MATERNAL MESSENGER-RNA ENCODING THE ORPHAN STEROID-RECEPTOR SPCOUP-TFIS LOCALIZED IN SEA-URCHIN EGGS

The SpCOUP-TF gene is a highly conserved sea urchin homologue of the v ertebrate COUP-TFs and the Drosophila seven up subfamily of transcript ion factors, which are members of the orphan steroid hormone receptors , Whole-mount in situ hybridization experiments, using three sea urchi n species, detect the maternal SpCOUP-TF mRNA deposited unevenly in th e oocytes, mature eggs and the blastomeres of the early embryo, The lo calization pattern indicates that, in all three sea urchin species, th e maternal SpCOUP-TF mRNA is placed in the egg in a fixed position rel ative to the embryonic axes, i.e. lateral to the animal-vegetal and at 45 degrees angle to the oral-aboral (ventral-dorsal) axis, The embryo nic expression of the SpCOUP-TF gene is spatially restricted in the or al ectoderm of the early embryo and, at later stages, in the cells of the ciliated band, the neurogenic cell lineage of the sea urchin embry o. The SpCOUP-TF mRNA, the first sea urchin maternal mRNA encoding a t ranscription factor that is specifically localized with respect to bot h embryonic axes in the egg, could be involved in early specification events in the sea urchin embryo.