Placozoa, and Cnidaria) to our understanding of: a. The evolution of metazoan developmental processes; b. Fundamental molecular mechanisms underlying metazoan features; and c. The complex interactions that animals establish with their environment.
Assuntos
Evolução Biológica , Animais , Cnidários/classificação , Ctenóforos/classificação , Evolução Molecular , Alemanha , Filogenia , Placozoa/classificação , Poríferos/classificaçãoRESUMO
Schierwater & DeSalle introduce the enigmatic phylum Placozoa.
Assuntos
Distribuição Animal , Características de História de Vida , Placozoa/fisiologia , Animais , Evolução Biológica , Placozoa/anatomia & histologia , Placozoa/classificação , Placozoa/genética , ReproduçãoRESUMO
The enigmatic placozoans, which hold a key position in the metazoan Tree of Life, have attracted substantial attention in many areas of biological and biomedical research. While placozoans have become an emerging model system, their ecology and particularly biogeography remain widely unknown. In this study, we use modelling approaches to explore habitat preferences, and distribution pattern of the placozoans phylum. We provide hypotheses for discrete ecological niche separation between genetic placozoan lineages, which may also help to understand biogeography patterns in other small marine invertebrates. We, here, used maximum entropy modelling to predict placozoan distribution using 20 environmental grids of 9.2 km2 resolution. In addition, we used recently developed metrics of niche overlap to compare habitat suitability models of three genetic clades. The predicted distributions range from 55°N to 44°S and are restricted to regions of intermediate to warm sea surface temperatures. High concentrations of salinity and low nutrient concentrations appear as secondary factors. Tests of niche equivalency reveal the largest differences between placozoan clades I and III. Interestingly, the genetically well-separated clades I and V appear to be ecologically very similar. Our habitat suitability models predict a wider latitudinal distribution for placozoans, than currently described, especially in the northern hemisphere. With respect to biogeography modelling, placozoans show patterns somewhere between higher metazoan taxa and marine microorganisms, with the first group usually showing complex biogeographies and the second usually showing "no biogeography."
Assuntos
Especiação Genética , Filogenia , Placozoa/genética , RNA Ribossômico 16S/genética , Distribuição Animal/fisiologia , Animais , Teorema de Bayes , Região do Caribe , Ecossistema , Mar Mediterrâneo , Oceano Pacífico , Filogeografia , Placozoa/classificação , TemperaturaRESUMO
Despite an explosion in the amount of sequence data, phylogenomics has failed to settle controversy regarding some critical nodes on the animal tree of life. Understanding relationships among Bilateria, Ctenophora, Cnidaria, Placozoa, and Porifera is essential for studying how complex traits such as neurons, muscles, and gastrulation have evolved. Recent studies have cast doubt on the historical viewpoint that sponges are sister to all other animal lineages with recent studies recovering ctenophores as sister. However, the ctenophore-sister hypothesis has been criticized as unrealistic and caused by systematic error. We review past phylogenomic studies and potential causes of systematic error in an effort to identify areas that can be improved in future studies. Increased sampling of taxa, less missing data, and a priori removal of sequences and taxa that may cause systematic error in phylogenomic inference will likely be the most fruitful areas of focus when assembling future datasets. Ultimately, we foresee metazoan relationships being resolved with higher support in the near future, and we caution against dismissing novel hypotheses merely because they conflict with historical viewpoints of animal evolution.
Assuntos
Invertebrados/genética , Filogenia , Animais , Cnidários/classificação , Cnidários/genética , Ctenóforos/classificação , Ctenóforos/genética , Invertebrados/classificação , Placozoa/classificação , Placozoa/genética , Poríferos/classificação , Poríferos/genéticaRESUMO
The heparan sulfate proteoglycan 2 (HSPG2)/perlecan gene is ancient and conserved in all triploblastic species. Its presence maintains critical cell boundaries in tissue and its large (up to ~900 kDa) modular structure has prompted speculation about the evolutionary origin of the gene. The gene's conservation amongst basal metazoans is unclear. After the recent sequencing of their genomes, the cnidarian Nematostella vectensis and the placozoan Trichoplax adhaerens have become favorite models for studying tissue regeneration and the evolution of multicellularity. More ancient basal metazoan phyla include the poriferan and ctenophore, whose evolutionary relationship has been clarified recently. Our in silico and PCR-based methods indicate that the HSPG2 gene is conserved in both the placozoan and cnidarian genomes, but not in those of the ctenophores and only partly in poriferan genomes. HSPG2 also is absent from published ctenophore and Capsaspora owczarzaki genomes. The gene in T. adhaerens is encoded as two separate but genetically juxtaposed genes that house all of the constituent pieces of the mammalian HSPG2 gene in tandem. These genetic constituents are found in isolated genes of various poriferan species, indicating a possible intronic recombinatory mechanism for assembly of the HSPG2 gene. Perlecan's expression during wound healing and boundary formation is conserved, as expression of the gene was activated during tissue regeneration and reformation of the basement membrane of N. vectensis. These data indicate that the complex HSPG2 gene evolved concurrently in a common ancestor of placozoans, cnidarians and bilaterians, likely along with the development of differentiated cell types separated by acellular matrices, and is activated to reestablish these tissue borders during wound healing.
Assuntos
Cnidários/genética , Ctenóforos/genética , Proteoglicanas de Heparan Sulfato/genética , Placozoa/genética , Poríferos/genética , Regeneração/genética , Sequência de Aminoácidos , Animais , Membrana Basal/metabolismo , Membrana Basal/ultraestrutura , Cnidários/classificação , Cnidários/metabolismo , Cnidários/ultraestrutura , Ctenóforos/classificação , Ctenóforos/metabolismo , Ctenóforos/ultraestrutura , Evolução Molecular , Expressão Gênica , Proteoglicanas de Heparan Sulfato/química , Proteoglicanas de Heparan Sulfato/metabolismo , Humanos , Modelos Genéticos , Dados de Sequência Molecular , Filogenia , Placozoa/classificação , Placozoa/metabolismo , Placozoa/ultraestrutura , Reação em Cadeia da Polimerase , Poríferos/classificação , Poríferos/metabolismo , Poríferos/ultraestrutura , Alinhamento de Sequência , Homologia de Sequência de AminoácidosRESUMO
A new genus, Dendrogramma, with two new species of multicellular, non-bilaterian, mesogleal animals with some bilateral aspects, D. enigmatica and D. discoides, are described from the south-east Australian bathyal (400 and 1000 metres depth). A new family, Dendrogrammatidae, is established for Dendrogramma. These mushroom-shaped organisms cannot be referred to either of the two phyla Ctenophora or Cnidaria at present, because they lack any specialised characters of these taxa. Resolving the phylogenetic position of Dendrogramma depends much on how the basal metazoan lineages (Ctenophora, Porifera, Placozoa, Cnidaria, and Bilateria) are related to each other, a question still under debate. At least Dendrogramma must have branched off before Bilateria and is possibly related to Ctenophora and/or Cnidaria. Dendrogramma, therefore, is referred to Metazoa incertae sedis. The specimens were fixed in neutral formaldehyde and stored in 80% ethanol and are not suitable for molecular analysis. We recommend, therefore, that attempts be made to secure new material for further study. Finally similarities between Dendrogramma and a group of Ediacaran (Vendian) medusoids are discussed.
Assuntos
Cnidários/classificação , Ctenóforos/classificação , Placozoa/classificação , Poríferos/classificação , Animais , Austrália , Cnidários/anatomia & histologia , Cnidários/ultraestrutura , Ctenóforos/anatomia & histologia , Ctenóforos/ultraestrutura , Microscopia Eletrônica de Varredura , Oceanos e Mares , Placozoa/anatomia & histologia , Placozoa/ultraestrutura , Poríferos/anatomia & histologia , Poríferos/ultraestruturaRESUMO
Unraveling the base of metazoan evolution is of crucial importance for rooting the metazoan Tree of Life. This subject has attracted substantial attention for more than a century and recently fueled a burst of modern phylogenetic studies. Conflicting scenarios from different studies and incongruent results from nuclear versus mitochondrial markers challenge current molecular phylogenetic approaches. Here we analyze the presently most comprehensive data sets of mitochondrial genomes from non-bilaterian animals to illuminate the phylogenetic relationships among early branching metazoan phyla. The results of our analyses illustrate the value of mitogenomics and support previously known topologies between animal phyla but also identify several problematic taxa, which are sensitive to long branch artifacts or missing data.
Assuntos
Evolução Molecular , Genoma Mitocondrial , Filogenia , Animais , Cnidários/classificação , Ctenóforos/classificação , Modelos Genéticos , Placozoa/classificação , Poríferos/classificação , Análise de Sequência de DNARESUMO
The enigmatic animal phylum Placozoa holds a key position in the metazoan Tree of Life. A simple bauplan makes it appear to be the most basal metazoan known and genetic evidence also points to a position close to the last common metazoan ancestor. Trichoplax adhaerens is the only formally described species in the phylum to date, making the Placozoa the only monotypic phylum in the animal kingdom. However, recent molecular genetic as well as morphological studies have identified a high level of diversity, and hence a potential high level of taxonomic diversity, within this phylum. Different taxa, possibly at different taxonomic levels, are awaiting description. In this review we firstly summarize knowledge on the morphology, phylogenetic position and ecology of the Placozoa. Secondly, we give an overview of placozoan morphological and genetic diversity and finally present an updated distribution of placozoan populations. We conclude that there is great potential and need to erect new taxa and to establish a firm system for this taxonomic tabula rasa.
Assuntos
Biodiversidade , Placozoa/classificação , Animais , Evolução Molecular , Variação Genética , Genoma Mitocondrial , Geografia , Dados de Sequência Molecular , Filogenia , Placozoa/anatomia & histologia , Placozoa/genética , Dinâmica Populacional , RNA Ribossômico 16S , RNA Ribossômico 28SRESUMO
The neighbourhoods of cytochrome P450 (CYP) genes in deuterostome genomes, as well as those of the cnidarians Nematostella vectensis and Acropora digitifera and the placozoan Trichoplax adhaerens were examined to find clues concerning the evolution of CYP genes in animals. CYP genes created by the 2R whole genome duplications in chordates have been identified. Both microsynteny and macrosynteny were used to identify genes that coexisted near CYP genes in the animal ancestor. We show that all 11 CYP clans began in a common gene environment. The evidence implies the existence of a single locus, which we term the 'cytochrome P450 genesis locus', where one progenitor CYP gene duplicated to create a tandem set of genes that were precursors of the 11 animal CYP clans: CYP Clans 2, 3, 4, 7, 19, 20, 26, 46, 51, 74 and mitochondrial. These early CYP genes existed side by side before the origin of cnidarians, possibly with a few additional genes interspersed. The Hox gene cluster, WNT genes, an NK gene cluster and at least one ARF gene were close neighbours to this original CYP locus. According to this evolutionary scenario, the CYP74 clan originated from animals and not from land plants nor from a common ancestor of plants and animals. The CYP7 and CYP19 families that are chordate-specific belong to CYP clans that seem to have originated in the CYP genesis locus as well, even though this requires many gene losses to explain their current distribution. The approach to uncovering the CYP genesis locus overcomes confounding effects because of gene conversion, sequence divergence, gene birth and death, and opens the way to understanding the biodiversity of CYP genes, families and subfamilies, which in animals has been obscured by more than 600 Myr of evolution.
Assuntos
Cnidários/genética , Sistema Enzimático do Citocromo P-450/genética , Evolução Molecular , Loci Gênicos , Placozoa/genética , Animais , Mapeamento Cromossômico , Cromossomos/genética , Cromossomos/metabolismo , Cnidários/classificação , Cnidários/enzimologia , Sistema Enzimático do Citocromo P-450/classificação , Sistema Enzimático do Citocromo P-450/metabolismo , Duplicação Gênica , Genes Homeobox , Ligação Genética , Humanos , Mitocôndrias/enzimologia , Mitocôndrias/genética , Família Multigênica , Filogenia , Placozoa/classificação , Placozoa/enzimologia , Retinoides/metabolismo , SinteniaRESUMO
Molecular phylogenetic analyses have produced a plethora of controversial hypotheses regarding the patterns of diversification of non-bilaterian animals. To unravel the causes for the patterns of extreme inconsistencies at the base of the metazoan tree of life, we constructed a novel supermatrix containing 122 genes, enriched with non-bilaterian taxa. Comparative analyses of this supermatrix and its two non-overlapping multi-gene partitions (including ribosomal and non-ribosomal genes) revealed conflicting phylogenetic signals. We show that the levels of saturation and long branch attraction artifacts in the two partitions correlate with gene sampling. The ribosomal gene partition exhibits significantly lower saturation levels than the non-ribosomal one. Additional systematic errors derive from significant variations in amino acid substitution patterns among the metazoan lineages that violate the stationarity assumption of evolutionary models frequently used to reconstruct phylogenies. By modifying gene sampling and the taxonomic composition of the outgroup, we were able to construct three different yet well-supported phylogenies. These results show that the accuracy of phylogenetic inference may be substantially improved by selecting genes that evolve slowly across the Metazoa and applying more realistic substitution models. Additional sequence-independent genomic markers are also necessary to assess the validity of the phylogenetic hypotheses.
Assuntos
Ctenóforos/classificação , Filogenia , Placozoa/classificação , Poríferos/classificação , Ribossomos/genética , Animais , Teorema de Bayes , Ctenóforos/genética , Genômica , Funções Verossimilhança , Modelos Genéticos , Placozoa/genética , Poríferos/genéticaRESUMO
The ever-lingering question: "What did the urmetazoan look like?" has not lost its charm, appeal or elusiveness for one and a half centuries. A solid amount of organismal data give what some feel is a clear answer (e.g. Placozoa are at the base of the metazoan tree of life (ToL)), but a diversity of modern molecular data gives almost as many answers as there are exemplars, and even the largest molecular data sets could not solve the question and sometimes even suggest obvious zoological nonsense. Since the problems involved in this phylogenetic conundrum encompass a wide array of analytical freedom and uncertainty it seems questionable whether a further increase in molecular data (quantity) can solve this classical deep phylogeny problem. This review thus strikes a blow for evaluating quality data (including morphological, molecule morphologies, gene arrangement, and gene loss versus gene gain data) in an appropriate manner.
Assuntos
Evolução Biológica , Filogenia , Animais , Placozoa/anatomia & histologia , Placozoa/classificação , Placozoa/genéticaRESUMO
The morphology and ultrastructure of 10 clonal placozoan lineages were studied. We scored several morphological characters at a cellular and intracellular level and identified a number of morphological differences among clones. Some differences appear clone specific and allow recognizing five distinct lineages based on morphological criteria only. These data will be crucial for a yet to be established placozoan systematics. Furthermore, we here describe three new diagnostic morphological characters for Placozoa: a new structure in the upper epithelium, called "concave disc," two distinct subpopulations of fiber cells, and especially small cells in the body margin. Besides the fiber cells appear to be arranged in several layers forming a complex, three-dimensional net not previously described. We also describe the marginal cells as the formerly suggested potential stem-cell type. The basic morphology is revised.