Botryllus schlosseri: a model ascidian for the study of asexual reproduction

Deep quantitative proteomics of North American Pacific coast star tunicate (Botryllus schlosseri)

Botryllus schlosseri, is a model marine invertebrate for studying immunity, regeneration, and stress-induced evolution. Conditions for validating its predicted proteome were optimized using nanoElute® 2 deep-coverage LCMS, revealing up to 4930 protein groups and 20,984 unique peptides per sample. Spectral libraries were generated and filtered to remove interferences, low-quality transitions, and only retain proteins with >3 unique peptides. The resulting DIA assay library enabled label-free quantitation of 3426 protein groups represented by 22,593 unique peptides. Quantitative comparisons of single systems from a laboratory-raised with two field-collected populations revealed (1) a more unique proteome in the laboratory-raised population, and (2) proteins with high/low individual variabilities in each population. DNA repair/replication, ion transport, and intracellular signaling processes were distinct in laboratory-cultured colonies. Spliceosome and Wnt signaling proteins were the least variable (highly functionally constrained) in all populations. In conclusion, we present the first colonial tunicate's deep quantitative proteome analysis, identifying functional protein clusters associated with laboratory conditions, different habitats, and strong versus relaxed abundance constraints. These results empower research on B. schlosseri with proteomics resources and enable quantitative molecular phenotyping of changes associated with transfer from in situ to ex situ and from in vivo to in vitro culture conditions.

A broad-taxa approach as an important concept in ecotoxicological studies and pollution monitoring

Aquatic invertebrates play a pivotal role in (eco)toxicological assessments because they offer ethical, cost-effective and repeatable testing options. Additionally, their significance in the food chain and their ability to represent diverse aquatic ecosystems make them valuable subjects for (eco)toxicological studies. To ensure consistency and comparability across studies, international (eco)toxicology guidelines have been used to establish standardised methods and protocols for data collection, analysis and interpretation. However, the current standardised protocols primarily focus on a limited number of aquatic invertebrate species, mainly from Arthropoda, Mollusca and Annelida. These protocols are suitable for basic toxicity screening, effectively assessing the immediate and severe effects of toxic substances on organisms. For more comprehensive and ecologically relevant assessments, particularly those addressing long-term effects and ecosystem-wide impacts, we recommended the use of a broader diversity of species, since the present choice of taxa exacerbates the limited scope of basic ecotoxicological studies. This review provides a comprehensive overview of (eco)toxicological studies, focusing on major aquatic invertebrate taxa and how they are used to assess the impact of chemicals in diverse aquatic environments. The present work supports the use of a broad-taxa approach in basic environmental assessments, as it better represents the natural populations inhabiting various ecosystems. Advances in omics and other biochemical and computational techniques make the broad-taxa approach more feasible, enabling mechanistic studies on non-model organisms. By combining these approaches with in vitro techniques together with the broad-taxa approach, researchers can gain insights into less-explored impacts of pollution, such as changes in population diversity, the development of tolerance and transgenerational inheritance of pollution responses, the impact on organism phenotypic plasticity, biological invasion outcomes, social behaviour changes, metabolome changes, regeneration phenomena, disease susceptibility and tissue pathologies. This review also emphasises the need for harmonised data-reporting standards and minimum annotation checklists to ensure that research results are findable, accessible, interoperable and reusable (FAIR), maximising the use and reusability of data. The ultimate goal is to encourage integrated and holistic problem-focused collaboration between diverse scientific disciplines, international standardisation organisations and decision-making bodies, with a focus on transdisciplinary knowledge co-production for the One-Health approach.

The stress granule component TIAR during the non-embryonic development of the colonial ascidian Botryllusschlosseri

TIAR, is a nucleic acid binding protein involved in the formation of cytoplasmic foci known as stress granules, in which mRNA translation is temporarily blocked in response to stressful conditions. TIAR is used as stress granules molecular marker in vertebrates, but it is not so deeply investigated in invertebrates, especially in marine organisms. In the present work, we investigated the role of TIAR in the colonial ascidian Botryllus schlosseri during its non-embryonic development, featured by the cyclical renewal of the colony. We studied the extent of transcription during the colonial blastogenetic cycle and the location of the transcripts in Botryllus tissues. Using an anti-TIAR antibody specific for ascidians, by immunocytochemistry and immunohistochemistry assays, we studied the expression of the protein in haemolymph cells and body tissues and by transmission electron microscopy we identified its subcellular localisation. The anti-TIAR antibody was also microinjected in the circulatory system of B. schlosseri to study its effect on non-embryonic development and immune responses. Results indicate a delay in the progression of the blastogenetic cycle in injected colonies. In addition, degranulation of circulating cytotoxic cells and phagocytosis by professional, circulating phagocytes, two fundamental processes of innate immunity, were also negatively affected.

The novel Orshina Rhythm in a colonial urochordate signifies the display of recurrent aging/rejuvenation sequels

When it comes to aging, some colonial invertebrates present disparate patterns from the customary aging phenomenon in unitary organisms, where a single senescence phenomenon along ontogeny culminates in their inevitable deaths. Here we studied aging processes in 81 colonies of the marine urochordate Botryllus schlosseri each followed from birth to death (over 720 days). The colonies were divided between three life history strategies, each distinct from the others based on the presence/absence of colonial fission: NF (no fission), FA (fission develops after the colony reaches maximal size), and FB (fission develops before the colony reaches maximal size). The study revealed recurring patterns in sexual reproductive statuses (hermaphroditism and male-only settings), colonial vigor, and size. These recurring patterns, collectively referred to as an Orshina, with one or more 'astogenic segments' on the genotype level. The combination of these segments forms the Orshina rhythm. Each Orshina segment lasts about three months (equivalent to 13 blastogenic cycles), and concludes with either the colonial death or rejuvenation, and is manipulated by absence/existing of fission events in NF/FA/FB strategies. These findings indicate that reproduction, life span, death, rejuvenation and fission events are important scheduled biological components in the constructed Orshina rhythm, a novel aging phenomenon.

New Data on the Rhamnose-Binding Lectin from the Colonial Ascidian Botryllus schlosseri: Subcellular Distribution, Secretion Mode and Effects on the Cyclical Generation Change

Botryllus schlosseri in a cosmopolitan ascidian, considered a reliable model organism for studies on the evolution of the immune system. B. schlosseri rhamnose-binding lectin (BsRBL) is synthesised by circulating phagocytes and behaves as an opsonin by interacting with foreign cells or particles and acting as a molecular bridge between them and the phagocyte surface. Although described in previous works, many aspects and roles of this lectin in Botryllus biology remain unknown. Here, we studied the subcellular distribution of BsRBL during immune responses using light and electron microscopy. In addition, following the hints from extant data, suggesting a possible role of BsRBL in the process of cyclical generation change or takeover, we investigated the effects of interfering with this protein, by injecting a specific antibody in the colonial circulation, starting one day before the generation change. Results confirm the requirement of the lectin for a correct generation change and open new queries on the roles of this lectin in Botryllus biology.

Transcriptome landscapes that signify Botrylloides leachi (Ascidiacea) torpor states

Harsh environments enforce the expression of behavioural, morphological, physiological, and reproductive rejoinders, including torpor. Here we study the morphological, cellular, and molecular alterations in torpor architype in the colonial urochordate Botrylloides aff. leachii by employing whole organism Transmission electron (TEM) and light microscope observations, RNA sequencing, real-time polymerase chain reaction (qPCR) quantification of selected genes, and immunolocalization of WNT, SMAD and SOX2 gene expressions. On the morphological level, torpor starts with gradual regression of all zooids and buds which leaves the colony surviving as condensed vasculature remnants that may be 'aroused' to regenerate fully functional colonies upon changes in the environment. Simultaneously, we observed altered distributions of hemolymph cell types. Phagocytes doubled in number, while the number of morula cells declined by half. In addition, two new circulating cell types were observed, multi-nucleated and bacteria-bearing cells. RNA sequencing technology revealed marked differences in gene expression between different organism compartments and states: active zooids and ampullae, and between mid-torpor and naive colonies, or naive and torpid colonies. Gene Ontology term enrichment analyses further showed disparate biological processes. In torpid colonies, we observed overall 233 up regulated genes. These genes included NR4A2, EGR1, MUC5AC, HMCN2 and. Also, 27 transcription factors were upregulated in torpid colonies including ELK1, HDAC3, RBMX, MAZ, STAT1, STAT4 and STAT6. Interestingly, genes involved in developmental processes such as SPIRE1, RHOA, SOX11, WNT5A and SNX18 were also upregulated in torpid colonies. We further validated the dysregulation of 22 genes during torpor by utilizing qPCR. Immunohistochemistry of representative genes from three signaling pathways revealed high expression of these genes in circulated cells along torpor. WNT agonist administration resulted in early arousal from torpor in 80% of the torpid colonies while in active colonies WNT agonist triggered the torpor state. Abovementioned results thus connote unique transcriptome landscapes associated with Botrylloides leachii torpor.

Yamanaka Factors in the Budding Tunicate Botryllus schlosseri Show a Shared Spatio-Temporal Expression Pattern in Chordates

In vertebrates, the four transcription factors Sox2, c-Myc, Pou5f1 and Klf4 are involved in the differentiation of several tissues during vertebrate embryogenesis; moreover, they are normally co-expressed in embryonic stem cells and play roles in pluripotency, self-renewal, and maintenance of the undifferentiated state in adult cells. The in vitro forced co-expression of these factors, named Yamanaka factors (YFs), induces pluripotency in human or mouse fibroblasts. Botryllus schlosseri is a colonial tunicate undergoing continuous stem cell-mediated asexual development, providing a valuable model system for the study of pluripotency in the closest living relatives of vertebrates. In this study, we identified B. schlosseri orthologs of human Sox2 and c-Myc genes, as well as the closest homologs of the vertebrate-specific Pou5f1 gene, through an in-depth evolutionary analysis of the YF gene families in tunicates and other deuterostomes. Then, we studied the expression of these genes during the asexual cycle of B. schlosseri using in situ hybridization in order to investigate their possible involvement in tissue differentiation and in pluripotency maintenance. Our results show a shared spatio-temporal expression pattern consistent with the reported functions of these genes in invertebrate and vertebrate embryogenesis. Moreover, Myc, SoxB1 and Pou3 were expressed in candidate stem cells residing in their niches, while Pou2 was found expressed exclusively in the immature previtellogenic oocytes, both in gonads and circulating in the colonial vascular system. Our data suggest that Myc, SoxB1 and Pou3 may be individually involved in the differentiation of the same territories seen in other chordates, and that, together, they may play a role in stemness even in this colonial ascidian.

The Onset of Whole-Body Regeneration in Botryllus schlosseri: Morphological and Molecular Characterization

Colonial tunicates are the only chordates that regularly regenerate a fully functional whole body as part of their asexual life cycle, starting from specific epithelia and/or mesenchymal cells. In addition, in some species, whole-body regeneration (WBR) can also be triggered by extensive injuries, which deplete most of their tissues and organs and leave behind only small fragments of their body. In this manuscript, we characterized the onset of WBR in Botryllus schlosseri, one colonial tunicate long used as a laboratory model. We first analyzed the transcriptomic response to a WBR-triggering injury. Then, through morphological characterization, in vivo observations via time-lapse, vital dyes, and cell transplant assays, we started to reconstruct the dynamics of the cells triggering regeneration, highlighting an interplay between mesenchymal and epithelial cells. The dynamics described here suggest that WBR in B. schlosseri is initiated by extravascular tissue fragments derived from the injured individuals rather than particular populations of blood-borne cells, as has been described in closely related species. The morphological and molecular datasets here reported provide the background for future mechanistic studies of the WBR ontogenesis in B. schlosseri and allow to compare it with other regenerative processes occurring in other tunicate species and possibly independently evolved.

Effects of Exposure to Trade Antifouling Paints and Biocides on Larval Settlement and Metamorphosis of the Compound Ascidian Botryllus schlosseri

To evaluate the effects of antifouling paints and biocides on larval settlement and metamorphosis, newly hatched swimming larvae of the compound ascidian Botryllus schlosseri, a dominant species of soft-fouling in coastal communities, were exposed to (i) substrata coated with seven antifouling paints on the market containing different biocidal mixtures and types of matrices and (ii) sea water containing various concentrations of eight biocidal constituents. All antifouling paints showed high performance, causing 100% mortality and metamorphic inhibition, with ≥75% not-settled dead larvae. All antifouling biocides prevented the settlement of larvae. The most severe larval malformations, i.e., (i) the formation of a bubble encasing the cephalenteron and (ii) the inhibition of tail resorption, were observed after exposure to metal and organometal compounds, including tributyltin (TBT) at 1 μM (325.5 µg L−1), zinc pyrithione (ZnP) at 1 μM (317.7 µg L−1), and CuCl at 0.1 μM (98.99 µg L−1), and to antimicrobials and fungicides, including Sea-Nine 211 at 1 μM (282.2 µg L−1) and Chlorothalonil at 1 μM (265.9 µg L−1). The herbicides seemed to be less active. Irgarol 1051 was not lethal at any of the concentrations tested. Diuron at 250 μM (58.2 mg L−1) and 2,3,5,6-tetrachloro-4-(methylsulphonyl)pyridine (TCMS pyridine) at 50 μM (14.8 mg L−1) completely inhibited larval metamorphosis. These results may have important implications for the practical use of different antifouling components, highlighting the importance of their testing for negative impacts on native benthic species.

Studying Tunicata WBR Using Botrylloides anceps

Tunicates are marine filter-feeding invertebrates that can be found worldwide and which are the closest phylogenetic group to the vertebrates (Craniata). Of particular interest, colonial tunicates are the only known chordates that can undergo Whole-Body Regeneration (WBR) via vascular budding. In Botrylloides anceps, a fully functional adult regenerates from a fragment of the vascular system in around 2 weeks after amputation. In this chapter, we present protocols to collect B. anceps colonies, confirm their species, breed them in the lab, monitor WBR and perform histological staining on cryosections.

Whole body regeneration and developmental competition in two botryllid ascidians

BACKGROUND

Botryllid ascidians are a group of marine invertebrate chordates that are colonial and grow by repeated rounds of asexual reproduction to form a colony of individual bodies, called zooids, linked by a common vascular network. Two distinct processes are responsible for zooid regeneration. In the first, called blastogenesis, new zooids arise from a region of multipotent epithelium from a pre-existing zooid. In the second, called whole body regeneration (WBR), mobile cells in the vasculature coalesce and are the source of the new zooid. In some botryllid species, blastogenesis and WBR occur concurrently, while in others, blastogenesis is used exclusively for growth, while WBR only occurs following injury or exiting periods of dormancy. In species such as Botrylloides diegensis, injury induced WBR is triggered by the surgical isolation of a small piece of vasculature. However, Botryllus schlosseri has unique requirements that must be met for successful injury induced WBR. Our goal was to understand why there would be different requirements between these two species.

RESULTS

While WBR in B. diegensis was robust, we found that in B. schlosseri, new zooid growth following injury is unlikely due to circulatory cells, but instead a result of ectopic development of tissues leftover from the blastogenic process. These tissues could be whole, damaged, or partially resorbed developing zooids, and we defined the minimal amount of vascular biomass to support ectopic regeneration. We did find a common theme between the two species: a competitive process exists which results in only a single zooid reaching maturity following injury. We utilized this phenomenon and found that competition is reversible and mediated by circulating factors and/or cells.

CONCLUSIONS

We propose that WBR does not occur in B. schlosseri and that the unique requirements defined in other studies only serve to increase the chances of ectopic development. This is likely a response to injury as we have discovered a vascular-based reversible competitive mechanism which ensures that only a single zooid completes development. This competition has been described in other species, but the unique response of B. schlosseri to injury provides a new model to study resource allocation and competition within an individual.

Comparative transcriptomic analysis reveals gene regulation mediated by caspase activity in a chordate organism

BACKGROUND

Apoptosis is a caspase regulated cell death present in all metazoans defined by a conserved set of morphological features. A well-described function of apoptosis is the removal of excessive cells during development and homeostasis. Recent studies have shown an unexpected signalling property of apoptotic cells, affecting cell fate and/or behaviour of neighbouring cells. In contrast to the apoptotic function of cell elimination, this new role of apoptosis is not well understood but seems caspase-dependent. To deepen our understanding of apoptotic functions, it is necessary to work on a biological model with a predictable apoptosis pattern affecting cell fate and/or behaviour. The tunicate Ciona intestinalis has a bi-phasic life cycle with swimming larvae which undergo metamorphosis after settlement. Previously, we have shown that the tail regression step during metamorphosis, characterized by a predictable polarized apoptotic wave, ensures elimination of most tail cells and controls primordial germ cells survival and migration.

RESULTS

We performed differential transcriptomic analysis between control metamorphosing larvae and larvae treated with the pan-caspase inhibitor Z-VAD-fmk in order to explore the transcriptional control of apoptotic cells on neighbouring cells that survive and migrate. When caspase activity was impaired, genes known to be involved in metamorphosis were downregulated along with other implicated in cell migration and survival molecular pathways.

CONCLUSION

We propose these results as a confirmation that apoptotic cells can control surrounding cells fate and as a reference database to explore novel apoptotic functions in animals, including those related to migration and differentiation.

A pan-metazoan concept for adult stem cells: the wobbling Penrose landscape

Adult stem cells (ASCs) in vertebrates and model invertebrates (e.g. Drosophila melanogaster) are typically long‐lived, lineage‐restricted, clonogenic and quiescent cells with somatic descendants and tissue/organ‐restricted activities. Such ASCs are mostly rare, morphologically undifferentiated, and undergo asymmetric cell division. Characterized by ‘stemness’ gene expression, they can regulate tissue/organ homeostasis, repair and regeneration. By contrast, analysis of other animal phyla shows that ASCs emerge at different life stages, present both differentiated and undifferentiated phenotypes, and may possess amoeboid movement. Usually pluri/totipotent, they may express germ‐cell markers, but often lack germ‐line sequestering, and typically do not reside in discrete niches. ASCs may constitute up to 40% of animal cells, and participate in a range of biological phenomena, from whole‐body regeneration, dormancy, and agametic asexual reproduction, to indeterminate growth. They are considered legitimate units of selection. Conceptualizing this divergence, we present an alternative stemness metaphor to the Waddington landscape: the ‘wobbling Penrose’ landscape. Here, totipotent ASCs adopt ascending/descending courses of an ‘Escherian stairwell’, in a lifelong totipotency pathway. ASCs may also travel along lower stemness echelons to reach fully differentiated states. However, from any starting state, cells can change their stemness status, underscoring their dynamic cellular potencies. Thus, vertebrate ASCs may reflect just one metazoan ASC archetype.

Characterisation and functional role of a novel C1qDC protein from a colonial ascidian

As an invertebrate, the compound ascidian Botryllus schlosseri faces nonself only with innate immunity. In this species, we already identified the key components of the lectin and alternative complement activation pathways. In the present work, by mining the transcriptome, we identified a single transcript codifying for a protein, member of the C1q-domain-containing protein family, with a signal peptide followed by two globular C1q (gC1q) domains. It shares a similar domain organisation with C1q/TNF-related proteins 4, the only vertebrate protein family with two gC1q domains. Our gC1q domain-containing protein, called BsC1qDC, is actively transcribed by immunocytes. The transcription is modulated during the Botryllus blastogenetic cycle and is upregulated following the injection of Bacillus clausii cells in the circulation. Furthermore, the injection of bsc1qdc iRNA in the vasculature results in decreased transcription of the gene and a significant impairment of phagocytosis and degranulation, suggesting the involvement of this molecule in immune responses.

Artificial seawater based long-term culture of colonial ascidians

Tunicates are highly diverse marine invertebrate filter-feeders that are vertebrates' closest relatives. These organisms, despite a drastically different body plan during their adulthood, have a tissue complexity related to that of vertebrates. Ascidians, which compose most of the Tunicata, are benthic sessile hermaphrodites that reproduce sexually through a motile tadpole larval stage. Over half of the known ascidians species are able to reproduce asexually by budding, typically leading to the formation of colonies where animals, called zooids, are interconnected through an external vascular system. In addition, colonial ascidians are established models for important biological processes including allorecognition, immunobiology, aging, angiogenesis and whole-body regeneration. However, the current paucity in breeding infrastructures limits the study of these animals to coastal regions. To promote a wider scientific spreading and popularity of colonial ascidians, we have developed a flexible recirculating husbandry setup for their long-term in-lab culture. Our system is inspired both by the flow-through aquariums used by coastal ascidian labs, as well as by the recirculating in-lab systems used for zebrafish research. Our hybrid system thus combines colony breeding, water filtering and food culturing in a semi-automated system where specimens develop on hanging microscopy glass slides. Temperature, light/dark cycles, flow speed and feeding rates can be controlled independently in four different breeding environments to provide room for species-specific optimization as well as for running experiments. This setup is complemented with a quarantine for the acclimatization of wild isolates. Herein we present our success in breeding Botrylloides diegensis, a species of colonial ascidians, for more than 3 years in recirculating artificial seawater over 600 ​km away from their natural habitat. We show that colonies adapt well to in-lab culturing provided that a suitable marine microbiome is present, and that a specific strain can be isolated, propagated and efficiently used for research over prolonged periods of time. The flexible and modular structure of our system can be scaled and adapted to the needs of specific species, such as Botryllus schlosseri, as well as of particular laboratory spaces. Overall, we show that Botrylloides diegensis can be proficiently bred in-land and suggest that our results can be extended to other species of colonial ascidians to promote research on these fascinating animals.

Stem Cells and Innate Immunity in Aquatic Invertebrates: Bridging Two Seemingly Disparate Disciplines for New Discoveries in Biology

The scopes related to the interplay between stem cells and the immune system are broad and range from the basic understanding of organism's physiology and ecology to translational studies, further contributing to (eco)toxicology, biotechnology, and medicine as well as regulatory and ethical aspects. Stem cells originate immune cells through hematopoiesis, and the interplay between the two cell types is required in processes like regeneration. In addition, stem and immune cell anomalies directly affect the organism's functions, its ability to cope with environmental changes and, indirectly, its role in ecosystem services. However, stem cells and immune cells continue to be considered parts of two branches of biological research with few interconnections between them. This review aims to bridge these two seemingly disparate disciplines towards much more integrative and transformative approaches with examples deriving mainly from aquatic invertebrates. We discuss the current understanding of cross-disciplinary collaborative and emerging issues, raising novel hypotheses and comments. We also discuss the problems and perspectives of the two disciplines and how to integrate their conceptual frameworks to address basic equations in biology in a new, innovative way.

Sensing the world and its dangers: An evolutionary perspective in neuroimmunology

Detecting danger is key to the survival and success of all species. Animal nervous and immune systems cooperate to optimize danger detection. Preceding studies have highlighted the benefits of bringing neurons into the defense game, including regulation of immune responses, wound healing, pathogen control, and survival. Here, we summarize the body of knowledge in neuroimmune communication and assert that neuronal participation in the immune response is deeply beneficial in each step of combating infection, from inception to resolution. Despite the documented tight association between the immune and nervous systems in mammals or invertebrate model organisms, interdependence of these two systems is largely unexplored across metazoans. This review brings a phylogenetic perspective of the nervous and immune systems in the context of danger detection and advocates for the use of non-model organisms to diversify the field of neuroimmunology. We identify key taxa that are ripe for investigation due to the emergence of key evolutionary innovations in their immune and nervous systems. This novel perspective will help define the primordial principles that govern neuroimmune communication across taxa.

Vascular Aging in the Invertebrate Chordate, Botryllus schlosseri

Vascular diseases affect over 1 billion people worldwide and are highly prevalent among the elderly, due to a progressive deterioration of the structure of vascular cells. Most of our understanding of these age-related cellular changes comes from in vitro studies on human cell lines. Further studies of the mechanisms underlying vascular aging in vivo are needed to provide insight into the pathobiology of age-associated vascular diseases, but are difficult to carry out on vertebrate model organisms. We are studying the effects of aging on the vasculature of the invertebrate chordate, Botryllus schlosseri. This extracorporeal vascular network of Botryllus is transparent and particularly amenable to imaging and manipulation. Here we use a combination of transcriptomics, immunostaining and live-imaging, as well as in vivo pharmacological treatments and regeneration assays to show that morphological, transcriptional, and functional age-associated changes within vascular cells are key hallmarks of aging in B. schlosseri, and occur independent of genotype. We show that age-associated changes in the cytoskeleton and the extracellular matrix reshape vascular cells into a flattened and elongated form and there are major changes in the structure of the basement membrane over time. The vessels narrow, reducing blood flow, and become less responsive to stimuli inducing vascular regression. The extracorporeal vasculature is highly regenerative following injury, and while age does not affect the regeneration potential, newly regenerated vascular cells maintain the same aged phenotype, suggesting that aging of the vasculature is a result of heritable epigenetic changes.

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

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

BsTLR1: A new member of the TLR family of recognition proteins from the colonial ascidian Botryllus schlosseri

Toll-like receptors (TLRs) represent a well-known family of conserved pattern recognition receptors the importance of which, in non-self recognition, was demonstrated in both vertebrates and invertebrates. Tunicates represent the vertebrate sister group and, as invertebrates, they rely only on innate immunity for their defence. As regards TLRs, two transcripts have been described and characterised in the solitary species Ciona intestinalis, referred to as CiTLR1 and CiTLR2. Using the Ciona TLR nucleotide sequences, we mined our available transcriptome of the colonial ascidian Botryllus schlosseri looking for similar sequences. We were able to identify a sequence, with similarity to CiTLR2 and, through in silico transduction and subsequent sequence analysis, we studied the domain content of the putative protein. The sequence, called BsTLR1, has a TIR and a transmembrane domain, four LLR and two LRR-CT domains. It is actively transcribed by both phagocytes and morula cells, the two circulating immunocyte types. In addition, we analysed bstlr1 transcription in vivo and in vitro, in different phases of the Botryllus blastogenetic cycle and under various experimental conditions. Our data show that there is a change in gene expression and mRNA location, according to the blastogenetic phase. Furthermore, we used a commercial antibody raised against the ectodomain of hTLR5 to study the possible functional role of Botryllus TLR(s). We observed that anti-hTLR5 significantly decreased in vitro phagocytosis and morula cell degranulation, two typical responses to the recognition of nonself. Collectively, our data add new information on the mechanisms of nonself recognition in a colonial ascidian.

Beyond Adult Stem Cells: Dedifferentiation as a Unifying Mechanism Underlying Regeneration in Invertebrate Deuterostomes

The diversity of regenerative phenomena seen in adult metazoans, as well as their underlying mechanistic bases, are still far from being comprehensively understood. Reviewing both ultrastructural and molecular data, the present work aims to showcase the increasing relevance of invertebrate deuterostomes, i.e., echinoderms, hemichordates, cephalochordates and tunicates, as invaluable models to study cellular aspects of adult regeneration. Our comparative approach suggests a fundamental contribution of local dedifferentiation -rather than mobilization of resident undifferentiated stem cells- as an important cellular mechanism contributing to regeneration in these groups. Thus, elucidating the cellular origins, recruitment and fate of cells, as well as the molecular signals underpinning tissue regrowth in regeneration-competent deuterostomes, will provide the foundation for future research in tackling the relatively limited regenerative abilities of vertebrates, with clear applications in regenerative medicine.

Insights into the Complement System of Tunicates: C3a/C5aR of the Colonial Ascidian Botryllus schlosseri

As an evolutionary ancient component of the metazoan immune defense toolkit, the complement system can modulate cells and humoral responses of both innate and (in jawed vertebrates) adaptive immunity. All the three known complement-activation pathways converge on the cleavage of C3 to C3a and C3b. The anaphylatoxin C3a behaves as a chemokine in inflammatory responses, whereas C3b exerts an opsonic role and, ultimately, can activate the lytic pathway. C3aR, one of the mammalian receptors for C3a, is a member of the G-protein-coupled receptor family sharing seven transmembrane alpha helixes. C3aR can act as a chemokine and recruit neutrophils, triggering degranulation and respiratory burst, which initiates an inflammatory reaction. Mining the transcriptome of the colonial ascidian Botryllus schlosseri, we identified a transcript showing homology with both mammalian C3aR and C5aR. The gene (bsc3/c5ar) is actively transcribed in morula cells, the circulating immunocyte triggering the inflammatory reactions in response to the recognition of nonself. Its transcription is modulated during the recurrent cycles of asexual reproduction known as blastogenetic cycles. Moreover, the treatment of hemocytes with C3aR agonist, induces a significant increase in the transcription of BsC3, revealing the presence of an autocrine feedback system able to modulate the expression of C3 in order to obtain a rapid clearance of potentially dangerous nonself cells or particles. The obtained results support the previously proposed role of complement as one of the main humoral components of the immune response in tunicates and stress the importance of morula cells in botryllid ascidian innate immunity.

What is the role of PIWI family proteins in adult pluripotent stem cells? Insights from asexually reproducing animals, planarians

Planarians have a remarkable regenerative ability owing to their adult pluripotent stem cells (aPSCs), which are called "neoblasts." Planarians maintain a considerable number of neoblasts throughout their adulthood to supply differentiated cells for the maintenance of tissue homeostasis and asexual reproduction (fission followed by regeneration). Thus, planarians serve as a good model to study the regulatory mechanisms of in vivo aPSCs. In asexually reproducing invertebrates, such as sponge, Hydra, and planaria, piwi family genes are the markers most commonly expressed in aPSCs. While piwi family genes are known as guardians against transposable elements in the germline cells of animals that only sexually propagate, their functions in the aPSC system have remained elusive. In this review, we introduce recent knowledge on the PIWI family proteins in the aPSC system in planarians and other organisms and discuss how PIWI family proteins contribute to the regulation of the aPSC system.

Phylogenetic analysis of phenotypic characters of Tunicata supports basal Appendicularia and monophyletic Ascidiacea

With approximately 3000 marine species, Tunicata represents the most disparate subtaxon of Chordata. Molecular phylogenetic studies support Tunicata as sister taxon to Craniota, rendering it pivotal to understanding craniate evolution. Although successively more molecular data have become available to resolve internal tunicate phylogenetic relationships, phenotypic data have not been utilized consistently. Herein these shortcomings are addressed by cladistically analyzing 117 phenotypic characters for 49 tunicate species comprising all higher tunicate taxa, and five craniate and cephalochordate outgroup species. In addition, a combined analysis of the phenotypic characters with 18S rDNA-sequence data is performed in 32 OTUs. The analysis of the combined data is congruent with published molecular analyses. Successively up-weighting phenotypic characters indicates that phenotypic data contribute disproportionally more to the resulting phylogenetic hypothesis. The strict consensus tree from the analysis of the phenotypic characters as well as the single most parsimonious tree found in the analysis of the combined dataset recover monophyletic Appendicularia as sister taxon to the remaining tunicate taxa. Thus, both datasets support the hypothesis that the last common ancestor of Tunicata was free-living and that ascidian sessility is a derived trait within Tunicata. "Thaliacea" is found to be paraphyletic with Pyrosomatida as sister taxon to monophyletic Ascidiacea and the relationship between Doliolida and Salpida is unresolved in the analysis of morphological characters; however, the analysis of the combined data reconstructs Thaliacea as monophyletic nested within paraphyletic "Ascidiacea". Therefore, both datasets differ in the interpretation of the evolution of the complex holoplanktonic life history of thaliacean taxa. According to the phenotypic data, this evolution occurred in the plankton, whereas from the combined dataset a secondary transition into the plankton from a sessile ascidian is inferred. Besides these major differences, both analyses are in accord on many phylogenetic groupings, although both phylogenetic reconstructions invoke a high degree of homoplasy. In conclusion, this study represents the first serious attempt to utilize the potential phylogenetic information present in phenotypic characters to elucidate the inter-relationships of this diverse marine taxon in a consistent cladistic framework.

Phylogenetic analysis and screening of antimicrobial and cytotoxic activities of culturable bacteria associated with the ascidian Botryllus schlosseri

AIMS

Isolating culturable bacteria associated with ascidian (Botryllus schlosseri) and investigating their bioactivities to discover new marine microbial resources with potential to produce novel bioactive natural products.

METHODS AND RESULTS

A total of 357 bacteria were isolated from the ascidian B. schlosseri from the coast of Weihai in the north Yellow Sea, China. Of these, 203 isolates were identified by 16S rRNA gene sequencing and they belonged to 52 genera from 30 families in five phyla. The antimicrobial activities and cytotoxic activities of all isolates were determined. Of the 357 isolates, 135 isolates demonstrated antimicrobial activities, and the crude extracts of five isolates showed strong cytotoxicity against human hepatocellular carcinoma Bel 7402 or human cervical carcinoma HeLa cells.

CONCLUSIONS

Our study revealed the diversity of bacteria associated with the ascidian B. schlosseri and reported a broad spectrum of antimicrobial and cytotoxic activities displayed by these isolates.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our results suggest that the culturable bacteria associated with the ascidian B. schlosseri may be a potential source for novel bioactive compounds.

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

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

Complement system and phagocytosis in a colonial protochordate

In the present work, we investigated, in the colonial ascidian Botryllus schlosseri, the role of complement C3 (BsC3) in phagocytosis. We studied the modulation of BsC3 transcription in the course of the colonial blastogenetic cycle, with particular reference to the takeover, when apoptotic cells in the tissues of old zooids are cleared by circulating phagocytes. In situ hybridisation with BsC3 riboprobes labelled only morula cells, the most abundant haemocytes. Anti-hC3 antibody recognised morula cells and also phagocytes when haemocytes were previously incubated with zymosan. The inhibition of C3 activation prevented the labelling of phagocytes. In phagocytosis assays with haemocytes from colonies injected with anti-hC3 antibody or bsc3 iRNA, the capability to ingest target cells was significantly (p < 0.001) reduced. Therefore, our results strongly support a key role of BsC3 in phagocytosis and open to new investigations on the nature of the receptors of the products of BsC3 activation.

Putative stem cells in the hemolymph and in the intestinal submucosa of the solitary ascidian Styela plicata

Background

In various ascidian species, circulating stem cells have been documented to be involved in asexual reproduction and whole-body regeneration. Studies of these cell population(s) are mainly restricted to colonial species. Here, we investigate the occurrence of circulating stem cells in the solitary Styela plicata, a member of the Styelidae, a family with at least two independent origins of coloniality.

Results

Using flow cytometry, we characterized a population of circulating putative stem cells (CPSCs) in S. plicata and determined two gates likely enriched with CPSCs based on morphology and aldehyde dehydrogenase (ALDH) activity. We found an ALDH + cell population with low granularity, suggesting a stem-like state. In an attempt to uncover putative CPSCs niches in S. plicata, we performed a histological survey for hemoblast-like cells, followed by immunohistochemistry with stem cell and proliferation markers. The intestinal submucosa (IS) showed high cellular proliferation levels and high frequency of undifferentiated cells and histological and ultrastructural analyses revealed the presence of hemoblast aggregations in the IS suggesting a possible niche. Finally, we document the first ontogenetic appearance of distinct metamorphic circulatory mesenchyme cells, which precedes the emergence of juvenile hemocytes.

Conclusions

We find CPSCs in the hemolymph of the solitary ascidian Styela plicata, presumably involved in the regenerative capacity of this species. The presence of proliferating and undifferentiated mesenchymal cells suggests IS as a possible niche.

Novel budding mode in Polyandrocarpa zorritensis: a model for comparative studies on asexual development and whole body regeneration

Background

In tunicates, the capacity to build an adult body via non-embryonic development (NED), i.e., asexual budding and whole body regeneration, has been gained or lost several times across the whole subphylum. A recent phylogeny of the family Styelidae revealed an independent acquisition of NED in the colonial species Polyandrocarpa zorritensis and highlighted a novel budding mode. In this paper, we provide the first detailed characterization of the asexual life cycle of P. zorritensis.

Results

Bud formation occurs along a tubular protrusion of the adult epidermis, the stolon, in a vascularized area defined as budding nest. The bud arises through a folding of the epithelia of the stolon with the contribution of undifferentiated mesenchymal cells. This previously unreported mode of bud onset leads to the formation of a double vesicle, which starts to develop into a zooid through morphogenetic mechanisms common to other Styelidae. The budding nest can also continue to accumulate nutrients and develop into a round-shaped structure, designated as spherule, which represents a dormant form able to survive low temperatures.

Conclusions

To understand the mechanisms of NED and their evolution, it is fundamental to start from a robust phylogenetic framework in order to select relevant species to compare. The anatomical description of P. zorritensis NED provides the foundation for future comparative studies on plasticity of budding and regeneration in tunicates.

Electronic supplementary material

The online version of this article (10.1186/s13227-019-0121-x) contains supplementary material, which is available to authorized users.

Differentiation and Induced Sensorial Alteration of the Coronal Organ in the Asexual Life of a Tunicate

Tunicates, the sister group of vertebrates, possess a mechanoreceptor organ, the coronal organ, which is considered the best candidate to address the controversial issue of vertebrate hair cell evolution. The organ, located at the base of the oral siphon, controls the flow of seawater into the organism and can drive the "squirting" reaction, i.e., the rapid body muscle contraction used to eject dangerous particles during filtration. Coronal sensory cells are secondary mechanoreceptors and share morphological, developmental, and molecular traits with vertebrate hair cells. In the colonial tunicate Botryllus schlosseri, we described coronal organ differentiation during asexual development. Moreover, we showed that the ototoxic aminoglycoside gentamicin caused morphological and mechanosensorial impairment in coronal cells. Finally, fenofibrate had a strong protective effect on coronal sensory cells due to gentamicin-induced toxicity, as occurs in vertebrate hair cells. Our results reinforce the hypothesis of homology between vertebrate hair cells and tunicate coronal sensory cells.

IAP genes partake weighty roles in the astogeny and whole body regeneration in the colonial urochordate Botryllus schlosseri

Inhibitors of Apoptosis Protein (IAP) genes participate in processes like apoptosis, proliferation, innate immunity, inflammation, cell motility, differentiation and in malignancies. Here we reveal 25 IAP genes in the tunicate Botryllus schlosseri's genome and their functions in two developmental biology phenomena, a new mode of whole body regeneration (WBR) induced by budectomy, and blastogenesis, the four-staged cycles of botryllid ascidian astogeny. IAP genes that were specifically upregulated during these developmental phenomena were identified, and protein expression patterns of one of these genes, IAP28, were followed. Most of the IAP genes upregulation recorded at blastogenetic stages C/D was in concert with the upregulation at 100 μM H₂O₂ apoptotic-induced treatment and in parallel to expressions of AIF1, Bax, Mcl1, caspase 2 and two orthologues of caspase 7. Wnt agonist altered the takeover duration along with reduced IAP expressions, and displacement of IAP28⁺ phagocytes. WBR was initiated solely at blastogenetic stage D, where zooidal absorption was attenuated and regeneration centers were formed either from remains of partially absorbed zooids or from deformed ampullae. Subsequently, bud-bearing zooids developed, in concert with a massive IAP28-dependent phagocytic wave that eliminated the old zooids, then proceeded with the establishment of morphologically normal-looking colonies. IAP4, IAP14 and IAP28 were also involved in WBR, in conjunction with the expression of the pro-survival PI3K-Akt pathway. IAPs function deregulation by Smac mimetics resulted in severe morphological damages, attenuation in bud growth and differentiation, and in destabilization of colonial coordination. Longtime knockdown of IAP functions prior to the budectomy, resulted in colonial death.

Sixty years of experimental studies on the blastogenesis of the colonial tunicate Botryllus schlosseri

In the second half of the eighteenth century, Schlosser and Ellis described the colonial ascidian Botryllus schlosseri garnering the interest of scientists around the world. In the 1950's scientists began to study B. schlosseri and soon recognized it as an important model organism for the study of developmental biology and comparative immunology. In this review, we summarize the history of B. schlosseri studies and experiments performed to characterize the colony life cycle and bud development. We describe experiments performed to analyze variations in bud productivity, zooid growth and bilateral asymmetry (i.e., the situs viscerum), and discuss zooid and bud removal experiments that were used to study the cross-talk between consecutive blastogenetic generations and vascular budding. We also summarize experiments that demonstrated that the ability of two distinct colonies to fuse or reject is controlled by a single polymorphic gene locus (BHF) with multiple, codominantly expressed alleles. Finally, we describe how the ability to fuse and create chimeras was used to show that within a chimera somatic and germline stem cells compete to populate niches and regenerate tissue or germline organs. Starting from the results of these 60 years of study, we can now use new technological advances to expand the study of B. schlosseri traits and understand functional relationships between its genome and life history phenotypes.

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

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

Whole-Body Regeneration in the Colonial Tunicate Botrylloides leachii

The colonial marine invertebrate Botrylloides leachii belongs to the Tunicata subphylum, the closest invertebrate relatives to the vertebrate group and the only known class of chordates that can undergo whole-body regeneration (WBR). This dramatic developmental process allows a minute isolated fragment of B. leachii's vascular system, or a colony excised of all adults, to restore a functional animal in as little as 10 days. In addition to this exceptional regenerative capacity, B. leachii can reproduce both sexually, through a tadpole larval stage, and asexually, through palleal budding. Thus, three alternative developmental strategies lead to the establishment of filter-feeding adults. Consequently, B. leachii is particularly well suited for comparative studies on regeneration and should provide novel insights into regenerative processes in chordates.Here, after a short introduction on regeneration, we overview the biology of B. leachii as well as the current state of knowledge on WBR in this species and in related species of tunicates. Finally, we highlight the possible future directions that research might take in the study of WBR, including thoughts on technological approaches that appear most promising in this context. Overall, we provide a synthesis of the current knowledge on WBR in B. leachii to support research in this chordate species.

Gonad development and hermaphroditism in the ascidian Botryllus schlosseri

The colonial ascidian Botryllus schlosseri is an ideal model organism for studying gonad development and hermaphroditism. B. schlosseri has been reared in laboratories for over half a century, and its unique biology allows investigators to probe the processes of germ cell migration and gonad formation, resorption, and regeneration. Following metamorphosis, colonies of B. schlosseri show a synchronized and sequential fertility program that, under standard laboratory conditions, begins with a juvenile stage with no visible gonads and subsequently develops testes at 9 weeks followed later by the production of oocytes-thus resulting in hermaphroditic individuals. The timing of oocyte production varies according to the season, and adult B. schlosseri colonies can cycle among infertile and both male and hermaphrodite fertile states in response to changing environmental conditions. Thus, these acidians are amenable to studying the molecular mechanisms controlling fertility, and recent genomic and transcriptomic databases are providing insight to the key genes involved. Here, we review the techniques and approaches developed to study germ cell migration and gonad formation in B. schlosseri, and include novel videos showing processes related to oocyte ovulation and sperm discharge. In the future, this valuable invertebrate model system may help understand the mechanisms of gonad development and regeneration in a chordate. Mol. Reprod. Dev. 84: 158-170, 2017. © 2016 Wiley Periodicals, Inc.

Coupling astogenic aging in the colonial tunicate Botryllus schlosseri with the stress protein mortalin

Botryllus schlosseri, a colonial marine invertebrate, exhibits three generations of short-lived astogenic modules that continuously grow and die throughout the colony's entire lifespan, within week-long repeating budding cycles (blastogenesis), each consisting of four stages (A-D). At stage D, aging is followed by the complete absorption of adult modules (zooids) via a massive apoptotic process. Here we studied in Botryllus the protein mortalin (HSP70s member), a molecule largely known for its association with aging and proliferation. In-situ hybridization and qPCR assays reveal that mortalin follows the cyclic pattern of blastogenesis. Colonies at blastogenic stage D display the highest mortalin levels, and young modules exhibit elevated mortalin levels compared to old modules. Manipulations of mortalin with the specific allosteric inhibitor MKT-077 has led to a decrease in the modules' growth rate and the development of abnormal somatic/germinal morphologies (primarily in vasculature and in organs such as the endostyle, the stomach and gonads). We therefore propose that mortalin plays a significant role in the astogeny and aging of colonial modules in B. schlosseri, by direct involvement in the regulation of blastogenesis.

Immunity in Protochordates: The Tunicate Perspective

Tunicates are the closest relatives of vertebrates, and their peculiar phylogenetic position explains the increasing interest toward tunicate immunobiology. They are filter-feeding organisms, and this greatly influences their defense strategies. The majority of the studies on tunicate immunity were carried out in ascidians. The tunic acts as a first barrier against pathogens and parasites. In addition, the oral siphon and the pharynx represent two major, highly vascularized, immune organs, where circulating hemocytes can sense non-self material and trigger immune responses that, usually, lead to inflammation and phagocytosis. Inflammation involves the recruitment of circulating cytotoxic, phenoloxidase (PO)-containing cells in the infected area, where they degranulate as a consequence of non-self recognition and release cytokines, complement factors, and the enzyme PO. The latter, acting on polyphenol substrata, produces cytotoxic quinones, which polymerize to melanin, and reactive oxygen species, which induce oxidative stress. Both the alternative and the lectin pathways of complement activation converge to activate C3: C3a and C3b are involved in the recruitment of hemocytes and in the opsonization of foreign materials, respectively. The interaction of circulating professional phagocytes with potentially pathogenic foreign material can be direct or mediated by opsonins, either complement dependent or complement independent. Together with cytotoxic cells, phagocytes are active in the encapsulation of large materials. Cells involved in immune responses, collectively called immunocytes, represent a large fraction of hemocytes, and the presence of a cross talk between cytotoxic cells and phagocytes, mediated by secreted humoral factors, was reported. Lectins play a pivotal role as pattern-recognition receptors and opsonizing agents. In addition, variable region-containing chitin-binding proteins, identified in the solitary ascidian Ciona intestinalis, control the settlement and colonization of bacteria in the gut.

Vascular budding in Symplegma brakenhielmi and the evolution of coloniality in styelid ascidians

Individuals of colonial animals (e.g. zooids) are in continuous turnover. In ascidians colonial or solitary species have evolved by convergence multiple times. Colonial Botryllus and Botrylloides are well-studied genera that exhibit colony-wide developmental mechanisms that regulate synchronous and orchestrated cycles of budding and turnover of zooids. The origins of modular developmental mechanisms that facilitated the evolution of coloniality in this group remain unclear. To reconstruct ancestral states of coloniality we studied Symplegma brakenhielmi, a sister taxon of the botryllids. S. brakenhielmi zooids are embedded in a common tunic and present a similar vascular system as the botrylloides, however development and turnover of zooids occurs asynchronously and in a more independent manner. We generated a table of common stages of budding in Symplegma and Botryllus for comparative studies of asexual development. We tested dependent processes of budding among individuals of the colony by systemic bud or zooid removals. Although our results showed a higher degree of independence in bud development in S. brakenhielmi, we found a subtle colony-wide regulatory mechanism of modular development, i.e. new buds expedited development after the removal of all buds in the colony. Next, we characterized external morphology, ultrastructure, and abundance of circulatory blood cells in the vascular system of S. brakenhielmi. Macrophage-like cells (MLCs) are involved in zooid resorption and turnover. Proportions of MLCs in the blood of S. brakenhielmi corresponded to the peak of occurrence of this cell type during the budding cycle of B. schlosseri. We found several new blood cell types in S. brakenhielmi, including two cell types that resemble circulatory progenitor stem cells of other botryllid colonial ascidians. These cells showed features of undifferentiated cells and expressed mitotic marker Phospho-histone H3. Comparative studies of S. brakenhielmi and B. schlosseri allow us to discuss possible changes in the regulation of modular development (i.e. regulation of life and death in the colony), and a possible contribution of circulatory blood cells in budding processes. We propose that the higher degree of developmental independence in S. brakenhielmi budding is a result of its ancestral solitary mode of development.

Protection from Oxidative Stress in Immunocytes of the Colonial Ascidian Botryllus schlosseri: Transcript Characterization and Expression Studies

Botryllus schlosseri is a cosmopolitan colonial ascidian that undergoes cyclical generation changes, or take-overs, during which adult zooids are resorbed and replaced by their buds. At take-over, adult tissues undergo diffuse apoptosis and effete cells are massively ingested by circulating phagocytes, with a consequent increase in oxygen consumption and in production of reactive oxygen species (ROS). The latter are responsible for the death of phagocytes involved in the clearance of apoptotic cells and corpses by phagocytosis-induced apoptosis. However, the majority of phagocytes and hemocytes do not die, even if they experience oxidative stress. This fact suggests the presence of detoxification mechanisms assuring their protection. To test this assumption, we searched for transcripts of genes involved in detoxification in the transcriptome of B. schlosseri. We identified and characterized transcripts for Cu/Zn superoxide dismutase (SOD), γ-glutamyl-cysteine ligase modulatory subunit (GCLM), glutathione synthase (GS), and two glutathione peroxidases (i.e., GPx3 and GPx5), all involved in protection from ROS. We also carried out a phylogenetic analysis of the putative amino acid sequences, confirming their similarity to their vertebrate counterparts, and studied the location of their mRNAs by in situ hybridization on hemocyte monolayers. We also analyzed gene transcription during the colonial blastogenetic cycle, which is the interval of time between one take-over and the next, by qRT-PCR. In addition, we investigated the effects of cadmium (Cd), an inducer of oxidative stress, on gene transcription. Our results indicated that i) antioxidant gene expression is modulated in the course of the blastogenetic cycle and upon exposure to Cd, and ii) hemocytes synthesize both enzymatic and nonenzymatic antioxidants, in line with the idea that they represent a major detoxification system for ascidians.

Recurrent phagocytosis-induced apoptosis in the cyclical generation change of the compound ascidian Botryllus schlosseri

Colonies of the marine, filter-feeding ascidian Botryllus schlosseri undergo cyclical generation changes or takeovers. These events are characterised by the progressive resorption of adult zooids and their replacement by their buds that grow to adult size, open their siphons and start filtering. During the take-over, tissues of adult zooids undergo extensive apoptosis; circulating, spreading phagocytes enter the effete tissues, ingest dying cells acquiring a giant size and a round morphology. Then, phagocytes re-enter the circulation where they represent a considerable fraction (more than 20%) of circulating haemocytes. In this study, we evidence that most of these circulating phagocytes show morphological and biochemical signs of apoptosis. Accordingly, these phagocytes express transcripts of orthologues of the apoptosis-related genes Bax, AIF1 and PARP1. Electron microscopy shows that giant phagocytes contain apoptotic phagocytes inside their own phagocytic vacuole. The transcript of the orthologues of the anti-apoptotic gene IAP7 was detected only in spreading phagocytes, mostly abundant in phases far from the take-over. Therefore, the presented data suggest that, at take-over, phagocytes undergo phagocytosis-induced apoptosis (PIA). In mammals, PIA is assumed to be a process assuring the killing and the complete elimination of microbes, by promoting the disposal of terminally differentiated phagocytes and the resolution of infection. In B. schlosseri, PIA assumes a so far undescribed role, being required for the control of asexual development and colony homeostasis.

Identification of differentially expressed genes from multipotent epithelia at the onset of an asexual development

Organisms that have evolved alternative modes of reproduction, complementary to the sexual mode, are found across metazoans. The chordate Botryllus schlosseri is an emerging model for asexual development studies. Botryllus can rebuild its entire body from a portion of adult epithelia in a continuous and stereotyped process called blastogenesis. Anatomy and ontogenies of blastogenesis are well described, however molecular signatures triggering this developmental process are entirely unknown. We isolated tissues at the site of blastogenesis onset and from the same epithelia where this process is never triggered. We linearly amplified an ultra-low amount of mRNA (<10ng) and generated three transcriptome datasets. To provide a conservative landscape of transcripts differentially expressed between blastogenic vs. non-blastogenic epithelia we compared three different mapping and analysis strategies with a de novo assembled transcriptome and partially assembled genome as references, additionally a self-mapping strategy on the dataset. A subset of differentially expressed genes were analyzed and validated by in situ hybridization. The comparison of different analyses allowed us to isolate stringent sets of target genes, including transcripts with potential involvement in the onset of a non-embryonic developmental pathway. The results provide a good entry point to approach regenerative event in a basal chordate.

Redeployment of germ layers related TFs shows regionalized expression during two non-embryonic developments

In all non-vertebrate metazoan phyla, species that evolved non-embryonic developmental pathways as means of propagation or regeneration can be found. In this context, new bodies arise through asexual reproduction processes (such as budding) or whole body regeneration, that lack the familiar temporal and spatial cues classically associated with embryogenesis, like maternal determinants, or gastrulation. The molecular mechanisms underlying those non-embryonic developments (i.e., regeneration and asexual reproduction), and their relationship to those deployed during embryogenesis are poorly understood. We have addressed this question in the colonial ascidian Botryllus schlosseri, which undergoes an asexual reproductive process via palleal budding (PB), as well as a whole body regeneration by vascular budding (VB). We identified early regenerative structures during VB and then followed the fate of differentiating tissues during both non-embryonic developments (PB and VB) by monitoring the expression of genes known to play key functions in germ layer specification with well conserved expression patterns in solitary ascidian embryogenesis. The expression patterns of FoxA1, GATAa, GATAb, Otx, Bra, Gsc and Tbx2/3 were analysed during both PB and VB. We found that the majority of these transcription factors were expressed during both non-embryonic developmental processes, revealing a regionalization of the palleal and vascular buds. Knockdown of GATAa by siRNA in palleal buds confirmed that preventing the correct development of one of these regions blocks further tissue specification. Our results indicate that during both normal and injury-induced budding, a similar alternative developmental program operates via early commitment of epithelial regions.

Temporally and spatially dynamic germ cell niches in Botryllus schlosseri revealed by expression of a TGF-beta family ligand and vasa

Background

Germ cells are specified during early development and are responsible for generating gametes in the adult. After germ cells are specified, they typically migrate to a particular niche in the organism where they reside for the remainder of its lifetime. For some model organisms, the specification and migration of germ cells have been extensively studied, but how these events occur in animals that reproduce both sexually and asexually is not well understood.

Results

We have identified a novel TGF-β family member in Botryllus schlosseri, tgfβ-f, and found that it is expressed by follicle cell progenitors and the differentiated follicle and support cells surrounding the maturing gametes. Using the expression of tgfβ-f and the germ cell marker vasa, we have found that nearly all germ cells in Botryllus are associated with tgfβ-f-expressing follicle progenitors in clusters consisting solely of those two cell types. These clusters were mostly small, consisting of ten or fewer cells, and generally contained between a 2:1 and 1:1 ratio of follicle progenitors to germ cells. Clusters of germ and follicle progenitor cells were primarily localized to niches in the primary and secondary buds, but could also be found in other locations including the vasculature. We analyzed the location of germ cell clusters throughout the asexual life cycle of Botryllus and found that at the stage when germ cells are first detected in the secondary bud niche, a dramatic change in the size and location of germ/follicle cell clusters also occurred.

Conclusions

Our findings suggest that germ/follicle cell clusters have predictable migratory patterns during the weekly asexual developmental cycle in Botryllus. An increased number of small clusters and the presence of clusters in the vasculature coinciding with the appearance of clusters in the secondary bud suggest that fragmentation of clusters and the migration of smaller clusters through the vasculature may be an important aspect of Botryllus reproductive biology, ensuring the transmission of the germline to subsequent asexual generations.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0047-5) contains supplementary material, which is available to authorized users.

Transcriptome dynamics in the asexual cycle of the chordate Botryllus schlosseri

Background

We performed an analysis of the transcriptome during the blastogenesis of the chordate Botryllus schlosseri, focusing in particular on genes involved in cell death by apoptosis. The tunicate B. schlosseri is an ascidian forming colonies characterized by the coexistence of three blastogenetic generations: filter-feeding adults, buds on adults, and budlets on buds. Cyclically, adult tissues undergo apoptosis and are progressively resorbed and replaced by their buds originated by asexual reproduction. This is a feature of colonial tunicates, the only known chordates that can reproduce asexually.

Results

Thanks to a newly developed web-based platform (http://botryllus.cribi.unipd.it), we compared the transcriptomes of the mid-cycle, the pre-take-over, and the take-over phases of the colonial blastogenetic cycle. The platform is equipped with programs for comparative analysis and allows to select the statistical stringency. We enriched the genome annotation with 11,337 new genes; 581 transcripts were resolved as complete open reading frames, translated in silico into amino acid sequences and then aligned onto the non-redundant sequence database. Significant differentially expressed genes were classified within the gene ontology categories. Among them, we recognized genes involved in apoptosis activation, de-activation, and regulation.

Conclusions

With the current work, we contributed to the improvement of the first released B. schlosseri genome assembly and offer an overview of the transcriptome changes during the blastogenetic cycle, showing up- and down-regulated genes. These results are important for the comprehension of the events underlying colony growth and regression, cell proliferation, colony homeostasis, and competition among different generations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2598-1) contains supplementary material, which is available to authorized users.

Characterization of Ambra1 in asexual cycle of a non-vertebrate chordate, the colonial tunicate Botryllus schlosseri, and phylogenetic analysis of the protein group in Bilateria

Ambra1 is a positive regulator of autophagy, a lysosome-mediated degradative process involved both in physiological and pathological conditions. Nowadays, Ambra1 has been characterized only in mammals and zebrafish. Through bioinformatics searches and targeted cloning, we report the identification of the complete Ambra1 transcript in a non-vertebrate chordate, the tunicate Botryllus schlosseri. Tunicata is the sister group of Vertebrata and the only chordate group possessing species that reproduce also by blastogenesis (asexual reproduction). B. schlosseri Ambra1 deduced amino acid sequence is shorter than vertebrate homologues but still contains the typical WD40 domain. qPCR analyses revealed that the level of B. schlosseri Ambra1 transcription is temporally regulated along the colonial blastogenetic cycle. By means of similarity searches we identified Wdr5 and Katnb1 as proteins evolutionarily associated to Ambra1. Phylogenetic analyses on Bilateria indicate that: (i) Wdr5 is the most related to Ambra1, so that they may derive from an ancestral gene, (ii) Ambra1 forms a group of ancient genes evolved before the radiation of the taxon, (iii) these orthologous Ambra1 share the two conserved WD40/YVTN repeat-like-containing domains, and (iv) they are characterized by ancient duplications of WD40 repeats within the N-terminal domain.