Tunicates have been recently revealed to be the closest living relatives of vertebrates. Yet, with more than 2500 described species, details of their evolutionary history are still obscure. From a molecular point of view, tunicate phylogenetic relationships have been mostly studied based on analyses of 18S rRNA sequences, which indicate several major clades at odds with the traditional class-level arrangements. Nonetheless, substantial uncertainty remains about the phylogenetic relationships and taxonomic status of key groups such as the Aplousobranchia, Appendicularia, and Thaliacea.
Thirty new complete 18S rRNA sequences were acquired from previously unsampled tunicate species, with special focus on groups presenting high evolutionary rate. The updated 18S rRNA dataset has been aligned with respect to the constraint on homology imposed by the rRNA secondary structure. A probabilistic framework of phylogenetic reconstruction was adopted to accommodate the particular evolutionary dynamics of this ribosomal marker. Detailed Bayesian analyses were conducted under the non-parametric CAT mixture model accounting for site-specific heterogeneity of the evolutionary process, and under RNA-specific doublet models accommodating the occurrence of compensatory substitutions in stem regions. Our results support the division of tunicates into three major clades: 1) Phlebobranchia + Thaliacea + Aplousobranchia, 2) Appendicularia, and 3) Stolidobranchia, but the position of Appendicularia could not be firmly resolved. Our study additionally reveals that most Aplousobranchia evolve at extremely high rates involving changes in secondary structure of their 18S rRNA, with the exception of the family Clavelinidae, which appears to be slowly evolving. This extreme rate heterogeneity precluded resolving with certainty the exact phylogenetic placement of Aplousobranchia. Finally, the best fitting secondary-structure and CAT-mixture models suggest a sister-group relationship between Salpida and Pyrosomatida within Thaliacea.
An updated phylogenetic framework for tunicates is provided based on phylogenetic analyses using the most realistic evolutionary models currently available for ribosomal molecules and an unprecedented taxonomic sampling. Detailed analyses of the 18S rRNA gene allowed a clear definition of the major tunicate groups and revealed contrasting evolutionary dynamics among major lineages. The resolving power of this gene nevertheless appears limited within the clades composed of Phlebobranchia + Thaliacea + Aplousobranchia and Pyuridae + Styelidae, which were delineated as spots of low resolution. These limitations underline the need to develop new nuclear markers in order to further resolve the phylogeny of this keystone group in chordate evolution.
Ascidians, primitive chordates that have retained features of the likely progenitors to all vertebrates, are a useful model to study the evolutionary relationship of chordates to other animals. We have selected the well characterized ribosomal RNA (rRNA) genes to investigate this relationship, and we describe here the cloning and characterization of an entire ribosomal DNA (rDNA) tandem repeat unit from a lower chordate, the ascidian Herdmania momus. rDNA copy number and considerable sequence differences were observed between two H. momus populations. Comparison of rDNA primary sequence and rRNA secondary structures from H. momus with those from other well characterized organisms, demonstrated that the ascidians are more closely related to other chordates than invertebrates. The rDNA tandem repeat makes up a larger percentage (7%) of the genome of this animal than in other higher eukaryotes. The total length of the spacer and transcribed region in H. momus rDNA is small compared to most higher eukaryotes, being less than 8 kb, and the intergenic spacer region consists of smaller internal repeats. Comparative analysis of rDNA sequences has allowed the construction of secondary structures for the 18S, 5.8S and 26S rRNAs.
Within Chordata, the subphyla Vertebrata and Cephalochordata (lancelets) are characterized by a remarkable stability of the mitochondrial (mt) genome, with constancy of gene content and almost invariant gene order, whereas the limited mitochondrial data on the subphylum Tunicata suggest frequent and extensive gene rearrangements, observed also within ascidians of the same genus.
To confirm this evolutionary trend and to better understand the evolutionary dynamics of the mitochondrial genome in Tunicata Ascidiacea, we have sequenced and characterized the complete mt genome of two congeneric ascidian species, Phallusia mammillata and Phallusia fumigata (Phlebobranchiata, Ascidiidae). The two mtDNAs are surprisingly rearranged, both with respect to one another and relative to those of other tunicates and chordates, with gene rearrangements affecting both protein-coding and tRNA genes. The new data highlight the extraordinary variability of ascidian mt genome in base composition, tRNA secondary structure, tRNA gene content, and non-coding regions (number, size, sequence and location). Indeed, both Phallusia genomes lack the trnD gene, show loss/acquisition of DHU-arm in two tRNAs, and have a G+C content two-fold higher than other ascidians. Moreover, the mt genome of P. fumigata presents two identical copies of trnI, an extra tRNA gene with uncertain amino acid specificity, and four almost identical sequence regions. In addition, a truncated cytochrome b, lacking a C-terminal tail that commonly protrudes into the mt matrix, has been identified as a new mt feature probably shared by all tunicates.
The frequent occurrence of major gene order rearrangements in ascidians both at high taxonomic level and within the same genus makes this taxon an excellent model to study the mechanisms of gene rearrangement, and renders the mt genome an invaluable phylogenetic marker to investigate molecular biodiversity and speciation events in this largely unexplored group of basal chordates.
Tunicates have been extensively studied because of their crucial phylogenetic location (the closest living relatives of vertebrates) and particular developmental plan. Recent genome efforts have disclosed that tunicates are also remarkable in their genome organization and molecular evolutionary patterns. Here, we review these latter aspects, comparing the similarities and specificities of two model species of the group: Oikopleura dioica and Ciona intestinalis. These species exhibit great genome plasticity and Oikopleura in particular has undergone a process of extreme genome reduction and compaction that can be explained in part by gene loss, but is mostly due to other mechanisms such as shortening of intergenic distances and introns, and scarcity of mobile elements. In Ciona, genome reorganization was less severe being more similar to the other chordates in several aspects. Rates and patterns of molecular evolution are also peculiar in tunicates, being Ciona about 50% faster than vertebrates and Oikopleura three times faster. In fact, the latter species is considered as the fastest evolving metazoan recorded so far. Two processes of increase in evolutionary rates have taken place in tunicates. One of them is more extreme, and basically restricted to genes encoding regulatory proteins (transcription regulators, chromatin remodeling proteins, and metabolic regulators), and the other one is less pronounced but affects the whole genome. Very likely adaptive evolution has played a very significant role in the first, whereas the functional and/or evolutionary causes of the second are less clear and the evidence is not conclusive. The evidences supporting the incidence of increased mutation and less efficient negative selection are presented and discussed.
positive selection; genome plasticity; Oikopleura dioica; Ciona
In chordates, neural induction is the first step of a complex developmental process through which ectodermal cells acquire a neural identity. In ascidians, FGF-mediated neural induction occurs at the 32-cell stage in two blastomere pairs, precursors respectively of anterior and posterior neural tissue. We combined molecular embryology and cis-regulatory analysis to unveil in the ascidian Ciona intestinalis the remarkably simple proximal genetic network that controls posterior neural fate acquisition downstream of FGF. We report that the combined action of two direct FGF targets, the TGFβ factor Nodal, acting via Smad- and Fox-binding sites, and the transcription factor Otx suffices to trigger ascidian posterior neural tissue formation. Moreover, we found that this strategy is conserved in the distantly related ascidian Phallusia mammillata, in spite of extreme sequence divergence in the cis-regulatory sequences involved. Our results thus highlight that the modes of gene regulatory network evolution differ with the evolutionary scale considered. Within ascidians, developmental regulatory networks are remarkably robust to genome sequence divergence. Between ascidians and vertebrates, major fate determinants, such as Otx and Nodal, can be co-opted into different networks. Comparative developmental studies in ascidians with divergent genomes will thus uncover shared ascidian strategies, and contribute to a better understanding of the diversity of developmental strategies within chordates.
The Chordate phylum groups vertebrates, tunicates (including ascidians) and cephalochordates (amphioxus). These animals share a typical body plan characterized by the presence during embryonic life of a notochord and a dorsal neural tube. Ascidians, however, took a significantly different evolutionary path from other chordates resulting in divergent morphological, embryological and genomic features. Their development is fast and stereotyped with very few cells and ascidian genomes have undergone compaction and extensive rearrangements when compared to vertebrates, but also between ascidian species. This raises the question of whether developmental mechanisms controlling typical chordate structure formation are conserved between ascidians and vertebrates. Here, we have studied the set of ascidian genes which control the formation of the posterior part of the nervous system. We uncovered original usages of the signaling molecule Nodal and the transcription factor Otx. For example, Otx, which is a specific determinant of anterior identity in most metazoans, has been co-opted for the formation of the ascidian posterior nervous system. These two factors define a regulatory signature found in enhancers of posterior neural genes in two genomically divergent ascidian species.
Tunicates are invertebrate members of the chordate phylum, and are considered to be the sister group of vertebrates. Tunicates are composed of ascidians, thaliaceans, and appendicularians. With the advent of inexpensive high-throughput sequencing, the number of sequenced tunicate genomes is expected to rise sharply within the coming years. To facilitate comparative genomics within the tunicates, and between tunicates and vertebrates, standardized rules for the nomenclature of tunicate genetic elements need to be established. Here we propose a set of nomenclature rules, consensual within the community, for predicted genes, pseudogenes, transcripts, operons, transcriptional cis-regulatory regions, transposable elements, and transgenic constructs. In addition, the document proposes guidelines for naming transgenic and mutant lines.
tunicates; genome annotation; gene; transposable element; cis-regulatory sequences
Ascidians are tunicates, the taxon recently proposed as sister group to the vertebrates. They possess a chordate-like swimming larva, which metamorphoses into a sessile adult. Several ascidian species form colonies of clonal individuals by asexual reproduction. During their life cycle, ascidians present three muscle types: striated in larval tail, striated in the heart, and unstriated in the adult body-wall.
In the colonial ascidian Botryllus schlosseri, we investigated organisation, differentiation and gene expression of muscle beginning from early buds to adults and during zooid regression. We characterised transcripts for troponin T (BsTnT-c), adult muscle-type (BsMA2) and cytoplasmic-type (BsCA1) actins, followed by in situ hybridisation (ISH) on sections to establish the spatio-temporal expression of BsTnT-c and BsMA2 during asexual reproduction and in the larva. Moreover, we characterised actin genomic sequences, which by comparison with other metazoans revealed conserved intron patterns.
Integration of data from ISH, phalloidin staining and TEM allowed us to follow the phases of differentiation of the three muscle kinds, which differ in expression pattern of the two transcripts. Moreover, phylogenetic analyses provided evidence for the close relationship between tunicate and vertebrate muscle genes. The characteristics and plasticity of muscles in tunicates are discussed.
Deuterostomes are a monophyletic group of animals that include the vertebrates, invertebrate chordates, ambulacrarians and xenoturbellids. Fossil representatives from most major deuterostome groups, including some phylum-level crown groups, are found in the Lower Cambrian, suggesting that evolutionary divergence occurred in the Late Precambrian, in agreement with some molecular clock estimates. Molecular phylogenies, larval morphology and the adult heart/kidney complex all support echinoderms and hemichordates as a sister grouping (Ambulacraria). Xenoturbellids are a relatively newly discovered phylum of worm-like deuterostomes that lacks a fossil record, but molecular evidence suggests that these animals are a sister group to the Ambulacraria. Within the chordates, cephalochordates share large stretches of chromosomal synteny with the vertebrates, have a complete Hox complex and are sister group to the vertebrates based on ribosomal and mitochondrial gene evidence. In contrast, tunicates have a highly derived adult body plan and are sister group to the vertebrates based on the analyses of concatenated genomic sequences. Cephalochordates and hemichordates share gill slits and an acellular cartilage, suggesting that the ancestral deuterostome also shared these features. Gene network data suggest that the deuterostome ancestor had an anterior–posterior body axis specified by Hox and Wnt genes, a dorsoventral axis specified by a BMP/chordin gradient, and was bilaterally symmetrical with left–right asymmetry determined by expression of nodal.
deuterostomes; chordate evolution; Ambulacraria
Hair cells are vertebrate secondary sensory cells located in the ear and in the lateral line organ. Until recently, these cells were considered to be mechanoreceptors exclusively found in vertebrates that evolved within this group. Evidence of secondary mechanoreceptors in some tunicates, the proposed sister group of vertebrates, has recently led to the hypothesis that vertebrate and tunicate secondary sensory cells share a common origin. Secondary sensory cells were described in detail in two tunicate groups, ascidians and thaliaceans, in which they constitute an oral sensory structure called the coronal organ. Among thaliaceans, the organ is absent in salps and it has been hypothesised that this condition is due to a different feeding system adopted by this group of animals. No information is available as to whether a comparable structure exists in the third group of tunicates, the appendicularians, although different sensory structures are known to be present in these animals.
We studied the detailed morphology of appendicularian oral mechanoreceptors. Using light and electron microscopy we could demonstrate that the mechanosensory organ called the circumoral ring is composed of secondary sensory cells. We described the ultrastructure of the circumoral organ in two appendicularian species, Oikopleura dioica and Oikopleura albicans, and thus taxonomically completed the data collection of tunicate secondary sensory cells. To understand the evolution of secondary sensory cells in tunicates, we performed a cladistic analysis using morphological data. We constructed a matrix consisting of 19 characters derived from detailed ultrastructural studies in 16 tunicate species and used a cephalochordate and three vertebrate species as outgroups.
Our study clearly shows that the circumoral ring is the appendicularian homologue of the coronal organ of other tunicate taxa. The cladistic analysis enabled us to reconstruct the features of the putative ancestral hair cell in tunicates, represented by a simple monociliated cell. This cell successively differentiated into the current variety of oral mechanoreceptors in the various tunicate lineages. Finally, we demonstrated that the inferred evolutionary changes coincide with major transitions in the feeding strategies in each respective lineage.
Cladistic analysis; Oikopleura albicans; Oikopleura dioica; Sensory cells
Urochordates are the closest relatives of vertebrates and at the larval stage, possess a characteristic bilateral chordate body plan. In vertebrates, the genes that orchestrate embryonic patterning are in part regulated by highly conserved non-coding elements (CNEs), yet these elements have not been identified in urochordate genomes. Consequently the evolution of the cis-regulatory code for urochordate development remains largely uncharacterised. Here, we use genome-wide comparisons between C. intestinalis and C. savignyi to identify putative urochordate cis-regulatory sequences. Ciona conserved non-coding elements (ciCNEs) are associated with largely the same key regulatory genes as vertebrate CNEs. Furthermore, some of the tested ciCNEs are able to activate reporter gene expression in both zebrafish and Ciona embryos, in a pattern that at least partially overlaps that of the gene they associate with, despite the absence of sequence identity. We also show that the ability of a ciCNE to up-regulate gene expression in vertebrate embryos can in some cases be localised to short sub-sequences, suggesting that functional cross-talk may be defined by small regions of ancestral regulatory logic, although functional sub-sequences may also be dispersed across the whole element. We conclude that the structure and organisation of cis-regulatory modules is very different between vertebrates and urochordates, reflecting their separate evolutionary histories. However, functional cross-talk still exists because the same repertoire of transcription factors has likely guided their parallel evolution, exploiting similar sets of binding sites but in different combinations.
Vertebrates share many aspects of early development with our closest chordate ancestors, the tunicates. However, whilst the repertoire of genes that orchestrate development is essentially the same in the two lineages, the genomic code that regulates these genes appears to be very different, even though it is highly conserved within vertebrates themselves. Using comparative genomics, we have identified a parallel developmental code in tunicates and confirmed that this code, despite a lack of sequence conservation, associates with a similar repertoire of genes. However, the organisation of the code spatially is very different in the two lineages, strongly suggesting that most of it arose independently in vertebrates and tunicates, and in most cases lacking any direct sequence ancestry. We have assayed elements of the tunicate code, and found that at least some of them can regulate gene expression in zebrafish embryos. Our results suggest that regulatory code has arisen independently in different animal lineages but possesses some common functionality because its evolution has been driven by a similar cohort of developmental transcription factors. Our work helps illuminate how complex, stable gene regulatory networks evolve and become fixed within lineages.
The vascular endothelial growth factor (VEGF)-VEGF Receptor (VEGFR) system is an important pathway for regulation of angiogenesis. However, its evolutionary development, particularly the step from invertebrates to vertebrates, is still largely unknown. Here, we molecularly cloned the VEGFR-like gene from Halocynthia roretzi, a species belonging to the Tunicata, the chordate subphylum recently considered the sister group of vertebrates. The cDNA encoded a homolog of human VEGFR, including the transmembrane domain, and the tyrosine kinase domain with a kinase-insert region, which was designated S. sq VEGFR (GenBank AB374180). Similar to Tunicates including ascidians in the phylogenetic tree, the Amphioxus, another chordate, is located close to vertebrates. However, S. sq VEGFR has a higher homology than the Amphioxus VEGFR-like molecule (GenBank AB025557) to human VEGFR in the kinase domain-2 region. The S. sq VEGFR mRNA was expressed at highest levels in circulatory system-containing tissues, suggesting that S. sq VEGFR plays an important role in the formation or maintenance of circulatory system in Tunicates, Halocynthia roretzi.
vascular endothelial growth factor receptor; Halocynthia roretzi; sea squirt; circulatory system
The phylogenetic position of Bryozoa is one of the most controversial issues in metazoan phylogeny. In an attempt to address this issue, the first bryozoan mitochondrial genome from Flustrellidra hispida (Gymnolaemata, Ctenostomata) was recently sequenced and characterized. Unfortunately, it has extensive gene translocation and extremely reduced size. In addition, the phylogenies obtained from the result were conflicting, so they failed to assign a reliable phylogenetic position to Bryozoa or to clarify lophophorate phylogeny. Thus, it is necessary to characterize further mitochondrial genomes from slowly-evolving bryozoans to obtain a more credible lophophorate phylogeny.
The complete mitochondrial genome (15,433 bp) of Bugula neritina (Bryozoa, Gymnolaemata, Cheilostomata), one of the most widely distributed cheliostome bryozoans, is sequenced. This second bryozoan mitochondrial genome contains the set of 37 components generally observed in other metazoans, differing from that of F. hispida (Bryozoa, Gymnolaemata, Ctenostomata), which has only 36 components with loss of tRNAser(ucn) genes. The B. neritina mitochondrial genome possesses 27 multiple noncoding regions. The gene order is more similar to those of the two remaining lophophorate phyla (Brachiopoda and Phoronida) and a chiton Katharina tunicate than to that of F. hispida. Phylogenetic analyses based on the nucleotide sequences or amino acid residues of 12 protein-coding genes showed consistently that, within the Lophotrochozoa, the monophyly of the bryozoan class Gymnolaemata (B. neritina and F. hispida) was strongly supported and the bryozoan clade was grouped with brachiopods. Echiura appeared as a subtaxon of Annelida, and Entoprocta as a sister taxon of Phoronida. The clade of Bryozoa + Brachiopoda was clustered with either the clade of Annelida-Echiura or that of Phoronida + Entoprocta.
This study presents the complete mitochondrial genome of a cheliostome bryozoan, B. neritina. The phylogenetic analyses suggest a close relationship between Bryozoa and Brachiopoda within the Lophotrochozoa. However, the sister group of Bryozoa + Brachiopoda is still ambiguous, although it has some attractions with Annelida-Echiura or Phoronida + Entoprocta. If the latter is a true phylogeny, lophophorate monophyly including Entoprocta is supported. Consequently, the present results imply that Brachiozoa (= Brachiopoda + Phoronida) and the recently-resurrected Bryozoa concept comprising Ectoprocta and Entoprocta may be refuted.
Little is known about the ancient chordates that gave rise to the first vertebrates, but the descendants of other invertebrate chordates extant at the time still flourish in the ocean. These invertebrates include the cephalochordates and tunicates, whose larvae share with vertebrate embryos a common body plan with a central notochord and a dorsal nerve cord. Tunicates are now thought to be the sister group of vertebrates. However, research based on several species of ascidians, a diverse and wide-spread class of tunicates, revealed that the molecular strategies underlying their development appear to diverge greatly from those found in vertebrates. Furthermore, the adult body plan of most tunicates, which arises following an extensive post-larval metamorphosis, shows little resemblance to the body plan of any other chordate. In this review, we compare the developmental strategies of ascidians and vertebrates and argue that the very divergence of these strategies reveals the surprising level of plasticity of the chordate developmental program and is a rich resource to identify core regulatory mechanisms that are evolutionarily conserved in chordates. Further, we propose that the comparative analysis of the architecture of ascidian and vertebrate gene regulatory networks may provide critical insight into the origin of the chordate body plan.
Traditional metazoan phylogeny classifies the Vertebrata as a subphylum of the phylum Chordata, together with two other subphyla, the Urochordata (Tunicata) and the Cephalochordata. The Chordata, together with the phyla Echinodermata and Hemichordata, comprise a major group, the Deuterostomia. Chordates invariably possess a notochord and a dorsal neural tube. Although the origin and evolution of chordates has been studied for more than a century, few authors have intimately discussed taxonomic ranking of the three chordate groups themselves. Accumulating evidence shows that echinoderms and hemichordates form a clade (the Ambulacraria), and that within the Chordata, cephalochordates diverged first, with tunicates and vertebrates forming a sister group. Chordates share tadpole-type larvae containing a notochord and hollow nerve cord, whereas ambulacrarians have dipleurula-type larvae containing a hydrocoel. We propose that an evolutionary occurrence of tadpole-type larvae is fundamental to understanding mechanisms of chordate origin. Protostomes have now been reclassified into two major taxa, the Ecdysozoa and Lophotrochozoa, whose developmental pathways are characterized by ecdysis and trochophore larvae, respectively. Consistent with this classification, the profound dipleurula versus tadpole larval differences merit a category higher than the phylum. Thus, it is recommended that the Ecdysozoa, Lophotrochozoa, Ambulacraria and Chordata be classified at the superphylum level, with the Chordata further subdivided into three phyla, on the basis of their distinctive characteristics.
chordate evolution; three-phylum system; Vertebrata; Urochordata; Cephalochordata
Comparative mitochondrial genomic analyses are rare among crustaceans below the family or genus level. The obliged subterranean crustacean amphipods of the family Metacrangonyctidae, found from the Hispaniola (Antilles) to the Middle East, including the Canary Islands and the peri-Mediterranean region, have an evolutionary history and peculiar biogeography that can respond to Tethyan vicariance. Indeed, recent phylogenetic analysis using all protein-coding mitochondrial sequences and one nuclear ribosomal gene have lent support to this hypothesis (Bauzà-Ribot et al. 2012).
We present the analyses of mitochondrial genome sequences of 21 metacrangonyctids in the genera Metacrangonyx and Longipodacrangonyx, covering the entire geographical range of the family. Most mitogenomes were attained by next-generation sequencing techniques using long-PCR fragments sequenced by Roche FLX/454 or GS Junior pyro-sequencing, obtaining a coverage depth per nucleotide of up to 281×. All mitogenomes were AT-rich and included the usual 37 genes of the metazoan mitochondrial genome, but showed a unique derived gene order not matched in any other amphipod mitogenome. We compare and discuss features such as strand bias, phylogenetic informativeness, non-synonymous/synonymous substitution rates and other mitogenomic characteristics, including ribosomal and transfer RNAs annotation and structure.
Next-generation sequencing of pooled long-PCR amplicons can help to rapidly generate mitogenomic information of a high number of related species to be used in phylogenetic and genomic evolutionary studies. The mitogenomes of the Metacrangonyctidae have the usual characteristics of the metazoan mitogenomes (circular molecules of 15,000-16,000 bp, coding for 13 protein genes, 22 tRNAs and two ribosomal genes) and show a conserved gene order with several rearrangements with respect to the presumed Pancrustacean ground pattern. Strand nucleotide bias appears to be reversed with respect to the condition displayed in the majority of crustacean mitogenomes since metacrangonyctids show a GC-skew at the (+) and (-) strands; this feature has been reported also in the few mitogenomes of Isopoda (Peracarida) known thus far. The features of the rRNAs, tRNAs and sequence motifs of the control region of the Metacrangonyctidae are similar to those of the few crustaceans studied at present.
Electronic supplementary material
The online version of this article (doi:10.1186/1471-2164-15-566) contains supplementary material, which is available to authorized users.
Crustacea; Amphipoda; Metacrangonyx; Mitogenome evolution; Mitochondrial RNA secondary structure
Botryllus schlosseri is a colonial urochordate that follows the chordate plan of development following sexual reproduction, but invokes a stem cell-mediated budding program during subsequent rounds of asexual reproduction. As urochordates are considered to be the closest living invertebrate relatives of vertebrates, they are ideal subjects for whole genome sequence analyses. Using a novel method for high-throughput sequencing of eukaryotic genomes, we sequenced and assembled 580 Mbp of the B. schlosseri genome. The genome assembly is comprised of nearly 14,000 intron-containing predicted genes, and 13,500 intron-less predicted genes, 40% of which could be confidently parceled into 13 (of 16 haploid) chromosomes. A comparison of homologous genes between B. schlosseri and other diverse taxonomic groups revealed genomic events underlying the evolution of vertebrates and lymphoid-mediated immunity. The B. schlosseri genome is a community resource for studying alternative modes of reproduction, natural transplantation reactions, and stem cell-mediated regeneration.
The tunicates are an evolutionary group that includes species such as sea squirts and sea tulips. Their name comes from the structure known as a ‘tunic’ that surrounds their sac-like bodies. As marine filter feeders, tunicates obtain nutrients by straining food particles from water, and they can live either alone or in colonies depending on the species. Charles Darwin suggested that tunicates may be the key to understanding the evolution of vertebrates, and indeed today they are regarded as the closest living relatives of this group.
Colonial tunicates can reproduce either sexually, or asexually by budding. Compatible colonies have the ability to recognize one another and to fuse their blood vessels to form a single organism, whereas incompatible colonies reject one another and remain separate. This recognition process bears some resemblance to the rejection of foreign organ transplants in mammals.
Here, Voskoboynik and co-workers present the first genome sequence of a colonial tunicate, Botryllus schlosseri. They used a novel sequencing approach that significantly increased the length of a DNA molecule that can be determined by next-generation sequencing, and allowed large DNA repeat regions to be easily resolved. In total, they sequenced 580 million base pairs of DNA, which they estimate contains roughly 27,000 genes.
By comparing the B. schlosseri genome with those of a number of vertebrates, Voskoboynik et al. identified multiple B. schlosseri genes that also participate in the development and functioning of the vertebrate eye, heart, and auditory system, as well as others that may have contributed to the evolution of the immune system and of blood cells. The genome of B. schlosseri thus provides an important new tool for studying the genetic basis of the evolution of vertebrates.
Botryllus schlosseri; tunicates; stem cell; hematopoiesis; vertebrate evolution; genome; Other
In spite of the recent accumulation of genomic data, the evolutionary pathway in the individual genes of present-day living taxa is still elusive for most genes. Among ion channels, inward K+ rectifier (IRK) channels are the fundamental and well-defined protein group. We analyzed the genomic structures of this group and compared them among a phylogenetically wide range with our sequenced Halocynthia roretzi, a tunicate, IRK genomic genes.
A total of 131 IRK genomic genes were analyzed. The phylogenic trees of amino acid sequences revealed a clear diversification of deuterostomic IRKs from protostomic IRKs and suggested that the tunicate IRKs are possibly representatives of the descendants of ancestor forms of three major groups of IRKs in the vertebrate. However, the exon-intron structures of the tunicate IRK genomes showed considerable similarities to those of Caenorhabditis. In the vertebrate clade, the members in each major group increased at least four times those in the tunicate by various types of global gene duplication. The generation of some major groups was inferred to be due to anti-tandem (palindromic) duplication in early history. The intron insertion points greatly decreased during the evolution of the vertebrates, remaining as a unique conservation of an intron insertion site in the portion of protein-protein interaction within the coding regions of all vertebrate G-protein-activated IRK genes.
From the genomic survey of a family of IRK genes, it was suggested that the ancient intron insertion sites and the unique palindromic genomic duplication evolutionally shaped this membrane protein family.
The complete mitochondrial genomes (mitogenomes) of Cnaphalocrocis medinalis and Chilo suppressalis (Lepidoptera: Pyralidae) were determined and analyzed. The circular genomes were 15,388 bp long for C. medinalis and 15,395 bp long for C. suppressalis. Both mitogenomes contained 37 genes, with gene order similar to that of other lepidopterans. Notably, 12 protein-coding genes (PCGs) utilized the standard ATN, but the cox1 gene used CGA as the initiation codon; the cox1, cox2, and nad4 genes in the two mitogenomes had the truncated termination codons T, T, and TA, respectively, but the nad5 gene was found to use T as the termination codon only in the C. medinalis mitogenome. Additionally, the codon distribution and Relative Synonymous Codon Usage of the 13 PCGs in the C. medinalis mitogenome were very different from those in other pyralid moth mitogenomes. Most of the tRNA genes had typical cloverleaf secondary structures. However, the dihydrouridine (DHU) arm of the trnS1(AGN) gene did not form a stable stem-loop structure. Forty-nine helices in six domains, and 33 helices in three domains were present in the secondary structures of the rrnL and rrnS genes of the two mitogenomes, respectively. There were four major intergenic spacers, except for the A+T-rich region, spanning at least 12 bp in the two mitogenomes. The A+T-rich region contained an 'ATAGT(A)'-like motif followed by a poly-T stretch in the two mitogenomes. In addition, there were a potential stem-loop structure, a duplicated 25-bp repeat element, and a microsatellite '(TA)13' observed in the A+T-rich region of the C. medinalis mitogenome. A poly-T motif, a duplicated 31-bp repeat element, and a 19-bp triplication were found in the C. suppressalis mitogenome. However, there are many differences in the A+T-rich regions between the C. suppressalis mitogenome sequence in the present study and previous reports. Finally, the phylogenetic relationships of these insects were reconstructed based on amino acid sequences of mitochondrial 13 PCGs using Bayesian inference and maximum likelihood methods. These molecular-based phylogenies support the traditional morphologically based view of relationships within the Pyralidae.
Mitochondrial genome; Cnaphalocrocis medinalis; Chilo suppressalis; Lepidoptera; Pyralidae; phylogenetic relationship.
To gain insight into the evolutionary features of the huntingtin (htt) gene in Chordata, we have sequenced and characterized the full-length htt mRNA in the ascidian Ciona intestinalis, a basal chordate emerging as new invertebrate model organism. Moreover, taking advantage of the availability of genomic and EST sequences, the htt gene structure of a number of chordate species, including the cogeneric ascidian Ciona savignyi, and the vertebrates Xenopus and Gallus was reconstructed.
The C. intestinalis htt transcript exhibits some peculiar features, such as spliced leader trans-splicing in the 98 nt-long 5' untranslated region (UTR), an alternative splicing in the coding region, eight alternative polyadenylation sites, and no similarities of both 5' and 3'UTRs compared to homologs of the cogeneric C. savignyi. The predicted protein is 2946 amino acids long, shorter than its vertebrate homologs, and lacks the polyQ and the polyP stretches found in the the N-terminal regions of mammalian homologs. The exon-intron organization of the htt gene is almost identical among vertebrates, and significantly conserved between Ciona and vertebrates, allowing us to hypothesize an ancestral chordate gene consisting of at least 40 coding exons.
During chordate diversification, events of gain/loss, sliding, phase changes, and expansion of introns occurred in both vertebrate and ascidian lineages predominantly in the 5'-half of the htt gene, where there is also evidence of lineage-specific evolutionary dynamics in vertebrates. On the contrary, the 3'-half of the gene is highly conserved in all chordates at the level of both gene structure and protein sequence. Between the two Ciona species, a fast evolutionary rate and/or an early divergence time is suggested by the absence of significant similarity between UTRs, protein divergence comparable to that observed between mammals and fishes, and different distribution of repetitive elements.
An elaborated tripartite brain is considered one of the important innovations of vertebrates. Other extant chordate groups have a more basic brain organization. For instance, cephalochordates possess a relatively simple brain possibly homologous to the vertebrate forebrain and hindbrain, whereas tunicates display the tripartite organization, but without the specialized brain centers. The difference in anatomical complexity is even more pronounced if one compares chordates with other deuterostomes that have only a diffuse nerve net or alternatively a rather simple central nervous system. To gain a new perspective on the evolutionary roots of the complex vertebrate brain, we made here a phylostratigraphic analysis of gene expression patterns in the developing zebrafish (Danio rerio). The recovered adaptive landscape revealed three important periods in the evolutionary history of the zebrafish brain. The oldest period corresponds to preadaptive events in the first metazoans and the emergence of the nervous system at the metazoan–eumetazoan transition. The origin of chordates marks the next phase, where we found the overall strongest adaptive imprint in almost all analyzed brain regions. This finding supports the idea that the vertebrate brain evolved independently of the brains within the protostome lineage. Finally, at the origin of vertebrates we detected a pronounced signal coming from the dorsal telencephalon, in agreement with classical theories that consider this part of the cerebrum a genuine vertebrate innovation. Taken together, these results reveal a stepwise adaptive history of the vertebrate brain where most of its extant organization was already present in the chordate ancestor.
brain evolution; vertebrates; phylostratigraphy; zebrafish; gene expression; development
Ascidians or sea squirts form a diverse group within chordates, which includes a few thousand members of marine sessile filter-feeding animals. Their mitochondrial genomes are characterized by particularly high evolutionary rates and rampant gene rearrangements. This extreme variability complicates standard polymerase chain reaction (PCR) based techniques for molecular characterization studies, and consequently only a few complete Ascidian mitochondrial genome sequences are available. Using the standard PCR and Sanger sequencing approach, we produced the mitochondrial genome of Ascidiella aspersa only after a great effort. In contrast, we produced five additional mitogenomes (Botrylloides aff. leachii, Halocynthia spinosa, Polycarpa mytiligera, Pyura gangelion, and Rhodosoma turcicum) with a novel strategy, consisting in sequencing the pooled total DNA samples of these five species using one Illumina HiSeq 2000 flow cell lane. Each mitogenome was efficiently assembled in a single contig using de novo transcriptome assembly, as de novo genome assembly generally performed poorly for this task. Each of the new six mitogenomes presents a different and novel gene order, showing that no syntenic block has been conserved at the ordinal level (in Stolidobranchia and in Phlebobranchia). Phylogenetic analyses support the paraphyly of both Ascidiacea and Phlebobranchia, with Thaliacea nested inside Phlebobranchia, although the deepest nodes of the Phlebobranchia–Thaliacea clade are not well resolved. The strategy described here thus provides a cost-effective approach to obtain complete mitogenomes characterized by a highly plastic gene order and a fast nucleotide/amino acid substitution rate.
Tunicates; Ascidians; mitochondrial genome; mitogenomics; next-generation sequencing; Illumina; gene order; rearrangements; phylogeny; mixture models; genome assembly
Mitogenomic phylogenies have revealed well-supported relationships for many eukaryote groups. In the order Lepidoptera, 113 species mitogenomes had been sequenced (May 14, 2014). However, these data are restricted to ten of the forty-three recognised superfamilies, while it has been challenging to recover large numbers of mitogenomes due to the time and cost required for primer design and sequencing. Nuclear rather than mitochondrial genes have been preferred to reconstruct deep-level lepidopteran phylogenies, without seriously evaluating the potential of entire mitogenomes. Next-generation sequencing methods remove these limitations by providing efficiently massive amounts of sequence data. In the present study, we simultaneously obtained a large number of nymphalid butterfly mitogenomes to evaluate the utility of mitogenomic phylogenies by comparing reconstructions to the now quite well established phylogeny of Nymphalidae.
We newly obtained 30 nymphalid mitogenomes via pyrosequencing on the Roche 454 GS Junior system, and combined these sequences with publicly accessible data to provide a 70-taxa dataset covering 37 genes for a 15,495 bp alignment. Polymorphic sites were not homogeneously distributed across the gene. Two gene regions, nad6 and 3’ end of nad5, were most variable, whereas the cox1 and 5’ ends of rrnL were most conserved. Phylogenetic relationships inferred by two likelihood methods were congruent and strongly supported (>0.95 posterior probability; ML bootstrap >85%), across the majority of nodes for multiple partitioning strategies and substitution models. Bayes factor results showed that the most highly partitioned dataset is the preferred strategy among different partitioning schemes. The most striking phylogenetic findings were that the subfamily Danainae not Libytheinae was sister of the remaining brush-footed butterflies and that, within Limenitidini, the genus Athyma was clearly polyphyletic. None of the single-gene phylogenies recovered the highly supported topologies generated on the basis of the whole mitogenomic data.
Thirty mitogenomes were assembled with 89% completeness from the contigs of pyrosequencing-derived reads. Entire mitogenomes or higher-quality sequences could be obtained by increasing pyrosequencing read coverage or by additional Sanger sequencing. Our mitogenomic phylogenies provide robust nodal support at a range of levels, demonstrating that mitogenomes are both accurate and efficient molecular markers for inferring butterfly phylogeny.
Electronic supplementary material
The online version of this article (doi:10.1186/1471-2164-15-468) contains supplementary material, which is available to authorized users.
Athyma; Bayes factor; DNA Barcoding; cox1 gene; Limenitidinae; Limenitidini; Mitogenomic phylogeny; Papilionoidea; Pyrosequencing
An analysis of amphioxus miRNAs suggests an expansion of miRNAs played a key role in the evolution of chordates to vertebrates
microRNAs (miRNAs) are endogenous small non-coding RNAs that regulate gene expression at the post-transcriptional level. While the number of known human and murine miRNAs is continuously increasing, information regarding miRNAs from other species such as amphioxus remains limited.
We combined Solexa sequencing with computational techniques to identify novel miRNAs in the amphioxus species B. belcheri (Gray). This approach allowed us to identify 113 amphioxus miRNA genes. Among them, 55 were conserved across species and encoded 45 non-redundant mature miRNAs, whereas 58 were amphioxus-specific and encoded 53 mature miRNAs. Validation of our results with microarray and stem-loop quantitative RT-PCR revealed that Solexa sequencing is a powerful tool for miRNA discovery. Analyzing the evolutionary history of amphioxus miRNAs, we found that amphioxus possesses many miRNAs unique to chordates and vertebrates, and these may thus represent key steps in the evolutionary progression from cephalochordates to vertebrates. We also found that amphioxus is more similar to vertebrates than are tunicates with respect to their miRNA phylogenetic histories.
Taken together, our results indicate that Solexa sequencing allows the successful discovery of novel miRNAs from amphioxus with high accuracy and efficiency. More importantly, our study provides an opportunity to decipher how the elaboration of the miRNA repertoire that occurred during chordate evolution contributed to the evolution of the vertebrate body plan.
Ascidians are a fascinating group of filter-feeding marine chordates characterized by rapid evolution of both sequences and structure of their nuclear and mitochondrial genomes. Moreover, they include several model organisms used to investigate complex biological processes in chordates. To study the evolutionary dynamics of ascidians at short phylogenetic distances, we sequenced 13 new mitogenomes and analyzed them, together with 15 other available mitogenomes, using a novel approach involving detailed whole-mitogenome comparisons of conspecific and congeneric pairs. The evolutionary rate was quite homogeneous at both intraspecific and congeneric level, and the lowest congeneric rates were found in cryptic (morphologically undistinguishable) and in morphologically very similar species pairs. Moreover, congeneric nonsynonymous rates (dN) were up to two orders of magnitude higher than in intraspecies pairs. Overall, a clear-cut gap sets apart conspecific from congeneric pairs. These evolutionary peculiarities allowed easily identifying an extraordinary intraspecific variability in the model ascidian Botryllus schlosseri, where most pairs show a dN value between that observed at intraspecies and congeneric level, yet consistently lower than that of the Ciona intestinalis cryptic species pair. These data suggest ongoing speciation events producing genetically distinct B. schlosseri entities. Remarkably, these ongoing speciation events were undetectable by the cox1 barcode fragment, demonstrating that, at low phylogenetic distances, the whole mitogenome has a higher resolving power than cox1. Our study shows that whole-mitogenome comparative analyses, performed on a suitable sample of congeneric and intraspecies pairs, may allow detecting not only cryptic species but also ongoing speciation events.
ascidian; mitochondrial genome; evolutionary rate; species identification
A skewed assemblage of two epi-, meso- and bathypelagic fish families makes up the order Myctophiformes – the blackchins Neoscopelidae and the lanternfishes Myctophidae. The six rare neoscopelids show few morphological specializations whereas the divergent myctophids have evolved into about 250 species, of which many show massive abundances and wide distributions. In fact, Myctophidae is by far the most abundant fish family in the world, with plausible estimates of more than half of the oceans combined fish biomass. Myctophids possess a unique communication system of species-specific photophore patterns and traditional intrafamilial classification has been established to reflect arrangements of photophores. Myctophids present the most diverse array of larval body forms found in fishes although this attribute has both corroborated and confounded phylogenetic hypotheses based on adult morphology. No molecular phylogeny is available for Myctophiformes, despite their importance within all ocean trophic cycles, open-ocean speciation and as an important part of neoteleost divergence. This study attempts to resolve major myctophiform phylogenies from both mitogenomic sequences and corroborating evidence in the form of unique mitochondrial gene order rearrangements.
Mitogenomic evidence from DNA sequences and unique gene orders are highly congruent concerning phylogenetic resolution on several myctophiform classification levels, corroborating evidence from osteology, larval ontogeny and photophore patterns, although the lack of larval morphological characters within the subfamily Lampanyctinae stands out. Neoscopelidae is resolved as the sister family to myctophids with Solivomer arenidens positioned as a sister taxon to the remaining neoscopelids. The enigmatic Notolychnus valdiviae is placed as a sister taxon to all other myctophids and exhibits an unusual second copy of the tRNA-Met gene – a gene order rearrangement reminiscent of that found in the tribe Diaphini although our analyses show it to be independently derived. Most tribes are resolved in accordance with adult morphology although Gonichthyini is found within a subclade of the tribe Myctophini consisting of ctenoid scaled species. Mitogenomic sequence data from this study recognize 10 reciprocally monophyletic lineages within Myctophidae, with five of these clades delimited from additional rearranged gene orders or intergenic non-coding sequences.
Mitogenomic results from DNA sequences and unique gene orders corroborate morphology in phylogeny reconstruction and provide a likely scenario for the phylogenetic history of Myctophiformes. The extent of gene order rearrangements found within the mitochondrial genomes of myctophids is unique for phylogenetic purposes.
Myctophiformes; Myctophidae; Neoscopelidae; Phylogeny; Mitogenomics; Gene rearrangements; Non-coding sequence