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Étude comparative d'expression de gènes chez un scyphozoaire (Aurelia aurita) et un hydrozoaire (Clytia hemisphaerica) : nouvelles perspectives sur certaines innovations évolutives qui caractérisent leurs stades méduse

Abstract : Three of the five classes of cnidarians possess pelagic medusae. The latter are classically considered homologous, i.e. inherited from a planktonic stage acquired in the ancestor of all medusozoans. However, an impressive array of fundamental differences opposes hydromedusae to cubomedusae/scyphomedusae, e.g. concerning their locomotory nervous systems, sensory organs, and ontogeny. Could there have been several convergent acquisitions of a medusa stage? The research presented here is based on comparative study of a hydromedusa (Clytia hemisphaerica) and a scyphomedusa (Aurelia aurita), involving gene phylogenies, and gene expression surveys using RNA-seq, in situ hybridisation, and qPCR. The main objectives were (i) an improved characterisation of the Clytia tentacle system; and comparisons between these two medusae, of (ii) the deployment of key actors in animal development and morphogenesis, and of (iii) some proteins associated with striated muscles which are absent in polyps and convergent with those of bilaterians. Comparison of Wnt expressions supports conservation among cnidarians of the roles held by several signalling pathways downstream of the Wnt ligands, in the organisation of the oral-aboral polarity and of some elements of the nervous system, and suggests that tentacles and tetraradial symmetry may not be homologous between hydro- and scyphomedusae. Independent recruitments of tropomyosin forms specific to the striated muscles strongly advocate convergent origins of striated muscles and thereby of pelagic locomotion between hydro- and scyphomedusae. These findings challenge the classical scenario of a single medusa acquisition during cnidarian evolution.
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Thomas Condamine. Étude comparative d'expression de gènes chez un scyphozoaire (Aurelia aurita) et un hydrozoaire (Clytia hemisphaerica) : nouvelles perspectives sur certaines innovations évolutives qui caractérisent leurs stades méduse. Biologie animale. Sorbonne Université, 2018. Français. ⟨NNT : 2018SORUS384⟩. ⟨tel-02611784⟩

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