Origine et diversification des mécanismes de régionalisation de la blastula chez les cordés – EvolAx

The molecular mechanisms controlling development at stages preceding gastrulation seem to extensively diverge during evolution, even across relatively closely related species such as the vertebrates. Analyses focused on the major deuterostome model organisms (sea urchin, ascidian, and in vertebrates, zebrafish, xenopus, mouse and chick) have highlighted striking similarities in the roles of FGF and Nodal signaling. However, robust interpretations in terms of conservations are generally hampered by extensively different morphologies and species- or taxon- specific divergence processes. Understanding the underlying unity of the mechanisms controlling early development is essential to gain insight into the origin of bilaterality in deuterostomes. Blastula patterning mechanisms also provide an excellent model system to elucidate the molecular bases of the plasticity of developmental processes during evolution. In order to gain insight into their origin and evolution in chordates, we propose an evo-devo approach, aimed at reconstructing the vertebrate and chordate ancestral states. This analysis will rely on the study of representatives of three groups, which occupy key phylogenetic positions in vertebrates and chordates but have nevertheless been largely ignored by geneticists until recently: the dogfish Scyliorhinus canicula, the lamprey Petromyzon marinus and the amphioxus Branchiostoma lanceolatum. The applicants are part of the handful of groups that have pioneered the use of these models at the international level and have an acknowledged expertise in this field. Large-scale molecular databases (ESTs or genomic) as well as infrastructures allowing reliable productions of embryos in large quantities now make developmental genetics approaches accessible in these species. Our experimental strategy will rely on three complementary analyses (1) an exhaustive molecular characterization of the blastula aimed at understanding its regionalization and identifying its signaling centers, (2) an in situ, quantitative analysis of key signaling activities (Bmp, Nodal/activin, FGF, Wnt) by whole-mount fluorescent immunostaining and (3) functional analyses using pharmacological treatments with antagonists of these signaling pathways. We will in particular test candidate inhibitors of canonical Wnt signaling. The activity of such molecules, whose study is underway in the laboratory of one of the partners, will be validated during the course of the project, using xenopus early development as a reference system. Taken together, the results of the project should lead to the understanding of the roles of key signaling pathways in the establishment of the blastula molecular pattern in the three species studied. Comparisons with more traditional deuterostome model organisms should provide new insights into the ancestral characteristics of early development in vertebrates and into their origin in chordates. They could also allow a better understanding of the molecular bases of taxa-specific evolutionary trends related, for instance, to embryo size or the presence of yolk. These data are essential to understand the origin of the mechanisms of axis specification at a broader evolutionary scale, in metazoans.