Identification of a novel gene involved in asexual organogenesis in the budding ascidian Botryllus schlosseri

Heterogeneity of primordial germ cells

Primordial germ cells (PGCs) must complete a complex and dynamic developmental program during embryogenesis to establish the germline. This process is highly conserved and involves a diverse array of tasks required of PGCs, including migration, survival, sex differentiation, and extensive epigenetic reprogramming. A common theme across many organisms is that PGC success is heterogeneous: only a portion of all PGCs complete all these steps while many other PGCs are eliminated from further germline contribution. The differences that distinguish successful PGCs as a population are not well understood. Here, we examine variation that exists in PGCs as they navigate the many stages of this developmental journey. We explore potential sources of PGC heterogeneity and their potential implications in affecting germ cell behaviors. Lastly, we discuss the potential for PGC development to function as a multistage selection process that assesses heterogeneity in PGCs to refine germline quality.

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.

How One Thing Led to Another

I started research in high school, experimenting on immunological tolerance to transplantation antigens. This led to studies of the thymus as the site of maturation of T cells, which led to the discovery, isolation, and clinical transplantation of purified hematopoietic stem cells (HSCs). The induction of immune tolerance with HSCs has led to isolation of other tissue-specific stem cells for regenerative medicine. Our studies of circulating competing germline stem cells in colonial protochordates led us to document competing HSCs. In human acute myelogenous leukemia we showed that all preleukemic mutations occur in HSCs, and determined their order; the final mutations occur in a multipotent progenitor derived from the preleukemic HSC clone. With these, we discovered that CD47 is an upregulated gene in all human cancers and is a "don't eat me" signal; blocking it with antibodies leads to cancer cell phagocytosis. CD47 is the first known gene common to all cancers and is a target for cancer immunotherapy.

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.

Ontology for the asexual development and anatomy of the colonial chordate Botryllus schlosseri

Ontologies provide an important resource to integrate information. For developmental biology and comparative anatomy studies, ontologies of a species are used to formalize and annotate data that are related to anatomical structures, their lineage and timing of development. Here, we have constructed the first ontology for anatomy and asexual development (blastogenesis) of a bilaterian, the colonial tunicate Botryllus schlosseri. Tunicates, like Botryllus schlosseri, are non-vertebrates and the only chordate taxon species that reproduce both sexually and asexually. Their tadpole larval stage possesses structures characteristic of all chordates, i.e. a notochord, a dorsal neural tube, and gill slits. Larvae settle and metamorphose into individuals that are either solitary or colonial. The latter reproduce both sexually and asexually and these two reproductive modes lead to essentially the same adult body plan. The Botryllus schlosseri Ontology of Development and Anatomy (BODA) will facilitate the comparison between both types of development. BODA uses the rules defined by the Open Biomedical Ontologies Foundry. It is based on studies that investigate the anatomy, blastogenesis and regeneration of this organism. BODA features allow the users to easily search and identify anatomical structures in the colony, to define the developmental stage, and to follow the morphogenetic events of a tissue and/or organ of interest throughout asexual development. We invite the scientific community to use this resource as a reference for the anatomy and developmental ontology of B. schlosseri and encourage recommendations for updates and improvements.

Phagocyte dynamics in a highly regenerative urochordate: insights into development and host defense

Phagocytosis is a cellular process by which particles and foreign bodies are engulfed and degraded by specialized cells. It is functionally involved in nutrient acquisition and represents a fundamental mechanism used to remove pathogens and cellular debris. In the marine invertebrate chordate Botryllus schlosseri, cell corpse engulfment by phagocytic cells is the recurrent mechanism of programmed cell clearance and a critical process for the successful execution of asexual regeneration and colony homeostasis. In the present study, we have utilized a naturally occurring process of vascular parabiosis coupled with intravascular microinjection of fluorescent bioparticles and liposomes as tools to investigate the dynamics of phagocyte behavior in real-time during cyclical body regeneration. Our findings indicate that B. schlosseri harbors two major populations of post-mitotic phagocytes, which display distinct phagocytic specificity and homing patterns: a static population that lines the circulatory system epithelia, and a mobile population that continuously recirculates throughout the colony and exhibits a characteristic homing pattern within mesenchymal niches called ventral islands (VI). We observed that a significant proportion of ventral island phagocytes (VIP) die and are engulfed by other VIP following takeover. Selective impairment of VIP activity curtailed zooid resorption and asexual development. Together, these findings strongly suggest that ventral islands are sites of phagocyte homing and turnover. As botryllid ascidians represent invertebrate chordates capable of whole body regeneration in a non-embryonic scenario, we discuss the pivotal role that phagocytosis plays in homeostasis, tissue renewal and host defense.

Asexual propagation and regeneration in colonial ascidians

Regeneration is widely distributed among the metazoans. However, clear differences exist as to the degree of regenerative capacity: some phyla can only replace missing body parts, whereas others can generate entirely new individuals. Ascidians are animals that possess a remarkable regenerative plasticity and exhibit a great diversity of mechanisms for asexual propagation and survival. They are marine invertebrate members of the subphylum Tunicata and represent modern-day descendants of the chordate ancestor; in their tadpole stage they exhibit a chordate body plan that is resorbed during metamorphosis. Solitary species grow into an adult that can reach several centimeters in length, whereas colonial species grow by asexual propagation, creating a colony of genetically identical individuals. In this review, we present an overview of the biology of colonial ascidians as a paradigm for study in stem cell and regenerative biology. Focusing on botryllid ascidians, we introduce the potential roles played by multipotent epithelia and multipotent/pluripotent stem cells as source of asexual propagation and regenerative plasticity in the different budding mechanisms, and consider the putative mechanism of body repatterning in a non-embryonic scenario. We also discuss the involvement of intra-colony homeostatic processes in regulating budding potential, and the functional link between allorecognition, chimerism, and regenerative potential.

Evolutionary crossroads in developmental biology: the tunicates

The tunicates, or urochordates, constitute a large group of marine animals whose recent common ancestry with vertebrates is reflected in the tadpole-like larvae of most tunicates. Their diversity and key phylogenetic position are enhanced, from a research viewpoint, by anatomically simple and transparent embryos, compact rapidly evolving genomes, and the availability of powerful experimental and computational tools with which to study these organisms. Tunicates are thus a powerful system for exploring chordate evolution and how extreme variation in genome sequence and gene regulatory network architecture is compatible with the preservation of an ancestral chordate body plan.

Natural apoptosis during the blastogenetic cycle of the colonial ascidian Botryllus schlosseri: a morphological analysis

Colonies of the compound ascidian Botryllus schiosseri undergo regular generation changes, during which adult zooids are progressively resorbed and replaced by growing buds. The generation change, or take-over, is characterized by massive cell death by apoptosis, as indicated by nuclear condensation, activation of caspases, overexpression of molecules recognized by antibodies against mammalian Bax, Fas, and FasL, changes in the expression of surface molecules by senescent cells of zooid tissues, and recruitment of circulating phagocytes in zooid tissues which ensure the complete clearing of dying cells. The entire process lasts 24-36 h at 20 degrees C and has been subdivided, on the basis of the degree of contraction of old zooids, into four substages. In the present work, we carried out a detailed morphological analysis of the events occurring in zooid tissues during the take-over substages. Results Indicate that traces of apoptosis can be found in the epidermis, peribranchial epithelium, and heart in the late substage but are easily found in the branchial basket 2-4 h after the beginning of the generation change, thus confirming the antero-posterior progression of cell death, at least in the alimentary system.

Molecular mechanisms of allorecognition in a basal chordate

Allorecognition has been described in many metazoan phyla, from the sponges to the mammals. In vertebrates, allorecognition is a result of a MHC-based recognition event central to adaptive immunity. However, the origin of the adaptive immune system and the potential relationship to more primitive allorecognition systems is unclear. The colonial ascidian, Botryllus schlosseri, has been used as a model organism for the study of allorecognition for over a century, as it undergoes a natural transplantation reaction controlled by a single, highly polymorphic locus. Herein we will summarize our current understanding of the molecular mechanisms that underlie this innate allorecognition reaction.

Sea squirts and immune tolerance

Transplantation specificity and protective immunity occur in both adaptive and innate branches of the vertebrate immune system. Understanding the mechanisms that underlie specificity and self-tolerance of immune function has major significance, from preventing a rejection reaction after transplantation to dissecting the causes of autoimmune disease. The core of vertebrate immunity is the ability to discriminate between highly polymorphic ligands, and this process is also found in allorecognition systems throughout the metazoa. Botryllus schlosseri is a tunicate, the modern-day descendents of the phylum that made the transition between invertebrates and vertebrates. In addition, B. schlosseri undergoes a natural transplantation reaction, which is controlled by a single, highly polymorphic locus called fuhc, reminiscent of major histocompatibility complex (MHC)-based allorecognition. The life-history characteristics of Botryllus make it an excellent model to dissect the functional and developmental mechanisms underlying allorecognition, and have the potential to reveal novel insights into issues from innate recognition strategies to the evolution of genetic polymorphism. In addition, we hypothesize that allorecognition in Botryllus must be based on conserved processes that are fundamental to all immune function: education and tolerance, or the ontogeny and maintenance of specificity.

Functional analysis of Pitx during asexual regeneration in a basal chordate

Embryogenesis in ascidians is the classic example of mosaic development, yet within this phyla a number of colonial species exist which as adults can reproduce entire bodies asexually. The colonial ascidian Botryllus schlosseri is an excellent model to study this process: on a weekly basis it regenerates all somatic and germline tissues, and while these processes have been characterized morphologically at high-resolution over the last 70 years, almost nothing is known regarding the genetic basis of asexual development and its relationship to embryogenesis. In this study, we functionally characterized the role of the paired-related homeobox transcription factor, Pitx, during this regenerative process. During ascidian embryogenesis Pitx seems to be multifunctional and involved in the formation of multiple tissues, including the stomodeum, pituitary gland, and determination of left-right asymmetry, similar to other deuterostomes. Previous spatial-temporal expression studies during asexual regeneration in Botryllus adults suggest the same roles in this developmental program. Here, we analyzed Pitx function using RNA interference at distinct stages of asexual development. Pitx phenotypes were described focusing on each developmental stage both in vivo, and via histological analysis, and were found to correspond to expression patterns; with the exception that normal asymmetries in the gut were not affected by knockdown. As mRNA destruction is not instantaneous, we found that by tailoring our short interfering double-stranded RNA delivery different developmental processes could be studied independently. This allows a reverse genetic approach to dissect asexual developmental pathways, even in cases involving multifunctional, ubiquitously expressed genes like Pitx.

A conserved role of the VEGF pathway in angiogenesis of an ectodermally-derived vasculature

Angiogenesis, the growth and remodeling of a vascular network, is an essential process during development, growth and disease. Here we studied the role of the vascular endothelial growth factor receptor (VEGFR) in experimentally-induced angiogenesis in the colonial ascidian Botryllus schlosseri (Tunicata, Ascidiacea). The circulatory system of B. schlosseri is composed of two distinct, but interconnected regions: a plot of sinuses and lacunae which line the body, and a transparent, macroscopic extracorporeal vascular network. The vessels of the extracorporeal vasculature are morphologically inverted in comparison to the vasculature in vertebrates: they consist of a single layer of ectodermally-derived cells with the basal lamina lining the lumen of the vessel. We found that when the peripheral circulatory system of a colony is surgically removed, it can completely regenerate within 24 to 48 h and this regeneration is dependent on proper function of the VEGF pathway: siRNA-mediated knockdown of the VEGFR blocked vascular regeneration, and interfered with vascular homeostasis. In addition, a small molecule, the VEGFR kinase inhibitor PTK787/ZK222584, phenocopied the siRNA knockdown in a reversible manner. Despite the disparate germ layer origins and morphology of the vasculature, the developmental program of branching morphogenesis during angiogenesis is controlled by similar molecular mechanisms, suggesting that the function of the VEGF pathway may be co-opted during the regeneration of an ectoderm-derived tubular structure.

Divergent roles of the DEAD-box protein BS-PL10, the urochordate homologue of human DDX3 and DDX3Y proteins, in colony astogeny and ontogeny

Proteins of the highly conserved PL-10 (Ded1P) subfamily of DEAD-box family, participate in a wide variety of biological functions. However, the entire spectrum of their functions in both vertebrates and invertebrates is still unknown. Here, we isolated the Botryllus schlosseri (Urochordata) homologue, BS-PL10, revealing its distributions and functions in ontogeny and colony astogeny. In botryllid ascidians, the colony grows by increasing the number of modular units (each called a zooid) through a whole colony synchronized and weekly cyclical astogenic budding process (blastogenesis). At the level of the colony, both BS-PL10 mRNA and its protein (78 kDa) fluctuate in a weekly pattern that corresponds with the animal's blastogenic cycle, increasing from blastogenic stage A to blastogenic stage D. At the organ/module level, a sharp decline is revealed. Primary and secondary developing buds express high levels of BS-PL10 mRNA and protein at all blastogeneic stages. These levels are reduced four to nine times in the new set of functional zooids. This portrait of colony astogeny differed from its ontogeny. Oocytes and sperm cells express high levels of BS-PL10 protein only at early stages of development. Young embryos reveal background levels with increased expressions in some organs at more developed stages. Results reveal that higher levels of BS-PL10 mRNA and protein are characteristic to multipotent soma and germ cells, but patterns deviate between two populations of differentiating stem cells, the stem cells involved in weekly blastogenesis and stem cells involved in embryogenesis. Two types of experimental manipulations, zooidectomy and siRNA assays, have confirmed the importance of BS-PL10 for cell differentiation and organogenesis. BS-PL10 (phylogenetically matching the animal's position in the evolutionary tree), is the only member of this subfamily in B. schlosseri, featuring a wide range of biological activities, some of which represent pivotal roles. The surprising weekly cyclical expression and the participation in cell differentiation posit this molecule as a model system for studying PL10 protein subfamily.

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

Botryllus schlosseri, a cosmopolitan colonial ascidian reared in the laboratory for more than 50 years, reproduces both sexually and asexually and is used as a model organism for studying a variety of biological problems. Colonies are formed of numerous, genetically identical individuals (zooids) and undergo cyclical generation changes in which the adult zooids die and are replaced by their maturing buds. Because the progression of the colonial life cycle is intimately correlated with blastogenesis, a shared staging method of bud development is required to compare data coming from different laboratories. With the present review, we aim (1) to introduce B. schlosseri as a valuable chordate model to study various biological problems and, especially, sexual and asexual development; (2) to offer a detailed description of bud development up to adulthood and the attainment of sexual maturity; (3) to re-examine Sabbadin's (1955) staging method and re-propose it as a simple tool for in vivo recognition of the main morphogenetic events and recurrent changes in the blastogenetic cycle, as it refers to the developmental stages of buds and adults.

Involvement of vasa homolog in germline recruitment from coelomic stem cells in budding tunicates

We investigated the mechanism by which germline cells are recruited in every asexual reproductive cycle of the budding tunicate Polyandrocarpa misakiensis using a vasa homolog (PmVas) as the germline-specific probe. A presumptive gonad of Polyandrocarpa arose as a loose cell aggregate in the ventral hemocoel of a 1-week-old developing zooid. It developed into a compact clump of cells and then separated into two lobes, each differentiating into the ovary and the testis. The ovarian tube that was formed at the bottom of the ovary embedded the oogonia and juvenile oocytes, forming the germinal epithelium. PmVas was expressed strongly by loose cell aggregates, compact clumps, and peripheral germ cells in the testis and germinal epithelium. No signals were detected in growing buds and less than 1-week-old zooids, indicating that germ cells arise de novo in developing zooids of P. misakiensis. Cells of the loose cell aggregates were 5-6 mum in diameter. They looked like undifferentiated hemoblasts in the hemocoel. To examine the involvement of PmVas in the germline recruitment at postembryonic stages, both growing buds and 1-week-old developing zooids were soaked with double-stranded PmVas RNA. The growing buds developed into fertile zooids expressing PmVas, whereas the 1-week-old zooids developed into sterile zooids that did not express PmVas. In controls (1-week-old zooids) soaked with double-stranded lacZ RNA, the gonad developed normally. These results strongly suggest that in P. misakiensis, PmVas plays a decisive role in switching from coelomic stem cells to germ cells.

fester, A candidate allorecognition receptor from a primitive chordate

Histocompatibility in the primitive chordate, Botryllus schlosseri, is controlled by a single, highly polymorphic locus, the FuHC. By taking a forward genetic approach, we have identified a locus encoded near the FuHC, called fester, which is polymorphic, polygenic, and inherited in distinct haplotypes. Somatic diversification occurs through extensive alternative splicing, with each individual expressing a unique repertoire of splice forms, both membrane bound and potentially secreted, all expressed in tissues intimately associated with histocompatibility. Functional studies, via both siRNA-mediated knockdown and direct blocking by monoclonal antibodies raised against fester, were able to disrupt predicted histocompatibility outcomes. The genetic and somatic diversity, coupled to the expression and functional data, suggests that fester is a receptor involved in histocompatibility.