DUBOC, V.*; R�TTINGER, E.; LAPRAZ, F.; BESNARDEAU, L.; LEPAGE, T.; UMR 7009 CNRS-UPMC : Role of Nodal in establishment of sea urchin larva body plan
Nodal factors play crucial roles during embryogenesis of chordates. They have been implicated in a number of developmental processes, including mesoderm and endoderm formation and patterning of the embryo along the anterior-posterior and left-right axes. We have recently isolated the first nodal gene from a non-chordate organism, the sea urchin. We have found that while Nodal signalling does not appear critical for the establishment of mesodermal and endodermal germ layers, it plays a crucial role during axis specification. Sea urchin nodal starts to be expressed in the presumptive oral ectoderm and controls the formation of the oral-aboral axis. The sea urchin nodal gene acts upstream of several transcription factors and secreted factors implicated in formation of the oral and aboral territories including gooseco�d, brachyury and BMP2/4. The oral ectoderm territory of the sea urchin embryo seems to behave very much like the organizer of vertebrates by emitting signals that pattern the embryo along the oral-aboral axis. A second conserved role for nodal signaling during deuterostome evolution is its involvement in the establishment of left-right asymmetries. Sea urchin larvae exhibit profound left-right asymmetry with the formation of the adult rudiment from the left coelomic pouch. We have shown that a nodal/lefty/pitx2 gene cassette regulates left-right asymmetry in the sea urchin but that intriguingly, the expression of these genes is reversed compared to vertebrates. We show that Nodal signals emitted from the right ectoderm of the larva regulate the asymmetrical morphogenesis of the coelomic pouches by inhibiting rudiment formation on the right side of the larva. Our results show that the mechanisms responsible for patterning the left-right axis are conserved in echinoderms. Altogether, it appears that the role of Nodal during axis specification is conserved in basal deuterostomes.
