Variability and Robustness of Gastrulation Processes in Deuterostomes : Sea Urchin Twins and Conjoined Zebrafish.

Abstract : The experimental induction of twins (sea urchin) or Siamese (zebrafish) challenges the plasticity and robustness of the developing embryo. We showed that individuals in twin and Siamese pairs follow different developmental paths that are in fact quite different than wild-type. We hypothesize that these different paths reveal the organism’s developmental plasticity. In both species, twinning is interpreted to rely on i) the lability of morphogen gradients setting the embryonic axes, ii) the repertoire of cell behaviors during blastulation and gastrulation in response to morphogen gradients and biomechanical cues and/or iii) the regulation of morphogenetic field size. We showed that the development of sea urchin twins explores a realm of possibilities different from that of typical embryos. The twins develop first as a single and flat cell layer before compacting into a bona fide blastula. Developing twins belong to 3 phenotypic categories depending on the number of germ layer precursors (correct number or not) and their contacts (correct contacts or not). However, the 3 classes lead to similar percentage of normal pluteus larvae. We demonstrate that cellular reorganization is not required to restore the normal contacts between the different cell types implying that development proceeds through fate changes. In addition, larvae from twins have a normal number of primary mesenchymal cells and this does not involve further cell division. This situation is reminiscent of the ablation of micromeres with transfating compensating for the loss, leading to a proper number of PMCs and is thus likely to involve the transfating of SMCs.Siamese zebrafish are observed through Nodal pathway overexpression, transplantation of the Spemann organizer at the onset of gastrulation, or in the maternal homozygous mutant Janus. Our work focused on the first case. The interplay of signaling pathways in setting embryonic axes and inducing a Spemann-type organizing center has been extensively studied in zebrafish. However, the current state of art does not explain the link between biochemical cues and individual cell behaviors, and from them the emergence of morphogenetic fields and the regulation of their size. If not in opposite positions, the extent of field size regulation is dependent on the position of the induced or transplanted organizer. The cell lineage of siamese zebrafish reconstructed from 3D+time live imaging reveals cell behaviors underlying the fusion of identical fields.
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Antonio Ortiz. Variability and Robustness of Gastrulation Processes in Deuterostomes : Sea Urchin Twins and Conjoined Zebrafish.. Embryology and Organogenesis. Université Paris-Saclay, 2018. English. ⟨NNT : 2018SACLS495⟩. ⟨tel-02179270⟩

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