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Oldenburg, Universiẗat, Diss., 2000. / Dateiformat: Dateien im PDF-Format.

Oldenburg, Universiẗat, Diss., 2000. / Dateiformat: zip, Dateien in unterschiedlichen Formaten.

Mainz, Universiẗat, Diss., 2003.

Universiẗat, Diss., 2003--Mainz.

La métamorphose est une étape cruciale du cycle de vie de beaucoup de métazoaires. Elle est définie comme la transition d’une larve en un juvénile, souvent accompagnée de changements drastiques de morphologie, physiologie et de niche écologique. Le rôle des hormones thyroïdiennes a été mis en évidence dans la métamorphose des amphibiens, mais aussi de nombreux autres chordés comme certains poissons téléostéens ou l'amphioxus, suggérant que ce système était déjà présent et impliqué dans la métamorphose chez l'ancêtre commun de tous les chordés.Nous avons étudié le lien entre hormones thyroïdiennes et métamorphose selon deux axes. Premièrement, nous avons étudié la coordination entre la métamorphose et le recrutement larvaire, chez le chirurgien bagnard Acanthurus triostegus. Ce travail nous a permis de mieux comprendre le rôle de l’hormone thyroïdienne comme signal déclencheur d'une transition écologique importante. Dans le second axe de travail nous avons examiné l’origine de la signalisation thyroïdienne. Chez l’annélide Platynereis dumerili, nous avons identifié un récepteur fonctionnel des hormones et un rôle de l’hormone thyroïdienne dans son développement. Cela qui nous permet de démontrer que la signalisation thyroïdienne était présente à l’origine des bilateriens. Nous avons également mis en évidence un rôle des hormones thyroïdiennes dans le développement de cette espèce. Enfin dans le troisième axe nous nous sommes penchés sur l’origine de la synthèse de l’hormone thyroïdienne en retraçant l’évolution de la thyroglobuline, la protéine nécessaire à la production d’hormone chez les vertébrés. Son absence dans les autres taxons, alors que les dérivés de l’hormone y ont un rôle biologique, pose la question des mécanismes ancestraux de synthèse de ces hormones.Ces travaux explorent l’évolution de la signalisation thyroïdienne et proposent d’aborder la question du lien avec la métamorphose d’un point de vue évolutif et non-seulement développemental, afin de mieux comprendre la diversité des métamorphoses observées dans le monde animal. / Metamorphosis is a critical life step of many metazoans. It is defined as the transition between a larva and a juvenile. It comes with major changes of morphology, physiology and ecology. The role of thyroid hormone has been proven in the metamorphosis of amphibians but also in many other chordates such as teleost fishes or the amphioxus. This suggests that this system was functional and involved in the metamorphosis of the common ancestor of all chordates.We studied the link between thyroid hormones and metamorphosis according to two axes. First, we studied the coordination between metamorphosis and the larval colonization, using the convicted surgeon fish Acanthurus triostegus as a model. This work allowed us to better understand the role of thyroid hormones as a trigger signal of a major ecological transition. On the second axis, we investigated the origin of thyroid hormone signalization. In the annelid Platynereis dumerilii, we identified a functional thyroid hormone receptor and a role of TH in its development. This proves that thyroid hormone signaling was present at the basis of bilaterians. We also assessed the question of the origin of thyroid hormone synthesis by tracing back the evolution of the thyroglobulin, the protein mandatory for thyroid hormone synthesis in vertebrates. It absence in the other taxa, whereas thyroid hormone derivatives have a biological role, asks the question of the ancestral mechanisms of thyroid hormone synthesis.These works investigate the evolution of the thyroid hormone signalization and suggest to tackle the question of the link with metamorphosis from an evolutionary perspective, and not only from a developmental one, in order to better understand the diversity of metamorphosis overserved in the animal kingdom.

Embryogenesis is one of the most delicate biological processes which requires precise control in various levels, including molecular distribution and gene expression, cellular orientation and specification, and tissue dynamics giving rise to proper morphology. The diverse animal morphology can be resulted from the difference during early embryonic cleavages. Spiral cleavage is a conserved embryonic patterning strategy used in the majority of the animal clade Spiralia. The specific cell positioning during cell division and quadrant-based clonal domain formation make the embryos with the blastomeres orientated in a spiral manner when viewing from the animal pole. Although spiral cleavage is conserved in many phyla, the detailed cellular, molecular and biophysical mechanisms for this left-right symmetry breaking event remain unclear. Here I studied the early development of the prototypic annelid spiral-cleaver Platynereis dumerilii, which performs two unequal embryonic cleavages followed by the first dextral spiral cleavages, and compared the mechanisms to other spiralians or to other cleavage types. First, I described the morphology of each cell cycle from the zygote until 64-cell stage by imaging the fluorescently labeled fixed embryos. Second, with mRNA injection, whole-embryo live-imaging with Selective Plane Illumination Microscopy (SPIM), and in silico cell tracking, I monitored these cleavages in 4-D, constructed the early cell lineages, and revealed the subtle asynchrony of the four quartets. Third, together with the spindle inclination angle measurement, I discovered the leading role of the D macromere during P. dumerilii spiral cleavage. I also confirmed that the dextral micromere orientation is neither affected by the eggshell nor the presence of all the neighbor macromeres, suggesting that this cellular property may be achieved by cell autonomous molecular mechanisms. In order to quantify the candidate cytoskeletal dynamics during spiral cleavage, I optimized the construction of the injected mRNAs and the injection protocol to achieve the highest translational level of the fluorescent protein within a given developmental time. Beside mRNA injection, I also established a protein expression and injection protocol for P. dumerilii protein injection in order to visualize the target gene as early as possible. Both techniques didn’t dramatically influence embryogenesis and allow for quantification of the protein dynamics. With these strategies, I discovered and measured the chiral counter rotational flow of cortical actomyosin in each spiral cleavage and revealed that it’s present in the first two spiral cleavages, especially of the macromeres. The biophysical force generated by actomyosin contributes in the cell deformation and spindle inclination, resulting in proper dextral micromere positioning, during the first spiral cleavage, confirmed by the chemical treatment to the P. dumerilii embryos. The asymmetric actomyosin distribution, nuclei migration, and the change of the cell axes during cytokinesis in the macromeres also suggests that the macromeres may play critical roles to lead spiral cleavage. This work is built on the knowledge of the spiral cleavage machinery and has extended it in multiple dimensions. The detailed phase-by-phase description of each cleavage increases the information of P. dumerilii embryogenesis. The established labeling and imaging techniques in this thesis are the important basis for investigation and comparisons of different spiralian development in the future. More broadly, the discovery of actomyosin dynamics shows conservation to the left-right symmetry breaking events of the animals which does not belong to Spiralia. These together bring insights to a global evolutionary speculation: a conserved mechanical force generation pathway, tuned by the upstream molecular signals, may be the key of the miscellaneous cleavage types, resulting in the astonishing variety of embryo patterning.

Gene regulatory networks, underlying the molecular regulation of eye development are conserved across many animal phyla. Genes from the Pax family of transcription factors are one of the most conserved members through the evolution, regulating the development of crucial parts of eye, including the photoreceptor cells. Pax transcription factors are considered to be regulators of opsins, molecules providing the conversion of the light stimulus into the electrochemical signalisation in the photoreceptors cells. In this thesis, pax6 and pax2/5/8 transcription factors are investigated as potential regulators of eye development in Platynereis dumerilii. pax6 and pax2/5/8 transcription factors are tested as potential regulators of the r-opsin in Platynereis, based on the observed early expression onsets of these genes. Wild-type expression analysis of pax6 and pax2/5/8 using the whole mount RNA in-situ hybridization is provided, accompanied by the initial analysis of the Platynereis pax6 knockout line. pax6 heterozygote mutants are shown to be viable and able to reproduce, however, homozygote mutation of pax6 in Platynereis is lethal. Our data suggest that transcription factors pax2/5/8, otx and six3 are not regulated by the pax6 in Platynereis. Concerning the r-opsin present in the Platynereis eyes, pax6...