Complete DNA sequence of the mitochondrial genome of the ascidian Halocynthia roretzi (Chordata, Urochordata)

Statistical analysis of synonymous and stop codons in pseudo-random and real sequences as a function of GC content

Knowledge of the frequencies of synonymous triplets in protein-coding and non-coding DNA stretches can be used in gene finding. These frequencies depend on the GC content of the genome or parts of it. An example of interest is provided by stop codons. This is relevant for the definition of Open Reading Frames. A generic case is provided by pseudo-random sequences, especially when they code for complex proteins or when they are non-coding and not subject to selection pressure. Here, we calculate, for such sequences and for all 25 known genetic codes, the frequency of each amino acid and stop codon based on their set of codons and as a function of GC content. The amino acids can be classified into five groups according to the GC content where their expected frequency reaches its maximum. We determine the overall Shannon information based on groups of synonymous codons and show that it becomes maximum at a percent GC of 43.3% (for the standard code). This is in line with the observation that in most fungi, plants, and animals, this genomic parameter is in the range from 35 to 50%. By analysing natural sequences, we show that there is a clear bias for triplets corresponding to stop codons near the 5'- and 3'-splice sites in the introns of various clades.

Insights into the Deep Phylogeny and Novel Gene Rearrangement of Mytiloidea from Complete Mitochondrial Genome

The extant marine mussels which belong to the Mytiloidea are widespread species inhabiting mostly coastal waters, with some distributed in the deep sea. To clarify the classification systems and phylogenetic relationships range from genus to family level within Mytiloidea, new sequence was used in a phylogenetic analysis including all the available Mytiloidea mitochondrial genomes. In this study, the complete mitochondrial genome of Vignadula atrata is 15,624 bp in length and contains 12 protein-coding genes (PCGs, atp8 is absent), two ribosomal RNA genes, and 22 transfer RNA genes. Phylogenetic analysis based on 12 PCGs showed that it has a close relationship to Bathymodiolus. The analysis of gene rearrangements in the Pteriomorphia showed that the arrangements are highly variable across species, novel gene rearrangements were found within Mytiloidea. The V. atrata mitogenome was provided in detail, with notes on the sequence and a key to the species of Vignadula. This study provides a perspective on the taxonomic histories of the marine mussels and refines the unclear relationship between the origin and evolution of species in Mytiloidea within Bivalvia.

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.

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.

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.

Host control of symbiont natural product chemistry in cryptic populations of the tunicate Lissoclinum patella

Natural products (secondary metabolites) found in marine invertebrates are often thought to be produced by resident symbiotic bacteria, and these products appear to play a major role in the symbiotic interaction of bacteria and their hosts. In these animals, there is extensive variation, both in chemistry and in the symbiotic bacteria that produce them. Here, we sought to answer the question of what factors underlie chemical variation in the ocean. As a model, we investigated the colonial tunicate Lissoclinum patella because of its rich and varied chemistry and its broad geographic range. We sequenced mitochondrial cytochrome c oxidase 1 (COXI) genes, and found that animals classified as L. patella fall into three phylogenetic groups that may encompass several cryptic species. The presence of individual natural products followed the phylogenetic relationship of the host animals, even though the compounds are produced by symbiotic bacteria that do not follow host phylogeny. In sum, we show that cryptic populations of animals underlie the observed chemical diversity, suggesting that the host controls selection for particular secondary metabolite pathways. These results imply novel approaches to obtain chemical diversity from the oceans, and also demonstrate that the diversity of marine natural products may be greatly impacted by cryptic local extinctions.

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

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.

Mitochondrial genome of a Japanese placozoan

Placozoans are marine invertebrates found in tropical and subtropical waters. Their body plan is among the simplest of free-living animals. The present study determined the mitochondrial genome sequence of a placozoan collected on the coast of Shirahama, Wakayama, Honshu, Japan, and compared it with those of Trichoplax adhaerens from the Red Sea and of three strains from the Caribbean Sea. The sequences of mitochondrial respiratory chain of the Japanese placozoan genes are very similar to those of the BZ49 strain from the Caribbean Sea. However, there are distinct differences in gene arrangement, such as the location of two open reading frames. This Japanese placozoan is therefore distinguishable from the other strains. Based on current knowledge of placozoan 16S diversity our 'Shirahama' strain most likely represents the H15 lineage, known from the Philippines. In the mitochondrial genome of placozoans, substitution rates are slower than in bilaterians, whereas the rate of rearrangements is faster.

Taurine-containing uridine modifications in tRNA anticodons are required to decipher non-universal genetic codes in ascidian mitochondria

Variations in the genetic code are found frequently in mitochondrial decoding systems. Four non-universal genetic codes are employed in ascidian mitochondria: AUA for Met, UGA for Trp, and AGA/AGG(AGR) for Gly. To clarify the decoding mechanism for the non-universal genetic codes, we isolated and analyzed mitochondrial tRNAs for Trp, Met, and Gly from an ascidian, Halocynthia roretzi. Mass spectrometric analysis identified 5-taurinomethyluridine (τm(5)U) at the anticodon wobble positions of tRNA(Met)(AUR), tRNA(Trp)(UGR), and tRNA(Gly)(AGR), suggesting that τm(5)U plays a critical role in the accurate deciphering of all four non-universal codes by preventing the misreading of pyrimidine-ending near-cognate codons (NNY) in their respective family boxes. Acquisition of the wobble modification appears to be a prerequisite for the genetic code alteration.

tRNA Modification and Genetic Code Variations in Animal Mitochondria

In animal mitochondria, six codons have been known as nonuniversal genetic codes, which vary in the course of animal evolution. They are UGA (termination codon in the universal genetic code changes to Trp codon in all animal mitochondria), AUA (Ile to Met in most metazoan mitochondria), AAA (Lys to Asn in echinoderm and some platyhelminth mitochondria), AGA/AGG (Arg to Ser in most invertebrate, Arg to Gly in tunicate, and Arg to termination in vertebrate mitochondria), and UAA (termination to Tyr in a planaria and a nematode mitochondria, but conclusive evidence is lacking in this case). We have elucidated that the anticodons of tRNAs deciphering these nonuniversal codons (tRNA^Trp for UGA, tRNA^Met for AUA, tRNA^Asn for AAA, and tRNA^Ser and tRNA^Gly for AGA/AGG) are all modified; tRNA^Trp has 5-carboxymethylaminomethyluridine or 5-taurinomethyluridine, tRNA^Met has 5-formylcytidine or 5-taurinomethyluridine, tRNA^Ser has 7-methylguanosine and tRNA^Gly has 5-taurinomethyluridine in their anticodon wobble position, and tRNA^Asn has pseudouridine in the anticodon second position. This review aims to clarify the structural relationship between these nonuniversal codons and the corresponding tRNA anticodons including modified nucleosides and to speculate on the possible mechanisms for explaining the evolutional changes of these nonuniversal codons in the course of animal evolution.

Characterization of a mitochondrial ORF from the gender-associated mtDNAs of Mytilus spp. (Bivalvia: Mytilidae): identification of the "missing" ATPase 8 gene

Bivalve species are characterized by extraordinary variability in terms of mitochondrial (mt) genome size, gene arrangement and tRNA gene number. Many species are thought to lack the mitochondrial protein-coding gene atp8. Of these species, the Mytilidae appears to be the only known taxon with doubly uniparental inheritance of mtDNA that does not possess the atp8 gene. This raises the question as to whether mytilids have completely lost the ATP8 protein, whether the gene has been transferred to the nucleus or whether they possess a highly modified version of the gene/protein that has led to its lack of annotation. In the present study, we re-investigated all complete (or nearly complete) F and M mytilid mt genomes previously sequenced for the presence of conserved open reading frames (ORFs) that might code for ATP8 and/or have other functional importance in these bivalves. We also revised the annotations of all available complete mitochondrial genomes of bivalves and nematodes that are thought to lack atp8 in an attempt to detect it. Our results indicate that a novel mytilid ORF of significant length (i.e., the ORF is >85 amino acids in length), with complete start and stop codons, is a candidate for the atp8 gene: (1) it possesses a pattern of evolution expected for a protein-coding gene evolving under purifying selection (i.e., the 3rd>1st>2nd codon pattern of evolution), (2) it is actively transcribed in Mytilus species, (3) it has one predicted transmembrane helix (as do other metazoan ATP8 proteins), (4) it has conserved functional motifs and (5), comparisons of its amino acid sequence with ATP8 sequences of other molluscan or bivalve species reveal similar hydropathy profiles. Furthermore, our revised annotations also confirmed the mt presence of atp8 in almost all bivalve species and in one nematode species. Our results thus support recognizing the presence of ATPase 8 in most bivalves mt genomes (if not all) rather than the continued characterization of these genomes as lacking this gene.

Unique features of animal mitochondrial translation systems. The non-universal genetic code, unusual features of the translational apparatus and their relevance to human mitochondrial diseases

In animal mitochondria, several codons are non-universal and their meanings differ depending on the species. In addition, the tRNA structures that decipher codons are sometimes unusually truncated. These features seem to be related to the shortening of mitochondrial (mt) genomes, which occurred during the evolution of mitochondria. These organelles probably originated from the endosymbiosis of an aerobic eubacterium into an ancestral eukaryote. It is plausible that these events brought about the various characteristic features of animal mt translation systems, such as genetic code variations, unusually truncated tRNA and rRNA structures, unilateral tRNA recognition mechanisms by aminoacyl-tRNA synthetases, elongation factors and ribosomes, and compensation for RNA deficits by enlarged proteins. In this article, we discuss molecular mechanisms for these phenomena. Finally, we describe human mt diseases that are caused by modification defects in mt tRNAs.

Tunicate mitogenomics and phylogenetics: peculiarities of the Herdmania momus mitochondrial genome and support for the new chordate phylogeny

Background

Tunicates represent a key metazoan group as the sister-group of vertebrates within chordates. The six complete mitochondrial genomes available so far for tunicates have revealed distinctive features. Extensive gene rearrangements and particularly high evolutionary rates have been evidenced with regard to other chordates. This peculiar evolutionary dynamics has hampered the reconstruction of tunicate phylogenetic relationships within chordates based on mitogenomic data.

Results

In order to further understand the atypical evolutionary dynamics of the mitochondrial genome of tunicates, we determined the complete sequence of the solitary ascidian Herdmania momus. This genome from a stolidobranch ascidian presents the typical tunicate gene content with 13 protein-coding genes, 2 rRNAs and 24 tRNAs which are all encoded on the same strand. However, it also presents a novel gene arrangement, highlighting the extreme plasticity of gene order observed in tunicate mitochondrial genomes. Probabilistic phylogenetic inferences were conducted on the concatenation of the 13 mitochondrial protein-coding genes from representatives of major metazoan phyla. We show that whereas standard homogeneous amino acid models support an artefactual sister position of tunicates relative to all other bilaterians, the CAT and CAT+BP site- and time-heterogeneous mixture models place tunicates as the sister-group of vertebrates within monophyletic chordates. Moreover, the reference phylogeny indicates that tunicate mitochondrial genomes have experienced a drastic acceleration in their evolutionary rate that equally affects protein-coding and ribosomal-RNA genes.

Conclusion

This is the first mitogenomic study supporting the new chordate phylogeny revealed by recent phylogenomic analyses. It illustrates the beneficial effects of an increased taxon sampling coupled with the use of more realistic amino acid substitution models for the reconstruction of animal phylogeny.

Unique mitogenomic features in the scleractinian family pocilloporidae (scleractinia: astrocoeniina)

The complete DNA sequences of three mitochondrial (mt) genomes were obtained from the scleractinian corals, Stylophora pistillata, Pocillopora damicornis, and Madracis mirabilis, and were compared to the published mt genomes to elucidate phylogenetically unique features of the family Pocilloporidae. The entire mt genomes of pocilloporid corals ranged from 16,951 to 17,425 bp with the A+T contents of their sense strands ranging from 68.4% to 70.2%. The gene order of protein-coding genes was identical to those of other scleractinian corals. The novel atp8 gene, first described in confamilial Seriatopora corals, was also confirmed using reverse transcription-polymerase chain reaction (RT-PCR), Northern blot, and sequence analyses in other genera of the Pocilloporidae. The intergenic spacer between atp6 and nad4, containing distinct repeated elements, conserved sequence blocks and domains, and functional structures, possesses typical characteristics of a putative control region for the four coral genera. A duplicated trnW, detected in the region close to the cox1 gene and which shares the highly conserved primary and secondary structures of its original counterpart, was discovered in both Seriatopora and Stylophora. These molecular characteristics are unique and provide the phylogenetic information for future evaluation of the status of the family Pocilloporidae in the evolutionary history of scleractinian corals.

Evolution of the mitochondrial genome of Metazoa as exemplified by comparison of congeneric species

The mitochondrial genome (mtDNA) of Metazoa is a good model system for evolutionary genomic studies and the availability of more than 1000 sequences provides an almost unique opportunity to decode the mechanisms of genome evolution over a large phylogenetic range. In this paper, we review several structural features of the metazoan mtDNA, such as gene content, genome size, genome architecture and the new parameter of gene strand asymmetry in a phylogenetic framework. The data reviewed here show that: (1) the plasticity of Metazoa mtDNA is higher than previously thought and mainly due to variation in number and location of tRNA genes; (2) an exceptional trend towards stabilization of genomic features occurred in deuterostomes and was exacerbated in vertebrates, where gene content, genome architecture and gene strand asymmetry are almost invariant. Only tunicates exhibit a very high degree of genome variability comparable to that found outside deuterostomes. In order to analyse the genomic evolutionary process at short evolutionary distances, we have also compared mtDNAs of species belonging to the same genus: the variability observed in congeneric species significantly recapitulates the evolutionary dynamics observed at higher taxonomic ranks, especially for taxa showing high levels of genome plasticity and/or fast nucleotide substitution rates. Thus, the analysis of congeneric species promises to be a valuable approach for the assessment of the mtDNA evolutionary trend in poorly or not yet sampled metazoan groups.

Complete nucleotide sequences of mitochondrial genomes of two solitary entoprocts, Loxocorone allax and Loxosomella aloxiata: implications for lophotrochozoan phylogeny

The complete nucleotide sequences of the mitochondrial (mt) genomes of the entoprocts Loxocorone allax and Loxosomella aloxiata were determined. Both species carry the typical gene set of metazoan mt genomes and have similar organizations of their mt genes. However, they show differences in the positions of two tRNA(Leu) genes. Additionally, the tRNA(Val) gene, and half of the long non-coding region, is duplicated and inverted in the Loxos. aloxiata mt genome. The initiation codon of the Loxos. aloxiata cytochrome oxidase subunit I gene is expected to be ACG rather than AUG. The mt gene organizations in these two entoproct species most closely resemble those of mollusks such as Katharina tunicata and Octopus vulgaris, which have the most evolutionarily conserved mt gene organization reported to date in mollusks. Analyses of the mt gene organization in the lophotrochozoan phyla (Annelida, Brachiopoda, Echiura, Entoprocta, Mollusca, Nemertea, and Phoronida) suggested a close phylogenetic relationship between Brachiopoda, Annelida, and Echiura. However, Phoronida was excluded from this grouping. Molecular phylogenetic analyses based on the sequences of mt protein-coding genes suggested a possible close relationship between Entoprocta and Phoronida, and a close relationship among Brachiopoda, Annelida, and Echiura.

The mitochondrial genome of the screamer louse Bothriometopus (phthiraptera: ischnocera): effects of extensive gene rearrangements on the evolution of the genome

Mitochondrial (mt) genome rearrangement has generally been studied with respect to the phenomenon itself, focusing on their phylogenetic distribution and causal mechanisms. Rearrangements have additional significance through effects on substitution, transcription, and mRNA processing. Lice are an ideal group in which to study the interactions between rearrangements and these factors due to the heightened rearrangement rate within this group. The entire mt genome of the screamer louse Bothriometopus was sequenced and compared to previously sequenced louse genomes. The mt genome is 15,564 bp, circular, and all genes are encoded on the same strand. The gene arrangement differs radically from both other louse species and the ancestral insect. Nucleotide composition is A+T biased, but there is no skew which may be due to reversal of replication direction or a transcriptional effect. Bothriometopus has both tRNA duplication and concerted evolution which has not been observed previously. Eleven of the 13 protein-coding genes have 3' end stem-loop structures which may allow mRNA processing without flanking tRNAs and so facilitate gene rearrangements. There are five candidate control regions capable of forming stem-loop structures. Two are structurally more similar to the control regions of other insect species than those of other lice. Analyses of Bothriometopus demonstrate that louse mt genomes, in addition to being extensively rearranged, differ significantly from most insect species in nucleotide composition biases, tRNA evolution, protein-coding gene structures and putative signaling sites such as the control region. These may be either a cause or a consequence of gene rearrangements.

The complete mitochondrial genomes of needle corals, Seriatopora spp. (Scleractinia: Pocilloporidae): an idiosyncratic atp8, duplicated trnW gene, and hypervariable regions used to determine species phylogenies and recently diverged populations

Complete DNA sequences were determined for the mitochondrial (mt) genomes of the needle corals, Seriatopora caliendrum (17,011bp) and S. hystrix (17,060bp). Gene arrangement of the Seriatopora mt genomes is similar to the 14 currently published scleractinian mitogenomes with three unusual features, including an idiosyncratic atp8, a duplicated trnW (tRNA(TRP)), and a putative control region located between atp6 and nad4. Atp8, located between duplicate trnW genes, showed relatively low amino acid similarity (25.6-34.6%) with those of published scleractinian corals. A reverse-transcription polymerase chain reaction confirmed the transcription of this novel atp8 gene in Seriatopora. A duplicated trnW was detected in the region close to the cox1 gene and shares the highly conserved primary and secondary structure of its original counterpart. The intergenic spacer between atp6 and nad4, which contains several distinct repeated elements, is being designated as the putative control region in the Seriatopora mt genomes. Evaluation of the molecular evolution of several protein-coding genes and intergenic spacers showed 3- to 4-fold higher divergence rates among populations or between species than those published for scleractinian mt genomes. This study not only successfully revealed the phylogenies of S. hystrix and S. caliendrum from the West Pacific Ocean by mtDNA, but also highlighted the potential utilities of mt hypervariable regions in phylogenetic construction below the species level for Seriatopora.

The mitochondrial genome of Phallusia mammillata and Phallusia fumigata (Tunicata, Ascidiacea): high genome plasticity at intra-genus level

Background

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.

Results

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.

Conclusion

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.

Phylogenetic position of a whale-fall lancelet (Cephalochordata) inferred from whole mitochondrial genome sequences

Background

The lancelet Asymmetron inferum (subphylum Cephalochordata) was recently discovered on the ocean floor off the southwest coast of Japan at a depth of 229 m, in an anaerobic and sulfide-rich environment caused by decomposing bodies of the sperm whale Physeter macrocephalus. This deep sulfide-rich habitat of A. inferum is unique among the lancelets. The distinguishing adaptation of this species to such an extraordinary habitat can be considered in a phylogenetic framework. As the first step of reconstruction of the evolutionary processes in this species, we investigated its phylogenetic position based on 11 whole mitochondrial genome sequences including the newly determined ones of the whale-fall lancelet A. inferum and two coral-reef congeners.

Results

Our phylogenetic analyses showed that extant lancelets are clustered into two major clades, the Asymmetron clade and the Epigonichthys + Branchiostoma clade. A. inferum was in the former and placed in the sister group to A. lucayanum complex. The divergence time between A. inferum and A. lucayanum complex was estimated to be 115 Mya using the penalized likelihood (PL) method or 97 Mya using the nonparametric rate smoothing (NPRS) method (the middle Cretaceous). These are far older than the first appearance of large whales (the middle Eocene, 40 Mya). We also discovered that A. inferum mitogenome (mitochondrial genome) has been subjected to large-scale gene rearrangements, one feature of rearrangements being unique among the lancelets and two features shared with A. lucayanum complex.

Conclusion

Our study supports the monophyly of genus Asymmetron assumed on the basis of the morphological characters. Furthermore, the features of the A. inferum mitogenome expand our knowledge of variation within cephalochordate mitogenomes, adding a new case of transposition and inversion of the trnQ gene. Our divergence time estimation suggests that A. inferum remained a member of the Mesozoic and the early Cenozoic large vertebrate-fall communities before shifting to become a whale-fall specialist.

The mitochondrial genome of Pocillopora (Cnidaria: Scleractinia) contains two variable regions: the putative D-loop and a novel ORF of unknown function

The complete mitochondrial genomes of two individuals attributed to different morphospecies of the scleractinian coral genus Pocillopora have been sequenced. Both genomes, respectively 17,415 and 17,422 nt long, share the presence of a previously undescribed ORF encoding a putative protein made up of 302 amino acids and of unknown function. Surprisingly, this ORF turns out to be the second most variable region of the mitochondrial genome (1% nucleotide sequence difference between the two individuals) after the putative control region (1.5% sequence difference). Except for the presence of this ORF and for the location of the putative control region, the mitochondrial genome of Pocillopora is organized in a fashion similar to the other scleractinian coral genomes published to date. For the first time in a cnidarian, a putative second origin of replication is described based on its secondary structure similar to the stem-loop structure of O(L), the origin of L-strand replication in vertebrates.

Conflict between translation initiation and elongation in vertebrate mitochondrial genomes

The strand-biased mutation spectrum in vertebrate mitochondrial genomes results in an AC-rich L-strand and a GT-rich H-strand. Because the L-strand is the sense strand of 12 protein-coding genes out of the 13, the third codon position is overall strongly AC-biased. The wobble site of the anticodon of the 22 mitochondrial tRNAs is either U or G to pair with the most abundant synonymous codon, with only one exception. The wobble site of Met-tRNA is C instead of U, forming the Watson-Crick match with AUG instead of AUA, the latter being much more frequent than the former. This has been attributed to a compromise between translation initiation and elongation; i.e., AUG is not only a methionine codon, but also an initiation codon, and an anticodon matching AUG will increase the initiation rate. However, such an anticodon would impose selection against the use of AUA codons because AUA needs to be wobble-translated. According to this translation conflict hypothesis, AUA should be used relatively less frequently compared to UUA in the UUR codon family. A comprehensive analysis of mitochondrial genomes from a variety of vertebrate species revealed a general deficiency of AUA codons relative to UUA codons. In contrast, urochordate mitochondrial genomes with two tRNA(Met) genes with CAU and UAU anticodons exhibit increased AUA codon usage. Furthermore, six bivalve mitochondrial genomes with both of their tRNA-Met genes with a CAU anticodon have reduced AUA usage relative to three other bivalve mitochondrial genomes with one of their two tRNA-Met genes having a CAU anticodon and the other having a UAU anticodon. We conclude that the translation conflict hypothesis is empirically supported, and our results highlight the fine details of selection in shaping molecular evolution.

The phylogeny of Mediterranean tortoises and their close relatives based on complete mitochondrial genome sequences from museum specimens

As part of an ongoing project to generate a mitochondrial database for terrestrial tortoises based on museum specimens, the complete mitochondrial genome sequences of 10 species and a approximately 14kb sequence from an eleventh species are reported. The sampling of the present study emphasizes Mediterranean tortoises (genus Testudo and their close relatives). Our new sequences are aligned, along with those of two testudinoid turtles from GenBank, Chrysemys picta and Mauremys reevesii, yielding an alignment of 14,858 positions, of which 3238 are parsimony informative. We develop a phylogenetic taxonomy for Testudo and related species based on well-supported, diagnosable clades. Several well-supported nodes are recovered, including the monophyly of a restricted Testudo, T. kleinmanni+T. marginata (the Chersus clade), and the placement of the enigmatic African pancake tortoise (Malacochersus tornieri) within the predominantly Palearctic greater Testudo group (Testudona tax. nov.). Despite the large amount of sequence reported, there is low statistical support for some nodes within Testudona and so we do not propose names for those groups. A preliminary and conservative estimation of divergence times implies a late Miocene diversification for the testudonan clade (6-10 million years ago), matching their first appearance in the fossil record. The multi-continental distribution of testudonan turtles can be explained by the establishment of permanent connections between Europe, Africa, and Asia at this time. The arrival of testudonan turtles to Africa occurred after one or more initial tortoise invasions gave rise to the diverse (>25 species) 'Geochelone complex.' Two unusual genomic features are reported for the mtDNA of one tortoise, M. tornieri: (1) nad4 has a shift of reading frame that we suggest is resolved by translational frameshifting of the mRNA on the ribosome during protein synthesis and (2) there are two copies of the control region and trnF, with the latter having experienced multiple-nucleotide substitutions in a pattern suggesting that each is being maintained by selection.

Evolution of the Mitochondrial Genome in Cephalochordata as Inferred from Complete Nucleotide Sequences from Two Epigonichthys Species

Complete mitochondrial (mt) DNA sequences of two lancelets, Epigonichthys maldivensis and E. lucayanus, were compared with those of two Branchiostoma lancelets and several deuterostomes previously surveyed. The mt-gene order of E. lucayanus was quite different from that of E. maldivensis, the latter being identical to the two Branchiostoma species. A remarkable genomic change in E. lucayanus mtDNA was an inversion, indicating the possibility of recombination of the mt-genome. Gene rearrangements, probably attributable to tandem genome duplications and subsequent random deletions, were observed in two parts. Short major unassignable sequences of the examined lancelets were regarded as a part of putative regulative elements, judging from some sequence similarity to the conserved sequence block (CSB) in mammalian mtDNA. The considerable mt-genome reorganization in E. lucayanus seemed to have affected the nucleotide substitution pattern, suggested by base composition analyses. The present analysis also suggested that AGR codons in lancelet mtDNA were likely to correspond to serine residue, rather than glycine. Furthermore, the AGG codon, so far reputed to be unassignable in lancelet mtDNA, was found twice in E. maldivensis, indicating the availability of all four AGN codons in some lancelets. This finding lends support to an alternative hypothesis regarding the evolutionary history of AGR-codon assignment in extant chordates, rather than that previously proposed. A molecular phylogenetic tree of the Epigonichthys and Branchiostoma species based on DNA sequences of the 13 mt-protein genes doubted the monophyly of the former genus, unlike the prevailing classification based on their different gonadal arrangements.

Ascidian molecular phylogeny inferred from mtDNA data with emphasis on the Aplousobranchiata

We explored the usefulness of mtDNA data in assessing phylogenetic relationships within the Ascidiacea. Although ascidians are a crucial group in studies of deuterostome evolution and the origin of chordates, little molecular work has been done to ascertain the evolutionary relationships within the class, and in the studies performed to date the key group Aplousobranchiata has not been adequately represented. We present a phylogenetic analysis based on mitochondrial cytochrome c oxidase subunit I (COI) sequences of 37 ascidian species, mainly Aplousobranchiata (26 species). Our data retrieve the main groups of ascidians, although Phlebobranchiata appeared paraphyletic in some analyses. Aplousobranch ascidians consistently appeared as a derived group, suggesting that their simple branchial structure is not a pleisiomorphic feature. Relationships between the main groups of ascidians were not conclusively determined, the sister group of Aplousobranchiata was the Stolidobranchiata or the Phlebobranchiata, depending on the analysis. Therefore, our data could not confirm an Enterogona clade (Aplousobranchiata+Phlebobranchiata). All of the tree topologies confirmed previous ideas, based on morphological and biochemical characters, suggesting that Cionidae and Diazonidae are members of the clade Aplousobranchiata, with Cionidae occupying a basal position within them in our analyses. Within the Aplousobranchiata, we found some stable clades that provide new data on the evolutionary relationships within this large group of ascidians, and that may prompt a re-evaluation of some morphological characters.

Sequencing and comparing whole mitochondrial genomes of animals

Comparing complete animal mitochondrial genome sequences is becoming increasingly common for phylogenetic reconstruction and as a model for genome evolution. Not only are they much more informative than shorter sequences of individual genes for inferring evolutionary relatedness, but these data also provide sets of genome-level characters, such as the relative arrangements of genes, which can be especially powerful. We describe here the protocols commonly used for physically isolating mitochondrial DNA (mtDNA), for amplifying these by polymerase chain reaction (PCR) or rolling circle amplification (RCA), for cloning, sequencing, assembly, validation, and gene annotation, and for comparing both sequences and gene arrangements. On several topics, we offer general observations based on our experiences with determining and comparing complete mitochondrial DNA sequences.

Forces maintaining organellar genomes: is any as strong as genetic code disparity or hydrophobicity?

It remains controversial why mitochondria and chloroplasts retain the genes encoding a small subset of their constituent proteins, despite the transfer of so many other genes to the nucleus. Two candidate obstacles to gene transfer, suggested long ago, are that the genetic code of some mitochondrial genomes differs from the standard nuclear code, such that a transferred gene would encode an incorrect amino acid sequence, and that the proteins most frequently encoded in mitochondria are generally very hydrophobic, which may impede their import after synthesis in the cytosol. More recently it has been suggested that both these interpretations suffer from serious "false positives" and "false negatives": genes that they predict should be readily transferred but which have never (or seldom) been, and genes whose transfer has occurred often or early, even though this is predicted to be very difficult. Here I consider the full known range of ostensibly problematic such genes, with particular reference to the sequences of events that could have led to their present location. I show that this detailed analysis of these cases reveals that they are in fact wholly consistent with the hypothesis that code disparity and hydrophobicity are much more powerful barriers to functional gene transfer than any other. The popularity of the contrary view has led to the search for other barriers that might retain genes in organelles even more powerfully than code disparity or hydrophobicity; one proposal, concerning the role of proteins in redox processes, has received widespread support. I conclude that this abandonment of the original explanations for the retention of organellar genomes has been premature. Several other, relatively minor, obstacles to gene transfer certainly exist, contributing to the retention of relatively many organellar genes in most lineages compared to animal mtDNA, but there is no evidence for obstacles as severe as code disparity or hydrophobicity. One corollary of this conclusion is that there is currently no reason to suppose that engineering nuclear versions of the remaining mammalian mitochondrial genes, a feat that may have widespread biomedical relevance, should require anything other than sequence alterations obviating code disparity and causing modest reductions in hydrophobicity without loss of enzymatic function.

Bidirectional replication initiates at sites throughout the mitochondrial genome of birds

Analysis of mitochondrial replication intermediates of Gallus gallus on fork-direction gels indicates that replication occurs in both directions around circular mitochondrial DNA. This finding was corroborated by a study of chick mitochondrial DNA on standard neutral two-dimensional agarose gels, which yielded archetypal initiation arcs in fragments covering the entire genome. There was, however, considerable variation in initiation arc intensity. The majority of initiation events map to regions flanking the major non-coding region, in particular the NADH dehydrogenase subunit 6 (ND6) gene. Initiation point mapping of the ND6 gene identified prominent free 5' ends of DNA, which are candidate start sites for DNA synthesis. Therefore we propose that the initiation zone of G. gallus mitochondrial DNA encompasses most, if not all, of the genome, with preferred initiation sites in regions flanking the major non-coding region. Comparison with mammals suggests a common mechanism of initiation of mitochondrial DNA replication in higher vertebrates.

Complete nucleotide sequence of the mitochondrial genome of Doliolum nationalis with implications for evolution of urochordates

The evolutionary history of the diverse lifestyles adopted by urochordates has attracted intense interest because it may effect the evolutionary history of vertebrates. Here, we report the complete mitochondrial (mt) DNA sequence of the pelagic thaliacean doliolid Doliolum nationalis. The doliolid mt genome shares the unusual tRNAs of trnM(uau) and trnG(ucu) with other ascidians, such as Halocynthia and Ciona. On the other hand, the gene order of the doliolid mt genome is significantly different from that of any ascidian species or vertebrate reported to date. Phylogenetic analyses of the amino acid sequences of 12 protein-coding genes strongly support the sister-grouping of doliolids and the Phlebobranch ascidian Ciona, with the Stolidobranch ascidian alocynthia as the outgroup, thereby providing strong support for the paraphyly of ascidians, as has been suggested by 18S rDNA studies. Given the paraphyletic nature of ascidians, it seems likely that the common ancestor of ascidians and thaliaceans was sessile, as are the present-day ascidians, and that the thaliaceans subsequently evolved a pelagic lifestyle.

Molecular systematics of primary reptilian lineages and the tuatara mitochondrial genome

We provide phylogenetic analyses for primary Reptilia lineages including, for the first time, Sphenodon punctatus (tuatara) using data from whole mitochondrial genomes. Our analyses firmly support a sister relationship between Sphenodon and Squamata, which includes lizards and snakes. Using Sphenodon as an outgroup for select squamates, we found evidence indicating a sister relationship, among our study taxa, between Serpentes (represented by Dinodon) and Varanidae. Our analyses support monophyly of Archosauria, and a sister relationship between turtles and archosaurs. This latter relationship is congruent with a growing set of morphological and molecular analyses placing turtles within crown Diapsida and recognizing them as secondarily anapsid (lacking a skull fenestration). Inclusion of Sphenodon, as the only surviving member of Sphenodontia (with fossils from the mid-Triassic), helps to fill a sampling gap within previous analyses of reptilian phylogeny. We also report a unique configuration for the mitochondrial genome of Sphenodon, including two tRNA(Lys) copies and an absence of ND5, tRNA(His), and tRNA(Thr) genes.

Complete mtDNA of Ciona intestinalis reveals extensive gene rearrangement and the presence of an atp8 and an extra trnM gene in ascidians

The complete mitochondrial genome (mtDNA) of the model organism Ciona intestinalis (Urochordata, Ascidiacea) has been amplified by long-PCR using specific primers designed on putative mitochondrial transcripts identified from publicly available mitochondrial-like expressed sequence tags. The C. intestinalis mtDNA encodes 39 genes: 2 rRNAs, 13 subunits of the respiratory complexes, including ATPase subunit 8 ( atp8), and 24 tRNAs, including 2 tRNA-Met with anticodons 5'-UAU-3'and 5'-CAU-3', respectively. All genes are transcribed from the same strand. This gene content seems to be a common feature of ascidian mtDNAs, as we have verified the presence of a previously undetected atp8 and of two trnM genes in the two other sequenced ascidian mtDNAs. Extensive gene rearrangement has been found in C. intestinalis with respect not only to the common Vertebrata/Cephalochordata/Hemichordata gene organization but also to other ascidian mtDNAs, including the cogeneric Ciona savignyi. Other features such as the absence of long noncoding regions, the shortness of rRNA genes, the low GC content (21.4%), and the absence of asymmetric base distribution between the two strands suggest that this genome is more similar to those of some protostomes than to deuterostomes.

Transcript mapping and genome annotation of ascidian mtDNA using EST data

Mitochondrial transcripts of two ascidian species were reconstructed through sequence assembly of publicly available ESTs resembling mitochondrial DNA sequences (mt-ESTs). This strategy allowed us to analyze processing and mapping of the mitochondrial transcripts and to investigate the gene organization of a previously uncharacterized mitochondrial genome (mtDNA). This new strategy would greatly facilitate the sequencing and annotation of mtDNAs. In Ciona intestinalis, the assembled mt-ESTs covered 22 mitochondrial genes ( approximately 12,000 bp) and provided the partial sequence of the mtDNA and the prediction of its gene organization. Such sequences were confirmed by amplification and sequencing of the entire Ciona mtDNA. For Halocynthia roretzi, for which the mtDNA sequence was already available, the inferred mt transcripts allowed better definition of gene boundaries (16S rRNA, ND1, ATP6, and tRNA-Ser genes) and the identification of a new gene (an additional Phe-tRNA). In both species, polycistronic and immature transcripts, creation of stop codons by polyadenylation, tRNA signal processing, and rRNA transcript termination signals were identified, thus suggesting that the main features of mitochondrial transcripts are conserved in Chordata.

Neural expression of the Huntington's disease gene as a chordate evolutionary novelty

Huntington's disease is a progressive neuro-degenerative disorder in humans, which is scharacterized by onset of dementia, muscular ataxia, and death. Huntington's disease is caused by the expansion of the polyglutamine (polyQ) tract in the N-terminus of the HD protein (Huntingtin). CAG expansion is a dominant gain of function mutation that affects striated neurons in the brain (Cattaneo, 2003, News Physiol Sci 18:34). The evolutionary origins of the vertebrate Hd gene are not well understood. In order to address the evolutionary history of the Hd gene, we have cloned and characterized the expression of the Hd gene in two invertebrate deuterostomes, an echinoderm and an ascidian, and have examined the expression patterns in a phylogenetic context. Echinoderms are basal deuterostomes and ascidians are basal chordates; both are useful for understanding the origins of and evolutionary trends in genes important in vertebrates such as the Huntigton's disease gene. Expression of Hd RNA is detected at all stages of development in both the echinoderm and ascidian studied. In the echinoderm Heliocidaris erythrogramma, Hd is expressed in coelomic mesodermal tissue derivatives, but not in the central nervous system. In the ascidian Halocynthia roretzi expression is located in both mesoderm and nervous tissue. We suggest that the primitive deuterostome expression pattern is not neural. Thus, neural expression of the Hd gene in deuterostomes may be a novel feature of the chordate lineage, and the original role(s) of HD in deuterostomes may have been non-neural.

The Ciona intestinalis genome: when the constraints are off

The recent genome sequencing of a non-vertebrate deuterostome, the ascidian tunicate Ciona intestinalis, makes a substantial contribution to the fields of evolutionary and developmental biology.1 Tunicates have some of the smallest bilaterian genomes, embryos with relatively few cells, fixed lineages and early determination of cell fates. Initial analyses of the C. intestinalis genome indicate that it has been evolving rapidly. Comparisons with other bilaterians show that C. intestinalis has lost a number of genes, and that many genes linked together in most other bilaterians have become uncoupled. In addition, a number of independent, lineage-specific gene duplications have been detected. These new results, although interesting in themselves, will take on a deeper significance once the genomes of additional invertebrate deuterostomes (e.g. echinoderms, hemichordates and amphioxus) have been sequenced. With such a broadened database, comparative genomics can begin to ask pointed questions about the relationship between the evolution of genomes and the evolution of body plans.

Evaluating hypotheses of deuterostome phylogeny and chordate evolution with new LSU and SSU ribosomal DNA data

We investigated evolutionary relationships among deuterostome subgroups by obtaining nearly complete large-subunit ribosomal RNA (LSU rRNA)-gene sequences for 14 deuterostomes and 3 protostomes and complete small-subunit (SSU) rRNA-gene sequences for five of these animals. With the addition of previously published sequences, we compared 28 taxa using three different data sets (LSU only, SSU only, and combined LSU + SSU) under minimum evolution (with LogDet distances), maximum likelihood, and maximum parsimony optimality criteria. Additionally, we analyzed the combined LSU + SSU sequences with spectral analysis of LogDet distances, a technique that measures the amount of support and conflict within the data for every possible grouping of taxa. Overall, we found that (1) the LSU genes produced a tree very similar to the SSU gene tree, (2) adding LSU to SSU sequences strengthened the bootstrap support for many groups above the SSU-only values (e.g., hemichordates plus echinoderms as Ambulacraria; lancelets as the sister group to vertebrates), (3) LSU sequences did not support SSU-based hypotheses of pterobranchs evolving from enteropneusts and thaliaceans evolving from ascidians, and (4) the combined LSU + SSU data are ambiguous about the monophyly of chordates. No tree-building algorithm united urochordates conclusively with other chordates, although spectral analysis did so, providing our only evidence for chordate monophyly. With spectral analysis, we also evaluated several major hypotheses of deuterostome phylogeny that were constructed from morphological, embryological, and paleontological evidence. Our rRNA-gene analysis refutes most of these hypotheses and thus advocates a rethinking of chordate and vertebrate origins.

The Mitochondrial Genome of the Sipunculid Phascolopsis gouldii Supports Its Association with Annelida Rather than Mollusca

We determined the sequence of about half (7,470 nt) of the mitochondrial genome of the sipunculid Phascolopsis gouldii, the first representative of this phylum to be so studied. All of the 19 identified genes are transcribed from the same DNA strand. The arrangement of these genes is remarkably similar to that of the oligochaete annelid Lumbricus terrestris. Comparison of both the inferred amino acid sequences and the gene arrangements of a variety of diverse metazoan taxa reveals that the phylum Sipuncula is more closely related to Annelida than to Mollusca. This requires reinterpretation of the homology of several embryological features and of patterns of animal body plan evolution.

Mitochondrial DNA variation in benzimidazole-resistant and -susceptible populations of the small ruminant parasite Teladorsagia circumcincta

The genetic diversity of the mtDNA ND4 gene in 11 Teladorsagia circumcincta populations from France and Morocco was assessed by sequencing. Some of these nematode populations were resistant to benzimidazole (BZ) anthelmintics, while others were susceptible. The nucleotide diversity in all populations studied was very high, probably due to a high mutation rate in nematodes, but there was no significant difference between them. This suggests that no strong, recurrent bottlenecks occur during the acquisition of BZ resistance by a worm population. The conservation of genetic variations during the acquisition of BZ resistance is probably due to the fact that anthelmintic treatments do not kill all the susceptible adult worms and to the presence of numerous free-living larvae that are not submitted to this anthelmintic pressure. There was no genetic subdivision between worm populations on a small geographical scale (less than 200 km), but significant F(ST)s were found on a larger geographical scale. This kind of subdivision cannot be explained by different genetic flows between populations because all these populations were isolated from each other. This subdivision is probably due to the breeding management practices and the large size of these worm populations, which limit genetic drift.

The complete mitochondrial genome of the articulate brachiopod Terebratalia transversa

We sequenced the complete mitochondrial DNA (mtDNA) of the articulate brachiopod Terebratalia transversa. The circular genome is 14,291 bp in size, relatively small compared with other published metazoan mtDNAs. The 37 genes commonly found in animal mtDNA are present; the size decrease is due to the truncation of several tRNA, rRNA, and protein genes, to some nucleotide overlaps, and to a paucity of noncoding nucleotides. Although the gene arrangement differs radically from those reported for other metazoans, some gene junctions are shared with two other articulate brachiopods, Laqueus rubellus and Terebratulina retusa. All genes in the T. transversa mtDNA, unlike those in most metazoan mtDNAs reported, are encoded by the same strand. The A+T content (59.1%) is low for a metazoan mtDNA, and there is a high propensity for homopolymer runs and a strong base-compositional strand bias. The coding strand is quite G+T-rich, a skew that is shared by the confamilial (laqueid) species L. rubellus but is the opposite of that found in T. retusa, a cancellothyridid. These compositional skews are strongly reflected in the codon usage patterns and the amino acid compositions of the mitochondrial proteins, with markedly different usages being observed between T. retusa and the two laqueids. This observation, plus the similarity of the laqueid noncoding regions to the reverse complement of the noncoding region of the cancellothyridid, suggests that an inversion that resulted in a reversal in the direction of first-strand replication has occurred in one of the two lineages. In addition to the presence of one noncoding region in T. transversa that is comparable with those in the other brachiopod mtDNAs, there are two others with the potential to form secondary structures; one or both of these may be involved in the process of transcript cleavage.

Numerous gene rearrangements in the mitochondrial genome of the wallaby louse, Heterodoxus macropus (Phthiraptera)

The complete arrangement of genes in the mitochondrial (mt) genome is known for 12 species of insects, and part of the gene arrangement in the mt genome is known for over 300 other species of insects. The arrangement of genes in the mt genome is very conserved in insects studied, since all of the protein-coding and rRNA genes and most of the tRNA genes are arranged in the same way. We sequenced the entire mt genome of the wallaby louse, Heterodoxus macropus, which is 14,670 bp long and has the 37 genes typical of animals and some noncoding regions. The largest noncoding region is 73 bp long (93% A+T), and the second largest is 47 bp long (92% A+T). Both of these noncoding regions seem to be able to form stem-loop structures. The arrangement of genes in the mt genome of this louse is unlike that of any other animal studied. All tRNA genes have moved and/or inverted relative to the ancestral gene arrangement of insects, which is present in the fruit fly Drosophila yakuba. At least nine protein-coding genes (atp6, atp8, cox2, cob, nad1-nad3, nad5, and nad6) have moved; moreover, four of these genes (atp6, atp8, nad1, and nad3) have inverted. The large number of gene rearrangements in the mt genome of H. macropus is unprecedented for an arthropod.

High direct estimate of the mutation rate in the mitochondrial genome of Caenorhabditis elegans

Mutations in the mitochondrial genome have been implicated in numerous human genetic disorders and offer important data for phylogenetic, forensic, and population genetic studies. Using a long-term series of Caenorhabditis elegans mutation accumulation lines, we performed a wide-scale screen for mutations in the mitochondrial genome that revealed a mutation rate that is two orders of magnitude higher than previous indirect estimates, a highly biased mutational spectrum, multiple mutations affecting coding function, as well as mutational hotspots at homopolymeric nucleotide stretches.