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

The Wnt's Tale: On the Evolution of a Signaling Pathway

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Mécanismes de régionalisation de l'embryon d'oursin le long des axes animal-végétatif et oral-aboral

Chez l'embryon d'oursin, modèle classique en Biologie du Développement, l'établissement des principaux feuillets fait intervenir de nombreux gènes codant généralement pour des composants de voie de signalisation. Nous avons isolé trois facteurs de transcription à T-box : ske-T, Brachyury et coquillette. Le profil d'expression de ske-T et de coquillette ainsi que l'analyse fonctionnelle de ce dernier indiquent que ces deux gènes sont requis durant la squelettogenèse. Brachyury, exprimé de façon dynamique dans les cellules en cours d'invagination aux deux extrémités du tube digestif, jouerait un rôle important durant la gastrulation. L'expression zygotique de Frizzled, récepteur membranaire des ligands Wnt, est spatialement limitée. Elle est détectée au pôle animal et dans un anneau de cellules localisées autour du pôle végétatif et qui forment ensuite le sommet du tube digestif. La surexpression de formes dominantes-négatives de Frizzled n'interfère pas avec la spécification initiale des principales lignées cellulaires mais entraîne la perte de formation du tube digestif et la désorganisation du squelette. Frizzled serait donc requis dans les mouvements cellulaires associés à la gastrulation et à la squelettogenèse. La kinase Nlk est successivement exprimée dans les cellules précurseurs du mésenchyme primaire, du mésenchyme secondaire, dans l'endoderme et dans certaines cellules de l'ectoderme. Les anomalies causées par la surexpression de cette kinase révèlent qu'elle agit en synergie avec la voie de Notch. Elle paraît impliquée dans les mécanismes de la spécification des cellules du mésenchyme secondaire et de mise en place des frontières embryonnaires.In the sea urchin embryo, a traditional model in Developmental Biology, the establishment of the germ layers involve many genes whose generally encoded components of signalling pathways. We have isolated three different T-box transcription factors : ske-T, Brachyury and coquillette. The expression pattern of ske-T and coquillette and the functional analysis of coquillette indicate that these two genes are implicated in the skeletogenesis. Brachyury, which is expressed in a dynamic pattern in the invaginating cells at the two ends of the archenteron, seems to play a crucial role during gastrulation. The zygotic expression of Frizzled, the receptor of the Wnt ligands, is spatially restricted. It is detected at the animal pole and in a ring of cells around the vegetal pole which later form the tip of the gut. The overexpression of dominant-negatives forms of Frizzled does not interfere with the initial specification of the main territories but it blocks the formation of the gut and perturbs the organization of the skeleton. Frizzled seems to be required for the cellular movements that occur during gastrulation and skeletogenesis. The NLK kinase is successively expressed in the precursors of the primary mesenchyme cells, secondary mesenchyme cells, in the endodermic cells and in some ectodermic cells. The perturbations caused by the overexpression of the wild type form of this kinase reveal that it should act in synergy with Notch signalling. Nlk appears to be involved in specification of the secondary mesenchyme cells and in the establishment of the embryonic borders.NICE-BU Sciences (060882101) / SudocVILLEFRANCHE/MER-Observ.Océano (061592201) / SudocSudocFranceF

The canonical Wnt pathway in embryonic axis polarity

International audienceThe canonical Wnt pathway plays crucial roles in multiple developmental processes, including in axis specification. Throughout the animal kingdom, this pathway has been reported to drive patterning of axes as different as the animal-vegetal axis in echinoderms to the dorsal-ventral axis in vertebrates. Intriguingly enough, this pathway appears structurally and functionally well conserved during evolution. However, differences between these phyla are observed that explain how a same pathway can mediate establishment of two such apparently distinct axes. This review compares the axis specification processes used in two evolutionarily distant embryos, the sea urchin and Xenopus

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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