The first comprehensive molecular phylogeny of Bryozoa (Ectoprocta) based on combined analyses of nuclear and mitochondrial genes

Transcriptomic Landscape of Polypide Development in the Freshwater Bryozoan Cristatella mucedo: From Budding to Degeneration

Colonial invertebrates consist of iterative semi-autonomous modules (usually termed zooids) whose lifespan is significantly shorter than that of the entire colony. Typically, module development begins with budding and ends with degeneration. Most studies on the developmental biology of colonial invertebrates have focused on blastogenesis, whereas the changes occurring throughout the entire zooidal life were examined only for a few tunicates. Here we provide the first description of transcriptomic changes during polypide development in the freshwater bryozoan Cristatella mucedo. For the first time for Bryozoa, we performed bulk RNA sequencing of six polypide stages in C. mucedo (buds, juvenile polypides, three mature stages, and degeneration stage) and generated a high-quality de novo reference transcriptome. Based on these data, we analyzed clusters of differentially expressed genes for enriched pathways and biological processes that may be involved in polypide budding, growth, active functioning, and degradation. Although stem cells have never been described in Bryozoa, our analysis revealed the expression of conservative "stemness" markers in developing buds and juvenile polypides. Our data also indicate that polypide degeneration is a complex regulated process involving autophagy and other types of programmed cell death. We hypothesize that the mTOR signaling pathway plays an important role in regulating the polypide lifespan.

Colonial Entoproct Epibiotic on a Sea Spider

We report an entoproct epibiotic on the surface of a sea spider (Pycnogonida). The pycnogonid was identified as Nymphon sp. (Nymphonidae). The entoproct was colonial, with three zooids, and was identified both morphologically and by a molecular phylogeny as Barentsia sp. (Barentsiidae). The largest zooid had eight tentacles and was about 0.7 mm long, smaller than for most colonial entoproct species. We determined partial sequences for the 18S rRNA and 28S rRNA genes from the entoproct. In an 18S-based maximum likelihood tree (1507 characters), the entoproct was the sister taxon to Barentsia gracilis.

First soft body morphological data on the tracemaker of the endolithic bryozoan trace fossil Terebripora ramosa d'Orbigny, 1842

Terebriporidae is one of the four extant endolithic ctenostome bryozoan families, with colonies immersed into carbonate substrates like molluscan shells. This monogeneric family comprises 17 species, with 11 extant and 6 fossil species. It is currently considered closely related to vesicularioid ctenostomes, a group characterized by colonies interconnected by polymorphic stolons and a distinct gizzard as part of their digestive systems. However, confusion persists regarding the correct species identities and affiliations of many terebriporid species, and even the description of the entire family is based solely on a few external features of their boring traces, rendering the family an ichnotaxon (trace fossil). Our molecular analysis does not support a vesicularioid affinity, but corroborate a close relationship to Immergentia, another genus of boring bryozoans. Consequently, this study aims to untangle the systematic confusion surrounding Terebriporidae by examining the tracemaker of the type species of the family, Terebripora ramosa from Chile, and investigating its morphology and histology using modern techniques. The current analysis could not confirm typical vesicularioid characters such as a gizzard or true polymorphic stolons. Instead, all characters point towards a closer relationship to Immergentiidae as suggested by a recent molecular phylogeny. In fact, these two taxa share several characters such as cystid appendages and duplicature bands, and appear closely related, with the only difference being a characteristic vane with tubulets present in the tracemaker of T. ramosa. The sister-group relationship of the tracemaker and the genus Immergentia infers that these borers share a common boring ancestor, but also emphasizes that additional species from the ichnogenus Terebripora need to be studied for more clarity.

Phoronida-A small clade with a big role in understanding the evolution of lophophorates

Phoronids, together with brachiopods and bryozoans, form the animal clade Lophophorata. Modern lophophorates are quite diverse-some can biomineralize while others are soft-bodied, they could be either solitary or colonial, and they develop through various eccentric larval stages that undergo different types of metamorphoses. The diversity of this clade is further enriched by numerous extinct fossil lineages with their own distinct body plans and life histories. In this review, I discuss how data on phoronid development, genetics, and morphology can inform our understanding of lophophorate evolution. The actinotrocha larvae of phoronids is a well documented example of intercalation of the new larval body plan, which can be used to study how new life stages emerge in animals with biphasic life cycle. The genomic and embryonic data from phoronids, in concert with studies of the fossil lophophorates, allow the more precise reconstruction of the evolution of lophophorate biomineralization. Finally, the regenerative and asexual abilities of phoronids can shed new light on the evolution of coloniality in lophophorates. As evident from those examples, Phoronida occupies a central role in the discussion of the evolution of lophophorate body plans and life histories.

Boring systematics: A genome skimmed phylogeny of ctenostome bryozoans and their endolithic family Penetrantiidae with the description of one new species

Ctenostomes are a group of gymnolaemate bryozoans with an uncalcified chitinous body wall having few external, skeletal characters. Hence, species identification is challenging and their systematics remain poorly understood, even more so when they exhibit an endolithic (boring) lifestyle. Currently, there are four Recent families of endolithic bryozoans that live inside mineralized substrates like mollusk shells. In particular, Penetrantiidae Silén, 1946 has received considerable attention and its systematic affinity to either cheilostomes or ctenostomes has been debated. Species delimitation of penetrantiids remains difficult, owing to a high degree of colonial and zooidal plasticity. Consequently, an additional molecular approach is essential to unravel the systematics of penetrantiids, their phylogenetic placement and their species diversity. We therefore sequenced the mitochondrial (mt) genomes and two nuclear markers of 27 ctenostome species including nine penetrantiids. Our phylogeny supports the Penetrantiidae as a monophyletic group placed as sister taxon to the remaining ctenostomes alongside paludicellids, arachnidioids and terebriporids. The boring family Terebriporidae d'Orbigny, 1847 were previously considered to be among vesicularioids, but our results suggest an arachnidioid affinity instead. Ctenostome paraphyly is supported by our data, as the cheilostomes nest within them. A Multiporata clade is also well supported, including the former victorelloid genus Sundanella. Altogether, this study provides new insights into ctenostome systematics, assists with species delimitation and contributes to our understanding of the bryozoan tree of life.

Unusual lophophore innervation in ctenostome Flustrellidra hispida (Bryozoa)

Since ctenostomes are traditionally regarded as an ancestral clade to some other bryozoan groups, the study of additional species may help to clarify questions on bryozoan evolution and phylogeny. One of these questions is the bryozoan lophophore evolution: whether it occurred through simplification or complication. The morphology and innervation of the ctenostome Flustrellidra hispida (Fabricius, 1780) lophophore have been studied with electron microscopy and immunocytochemistry with confocal laser scanning microscopy. Lophophore nervous system of F. hispida consists of several main nerve elements: cerebral ganglion, circumoral nerve ring, and the outer nerve ring. Serotonin-like immunoreactive perikarya, which connect with the circumoral nerve ring, bear the cilium that directs to the abfrontal side of the lophophore and extends between tentacle bases. The circumoral nerve ring gives rise to the intertentacular and frontal tentacle nerves. The outer nerve ring gives rise to the abfrontal neurites, which connect to the outer groups of perikarya and contribute to the formation of the abfrontal tentacle nerve. The outer nerve ring has been described before in other bryozoans, but it never contributes to the innervation of tentacles. The presence of the outer nerve ring participating in the innervation of tentacles makes the F. hispida lophophore nervous system particularly similar to the lophophore nervous system of phoronids. This similarity allows to suggest that organization of the F. hispida lophophore nervous system may reflect the ancestral state for all bryozoans. The possible scenario of evolutionary transformation of the lophophore nervous system within bryozoans is suggested.

Phylogenomics reveals deep relationships and diversification within phylactolaemate bryozoans

Bryozoans are mostly sessile colonial invertebrates that inhabit all kinds of aquatic ecosystems. Extant bryozoan species fall into two clades with one of them, Phylactolaemata, being the only exclusively freshwater clade. Phylogenetic relationships within the class Phylactolaemata have long been controversial owing to their limited distinguishable characteristics that reflect evolutionary relationships. Here, we present the first phylogenomic analysis of Phylactolaemata using transcriptomic data combined with dense taxon sampling of six families to better resolve the interrelationships and to estimate divergence time. Using maximum-likelihood and Bayesian inference approaches, we recovered a robust phylogeny for Phylactolaemata in which the interfamilial relationships are fully resolved. We show Stephanellidae is the sister taxon of all other phylactolaemates and confirm that Lophopodidae represents the second offshoot within the phylactolaemate tree. Plumatella fruticosa clearly falls outside Plumatellidae as previous investigations have suggested, and instead clusters with Pectinatellidae and Cristatellidae as the sister taxon of Fredericellidae. Our results demonstrate that cryptic speciation is very likely in F. sultana and in two species of Plumatella (P. repens and P. casmiana). Divergence time estimates show that Phylactolaemata appeared at the end of the Ediacaran and started to diverge in the Silurian, although confidence intervals were large for most nodes. The radiation of most extant phylactolaemate families occurred mainly in the Palaeogene and Neogene highlighting post-extinction diversification.

Morphospecies and molecular diversity of ‘lace corals’: the genus Reteporella (Bryozoa: Cheilostomatida) in the central North Atlantic Azores Archipelago

Background

As in most bryozoans, taxonomy and systematics of species in the genus Reteporella Busk, 1884 (family Phidoloporidae) has hitherto almost exclusively been based on morphological characters. From the central North Atlantic Azores Archipelago, nine Reteporella species have historically been reported, none of which have as yet been revised. Aiming to characterise the diversity and biogeographic distribution of Azorean Reteporella species, phylogenetic reconstructions were conducted on a dataset of 103 Azorean Reteporella specimens, based on the markers cytochrome C oxidase subunit 1, small and large ribosomal RNA subunits. Morphological identification was based on scanning electron microscopy and complemented the molecular inferences.

Results

Our results reveal two genetically distinct Azorean Reteporella clades, paraphyletic to eastern Atlantic and Mediterranean taxa. Moreover, an overall concordance between morphological and molecular species can be shown, and the actual bryozoan diversity in the Azores is greater than previously acknowledged as the dataset comprises three historically reported species and four putative new taxa, all of which are likely to be endemic. The inclusion of Mediterranean Reteporella specimens also revealed new species in the Adriatic and Ligurian Sea, whilst the inclusion of additional phidoloporid taxa hints at the non-monophyly of the genus Reteporella.

Conclusion

Being the first detailed genetic study on the genus Reteporella, the high divergence levels inferred within the genus Reteporella and family Phidoloporidae calls for the need of further revision. Nevertheless, the overall concordance between morphospecies and COI data suggest the potential adequacy of a 3% cut-off to distinguish Reteporella species. The discovery of new species in the remote Azores Archipelago as well as in the well-studied Mediterranean Sea indicates a general underestimation of bryozoan diversity. This study constitutes yet another example of the importance of integrative taxonomical approaches on understudied taxa, contributing to cataloguing genetic and morphological diversity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-02080-z.

A broadly resolved molecular phylogeny of New Zealand cheilostome bryozoans as a framework for hypotheses of morphological evolution

Larger molecular phylogenies based on ever more genes are becoming commonplace with the advent of cheaper and more streamlined sequencing and bioinformatics pipelines. However, many groups of inconspicuous but no less evolutionarily or ecologically important marine invertebrates are still neglected in the quest for understanding species- and higher-level phylogenetic relationships. Here, we alleviate this issue by presenting the molecular sequences of 165 cheilostome bryozoan species from New Zealand waters. New Zealand is our geographic region of choice as its cheilostome fauna is taxonomically, functionally and ecologically diverse, and better characterized than many other such faunas in the world. Using this most taxonomically broadly-sampled and statistically-supported cheilostome phylogeny comprising 214 species, when including previously published sequences, and 17 genes (2 nuclear and 15 mitochondrial) we tested several existing systematic hypotheses based solely on morphological observations. We find that lower taxonomic level hypotheses (species and genera) are robust while our inferred trees did not reflect current higher-level systematics (family and above), illustrating a general need for the rethinking of current hypotheses. To illustrate the utility of our new phylogeny, we reconstruct the evolutionary history of frontal shields (i.e., a calcified body-wall layer in ascus-bearing cheilostomes) and ask if its presence has any bearing on the diversification rates of cheilostomes.

Mitochondrial gene order of the freshwater bryozoan Cristatella mucedo retains ancestral lophotrochozoan features

Bryozoans are aquatic colonial suspension-feeders abundant in many marine and freshwater benthic communities. At the same time, the phylum is under studied on both morphological and molecular levels, and its position on the metazoan tree of life is still disputed. Bryozoa include the exclusively marine Stenolaemata, predominantly marine Gymnolaemata and exclusively freshwater Phylactolaemata. Here we report the mitochondrial genome of the phylactolaemate bryozoan Cristatella mucedo. This species has the largest (21,008 bp) of all currently known bryozoan mitogenomes, containing a typical metazoan gene compendium as well as a number of non-coding regions, three of which are longer than 1500 bp. The trnS1/trnG/nad3 region is presumably duplicated in this species. Comparative analysis of the gene order in C. mucedo and another phylactolaemate bryozoan, Pectinatella magnifica, confirmed their close relationships, and revealed a stronger similarity to mitogenomes of phoronids and other lophotrochozoan species than to marine bryozoans, indicating the ancestral nature of their gene arrangement. We suggest that the ancestral gene order underwent substantial changes in different bryozoan cladesshowing mosaic distribution of conservative gene blocks regardless of their phylogenetic position. Altogether, our results support the early divergence of Phylactolaemata from the rest of Bryozoa.

A molecular phylogeny of historical and contemporary specimens of an under-studied micro-invertebrate group

Resolution of relationships at lower taxonomic levels is crucial for answering many evolutionary questions, and as such, sufficiently varied species representation is vital. This latter goal is not always achievable with relatively fresh samples. To alleviate the difficulties in procuring rarer taxa, we have seen increasing utilization of historical specimens in building molecular phylogenies using high throughput sequencing. This effort, however, has mainly focused on large-bodied or well-studied groups, with small-bodied and under-studied taxa under-prioritized. Here, we utilize both historical and contemporary specimens, to increase the resolution of phylogenetic relationships among a group of under-studied and small-bodied metazoans, namely, cheilostome bryozoans. In this study, we pioneer the sequencing of air-dried cheilostomes, utilizing a recently developed library preparation method for low DNA input. We evaluate a de novo mitogenome assembly and two iterative methods, using the sequenced target specimen as a reference for mapping, for our sequences. In doing so, we present mitochondrial and ribosomal RNA sequences of 43 cheilostomes representing 37 species, including 14 from historical samples ranging from 50 to 149 years old. The inferred phylogenetic relationships of these samples, analyzed together with publicly available sequence data, are shown in a statistically well-supported 65 taxa and 17 genes cheilostome tree, which is also the most broadly sampled and largest to date. The robust phylogenetic placement of historical samples whose contemporary conspecifics and/or congenerics have been sequenced verifies the appropriateness of our workflow and gives confidence in the phylogenetic placement of those historical samples for which there are no close relatives sequenced. The success of our workflow is highlighted by the circularization of a total of 27 mitogenomes, seven from historical cheilostome samples. Our study highlights the potential of utilizing DNA from micro-invertebrate specimens stored in natural history collections for resolving phylogenetic relationships among species.

Differential Gene Expression Between Polymorphic Zooids of the Marine Bryozoan Bugulina stolonifera

Bryozoans are a diverse phylum of marine and freshwater colonial invertebrates containing approximately 6,300 described living species. Bryozoans grow by budding new physiologically connected colony members (zooids) from a founding individual that forms from a metamorphosed larva. In some species these zooids come in different shapes and sizes and are specialized to serve different tasks within the colony. A complex interaction of genotype, environment, and developmental pathway shapes zooid fate, however, the specific mechanisms underlying the establishment of this division of labor remain unknown. Here, the first characterization of differential gene expression between polymorphic zooids of a bryozoan colony is presented. The development of different zooid types of lab-cultured Bugulina stolonifera colonies including feeding autozooids, avicularia (derived non-feeding zooids that are homologous to feeding autozooids but shaped like a bird's beak), and rhizoids (a branching network of non-feeding anchoring zooids) was explored using RNA sequencing, de novo transcriptome assembly, and differential gene expression analyses. High throughput sequencing of cDNA libraries yielded an average of 14.9 ± 1.3 (SE) million high-quality paired-end reads per sample. Data for the first de novo transcriptome assemblies of B. stolonifera and the first characterization of genes involved in the formation and maintenance of zooid types within a bryozoan colony are presented. In a comparison between autozooid and avicularium tissues, 1,097 significant differentially expressed genes were uncovered. This work provides a much-needed foundation for understanding the mechanisms involved in the development of polymorphic zooids and the establishment of division of labor in bryozoans.

Key novelties in the evolution of the aquatic colonial phylum Bryozoa: evidence from soft body morphology

Molecular techniques are currently the leading tools for reconstructing phylogenetic relationships, but our understanding of ancestral, plesiomorphic and apomorphic characters requires the study of the morphology of extant forms for testing these phylogenies and for reconstructing character evolution. This review highlights the potential of soft body morphology for inferring the evolution and phylogeny of the lophotrochozoan phylum Bryozoa. This colonial taxon comprises aquatic coelomate filter-feeders that dominate many benthic communities, both marine and freshwater. Despite having a similar bauplan, bryozoans are morphologically highly diverse and are represented by three major taxa: Phylactolaemata, Stenolaemata and Gymnolaemata. Recent molecular studies resulted in a comprehensive phylogenetic tree with the Phylactolaemata sister to the remaining two taxa, and Stenolaemata (Cyclostomata) sister to Gymnolaemata. We plotted data of soft tissue morphology onto this phylogeny in order to gain further insights into the origin of morphological novelties and character evolution in the phylum. All three larger clades have morphological apomorphies assignable to the latest molecular phylogeny. Stenolaemata (Cyclostomata) and Gymnolaemata were united as monophyletic Myolaemata because of the apomorphic myoepithelial and triradiate pharynx. One of the main evolutionary changes in bryozoans is a change from a body wall with two well-developed muscular layers and numerous retractor muscles in Phylactolaemata to a body wall with few specialized muscles and few retractors in the remaining bryozoans. Such a shift probably pre-dated a body wall calcification that evolved independently at least twice in Bryozoa and resulted in the evolution of various hydrostatic mechanisms for polypide protrusion. In Cyclostomata, body wall calcification was accompanied by a unique detachment of the peritoneum from the epidermis to form the hydrostatic membraneous sac. The digestive tract of the Myolaemata differs from the phylactolaemate condition by a distinct ciliated pylorus not present in phylactolaemates. All bryozoans have a mesodermal funiculus, which is duplicated in Gymnolaemata. A colonial system of integration (CSI) of additional, sometimes branching, funicular cords connecting neighbouring zooids via pores with pore-cell complexes evolved at least twice in Gymnolaemata. The nervous system in all bryozoans is subepithelial and concentrated at the lophophoral base and the tentacles. Tentacular nerves emerge intertentacularly in Phylactolaemata whereas they partially emanate directly from the cerebral ganglion or the circum-oral nerve ring in myolaemates. Overall, morphological evidence shows that ancestral forms were small, colonial coelomates with a muscular body wall and a U-shaped gut with ciliary tentacle crown, and were capable of asexual budding. Coloniality resulted in many novelties including the origin of zooidal polymorphism, an apomorphic landmark trait of the Myolaemata.

The nervous system in the cyclostome bryozoan Crisia eburnea as revealed by transmission electron and confocal laser scanning microscopy

INTRODUCTION

Among bryozoans, cyclostome anatomy is the least studied by modern methods. New data on the nervous system fill the gap in our knowledge and make morphological analysis much more fruitful to resolve some questions of bryozoan evolution and phylogeny.

RESULTS

The nervous system of cyclostome Crisia eburnea was studied by transmission electron microscopy and confocal laser scanning microscopy. The cerebral ganglion has an upper concavity and a small inner cavity filled with cilia and microvilli, thus exhibiting features of neuroepithelium. The cerebral ganglion is associated with the circumoral nerve ring, the circumpharyngeal nerve ring, and the outer nerve ring. Each tentacle has six longitudinal neurite bundles. The body wall is innervated by thick paired longitudinal nerves. Circular nerves are associated with atrial sphincter. A membranous sac, cardia, and caecum all have nervous plexus.

CONCLUSION

The nervous system of the cyclostome C. eburnea combines phylactolaemate and gymnolaemate features. Innervation of tentacles by six neurite bundles is similar of that in Phylactolaemata. The presence of circumpharyngeal nerve ring and outer nerve ring is characteristic of both, Cyclostomata and Gymnolaemata. The structure of the cerebral ganglion may be regarded as a result of transformation of hypothetical ancestral neuroepithelium. Primitive cerebral ganglion and combination of nerve plexus and cords in the nervous system of C. eburnea allows to suggest that the nerve system topography of C. eburnea may represent an ancestral state of nervous system organization in Bryozoa. Several scenarios describing evolution of the cerebral ganglion in different bryozoan groups are proposed.

Skeletal carbonate mineralogy of Scottish bryozoans

This paper describes the skeletal carbonate mineralogy of 156 bryozoan species collected from Scotland (sourced both from museum collections and from waters around Scotland) and collated from literature. This collection represents 79% of the species which inhabit Scottish waters and is a greater number and proportion of extant species than any previous regional study. The study is also of significance globally where the data augment the growing database of mineralogical analyses and offers first analyses for 26 genera and four families. Specimens were collated through a combination of field sampling and existing collections and were analysed by X-ray diffraction (XRD) and micro-XRD to determine wt% MgCO₃ in calcite and wt% aragonite. Species distribution data and phylogenetic organisation were applied to understand distributional, taxonomic and phylo-mineralogical patterns. Analysis of the skeletal composition of Scottish bryozoans shows that the group is statistically different from neighbouring Arctic fauna but features a range of mineralogy comparable to other temperate regions. As has been previously reported, cyclostomes feature low Mg in calcite and very little aragonite, whereas cheilostomes show much more variability, including bimineralic species. Scotland is a highly variable region, open to biological and environmental influx from all directions, and bryozoans exhibit this in the wide range of within-species mineralogical variability they present. This plasticity in skeletal composition may be driven by a combination of environmentally-induced phenotypic variation, or physiological factors. A flexible response to environment, as manifested in a wide range of skeletal mineralogy within a species, may be one characteristic of successful invasive bryozoans.

Complete mitochondrial genome of the freshwater bryozoan Pectinatella magnifica (Phylactolaemata: Plumatellida) assembled from next-generation sequencing data

The complete mitochondrial genome of the freshwater bryozoan Pectinatella magnifica was sequenced. The circular mitochondrial genome is 17,539 bp and consists of 13 protein-coding, two ribosomal RNA, and 22 transfer RNA genes (GenBank accession no. MG546680). The Bayesian comparative analysis of molecular evolution rates revealed no acceleration of the mitochondrial DNA (mtDNA) evolution of P. magnifica. Results of maximum likelihood analysis showed that this species clustered with other species of the phylum Bryozoa.

AFLP reveals low genetic diversity of the bryozoan Pectinatella magnifica (Leidy, 1851) in the Czech Republic

Background

Non-native species have aroused scientific interest because of their ability to successfully colonise areas to which they have been introduced, despite their sometimes limited genetic variation compared to their native range. These species establish themselves with the aid of some pre-existing features favouring them in the new environment. Pectinatella magnifica (Leidy, 1851), the freshwater magnificent bryozoan, is non-native in Europe and Asia. This study was designed to determine the genetic diversity and population structure of P. magnifica colonies collected from the Protected Landscape Area (PLA) and UNESCO Biosphere Reserve Třeboňsko (the Czech Republic) in the 2009 and 2011-2014 periods using Amplified Fragment Length Polymorphism (AFLP).

Findings

The vast majority of the examined non-native colonies, except three colonies sampled in 2012, expressed very low levels of genetic variation, not differentiating from the USA native colony. The Bayesian clustering approach grouped the 28 accessions into two genetically different populations.

Conclusions

The data suggest relatively low gene diversity within all colonies, which might reflect the recent expansion of P. magnifica in the Czech Republic.

Reconstructing the muscular ground pattern of phylactolaemate bryozoans: first data from gelatinous representatives

BACKGROUND

Phylactolaemata is commonly regarded the earliest branch within Bryozoa and thus the sister group to the other bryozoan taxa, Cyclostomata and Gymnolaemata. Therefore, the taxon is important for the reconstruction of the bryozoan morphological ground pattern. In this study the myoanatomy of Pectinatella magnifica, Cristatella mucedo and Hyalinella punctata was analysed by means of histology, f-actin staining and confocal laser-scanning microscopy in order to fill gaps in knowledge concerning the myoanatomy of Phylactolaemata.

RESULTS

The retractor muscles and muscles of the aperture, gut, body wall, tentacle sheath, lophophore constitute the most prominent muscular subsets in these species. The lophophore shows longitudinal muscle bands in the tentacles, lophophoral arm muscles, epistome musculature and hitherto undescribed muscles of the ring canal. In general the muscular system of the three species is very similar with differences mainly in the body wall, tentacle sheath and epistome. The body wall contains an orthogonal grid of musculature. The epistome exhibits either a muscular meshwork in the epistomal wall or muscle fibers traversing the epistomal cavity. The whole tentacle sheath possesses a regular mesh of muscles in Pectinatella and Cristatella, whereas circular muscles are limited to the tentacle sheath base in Hyalinella.

CONCLUSION

This study is the first to describe muscles of the ring canal and contributes to reconstructing muscular features for the last common ancestor of all bryozoans. The data available suggest that two longitudinal muscle bands in the tentacles, as well as retractor muscles and longitudinal and circular muscles in the tentacle sheath, were present in the last common bryozoan ancestor. Comparisons among bryozoans shows that several apomorphies are present in the myoanatomy of each class- level taxon such as the epistomal musculature and musculature of the lophophoral arms in phylactolaemates, annular muscles in cyclostomes and parietal muscles in gymnolaemates.

Innervation of the lophophore suggests that the phoronid Phoronis ovalis is a link between phoronids and bryozoans

The validity of the Lophophorata as a monophyletic group remains controversial. New data on the innervation of the lophophore, which is a unique feature of the lophophorates, may help clarify the status of the Lophophorata and provide new information on the early evolution of the group. In this paper, the organization of the nervous system of the lophophore is described in adults of the minute phoronid Phoronis ovalis. The lophophore nervous system includes a dorsal ganglion, a tentacular nerve ring, an inner ganglion, an inner nerve ring, and six nerves in each tentacle. The inner ganglion and inner nerve ring, which is associated with sensory cells, are described for the first time in adult phoronids. The general plan of the nervous system of the lophophore and tentacles is similar in P. ovalis and bryozoans. These new results suggest the presence of two nerve centers and two nerve rings in the last common ancestor of phoronids and bryozoans. During evolution, bryozoans may have lost the outer nerve center and outer nerve ring, whereas phoronids may have lost the inner nerve center and inner nerve ring. These morphological results evidence the lophophorates are monophyletic.

The life of the freshwater bryozoan Stephanella hina (Bryozoa, Phylactolaemata)-a crucial key to elucidating bryozoan evolution

BACKGROUND

Phylactolaemata is the earliest branch and the sister group to all extant bryozoans. It is considered a small relict group that, perhaps due to the invasion of freshwater, has retained ancestral features. Reconstruction of the ground pattern of Phylactolaemata is thus essential for reconstructing the ground pattern of all Bryozoa, and for inferring phylogenetic relationships to possible sister taxa. It is well known that Stephanella hina, the sole member of the family Stephanelllidae, is probably one of the earliest offshoots among the Phylactolaemata and shows some morphological peculiarities. However, key aspects of its biology are largely unknown. The aim of the present study was to analyze live specimens of this species, in order to both document its behavior and describe its colony morphology.

RESULTS

The colony morphology of Stephanella hina consists of zooidal arrangements with lateral budding sites reminiscent of other bryozoan taxa, i.e., Steno- and Gymnolaemata. Zooids protrude vertically from the substrate and are covered in a non-rigid jelly-like ectocyst. The latter is a transparent, sticky hull that for the most part shows no distinct connection to the endocyst. Interestingly, individual zooids can be readily separated from the rest of the colony. The loose tube-like ectocyst can be removed from the animals that produces individuals that are unable to retract their lophophore, but merely shorten their trunk by contraction of the retractor muscles.

CONCLUSIONS

These observations indicate that S. hina is unique among Phylactolaemata and support the notion that bryozoans evolved from worm-like ancestors. In addition, we raise several arguments for its placement into a separate family, Stephanellidae, rather than among the Plumatellidae, as previously suggested.

The nervous system of the lophophore in the ctenostome Amathia gracilis provides insight into the morphology of ancestral ectoprocts and the monophyly of the lophophorates

BACKGROUND

The Bryozoa (=Ectoprocta) is a large group of bilaterians that exhibit great variability in the innervation of tentacles and in the organization of the cerebral ganglion. Investigations of bryozoans from different groups may contribute to the reconstruction of the bryozoan nervous system bauplan. A detailed investigation of the polypide nervous system of the ctenostome bryozoan Amathia gracilis is reported here.

RESULTS

The cerebral ganglion displays prominent zonality and has at least three zones: proximal, central, and distal. The proximal zone is the most developed and contains two large perikarya giving rise to the tentacle sheath nerves. The neuroepithelial organization of the cerebral ganglion is revealed. The tiny lumen of the cerebral ganglion is represented by narrow spaces between the apical projections of the perikarya of the central zone. The cerebral ganglion gives rise to five groups of main neurite bundles of the lophophore and the tentacle sheath: the circum-oral nerve ring, the lophophoral dorso-lateral nerves, the pharyngeal and visceral neurite bundles, the outer nerve ring, and the tentacle sheath nerves. Serotonin-like immunoreactive nerve system of polypide includes eight large perikarya located between tentacles bases. There are two analmost and six oralmost perikarya with prominent serotonergic "gap" between them. Based on the characteristics of their innervations, the tentacles can be subdivided into two groups: four that are near the anus and six that are near the mouth. Two longitudinal neurite bundles - medio-frontal and abfrontal - extend along each tentacle.

CONCLUSION

The zonality of the cerebral ganglion, the presence of three commissures, and location of the main nerves emanating from each zone might have caused by directive innervation of the various parts of the body: the tentacles sheath, the lophohpore, and the digestive tract. Two alternative scenarios of bryozoan lophophore evolution are discussed. The arrangement of large serotonin-like immunoreactive perikarya differs from the pattern previously described in ctenostome bryozoans. In accordance with its position relative to the same organs (tentacles, anus, and mouth), the lophophore outer nerve ring corresponds to the brachiopod lower brachial nerve and to the phoronid tentacular nerve ring. The presence of the outer nerve ring makes the lophophore innervation within the group (clade) of lophophorates similar and provides additional morphological evidence of the lophophore homology and monophyly of the lophophorates.

Matrotrophy and placentation in invertebrates: a new paradigm

Matrotrophy, the continuous extra-vitelline supply of nutrients from the parent to the progeny during gestation, is one of the masterpieces of nature, contributing to offspring fitness and often correlated with evolutionary diversification. The most elaborate form of matrotrophy-placentotrophy-is well known for its broad occurrence among vertebrates, but the comparative distribution and structural diversity of matrotrophic expression among invertebrates is wanting. In the first comprehensive analysis of matrotrophy across the animal kingdom, we report that regardless of the degree of expression, it is established or inferred in at least 21 of 34 animal phyla, significantly exceeding previous accounts and changing the old paradigm that these phenomena are infrequent among invertebrates. In 10 phyla, matrotrophy is represented by only one or a few species, whereas in 11 it is either not uncommon or widespread and even pervasive. Among invertebrate phyla, Platyhelminthes, Arthropoda and Bryozoa dominate, with 162, 83 and 53 partly or wholly matrotrophic families, respectively. In comparison, Chordata has more than 220 families that include or consist entirely of matrotrophic species. We analysed the distribution of reproductive patterns among and within invertebrate phyla using recently published molecular phylogenies: matrotrophy has seemingly evolved at least 140 times in all major superclades: Parazoa and Eumetazoa, Radiata and Bilateria, Protostomia and Deuterostomia, Lophotrochozoa and Ecdysozoa. In Cycliophora and some Digenea, it may have evolved twice in the same life cycle. The provisioning of developing young is associated with almost all known types of incubation chambers, with matrotrophic viviparity more widespread (20 phyla) than brooding (10 phyla). In nine phyla, both matrotrophic incubation types are present. Matrotrophy is expressed in five nutritive modes, of which histotrophy and placentotrophy are most prevalent. Oophagy, embryophagy and histophagy are rarer, plausibly evolving through heterochronous development of the embryonic mouthparts and digestive system. During gestation, matrotrophic modes can shift, intergrade, and be performed simultaneously. Invertebrate matrotrophic adaptations are less complex structurally than in chordates, but they are more diverse, being formed either by a parent, embryo, or both. In a broad and still preliminary sense, there are indications of trends or grades of evolutionarily increasing complexity of nutritive structures: formation of (i) local zones of enhanced nutritional transport (placental analogues), including specialized parent-offspring cell complexes and various appendages increasing the entire secreting and absorbing surfaces as well as the contact surface between embryo and parent, (ii) compartmentalization of the common incubatory space into more compact and 'isolated' chambers with presumably more effective nutritional relationships, and (iii) internal secretory ('milk') glands. Some placental analogues in onychophorans and arthropods mimic the simplest placental variants in vertebrates, comprising striking examples of convergent evolution acting at all levels-positional, structural and physiological.

Comparative morphology of the nervous system in three phylactolaemate bryozoans

Background

Though some elements of the bryozoan nervous system were discovered 180 years ago, few studies of their neuromorphology have been undertaken since that time. As a result the general picture of the bryozoan nervous system structure is incomplete in respect of details and fragmentary in respect of taxonomic coverage.

Results

The nervous system of three common European freshwater bryozoans – Cristatella mucedo, Plumatella repens (both with a horseshoe-shaped lophophore) and Fredericella sultana (with a circular lophophore) had numerous differences in the details of the structure but the general neuroarchitecture is similar. The nervous system of the zooid consists of the cerebral ganglion, a circumpharyngeal ring and lophophoral nerve tracts (horns), both sending numerous nerves to the tentacles, and the nerve plexuses of the body wall and of the gut. A number of the important details (distal branching of the additional radial nerve, pattern of distribution of nerve cells and neurites in the ganglion, etc.) were described for the first time. The number and position of the tentacle nerves in Cristatella mucedo was ascertained and suggestions about their function were made. The revealed distribution of various neuromediators in the nervous system allowed us to suggest functional affinities of some major nerves.

Conclusions

Despite the basic similarity, both the ganglion and the lophophore nervous system in Phylactolaemata have a more complex structure than in marine bryozoans (classes Gymnolaemata and Stenolaemata). First of all, their neuronal network has a denser and more complex branching pattern: most phylactolaemates have two large nerve tracts associated with lophophore arms, they have more nerves in the tentacles, additional and basal branches emitting from the main radial nerves, etc. This, in part, can be explained by the horseshoe shape of the lophophore and a larger size of the polypide in freshwater species. The structure of the nervous system in Fredericella sultana suggests that it underwent a secondary simplification following the reduction of the lophophore arms. Colony locomotion in Cristatella mucedo is based on co-ordinated activity of two perpendicular muscle layers of the sole and the plexus of motor neurons sandwiched between them. The trigger of this activity and the co-ordination mechanism remain enigmatic.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-015-0112-2) contains supplementary material, which is available to authorized users.

Bryozoans are returning home: recolonization of freshwater ecosystems inferred from phylogenetic relationships

Bryozoans are aquatic invertebrates that inhabit all types of aquatic ecosystems. They are small animals that form large colonies by asexual budding. Colonies can reach the size of several tens of centimeters, while individual units within a colony are the size of a few millimeters. Each individual within a colony works as a separate zooid and is genetically identical to each other individual within the same colony. Most freshwater species of bryozoans belong to the Phylactolaemata class, while several species that tolerate brackish water belong to the Gymnolaemata class. Tissue samples for this study were collected in the rivers of Adriatic and Danube basin and in the wetland areas in the continental part of Croatia (Europe). Freshwater and brackish taxons of bryozoans were genetically analyzed for the purpose of creating phylogenetic relationships between freshwater and brackish taxons of the Phylactolaemata and Gymnolaemata classes and determining the role of brackish species in colonizing freshwater and marine ecosystems. Phylogenetic relationships inferred on the genes for 18S rRNA, 28S rRNA, COI, and ITS2 region confirmed Phylactolaemata bryozoans as radix bryozoan group. Phylogenetic analysis proved Phylactolaemata bryozoan's close relations with taxons from Phoronida phylum as well as the separation of the Lophopodidae family from other families within the Plumatellida genus. Comparative analysis of existing knowledge about the phylogeny of bryozoans and the expansion of known evolutionary hypotheses is proposed with the model of settlement of marine and freshwater ecosystems by the bryozoans group during their evolutionary past. In this case study, brackish bryozoan taxons represent a link for this ecological phylogenetic hypothesis. Comparison of brackish bryozoan species Lophopus crystallinus and Conopeum seurati confirmed a dual colonization of freshwater ecosystems throughout evolution of this group of animals.

Description and molecular phylogeny of a new species of Phoronis (Phoronida) from Japan, with a redescription of topotypes of P. ijimai Oka, 1897

We describe Phoronis emigisp. n. as the eighth member of the genus based on specimens collected from a sandy bottom at 33.2 m depth in Tomioka Bay, Amakusa, Japan. The new species is morphologically similar to P. psammophila Cori, 1889, but can be distinguished from the latter by the number of longitudinal muscle bundles in the body wall (56–72 vs. 25–50 in P. psammophila) and the position of the nephridiopores (situated level with the anus vs. lower than the anus in P. psammophila). Using sequences of the nuclear 18S and 28S rRNA genes and the mitochondrial cytochrome c oxidase subunit I (COI) gene, we inferred the relationship of P. emigi to other phoronids by the maximum likelihood method and Bayesian analysis. The analyses showed that P. emigi is closely related to P. hippocrepia Wright, 1856 and P. psammophila Cori, 1889. We describe the morphology of the topotypes and additional material for P. ijimai Oka, 1897. Neither our morphological observations of P. ijimai, nor the phylogenetic analyses based on 18S and COI sequences, contradicts that P. vancouverensis Pixell, 1912 is conspecific with P. ijimai, a synonymy that has long been disputed.

From incipient to substantial: evolution of placentotrophy in a phylum of aquatic colonial invertebrates

Matrotrophy has long been known in invertebrates, but it is still poorly understood and has never been reviewed. A striking example of matrotrophy (namely, placentotrophy) is provided by the Bryozoa, a medium-sized phylum of the aquatic colonial filter feeders. Here I report on an extensive anatomical study of placental analogues in 21 species of the bryozoan order Cheilostomata, offering the first review on matrotrophy among aquatic invertebrates. The first anatomical description of incipient placentotrophy in invertebrates is presented together with the evidence for multiple independent origins of placental analogues in this order. The combinations of contrasting oocytic types (macrolecithal or microlecithal) and various degrees of placental development and embryonic enlargement during incubation, found in different bryozoan species, are suggestive of a transitional series from the incipient to the substantial placentotrophy accompanied by an inverse change in oogenesis, a situation reminiscent of some vertebrates. It seems that matrotrophy could trigger the evolution of sexual zooidal polymorphism in some clades. The results of this study show that this phylum, with its wide variety of reproductive patterns, incubation devices, and types of the simple placenta-like systems, offers a promising model for studying parallel evolution of placentotrophy in particular, and matrotrophy in general.

Complete mitochondrial genome of Membranipora grandicella (Bryozoa: Cheilostomatida) determined with next-generation sequencing: the first representative of the suborder Malacostegina

Next-generation sequencing (NGS) has proven a valuable platform for fast and easy obtaining of large numbers of sequences at relatively low cost. In this study we use shot-gun sequencing method on Illumina HiSeq 2000, to obtain enough sequences for the assembly of the bryozoan Membranipora grandicella (Bryozoa: Cheilostomatida) mitochondrial genome, which is the first representative of the suborder Malacostegina. The complete mitochondrial genome is 15,861 bp in length, which is relatively larger than other studied bryozoans. The mitochondrial genome contains 13 protein-coding genes, 2 ribosomal RNAs and 20 transfer RNAs. To investigate the phylogenetic position and the inner relationships of the phylum Bryozoa, phylogenetic trees were constructed with amino acid sequences of 11 PCGs from 30 metazoans. Two superclades of protostomes, namely Lophotrochozoa and Ecdysozoa, are recovered as monophyletic with strong support in both ML and Bayesian analyses. Somewhat to surprise, Bryozoa appears as the sister group of Chaetognatha with moderate or high support. The relationship among five bryozoans is Tubulipora flabellaris + (M. grandicella + (Flustrellidra hispida + (Bugula neritina + Watersipora subtorquata))), which supports for the view that Cheilostomatida is not a natural, monophyletic clade. NGS proved to be a quick and easy method for sequencing a complete mitochondrial genome.

The evolutionary history of division of labour

Functional specialization, or division of labour (DOL), of parts within organisms and colonies is common in most multi-cellular, colonial and social organisms, but it is far from ubiquitous. Several mechanisms have been proposed to explain the evolutionary origins of DOL; the basic feature common to all of them is that functional differences can arise easily. These mechanisms cannot explain the many groups of colonial and social animals that exhibit no DOL despite up to 500 million years of evolution. Here, I propose a new hypothesis, based on a multi-level selection theory, which predicts that a reproductive DOL is required to evolve prior to subsequent functional specialization. I test this hypothesis using a dataset consisting of the type of DOL for living and extinct colonial and social animals. The frequency distribution of DOL and the sequence of its acquisition confirm that reproductive specialization evolves prior to functional specialization. A corollary of this hypothesis is observed in colonial, social and also within multi-cellular organisms; those species without a reproductive DOL have a smaller range of internal variation, in terms of the number of polymorphs or cell types, than species with a reproductive DOL.

A molecular phylogeny of bryozoans

We present the most comprehensive molecular phylogeny of bryozoans to date. Our concatenated alignment of two nuclear ribosomal and five mitochondrial genes includes 95 taxa and 13,292 nucleotide sites, of which 8297 were included. The number of new sequences generated during this project are for each gene:ssrDNA (32), lsrDNA (22), rrnL (38), rrnS (35), cox1 (37), cox3 (34), and cytb (44). Our multi-gene analysis provides a largely stable topology across the phylum. The major groups were unambiguously resolved as (Phylactolaemata (Cyclostomata (Ctenostomata, Cheilostomata))), with Ctenostomata paraphyletic. Within Phylactolaemata, (Stephanellidae, Lophopodidae) form the earliest divergent clade. Fredericellidae is not resolved as a monophyletic family and forms a clade together with Plumatellidae, Cristatellidae and Pectinatellidae, with the latter two as sister taxa. Hyalinella and Gelatinella nest within the genus Plumatella. Cyclostome taxa fall into three major clades: i. (Favosipora (Plagioecia, Rectangulata)); ii. (Entalophoroecia ((Diplosolen, Cardioecia) (Frondipora, Cancellata))); and iii. (Articulata ((Annectocyma, Heteroporidae) (Tubulipora (Tennysonia, Idmidronea)))), with suborders Tubuliporina and Cerioporina, and family Plagioeciidae each being polyphyletic. Ctenostomata is composed of three paraphyletic clades to the inclusion of Cheilostomata: ((Alcyonidium, Flustrellidra) (Paludicella (Anguinella, Triticella)) (Hislopia (Bowerbankia, Amathia)) Cheilostomata); Flustrellidra nests within the genus Alcyonidium, and Amathia nests within the genus Bowerbankia. Suborders Carnosa and Stolonifera are not monophyletic. Within the cheilostomes, Malacostega is paraphyletic to the inclusion of all other cheilostomes. Conopeum is the most early divergent cheilostome, forming the sister group to ((Malacostega, Scrupariina, Inovicellina) ((Hippothoomorpha, Flustrina) (Lepraliomorpha, Umbonulomorpha))); Flustrina is paraphyletic to the inclusion of the hippothoomorphs; neither Lepraliomorpha nor Umbonulomorpha is monophyletic. Ascophorans are polyphyletic, with hippothoomorphs grouping separately from lepraliomorphs and umbonulomorphs; no cribrimorphs were included in the analysis. Results are discussed in the light of molecular and morphological evidence. Ancestral state reconstruction of larval strategy in Gymnolaemata revealed planktotrophy and lecithotrophy as equally parsimonious solutions for the ancestral condition. More comprehensive taxon sampling is expected to clarify this result. We discuss the extent of non-bryozoan contaminant sequences deposited in GenBank and their impact on the reconstruction of metazoan phylogenies and those of bryozoan interrelationships.

The complete mitochondrial genome of Flustra foliacea (Ectoprocta, Cheilostomata) - compositional bias affects phylogenetic analyses of lophotrochozoan relationships

Background

The phylogenetic relationships of the lophophorate lineages, ectoprocts, brachiopods and phoronids, within Lophotrochozoa are still controversial. We sequenced an additional mitochondrial genome of the most species-rich lophophorate lineage, the ectoprocts. Although it is known that there are large differences in the nucleotide composition of mitochondrial sequences of different lineages as well as in the amino acid composition of the encoded proteins, this bias is often not considered in phylogenetic analyses. We applied several approaches for reducing compositional bias and saturation in the phylogenetic analyses of the mitochondrial sequences.

Results

The complete mitochondrial genome (16,089 bp) of Flustra foliacea (Ectoprocta, Gymnolaemata, Cheilostomata) was sequenced. All protein-encoding, rRNA and tRNA genes are transcribed from the same strand. Flustra shares long intergenic sequences with the cheilostomate ectoproct Bugula, which might be a synapomorphy of these taxa. Further synapomorphies might be the loss of the DHU arm of the tRNA L(UUR), the loss of the DHU arm of the tRNA S(UCN) and the unique anticodon sequence GAG of the tRNA L(CUN). The gene order of the mitochondrial genome of Flustra differs strongly from that of the other known ectoprocts. Phylogenetic analyses of mitochondrial nucleotide and amino acid data sets show that the lophophorate lineages are more closely related to trochozoan phyla than to deuterostomes or ecdysozoans confirming the Lophotrochozoa hypothesis. Furthermore, they support the monophyly of Cheilostomata and Ectoprocta. However, the relationships of the lophophorate lineages within Lophotrochozoa differ strongly depending on the data set and the used method. Different approaches for reducing heterogeneity in nucleotide and amino acid data sets and saturation did not result in a more robust resolution of lophotrochozoan relationships.

Conclusion

The contradictory and usually weakly supported phylogenetic reconstructions of the relationships among lophotrochozoan phyla based on mitochondrial sequences indicate that these alone do not contain enough information for a robust resolution of the relations of the lophotrochozoan phyla. The mitochondrial gene order is also not useful for inferring their phylogenetic relationships, because it is highly variable in ectoprocts, brachiopods and some other lophotrochozoan phyla. However, our study revealed several rare genomic changes like the evolution of long intergenic sequences and changes in the structure of tRNAs, which may be helpful for reconstructing ectoproct phylogeny.

Myoanatomy and serotonergic nervous system of plumatellid and fredericellid Phylactolaemata (Lophotrochozoa, Ectoprocta)

The phylogenetic position of the Ectoprocta within the Lophotrochozoa is discussed controversially. For gaining more insight into ectoproct relationships and comparing it with other potentially related phyla, we analysed the myoanatomy and serotonergic nervous system of adult representatives of the Phylactolaemata (Plumatella emarginata, Plumatellavaihiriae, Plumatella fungosa, Fredericella sultana). The bodywall contains a mesh of circular and longitudinal muscles. On its distal end, the orifice possesses a prominent sphincter and continues into the vestibular wall, which has longitudinal and circular musculature. The tentacle sheath carries mostly longitudinal muscle fibres in Plumatella sp., whereas F. sultana also possesses regular circular muscle fibres. Three groups of muscles are associated with the lophophore: 1) Lophophoral arm muscles (missing in Fredericella), 2) epistome musculature and 3) tentacle musculature. The epistome flap is encompassed by smooth muscle fibres. A few fibres extend medially over the ganglion to its proximal floor. Abfrontal tentacle muscles have diagonally arranged muscle fibres in their proximal region, whereas the distal region is formed by a stack of muscles that resemble an inverted 'V'. Frontal tentacle muscles show more variation and either possess one or two bases. The digestive tract possesses circular musculature which is striated except at the intestine where it is composed of smooth muscle fibres. The serotonergic nervous system is concentrated in the cerebral ganglion. From the latter a serotonergic nerve extends to each tentacle base. In Plumatella the inner row of tentacles at the lophophoral concavity lacks serotonergic nerves. Bodywall musculature is a common feature in many lophotrochozoan phyla, but among other filter feeders like the Ectoprocta is only present in the 'lophophorate' Phoronida. The longitudinal tentacle musculature is reminiscent of the condition found in phoronids and brachiopods, but differs to entoproct tentacles. Although this study shows some support for the 'Lophophorata', more comparative analyses of possibly related phyla are required.

Gene expression in bryozoan larvae suggest a fundamental importance of pre-patterned blastemic cells in the bryozoan life-cycle

BACKGROUND

Bryozoa is a clade of aquatic protostomes. The bryozoan life cycle typically comprises a larval stage, which metamorphoses into a sessile adult that proliferates by asexual budding to form colonies. The homology of bryozoan larvae with other protostome larvae is enigmatic. Bryozoan larvae exhibit blastemic tissues that contribute to build the adult during morphogenesis. However, it remains unclear if the cells of these tissues are pre-determined according to their future fate or if the cells are undifferentiated, pluripotent stem cells. Gene expression studies can help to identify molecular patterning of larval and adult tissues and enlighten the evolution of bryozoan life cycle stages.

RESULTS

We investigated the spatial expression of 13 developmental genes in the larval stage of the gymnolaemate bryozoan Bugula neritina. We found most genes expressed in discrete regions in larval blastemic tissues that form definitive components of the adult body plan. Only two of the 13 genes, BnTropomyosin and BnFoxAB, were exclusively expressed in larval tissues that are discarded during metamorphosis.

CONCLUSIONS

Our results suggest that the larval blastemas in Bugula are pre-patterned according to their future fate in the adult. The gene expression patterns indicate that some of the bryozoan blastemas can be interpreted to correspond to homologous adult tissues of other animals. This study challenges an earlier proposed view that metazoan larvae share homologous undifferentiated "set-aside cells", and instead points to an independent origin of the bryozoan larval stage with respect to other lophotrochozoans.

Myoanatomy and serotonergic nervous system of the ctenostome Hislopia malayensis: evolutionary trends in bodyplan patterning of ectoprocta

BACKGROUND

Ectoprocta is a large lophotrochozoan clade of colonial suspension feeders comprising over 5.000 extant species. Their phylogenetic position within the Lophotrochzoa remains controversially discussed, but also the internal relationships of the major ectoproct subclades -Phylactolaemata, Stenolaemata, and Gymnolaemata - remains elusive. To gain more insight into the basic configuration of ectoproct muscle systems for phylogenetic considerations, we analysed the adult myoanatomy and the serotonergic nervous system as well as myogenesis in budding stages of the ctenostome Hislopia malayensis.

RESULTS

In adults, the serotonergic nervous system is restricted to the lophophoral base with a high concentration in the cerebral ganglion and serotonergic perikarya between each pair of tentacles. Prominent smooth apertural muscles extend from the basal cystid wall to each lateral side of the vestibular wall. The musculature of the tentacle sheath consists of regular strands of smooth longitudinal muscles. Each tentacle is supplied with two bands of longitudinal muscles that show irregular striation. At the lophophoral base several muscles are present: (i) Short muscle fibres that proximally diverge from a single point from where they split distally into two separate strands. (ii) Proximally of the first group are smooth, longitudinal fibres that extend to the proximal-most side of the lophophoral base. (iii) Smooth muscle fibres, the buccal dilatators, traverse obliquely towards the pharynx, and (iv) a circular ring of smooth muscle fibres situated distally of the buccal dilatators. Retractor muscles are mainly smooth with short distal striated parts. The foregut consists mainly of striated ring musculature with only few longitudinal muscle fibres in the esophagus, while the remaining parts of the digestive tract solely exhibit smooth musculature. During budding, apertural and retractor muscles are first to appear, while the parietal muscles appear at a later stage.

CONCLUSIONS

The apertural muscles show high similarity within Ectoprocta and always consist of two sets of muscles. Gymnolaemates and Phylactolaemates show clear differences within their digestive tract musculature, the former showing smooth and longitudinal muscles to a much greater extent than the latter. The complex musculature at the lophophoral base appears promising for inferring phylogenetic relationships, but sufficient comparative data are currently lacking.

Plumatella trasimenica and Plumatella timwoodi, two new species belonging to the 'repens group' from central Italy (Bryozoa : Phylactolaemata : Plumatellidae)

Two new species – Plumatella trasimenica, from Lake Trasimeno and from an external sedimentation pond of a tropical fish farm, and Plumatella timwoodi, from Lake Piediluco – are described. These species belong to the ‘repens group’ (P. bushnelli Wood, 2001; P. geimermassardi Wood & Okamura, 2004; P. nitens Wood, 1996; P. repens (Linné, 1758); P. rugosa Wood, Geimer & Massard, 1998; P. similirepens Wood, 2001; P. viganoi Taticchi, 2010), because the annulus-chamber pores of the floatoblast are always characterised by the presence of long spines regularly distributed around the pores. Here, further morphological features, which distinguish them from the other species of the repens group, are described for both new species. The coexistence in the same sampling sites of species of the repens-group indicates that the new species are not ecomorphs of similar species.