The complete mitochondrial genome of the orbiniid polychaete Orbinia latreillii (Annelida, Orbiniidae)--A novel gene order for Annelida and implications for annelid phylogeny

Mitochondrial genome of Leocrates chinensis Kinberg, 1866 (Annelida: Hesionidae)

We report the complete mitochondrial genome of Leocrates chinensis Kinberg, 1866 - the type species of the genus. It is 15061 bp long, and contains 13 protein-coding genes (PCGs), 22 tRNA genes (tRNAs), and 2 rRNA genes (rRNAs), and 1 putative control region. Phylogenetic analysis indicated that L. chinensis was placed as sister to Sirsoe methanicola (BS = 100) of the same family Hesionidae.

First mitochondrial genomes of Chrysopetalidae (Annelida) from shallow-water and deep-sea chemosynthetic environments

Among Annelida, Chrysopetalidae is an ecologically and morphologically diverse group, which includes shallow-water, deep-sea, free-living, and symbiotic species. Here, the four first mitochondrial genomes of this group are presented and described. One of the free-living shallow-water species Chrysopetalum debile (Chrysopetalinae), one of the yet undescribed free-living deep-sea species Boudemos sp., and those of the two deep-sea bivalve endosymbionts Craseoschema thyasiricola and Iheyomytilidicola lauensis (Calamyzinae). An updated phylogeny of Chrysopetalidae is performed, which supports previous phylogenetic hypotheses within Chrysopetalinae and indicates a complex ecological evolution within Calamyzinae. Additionally, analyses of natural selection pressure in the four mitochondrial genomes and additional genes from the two shallow-water species Bhawania goodei and Arichlidon gathofi were performed. Relaxed selection pressure in the mitochondrion of deep-sea and symbiotic species was found, with many sites under selection identified in the COX3 gene of deep-sea species.

Molluscan mitochondrial genomes break the rules

The first animal mitochondrial genomes to be sequenced were of several vertebrates and model organisms, and the consistency of genomic features found has led to a 'textbook description'. However, a more broad phylogenetic sampling of complete animal mitochondrial genomes has found many cases where these features do not exist, and the phylum Mollusca is especially replete with these exceptions. The characterization of full mollusc mitogenomes required considerable effort involving challenging molecular biology, but has created an enormous catalogue of surprising deviations from that textbook description, including wide variation in size, radical genome rearrangements, gene duplications and losses, the introduction of novel genes, and a complex system of inheritance dubbed 'doubly uniparental inheritance'. Here, we review the extraordinary variation in architecture, molecular functioning and intergenerational transmission of molluscan mitochondrial genomes. Such features represent a great potential for the discovery of biological history, processes and functions that are novel for animal mitochondrial genomes. This provides a model system for studying the evolution and the manifold roles that mitochondria play in organismal physiology, and many ways that the study of mitochondrial genomes are useful for phylogeny and population biology. This article is part of the Theo Murphy meeting issue 'Molluscan genomics: broad insights and future directions for a neglected phylum'.

Mitochondrial evolution in the Demospongiae (Porifera): Phylogeny, divergence time, and genome biology

The sponge class Demospongiae is the most speciose and morphologically diverse in the phylum Porifera, and the species within it are vital components of a range of ecosystems worldwide. Despite their ubiquity, a number of recalcitrant problems still remain to be solved regarding their phylogenetic inter-relationships, the timing of their appearance, and their mitochondrial biology, the latter of which is only beginning to be investigated. Here we generated 14 new demosponge mitochondrial genomes which, alongside previously published mitochondrial resources, were used to address these issues. In addition to phylogenomic analysis, we have used syntenic data and analysis of coding regions to forge a framework for understanding the inter-relationships between Demospongiae sub-classes and orders. We have also leveraged our new resources to study the mitochondrial biology of these clades in terms of codon usage, optimisation and gene expression, to understand how these vital cellular components may have contributed to the success of the Porifera. Our results strongly support a sister relationship between Keratosa and (Verongimorpha + Heteroscleromorpha), contradicting previous studies using nuclear markers. Our study includes one species of Clionaida, and show for the first time support for a grouping of Suberitida+(Clionaida+(Tethyida + Poecilosclerida). The findings of our phylogenetic analyses are supported by in-depth examination of structural and coding-level evidence from our mitochondrial data. A time-calibrated phylogeny estimated the origin of Demospongiae in the Cambrian (~529 Mya), and suggests that most demosponge order crown-groups emerged in the Mesozoic. This work therefore provides a robust basis for considering demosponge phylogenetic relationships, as well as essential mitochondrial data for understanding the biological basis for their success and diversity.

Phylogeny, evolution and mitochondrial gene order rearrangement in scale worms (Aphroditiformia, Annelida)

Next-generation sequencing (NGS) has become a powerful tool in phylogenetic and evolutionary studies. Here we applied NGS to recover two ribosomal RNA genes (18S and 28S) from 16 species and 15 mitochondrial genomes from 16 species of scale worms representing six families in the suborder Aphroditiformia (Phyllodocida, Annelida), a complex group of polychaetes characterized by the presence of dorsal elytra or scales. The phylogenetic relationship of the several groups of scale worms remains unresolved due to insufficient taxon sampling and low resolution of individual gene markers. Phylogenetic tree topology based on mitochondrial genomes is comparable with that based on concatenated sequences from two mitochondrial genes (cox1 and 16S) and two ribosomal genes (18S and 28S) genes, but has higher statistical support for several clades. Our analyses show that Aphroditiformia is monophyletic, indicating the presence of elytra is an apomorphic trait. Eulepethidae and Aphroditidae together form the sister group to all other families in this suborder, whereas Acoetidae is sister to Iphionidae. Polynoidae is monophyletic, but within this family the deep-sea subfamilies Branchinotogluminae and Macellicephalinae are paraphyletic. Mitochondrial genomes in most scale-worm families have a conserved gene order, but within Polynoidae there are two novel arrangement patterns in the deep-sea clade. Mitochondrial protein-coding genes in polynoids as a whole have evolved under strong purifying selection, but substitution rates in deep-sea species are much higher than those in shallow-water species, indicating that purifying selection is relaxed in deep-sea polynoids. There are positive selected amino acids for some mitochondrial genes of the deep-sea clade, indicating they may involve in the adaption of deep-sea polynoids. Overall, our study (1) provided more evidence for reconstruction of the phylogeny of Aphroditiformia, (2) provided evidence to refute the assumption that mitochondrial gene order in Errantia is conserved, and (3) indicated that the deep-sea extreme environment may have affected the mitochondrial genome evolution rate and gene order arrangement in Polynoidae.

Comparative Mitogenomics of Leeches (Annelida: Clitellata): Genome Conservation and Placobdella-Specific trnD Gene Duplication

Mitochondrial DNA sequences, often in combination with nuclear markers and morphological data, are frequently used to unravel the phylogenetic relationships, population dynamics and biogeographic histories of a plethora of organisms. The information provided by examining complete mitochondrial genomes also enables investigation of other evolutionary events such as gene rearrangements, gene duplication and gene loss. Despite efforts to generate information to represent most of the currently recognized groups, some taxa are underrepresented in mitochondrial genomic databases. One such group is leeches (Annelida: Hirudinea: Clitellata). Herein, we expand our knowledge concerning leech mitochondrial makeup including gene arrangement, gene duplication and the evolution of mitochondrial genomes by adding newly sequenced mitochondrial genomes for three bloodfeeding species: Haementeria officinalis, Placobdella lamothei and Placobdella parasitica. With the inclusion of three new mitochondrial genomes of leeches, a better understanding of evolution for this organelle within the group is emerging. We found that gene order and genomic arrangement in the three new mitochondrial genomes is identical to previously sequenced members of Clitellata. Interestingly, within Placobdella, we recovered a genus-specific duplication of the trnD gene located between cox2 and atp8. We performed phylogenetic analyses using 12 protein-coding genes and expanded our taxon sampling by including GenBank sequences for 39 taxa; the analyses confirm the monophyletic status of Clitellata, yet disagree in several respects with other phylogenetic hypotheses based on morphology and analyses of non-mitochondrial data.

Fifteen new earthworm mitogenomes shed new light on phylogeny within the Pheretima complex

The Pheretima complex within the Megascolecidae family is a major earthworm group. Recently, the systematic status of the Pheretima complex based on morphology was challenged by molecular studies. In this study, we carry out the first comparative mitogenomic study in oligochaetes. The mitogenomes of 15 earthworm species were sequenced and compared with other 9 available earthworm mitogenomes, with the main aim to explore their phylogenetic relationships and test different analytical approaches on phylogeny reconstruction. The general earthworm mitogenomic features revealed to be conservative: all genes encoded on the same strand, all the protein coding loci shared the same initiation codon (ATG), and tRNA genes showed conserved structures. The Drawida japonica mitogenome displayed the highest A + T content, reversed AT/GC-skews and the highest genetic diversity. Genetic distances among protein coding genes displayed their maximum and minimum interspecific values in the ATP8 and CO1 genes, respectively. The 22 tRNAs showed variable substitution patterns between the considered earthworm mitogenomes. The inclusion of rRNAs positively increased phylogenetic support. Furthermore, we tested different trimming tools for alignment improvement. Our analyses rejected reciprocal monophyly among Amynthas and Metaphire and indicated that the two genera should be systematically classified into one.

The complete mitochondrial genome sequence of the tubeworm Lamellibrachia satsuma and structural conservation in the mitochondrial genome control regions of Order Sabellida

The control region of the mitochondrial genomes shows high variation in conserved sequence organizations, which follow distinct evolutionary patterns in different species or taxa. In this study, we sequenced the complete mitochondrial genome of Lamellibrachia satsuma from the cold-seep region of Kagoshima Bay, as a part of whole genome study and extensively studied the structural features and patterns of the control region sequences. We obtained 15,037 bp of mitochondrial genome using Illumina sequencing and identified the non-coding AT-rich region or control region (354 bp, AT=83.9%) located between trnH and trnR. We found 7 conserved sequence blocks (CSB), scattered throughout the control region of L. satsuma and other taxa of Annelida. The poly-TA stretches, which commonly form the stem of multiple stem-loop structures, are most conserved in the CSB-I and CSB-II regions. The mitochondrial genome of L. satsuma encodes a unique repetitive sequence in the control region, which forms a unique secondary structure in comparison to Lamellibrachia luymesi. Phylogenetic analyses of all protein-coding genes indicate that L. satsuma forms a monophyletic clade with L. luymesi along with other tubeworms found in cold-seep regions (genera: Lamellibrachia, Escarpia, and Seepiophila). In general, the control region sequences of Annelida could be aligned with certainty within each genus, and to some extent within the family, but with a higher rate of variation in conserved regions.

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.

The complete mitochondrial genome of the polychaete, Goniada japonica (Phyllodocida, Goniadidae)

The study determined the complete mitochondrial genome (mitogenome) of the polychaete, Goniada japonica, which was first reported in the family of Goniadidae. A total of 7162 reads were generated by Illumina HiSeq2500 platform (Illumina Inc., San Diego, CA) with an average depth of 58.41×. The mitogenome of G. japonica was 15,327 bp in size and consists of 37 typical genes (13 protein-coding genes, 2 rRNA genes and 22 tRNA genes) and a putative control region. All the 37 genes were encoded on the heavy strand whose nucleotide compositions were 35.08% of A, 33.69% of T, 11.66% of G, and 19.57% of C, showing a lower content of G + C (31.23%). The gene order of 15 major coding genes was identical to that of the Nereididae species. Phylogenetic analysis showed that G. japonica has a closer relationship with Tylorrhynchus heterochaetus of Nereididae.

Complete mitochondrial genome of Hirudo nipponia (Annelida, Hirudinea)

The complete mitochondrial genome (14,414 bp) of the blood-feeding leech Hirudo nipponia, which was an important natural medicinal resource, was sequenced and characterized. The genome encodes 13 protein-coding genes, 2 rRNAs and 22 tRNAs. The content of A + T was 72.60% for H. nipponia (31.69% A, 40.91% T, 15.45% G and 11.95% C). All protein-coding genes started with ATN except for nad3 and nad5, which used GTG as start codon. Eight protein-coding genes stopped with termination codon TAA. Five protein-coding genes used incomplete stop codon TA or T. The A + T-rich region was located between tRNA-Arg and tRNA-His with a length of 83 bp. This is the first report about completely sequenced mitochondrial genome from the family Hirudinidae. The complete mitochondrial genomes of H. nipponia would be useful for the exploration of Hirudinea polygenetic relationships.

Complete mitochondrial genome of the marine polychaete, Marphysa sanguinea (Polychaeta, Eunicida)

The complete mitochondrial genome of Marphysa sanguinea from northern China was determined. This entire sequence was 15,159 bp in length, containing 13 protein-coding genes (PCGs), 2 rRNA genes, 22 tRNA genes and a putative control region (D-loop). The base composition of the genome was 30.7% A, 30.3% T, 26.5% C and 12.6% G. Some PCGs shared nucleotides with neighboring genes, and all 13 PCGs had ATG as a start codon. Two complete stop codons (TAA, TAG) were used by the PCGs. Compared to other marine polychaetes and oligophaetes, most of the PCGS were without any rearrangement, except for ND2 and ND3, in which the order was reversed.

The impact of paralogy on phylogenomic studies - a case study on annelid relationships

Phylogenomic studies based on hundreds of genes derived from expressed sequence tags libraries are increasingly used to reveal the phylogeny of taxa. A prerequisite for these studies is the assignment of genes into clusters of orthologous sequences. Sophisticated methods of orthology prediction are used in such analyses, but it is rarely assessed whether paralogous sequences have been erroneously grouped together as orthologous sequences after the prediction, and whether this had an impact on the phylogenetic reconstruction using a super-matrix approach. Herein, I tested the impact of paralogous sequences on the reconstruction of annelid relationships based on phylogenomic datasets. Using single-partition analyses, screening for bootstrap support, blast searches and pruning of sequences in the supermatrix, wrongly assigned paralogous sequences were found in eight partitions and the placement of five taxa (the annelids Owenia, Scoloplos, Sthenelais and Eurythoe and the nemertean Cerebratulus) including the robust bootstrap support could be attributed to the presence of paralogous sequences in two partitions. Excluding these sequences resulted in a different, weaker supported placement for these taxa. Moreover, the analyses revealed that paralogous sequences impacted the reconstruction when only a single taxon represented a previously supported higher taxon such as a polychaete family. One possibility of a priori detection of wrongly assigned paralogous sequences could combine 1) a screening of single-partition analyses based on criteria such as nodal support or internal branch length with 2) blast searches of suspicious cases as presented herein. Also possible are a posteriori approaches in which support for specific clades is investigated by comparing alternative hypotheses based on differences in per-site likelihoods. Increasing the sizes of EST libraries will also decrease the likelihood of wrongly assigned paralogous sequences, and in the case of orthology prediction methods like HaMStR it is likewise decreased by using more than one reference taxon.

Mitochondrial genomes to the rescue--Diurodrilidae in the myzostomid trap

Diurodrilidae is a taxon of Lophotrochozoa comprising about six, exclusively interstitial species, which are up to 500μm long and dorsoventrally flattened. Traditionally, Diurodrilidae had been regarded as an annelid family. However, recently Diurodrilidae had been excluded from Annelida and been placed in closer relationship to platyzoan taxa based on both morphological and nuclear rRNA data. Since both, Diurodrilidae and platyzoan taxa, exhibit long branches in the molecular analyses, the close relationship might be due to a long branch attraction artifact. The annelid taxon Myzostomida had been trapped in a similar long branch attraction artifact with platyzoan taxa using nuclear rRNA data, but determination of the nearly complete mitochondrial genome of myzostomids revealed their annelid affinity. Therefore, we determined the nearly complete mitochondrial genome of Diurodrilus subterraneus as well as new nuclear rRNA data for D. subterraneus and some platyzoan taxa. All our analyses of nuclear rRNA and mitochondrial sequence and gene order data presented herein clearly place Diurodrilidae within Annelida and with strong nodal support values in some analyses. Therefore, the previously suggested exclusion of Diurodrilidae from Annelida and its close relationship with platyzoan taxa can be attributed to a long branch artifact. Morphological data do not unambiguously support a platyzoan affinity of Diurodrilidae, but instead would also be in line with a progenetic origin of Diurodrilidae within Annelida.

Cellular and muscular growth patterns during sipunculan development

Sipuncula is a lophotrochozoan taxon with annelid affinities, albeit lacking segmentation of the adult body. Here, we present data on cell proliferation and myogenesis during development of three sipunculan species, Phascolosoma agassizii, Thysanocardia nigra, and Themiste pyroides. The first anlagen of the circular body wall muscles appear simultaneously and not subsequently as in the annelids. At the same time, the rudiments of four longitudinal retractor muscles appear. This supports the notion that four introvert retractors were part of the ancestral sipunculan bodyplan. The longitudinal muscle fibers form a pattern of densely arranged fibers around the retractor muscles, indicating that the latter evolved from modified longitudinal body wall muscles. For a short time interval, the distribution of S-phase mitotic cells shows a metameric pattern in the developing ventral nerve cord during the pelagosphera stage. This pattern disappears close to metamorphic competence. Our findings are congruent with data on sipunculan neurogenesis, as well as with recent molecular analyses that place Sipuncula within Annelida, and thus strongly support a segmental ancestry of Sipuncula.

The complete mitochondrial genome sequence of Whitmania pigra (Annelida, Hirudinea): the first representative from the class Hirudinea

The mitochondrial genome is a significant tool for investigating the evolutionary history of metazoan animals. The currently available mitochondrial genome data in GenBank is limited to understand the detail evolutionary relationship among the metazoan animals, especially in the phylum Annelida. Here we present the mitochondrial gene organization, gene order and codon usage of the leech Whitmania pigra (Annelida), which is the first representative from the class Hirudinea. It is a circular molecule of 14,426bp, and encodes 13 protein-coding genes, 2 ribosomal RNA genes, and 22 transfer RNA genes. All 37 genes of W. pigra mitochondrial genome are transcribed from the same strand, which is identical to studied annelids, two echiurans, two sipunculans and many other lophotrochozoans. Five conserved gene clusters can be found in mitochondrial genomes of nine studied annelids, including (1) cox1-N-cox2; (2) cox3-Q-nad6-cob-W-atp6; (3) H-nad5-F-E-P-T-nad4L-nad4; (4) srRNA-V-lrRNA; and (5) nad3-S(1)-nad2. Compared with that of other studied annelids, translocations of transfer RNAs were found in the gene arrangement of W. pigra mitochondrial genome. Phylogenetic analysis strongly support that the species from Hirudinina and Oligochaeta form a monophyletic group Clitellata (BPM=100, BPP=100), which is consistent with previous research based on morphological and other molecular data. Both gene order data and amino acid sequences reveal that echiurans are derived annelids and sipunculans should be clustered with annelids and echiurans.

MicroRNAs resolve an apparent conflict between annelid systematics and their fossil record

Both the monophyly and inter-relationships of the major annelid groups have remained uncertain, despite intensive research on both morphology and molecular sequences. Morphological cladistic analyses indicate that Annelida is monophyletic and consists of two monophyletic groups, the clitellates and polychaetes, whereas molecular phylogenetic analyses suggest that polychaetes are paraphyletic and that sipunculans are crown-group annelids. Both the monophyly of polychaetes and the placement of sipunculans within annelids are in conflict with the annelid fossil record--the former because Cambrian stem taxa are similar to modern polychaetes in possessing biramous parapodia, suggesting that clitellates are derived from polychaetes; the latter because although fossil sipunculans are known from the Early Cambrian, crown-group annelids do not appear until the latest Cambrian. Here we apply a different data source, the presence versus absence of specific microRNAs--genes that encode approximately 22 nucleotide non-coding regulatory RNAs--to the problem of annelid phylogenetics. We show that annelids are monophyletic with respect to sipunculans, and polychaetes are paraphyletic with respect to the clitellate Lumbricus, conclusions that are consistent with the fossil record. Further, sipunculans resolve as the sister group of the annelids, rooting the annelid tree, and revealing the polarity of the morphological change within this diverse lineage of animals.

The mitochondrial genome structure of Xenoturbella bocki (phylum Xenoturbellida) is ancestral within the deuterostomes

Background

Mitochondrial genome comparisons contribute in multiple ways when inferring animal relationships. As well as primary sequence data, rare genomic changes such as gene order, shared gene boundaries and genetic code changes, which are unlikely to have arisen through convergent evolution, are useful tools in resolving deep phylogenies. Xenoturbella bocki is a morphologically simple benthic marine worm recently found to belong among the deuterostomes. Here we present analyses comparing the Xenoturbella bocki mitochondrial gene order, genetic code and control region to those of other metazoan groups.

Results

The complete mitochondrial genome sequence of Xenoturbella bocki was determined. The gene order is most similar to that of the chordates and the hemichordates, indicating that this conserved mitochondrial gene order might be ancestral to the deuterostome clade. Using data from all phyla of deuterostomes, we infer the ancestral mitochondrial gene order for this clade. Using inversion and breakpoint analyses of metazoan mitochondrial genomes, we test conflicting hypotheses for the phylogenetic placement of Xenoturbella and find a closer affinity to the hemichordates than to other metazoan groups. Comparative analyses of the control region reveal similarities in the transcription initiation and termination sites and origin of replication of Xenoturbella with those of the vertebrates. Phylogenetic analyses of the mitochondrial sequence indicate a weakly supported placement as a basal deuterostome, a result that may be the effect of compositional bias.

Conclusion

The mitochondrial genome of Xenoturbella bocki has a very conserved gene arrangement in the deuterostome group, strikingly similar to that of the hemichordates and the chordates, and thus to the ancestral deuterostome gene order. Similarity to the hemichordates in particular is suggested by inversion and breakpoint analysis. Finally, while phylogenetic analyses of the mitochondrial sequences support a basal deuterostome placement, support for this decreases with the use of more sophisticated models of sequence evolution.

A close phylogenetic relationship between Sipuncula and Annelida evidenced from the complete mitochondrial genome sequence of Phascolosoma esculenta

Background

There are many advantages to the application of complete mitochondrial (mt) genomes in the accurate reconstruction of phylogenetic relationships in Metazoa. Although over one thousand metazoan genomes have been sequenced, the taxonomic sampling is highly biased, left with many phyla without a single representative of complete mitochondrial genome. Sipuncula (peanut worms or star worms) is a small taxon of worm-like marine organisms with an uncertain phylogenetic position. In this report, we present the mitochondrial genome sequence of Phascolosoma esculenta, the first complete mitochondrial genome of the phylum.

Results

The mitochondrial genome of P.esculenta is 15,494 bp in length. The coding strand consists of 32.1% A, 21.5% C, 13.0% G, and 33.4% T bases (AT = 65.5%; AT skew = -0.019; GC skew = -0.248). It contains thirteen protein-coding genes (PCGs) with 3,709 codons in total, twenty-two transfer RNA genes, two ribosomal RNA genes and a non-coding AT-rich region (AT = 74.2%). All of the 37 identified genes are transcribed from the same DNA strand. Compared with the typical set of metazoan mt genomes, sipunculid lacks trnR but has an additional trnM. Maximum Likelihood and Bayesian analyses of the protein sequences show that Myzostomida, Sipuncula and Annelida (including echiurans and pogonophorans) form a monophyletic group, which supports a closer relationship between Sipuncula and Annelida than with Mollusca, Brachiopoda, and some other lophotrochozoan groups.

Conclusion

This is the first report of a complete mitochondrial genome as a representative within the phylum Sipuncula. It shares many more similar features with the four known annelid and one echiuran mtDNAs. Firstly, sipunculans and annelids share quite similar gene order in the mitochondrial genome, with all 37 genes located on the same strand; secondly, phylogenetic analyses based on the concatenated protein sequences also strongly support the sipunculan + annelid clade (including echiurans and pogonophorans). Hence annelid "key-characters" including segmentation may be more labile than previously assumed.

Mitochondrial genome sequence and gene order of Sipunculus nudus give additional support for an inclusion of Sipuncula into Annelida

Background

Mitochondrial genomes are a valuable source of data for analysing phylogenetic relationships. Besides sequence information, mitochondrial gene order may add phylogenetically useful information, too. Sipuncula are unsegmented marine worms, traditionally placed in their own phylum. Recent molecular and morphological findings suggest a close affinity to the segmented Annelida.

Results

The first complete mitochondrial genome of a member of Sipuncula, Sipunculus nudus, is presented. All 37 genes characteristic for metazoan mtDNA were detected and are encoded on the same strand. The mitochondrial gene order (protein-coding and ribosomal RNA genes) resembles that of annelids, but shows several derivations so far found only in Sipuncula. Sequence based phylogenetic analysis of mitochondrial protein-coding genes results in significant bootstrap support for Annelida sensu lato, combining Annelida together with Sipuncula, Echiura, Pogonophora and Myzostomida.

Conclusion

The mitochondrial sequence data support a close relationship of Annelida and Sipuncula. Also the most parsimonious explanation of changes in gene order favours a derivation from the annelid gene order. These results complement findings from recent phylogenetic analyses of nuclear encoded genes as well as a report of a segmental neural patterning in Sipuncula.

The complete mitochondrial genome of Symphylella sp. (Myriapoda: Symphyla): Extensive gene order rearrangement and evidence in favor of Progoneata

We determined the complete 14,667bp mitochondrial DNA sequence of Symphylella sp., the first representative of the Scolopendrellidae (Arthropoda: Myriapoda: Symphyla). With respect to the ancestral arthropod mitochondrial gene order, two protein-coding genes, the rRNAs and 10 of the tRNAs appear to be rearranged. This rearrangement is novel in the arthropods and genes with identical transcriptional polarity are clustered except for trnE, trnN and putative control region (CR), resembling two previously reported diplopod genomes. A duplication/loss (random and non-random)-recombination model was proposed to account for the generation of the gene order in Symphylella sp. All phylogenetic analysis yielded strong support for a clade of Symphyla plus Diplopoda, i.e., Progoneata. However, the phylogenetic position of Myriapoda within Arthropoda remains unclear. The amino acid dataset gives strong support for an affinity to Pancrustacea, while the nucleotide dataset weakly supports Myriapoda grouped with Chelicerata.

Phylogenetic information from three mitochondrial genomes of Terebelliformia (Annelida) worms and duplication of the methionine tRNA

Mitochondrial genomes have been useful for inferring animal phylogeny across a wide range of clades, however they are still poorly sampled in some animal taxa, limiting our knowledge of mtDNA evolution. For example, despite being one of the most diverse animal phyla, only 5 complete annelid mitochrondial genomes have been published. To address this paucity of information, we obtained complete mitochondrial genomic sequences from Pista cristata (Terebellidae) and Terebellides stroemi (Trichobranchidae) as well as one nearly complete mitochondrial genome from Eclysippe vanelli (Ampharetidae). These taxa are within Terebelliformia (Annelida), which include spaghetti worms, icecream cone worms and their relatives. In contrast to the 37 genes found in most bilaterian metazoans, we recover 38 genes in the mitochondrial genomes of T. stroemi and P. cristata due to the presence of a second methionine tRNA (trnM). Interestingly, the two trnMs are located next to each other and are possibly a synapomorphy of these two taxa. The E. vanelli partial mitochondrial genome lacks this additional trnM at the same position, but it may be present in the region not sampled. Compared to other annelids, gene orders of these three mitochondrial genomes are generally conserved except for the atp6-mSSU region. Phylogenetic analyses reveal that mtDNA data strongly supports a Trichobranchidae/Terebellidae clade.

Annelid phylogeny and the status of Sipuncula and Echiura

BACKGROUND

Annelida comprises an ancient and ecologically important animal phylum with over 16,500 described species and members are the dominant macrofauna of the deep sea. Traditionally, two major groups are distinguished: Clitellata (including earthworms, leeches) and "Polychaeta" (mostly marine worms). Recent analyses of molecular data suggest that Annelida may include other taxa once considered separate phyla (i.e., Echiura, and Sipuncula) and that Clitellata are derived annelids, thus rendering "Polychaeta" paraphyletic; however, this contradicts classification schemes of annelids developed from recent analyses of morphological characters. Given that deep-level evolutionary relationships of Annelida are poorly understood, we have analyzed comprehensive datasets based on nuclear and mitochondrial genes, and have applied rigorous testing of alternative hypotheses so that we can move towards the robust reconstruction of annelid history needed to interpret animal body plan evolution.

RESULTS

Sipuncula, Echiura, Siboglinidae, and Clitellata are all nested within polychaete annelids according to phylogenetic analyses of three nuclear genes (18S rRNA, 28S rRNA, EF1alpha; 4552 nucleotide positions analyzed) for 81 taxa, and 11 nuclear and mitochondrial genes for 10 taxa (additional: 12S rRNA, 16S rRNA, ATP8, COX1-3, CYTB, NAD6; 11,454 nucleotide positions analyzed). For the first time, these findings are substantiated using approximately unbiased tests and non-scaled bootstrap probability tests that compare alternative hypotheses. For echiurans, the polychaete group Capitellidae is corroborated as the sister taxon; while the exact placement of Sipuncula within Annelida is still uncertain, our analyses suggest an affiliation with terebellimorphs. Siboglinids are in a clade with other sabellimorphs, and clitellates fall within a polychaete clade with aeolosomatids as their possible sister group. None of our analyses support the major polychaete clades reflected in the current classification scheme of annelids, and hypothesis testing significantly rejects monophyly of Scolecida, Palpata, Canalipalpata, and Aciculata.

CONCLUSION

Using multiple genes and explicit hypothesis testing, we show that Echiura, Siboglinidae, and Clitellata are derived annelids with polychaete sister taxa, and that Sipuncula should be included within annelids. The traditional composition of Annelida greatly underestimates the morphological diversity of this group, and inclusion of Sipuncula and Echiura implies that patterns of segmentation within annelids have been evolutionarily labile. Relationships within Annelida based on our analyses of multiple genes challenge the current classification scheme, and some alternative hypotheses are provided.

Mitochondrial sequence data expose the putative cosmopolitan polychaete Scoloplos armiger (Annelida, Orbiniidae) as a species complex

BACKGROUND

Polychaetes assigned as Scoloplos armiger (Orbiniidae) show a cosmopolitan distribution and have been encountered in all zoogeographic regions. Sibling S. armiger-like species have been revealed by recent studies using RAPDs and AFLP genetic data. We sequenced an approximately 12 kb fragment of the Scoloplos cf. armiger mitochondrial genome and developed primers for variable regions including the 3' end of the cox3 gene, trnQ, and most of nad6. A phylogenetic analysis of this 528-nucleotide fragment was carried out for S. armiger-like individuals from the Eastern North Atlantic as well as Pacific regions. The aim of this study is to test the cosmopolitan status, as well as to clarify the systematics of this species complex in the Eastern North Atlantic, while using a few specimens from the Pacific Ocean for comparision.

RESULTS

Phylogenetic analysis of the cox3-trnQ-nad6 data set recovered five different clades of Scoloplos cf. armiger. The fragment of the mitochondrial genome of Scoloplos cf. armiger is 12,042 bp long and contains 13 protein coding genes, 15 of the 22 expected tRNAs, and the large ribosomal subunit (rrnl).

CONCLUSION

The sequenced cox3-trnQ-nad6 fragment proved to be very useful in phylogenetic analyses of Scoloplos cf. armiger. Due to its larger sampling scale this study goes beyond previous analyses which used RAPD and AFLP markers. The results of this study clearly supports that Scoloplos armiger represents a species complex and not a cosmopolitan species. We find at least two S. armiger-like species within the Pacific region and three different S. armiger-like species in the North Atlantic. Implications for the taxonomy and the impact on ecological studies are discussed.