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.

Centrioles are microtubule-based organelles important for the formation of cilia, flagella and centrosomes. Despite progress in understanding the underlying assembly mechanisms, how centriole integrity is ensured is incompletely understood, including in sperm cells, where such integrity is particularly critical. We identified C. elegans sas-1 in a genetic screen as a locus required for bipolar spindle assembly in the early embryo. Our analysis reveals that sperm-derived sas-1 mutant centrioles lose their integrity shortly after fertilization, and that a related defect occurs when maternal sas-1 function is lacking. We establish that sas-1 encodes a C2 domain containing protein that localizes to centrioles in C. elegans, and which can bind and stabilize microtubules when expressed in human cells. Moreover, we uncover that SAS-1 is related to C2CD3, a protein required for complete centriole formation in human cells and affected in a type of oral-facial-digital (OFD) syndrome. Centrioles are microtubule-based organelles critical for forming cilia, flagella and centrosomes. Centrioles are very stable, but how such stability is ensured is poorly understood. We identified sas-1 as a component that contributes to centriole stability in C. elegans. Centrioles that lack sas-1 function loose their integrity, and our analysis reveals that sas-1 is particularly important for sperm-derived centrioles. Moreover, we show that SAS-1 binds and stabilizes microtubules in human cells, together leading us to propose that SAS-1 acts by stabilizing centriolar microtubules. We identify C2CD3 as a human homolog of SAS-1. C2CD3 is needed for the presence of the distal part of centrioles in human cells, and we thus propose that this protein family is broadly needed to maintain centriole structure.

Whole-body regeneration (WBR)—the formation of an entire adult from only a small fragment of its own tissue—is extremely rare among chordates. Exceptionally, in the colonial ascidian Botrylloides leachii (Savigny, 1816) a fully functional adult is formed from their common vascular system after ablation of all adults from the colony in just 10 d, thanks to their high blastogenetic potential. While previous studies have identified key genetic markers and morphological changes, no study has yet focused on the hematological aspects of regeneration despite the major involvement of the remaining vascular system and the contained hemocytes in this process. To dissect this process, we analyzed colony blood flow patterns using time-lapse microscopy to obtain a quantitative description of the velocity, reversal pattern, and average distance traveled by hemocytes. We also observed that flows present during regeneration are powered by temporally and spatially synchronized contractions of the terminal ampullae. In addition, we revised previous studies of B. leachii hematology as well as asexual development using histological sectioning and compared the role played by hemocytes during WBR. We found that regeneration starts with a rapid healing response characterized by hemocyte aggregation and infiltration of immunocytes, followed by increased activity of hemoblasts, recruitment of macrophage-like cells for clearing the tissues of debris, and their subsequent disappearance from the circulation concomitant with the maturation of a single regenerated adult. Overall, we provide a detailed account of the hematological properties of regenerating B. leachii colonies, providing novel lines of inquiry toward the decipherment of regeneration in chordates.

The early Caenorhabditis elegans embryo is an attractive model to investigate evolutionarily conserved cellular mechanisms. However, there is a paucity of automated methods to gather quantitative information with subcellular precision in this system. We developed ASSET (Algorithm for the Segmentation and the Standardization of C. elegans Time‐lapse recordings) to fill this need. ASSET automatically detects the eggshell and the cell cortex from DIC time‐lapse recordings of live one‐cell‐stage embryos and can also track subcellular structures using fluorescent time‐lapse microscopy. Importantly, ASSET standardizes the data into an absolute coordinate system to allow robust quantitative comparisons between embryos. We illustrate how ASSET can efficiently gather quantitative data on the motion of centrosomes and precisely track cortical invaginations, revealing hitherto unnoticed differences between wild‐type and saps‐1(RNAi) embryos. In summary, we establish ASSET as a novel tool for the efficient quantification and standardization of images from early C. elegans embryos. Developmental Dynamics 239:3285–3296, 2011.