L’origine des processus fondamentaux du développement animal: approche évo-dévo sur les modèles émergents Clytia hemisphaerica (cnidaire) et Pleurobrachia pileus (cténaire) – DiploDevo

Our knowledge of the core mechanisms that regulate animal development relies heavily on intensive experimental work in the classical model organisms, e.g. fruifly, nematode and the vertebrates, all of them belonging to the Bilateria clade of 'higher animals'. It is, however, becoming more and more apparent that to understand development from an evolutionary perspective, a diversification of model organisms for developmental studies is needed. Animal phyla that are phylogenetically positioned outside the Bilateria (i.e. Porifera Ctenophora, Cnidaria and Placozoa) have begun in recent year to trigger much interest because it is in this basal region of the animal tree that many of the major body plan properties and fundamental developmental processes originated. The current project addresses major evolutionary developmental biology questions, using newly-emerging experimental models covering the two sister-phyla to the Bilateria: the hydrozoan cnidarian Clytia hemisphaerica and the ctenophore Pleurobrachia pileus. Our two groups (Paris group = Partner 1: Michael Manuel, project coordinator; Villefranche group = Partner 2: Evelyn Houliston) seek to combine their efforts, taking advantage of their complementary experience and research themes, to develop and consolidate a position of international predominance in the field. Clytia is a hydrozoan and possesses significant features absent in both major existing cnidarian models Nematostella and Hydra, notably a free-swimming sexual medusa form, alternating in life cycle with vegetatively propagating fixed polyp colonies. Recently published work on Clytia by the Villefranche group has provided a benchmark for molecular analysis of cnidarian embryogenesis. Futhermore, the recent characterisation by the Paris group of nematogenesis (the production of a specialised neuro-sensory cell type called nematocytes) in the Clytia medusa has opened the way to analysis of genes involved at the successive steps of this process. Pleurobrachia pileus is a cosmopolitan member of the seriously understudied phylum Ctenophora, a key phylum, in particular with respect to the evolution of embryonic and adult polarity, and of the neuro-sensory system. Over the next three years we first propose to reinforce the genomic and experimental resources for our two model species, including through 'new generation' high-throughput sequencing and the development of transgenic lines. This effort is aimed at providing a solid platform for the fulfilment of our specific scientific aims, whose global justification is to provide a better understanding of the early evolution of developmental mechanisms. We have chosen to focus our research program around three major and complementary evolutionary questions: (i) the evolutionary origin of the germ line and the relationships between somatic stem cells and germ cells (ii) the evolution of mechanisms that generate embryo and adult polarity and so mediate generation of the developing body plan and (iii) the evolutionary origin of a particularly important animal cell type, the neuro-sensory cell. Our approach involves the generation of original gene expression data (through differential expression analysis, and detailed characterisation by in situ hybridisation for a subset of genes) and gene function analysis in both species, and their comparison with published data on orthologous genes from other organisms under a phylogenetic framework. By accumulating data from a diversity of gene families, we aim to distinguish ancient conserved features of the eumetazoan clade (Cnidaria, Ctenophora and Bilateria) from derived characteristics of particular lineages, and so shed new light on conservation and divergence in fundamental developmental processes.