Regional specification in the early embryo of the brittle star Ophiopholis aculeata
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Early embryogenesis has been examined experimentally in four of the seven echinoderm and hemichordate classes. Although these studies suggest that the mechanisms which underlie regional specification have been highly conserved within the echinoderm + hemichordate clade, nothing is known about these mechanisms within the other echinoderm classes, including the Ophiuroidea. In this study, early embryogenesis was examined in the ophiuroid Ophiopholis aculeata. Several aspects of early development in this ophiuroid differ from those of other echinoderms and hemichordates. In O. aculeata, the first two cleavage planes do not coincide with the animal-vegetal axis but rather form approximately 45 degrees off this axis. Fate maps of 2-, 4- and 8-cell embryos were constructed using microinjected lineage tracers and indicate that ectoderm, endoderm, and mesoderm segregate unequally at first cleavage. The distribution of vi developmental potential in the embryo was examined by isolating different regions of the early embryo and indicate that endomesodermal developmental potential segregates unequally at first, second, and third cleavage. In other echinoderm and hemichordate embryos, similar unequal segregations of larval fates and developmental potential typically do not occur until third cleavage. These results indicate that there has been an evolutionary shift in the orientation of the early cleavage program in O. aculeata with respect to the distribution of larval fates and developmental potential. Experiments were also performed to gain insight into the molecular mechanisms which underlie axial specification in O. aculeata. Evidence for a role of nuclear β- catenin was assessed by examining its localization and by perturbing its activity. β- catenin localization was observed during gastrula-stages when it was seen in adherens junctions of epithelial cells; there was no indication of any nuclear localization of β- catenin throughout embryogenesis. Treatment of embryos with LiCl (an inhibitor of β- catenin degradation) produced results largely inconsistent with its role in endomesodermal specification. Dorsal-ventral patterning mechanisms were investigated by treating embryos with NiCl2 (which radialize echinoid embryos, potentially by disrupting TGFβ signaling). Nickel-treatment disrupted ectodermal development but never radialized embryos completely. These results suggest that the molecular mechanisms of axial specification in O. aculeata may differ from those of echinoid embryos.