Procurement and Culture of Ascidian Embryos

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.

Colonial ascidians strongly preyed upon, yet dominate the substrate in a subtropical fouling community

Higher diversity and dominance at lower latitudes has been suggested for colonial species. We verified this pattern in species richness of ascidians, finding that higher colonial-to-solitary species ratios occur in the tropics and subtropics. At the latitudinal region with the highest ratio, in southeastern Brazil, we confirmed that colonial species dominate space on artificial plates in two independent studies of five fouling communities. We manipulated settlement plates to measure effects of predation and competition on growth and survivorship of colonial versus solitary ascidians. Eight species were subjected to a predation treatment, i.e. caged versus exposed to predators, and a competition treatment, i.e. leaving versus removing competitors, to assess main and interactive effects. Predation had a greater effect on growth and survivorship of colonial compared to solitary species, whereas competition did not show consistent patterns. We hypothesize that colonial ascidians dominate at this subtropical site despite being highly preyed upon because they regrow when partially consumed and can adjust in shape and space to grow into refuges. We contend that these means of avoiding mortality from predation can have large influences on diversification patterns of colonial species at low latitudes, where predation intensity is greater.

Raman spectroscopic imaging of the whole Ciona intestinalis embryo during development

Intracellular composition and the distribution of bio-molecules play central roles in the specification of cell fates and morphogenesis during embryogenesis. Consequently, investigation of changes in the expression and distribution of bio-molecules, especially mRNAs and proteins, is an important challenge in developmental biology. Raman spectroscopic imaging, a non-invasive and label-free technique, allows simultaneous imaging of the intracellular composition and distribution of multiple bio-molecules. In this study, we explored the application of Raman spectroscopic imaging in the whole Ciona intestinalis embryo during development. Analysis of Raman spectra scattered from C. intestinalis embryos revealed a number of localized patterns of high Raman intensity within the embryo. Based on the observed distribution of bio-molecules, we succeeded in identifying the location and structure of differentiated muscle and endoderm within the whole embryo, up to the tailbud stage, in a label-free manner. Furthermore, during cell differentiation, we detected significant differences in cell state between muscle/endoderm daughter cells and daughter cells with other fates that had divided from the same mother cells; this was achieved by focusing on the Raman intensity of single Raman bands at 1002 or 1526 cm(-1), respectively. This study reports the first application of Raman spectroscopic imaging to the study of identifying and characterizing differentiating tissues in a whole chordate embryo. Our results suggest that Raman spectroscopic imaging is a feasible label-free technique for investigating the developmental process of the whole embryo of C. intestinalis.

Embryological methods in ascidians: the Villefranche-sur-Mer protocols

Ascidians (marine invertebrates: urochordates) are thought to be the closest sister groups of vertebrates. They are particularly attractive models because of their non-duplicated genome and the fast and synchronous development of large populations of eggs into simple tadpoles made of about 3,000 cells. As a result of stereotyped asymmetric cleavage patterns all blastomeres become fate restricted between the 16- and 110 cell stage through inheritance of maternal determinants and/or cellular interactions. These advantageous features have allowed advances in our understanding of the nature and role of maternal determinants, inductive interactions, and gene networks that are involved in cell lineage specification and differentiation of embryonic tissues. Ascidians have also contributed to our understanding of fertilization, cell cycle control, self-recognition, metamorphosis, and regeneration. In this chapter we provide basic protocols routinely used at the marine station in Villefranche-sur-Mer using the cosmopolitan species of reference Ciona intestinalis and the European species Phallusia mammillata. These two models present complementary advantages with regard to molecular, functional, and imaging approaches. We describe techniques for basic culture of embryos, micro-injection, in vivo labelling, micro-manipulations, fixation, and immuno-labelling. These methods allow analysis of calcium signals, reorganizations of cytoplasmic and cortical domains, meiotic and mitotic cell cycle and cleavages as well as the roles of specific genes and cellular interactions. Ascidians eggs and embryos are also an ideal material to isolate cortical fragments and to isolate and re-associate individual blastomeres. We detail the experimental manipulations which we have used to understand the structure and role of the egg cortex and of specific blastomeres during development.

Localization and anchorage of maternal mRNAs to cortical structures of ascidian eggs and embryos using high resolution in situ hybridization

In several species, axis formation and tissue differentiation are the result of developmental cascades which begin with the localization and translation of key maternal mRNAs in eggs. Localization and anchoring of mRNAs to cortical structures can be observed with high sensitivity and resolution by fluorescent in situ hybridization coupled with labeling of membranes and macromolecular complexes. Oocytes and embryos of ascidians--marine chordates closely related to vertebrates--are ideal models to understand how maternal mRNAs pattern the simple ascidian tadpole. More than three dozen cortically localized maternal mRNAs have been identified in ascidian eggs. They include germ cell markers such as vasa or pem-3 and determinants of axis (pem-1), unequal cleavage (pem-1), and muscle cells (macho-1). High resolution localization of mRNAs, proteins, and lipids in whole eggs and embryos and their cortical fragments shows that maternal mRNA determinants pem-1 and macho-1 are anchored to cortical endoplasmic reticulum and segregate with it into small posterior somatic cells. In contrast, mRNAs such as vasa are associated with granular structures which are inherited by the same somatic cells plus adjacent germ cell precursors. In this chapter, we provide detailed protocols for simultaneous localization of mRNAs and proteins to determine their association with cellular structures in eggs and embryos. Using preparations of isolated cortical fragments with intact membranous structures allows unprecedented high resolution analysis and identification of cellular anchoring sites for key mRNAs. This information is necessary for understanding the mechanisms for localizing mRNAs and partitioning them into daughter cells after cleavage.

A complement response may activate metamorphosis in the ascidian Boltenia villosa

Ascidian metamorphosis transforms a free-swimming larval chordate ascidian into a sessile adult through a distinct series of metamorphic events. Initially, larvae must become competent to respond to settlement cues. Settlement is then marked by dramatic body plan remodeling and may be accompanied by attachment to the substrate. Subtractive hybridization has revealed that many innate immunity transcripts are upregulated during metamorphosis in the ascidian Boltenia villosa. Several of these genes have well-known roles in the mannose-binding lectin (MBL)-complement pathway of innate immunity, including MBL and MBL-activated serine protease (MASP). MBL recognizes and binds to bacterial pathogens, activates MASP, and triggers the complement cascade. In B. villosa, larvae upregulate BvMASP at the time of competency to initiate settlement. We show that several bacterial strains can induce settlement and that the timing of BvMASP expression in the papillae-associated tissue (PAT) cells is tightly correlated with larval competency. We further demonstrate that serine protease inhibitors used to block the complement response also block metamorphosis, allowing tail resorption, but preventing further morphological changes. Based on these experiments, we propose that the MBL-complement pathway may be important for competency, bacterial substrate detection and body plan remodeling during metamorphosis.

Regulatory roles of nitric oxide during larval development and metamorphosis in Ciona intestinalis

Metamorphosis in the ascidian Ciona intestinalis is a very complex process which converts a swimming tadpole to an adult. The process involves reorganisation of the body plan and a remarkable regression of the tail, which is controlled by caspase-dependent apoptosis. However, the endogenous signals triggering apoptosis and metamorphosis are little explored. Herein, we report evidence that nitric oxide (NO) regulates tail regression in a dose-dependent manner, acting on caspase-dependent apoptosis. An increase or decrease of NO levels resulted in a delay or acceleration of tail resorption, without affecting subsequent juvenile development. A similar hastening effect was induced by suppression of cGMP-dependent NO signalling. Inhibition of NO production resulted in an increase in caspase-3-like activity with respect to untreated larvae. Detection of endogenously activated caspase-3 and NO revealed the existence of a spatial correlation between the diminution of the NO signal and caspase-3 activation during the last phases of tail regression. Real-time PCR during development, from early larva to early juveniles, showed that during all stages examined, NO synthase (NOS) is always more expressed than arginase and it reaches the maximum value at late larva, the stage immediately preceding tail resorption. The spatial expression pattern of NOS is very dynamic, moving rapidly along the body in very few hours, from the anterior part of the trunk to central nervous system (CNS), tail and new forming juvenile digestive organs. NO detection revealed free diffusion from the production sites to other cellular districts. Overall, the results of this study provide a new important link between NO signalling and apoptosis during metamorphosis in C. intestinalis and hint at novel roles for the NO signalling system in other developmental and metamorphosis-related events preceding and following tail resorption.