Orthonectids Are Highly Degenerate Annelid Worms

Evolution of Orthonectida body plan

Orthonectida is an enigmatic group of animals with still uncertain phylogenetic position. Orthonectids parasitize various marine invertebrates. Their life cycle comprises a parasitic plasmodium and free-living males and females. Sexual individuals develop inside the plasmodium; after egress from the host they copulate in the external environment, and the larva, which has developed inside the female infects a new host. In a series of studied orthonectid species simplification of free-living sexual individuals can be clearly traced. The number of longitudinal and transverse muscle fibers is gradually reduced. In the nervous system, simplification is even more pronounced. The number of neurons constituting the ganglion is dramatically reduced from 200 in Rhopalura ophiocomae to 4-6 in Intoshia variabili. The peripheral nervous system undergoes gradual simplification as well. The morphological simplification is accompanied with genome reduction. However, not only genes are lost from the genome, it also undergoes compactization ensured by extreme reduction of intergenic distances, short intron sizes, and elimination of repetitive elements. The main trend in orthonectid evolution is simplification and miniaturization of free-living sexual individuals coupled with reduction and compactization of the genome.

RNA-seq analysis of parasitism by Intoshia linei (Orthonectida) reveals protein effectors of defence, communication, feeding and growth

Orthonectida is a group of multicellular endoparasites of a wide range of marine invertebrates. Their parasitic stage is a multinuclear shapeless plasmodium infiltrating host tissues. The development of the following worm-like sexual generation takes place within the cytoplasm of the plasmodium. The existence of the plasmodial stage and the development of a sexual stage within the plasmodium are unique features to Bilateria. However, the molecular mechanisms that maintain this peculiar organism, and hence enable parasitism in orthonectids, are unknown. Here, we present the first-ever RNA-seq analysis of the plasmodium, aimed at the identification and characterization of the plasmodium-specific protein-coding genes and corresponding hypothetical proteins that distinguish the parasitic plasmodium stage from the sexual stage of the orthonectid Intoshia linei Giard, 1877, parasite of nemertean Lineus ruber Müller, 1774. We discovered 119 plasmodium-specific proteins, 82 of which have inferred functions based on known domains. Thirty-five of the detected proteins are orphans, at least part of which may reflect the unique evolutionary adaptations of orthonectids to parasitism. Some of the identified proteins are known effector molecules of other endoparasites suggesting convergence. Our data indicate that the plasmodium-specific proteins might be involved in the plasmodium defense against the host, host-parasite communication, feeding and nutrient uptake, growth within the host, and support of the sexual stage development. These molecular processes in orthonectids have not been described before, and the particular protein effectors remained unknown until now.

Spiralian genomics and the evolution of animal genome architecture

Recent developments in sequencing technologies have greatly improved our knowledge of phylogenetic relationships and genomic architectures throughout the tree of life. Spiralia, a diverse clade within Protostomia, is essential for understanding the evolutionary history of parasitism, gene conversion, nervous systems and animal body plans. In this review, we focus on the current hypotheses of spiralian phylogeny and investigate the impact of long-read sequencing on the quality of genome assemblies. We examine chromosome-level assemblies to highlight key genomic features that have driven spiralian evolution, including karyotype, synteny and the Hox gene organization. In addition, we show how chromosome rearrangement has influenced spiralian genomic structures. Although spiralian genomes have undergone substantial changes, they exhibit both conserved and lineage-specific features. We recommend increasing sequencing efforts and expanding functional genomics research to deepen insights into spiralian biology.

Plasmodium structure of Intoshia linei (Orthonectida)

Orthonectids are enigmatic parasitic bilaterians whose exact position on the phylogenetic tree is still uncertain. Despite ongoing debate about their phylogenetic position, the parasitic stage of orthonectids known as "plasmodium" remains underexplored. There is still no consensus on the origin of the plasmodium: whether it is an altered host cell or a parasitic organism that develops in the host extracellular environment. To determine the origin of the orthonectid parasitic stage, we studied in detail the fine structure of the Intoshia linei orthonectid plasmodium using a variety of morphological methods. The orthonectid plasmodium is a shapeless multinucleated organism separated from host tissues by a double membrane envelope. Besides numerous nuclei, its cytoplasm contains organelles typical for other bilaterians, reproductive cells, and maturing sexual specimens. Reproductive cells, as well as developing orthonectid males and females, are covered by an additional membrane. The plasmodium forms protrusions directed to the surface of the host body and used by mature individuals for egress from the host. The obtained results indicate that the orthonectid plasmodium is an extracellular parasite. A possible mechanism for its formation might involve spreading parasitic larva cells across the host tissues with subsequent generation of a cell-within-cell complex. The cytoplasm of the plasmodium originates from the outer cell, which undergoes multiple nuclear divisions without cytokinesis, while the inner cell divides, giving rise to reproductive cells and embryos. The term "plasmodium" should be avoided and the term "orthonectid plasmodium" could be temporarily used instead.

Renewed perspectives on the sedentary-pelagic last common bilaterian ancestor

Various evaluations of the last common bilaterian ancestor (lcba) currently suggest that it resembled either a microscopic, non-segmented motile adult; or, on the contrary, a complex segmented adult motile urbilaterian. These fundamental inconsistencies remain largely unexplained. A majority of multidisciplinary data regarding sedentary adult ancestral bilaterian organization is overlooked. The sedentary-pelagic model is supported now by a number of novel developmental, paleontological and molecular phylogenetic data: (1) data in support of sedentary sponges, in the adult stage, as sister to all other Metazoa; (2) a similarity of molecular developmental pathways in both adults and larvae across sedentary sponges, cnidarians, and bilaterians; (3) a cnidarian-bilaterian relationship, including a unique sharing of a bona fide Hox-gene cluster, of which the evolutionary appearance does not connect directly to a bilaterian motile organization; (4) the presence of sedentary and tube-dwelling representatives of the main bilaterian clades in the early Cambrian; (5) an absence of definite taxonomic attribution of Ediacaran taxa reconstructed as motile to any true bilaterian phyla; (6) a similarity of tube morphology (and the clear presence of a protoconch-like apical structure of the Ediacaran sedentary Cloudinidae) among shells of the early Cambrian, and later true bilaterians, such as semi-sedentary hyoliths and motile molluscs; (7) recent data that provide growing evidence for a complex urbilaterian, despite a continuous molecular phylogenetic controversy. The present review compares the main existing models and reconciles the sedentary model of an urbilaterian and the model of a larva-like lcba with a unified sedentary(adult)-pelagic(larva) model of the lcba.

Different phylogenomic methods support monophyly of enigmatic 'Mesozoa' (Dicyemida + Orthonectida, Lophotrochozoa)

Dicyemids and orthonectids were traditionally classified in a group called Mesozoa, but their placement in a single clade has been contested and their position(s) within Metazoa is uncertain. Here, we assembled a comprehensive matrix of Lophotrochozoa (Metazoa) and investigated the position of Dicyemida (= Rhombozoa) and Orthonectida, employing multiple phylogenomic approaches. We sequenced seven new transcriptomes and one draft genome from dicyemids (Dicyema, Dicyemennea) and two transcriptomes from orthonectids (Rhopalura). Using these and published data, we assembled and analysed contamination-filtered datasets with up to 987 genes. Our results recover Mesozoa monophyletic and as a close relative of Platyhelminthes or Gnathifera. Because of the tendency of the long-branch mesozoans to group with other long-branch taxa in our analyses, we explored the impact of approaches purported to help alleviate long-branch attraction (e.g. taxon removal, coalescent inference, gene targeting). None of these were able to break the association of Orthonectida with Dicyemida in the maximum-likelihood trees. Contrastingly, the Bayesian analysis and site-specific frequency model in maximum-likelihood did not recover a monophyletic Mesozoa (but only when using a specific 50 gene matrix). The classic hypothesis on monophyletic Mesozoa is possibly reborn and should be further tested.

Endless forms most stupid, icky, and small: The preponderance of noncharismatic invertebrates as integral to a biologically sound view of life

Big, beautiful organisms are useful for biological education, increasing evolution literacy, and biodiversity conservation. But if educators gloss over the ubiquity of streamlined and miniaturized organisms, they unwittingly leave students and the public vulnerable to the idea that the primary evolutionary plot of every metazoan lineage is "progressive" and "favors" complexity. We show that simple, small, and intriguingly repulsive invertebrate animals provide a counterpoint to misconceptions about evolution. Our examples can be immediately deployed in biology courses and outreach. This context emphasizes that chordates are not the pinnacle of evolution. Rather, in the evolution of animals, miniaturization, trait loss, and lack of perfection are at least as frequent as their opposites. Teaching about invertebrate animals in a "tree thinking" context uproots evolution misconceptions (for students and the public alike), provides a mental scaffold for understanding all animals, and helps to cultivate future ambassadors and experts on these little-known, weird, and fascinating taxa.

Characterization of the interactions between Codanin-1 and C15Orf41, two proteins implicated in congenital dyserythropoietic anemia type I disease

Background

Congenital dyserythropoietic anemia type I (CDA I), is an autosomal recessive disease with macrocytic anemia in which erythroid precursors in the bone marrow exhibit pathognomonic abnormalities including spongy heterochromatin and chromatin bridges. We have shown previously that the gene mutated in CDA I encodes Codanin-1, a ubiquitously expressed and evolutionarily conserved large protein. Recently, an additional etiologic factor for CDA I was reported, C15Orf41, a predicted nuclease. Mutations in both CDAN1 and C15Orf41 genes results in very similar erythroid phenotype. However, the possible relationships between these two etiologic factors is not clear.

Results

We demonstrate here that Codanin-1 and C15Orf41 bind to each other, and that Codanin-1 stabilizes C15Orf41. C15Orf41 protein is mainly nuclear and Codanin-1 overexpression shifts it to the cytoplasm. Phylogenetic analyses demonstrated that even though Codanin-1 is an essential protein in mammals, it was lost from several diverse and unrelated animal taxa. Interestingly, C15Orf41 was eliminated in the exact same animal taxa. This is an extreme case of the Phylogenetic Profiling phenomenon, which strongly suggests common pathways for these two proteins. Lastly, as the 3D structure is more conserved through evolution than the protein sequence, we have used the Phyre2 alignment program to find structurally homologous proteins. We found that Codanin-1 is highly similar to CNOT1, a conserved protein which serves as a scaffold for proteins involved in mRNA stability and transcriptional control.

Conclusions

The physical interaction and the stabilization of C15Orf41 by Codanin-1, combined with the phylogenetic co-existence and co-loss of these two proteins during evolution, suggest that the major function of the presumptive scaffold protein, Codanin-1, is to regulate C15Orf41 activities. The similarity between Codanin-1 and CNOT1 suggest that Codanin-1 is involved in RNA metabolism and activity, and opens up a new avenue for the study of the molecular pathways affected in CDAI.

Extreme Genome and Nervous System Streamlining in the Invertebrate Parasite Intoshia variabili

Orthonectida is a small, rare, and in many aspects enigmatic group of organisms with a unique life cycle and a highly simplified adult free-living stage parasitizing various marine invertebrates [1, 2]. Phylogenetic relationships of Orthonectida have remained controversial for a long time. According to recent data, they are close to Annelida, specifically to Clitellata [3-5]. Several studies have shown that parasitism can not only lead to a dramatic reduction of the body plan and morphological structures but also affect organisms at the genomic level [6, 7]. Comparative studies of parasites and closely related non-parasitic species could clarify the genome reduction degree and evolution of parasitism. Here, we report on the morphology, genome structure, and content of the smallest known Orthonectida species Intoshia variabili, inhabiting the flatworm Graffiellus croceus. This orthonectid with an extremely simplified nervous system demonstrates the smallest known genome (15.3 Mbp) and one of the lowest reported so far gene numbers (5,120 protein-coding genes) among metazoans. The genome is extremely compact, due to a significant reduction of gene number, intergenic regions, intron length, and repetitive elements. The small genome size is probably a result of extreme genome reduction due to their parasitic lifestyle, as well as of simplification and miniaturization of the free-living stages. Our data could provide further insights into the evolution of parasitism and could help to define a minimal bilaterian gene set.

Computational discovery of hidden breaks in 28S ribosomal RNAs across eukaryotes and consequences for RNA Integrity Numbers

In some eukaryotes, a 'hidden break' has been described in which the 28S ribosomal RNA molecule is cleaved into two subparts. The break is common in protostome animals (arthropods, molluscs, annelids etc.), but a break has also been reported in some vertebrates and non-metazoan eukaryotes. We present a new computational approach to determine the presence of the hidden break in 28S rRNAs using mapping of RNA-Seq data. We find a homologous break is present across protostomes although it has been lost in a small number of taxa. We show that rare breaks in vertebrate 28S rRNAs are not homologous to the protostome break. A break is found in just 4 out of 331 species of non-animal eukaryotes studied and, in three of these, the break is located in the same position as the protostome break suggesting a striking instance of convergent evolution. RNA Integrity Numbers (RIN) rely on intact 28S rRNA and will be consistently underestimated in the great majority of animal species with a break.

Gene expression profiles of dicyemid life-cycle stages may explain how dispersing larvae locate new hosts

Metazoans have evolved a great variety of life histories in response to environmental conditions. A unique example is encountered in dicyemid mesozoans. In addition to a highly simplified adult body comprising only ~ 30 cells, dicyemids exhibit a parasitic lifestyle that includes nematogens (asexual reproductive adults), rhombogens (sexual reproductive adults), vermiform larvae generated by nematogens, and infusoriform larvae generated by rhombogens. However, due to the difficulties of observing microscopic endoparasites, the complex life cycle and biological functions of life-cycle stages of dicyemids have remained mysterious. Taking advantage of the recently decoded genome of Dicyema japonicum, we examined genes that undergird this lifestyle. Using stage-specific gene expression profiles, we found that biological processes associated with molecular transport, developmental regulation, and sensory response are specified at different stages. Together with the expression of potential neurotransmitters, we further suggest that apical cells in infusoriform larva probably serve sensory functions, although dicyemids have no nervous system. Gene expression profiles show that more genes are expressed in free-living infusoriform larvae than in the other three stages, and that some of these genes are likely involved in locating new hosts. These data provide molecular information about the unique lifestyle of dicyemids and illustrate how an extremely simplified endoparasite adapted and retained gene sets and morphological characters to complete its life cycle.

SUPPLEMENTARY INFORMATION

Supplementary information accompanies this paper at 10.1186/s40851-019-0146-y.

Morphology of the nervous system of monogonont rotifer Epiphanes senta with a focus on sexual dimorphism between feeding females and dwarf males

Background

Monogononta is a large clade of rotifers comprised of diverse morphological forms found in a wide range of ecological habitats. Most monogonont species display cyclical parthenogenesis, where generations of asexually reproducing females are interspaced by mixis events when sexual reproduction occurs between mictic females and dwarf, haploid males. The morphology of monogonont feeding females is relatively well described, however data on male anatomy are very limited. Thus far, male musculature of only two species has been described with confocal laser scanning microscopy (CLSM) and it remains unknown how dwarfism influences the neuroanatomy of males on detailed level.

Results

Here, we provide a CLSM-based description of the nervous system of both sexes of Epiphanes senta, a freshwater monogonont rotifer. The general nervous system architecture is similar between males and females and shows a similar level of complexity. However, the nervous system in males is more compact and lacks a stomatogastric part.

Conclusion

Comparison of the neuroanatomy between male and normal-sized feeding females provides a better understanding of the nature of male dwarfism in Monogononta. We propose that dwarfism of monogonont non-feeding males is the result of a specific case of heterochrony, called “proportional dwarfism” as they, due to their inability to feed, retain a juvenile body size, but still develop a complex neural architecture comparable to adult females. Reduction of the stomatogastric nervous system in the males correlates with the loss of the entire digestive tract and associated morphological structures.

Electronic supplementary material

The online version of this article (10.1186/s12983-019-0334-9) contains supplementary material, which is available to authorized users.

Revisiting metazoan phylogeny with genomic sampling of all phyla

Proper biological interpretation of a phylogeny can sometimes hinge on the placement of key taxa-or fail when such key taxa are not sampled. In this light, we here present the first attempt to investigate (though not conclusively resolve) animal relationships using genome-scale data from all phyla. Results from the site-heterogeneous CAT + GTR model recapitulate many established major clades, and strongly confirm some recent discoveries, such as a monophyletic Lophophorata, and a sister group relationship between Gnathifera and Chaetognatha, raising continued questions on the nature of the spiralian ancestor. We also explore matrix construction with an eye towards testing specific relationships; this approach uniquely recovers support for Panarthropoda, and shows that Lophotrochozoa (a subclade of Spiralia) can be constructed in strongly conflicting ways using different taxon- and/or orthologue sets. Dayhoff-6 recoding sacrifices information, but can also reveal surprising outcomes, e.g. full support for a clade of Lophophorata and Entoprocta + Cycliophora, a clade of Placozoa + Cnidaria, and raising support for Ctenophora as sister group to the remaining Metazoa, in a manner dependent on the gene and/or taxon sampling of the matrix in question. Future work should test the hypothesis that the few remaining uncertainties in animal phylogeny might reflect violations of the various stationarity assumptions used in contemporary inference methods.

A nemertean excitatory peptide/CCHamide regulates ciliary swimming in the larvae of Lineus longissimus

Background

The trochozoan excitatory peptide (EP) and its ortholog, the arthropod CCHamide, are neuropeptides that are only investigated in very few animal species. Previous studies on different trochozoan species focused on their physiological effect in adult specimens, demonstrating a myo-excitatory effect, often on tissues of the digestive system. The function of EP in the planktonic larvae of trochozoans has not yet been studied.

Results

We surveyed transcriptomes from species of various spiralian (Orthonectida, Nemertea, Brachiopoda, Entoprocta, Rotifera) and ecdysozoan taxa (Tardigrada, Onychophora, Priapulida, Loricifera, Nematomorpha) to investigate the evolution of EPs/CCHamides in protostomes. We found that the EPs of several pilidiophoran nemerteans show a characteristic difference in their C-terminus. Deorphanization of a pilidiophoran EP receptor showed, that the two splice variants of the nemertean Lineus longissimus EP activate a single receptor. We investigated the expression of EP in L. longissimus larvae and juveniles with customized antibodies and found that EP positive nerves in larvae project from the apical organ to the ciliary band and that EP is expressed more broadly in juveniles in the neuropil and the prominent longitudinal nerve cords. While exposing juvenile L. longissimus specimens to synthetic excitatory peptides did not show any obvious effect, exposure of larvae to either of the two EPs increased the beat frequency of their locomotory cilia and shifted their vertical swimming distribution in a water column upwards.

Conclusion

Our results show that EP/CCHamide peptides are broadly conserved in protostomes. We show that the EP increases the ciliary beat frequency of L. longissimus larvae, which shifts their vertical distribution in a water column upwards. Endogenous EP may be released at the ciliary band from the projections of apical organ EP positive neurons to regulate ciliary beating. This locomotory function of EP in L. longissimus larvae stands in contrast to the repeated association of EP/CCHamides with its myo-excitatory effect in adult trochozoans and the general association with the digestive system in many protostomes.

Electronic supplementary material

The online version of this article (10.1186/s12983-019-0326-9) contains supplementary material, which is available to authorized users.

Dicyemida and Orthonectida: Two Stories of Body Plan Simplification

Two enigmatic groups of morphologically simple parasites of invertebrates, the Dicyemida (syn. Rhombozoa) and the Orthonectida, since the 19th century have been usually considered as two classes of the phylum Mesozoa. Early molecular evidence suggested their relationship within the Spiralia (=Lophotrochozoa), however, high rates of dicyemid and orthonectid sequence evolution led to contradicting phylogeny reconstructions. Genomic data for orthonectids revealed that they are highly simplified spiralians and possess a reduced set of genes involved in metazoan development and body patterning. Acquiring genomic data for dicyemids, however, remains a challenge due to complex genome rearrangements including chromatin diminution and generation of extrachromosomal circular DNAs, which are reported to occur during the development of somatic cells. We performed genomic sequencing of one species of Dicyema, and obtained transcriptomic data for two Dicyema spp. Homeodomain (homeobox) transcription factors, G-protein-coupled receptors, and many other protein families have undergone a massive reduction in dicyemids compared to other animals. There is also apparent reduction of the bilaterian gene complements encoding components of the neuromuscular systems. We constructed and analyzed a large dataset of predicted orthologous proteins from three species of Dicyema and a set of spiralian animals including the newly sequenced genome of the orthonectid Intoshia linei. Bayesian analyses recovered the orthonectid lineage within the Annelida. In contrast, dicyemids form a separate clade with weak affinity to the Rouphozoa (Platyhelminthes plus Gastrotricha) or (Entoprocta plus Cycliophora) suggesting that the historically proposed Mesozoa is a polyphyletic taxon. Thus, dramatic simplification of body plans in dicyemids and orthonectids, as well as their intricate life cycles that combine metagenesis and heterogony, evolved independently in these two lineages.

Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria

Xenoturbella and the acoelomorph worms (Xenacoelomorpha) are simple marine animals with controversial affinities. They have been placed as the sister group of all other bilaterian animals (Nephrozoa hypothesis), implying their simplicity is an ancient characteristic [1, 2]; alternatively, they have been linked to the complex Ambulacraria (echinoderms and hemichordates) in a clade called the Xenambulacraria [3-5], suggesting their simplicity evolved by reduction from a complex ancestor. The difficulty resolving this problem implies the phylogenetic signal supporting the correct solution is weak and affected by inadequate modeling, creating a misleading non-phylogenetic signal. The idea that the Nephrozoa hypothesis might be an artifact is prompted by the faster molecular evolutionary rate observed within the Acoelomorpha. Unequal rates of evolution are known to result in the systematic artifact of long branch attraction, which would be predicted to result in an attraction between long-branch acoelomorphs and the outgroup, pulling them toward the root [6]. Other biases inadequately accommodated by the models used can also have strong effects, exacerbated in the context of short internal branches and long terminal branches [7]. We have assembled a large and informative dataset to address this problem. Analyses designed to reduce or to emphasize misleading signals show the Nephrozoa hypothesis is supported under conditions expected to exacerbate errors, and the Xenambulacraria hypothesis is preferred in conditions designed to reduce errors. Our reanalyses of two other recently published datasets [1, 2] produce the same result. We conclude that the Xenacoelomorpha are simplified relatives of the Ambulacraria.

Comparative analysis of the mitochondrial genomes of Orthonectida: insights into the evolution of an invertebrate parasite species

Among invertebrates, only a few groups still have uncertain phylogenetic position, Orthonectida, a small group of rare multi-cellular parasites of marine invertebrates, being one of them. Recent molecular and morphological findings suggest that orthonectids belong to Lophotrochozoa and are close to Annelida. Nevertheless, phylogenetic relationships between orthonectids and annelids are unclear, and the phylogeny within the group itself has never been studied. Sequencing of mitochondrial genomes is used here to clarify this issue. Complete mt genomes of the orthonectids Intoshia variabili and Rhopalura litoralis were characterized and compared with Intoshia linei mt genome. Our results show that Orthonectida mt genomes have undergone reduction and gene loss, and that they have complicated organization revealed in strand asymmetry in nucleotide composition, in some features of intergenic non-coding regions, tRNA duplication and folding. Moreover, all species of Orthonectida have a unique gene order with complicated rearrangement landscape. Significant differences in mitochondrial genomes in the three orthonectid species could be explained by the fact that their host species belong to different taxa (flat worms, nemertines and gastropods). Among the analyzed mt genomes of Orthonectida, I. linei possesses the closest gene order to the ancestral genome. All Orthonectida species are monophyletic, and in the phylogenetic tree are close to Pleistoannelida, and specifically, to Clitellata.

A New Species of Orthonectida That Parasitizes Xenoturbella bocki: Implications for Studies on Xenoturbella

Orthonectida is a phylum of marine invertebrates known to parasitize many invertebrate animals. Because of its simple body plan, it was suggested that it belong to Mesozoa, together with Dicyemida, and that it represent the evolutionary step between unicellular organisms and multicellular animals. Recent studies, including analyses of its genomes, have clarified its phylogenetic position as a member of the Protostomia, but details such as the species diversity within the phylum and how it infects the host remain unknown. Here we report orthonectids discovered from the marine worm Xenoturbella bocki. Orthonectids were found from sections of four xenoturbellid specimens, collected eight years apart. Live females were also discovered on three separate occasions. These recurring instances of orthonectids found from Xenoturbella show that they are parasitic to the animal and not just chance contaminations. Based on morphological characters such as the presence of sexual dimorphism, the arrangement of oocytes within the female body, and the presence of crystalline inclusions in the male epidermal cells, we regard this orthonectid as a new species, Rhopalura xenoturbellae sp. nov. Since orthonectids are present within the xenoturbellid adult body, caution is needed when interpreting morphological, molecular, and experimental data from X. bocki. Further studies on R. xenoturbellae will yield important information on the fundamental biological details of orthonectids that remain unknown.

On the origin of leeches by evolution of development

Leeches are a unique group of annelids arising from an ancestor that would be characterized as a freshwater oligochaete worm. Comparative biology of the oligochaetes and the leeches reveals that body plan changes in the oligochaete-to-leech transition probably occurred by addition or modification of the terminal steps in embryonic development and that they were likely driven by a change in the feeding behavior in the ancestor of leeches. In this review article, developmental changes that are associated with the evolution of several leech-specific traits are discussed. These include (1) the evolution of suckers, (2) the loss of chaetae, (3) the loss of septa, and (4) a fixed number of segments. An altered developmental fate of the teloblast is further proposed to be a key factor contributing to the fixation of the segment number, and the evolutionary change in teloblast development may also account for the loss of the ability to regenerate the lost body segments in the leech.

The mitochondrial genomes of the mesozoans Intoshia linei, Dicyema sp. and Dicyema japonicum

The Dicyemida and Orthonectida are two groups of tiny, simple, vermiform parasites that have historically been united in a group named the Mesozoa. Both Dicyemida and Orthonectida have just two cell layers and appear to lack any defined tissues. They were initially thought to be evolutionary intermediates between protozoans and metazoans but more recent analyses indicate that they are protostomian metazoans that have undergone secondary simplification from a complex ancestor. Here we describe the first almost complete mitochondrial genome sequence from an orthonectid, Intoshia linei, and describe nine and eight mitochondrial protein-coding genes from Dicyema sp. and Dicyema japonicum, respectively. The 14,247 base pair long I. linei sequence has typical metazoan gene content, but is exceptionally AT-rich, and has a unique gene order. The data we have analysed from the Dicyemida provide very limited support for the suggestion that dicyemid mitochondrial genes are found on discrete mini-circles, as opposed to the large circular mitochondrial genomes that are typical of the Metazoa. The cox1 gene from dicyemid species has a series of conserved, in-frame deletions that is unique to this lineage. Using cox1 genes from across the genus Dicyema, we report the first internal phylogeny of this group.