Are Hox Genes Ancestrally Involved in Axial Patterning? Evidence from the Hydrozoan Clytia hemisphaerica (Cnidaria)

Background: The early evolution and diversification of Hox-related genes in eumetazoans has been the subject of conflicting hypotheses concerning the evolutionary conservation of their role in axial patterning and the pre-bilaterian origin of the Hox and ParaHox clusters. The diversification of Hox/ParaHox genes clearly predates the origin of bilaterians. However, the existence of a "Hox code' predating the cnidarian-bilaterian ancestor and supporting the deep homology of axes is more controversial. This assumption was mainly based on the interpretation of Hox expression data from the sea anemone, but growing evidence from other cnidarian taxa puts into question this hypothesis. Methodology/Principal Findings: Hox, ParaHox and Hox-related genes have been investigated here by phylogenetic analysis and in situ hybridisation in Clytia hemisphaerica, an hydrozoan species with medusa and polyp stages alternating in the life cycle. Our phylogenetic analyses do not support an origin of ParaHox and Hox genes by duplication of an ancestral ProtoHox cluster, and reveal a diversification of the cnidarian HOX9-14 genes into three groups called A, B, C. Among the 7 examined genes, only those belonging to the HOX9-14 and the CDX groups exhibit a restricted expression along the oralaboral axis during development and in the planula larva, while the others are expressed in very specialised areas at the medusa stage. Conclusions/Significance: Cross species comparison reveals a strong variability of gene expression along the oral-aboral axis and during the life cycle among cnidarian lineages. The most parsimonious interpretation is that the Hox code, collinearity and conservative role along the antero-posterior axis are bilaterian innovations

L'évolution du système nerveux des Cœlentérés (Cnidaires et Cténaires) (un concept phylogénétique confronté à la démarche Evo-Dévo)

L'acquisition d'un système nerveux est une synapomorphie des eumétazoaires, groupe monophylétique rassemblant cnidaires, cténaires et bilatéria. Plusieurs hypothèses phylogénétiques ont été proposées pour ce grand clade, mais les résultats récents de phylogénomique soutiennent l'existence d'un clade [cnidaires+cténaires] groupe-frère des bilatéria. Cette nouvelle hypothèse réhabilite le groupe des cœlentérés favorisé par la littérature zoologique classique et implique de nouveaux senarii pour l'évolution du système nerveux des métazoaires. Cette thèse se propose donc d'analyser quelques caractéristiques du système nerveux des cnidaires et des cténaires dans ce nouveau cadre phylogénétique. L'anatomo-morphologie du système nerveux ainsi que les profils d'expression de gènes impliqués dans sa mise en place chez les bilatéria, en particulier les gènes Hox, ont été étudiés chez le cténaire Pleurobrachia pileus et le cnidaire Clytia hemisphaericaPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Insights into the early evolution of SOX genes from expression analyses in a ctenophore.

International audienc

Insights into the early evolution of SOX genes from expression analyses in a ctenophore.

International audienc

New insights on ctenophore neural anatomy: immunofluorescence study in Pleurobrachia pileus (Müller, 1776).

International audienc

Tooth and scale morphogenesis in shark: an alternative process to the mammalian enamel knot system

International audienceBackground: The gene regulatory network involved in tooth morphogenesis has been extremely well described in mammals and its modeling has allowed predictions of variations in regulatory pathway that may have led to evolution of tooth shapes. However, very little is known outside of mammals to understand how this regulatory framework may also account for tooth shape evolution at the level of gnathostomes. In this work, we describe expression patterns and proliferation/apoptosis assays to uncover homologous regulatory pathways in the catshark Scyliorhinus canicula.Results: Because of their similar structural and developmental features, gene expression patterns were described over the four developmental stages of both tooth and scale buds in the catshark. These gene expression patterns differ from mouse tooth development, and discrepancies are also observed between tooth and scale development within the catshark. However, a similar nested expression of Shh and Fgf suggests similar signaling involved in morphogenesis of all structures, although apoptosis assays do not support a strictly equivalent enamel knot system in sharks. Similarities in the topology of gene expression pattern, including Bmp signaling pathway, suggest that mouse molar development is more similar to scale bud development in the catshark.Conclusions: These results support the fact that no enamel knot, as described in mammalian teeth, can be described in the morphogenesis of shark teeth or scales. However, homologous signaling pathways are involved in growth and morphogenesis with variations in their respective expression patterns. We speculate that variations in this topology of expression are also a substrate for tooth shape evolution, notably in regulating the growth axis and symmetry of the developing structure

Additional file 3: of Tooth and scale morphogenesis in shark: an alternative process to the mammalian enamel knot system

Apoptosis detection in developing scale buds in the catshark Scyliorhinus canicula. Animation of successive merged views by confocal z-imaging of a of a thick transversal slice of a tail: apoptotic nuclei (TUNEL, red), and counter-staining against actin (phalloidin, red) and DNA (DAPI, blue). (AVI 23043 kb

Additional file 2: of Tooth and scale morphogenesis in shark: an alternative process to the mammalian enamel knot system

Apoptosis detection in developing tooth buds in the catshark Scyliorhinus canicula. Animation of longitudinal successive merged views (from surface to internal) by confocal z-imaging of a whole-mount jaw after staining of apoptotic nuclei (TUNEL, red), and counter-staining against actin (phalloidin, red) and DNA (DAPI, blue). Please note the scale on the merged image is not correct. (AVI 25347 kb