Abstracts
Résumé
Les métazoaires triblastiques, dans leur grande majorité, sont des bilatériens. Le degré d’organisation de leur ancêtre hypothétique commun, Urbilateria, est un problème majeur de l’évolution. Pour certains, qui le font dériver d’une larve planula cnidarienne, ce devait être un organisme simple, tandis que d’autres, qui situent son origine dans une forme polype, pensent qu’il devait déjà présenter un plan d’organisation relativement complexe. L’analyse comparée des caractères morpho-anatomiques, d’une part, et l’étude détaillée des modalités de leur mise en place au cours de l’ontogenèse, envisagée aux niveaux structural et génétique, d’autre part, réalisées sur un ensemble vaste et diversifié de bilatériens, apportent une série d’arguments qui rendent plus probable le second scénario envisagé. L’article que nous présentons et la discussion que nous engageons s’intéressent à la segmentation antéropostérieure du corps : Urbilateria était-il ou non segmenté ?
Summary
In order to establish the portrait of Urbilateria, the common ancestor of triblastic metazoan, this paper focuses on the antero-posterior segmentation frequently considered as characterising the bilaterian bauplan. The synthesis presented here describes the morphological, anatomical and functional aspects of this organisation. Furthermore it analyses the conditions of its emergence during the ontogenesis of Annelids, Arthropods and Chordates and identifies its genetic bases. The provided data exhibit the unitary character of the segmentation modalities among protostomian and deuterostomian organisms. This process occurs in two phases, involving a posterior proliferative zone after the gastrulation. It shows the similarity of the expression patterns of orthologous genes, the implication of comparable signalisation and regulation pathways. The congruence of the results obtained at both structural and molecular levels reinforce the segmental organisation conception of the common ancestor of Bilaterians.
Appendices
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