Deep sequencing of mixed total DNA without barcodes allows efficient assembly of highly plastic ascidian mitochondrial genomes

Evolution of myxozoan mitochondrial genomes: insights from myxobolids

BACKGROUND

Myxozoa is a class of cnidarian parasites that encompasses over 2,400 species. Phylogenetic relationships among myxozoans remain highly debated, owing to both a lack of informative morphological characters and a shortage of molecular markers. Mitochondrial (mt) genomes are a common marker in phylogeny and biogeography. However, only five complete myxozoan mt genomes have been sequenced: four belonging to two closely related genera, Enteromyxum and Kudoa, and one from the genus Myxobolus. Interestingly, while cytochrome oxidase genes could be identified in Enteromyxum and Kudoa, no such genes were found in Myxobolus squamalis, and another member of the Myxobolidae (Henneguya salminicola) was found to have lost its entire mt genome. To evaluate the utility of mt genomes to reconstruct myxozoan relationships and to understand if the loss of cytochrome oxidase genes is a characteristic of myxobolids, we sequenced the mt genome of five myxozoans (Myxobolus wulii, M. honghuensis, M. shantungensis, Thelohanellus kitauei and, Sphaeromyxa zaharoni) using Illumina and Oxford Nanopore platforms.

RESULTS

Unlike Enteromyxum, which possesses a partitioned mt genome, the five mt genomes were encoded on single circular chromosomes. An mt plasmid was found in M. wulii, as described previously in Kudoa iwatai. In all new myxozoan genomes, five protein-coding genes (cob, cox1, cox2, nad1, and nad5) and two rRNAs (rnl and rns) were recognized, but no tRNA. We found that Myxobolus and Thelohanellus species shared unidentified reading frames, supporting the view that these mt open reading frames are functional. Our phylogenetic reconstructions based on the five conserved mt genes agree with previously published trees based on the 18S rRNA gene.

CONCLUSIONS

Our results suggest that the loss of cytochrome oxidase genes is not a characteristic of all myxobolids, the ancestral myxozoan mt genome was likely encoded on a single circular chromosome, and mt plasmids exist in a few lineages. Our findings indicate that myxozoan mt sequences are poor markers for reconstructing myxozoan phylogenetic relationships because of their fast-evolutionary rates and the abundance of repeated elements, which complicates assembly.

Phylogeographic and Morphological Analysis of Botrylloides niger Herdman, 1886 from the Northeastern Mediterranean Sea

Botrylloides niger (class Ascidiacea) is an invasive marine filter-feeding invertebrate that is believed to originate from the West Atlantic region. This species of colonial tunicate has been observed in several locations along the coasts of Israel and around the Suez Canal, but it has not yet been reported on the coasts of the Northeastern Mediterranean Sea (NEMS), suggesting an ongoing Lessepsian migration. However, the extent of this invasion might be concealed by reports of other potentially misidentified species of Botrylloides, given that the strong morphological similarities within this genus renders taxonomical identification particularly challenging. In this study, we performed a phylogeographic and morphological analysis of B. niger in the NEMS. We collected 238 samples from 8 sampling stations covering 824 km of the coastlines of NEMS. We reported 14 different morphotypes, of which the orange-brown, orange, and brown-striped morphs were the most abundant. Using the mitochondrial cytochrome C oxidase I (COI) as a DNA barcode marker, we identified 4 haplotypes. The COI haplotypes clustered with the reference B. niger sequences from GenBank and differed significantly from the sister Botrylloides species. We confirmed our identification using three additional barcoding markers (Histone 3, 18S rRNA, and 28S rRNA), which all matched with over 99% similarity to reference sequences. In addition, we monitored a station for a year and conducted a temporal analysis of the collected colonies. The colonies were absent during the winter and spring, while new colonies were established in the summer and expanded during autumn. We performed demographic population analysis on our spatial data that identified a possible population subdivision at a sampling site, which might have been caused by local freshwater input. Herein, we present the first report on the presence of Botrylloides niger in the NEMS. This study represents a key step toward understanding the diversity and the propagation of this highly invasive species of colonial ascidians, both within the Mediterranean basin as well as globally.

Complete sequence and comparative analysis of the mitochondrial genome of the rare and endangered Clematis acerifolia, the first clematis mitogenome to provide new insights into the phylogenetic evolutionary status of the genus

Clematis is one of the large worldwide genera of the Ranunculaceae Juss. Family, with high ornamental and medicinal value. China is the modern distribution centre of Clematis with abundant natural populations. Due to the complexity and high morphological diversity of Clematis, the genus is difficult to classify systematically, and in particular, the phylogenetic position of the endangered Clematis acerifolia is highly controversial. The use of the mitochondrial complete genome is a powerful molecular method that is frequently used for inferring plants phylogenies. However, studies on Clematis mitogenome are rare, thus limiting our full understanding of its phylogeny and genome evolution. Here, we sequenced and annotated the C. acerifolia mt genome using Illumina short- and Nanopore long-reads, characterized the species first complete mitogenome, and performed a comparative phylogenetic analysis with its close relatives. The total length of the C. acerifolia mitogenome is 698,247 bp and the main structure is multi-branched (linear molecule 1 and circular molecule 2). We annotated 55 genes, including 35 protein-coding, 17 tRNA, and 3 rRNA genes. The C. acerifolia mitogenome has extremely unconserved structurally, with extensive sequence transfer between the chloroplast and mitochondrial organelles, sequence repeats, and RNA editing. The phylogenetic position of C. acerifolia was determined by constructing the species mitogenome with 24 angiosperms. Further, our C. acerifolia mitogenome characteristics investigation included GC contents, codon usage, repeats and synteny analysis. Overall, our results are expected to provide fundamental information for C. acerifolia mitogenome evolution and confirm the validity of mitochondrial analysis in determining the phylogenetic positioning of Clematis plants.

The complete mitochondrial genome of the forest crested lizard, Calotes emma (Squamata, Agamidae) in China by the next generation sequencing

The whole mitogenome can prove useful tools for phylogenetic reconstruction and efficiently recover with reasonable taxon sampling. Calotes emma is widely distributed and arboreal in habits. However, studies of C. emma are still very limited, including population genetics and evolutionary biology. In this study, we reported the complete mitochondrial genome of the C. emma by next-generation sequencing for future more researches on systematics and evolution of C. emma from the perspective of mitochondrial DNA. The length of mitogenome was 17,688 bp, including 13 protein-coding genes (PCGs), 2 ribosomal RNA (rRNA) genes, 22 tRNA genes and a control region. The phylogenetic tree recovered the monophyly of the Calotes and revealed that newly sequenced C. emma well supported as the sister taxon to C. mystaceus by very high posterior probabilities (1.0). The complete mitochondrial genome of C.emma in this study will be helpful for understanding the phylogenetic systematics and relationships, and molecular evolution of Calotes in Agamidae.

Morphological Study and 3D Reconstruction of the Larva of the Ascidian Halocynthia roretzi

The swimming larva represents the dispersal phase of ascidians, marine invertebrates belonging to tunicates. Due to its adhesive papillae, the larva searches the substrate, adheres to it, and undergoes metamorphosis, thereby becoming a sessile filter feeding animal. The larva anatomy has been described in detail in a few species, revealing a different degree of adult structure differentiation, called adultation. In the solitary ascidian Halocynthia roretzi, a species reared for commercial purposes, embryogenesis has been described in detail, but information on the larval anatomy is still lacking. Here, we describe it using a comparative approach, utilizing 3D reconstruction, as well as histological/TEM observations, with attention to its papillae. The larva is comparable to those of other solitary ascidians, such as Ciona intestinalis. However, it displays a higher level of adultation for the presence of the atrium, opened outside by means of the atrial siphon, and the peribranchial chambers. It does not reach the level of complexity of the larva of Botryllus schlosseri, a phylogenetically close colonial ascidian. Our study reveals that the papillae of H. roretzi, previously described as simple and conform, exhibit dynamic changes during settlement. This opens up new considerations on papillae morphology and evolution and deserves to be further investigated.

Bivalve molluscs as model systems for studying mitochondrial biology

The class Bivalvia is a highly successful and ancient taxon including ∼25,000 living species. During their long evolutionary history bivalves adapted to a wide range of physicochemical conditions, habitats, biological interactions, and feeding habits. Bivalves can have strikingly different size, and despite their apparently simple body plan, they evolved very different shell shapes, and complex anatomic structures. One of the most striking features of this class of animals is their peculiar mitochondrial biology: some bivalves have facultatively anaerobic mitochondria that allow them to survive prolonged periods of anoxia/hypoxia. Moreover, more than 100 species have now been reported showing the only known evolutionarily stable exception to the strictly maternal inheritance of mitochondria in animals, named doubly uniparental inheritance. Mitochondrial activity is fundamental to eukaryotic life, and thanks to their diversity and uncommon features, bivalves represent a great model system to expand our knowledge about mitochondrial biology, so far limited to a few species. We highlight recent works studying mitochondrial biology in bivalves at either genomic or physiological level. A link between these two approaches is still missing, and we believe that an integrated approach and collaborative relationships are the only possible ways to be successful in such endeavour.

An elongated COI fragment to discriminate botryllid species and as an improved ascidian DNA barcode

Botryllids are colonial ascidians widely studied for their potential invasiveness and as model organisms, however the morphological description and discrimination of these species is very problematic, leading to frequent specimen misidentifications. To facilitate species discrimination and detection of cryptic/new species, we developed new barcoding primers for the amplification of a COI fragment of about 860 bp (860-COI), which is an extension of the common Folmer's barcode region. Our 860-COI was successfully amplified in 177 worldwide-sampled botryllid colonies. Combined with morphological analyses, 860-COI allowed not only discriminating known species, but also identifying undescribed and cryptic species, resurrecting old species currently in synonymy, and proposing the assignment of clade D of the model organism Botryllus schlosseri to Botryllus renierii. Importantly, within clade A of B. schlosseri, 860-COI recognized at least two candidate species against only one recognized by the Folmer's fragment, underlining the need of further genetic investigations on this clade. This result also suggests that the 860-COI could have a greater ability to diagnose cryptic/new species than the Folmer's fragment at very short evolutionary distances, such as those observed within clade A. Finally, our new primers simplify the amplification of 860-COI even in non-botryllid ascidians, suggesting their wider usefulness in ascidians.

Mitochondrial genomes of stick insects (Phasmatodea) and phylogenetic considerations

Phasmatodea represents an order of hemimetabolous insects. This group includes species with extreme forms of masquerade crypsis, whereby they imitate twigs, bark, lichen, moss, and leaves. In this study, we sequenced and annotated three mitochondrial genomes (mitogenomes) from Phasmatodea. The lengths of the novel mitogenomes range from 14,162 bp to 15,879 bp. The gene content and organization correspond to those inferred for the ancestral insect. We conducted phylogenetic analyses together with the existing mitogenomes of polyneopterans and mayflies. In most cases, the Phasmatodea was non-monophyletic, with Embioptera and Zoraptera nested inside. The mitogenome sequences from Embioptera and Zoraptera suffered from high substitution rates and displayed very long branches in phylogenetic trees. The monophyletic Phasmatodea was recovered only when the analysis employed the site-heterogeneous CAT-GTR model in PhyloBayes and used the nucleotide dataset PCG_nt. The Euphasmatodea was well established by various data types and inference methods. In addition, the clade Heteropterygidae and the subfamilies Lonchodinae and Necrosciinae were strongly supported. The Australasian clade Lanceocercata was recovered across analyses. However, the Clitumninae was non-monophyletic.

Evolution of Allorecognition in the Tunicata

Allorecognition, the ability to distinguish self or kin from unrelated conspecifics, plays several important biological roles in invertebrate animals. Two of these roles include negotiating limited benthic space for colonial invertebrates, and inbreeding avoidance through self-incompatibility systems. Subphylum Tunicata (Phylum Chordata), the sister group to the vertebrates, is a promising group in which to study allorecognition. Coloniality has evolved many times independently in the tunicates, and the best known invertebrate self-incompatibility systems are in tunicates. Recent phylogenomic studies have coalesced around a phylogeny of the Tunicata as well as the Order Stolidobranchia within the Tunicata, providing a path forward for the study of allorecognition in this group.

Inferring Tunicate Relationships and the Evolution of the Tunicate Hox Cluster with the Genome of Corella inflata

Tunicates, the closest living relatives of vertebrates, have served as a foundational model of early embryonic development for decades. Comparative studies of tunicate phylogeny and genome evolution provide a critical framework for analyzing chordate diversification and the emergence of vertebrates. Toward this goal, we sequenced the genome of Corella inflata (Ascidiacea, Phlebobranchia), so named for the capacity to brood self-fertilized embryos in a modified, "inflated" atrial chamber. Combining the new genome sequence for Co. inflata with publicly available tunicate data, we estimated a tunicate species phylogeny, reconstructed the ancestral Hox gene cluster at important nodes in the tunicate tree, and compared patterns of gene loss between Co. inflata and Ciona robusta, the prevailing tunicate model species. Our maximum-likelihood and Bayesian trees estimated from a concatenated 210-gene matrix were largely concordant and showed that Aplousobranchia was nested within a paraphyletic Phlebobranchia. We demonstrated that this relationship is not an artifact due to compositional heterogeneity, as had been suggested by previous studies. In addition, within Thaliacea, we recovered Doliolida as sister to the clade containing Salpida and Pyrosomatida. The Co. inflata genome provides increased resolution of the ancestral Hox clusters of key tunicate nodes, therefore expanding our understanding of the evolution of this cluster and its potential impact on tunicate morphological diversity. Our analyses of other gene families revealed that several cardiovascular associated genes (e.g., BMP10, SCL2A12, and PDE2a) absent from Ci. robusta, are present in Co. inflata. Taken together, our results help clarify tunicate relationships and the genomic content of key ancestral nodes within this phylogeny, providing critical insights into tunicate evolution.

An integrative taxonomic framework for the study of the genus Ciona (Ascidiacea) and description of a new species, Ciona intermedia

The genus Ciona is an interesting ‘taxonomic case’ because its evolutionary history and taxonomy have not yet been resolved completely. In this study, we present new findings, describing specimens of an unidentified Ciona species collected along the north-eastern coasts of Sardinia (Tyrrhenian Sea, Mediterranean Sea). Applying an integrative taxonomic approach, based on the joint examination of morphological and molecular traits, we identify these specimens as a new species, Ciona intermedia sp. nov. Morphological comparisons and peculiarities of the habitat first revealed that these Ciona specimens have intermediate characters compared with other Ciona species. Molecular characterization (based on three mitochondrial regions: two already used for discriminating Ciona cryptic species and a newly developed one) confirmed that our specimens could not be assigned to any previously molecularly-characterized species. Both molecular phylogenetic reconstructions and morphological data clearly indicate C. intermedia as sister clade of Ciona edwardsi. Our findings add further complexity to the taxonomy of Ciona, underlying the importance of an integrative taxonomic approach for the study of the evolutionary history of this enigmatic genus.

Recent advances in understanding mitochondrial genome diversity

Ever since its discovery, the double-stranded DNA contained in the mitochondria of eukaryotes has fascinated researchers because of its bacterial endosymbiotic origin, crucial role in encoding subunits of the respiratory complexes, compact nature, and specific inheritance mechanisms. In the last few years, high-throughput sequencing techniques have accelerated the sequencing of mitochondrial genomes (mitogenomes) and uncovered the great diversity of organizations, gene contents, and modes of replication and transcription found in living eukaryotes. Some early divergent lineages of unicellular eukaryotes retain certain synteny and gene content resembling those observed in the genomes of alphaproteobacteria (the inferred closest living group of mitochondria), whereas others adapted to anaerobic environments have drastically reduced or even lost the mitogenome. In the three main multicellular lineages of eukaryotes, mitogenomes have pursued diverse evolutionary trajectories in which different types of molecules (circular versus linear and single versus multipartite), gene structures (with or without self-splicing introns), gene contents, gene orders, genetic codes, and transfer RNA editing mechanisms have been selected. Whereas animals have evolved a rather compact mitochondrial genome between 11 and 50 Kb in length with a highly conserved gene content in bilaterians, plants exhibit large mitochondrial genomes of 66 Kb to 11.3 Mb with large intergenic repetitions prone to recombination, and fungal mitogenomes have intermediate sizes of 12 to 236 Kb.

Toward a resolution of the cosmopolitan Botryllus schlosseri species complex (Ascidiacea, Styelidae): mitogenomics and morphology of clade E (Botryllus gaiae)

Botryllus schlosseri is a model colonial ascidian and a marine invader. It is currently recognized as a species complex comprising five genetically divergent clades, with clade A globally distributed and clade E found only in Europe. This taxon has also been recently redescribed by designation of a clade A specimen as the neotype. To clarify the taxonomic status of clade E and its relationship to clade A, we examine the entire mitochondrial genome and study the morphology of clade E. The mitogenome of clade E has an identical gene order to clade A, but substantially differs in the size of several non-coding regions. Remarkably, the nucleotide divergence of clade A-clade E is incompatible with the intraspecies ascidian divergence, but similar to the congeneric one and almost identical to the divergence between species once considered morphologically indistinguishable (e.g. the pair Ciona intestinalis (Linnaeus, 1767)-Ciona robusta Hoshino & Tokioka, 1967, and the pair Botrylloides niger Herdman, 1886-Botrylloides leachii (Savigny, 1816)). Clade E differs morphologically from the Botryllusschlosseri neotype mainly in the number and appearance of the stomach folds, and the shape of the anal opening, the first intestinal loop and the typhlosole. Our integrative taxonomical approach clearly distinguishes clade E as a species separate from Botryllusschlosseri, with unique morphological and molecular characters. Therefore, we here describe clade E as the new species Botryllus gaiae sp. nov.

Complete mitochondrial genome sequence of the Himalayan Griffon, Gyps himalayensis (Accipitriformes: Accipitridae): Sequence, structure, and phylogenetic analyses

This is the first study to describe the mitochondrial genome of the Himalayan Griffon, Gyps himalayensis, which is an Old World vulture belonging to the family Accipitridae and occurring along the Himalayas and the adjoining Tibetan Plateau. Its mitogenome is a closed circular molecule 17,381 bp in size containing 13 protein-coding genes, 22 tRNA coding genes, two rRNA-coding genes, a control region (CR), and an extra pseudo-control region (CCR) that are conserved in most Accipitridae mitogenomes. The overall base composition of the G. himalayensis mitogenome is 24.55% A, 29.49% T, 31.59% C, and 14.37% G, which is typical for bird mitochondrial genomes. The alignment of the Accipitridae species control regions showed high levels of genetic variation and abundant AT content. At the 5' end of the domain I region, a long continuous poly-C sequence was found. Two tandem repeats were found in the pseudo-control regions. Phylogenetic analysis with Bayesian inference and maximum likelihood based on 13 protein-coding genes indicated that the relationships at the family level were (Falconidae + (Cathartidae + (Sagittariidae + (Accipitridae + Pandionidae))). In the Accipitridae clade, G. himalayensis is more closely related to Aegypius monachus than to Spilornis cheela. The complete mitogenome of G. himalayensis provides a potentially useful resource for further exploration of the taxonomic status and phylogenetic history of Gyps species.

High and Variable Rates of Repeat-Mediated Mitochondrial Genome Rearrangement in a Genus of Plants

For 30 years, it has been clear that angiosperm mitochondrial genomes evolve rapidly in sequence arrangement (i.e., synteny), yet absolute rates of rearrangement have not been measured in any plant group, nor is it known how much these rates vary. To investigate these issues, we sequenced and reconstructed the rearrangement history of seven mitochondrial genomes in Monsonia (Geraniaceae). We show that rearrangements (occurring mostly as inversions) not only take place at generally high rates in these genomes but also uncover significant variation in rearrangement rates. For example, the hyperactive mitochondrial genome of Monsonia ciliata has accumulated at least 30 rearrangements over the last million years, whereas the branch leading to M. ciliata and its sister species has sustained rearrangement at a rate that is at least ten times lower. Furthermore, our analysis of published data shows that rates of mitochondrial genome rearrangement in seed plants vary by at least 600-fold. We find that sites of rearrangement are highly preferentially located in very close proximity to repeated sequences in Monsonia. This provides strong support for the hypothesis that rearrangement in angiosperm mitochondrial genomes occurs largely through repeat-mediated recombination. Because there is little variation in the amount of repeat sequence among Monsonia genomes, the variable rates of rearrangement in Monsonia probably reflect variable rates of mitochondrial recombination itself. Finally, we show that mitochondrial synonymous substitutions occur in a clock-like manner in Monsonia; rates of mitochondrial substitutions and rearrangements are therefore highly uncoupled in this group.

A new mitochondrial gene order in the banded cusk-eel Raneya brasiliensis (Actinopterygii, Ophidiiformes)

The complete mitochondrial genome of the banded cusk-eel, Raneya brasilensis (Kaup, 1856), was obtained using next-generation sequencing approaches. The genome sequence was 16,881 bp and exhibited a novel gene order for a vertebrate. Specifically, the WANCY and the nd6 – D-loop regions were re-ordered, supporting the hypothesis that these two regions are hotspots for gene rearrangements in Actinopterygii. Phylogenetic reconstructions confirmed that R. brasiliensis is nested within Ophidiiformes. Mitochondrial genomes are required from additional ophidiins to determine whether the gene rearrangements that we observed are specific to the genus Raneya or to the subfamily Ophidiinae.

The first complete mitochondrial genome sequence of Nanorana parkeri and Nanorana ventripunctata (Amphibia: Anura: Dicroglossidae), with related phylogenetic analyses

Members of the Nanorana genus (family Dicroglossidae) are often referred to as excellent model species with which to study amphibian adaptations to extreme environments and also as excellent keystone taxa for providing insights into the evolution of the Dicroglossidae. However, a complete mitochondrial genome is currently only available for Nanorana pleskei. Thus, we analyzed the complete mitochondrial genomes of Nanorana parkeri and Nanorana ventripunctata to investigate their evolutionary relationships within Nanorana and their phylogenetic position in the family Dicroglossidae. Our results showed that the genomes of N. parkeri (17,837 bp) and N. ventripunctata (18,373 bp) encode 13 protein‐coding genes (PCGs), two ribosomal RNA genes, 23 transfer RNA (tRNA) genes, and a noncoding control region. Overall sequences and genome structure of the two species showed high degree of similarity with N. pleskei, although the motif structures and repeat sequences of the putative control region showed clear differences among these three Nanorana species. In addition, a tandem repeat of the tRNA‐Met gene was found located between the tRNA‐Gln and ND2 genes. On both the 5′ and 3′‐sides, the control region possessed distinct repeat regions; however, the CSB‐2 motif was not found in N. pleskei. Based on the nucleotide sequences of 13 PCGs, our phylogenetic analyses, using Bayesian inference and maximum‐likelihood methods, illustrate the taxonomic status of Nanorana with robust support showing that N. ventripunctata and N. pleskei are more closely related than they are to N. parkeri. In conclusion, our analyses provide a more robust and reliable perspective on the evolutionary history of Dicroglossidae than earlier analyses, which used only a single species (N. pleskei).

Extensive mitochondrial gene rearrangements in Ctenophora: insights from benthic Platyctenida

BACKGROUND

Complete mitochondrial (mt) genomes have been sequenced for thousands of animals and represent a molecule of choice for many evolutionary studies. Nevertheless, some animal groups have remained under-sampled. Ctenophora (comb jellies) is one such example, with only two complete mt sequences determined hitherto for this phylum, which encompasses ca. 150-200 described species. This lack of data derives from the extremely fast mt evolutionary rate in this lineage, complicating primer design and DNA amplification. Indeed, in the two ctenophore mt genomes sequenced to date, i.e. those of Mnemiopsis leidyi (order Lobata) and Pleurobrachia bachei (order Cydippida), both rRNA and protein coding genes exhibit an extraordinary size reduction and have highly derived sequences. Additionally, all tRNAs, and the atp6 and atp8 genes are absent. In order to determine whether these characteristics are shared by other ctenophores, we obtained the complete mt genomes of three benthic ctenophores belonging to the so far unsampled order of Platyctenida: Coeloplana loyai, Coeloplana yulianicorum and Vallicula multiformis.

RESULTS

The mt genomes of benthic ctenophores reveal the same peculiarities found in Mnemiopsis and Pleurobrachia, demonstrating that the fast evolutionary rate is a general trait of the ctenophore mt genomes. Our results also indicate that this high evolutionary rate not only affects the nucleotide substitution but also gene rearrangements. Indeed, gene order was highly rearranged among representatives of the different taxonomic orders in which it was close to random, but also quite variable within Platyctenida, in which the genera Coeloplana and Vallicula share only four conserved synteny blocks. However, the two congeneric Coeloplana species display exactly the same gene order. Because of the extreme evolutionary rate, our phylogenetic analyses were unable to resolve the phylogenetic position of ctenophores within metazoans or the relationships among the different Ctenophora orders. Comparative sequence-analyses allowed us to correct the annotation of the Pleurobrachia mt genome, confirming the absence of tRNAs, the presence of both rRNA genes, and the existence of a reassignment of codon TGA from tryptophan to serine for this species.

CONCLUSIONS

Since Platyctenida is an early diverging lineage among Ctenophora, our findings suggest that the mt traits described above are ancestral characteristics of this phylum.

A phylogenomic framework and timescale for comparative studies of tunicates

Background

Tunicates are the closest relatives of vertebrates and are widely used as models to study the evolutionary developmental biology of chordates. Their phylogeny, however, remains poorly understood, and to date, only the 18S rRNA nuclear gene and mitogenomes have been used to delineate the major groups of tunicates. To resolve their evolutionary relationships and provide a first estimate of their divergence times, we used a transcriptomic approach to build a phylogenomic dataset including all major tunicate lineages, consisting of 258 evolutionarily conserved orthologous genes from representative species.

Results

Phylogenetic analyses using site-heterogeneous CAT mixture models of amino acid sequence evolution resulted in a strongly supported tree topology resolving the relationships among four major tunicate clades: (1) Appendicularia, (2) Thaliacea + Phlebobranchia + Aplousobranchia, (3) Molgulidae, and (4) Styelidae + Pyuridae. Notably, the morphologically derived Thaliacea are confirmed as the sister group of the clade uniting Phlebobranchia + Aplousobranchia within which the precise position of the model ascidian genus Ciona remains uncertain. Relaxed molecular clock analyses accommodating the accelerated evolutionary rate of tunicates reveal ancient diversification (~ 450–350 million years ago) among the major groups and allow one to compare their evolutionary age with respect to the major vertebrate model lineages.

Conclusions

Our study represents the most comprehensive phylogenomic dataset for the main tunicate lineages. It offers a reference phylogenetic framework and first tentative timescale for tunicates, allowing a direct comparison with vertebrate model species in comparative genomics and evolutionary developmental biology studies.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0499-2) contains supplementary material, which is available to authorized users.

De novo draft assembly of the Botrylloides leachii genome provides further insight into tunicate evolution

Tunicates are marine invertebrates that compose the closest phylogenetic group to the vertebrates. These chordates present a particularly diverse range of regenerative abilities and life-history strategies. Consequently, tunicates provide an extraordinary perspective into the emergence and diversity of these traits. Here we describe the genome sequencing, annotation and analysis of the Stolidobranchian Botrylloides leachii. We have produced a high-quality 159 Mb assembly, 82% of the predicted 194 Mb genome. Analysing genome size, gene number, repetitive elements, orthologs clustering and gene ontology terms show that B. leachii has a genomic architecture similar to that of most solitary tunicates, while other recently sequenced colonial ascidians have undergone genome expansion. In addition, ortholog clustering has identified groups of candidate genes for the study of colonialism and whole-body regeneration. By analysing the structure and composition of conserved gene linkages, we observed examples of cluster breaks and gene dispersions, suggesting that several lineage-specific genome rearrangements occurred during tunicate evolution. We also found lineage-specific gene gain and loss within conserved cell-signalling pathways. Such examples of genetic changes within conserved cell-signalling pathways commonly associated with regeneration and development that may underlie some of the diverse regenerative abilities observed in tunicates. Overall, these results provide a novel resource for the study of tunicates and of colonial ascidians.

Deep-level phylogeny of Cicadomorpha inferred from mitochondrial genomes sequenced by NGS

Recent development and advancement of next-generation sequencing (NGS) technologies have enabled the determination of mitochondrial genome (mitogenome) at extremely efficiency. In this study, complete or partial mitogenomes for 19 cicadomorphan species and six fulgoroid species were reconstructed by using the method of high-throughput sequencing from pooled DNA samples. Annotation analyses showed that the mitogenomes obtained have the typical insect mitogenomic content and structure. Combined with the existing hemipteran mitogenomes, a series of datasets with all 37 mitochondrial genes (up to 14,381 nt total) under different coding schemes were compiled to test previous hypotheses of deep-level phylogeny of Cicadomorpha. Thirty-seven species representing Cicadomorpha constituted the ingroup. A taxon sampling with nine species from Fulgoroidea and six from Heteroptera comprised the outgroup. The phylogenetic reconstructions congruently recovered the monophyly of each superfamily within Cicadomorpha. Furthermore, the hypothesis (Membracoidea + (Cicadoidea + Cercopoidea)) was strongly supported under the heterogeneous CAT model.

C. Costa M, A. Andow D, Paula DP. Mitogenome sequence accuracy using different elucidation methods

Mitogenome sequences are highly desired because they are used in several biological disciplines. Their elucidation has been facilitated through the development of massive parallel sequencing, accelerating their deposition in public databases. However, sequencing, assembly and annotation methods might induce variability in their quality, raising concerns about the accuracy of the sequences that have been deposited in public databases. In this work we show that different sequencing methods (number of species pooled in a library, insert size and platform) and assembly and annotation methods generated variable completeness and similarity of the resulting mitogenome sequences, using three species of predaceous ladybird beetles as models. The identity of the sequences varied considerably depending on the method used and ranged from 38.19 to 90.1% for Cycloneda sanguinea, 72.85 to 91.06% for Harmonia axyridis and 41.15 to 93.60% for Hippodamia convergens. Dissimilarities were frequently found in the non-coding A+T rich region, but were also common in coding regions, and were not associated with low coverage. Mitogenome completeness and sequence identity were affected by the sequencing and assembly/annotation methods, and high within-species variation was also found for other mitogenome depositions in GenBank. This indicates a need for methods to confirm sequence accuracy, and guidelines for verifying mitogenomes should be discussed and developed by the scientific community.

Regulatory cis- and trans-elements of mitochondrial D-loop-driven reporter genes in budding tunicates

To unveil the underlying mechanism of mitochondrial gene regulation associated with ageing and budding in the tunicate Polyandrocarpa misakiensis, mitochondrial non-coding-region (NCR)-containing reporter genes were constructed. PmNCR2.3K/GFP was expressed spatiotemporally in a pattern quite similar to mitochondrial 16SrRNA. The reporter gene expression was sensitive to high dose of rifampicin similar to mitochondrial genes, suggesting that the transcription indeed occurs in mitochondria. However, the gene expression also occurred in vivo in the cell nucleus and in vitro in the nuclear extracts. Mitochondrial transcription factor A (PmTFAM) enhanced reporter gene expression, depending on the NCR length. A budding-specific polypeptide TC14-3 is an epigenetic histone methylation inducer. It heavily enhanced reporter gene expression that was interfered by histone methylation inhibitors and PmTFAM RNAi. Our results indicate for the first time that the nuclear histone methylation is involved in mitochondrial gene activity via TFAM gene regulation.

Next-generation sequencing of mixed genomic DNA allows efficient assembly of rearranged mitochondrial genomes in Amolops chunganensis and Quasipaa boulengeri

Recent improvements in next-generation sequencing (NGS) technologies can facilitate the obtainment of mitochondrial genomes. However, it is not clear whether NGS could be effectively used to reconstruct the mitogenome with high gene rearrangement. These high rearrangements would cause amplification failure, and/or assembly and alignment errors. Here, we choose two frogs with rearranged gene order, Amolops chunganensis and Quasipaa boulengeri, to test whether gene rearrangements affect the mitogenome assembly and alignment by using NGS. The mitogenomes with gene rearrangements are sequenced through Illumina MiSeq genomic sequencing and assembled effectively by Trinity v2.1.0 and SOAPdenovo2. Gene order and contents in the mitogenome of A. chunganensis and Q. boulengeri are typical neobatrachian pattern except for rearrangements at the position of "WANCY" tRNA genes cluster. Further, the mitogenome of Q. boulengeri is characterized with a tandem duplication of trnM. Moreover, we utilize 13 protein-coding genes of A. chunganensis, Q. boulengeri and other neobatrachians to reconstruct the phylogenetic tree for evaluating mitochondrial sequence authenticity of A. chunganensis and Q. boulengeri. In this work, we provide nearly complete mitochondrial genomes of A. chunganensis and Q. boulengeri.

Phylogenetics, biogeography and population genetics of the ascidian Botryllus schlosseri in the Mediterranean Sea and beyond

The wide distribution of the ascidian Botryllus schlosseri along the Mediterranean coasts has been documented since the eighteenth century. However, despite copious documentation, analyses of dispersal modes and genetic profiles were limited to local populations or restricted regions. In order to get a pan-Mediterranean overview, 288 specimens from 11 populations of B. schlosseri from the western and eastern Mediterranean basins were sampled and analyzed using five microsatellite loci and COI sequences. Both molecular markers revealed high polymorphisms, with 182 microsatellites alleles and 54 COI haplotypes. Overall, F_st, D_est, and COI Ф_pt values were 0.146, 0.635 and 0.322, respectively, reflecting a high genetic diversity and a significant genetic structure as compared to other B. schlosseri populations worldwide, reflected by substantially higher values for effective number of alleles (N_e) in the Mediterranean. A phylogenetic analysis of the COI sequences resulted in four distinct clades and two molecular operational taxonomic units (OTUs). We recorded a stronger genetic structure among the populations of the eastern basin compared to the western basin (microsatellites F_st=0.217 versus 0.082; COI Ф_pt=0.416 versus 0.171), suggesting either a restricted connectivity between the basins or a stronger genetic drift in each basin. The occurrence of two OTUs and different ecological conditions may also contribute to this finding. Mean Nei's genetic distance in the eastern Mediterranean populations was more than three times higher compared to the western basin. No correlation was observed between geographic and genetic distances (Mantel test), suggesting that maritime transport is the main dispersal vector of B. schlosseri colonies. The possibility that the Mediterranean is a center of diversity for B. schlosseri, and probably its site of origin, is further discussed.

Animal Mitochondrial DNA as We Do Not Know It: mt-Genome Organization and Evolution in Nonbilaterian Lineages

Animal mitochondrial DNA (mtDNA) is commonly described as a small, circular molecule that is conserved in size, gene content, and organization. Data collected in the last decade have challenged this view by revealing considerable diversity in animal mitochondrial genome organization. Much of this diversity has been found in nonbilaterian animals (phyla Cnidaria, Ctenophora, Placozoa, and Porifera), which, from a phylogenetic perspective, form the main branches of the animal tree along with Bilateria. Within these groups, mt-genomes are characterized by varying numbers of both linear and circular chromosomes, extra genes (e.g. atp9, polB, tatC), large variation in the number of encoded mitochondrial transfer RNAs (tRNAs) (0-25), at least seven different genetic codes, presence/absence of introns, tRNA and mRNA editing, fragmented ribosomal RNA genes, translational frameshifting, highly variable substitution rates, and a large range of genome sizes. This newly discovered diversity allows a better understanding of the evolutionary plasticity and conservation of animal mtDNA and provides insights into the molecular and evolutionary mechanisms shaping mitochondrial genomes.

Mitochondrial metagenomics: letting the genes out of the bottle

‘Mitochondrial metagenomics’ (MMG) is a methodology for shotgun sequencing of total DNA from specimen mixtures and subsequent bioinformatic extraction of mitochondrial sequences. The approach can be applied to phylogenetic analysis of taxonomically selected taxa, as an economical alternative to mitogenome sequencing from individual species, or to environmental samples of mixed specimens, such as from mass trapping of invertebrates. The routine generation of mitochondrial genome sequences has great potential both for systematics and community phylogenetics. Mapping of reads from low-coverage shotgun sequencing of environmental samples also makes it possible to obtain data on spatial and temporal turnover in whole-community phylogenetic and species composition, even in complex ecosystems where species-level taxonomy and biodiversity patterns are poorly known. In addition, read mapping can produce information on species biomass, and potentially allows quantification of within-species genetic variation. The success of MMG relies on the formation of numerous mitochondrial genome contigs, achievable with standard genome assemblers, but various challenges for the efficiency of assembly remain, particularly in the face of variable relative species abundance and intra-specific genetic variation. Nevertheless, several studies have demonstrated the power of mitogenomes from MMG for accurate phylogenetic placement, evolutionary analysis of species traits, biodiversity discovery and the establishment of species distribution patterns; it offers a promising avenue for unifying the ecological and evolutionary understanding of species diversity.

High-Throughput Sequencing of Complete Mitochondrial Genomes

Next-generation sequencing has revolutionized mitogenomics, turning a cottage industry into a high throughput process. This chapter outlines methodologies used to sequence, assemble, and annotate mitogenomes of non-model organisms using Illumina sequencing technology, utilizing either long-range PCR amplicons or gDNA as starting template. Instructions are given on how to extract DNA, conduct long-range PCR amplifications, generate short Sanger barcode tag sequences, prepare equimolar sample pools, construct and assess quality library preparations, assemble Illumina reads using either seeded reference mapping or de novo assembly, and annotate mitogenomes in the absence of an automated pipeline. Notes and recommendations, derived from our own experience, are given throughout this chapter.

The Utility of Genome Skimming for Phylogenomic Analyses as Demonstrated for Glycerid Relationships (Annelida, Glyceridae)

Glyceridae (Annelida) are a group of venomous annelids distributed worldwide from intertidal to abyssal depths. To trace the evolutionary history and complexity of glycerid venom cocktails, a solid backbone phylogeny of this group is essential. We therefore aimed to reconstruct the phylogenetic relationships of these annelids using Illumina sequencing technology. We constructed whole-genome shotgun libraries for 19 glycerid specimens and 1 outgroup species (Glycinde armigera). The chosen target genes comprise 13 mitochondrial proteins, 2 ribosomal mitochondrial genes, and 4 nuclear loci (18SrRNA, 28SrRNA, ITS1, and ITS2). Based on partitioned maximum likelihood as well as Bayesian analyses of the resulting supermatrix, we were finally able to resolve a robust glycerid phylogeny and identified three clades comprising the majority of taxa. Furthermore, we detected group II introns inside the cox1 gene of two analyzed glycerid specimens, with two different insertions in one of these species. Moreover, we generated reduced data sets comprising 10 million, 4 million, and 1 million reads from the original data sets to test the influence of the sequencing depth on assembling complete mitochondrial genomes from low coverage genome data. We estimated the coverage of mitochondrial genome sequences in each data set size by mapping the filtered Illumina reads against the respective mitochondrial contigs. By comparing the contig coverage calculated in all data set sizes, we got a hint for the scalability of our genome skimming approach. This allows estimating more precisely the number of reads that are at least necessary to reconstruct complete mitochondrial genomes in Glyceridae and probably non-model organisms in general.

First complete mitochondrial genome of the South American annual fish Austrolebias charrua (Cyprinodontiformes: Rivulidae): peculiar features among cyprinodontiforms mitogenomes

Background

Among teleosts, the South American genus Austrolebias (Cyprinodontiformes: Rivulidae) includes 42 taxa of annual fishes divided into five different species groups. It is a monophyletic genus, but morphological and molecular data do not resolve the relationship among intrageneric clades and high rates of substitution have been previously described in some mitochondrial genes. In this work, the complete mitogenome of a species of the genus was determined for the first time. We determined its structure, gene order and evolutionary peculiar features, which will allow us to evaluate the performance of mitochondrial genes in the phylogenetic resolution at different taxonomic levels.

Results

Regarding gene content and order, the circular mitogenome of A. charrua (17,271 pb) presents the typical pattern of vertebrate mitogenomes. It contains the full complement of 13 proteins-coding genes, 22 tRNA, 2 rRNA and one non-coding control region. Notably, the tRNA-Cys was only 57 bp in length and lacks the D-loop arm. In three full sibling individuals, heteroplasmatic condition was detected due to a total of 12 variable sites in seven protein-coding genes. Among cyprinodontiforms, the mitogenome of A. charrua exhibits the lowest G+C content (37 %) and GCskew, as well as the highest strand asymmetry with a net difference of T over A at 1st and 3rd codon positions. Considering the 12 coding-genes of the H strand, correspondence analyses of nucleotide composition and codon usage show that A and T at 1st and 3rd codon positions have the highest weight in the first axis, and segregate annual species from the other cyprinodontiforms analyzed. Given the annual life-style, their mitogenomes could be under different selective pressures. All 13 protein-coding genes are under strong purifying selection and we did not find any significant evidence of nucleotide sites showing episodic selection (dN >dS) at annual lineages. When fast evolving third codon positions were removed from alignments, the “supergene” tree recovers our reference species phylogeny as well as the Cytb, ND4L and ND6 genes. Therefore, third codon positions seem to be saturated in the aforementioned coding regions at intergeneric Cyprinodontiformes comparisons.

Conclusions

The complete mitogenome obtained in present work, offers relevant data for further comparative studies on molecular phylogeny and systematics of this taxonomic controversial endemic genus of annual fishes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2090-3) contains supplementary material, which is available to authorized users.

Comparative mitogenomics of the assassin bug genus Peirates (Hemiptera: Reduviidae: Peiratinae) reveal conserved mitochondrial genome organization of P. atromaculatus, P. fulvescens and P. turpis

In this study, we sequenced four new mitochondrial genomes and presented comparative mitogenomic analyses of five species in the genus Peirates (Hemiptera: Reduviidae). Mitochondrial genomes of these five assassin bugs had a typical set of 37 genes and retained the ancestral gene arrangement of insects. The A+T content, AT- and GC-skews were similar to the common base composition biases of insect mtDNA. Genomic size ranges from 15,702 bp to 16,314 bp and most of the size variation was due to length and copy number of the repeat unit in the putative control region. All of the control region sequences included large tandem repeats present in two or more copies. Our result revealed similarity in mitochondrial genomes of P. atromaculatus, P. fulvescens and P. turpis, as well as the highly conserved genomic-level characteristics of these three species, e.g., the same start and stop codons of protein-coding genes, conserved secondary structure of tRNAs, identical location and length of non-coding and overlapping regions, and conservation of structural elements and tandem repeat unit in control region. Phylogenetic analyses also supported a close relationship between P. atromaculatus, P. fulvescens and P. turpis, which might be recently diverged species. The present study indicates that mitochondrial genome has important implications on phylogenetics, population genetics and speciation in the genus Peirates.

Fishing for barcodes in the Torrent: from COI to complete mitogenomes on NGS platforms

The adoption of Next-Generation Sequencing (NGS) by the field of DNA barcoding of Metazoa has been hindered by the fit between the classical COI barcode and the Sanger-based sequencing method. Here we describe a framework for the sequencing and multiplexing of mitogenomes on NGS platforms that implements (I) a universal long-range PCR-based amplification technique, (II) a two-level multiplexing approach (i.e. divergence-based and specific tag indexing), and (III) a dedicated demultiplexing and assembling script from an Ion Torrent sequencing platform. We provide a case study of mitogenomes obtained for two vouchered individuals of daces Leuciscus burdigalensis and L. oxyrrhis and show that this workflow enables to recover over 100 mitogenomes per sequencing chip on a PGM sequencer, bringing the individual cost down below 7,50€ per mitogenome (as of current 2015 sequencing costs). The use of several kilobases for identification purposes, as involved in the improved DNA-barcode we propose, stress the need for data reliability, especially through metadata. Based on both scientific and economic considerations, this framework presents a relevant approach for multiplexing samples, adaptable on any desktop NGS platform. It enables to extend from the prevalent barcoding approach by shifting from the single COI to complete mitogenome sequencing

Comparison of whole mitochondrial genome sequences from two clades of the invasive ascidian, Didemnum vexillum

The mitochondria are the main source of cellular energy production and have an important role in development, fertility, and thermal limitations. Adaptive mitochondrial DNA mutations have the potential to be of great importance in determining aspects of the life history of an organism. Phylogenetic analyses of the globally invasive marine ascidian Didemnum vexillum using the mitochondrial cytochrome c oxidase 1 (COX1) coding region, revealed two distinct clades. Representatives of one clade (denoted by 'B') are geographically restricted to D. vexillum's native region (north-west Pacific Ocean, including Japan), whereas members of the other clade (denoted by 'A') have been introduced and become invasive in temperate coastal areas around the world. Persistence of clade B's restricted distribution may reflect it being inherently less invasive than clade A. To investigate this we sought to determine if the two clades differ significantly in other mitochondrial genes of functional significance, specifically, alterations in amino acids encoded in mitochondrial enzyme subunits. Differences in functional mitochondrial genes could indicate an increased ability for clade A colonies to tolerate a wider range of environmental temperature. Full mitochondrial genomic sequences from D. vexillum clades A and B were obtained and they predict significant sequence differences in genes encoding for enzymes involved in oxidative phosphorylation. Diversity levels were relatively high and showed divergence across almost all genes, with p-distance values between the two clades indicating recent divergence. Both clades showed an excess of rare variants, which is consistent with balancing selection or a recent population expansion. Results presented here will inform future research focusing on examining the functional properties of the corresponding mitochondrial respiration enzymes, of A and B clade enzymes. By comparing closely related taxa that have differing distributions it is possible to identify genes and phenotypes suited to particular environments. The examination of mitochondrial genotypes, and associated enzyme functioning, across populations may aid in our understanding of thermal tolerance and environmental adaptation.

Ascidian mitogenomics: comparison of evolutionary rates in closely related taxa provides evidence of ongoing speciation events

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.

Multiplex sequencing of pooled mitochondrial genomes-a crucial step toward biodiversity analysis using mito-metagenomics

The advent in high-throughput-sequencing (HTS) technologies has revolutionized conventional biodiversity research by enabling parallel capture of DNA sequences possessing species-level diagnosis. However, polymerase chain reaction (PCR)-based implementation is biased by the efficiency of primer binding across lineages of organisms. A PCR-free HTS approach will alleviate this artefact and significantly improve upon the multi-locus method utilizing full mitogenomes. Here we developed a novel multiplex sequencing and assembly pipeline allowing for simultaneous acquisition of full mitogenomes from pooled animals without DNA enrichment or amplification. By concatenating assemblies from three de novo assemblers, we obtained high-quality mitogenomes for all 49 pooled taxa, with 36 species >15 kb and the remaining >10 kb, including 20 complete mitogenomes and nearly all protein coding genes (99.6%). The assembly quality was carefully validated with Sanger sequences, reference genomes and conservativeness of protein coding genes across taxa. The new method was effective even for closely related taxa, e.g. three Drosophila spp., demonstrating its broad utility for biodiversity research and mito-phylogenomics. Finally, the in silico simulation showed that by recruiting multiple mito-loci, taxon detection was improved at a fixed sequencing depth. Combined, these results demonstrate the plausibility of a multi-locus mito-metagenomics approach as the next phase of the current single-locus metabarcoding method.

The complete mitochondrial genome of the devil firefish Pterois miles (Bennett, 1828) (Scorpaenidae)

The complete mitochondrial genome of the devil firefish Pterois miles (Bennett, 1828) was obtained using next generation sequencing approaches. The genome sequence was comprised of 16,497 bp exhibiting the standard vertebrate mitochondrial gene arrangement. Regions of gene overlap, tRNA lengths, as well as start and stop codons were similar to those observed in closely related families (i.e. Sebastidae, Peristediidae). Phylogenetic reconstructions support the polyphyly of Scorpaeniformes, and confirm the close relationship of Scorpaenidae and Sebastidae.

Bulk de novo mitogenome assembly from pooled total DNA elucidates the phylogeny of weevils (Coleoptera: Curculionoidea)

Complete mitochondrial genomes have been shown to be reliable markers for phylogeny reconstruction among diverse animal groups. However, the relative difficulty and high cost associated with obtaining de novo full mitogenomes have frequently led to conspicuously low taxon sampling in ensuing studies. Here, we report the successful use of an economical and accessible method for assembling complete or near-complete mitogenomes through shot-gun next-generation sequencing of a single library made from pooled total DNA extracts of numerous target species. To avoid the use of separate indexed libraries for each specimen, and an associated increase in cost, we incorporate standard polymerase chain reaction-based "bait" sequences to identify the assembled mitogenomes. The method was applied to study the higher level phylogenetic relationships in the weevils (Coleoptera: Curculionoidea), producing 92 newly assembled mitogenomes obtained in a single Illumina MiSeq run. The analysis supported a separate origin of wood-boring behavior by the subfamilies Scolytinae, Platypodinae, and Cossoninae. This finding contradicts morphological hypotheses proposing a close relationship between the first two of these but is congruent with previous molecular studies, reinforcing the utility of mitogenomes in phylogeny reconstruction. Our methodology provides a technically simple procedure for generating densely sampled trees from whole mitogenomes and is widely applicable to groups of animals for which bait sequences are the only required prior genome knowledge.

The complete mitochondrial genome of the gilthead seabream Sparus aurata L. (Sparidae)

The complete mitochondrial genome of the gilthead seabream Sparus aurata Linnaeus 1758, one of the world's most important mariculture species, was sequenced using next generation sequencing technology. The genome sequence is comprised of 16,652 bp exhibiting the canonical vertebrate mitochondria gene order. Regions of gene overlap, tRNA length, as well as start and stop codon were similar to those observed in other Sparidae. Phylogenetic reconstructions based on mitochondrial protein coding genes corroborate the view that Sparidae is paraphyletic and includes Centracanthidae.

Evaluation of a pooled strategy for high-throughput sequencing of cosmid clones from metagenomic libraries

High-throughput sequencing methods have been instrumental in the growing field of metagenomics, with technological improvements enabling greater throughput at decreased costs. Nonetheless, the economy of high-throughput sequencing cannot be fully leveraged in the subdiscipline of functional metagenomics. In this area of research, environmental DNA is typically cloned to generate large-insert libraries from which individual clones are isolated, based on specific activities of interest. Sequence data are required for complete characterization of such clones, but the sequencing of a large set of clones requires individual barcode-based sample preparation; this can become costly, as the cost of clone barcoding scales linearly with the number of clones processed, and thus sequencing a large number of metagenomic clones often remains cost-prohibitive. We investigated a hybrid Sanger/Illumina pooled sequencing strategy that omits barcoding altogether, and we evaluated this strategy by comparing the pooled sequencing results to reference sequence data obtained from traditional barcode-based sequencing of the same set of clones. Using identity and coverage metrics in our evaluation, we show that pooled sequencing can generate high-quality sequence data, without producing problematic chimeras. Though caveats of a pooled strategy exist and further optimization of the method is required to improve recovery of complete clone sequences and to avoid circumstances that generate unrecoverable clone sequences, our results demonstrate that pooled sequencing represents an effective and low-cost alternative for sequencing large sets of metagenomic clones.

Molecular and morphological discrimination between an invasive ascidian, Ascidiella aspersa, and its congener A. scabra (Urochordata: Ascidiacea)

The solitary ascidian Ascidiella aspersa (Müller, 1776) has sometimes been regarded as conspecific with A. scabra (Müller, 1776), although previous detailed morphological comparisons have indicated that the two are distinguishable by internal structures. Resolution of this taxonomic issue is important because A. aspersa has been known as a notoriously invasive ascidian, doing much damage to aquaculture e.g. in Hokkaido, Japan. We collected many specimens from European waters (including the Swedish coast, near the type localities of these two species) and Hokkaido, Japan (as an alien population) and made molecular phylogenetic analyses using the mitochondrial cytochrome c oxidase subunit I (COI) gene, and found that in terms of COI sequences all the analyzed specimens were clustered into two distinct groups, one of which is morphologically referable to A. aspersa and the other to A. scabra. Thus, these two species should be regarded as distinct from each other.

Bacterial endosymbiosis in a chordate host: long-term co-evolution and conservation of secondary metabolism

Intracellular symbiosis is known to be widespread in insects, but there are few described examples in other types of host. These symbionts carry out useful activities such as synthesizing nutrients and conferring resistance against adverse events such as parasitism. Such symbionts persist through host speciation events, being passed down through vertical transmission. Due to various evolutionary forces, symbionts go through a process of genome reduction, eventually resulting in tiny genomes where only those genes essential to immediate survival and those beneficial to the host remain. In the marine environment, invertebrates such as tunicates are known to harbor complex microbiomes implicated in the production of natural products that are toxic and probably serve a defensive function. Here, we show that the intracellular symbiont Candidatus Endolissoclinum faulkneri is a long-standing symbiont of the tunicate Lissoclinum patella, that has persisted through cryptic speciation of the host. In contrast to the known examples of insect symbionts, which tend to be either relatively recent or ancient relationships, the genome of Ca. E. faulkneri has a very low coding density but very few recognizable pseudogenes. The almost complete degradation of intergenic regions and stable gene inventory of extant strains of Ca. E. faulkneri show that further degradation and deletion is happening very slowly. This is a novel stage of genome reduction and provides insight into how tiny genomes are formed. The ptz pathway, which produces the defensive patellazoles, is shown to date to before the divergence of Ca. E. faulkneri strains, reinforcing its importance in this symbiotic relationship. Lastly, as in insects we show that stable symbionts can be lost, as we describe an L. patella animal where Ca. E. faulkneri is displaced by a likely intracellular pathogen. Our results suggest that intracellular symbionts may be an important source of ecologically significant natural products in animals.

Next-generation sequencing and phylogenetic signal of complete mitochondrial genomes for resolving the evolutionary history of leaf-nosed bats (Phyllostomidae)

Leaf-nosed bats (Phyllostomidae) are one of the most studied groups within the order Chiroptera mainly because of their outstanding species richness and diversity in morphological and ecological traits. Rapid diversification and multiple homoplasies have made the phylogeny of the family difficult to solve using morphological characters. Molecular data have contributed to shed light on the evolutionary history of phyllostomid bats, yet several relationships remain unresolved at the intra-familial level. Complete mitochondrial genomes have proven useful to deal with this kind of situation in other groups of mammals by providing access to a large number of molecular characters. At present, there are only two mitogenomes available for phyllostomid bats hinting at the need for further exploration of the mitogenomic approach in this group. We used both standard Sanger sequencing of PCR products and next-generation sequencing (NGS) of shotgun genomic DNA to obtain new complete mitochondrial genomes from 10 species of phyllostomid bats, including representatives of major subfamilies, plus one outgroup belonging to the closely-related mormoopids. We then evaluated the contribution of mitogenomics to the resolution of the phylogeny of leaf-nosed bats and compared the results to those based on mitochondrial genes and the RAG2 and VWF nuclear makers. Our results demonstrate the advantages of the Illumina NGS approach to efficiently obtain mitogenomes of phyllostomid bats. The phylogenetic signal provided by entire mitogenomes is highly comparable to the one of a concatenation of individual mitochondrial and nuclear markers, and allows increasing both resolution and statistical support for several clades. This enhanced phylogenetic signal is the result of combining markers with heterogeneous evolutionary rates representing a large number of nucleotide sites. Our results illustrate the potential of the NGS mitogenomic approach for resolving the evolutionary history of phyllostomid bats based on a denser species sampling.