Analysis of the P. lividus sea urchin genome highlights contrasting trends of genomic and regulatory evolution in deuterostomes

Sea urchins are emblematic models in developmental biology and display several characteristics that set them apart from other deuterostomes. To uncover the genomic cues that may underlie these specificities, we generated a chromosome-scale genome assembly for the sea urchin Paracentrotus lividus and an extensive gene expression and epigenetic profiles of its embryonic development. We found that, unlike vertebrates, sea urchins retained ancestral chromosomal linkages but underwent very fast intrachromosomal gene order mixing. We identified a burst of gene duplication in the echinoid lineage and showed that some of these expanded genes have been recruited in novel structures (water vascular system, Aristotle's lantern, and skeletogenic micromere lineage). Finally, we identified gene-regulatory modules conserved between sea urchins and chordates. Our results suggest that gene-regulatory networks controlling development can be conserved despite extensive gene order rearrangement

Expression pattern of Brachyury in the embryo of the sea urchin Paracentrotus lividus

International audienceBrachyury is a key transcription factor whose homologs have been identified in many animal species. Different Brachyury expression patterns have been observed amongst echinoderms. We have isolated PlBra, the Brachyury ortholog from the sea urchin Paracentrotus lividus and analyzed its expression during development. PlBra is first expressed at the end of cleavage in a ring of cells at the border between the presumptive endoderm and mesoderm territories. At later stages, PlBra is expressed around the blastopore and in the stomodaeum area as in most basal deuterostomes

Coquillette, a sea urchin T-box gene of the Tbx2 subfamily, is expressed asymmetrically along the oral-aboral axis of the embryo and is involved in skeletogenesis.

International audienceTranscription factors of the T-domain family regulate many developmental processes. We have isolated from the sea urchin a new member of the Tbx2 subfamily: coquillette. Coquillette has a late zygotic expression whose localization is dynamic: at the blastula stage it is restricted to the aboral side of most of the presumptive ectoderm and endoderm territories and from gastrulation on, to the aboral-most primary mesenchyme cells. Perturbation of coquillette function delays gastrulation and strongly disorganizes the skeleton of the larva. Coquillette is sensitive to alteration of the oral-aboral (OA) axis and we identify goosecoid, which controls oral and aboral fates in the ectoderm, as a probable upstream regulator. Coquillette appears to be an integral part of the patterning system along the OA axis

A comprehensive survey of wnt and frizzled expression in the sea urchin Paracentrotus lividus

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Envelysin

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Frizzled5/8 is required in secondary mesenchyme cells to initiate archenteron invagination during sea urchin development

International audienceWnt signaling pathways play key roles in numerous developmental processes both in vertebrates and invertebrates. Their signals are transduced by Frizzled proteins, the cognate receptors of the Wnt ligands. This study focuses on the role of a member of the Frizzled family, Fz5/8, during sea urchin embryogenesis. During development, Fz5/8 displays restricted expression, beginning at the 60-cell stage in the animal domain and then from mesenchyme blastula stage, in both the animal domain and a subset of secondary mesenchyme cells (SMCs). Loss-of-function analyses in whole embryos and chimeras reveal that Fz5/8 is not involved in the specification of the main embryonic territories. Rather, it appears to be required in SMCs for primary invagination of the archenteron, maintenance of endodermal marker expression and apical localization of Notch receptors in endodermal cells. Furthermore, among the three known Wnt pathways, Fz5/8 appears to signal via the planar cell polarity pathway. Taken together, the results suggest that Fz5/8 plays a crucial role specifically in SMCs to control primary invagination during sea urchin gastrulation

Nemo-like kinase (NLK) acts downstream of Notch/Delta signalling to downregulate TCF during mesoderm induction in the sea urchin embryo

International audienceStudies in Caenorhabditis elegans and vertebrates have established that the MAP kinase-related protein NLK counteracts Wnt signalling by downregulating the transcription factor TCF. Here, we present evidence that during early development of the sea urchin embryo, NLK is expressed in the mesodermal precursors in response to Notch signalling and directs their fate by downregulating TCF. The expression pattern of nlk is strikingly similar to that of Delta and the two genes regulate the expression of each other. nlk overexpression, like ectopic activation of Notch signalling, provoked massive formation of mesoderm and associated epithelial mesenchymal transition. NLK function was found to be redundant with that of the MAP kinase ERK during mesoderm formation and to require the activity of the activating kinase TAK1. In addition, the sea urchin NLK, like its vertebrate counterpart, antagonizes the activity of the transcription factor TCF. Finally, activating the expression of a TCF-VP16 construct at blastula stages strongly inhibits endoderm and mesoderm formation, indicating that while TCF activity is required early for launching the endomesoderm gene regulatory network, it has to be downregulated at blastula stage in the mesodermal lineage. Taken together, our results indicate that the evolutionarily conserved TAK/NLK regulatory pathway has been recruited downstream of the Notch/Delta pathway in the sea urchin to switch off TCF-beta-catenin signalling in the mesodermal territory, allowing precursors of this germ layer to segregate from the endomesoderm

A wnt2 ortholog in the sea urchin Paracentrotus lividus

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Numerical investigation of an experimental Kyropoulos process to grow silicon ingots for photovoltaic application

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