MANIPULATING CELL FATES IN THE SEA-URCHIN EMBRYO

This article describes the mechanisms associated with ectoderm differe ntiation during sea urchin embryogenesis. Using animal half explants a nd molecular markers for oral and aboral ectoderm cells, we present ev idence that in Lytechinus pictus, two signals from underlying vegetal blastomeres are required for normal ectoderm differentiation. One sign al induces the expression of aboral ectoderm genes and a second restri cts oral ectoderm expression to the oral side of the embryo. Strongylo centrotus purpuratus animal halves behave somewhat differently than Ly techinus pictus in their ability to express aboral ectoderm-specific g enes. In order to determine how ectoderm genes are activated in early development, we have analyzed the transcriptional control region of th e aboral ectoderm-specific Spec2a gene. This control region confers ab oral ectoderm-specific expression on Spec2a by means of an aboral ecto derm/mesenchyme cell-specific enhancer and a mesenchyme cell repressor element. The Spec2a enhancer contains multiple DNA-binding sites for SpOtx, a homeobox transcription factor related to Drosophila orthodent icle and mouse Otx1 and Otx2. SpOtx is present throughout development but unexpectedly, at stages before late cleavage is found mainly in th e cytoplasm. Between late cleavage and early blastula stages, SpOtx tr anslocates into the nucleus. Overexpressing SpOtx by injecting SpOtx m RNA into living eggs results in a thin epithelial ball with essentiall y all cells displaying aboral ectoderm properties. We suggest a model whereby overexpression of SpOtx leads to premature nuclear translocati on and conversion of all progenitor cells in the embryo to an aboral e ctoderm fate. Our results imply that SpOtx plays a major role in the d ifferentiation of aboral ectoderm cells during sea urchin development.